Mechanical Ventilation:
Application & Effect
Mechanical ventilation involves a number of relationships which must be understood and appreciated in order to deliver safe and effective ventilatory support for each patient. The following are brief overviews of the more important of these relationships:
Respiratory Rate
Definition:
• The frequency with which breaths occur, usually measured over a period of one minute.
“Normal” Values:
• “Normal” spontaneous respiratory rates (RR) vary between age groups.
• For example, infants may normally breathe 30-60 breaths per minute (bpm), children 20-25 bpm, and adults at 10-15 bpm.
• Each breath consists of an “inspiratory phase/time (Ti)” and an “expiratory phase/time (Te)”. Te is normally longer than Ti, and includes a slight pause between breaths.
• The duration of “normal” Ti and Te will vary between age groups, may change dramatically with changes in medical condition, and may significantly effect variables such as cardiac output, venous return and air trapping within the lungs.
Factors & Effects:
• A comfortable respiratory rate for any individual may be affected by many factors:
• the depth of respiration (tidal volume)
• lung function (ie. fibrosis, COPD, Asthma)
• metabolic rate (ie. sepsis, exercise, high work of breathing)
• blood flow and volume (ie. low cardiac output, embolism)
• effective hemoglobin levels or presence of abnormal hemoglobin (ie. methemoglobin, carboxyhemoglobin (smoke inhalation))
• ambient conditions (ie. high altitude, air pollution, impure medical gas supply)
• pain or anxiety, etc.
• An increase in respiratory rate generally reflects an attempt to increase levels of oxygen (O2) or decrease levels of carbon dioxide (CO2) within the blood by delivering more gas to the lungs, secondary to factors such as those listed above.
• A decrease in respiratory rate may reflect:
• decreased metabolic needs (sleep being the best example)
• neuro-muscular illness (ie Myasthenia Gravis, Guilliane-Barre syndrome)
• extreme fatigue (ie. severe asthma)
• chemical depression (ie. morphine, residual anesthetic)
• a period of recovery following hyperventilation secondary to transient pain or anxiety.
Clinical Wisdom:
• First, select an appropriate tidal volume or ventilating pressure, and THEN select a rate to maintain the desired acid-base balance.
• Coordinate ventilator settings (Ti, rate) to maximize patient comfort and minimize work of breathing.
• Changes in rate are significant. Look to the patient for the cause.
Pressure
Definition:
• Pressure is a measure of the force exerted by one medium onto another. For example, air filling a lung exerts pressure onto the walls of the alveoli, which increases as more air occupies the space within those walls. This measurement may be expressed in many ways, such as centimeters of water (cmH2O), millimeters of mercury (mmHg), pounds per square inch (psi), etc.. The most common reference for pressure associated with mechanical ventilation is cmH2O.
• Negative pressure exerts a force which “pulls” toward rather than pushes against. Normal breathing is an example of negative pressure at work. As the respiratory muscles and the diaphragm coordinate to enlarge the chest cavity, a negative pressure is generated within the airways. This vacuum draws air into the lungs.
• Positive pressure is the force associated with conventional mechanical ventilation. Gas is “pushed” into the lungs, exerting a positive force within the airways and alveoli. This expands both the lungs and the chest wall. While this may be an effective method of providing ventilation, it works contrary to the normal physiology of the structures within the chest cavity. Thus, the need to provide adequate ventilation must be balanced with any negative effects it may produce.
• Compliance is the relationship between a specific volume and the pressure it generates within a confined space, expressed as V/P (the change in volume divided by the change in pressure).
• Closing Pressure is the pressure at which the airways close and the flow of air ceases. In disease states, this pressure may be dramatically elevated by inflammation, secretions, fibrosis, etc. Once the airways close, a pressure greater than the closing pressure is generally needed to re-open them (opening pressure), secondary to cohesive and adhesive forces within the airways. Thus, there is significant clinical advantage to maintaining airway pressures above the closing pressure.
“Normal” Values:
• A healthy human lung is very compliant, normal values for resting (static) lung compliance being in the range of 50-85 mL/cmH2O.
• Mechanical ventilation of a healthy lung with an appropriate volume of gas commonly generates pressures of approximately 15-20 cmH2O.
Factors & Effects:
• Accumulation of secretions in the airways may dramatically narrow points along the lumen of the airways, which will increase resistance to air flow and increase the pressure required to sustain that flow. If not removed, the airway may become occluded by “plugs” of dried or thick secretions. Alveoli behind the plugs will eventually collapse. An effective regimen to maintain broncho-pulmonary toilet is, therefore, essential.
• Mild manual hyperinflation of the lungs both prior to and after suctioning procedures will help to recruit collapsed alveoli, and assist with expulsion of foreign materials during exhalation.
• A strong cough by the patient remains the most effective way of clearing secretions.
• Inflammation of the airways will also dramatically increase resistance to the flow and distribution of gas through those airways, as well as hinder attempts to clear secretions. Long-term inflammatory processes are often associated with fibrotic changes in the lung parenchyma. Both processes are generally associated with significantly elevated ventilating pressures, along with the increased risk of barotrauma and infection.
• External factors such as patient position, obesity, splinting, partial or total occlusion of the artificial airway, or simply an artificial airway that is too small for the patient’s needs can account for elevated ventilating pressures in patients with healthy lungs.
• External factors can dramatically increase the patient’s work of breathing. Fortunately, most are also easily recognized by an attentive caregiver and can be quickly corrected or minimized.
Clinical Wisdom:
• Peak pressures above 35 cmH2O are associated with a sharp increase in the incidence of barotrauma. Thus, during mechanical ventilation, it is important to achieve the lowest possible peak inspiratory pressures while maintaining adequate acid-base status.
• Maintain a base pressure (PEEP: Positive End-Expiratory Pressure) above the patient’s closing pressure to maintain airway patency, facilitate ventilation and minimize the risk of barotrauma.
• Strive to balance the many factors which affect pressure during mechanical ventilation: Ventilator mode, flow, volume, PEEP, Ti, the sensitivity of the ventilator and the type of wave form utilized. Familiarity with the type of ventilator being used and coordination with patient efforts is often the key to this balance.
• A sudden and dramatic elevation of ventilating pressures is very often due to the patient purposely occluding (ie biting) the artificial airway. This is also often the best way for a conscious, ventilated patient to get your attention.
• On all patients, ensure that pressure-relief controls and/or devices are appropriately set for the individual patient.
Volume
Definition:
• The specific amount of gas delivered to the patient during a given breath or over a period of time.
• The volume delivered may directed by a set value, a spontaneous effort (or combination of the two) or as a result of a controlled pressure or flow over a period of time (ie a pressure-controlled breath).
• Tidal Volume (Vt): The amount of gas in an individual breath.
• Minute Volume (VE) The amount of gas delivered to the patient over a period of one minute.
• Dead-Space Volume (Vds): The amount of gas in an individual breath which does not participate in gas exchange within the lungs. This volume is measured from the point where fresh gas enters the patient’s airway (ie connection between the ventilator circuit and the endotracheal tube). Also referred to as “physiological” dead space, this volume is further divided into “anatomical” dead space and “alveolar” dead space.
• Anatomical dead space is the volume that occupies anatomical structures which would not normally participate in gas exchange (ie structures above the alveoli such as the bronchi, trachea, etc).
• Alveolar dead space is the volume within the alveoli that does not participate in gas exchange, usually due to abnormalities in perfusion.
• Dead space can be added to a circuit to replace lost anatomical dead space (ie tracheostomy, which bypasses the upper airway) or to attempt to increase carbon dioxide retention in the presence of persistent hyperventilation.
• Compressible Volume: The volume which remains in the pressurized ventilator circuit during a mechanical breath. This volume will be dependent on the compliance of the ventilator circuit and the volume of the unfilled portion of the humidifier chamber, if applicable. Ventilator circuits typically have a compliance of 3 ml/cmH2O or less. Thus, if a breath of 500 ml is delivered through a circuit with a compliance of 2 ml/cmH2O and generates a peak pressure of 25 cmH2O, at least 50 ml of this mechanical breath will be “lost” in the circuit and not reach the patient’s airways. This volume becomes more significant with smaller tidal volumes, such as with pediatric or infant patients, as it is likely to represent a much larger percentage of each mechanical breath.
“Normal” Values:
• There is no set “normal” tidal volume (Vt), as each patient must be assessed individually in this regard. Approximate values range from 6-15 ml/kg and appropriate Vt is dependent on the state of the lungs, the type of surgery involved, the desired goals and duration of ventilation, and the preferred practices of the facility or physicians involved. Ml/kg requirements are best determined by an approximated “lean” body weight, and a Vt of greater than 1 liter is rarely necessary, regardless of body weight. Visualization of appropriate chest expansion is an excellent method of verifying appropriate volume once the patient is connected to the ventilator.
• Normal anatomical dead space is approximately one third of an individual’s resting Vt. This value is reduced by approximately 50% in patients with tracheostomy.
• In a healthy lung, alveolar dead space is insignificant. However, in compromised lungs, this value may increase dramatically and account for a substantial portion of the overall dead space volume.
Factors & Effects:
• Obesity may interfere with determination of an appropriate Vt, as the excursion of both chest wall and diaphragm may be hindered by the excess of surrounding tissues.
• Compressible volume may be significant when ventilating infants and children, or adults with poorly compliant lungs. Again, visualization of appropriate chest expansion on inspiration is a vital clinical tool in determining appropriate volume.
• Hyperinflation of the lungs (excess volume) may cause lung injury or compromise cardiac function. The patient is particularly at risk during manual ventilation, where the volumes being delivered are often far less exact or consistent than those delivered by the ventilator.
Clinical Wisdom:
• Assessment of the individual patient is vital in determining initial settings for volume.
• If in doubt, err on the low side when setting initial volumes on the ventilator. If chest expansion is poor with the initial settings, volume can quickly be increased and risk of volutrauma is minimized.
• Visualization of chest expansion is an excellent tool for assessing the appropriateness of the volumes being used during both mechanical or manual ventilation. A good clinician uses this tool frequently.
• Auscultation for bilateral air entry is also an important assessment tool, as an appropriate volume may not produce appropriate chest excursion if the gas is not properly distributed throughout the lungs.
• Appropriate volumes for an unconscious patient may not necessarily be sufficient for a conscious patient. Thus, it is important to ensure that the ventilator is properly set up to both deliver a comfortable level of ventilation, and to accommodate any patient efforts to breath. (Note: More often, it is the rate that determines patient comfort once an appropriate volume has been set. Demand valves or similar features, which allow a patient to draw gas from the ventilator over and above the set amount, are a common feature of modern ventilators and allow the patient to make breath by breath adjustments in volume. Therefore, adjusting the volume setting is an option, but not necessarily the first or best option.)
• As a guideline only:
• For patients with essentially healthy lungs, an initial Vt of 10-12 ml/kg (to a maximum of 1000 ml) should return appropriate chest excursion in most patients, without producing excessive ventilating pressures.
• For infants being volume-ventilated, this initial volume may be as high as 15-20 ml/kg, to compensate for compressible volume in the circuit and produce satisfactory chest expansion.
• For patients with compromised lung status (ie fibrosis, pneumonia), a lower initial Vt of approximately 8-10 ml/kg is recommended, as the lung parenchyma is likely to be much more susceptible to ventilation-associated injury.
Positive End-Expiratory Pressure (PEEP)
Definition:
• PEEP is a pressure which is maintained in the lungs at the end of an exhaled breath.
• The purpose of PEEP is to prevent the alveoli from emptying completely and collapsing with exhalation, or to reopen areas of atelectasis. In turn, ventilation to these areas, with corresponding oxygenation, is generally improved (improved ventilation-perfusion ratio (Qs/Qt)).
• The body maintains a physiological PEEP of approximately +2 cmH2O, which is facilitated by closure of the glottis prior to complete relaxation of the muscles which affect expansion of the chest cage.
• PEEP is also a “parameter” which can be set and regulated by modern ventilators. Note: PEEP is often confused with CPAP (Continuous Positive Airway Pressure), which is a “mode” of ventilation normally centered around a certain level of PEEP.
• “Best” PEEP is defined as the level of PEEP which provides the best lung compliance.
• “Optimal” PEEP is defined as the level of PEEP which provides for optimal lung function (ie oxygen transport) with minimal decrease in cardiac output.
• Best PEEP and Optimal PEEP should be the same in a healthy lung. However, oxygenation can often be dramatically increased with PEEP levels only slightly higher than “best” PEEP, particularly in a compromised lung. Note: At high PEEP levels, optimal PEEP does not often correlate with A-aO2 gradients, PaO2 or Qs/Qt.
“Normal” Values:
• PEEP levels of 3-5 cmH2O are common practice and are generally effective in preventing atelectasis when combined with adequate inspiratory volumes.
• PEEP levels of 7.5-12.5 cmH2O are often applied to slow post-operative bleeding following thoracic surgery. Once bleeding has slowed or stopped, PEEP levels may then be reduced to a lower level, as tolerated.
Factors & Effects:
• As a positive pressure, PEEP above the physiologic level works contrary to the normal physiology of the thoracic cage (negative pressure generated during spontaneous inspiration enhances venous return to the heart) and may significantly affect venous return and cardiac function. Therefore, the addition of PEEP to a ventilatory regimen must be balanced with these functions.
• For infants in the perinatal period, elevated PEEP levels may be associated with an increased risk of cerebral hemorrhage secondary to decreased venous return to the heart and cerebral vascular congestion. This risk is most significant in preterm infants. While rare in a short-term, postoperative course of ventilation and more often associated with high peak inspiratory pressures, it should still be kept in mind.
Clinical Wisdom:
• The presence of an endotracheal tube denies the patient the ability to close the glottis and maintain the normal physiological level of PEEP in the lungs. Thus, the inclusion of even a small amount of PEEP via the ventilator is recommended for most patients. In cases where PEEP is contraindicated, such as caval-pulmonary shunts, adequate tidal volumes and rapid weaning to extubation will be the best measures to protect against progressive atelectasis in the absence of PEEP.
• With increases in PEEP, assess the patient for cardiac-related effects, particularly changes in blood pressure. In patients with low blood pressure, such increases should be made judiciously and in small increments.
• In cases where excessive chest expansion during ventilation is a concern (ie CABG with LIMA/RIMA graft), a moderate level of PEEP is not contraindicated. The advantages of maintaining PEEP in such cases (prevention of atelectasis, lower opening pressures, improved oxygenation) far outweigh any possible risk to the graft.
The Mechanical Ventilator
The First Rule of health care is “Do No Harm”.
With this in mind, the First Rule of mechanical ventilation is “Know Your Equipment”.
The ventilator is a versatile tool in the hands of a skilled operator. While clinical expertise with mechanical ventilators is attained through patient application, a fair degree of technical expertise can be attained prior to patient application. Time spent in familiarizing oneself with a new ventilator using a test lung or patient simulator is common practice among Respiratory Therapists. However, this is less often the case with physicians and nurses. This may be due to lack of opportunity or secondary to a division of responsibilities where ventilation is concerned. In such cases, the caregiver may have to take the initiative and arrange for both instruction and “play time” with an unfamiliar ventilator. Even if the physician or nurse is not the primary operator of the ventilator in the clinical setting, “hands on” experience with the ventilator in a test setting may contribute significantly to patient care on several levels.
With its capabilities and limitations revealed, the ventilator is immediately less intimidating. The caregiver becomes more comfortable with the ventilator and thus, is better able to evaluate the ventilator’s contribution to the care of the patient. Minor problems with the ventilator may be approached with greater calm and confidence. Changes in patient ventilatory status, often mirrored in the ventilator’s performance, are more easily recognized. As well, the ventilator’s potential is better understood, which allows for more versatile patient care. All, good things.
Play Time: This term is not to be used lightly, and refers only to operation of a ventilator while using a test lung. However, within this one condition, it is an accurate description. This is time spent “playing” with the ventilator, and is meant not only for discovering what the machine WILL do, but also what it will NOT do, and how it reacts to parameters both within and outside of normal clinical use. This is the one situation where you may actually play with the settings and discover what happens if you do this, or do that. If you have this opportunity, play. Discover what happens if one component of the circuit comes loose, or is occluded … or what happens when the power cord is disconnected from the wall outlet. With the exception of bursting the test lung (try to avoid this, but better a test lung than a real lung), it is unlikely that you will damage the ventilator or its circuit. You will, however, rapidly become more familiar with the ventilator and have a better appreciation of what it can do for (or to) your patient. This will be the only setting where playing is allowed. Once the ventilator is attached to a patient, the time for playing will have passed.
The Ventilation Experience: If you have the opportunity during your “play time”, and using settings within normal clinical ranges, become the patient and try breathing through an endotracheal tube while attached to the ventilator. With an ETT held securely in your mouth and your nares occluded, test out each mode, various levels of PEEP and sensitivity, various flow patterns and alarm settings. Try it for five minutes. It is highly likely that, by the end of those five minutes, you will have a much better appreciation of what your patients experience. NOTE: If you are breathing non-humidified gas during this trial, be aware that you may experience some discomfort in your airways. For this reason, it is recommended that you utilize a humidification device and an FiO2 of 0.21. “Do No Harm” applies to yourself , as well as to your patients. As well, be aware that the ventilator circuit will be contaminated by this practice.
Once again, this is a common practice during a Respiratory Therapist’s training, but is rarely experienced by physicians and nurses. This is unfortunate, as this simulation is a very valuable and memorable learning experience.
Ventilators are not all the same: Familiarity with one model of ventilator does not ensure familiarity or competency with other models. While they may share some features, the operating characteristics between one model and another may differ considerably. It is important that the caregiver become familiar with each model of ventilator used in their sphere of responsibility.
Manual Ventilation
The use of a manual resuscitation device is a relatively simple and often under-rated procedure. Simple, in that the operation of a self-inflating “bag” is not difficult, and under-rated in that the technique used is often given little consideration despite the wide array of effects that it can produce. The following are considerations for proper use of the manual resuscitation device:
• Manual resuscitation devices often come in various sizes (ie infant, pediatric, adult). The operator should be aware of the volumes of both the main body and the reservoir of the device in use. The device should be able to deliver approximately twice the tidal volume being delivered by the ventilator, or better, in order to guarantee a sufficient range of ventilation for the patient.
• Compression of the “bag” during manual ventilation should produce chest expansion comparable to that of a ventilator breath. This may not require full compression of the bag. A common mistake is to try to compress the bag fully, often with two hands, in order to give a “big breath”, without thought to the volume being delivered. This practice may produce dangerously high volumes, particularly in smaller patients. Chest expansion must be visualized to ensure that compression levels are appropriate.
• At the end of compression, the bag must be released and the patient allowed to exhale. A common mistake is to hold the compression for a time. This practice places considerable stress on the alveoli, particularly if they are already over-distended with excess volume. This may also compromise cardiac function by impeding venous return. On a self-inflating device, the patient will not be able to exhale as long as this compression is held. On a flow-inflating device, exhalation is possible during the period of compression, but the effort required to do so is considerable. Thus, this practice is not recommended.
• Inspiratory time (Ti), or the time used to compress the bag and deliver the breath, should reflect a normal breath as much as possible. A breath delivered too quickly involves highly turbulent gas flows which may not disperse throughout the lung as evenly as would be possible with a slower flow. This tends to produce areas of hyperinflation and areas of hypoventilation within the lung. Again, the hyperinflated alveoli are stressed. A normal Ti will allow the gas to distribute itself more effectively throughout the lung. An abnormally long Ti may be uncomfortable for a conscious patient. An effective gauge of appropriate inspiratory time for an adult is to breath along with the breath you are delivering to the patient. If it feels comfortable to you, there is a good possibility that it will also be comfortable for your patient. For smaller patients, the goal is to mimic the cycling of the ventilator as much as possible.
• Expiratory time (Te) is the time allowed for the patient to exhale between breaths. Te should be equal to or longer than Ti. A common mistake is to compress the bag immediately upon re-inflation. This may not give the patient sufficient time to exhale. This practice, known as “stacking breaths”, is very uncomfortable to a conscious patient and is likely to produce excessive intrathoracic pressures and alveolar over-distention under the best of circumstances. During higher rates of ventilation, both Ti and Te will need to be shortened to provide the proper balance between the two.
• The gas source, in most cases, will be 100% oxygen delivered via flowmeter. It is important to verify the gas source before using the device, particularly in an emergency situation. In some units (ie NICU), the gas source may be an air-oxygen blender. This device will allow for more specific control of the FiO2 in use. Again, the gas source and setting should be verified before using the device on the patient.
• Gas flow to the bag should be sufficient to maintain partial inflation of the reservoir. A slight deflation of the reservoir during re-inflation of the bag indicates an appropriate flow. At this flow, the reservoir should re-inflate during the breath. If the reservoir collapses completely between breaths, the flow is too low and the delivered FiO2 may be compromised. If the reservoir does not deflate at all between breaths, flow is too high and gas is being wasted. This is of particular importance if a tank is being used as the gas source. Once an appropriate rhythm of ventilation has been established, the flow may be adjusted to conserve resources.
Clinical Wisdom:
• Prior to beginning manual ventilation, ensure that your equipment is properly assembled and functioning as required.
• During manual ventilation, focus on providing appropriate ventilation.
• During manual ventilation, support the resuscitation device and minimize movement of the airway. This will reduce the risk to the security of the airway and be much more comfortable for the patient.
• During manual ventilation, coordinate your efforts with the respiratory efforts of the patient as much as possible. Ventilating opposite patient efforts to breathe will tire and stress the patient, and provide very ineffective ventilation.
• During long-term manual ventilation (ie during transport), frequently assess the patient for changes in ventilatory status.
• The resuscitation bag is a life-supporting device. If you are unsure of your technique, seek advice/instruction from a colleague with more experience. This is particularly true with regards to flow-inflating devices, which require more skill to master.
Initial Ventilator Set-Up & Patient Application
The procedures for preparation of the bedside for a ventilated post-operative patient and for application of the ventilator upon admission may vary considerably between facilities, secondary to differences in personnel, equipment, experience and custom. However, the following are considerations which may assist you in setting up for the post-operative cardiac surgical patient and in applying the ventilator safely and effectively:
Determination of Ventilator Type & Initial Settings
• In a facility where you have the choice between two or more ventilator types, there should be a consistent method for determining the appropriate machine to use, be it by patient weight, complexity of surgery or other factors.
• If your inventory consists of purely “infant” and “adult” ventilators, a choice may have to be made between “pressure” ventilation and “volume” ventilation. A recommended cut-off value for this determination is a weight of 6.0 kg. For patients less than 6 kg, pressure-oriented ventilation may be more effective, as the small tidal volumes required for these patients may be difficult for volume-oriented ventilators to deliver.
• The complexity of the surgery involved or anticipated difficulty of post-operative ventilation should also be considered in your choice of ventilator, mode of ventilation and associated parameters.
• Newer ventilators may have the versatility to ventilate a broad spectrum of patients, from neonatal to adult. If you have limited numbers of these ventilators, you may need to reserve use of these units for the patients who will have most need of their capabilities.
Application & Effect
Mechanical ventilation involves a number of relationships which must be understood and appreciated in order to deliver safe and effective ventilatory support for each patient. The following are brief overviews of the more important of these relationships:
Respiratory Rate
Definition:
• The frequency with which breaths occur, usually measured over a period of one minute.
“Normal” Values:
• “Normal” spontaneous respiratory rates (RR) vary between age groups.
• For example, infants may normally breathe 30-60 breaths per minute (bpm), children 20-25 bpm, and adults at 10-15 bpm.
• Each breath consists of an “inspiratory phase/time (Ti)” and an “expiratory phase/time (Te)”. Te is normally longer than Ti, and includes a slight pause between breaths.
• The duration of “normal” Ti and Te will vary between age groups, may change dramatically with changes in medical condition, and may significantly effect variables such as cardiac output, venous return and air trapping within the lungs.
Factors & Effects:
• A comfortable respiratory rate for any individual may be affected by many factors:
• the depth of respiration (tidal volume)
• lung function (ie. fibrosis, COPD, Asthma)
• metabolic rate (ie. sepsis, exercise, high work of breathing)
• blood flow and volume (ie. low cardiac output, embolism)
• effective hemoglobin levels or presence of abnormal hemoglobin (ie. methemoglobin, carboxyhemoglobin (smoke inhalation))
• ambient conditions (ie. high altitude, air pollution, impure medical gas supply)
• pain or anxiety, etc.
• An increase in respiratory rate generally reflects an attempt to increase levels of oxygen (O2) or decrease levels of carbon dioxide (CO2) within the blood by delivering more gas to the lungs, secondary to factors such as those listed above.
• A decrease in respiratory rate may reflect:
• decreased metabolic needs (sleep being the best example)
• neuro-muscular illness (ie Myasthenia Gravis, Guilliane-Barre syndrome)
• extreme fatigue (ie. severe asthma)
• chemical depression (ie. morphine, residual anesthetic)
• a period of recovery following hyperventilation secondary to transient pain or anxiety.
Clinical Wisdom:
• First, select an appropriate tidal volume or ventilating pressure, and THEN select a rate to maintain the desired acid-base balance.
• Coordinate ventilator settings (Ti, rate) to maximize patient comfort and minimize work of breathing.
• Changes in rate are significant. Look to the patient for the cause.
Pressure
Definition:
• Pressure is a measure of the force exerted by one medium onto another. For example, air filling a lung exerts pressure onto the walls of the alveoli, which increases as more air occupies the space within those walls. This measurement may be expressed in many ways, such as centimeters of water (cmH2O), millimeters of mercury (mmHg), pounds per square inch (psi), etc.. The most common reference for pressure associated with mechanical ventilation is cmH2O.
• Negative pressure exerts a force which “pulls” toward rather than pushes against. Normal breathing is an example of negative pressure at work. As the respiratory muscles and the diaphragm coordinate to enlarge the chest cavity, a negative pressure is generated within the airways. This vacuum draws air into the lungs.
• Positive pressure is the force associated with conventional mechanical ventilation. Gas is “pushed” into the lungs, exerting a positive force within the airways and alveoli. This expands both the lungs and the chest wall. While this may be an effective method of providing ventilation, it works contrary to the normal physiology of the structures within the chest cavity. Thus, the need to provide adequate ventilation must be balanced with any negative effects it may produce.
• Compliance is the relationship between a specific volume and the pressure it generates within a confined space, expressed as V/P (the change in volume divided by the change in pressure).
• Closing Pressure is the pressure at which the airways close and the flow of air ceases. In disease states, this pressure may be dramatically elevated by inflammation, secretions, fibrosis, etc. Once the airways close, a pressure greater than the closing pressure is generally needed to re-open them (opening pressure), secondary to cohesive and adhesive forces within the airways. Thus, there is significant clinical advantage to maintaining airway pressures above the closing pressure.
“Normal” Values:
• A healthy human lung is very compliant, normal values for resting (static) lung compliance being in the range of 50-85 mL/cmH2O.
• Mechanical ventilation of a healthy lung with an appropriate volume of gas commonly generates pressures of approximately 15-20 cmH2O.
Factors & Effects:
• Accumulation of secretions in the airways may dramatically narrow points along the lumen of the airways, which will increase resistance to air flow and increase the pressure required to sustain that flow. If not removed, the airway may become occluded by “plugs” of dried or thick secretions. Alveoli behind the plugs will eventually collapse. An effective regimen to maintain broncho-pulmonary toilet is, therefore, essential.
• Mild manual hyperinflation of the lungs both prior to and after suctioning procedures will help to recruit collapsed alveoli, and assist with expulsion of foreign materials during exhalation.
• A strong cough by the patient remains the most effective way of clearing secretions.
• Inflammation of the airways will also dramatically increase resistance to the flow and distribution of gas through those airways, as well as hinder attempts to clear secretions. Long-term inflammatory processes are often associated with fibrotic changes in the lung parenchyma. Both processes are generally associated with significantly elevated ventilating pressures, along with the increased risk of barotrauma and infection.
• External factors such as patient position, obesity, splinting, partial or total occlusion of the artificial airway, or simply an artificial airway that is too small for the patient’s needs can account for elevated ventilating pressures in patients with healthy lungs.
• External factors can dramatically increase the patient’s work of breathing. Fortunately, most are also easily recognized by an attentive caregiver and can be quickly corrected or minimized.
Clinical Wisdom:
• Peak pressures above 35 cmH2O are associated with a sharp increase in the incidence of barotrauma. Thus, during mechanical ventilation, it is important to achieve the lowest possible peak inspiratory pressures while maintaining adequate acid-base status.
• Maintain a base pressure (PEEP: Positive End-Expiratory Pressure) above the patient’s closing pressure to maintain airway patency, facilitate ventilation and minimize the risk of barotrauma.
• Strive to balance the many factors which affect pressure during mechanical ventilation: Ventilator mode, flow, volume, PEEP, Ti, the sensitivity of the ventilator and the type of wave form utilized. Familiarity with the type of ventilator being used and coordination with patient efforts is often the key to this balance.
• A sudden and dramatic elevation of ventilating pressures is very often due to the patient purposely occluding (ie biting) the artificial airway. This is also often the best way for a conscious, ventilated patient to get your attention.
• On all patients, ensure that pressure-relief controls and/or devices are appropriately set for the individual patient.
Volume
Definition:
• The specific amount of gas delivered to the patient during a given breath or over a period of time.
• The volume delivered may directed by a set value, a spontaneous effort (or combination of the two) or as a result of a controlled pressure or flow over a period of time (ie a pressure-controlled breath).
• Tidal Volume (Vt): The amount of gas in an individual breath.
• Minute Volume (VE) The amount of gas delivered to the patient over a period of one minute.
• Dead-Space Volume (Vds): The amount of gas in an individual breath which does not participate in gas exchange within the lungs. This volume is measured from the point where fresh gas enters the patient’s airway (ie connection between the ventilator circuit and the endotracheal tube). Also referred to as “physiological” dead space, this volume is further divided into “anatomical” dead space and “alveolar” dead space.
• Anatomical dead space is the volume that occupies anatomical structures which would not normally participate in gas exchange (ie structures above the alveoli such as the bronchi, trachea, etc).
• Alveolar dead space is the volume within the alveoli that does not participate in gas exchange, usually due to abnormalities in perfusion.
• Dead space can be added to a circuit to replace lost anatomical dead space (ie tracheostomy, which bypasses the upper airway) or to attempt to increase carbon dioxide retention in the presence of persistent hyperventilation.
• Compressible Volume: The volume which remains in the pressurized ventilator circuit during a mechanical breath. This volume will be dependent on the compliance of the ventilator circuit and the volume of the unfilled portion of the humidifier chamber, if applicable. Ventilator circuits typically have a compliance of 3 ml/cmH2O or less. Thus, if a breath of 500 ml is delivered through a circuit with a compliance of 2 ml/cmH2O and generates a peak pressure of 25 cmH2O, at least 50 ml of this mechanical breath will be “lost” in the circuit and not reach the patient’s airways. This volume becomes more significant with smaller tidal volumes, such as with pediatric or infant patients, as it is likely to represent a much larger percentage of each mechanical breath.
“Normal” Values:
• There is no set “normal” tidal volume (Vt), as each patient must be assessed individually in this regard. Approximate values range from 6-15 ml/kg and appropriate Vt is dependent on the state of the lungs, the type of surgery involved, the desired goals and duration of ventilation, and the preferred practices of the facility or physicians involved. Ml/kg requirements are best determined by an approximated “lean” body weight, and a Vt of greater than 1 liter is rarely necessary, regardless of body weight. Visualization of appropriate chest expansion is an excellent method of verifying appropriate volume once the patient is connected to the ventilator.
• Normal anatomical dead space is approximately one third of an individual’s resting Vt. This value is reduced by approximately 50% in patients with tracheostomy.
• In a healthy lung, alveolar dead space is insignificant. However, in compromised lungs, this value may increase dramatically and account for a substantial portion of the overall dead space volume.
Factors & Effects:
• Obesity may interfere with determination of an appropriate Vt, as the excursion of both chest wall and diaphragm may be hindered by the excess of surrounding tissues.
• Compressible volume may be significant when ventilating infants and children, or adults with poorly compliant lungs. Again, visualization of appropriate chest expansion on inspiration is a vital clinical tool in determining appropriate volume.
• Hyperinflation of the lungs (excess volume) may cause lung injury or compromise cardiac function. The patient is particularly at risk during manual ventilation, where the volumes being delivered are often far less exact or consistent than those delivered by the ventilator.
Clinical Wisdom:
• Assessment of the individual patient is vital in determining initial settings for volume.
• If in doubt, err on the low side when setting initial volumes on the ventilator. If chest expansion is poor with the initial settings, volume can quickly be increased and risk of volutrauma is minimized.
• Visualization of chest expansion is an excellent tool for assessing the appropriateness of the volumes being used during both mechanical or manual ventilation. A good clinician uses this tool frequently.
• Auscultation for bilateral air entry is also an important assessment tool, as an appropriate volume may not produce appropriate chest excursion if the gas is not properly distributed throughout the lungs.
• Appropriate volumes for an unconscious patient may not necessarily be sufficient for a conscious patient. Thus, it is important to ensure that the ventilator is properly set up to both deliver a comfortable level of ventilation, and to accommodate any patient efforts to breath. (Note: More often, it is the rate that determines patient comfort once an appropriate volume has been set. Demand valves or similar features, which allow a patient to draw gas from the ventilator over and above the set amount, are a common feature of modern ventilators and allow the patient to make breath by breath adjustments in volume. Therefore, adjusting the volume setting is an option, but not necessarily the first or best option.)
• As a guideline only:
• For patients with essentially healthy lungs, an initial Vt of 10-12 ml/kg (to a maximum of 1000 ml) should return appropriate chest excursion in most patients, without producing excessive ventilating pressures.
• For infants being volume-ventilated, this initial volume may be as high as 15-20 ml/kg, to compensate for compressible volume in the circuit and produce satisfactory chest expansion.
• For patients with compromised lung status (ie fibrosis, pneumonia), a lower initial Vt of approximately 8-10 ml/kg is recommended, as the lung parenchyma is likely to be much more susceptible to ventilation-associated injury.
Positive End-Expiratory Pressure (PEEP)
Definition:
• PEEP is a pressure which is maintained in the lungs at the end of an exhaled breath.
• The purpose of PEEP is to prevent the alveoli from emptying completely and collapsing with exhalation, or to reopen areas of atelectasis. In turn, ventilation to these areas, with corresponding oxygenation, is generally improved (improved ventilation-perfusion ratio (Qs/Qt)).
• The body maintains a physiological PEEP of approximately +2 cmH2O, which is facilitated by closure of the glottis prior to complete relaxation of the muscles which affect expansion of the chest cage.
• PEEP is also a “parameter” which can be set and regulated by modern ventilators. Note: PEEP is often confused with CPAP (Continuous Positive Airway Pressure), which is a “mode” of ventilation normally centered around a certain level of PEEP.
• “Best” PEEP is defined as the level of PEEP which provides the best lung compliance.
• “Optimal” PEEP is defined as the level of PEEP which provides for optimal lung function (ie oxygen transport) with minimal decrease in cardiac output.
• Best PEEP and Optimal PEEP should be the same in a healthy lung. However, oxygenation can often be dramatically increased with PEEP levels only slightly higher than “best” PEEP, particularly in a compromised lung. Note: At high PEEP levels, optimal PEEP does not often correlate with A-aO2 gradients, PaO2 or Qs/Qt.
“Normal” Values:
• PEEP levels of 3-5 cmH2O are common practice and are generally effective in preventing atelectasis when combined with adequate inspiratory volumes.
• PEEP levels of 7.5-12.5 cmH2O are often applied to slow post-operative bleeding following thoracic surgery. Once bleeding has slowed or stopped, PEEP levels may then be reduced to a lower level, as tolerated.
Factors & Effects:
• As a positive pressure, PEEP above the physiologic level works contrary to the normal physiology of the thoracic cage (negative pressure generated during spontaneous inspiration enhances venous return to the heart) and may significantly affect venous return and cardiac function. Therefore, the addition of PEEP to a ventilatory regimen must be balanced with these functions.
• For infants in the perinatal period, elevated PEEP levels may be associated with an increased risk of cerebral hemorrhage secondary to decreased venous return to the heart and cerebral vascular congestion. This risk is most significant in preterm infants. While rare in a short-term, postoperative course of ventilation and more often associated with high peak inspiratory pressures, it should still be kept in mind.
Clinical Wisdom:
• The presence of an endotracheal tube denies the patient the ability to close the glottis and maintain the normal physiological level of PEEP in the lungs. Thus, the inclusion of even a small amount of PEEP via the ventilator is recommended for most patients. In cases where PEEP is contraindicated, such as caval-pulmonary shunts, adequate tidal volumes and rapid weaning to extubation will be the best measures to protect against progressive atelectasis in the absence of PEEP.
• With increases in PEEP, assess the patient for cardiac-related effects, particularly changes in blood pressure. In patients with low blood pressure, such increases should be made judiciously and in small increments.
• In cases where excessive chest expansion during ventilation is a concern (ie CABG with LIMA/RIMA graft), a moderate level of PEEP is not contraindicated. The advantages of maintaining PEEP in such cases (prevention of atelectasis, lower opening pressures, improved oxygenation) far outweigh any possible risk to the graft.
The Mechanical Ventilator
The First Rule of health care is “Do No Harm”.
With this in mind, the First Rule of mechanical ventilation is “Know Your Equipment”.
The ventilator is a versatile tool in the hands of a skilled operator. While clinical expertise with mechanical ventilators is attained through patient application, a fair degree of technical expertise can be attained prior to patient application. Time spent in familiarizing oneself with a new ventilator using a test lung or patient simulator is common practice among Respiratory Therapists. However, this is less often the case with physicians and nurses. This may be due to lack of opportunity or secondary to a division of responsibilities where ventilation is concerned. In such cases, the caregiver may have to take the initiative and arrange for both instruction and “play time” with an unfamiliar ventilator. Even if the physician or nurse is not the primary operator of the ventilator in the clinical setting, “hands on” experience with the ventilator in a test setting may contribute significantly to patient care on several levels.
With its capabilities and limitations revealed, the ventilator is immediately less intimidating. The caregiver becomes more comfortable with the ventilator and thus, is better able to evaluate the ventilator’s contribution to the care of the patient. Minor problems with the ventilator may be approached with greater calm and confidence. Changes in patient ventilatory status, often mirrored in the ventilator’s performance, are more easily recognized. As well, the ventilator’s potential is better understood, which allows for more versatile patient care. All, good things.
Play Time: This term is not to be used lightly, and refers only to operation of a ventilator while using a test lung. However, within this one condition, it is an accurate description. This is time spent “playing” with the ventilator, and is meant not only for discovering what the machine WILL do, but also what it will NOT do, and how it reacts to parameters both within and outside of normal clinical use. This is the one situation where you may actually play with the settings and discover what happens if you do this, or do that. If you have this opportunity, play. Discover what happens if one component of the circuit comes loose, or is occluded … or what happens when the power cord is disconnected from the wall outlet. With the exception of bursting the test lung (try to avoid this, but better a test lung than a real lung), it is unlikely that you will damage the ventilator or its circuit. You will, however, rapidly become more familiar with the ventilator and have a better appreciation of what it can do for (or to) your patient. This will be the only setting where playing is allowed. Once the ventilator is attached to a patient, the time for playing will have passed.
The Ventilation Experience: If you have the opportunity during your “play time”, and using settings within normal clinical ranges, become the patient and try breathing through an endotracheal tube while attached to the ventilator. With an ETT held securely in your mouth and your nares occluded, test out each mode, various levels of PEEP and sensitivity, various flow patterns and alarm settings. Try it for five minutes. It is highly likely that, by the end of those five minutes, you will have a much better appreciation of what your patients experience. NOTE: If you are breathing non-humidified gas during this trial, be aware that you may experience some discomfort in your airways. For this reason, it is recommended that you utilize a humidification device and an FiO2 of 0.21. “Do No Harm” applies to yourself , as well as to your patients. As well, be aware that the ventilator circuit will be contaminated by this practice.
Once again, this is a common practice during a Respiratory Therapist’s training, but is rarely experienced by physicians and nurses. This is unfortunate, as this simulation is a very valuable and memorable learning experience.
Ventilators are not all the same: Familiarity with one model of ventilator does not ensure familiarity or competency with other models. While they may share some features, the operating characteristics between one model and another may differ considerably. It is important that the caregiver become familiar with each model of ventilator used in their sphere of responsibility.
Manual Ventilation
The use of a manual resuscitation device is a relatively simple and often under-rated procedure. Simple, in that the operation of a self-inflating “bag” is not difficult, and under-rated in that the technique used is often given little consideration despite the wide array of effects that it can produce. The following are considerations for proper use of the manual resuscitation device:
• Manual resuscitation devices often come in various sizes (ie infant, pediatric, adult). The operator should be aware of the volumes of both the main body and the reservoir of the device in use. The device should be able to deliver approximately twice the tidal volume being delivered by the ventilator, or better, in order to guarantee a sufficient range of ventilation for the patient.
• Compression of the “bag” during manual ventilation should produce chest expansion comparable to that of a ventilator breath. This may not require full compression of the bag. A common mistake is to try to compress the bag fully, often with two hands, in order to give a “big breath”, without thought to the volume being delivered. This practice may produce dangerously high volumes, particularly in smaller patients. Chest expansion must be visualized to ensure that compression levels are appropriate.
• At the end of compression, the bag must be released and the patient allowed to exhale. A common mistake is to hold the compression for a time. This practice places considerable stress on the alveoli, particularly if they are already over-distended with excess volume. This may also compromise cardiac function by impeding venous return. On a self-inflating device, the patient will not be able to exhale as long as this compression is held. On a flow-inflating device, exhalation is possible during the period of compression, but the effort required to do so is considerable. Thus, this practice is not recommended.
• Inspiratory time (Ti), or the time used to compress the bag and deliver the breath, should reflect a normal breath as much as possible. A breath delivered too quickly involves highly turbulent gas flows which may not disperse throughout the lung as evenly as would be possible with a slower flow. This tends to produce areas of hyperinflation and areas of hypoventilation within the lung. Again, the hyperinflated alveoli are stressed. A normal Ti will allow the gas to distribute itself more effectively throughout the lung. An abnormally long Ti may be uncomfortable for a conscious patient. An effective gauge of appropriate inspiratory time for an adult is to breath along with the breath you are delivering to the patient. If it feels comfortable to you, there is a good possibility that it will also be comfortable for your patient. For smaller patients, the goal is to mimic the cycling of the ventilator as much as possible.
• Expiratory time (Te) is the time allowed for the patient to exhale between breaths. Te should be equal to or longer than Ti. A common mistake is to compress the bag immediately upon re-inflation. This may not give the patient sufficient time to exhale. This practice, known as “stacking breaths”, is very uncomfortable to a conscious patient and is likely to produce excessive intrathoracic pressures and alveolar over-distention under the best of circumstances. During higher rates of ventilation, both Ti and Te will need to be shortened to provide the proper balance between the two.
• The gas source, in most cases, will be 100% oxygen delivered via flowmeter. It is important to verify the gas source before using the device, particularly in an emergency situation. In some units (ie NICU), the gas source may be an air-oxygen blender. This device will allow for more specific control of the FiO2 in use. Again, the gas source and setting should be verified before using the device on the patient.
• Gas flow to the bag should be sufficient to maintain partial inflation of the reservoir. A slight deflation of the reservoir during re-inflation of the bag indicates an appropriate flow. At this flow, the reservoir should re-inflate during the breath. If the reservoir collapses completely between breaths, the flow is too low and the delivered FiO2 may be compromised. If the reservoir does not deflate at all between breaths, flow is too high and gas is being wasted. This is of particular importance if a tank is being used as the gas source. Once an appropriate rhythm of ventilation has been established, the flow may be adjusted to conserve resources.
Clinical Wisdom:
• Prior to beginning manual ventilation, ensure that your equipment is properly assembled and functioning as required.
• During manual ventilation, focus on providing appropriate ventilation.
• During manual ventilation, support the resuscitation device and minimize movement of the airway. This will reduce the risk to the security of the airway and be much more comfortable for the patient.
• During manual ventilation, coordinate your efforts with the respiratory efforts of the patient as much as possible. Ventilating opposite patient efforts to breathe will tire and stress the patient, and provide very ineffective ventilation.
• During long-term manual ventilation (ie during transport), frequently assess the patient for changes in ventilatory status.
• The resuscitation bag is a life-supporting device. If you are unsure of your technique, seek advice/instruction from a colleague with more experience. This is particularly true with regards to flow-inflating devices, which require more skill to master.
Initial Ventilator Set-Up & Patient Application
The procedures for preparation of the bedside for a ventilated post-operative patient and for application of the ventilator upon admission may vary considerably between facilities, secondary to differences in personnel, equipment, experience and custom. However, the following are considerations which may assist you in setting up for the post-operative cardiac surgical patient and in applying the ventilator safely and effectively:
Determination of Ventilator Type & Initial Settings
• In a facility where you have the choice between two or more ventilator types, there should be a consistent method for determining the appropriate machine to use, be it by patient weight, complexity of surgery or other factors.
• If your inventory consists of purely “infant” and “adult” ventilators, a choice may have to be made between “pressure” ventilation and “volume” ventilation. A recommended cut-off value for this determination is a weight of 6.0 kg. For patients less than 6 kg, pressure-oriented ventilation may be more effective, as the small tidal volumes required for these patients may be difficult for volume-oriented ventilators to deliver.
• The complexity of the surgery involved or anticipated difficulty of post-operative ventilation should also be considered in your choice of ventilator, mode of ventilation and associated parameters.
• Newer ventilators may have the versatility to ventilate a broad spectrum of patients, from neonatal to adult. If you have limited numbers of these ventilators, you may need to reserve use of these units for the patients who will have most need of their capabilities.
