Modes of Ventilation

Choosing the mode of ventilation is often difficult for several reasons. First, confusion often exists over ventilator terminology, with many of the commonly used terms no more than abbreviations (e.g., PEEP and IMV). Second, no widely accepted best mode of mechanical ventilation is available; intensivists frequently debate in the literature about which mode is best for which patients. Third, with the exception of respiratory therapists, most hospital personnel, including all but a handful of physicians and nurses, do not understand the altered physiology of mechanically ventilated patients.

The best way to clarify this situation is to provide list of modes of ventilation, and ask you to study them.

• Controlled mandatory ventilation (CMV) and Assist-control ventilation (ACV). These are variations on the same thing. In CMV and ACV every breath the patient receives is a full ventilator breath; the rate set on the ventilator is the minimum rate the patient will receive. In CMV the patient does not assist, which means he does not generate any negative inspiratory pressure. For all practical purposes, it means he does not attempt to breathe. Therefore, the rate set on the ventilator in CMV is also the the maximum rate as well. (In most cases, the patient is either paralyzed or sedated. In ACV the patient assists the initiation of each breath, by generating a slight negative pressure at the beginning of inspiration; this causes the ventilator to 'trigger' and give a full ventilator breath. Commonly, in ACV, the total respiratory rate is greater than the set rate because the patient is trying to breathe at a rate greater than what is set. What ever the total rate, it cannot be less than the set rate, and if more, every breath is always a full ventilator breath.

• Controlled Ventilation and Assist-Control Ventilation

  Probably the earliest mode of intermittent positive pressure ventilation (IPPV) was controlled ventilation. Note the pressure relationships during controlled ventilation ( Fig. 10-3, B); there is no negative pressure because all ventilation is accomplished by the machine with no patient effort required. In contrast to normal breathing, airway pressure during controlled ventilation is positive during inspiration and also positive during expiration. To achieve an adequate tidal volume, the ventilator "pushes" air into the lungs, resulting in a peak airway pressure higher than that reached during any part of normal breathing. Peak airway pressure marks the end of inspiration and the beginning of expiration; as in normal breathing, expiration is passive. As the lungs' elastic recoil brings them toward functional residual capacity, air is exhaled. Airway pressure at the end of expiration is again atmospheric (zero) until the next machine breath begins.

  TERMINOLOGY ALERT. All ventilator terminology states that baseline airway pressure is zero. In fact, it is atmospheric or ambient airway pressure, which varies depending on the altitude. It is simply a matter of convenience to call any ambient pressure zero, so that ventilator pressures can be plus or minus zero and the numbers be relatively small. Thus if the atmospheric pressure is 760 mm Hg = 988 cm H20, then a peak airway pressure of 30 cm H20 in absolute terms would be 1118 cm H20. Obviously it is much easier to use terms relative to ambient than the absolute pressures. This point may be completely obvious to you, but I have found many people don't really know what "zero" baseline pressure means in relation to mechanical ventilation.

  As a result of the higher peak pressure and the fact that airway pressure is positive throughout the breathing cycle, mean airway pressure during IPPV is higher than in normal breathing. The higher mean airway pressure improves gas exchange and accounts for some of the complications of mechanical ventilation .

  Controlled ventilation is the mode of ventilation used when the patient is completely paralyzed or otherwise unable either to breathe on his own or to initiate ventilator breaths. Controlled ventilation becomes unnecessarily limiting when the patient can contribute to his own minute ventilation.

  ASSIST-CONTROL VENTILATION

  Assist-control ventilation was the next mode to evolve after controlled ventilation (Fig. 10­4, A). Assist­ control ventilation allows the patient to initiate ventilator breaths, providing the advantage of cycling the ventilator when the patient is ready and of lessening the need to suppress the patient's own drive to breathe. The transient negative pressure shown in Fig. 10-4, A, represents the patient's inspiratory effort; the machine senses the negative pressure and obliges by pushing in the next ventilator breath.

  With the assist-control mode, both a tidal volume and a minimal respiratory rate must be selected in case the patient stops breathing. For example, if the ventilator is set for 12 breaths/min with a tidal volume of 700 cc, the patient will receive this as a minimum. If the patient chooses to breathe 20 time a minute, 20 will be the respiratory rate, and the volume of each breath will be 700 cc. Assist­control ventilation thus has the advantage of allowing the patient to choose his own respiratory rate, with each initiated breath guaranteeing an adequate tidal volume. If the respiratory rate of the patient is controlled by properly functioning chemoreceptors, the assist­control mode should result in optimal alveolar ventilation.

  Of course each breath is only initiated by the patient but is completed by the machine. Respiratory muscles are not fully used during assist control breathing, so muscle atrophy resulting from disuse could develop if assist­control ventilation is used for long periods. Another potential problem arises if the patient breathes too fast, which may occur, for example, with central nervous system disorders, toxic or febrile states, and sepsis. In these conditions, tachypnea occurring without the patient breathing through a ventilator might be accompanied by shallow tidal volumes. Because the ventilator delivers a full tidal volume for each patient­initiated breath, the minute ventilation is much higher than needed and can lead to severe respiratory alkalosis.

  Fig. 10­4 Airway pressures during assist­control ventilation, A, and intermittent mandatory ventilation. Pressures are in cm H₂O

  Another problem that results from a rapid respiratory rate is a shortened time interval for expiration. When the patient is tachypneic, there is less time for exhalation of the full tidal volume; each patient­initiated ventilator breath occurs too soon, and the patient often appears to be ''fighting the ventilator.'' Partly for these reasons, assist control ventilation is no longer used as often as the next mode of ventilation, intermittent mandatory ventilation.

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  INTERMITTENT MANDATORY VENTILATION

  Intermittent mandatory ventilation (IMV) allows the patient to interact with the ventilator in a more physiologic manner than with the control or assist control modes. In the IMV mode, both the tidal volume and the number of machine breaths are also set, but they are only delivered intermittently, alternating with the patient's own breathing efforts. Between the ventilator's intermittent mandatory breaths, the patient breathes spontaneously, setting his own tidal volume and respiratory rate (Fig. 10-4, B). IMV is made possible by a valve that can alternate the inspiratory circuit between the ventilator and room air. To maintain a constant fraction of inspired oxygen (FIO₂), the room air is enriched with the same FIO₂ as is delivered from the ventilator (Fig. 10­5).

  Many machines are designed so that the ventilator breath is delivered in synchrony with the patient's own inspiratory effort; this is called synchronous IMV or SIMV. IMV is only for the patient who has an intact respiratory center and who can accomplish the work of breathing. IMV is popular for weaning chronically ventilated patients from the ventilator; the number of IMV (machine) breaths can be gradually decreased, over a period of weeks if necessary, all the way down to l/min or less.

  Although IMV was introduced as a weaning technique, it is also a common ventilatory mode in patients not being weaned. The difference between the two uses of IMV (weaning vs. full ventilatory maintenance) is in the number of IMV breaths/minute (assuming the patient is spontaneously breathing). Arbitrarily, seven or less IMV breaths/minute is considered a weaning mode, with the patient's spontaneous breathing constituting a major part of the total ventilation. Eight or more IMV breaths/minute is tantamount to full­ventilatory support in most people, even though the patient is breathing spontaneously between ventilator breaths. Although eight breaths/min is an arbitrary dividing line, this division serves to emphasize that IMV can be used for two different purposes.

  Since its introduction in 1973, there has been a minor controversy over whether IMV is really better than assist­control ventilation as a full­ventilatory mode or is better than the old "trial and error" technique as a weaning technique. In the trial and error method, the patient is judged ready to be weaned and then simply is disconnected from the ventilator; close observation of the patient after disconnection is obviously important (see the section on Ventilator Weaning later in this chapter).

  Drs. Shapiro and Cane (1984) have addressed the issue of IMV vs. assist­control­ventilation by calling attention to the fact that IMV is the only clinically available technique for providing partial ventilatory support, i.e., for ventilator weaning. They define partial­ventilatory support as a ventilator rate of 7/min or less with the patient ''providing a physiologically significant degree of spontaneous ventilation.'' Anything more than this rate is "full­ventilatory support'' and can be provided by controlled ventilation, assist­control ventilation or IMV.

  For full­ventilatory support, there is no proof that IMV or assist­control ventilation is a better technique. From a practical standpoint, IMV is now the most common technique for full­ventilatory support, replacing both control and assist­control modes in popularity. For partial­ventilatory support, debating IMV vs. assist­control ventilation is pointless since the latter is not a weaning mode.

  Example of an intermittent mandatory ventilation (IMV) circuit. IMV is made possible by a one­way valve that closes when the ventilator delivers air but that can be opened by the patient's spontaneous breathing between ventilator­delivered breaths. In this figure the one­way valve separates the primary ventilator circuit from a parallel nonventilator circuit that delivers the same FIO₂ as the ventilator. When the ventilator cycles, positive pressure is delivered to the patient and the one­way valve closes. Between mechanical breaths, the patient's spontaneous breathing can open the valve so that the patient can inhale air of the same FIO₂, and humidification as the air from the ventilator, but under ambient pressure. If desired, positive airway pressure can also be delivered through the IMV circuit; because the patient is breathing spontaneously. positive pressure through an IMV circuit represents continuous positive airway pressure alternating with ventilator breaths. (From McPherson, S.P., and Spearman, C.B.: Respiratory therapy equipment, ed. 3, St. Louis, 1985, The C.V. Mosby Co.)

  In summary, the three choices for full support are control ventilation, assist­control ventilation. and IMV. If only partial­ventilatory support is to be provided, IMV is the only available mode. When using IMV, the number of ventilator breaths/ minute determines if the patient is receiving partial(7 or less) or full­ (8 or more) ventilatory support.

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  Choosing the FIO2

  Once the decision is made to institute mechanical ventilation, the fraction of inspired oxygen (FIO₂) and the mode of ventilation must be chosen. Choosing an FIO₂ is no more difficult than picking a number, albeit one based on the patient's clinical problem and the reason for intubation. For example, a patient who is intubated mainly for hypercapnia will usually be adequately oxygenated with an FIO₂ under 0.40. A patient intubated because of severe hypoxemia or during cardiopulmonary resuscitation may need an initial FIO₂ of 1.00. Blood gas measurements should be obtained in the first half hour after treatment, and adjustments made to keep the PaO₂ between 60 and 90 mm Hg at the lowest FIO₂ possible.

  Clinical problem 2 A decision is made to wean a 67­year­old man from the ventilator. Before weaning is begun, the machine is in the assist­control mode. The patient is initiating 16 breaths/min and is receiving 700 cc/breath. He is switched to IMV at a rate of 1 2/min and within a half hour is noted to be in respiratory distress with a total respiratory rate (machine initiated plus spontaneous) of 20/min. Blood gas measurements obtained before and after the change to IMV are shown below. How would you explain the changes? Ventilatory Machine breaths Spontaneous mode (and tidal volume) breaths pH PaCO2 PaO2 FIO2 Assist­control 16 (700 cc) 0 7.45 38 78 0.40 IMV 12 (700 cc) 8 7.39 47 65 0.40

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  VENTILATOR SETTINGS

  Table 10­1 lists the principal ventilator settings for intermittent positive pressure ventilation when using conventional volume ventilators. These machines are by far the most commonly used for adult mechanical ventilation and are called volume ventilators because they will generate whatever pressure is necessary (up to a limit) to provide the preset tidal volume. Pressure­cycled ventilators, which have the pressure preset, are rarely used for mechanical ventilation today.

  Each ventilator setting is adjusted by using a knob or a dial on the ventilator console (see Fig. 10­­2). These settings determine whether the patient is receiving full­ or partial­ventilatory support and, if full­ventilatory support, whether it is the control or assist­control mode. Also, the amount of positive end­expiratory pressure can be set for any of the ventilatory modes.

  Table 10­1.

  Principal ventilator settings for intermittent positive pressure ventilation while using conventional volume ventilators*

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  Setting Typical range

  Fraction of inspired oxygen 0.21­1.00

  Tidal volume 400­1000 cc

  Inspiratory pressure limit Up to 80 cm H₂O

  Respiratory rate FVS, 8­30 or more breaths/min

  PVS, 7 or less breaths/min

  PVS can be accomplished only in the IMV mode, which allows the patient to breathe spontaneously between ventilator breaths peak inspiratory flow rate 20­100 L/min (to achieve inspiratory flow time of

  0.5­1.5 sec)

  Inspiratory sensitivity Control mode, no sensitivity

  Assist­control mode, sensitivity dialed in (a variable control) control)

  Positive end­expiratory pressure 1­30 or more cm H₂O

  Inspiratory plateau or hold

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  Fraction of inspired oxygen.

  A precise FIO₂ can be dialed on most volume ventilators. In addition, alarms can be set to sound if the delivered FIO₂ falls outside a certain range. For example, if the FIO₂ is set at 0.40, the limits can be set between 0.30 and 0.60; any delivered FIO₂ outside this range will sound an alarm.

  Tidal volume and inspiratory pressure limit.

  A volume ventilator delivers a preset tidal volume regardless of the condition in the airways; presetting assures that the volume needed will be received by the patient. However, a potential danger exists if a major airway becomes unexpectedly obstructed or if something impedes air entry. In such cases delivery of the preset volume can result in a dangerously high airway pressure; to guard against this, an inspiratory pressure limit is always set along with the tidal volume. For example, setting the tidal volume for delivery of 700 cc might achieve a peak airway pressure of 30 cm H₂O; a pressure limit of 50 cm H₂O can be set at the same time. If, for example, the endotracheal tube slips into the patient's right main stem bronchus, the machine will attempt to deliver 700 cc to just one lung (half the previous lung volume), and the peak inspiratory pressure will acutely rise. Conceivably the elevated airway pressure could rupture the right lung or cause other damage. Instead, however, when 50 cm H₂O airway pressure is reached, the machine stops inspiration and an alarm sounds, perhaps after delivering only 400 cc. With this warning, the therapist or nurse can quickly investigate the problem. The alarm will sound each time airway pressure reaches the preset inspiratory pressure limit.

  Respiratory rate.

  The respiratory rate is set by using a dial on the machine. For controlled ventilation, the rate equals the total number of ventilator breaths the patient will receive. For assist control ventilation, the rate represents the minimal number of breaths; depending on the inspiratory sensitivity (also set by the machine), the patient may initiate more than the minimal amount. For intermittent mandatory ventilation, the respiratory rate is also the total number of ventilator breaths per minute; however, between the machine breaths, the patient may breathe spontaneously (Fig. 10­4 and 10­5).

  Peak inspiratory flow rate.

  The peak inspiratory flow rate determines how fast each breath will be delivered to the patient and is therefore a determinant of inspiratory time. The faster the flow rate, the shorter the inspiratory time, and the more breaths that can be delivered per minute. Optimal inspiratory flow time is between 0.5 and 1.5 seconds and is usually achieved with a peak inspiratory flow rate between 40 and 70 L/min.

  Sensitivity.

  Many volume ventilators include a dial labeled "sensitivity" or ' inspiratory effort"; this setting determines how easily a patient can initiate a machine­delivered breath. When the sensitivity dial is turned all the way to the off position, no amount of patient effort will initiate a machine breath, and the machine is in the controlled ventilatory mode. As sensitivity is "dialed in,'' the ventilator changes to the assist­control mode, and it becomes much easier for the patient to initiate a machine breath. The sensitivity dial is not calibrated in units, but rather is adjusted by trial and error to the patient's own inspiratory efforts. However, the patient's inspiratory effort will show up as a negative (subatmospheric) deflection on the ventilator's pressure dial, usually between - 0.5 and ­ 2.5 cm H₂O.

  Fig. 10­6.

  Pressure tracing with positive end­expiratory pressure (PEEP). In this example a PEEP of 5 cm H₂O has been applied.

  Positive end­expiratory pressure.

  On most ventilators manufactured today, PEEP can be simply set by dialing to a desired setting. The dial regulates an expiratory resistance valve that effectively keeps airway pressure above atmospheric pressure at end­expiration. PEEP may be used in the control, assist­control, or IMV modes. A PEEP pressure tracing is shown in Fig. 10­6; the use and complications of PEEP will be discussed in later sections.

  Inspiratory plateau or hold.

  The ispiratory plateau or hold dial adds resistance to the expiratory circuit; the effect is to prolong inspiration and create a transient plateau pressure. Airway pressure is still zero at end­ expiration ( Fig. 10­7) in contrast to PEEP, which maintains a positive airway pressure at the end of expiration (see Fig. 10­6). Inspiratory plateau was originally used to improve oxygenation by providing a longer time for gas exchange, but PEEP is now used instead. Today, the principal use of inspiratory plateau is in measuring static compliance (see the section on Ventilator Compliance).

  These ventilator settings (Table 10­1) represent the principal ones on volume ventilators. A glance at any modern ventilator may reveal several more knobs, alarms, and circuits than can be discussed here. Generally, other settings are determined by the respiratory therapy personnel whose knowledge of ventilators exceeds that of most physicians because the therapists work with ventilators daily. More detail on mechanical ventilators is available in the texts listed in the references.

  The ranges for most ventilator settings are large (Table 10­1). Patients vary widely in their ventilatory requirements, and it is virtually impossible to predict settings that will provide a given patient with optimal blood gases. What settings should be chosen when initiating mechanical ventilation? Generally, the initial tidal volume is set at approximately 10 to 15 cc/kg body weight, the respiratory rate is set between 10 to 16 per minute (control, assist­control, or IMV mode, depending on the clinical state), and the inspiratory flow rate is set at 40 to 60 L/min. PEEP is usually added later, depending on the patient's clinical course and the results of arterial blood gas analysis. Whatever settings are chosen, it is important to check the blood gas measurements within 30 minutes and to repeat the measurements within a few hours to assure a ventilatory steady state. Any time a ventilator setting that might affect PaO₂ or PaCO₂ is changed, the blood gas analysis should be repeated within 30 to 60 min.

  Fig. 10­7.

  Effect of inspiratory plateau. The principal effect is to prolong inspiration. In contrast to PEEP, inspiratory plateau allows the end­expiratory pressure to return to zero. The difference between peak pressure and plateau pressure is caused by airways resistance. The difference between plateau pressure and end­expiratory pressure is that amount of pressure needed to distend the system (tubing, lungs, chest wall), and the difference can be used to calculate system compliance. See text for further discussion.

  Clinical problem 3

  A 60­year­ old patient is in the hospital for treatment of a myocardial infarction. During the night she suffers acute pulmonary edema and requires cardiopulmonary resuscitation. Before the patient is intubated and mechanical ventilation is begun, her blood gas measurements show pH of 7.06, PaCO₂ of 61 mm Hg, and PaO₂ of 50 mm Hg while breathing 100% oxygen delivered by manual ventilation with an Ambu bag. The patient's estimated body weight is 50 kg (110 Lbs). What initial ventilator settings would you choose for the following:

  a. FIO₂

  b. Tidal volume

  c. Inspiratory pressure limit

  d. Respiratory rate

  e. Peak inspiratory flow rate

  f. Inspiratory sensitivity

  Would you provide PEEP?

  Clinical problem 4

  A 72­year­old man with severe chronic obstructive pulmonary disease is in the intensive care unit. His pH is 7.24, PaCO₂ is 84 mm Hg, and PaO₂ is 58 mm Hg while breathing 28% oxygen through a Venturi mask. His chest x­ray suggests severe emphysema. Despite optimal drug therapy, his blood gas measurements cannot be improved, and he is almost unarousable. To prevent respiratory arrest, he is intubated and given mechanical ventilation. His estimated body weight is 70 kg (150 lbs). What initial ventilator settings would you choose for the following:

  a. FIO₂

  b. Tidal volume

  c Inspiratory pressure limit

  d. Respiratory rate

  e. Peak inspiratory flow rate

  f. Inspiratory sensitivity

  Would you provide PEEP?

  Clinical problem 5

  A comatose 20­year­ old patient is brought to the emergency room following an overdose of sleeping pills. Because of very shallow respirations and cyanosis, the patient is intubated before his blood gas results are known. Initial ventilator settings include a tidal volume (VT) of 700 cc, a respiratory rate (RR) of 12/min, and an FIO₂ of 0.50. The patient has no spontaneous breathing. Blood gas results obtained (1) before intubation and (2) 20 minutes later show the following:

  pH---PaCO₂---PaO₂ FIO₂ VT RR

  (1) 7.10 79 38 Room air 0 0

  (2) 7.25 56 117 50% oxygen 700 12 Following the second blood gas analysis, would you change the FIO₂, the tidal volume, or the respiratory rate'? If so, what settings would you choose?

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  Continue with Part III of Chapter 10: Mechanical Ventilation
  From Pulmonary Physiology in Clinical Practice, copyright 1999 by

  Lawrence Martin, M.D.

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