Chapter 10: Mechanical Ventilation (continued) - Part III of III

from Pulmonary Physiology in Clinical Practice, copyright 1999 by
Lawrence Martin, M.D.


Headings in PART I

Introduction (READ THIS FIRST!)
Intubation and mechanical ventilation
Indications for mechanical ventilation
Normal breathing vs. ventilator breathing
Choosing the settings

Headings in PART II


Modes of mechanical ventilation
Controlled ventilation and Assist-Control Ventilation
Intermittent mandatory ventilation
More on ventilator settings

Headings in PART III

Ventilator compliance
Pressure support ventilation
Positive end-expiratory pressure
Non-invasive mechanical ventilation>
Choosing the FIO2
Complications of mechanical ventilation
Weaning or Liberating the patient from the Ventilator

VENTILATOR COMPLIANCE

The concept of compliance was introduced in Chapter 3. Since compliance represents a change in volume/change in pressure, it is relatively easy to measure in ventilated patients. Ventilator compliance is a measure of the distensibility of the entire system, which includes the ventilator tubing, the chest wall, and the lungs. Compliance is often a valuable parameter to follow since any significant change may represent a change in the patient's respiratory condition.

In measuring system compliance, the volume is the amount of air delivered by the ventilator per breath, i.e., the tidal volume, which is easily measured by collecting the expired air from each breath. Determining the correct pressure to use in compliance measurement is a little more difficult.

If the patient is not receiving positive end expiratory pressure, the pressure at the beginning of inspiration is zero (atmospheric); the final pressure is the peak airway pressure. For purposes of discussion, assume a patient's tidal volume is 500 cc and his peak pressure is 20 cm H2O. The net change is the pressure at end-inspiration (20 cm H2O) minus the pressure at end-expiration (0 cm H2O) or 20 cm H2O. Is the compliance then 500 cc/20 cm H2O or 25 cc/cm H2O? Not exactly.

As discussed in Chapter 3, compliance is a static measurement. If the pressure change is measured during ventilator breathing, the amount of pressure being measured is that required to not only distend the lungs, but also to overcome airways resistance. If two patients with equally distensible lungs have different airways resistance, their compliance, measured during breathing, will be different.

To eliminate the pressure caused by airways resistance, the inspiratory plateau dial is quickly turned to the maximum at the end of inspiration; this setting creates a plateau of pressure. The plateau lasts for only a fraction of a second, but is long enough to be observed on the ventilator's pressure gauge. The difference between the peak airway pressure and plateau pressure reflects the amount of pressure needed to overcome airways resistance. The difference between plateau pressure and end­expiratory pressure is that amount of pressure needed to distend the lungs and is therefore used in the compliance measurement (Fig. 10­7).

Clinical problem 6

A 29-year-old man is intubated following an overdose of sleeping pills. He is receiving controlled ventilation at a tidal volume of 800 cc. The patient's peak airway pressure is 30 cm H2O, his plateau pressure is 20 cm H2O, and his end-expiratory pressure is 0. What is the static system compliance?


Clinical problem 7

a. A 35-year-old patient is receiving mechanical ventilation for severe pneumonia. The ventilator settings are as follows: FIO2, 0.60; tidal volume, 900 cc; and respiratory rate, 12/min. The peak pressure is 45 cm H2O, the plateau pressure is 40 cm H2O, and the PEEP pressure is 5 cm H2O. What is the compliance of this system?

b. The peak pressure limit for the above patient is set at 60 cm H2O. One hour later the machine alarm indicates that the pressure limit has been exceeded. The respiratory therapist increases the limit to 80 cm H2O and takes new readings. The tidal volume is still 900 cc, but the peak pressure is now 68 cm H2O, the plateau pressure is 64 cm H2O, and the PEEP pressure is 5 cm H2O. What is the system compliance at this point? How do you explain the change?

Pressure Support Ventilation

This is ventilation delivered during spontaneous breathing, which means the patient is doing the vast majority of the work of breathing. The machine gives a "boost" to the patient be providing a small amount of inspiratory pressure, usually 5 to 10 cm H20. This low level of inspiration contrasts with the much higher inspiratory pressures during IMV, CMV or ACV modes of ventilation. With IMV, CMV or ACV, the patient could be paralyzed, but the ventilator would still deliver the full tidal volume breath. With PSV, a paralyzed patient would not receive sufficient tidal volume (in most cases) since the inspiratory pressures are too small for this purpose. PSV is really a weaning mode of ventilation, used with IMV or full spontaneous breathing to facilitate weaning.

PSV cannot be used during CMV or ACV, since every breath is a full ventilator breath. It is used with IMV or spontaneous breathing ("CPAP" mode on 7200 ventilator).

If a patient can maintain comfort and good blood gases on PSV alone, that patient can be removed from the ventilator; stated another way, a patient who can maintain good blood gases with PSV alone does not need continuous mechanical ventilation.

Positive end-expiratory pressure

For most ventilated patients, the machine­delivered fraction of inspired oxygen ( FIO2) will provide an adequate PaO2. When this FIO2 is above 0.60 and the PaO2 remains inadequate, positive end-expiratory pressure (PEEP) is often employed. PEEP is used only to improve the PaO2, not the PaCO2. PEEP is an alteration of ventilator pressures

so that airway pressure is positive (above atmospheric) throughout the breathing cycle. It may be used with any of the ventilatory modes discussed so far.

PEEP was first introduced into clinical medicine in 1967 when physicians working in a Denver intensive care unit described their experience with adult respiratory distress syndrome (ARDS) patients (Ashbaugh, Bigelow, Petty, et al., 1967). Two of the patients received PEEP on an empiric basis, and their PaO2 improved. PEEP has since been used routinely in the management of ARDS. A pressure curve for PEEP is shown in Fig. 10-6. Normally, airway pressure at end­expiration is atmospheric (measured at the mouth); with PEEP it is above atmospheric. As commonly employed, PEEP pressures are usually between 5 and 20 cm H2O above atmospheric pressure.

The mechanism by which PEEP improves oxygenation is not known for sure. Since PEEP increases functional residual capacity, it probably leads to better oxygenation by preventing end-expiratory collapse (Fig. 10­9). (Lung water studies have shown that PEEP does not diminish total lung water, but just redistributes it within the alveoli. Therefore PEEP cannot be considered a primary treatment for pulmonary edema.)

It is important to recognize that PEEP is measured in the upper airways and does not equal airway pressure in the alveolus. The PEEP is considerably dissipated by the time it reaches the alveoli. Yet it is the positive pressure at the alveolar level that both improves oxygenation and leads to complications. There is no practical way to know how much of the measured PEEP is present in the alveoli; the amount of dissipation depends on complex factors, including lung compliance and airways resistance. Nonetheless, as learned by experience and observation, a PEEP of less than 10 cm H2O usually improves PaO2 without significant complications; above this level, PEEP is more likely to be accompanied by complications, either barotrauma or a decrease in cardiac output (discussed later in this chapter).

Fig. 10-9. Effect of PEEP on oxygenation. During expiration with PEEP, airways that would otherwise collapse are kept open, allowing continued oxygen transfer. In Example A. the PaO2 and SaO2 improve while FIO2. is unchanged. In Example B. the Pao. is maintained at an acceptable level while the FIO2. is decreased from 0.70 to 0.50.

Continuous positive airway pressure (CPAP)

Another way of delivering positive airway pressure is through continuous positive airway pressure (CPAP). CPAP is positive end expiratory pressure (PEEP) without air being pushed in by a machine. With CPAP, inspiration is accomplished entirely by the patient's own muscular effort.

Note that CPAP requires a tight seal to prevent escape of the positive air pressure, so it must be delivered through an endotracheal tube or a tightly sealed face mask. The changes in airway pressure with CPAP are similar to those in normal breathing except that both inspiratory and expiratory pressures are maintained above atmospheric pressure (Fig. 10-10).

Despite its theoretical advantages (no ventilator is necessary and mean airway pressure is less than with intermittent positive pressure ventilation [IPPV]), CPAP is not widely used in adult medicine. CPAP is not a ventilatory mode (just as PEEP is not), and it is used only to augment oxygenation; machine­delivered ventilation is invariably more effective for this purpose. It is difficult to keep a tight­fitting face mask in place, and the mask is often uncomfortable for the patient. Despite its limitations, there are occasions when nonintubated patients can be aided by CPAP. The technique is always worth a try if a completely alert patient is suffering from oxygenation failure and avoiding mechanical ventilation is desired.

CPAP is also sometimes used as a weaning technique during the interval between discontinuing a patient from IPPV and his extubation.

Complications of Mechanical Ventilation

Whenever machines take over a vital function, there is risk of complications; perhaps a complication does not occur every time in every patient, but one occurs often enough to cause a healthy wariness. Used appropriately, mechanical ventilation can be life­saving and is well worth the risks; used inappropriately, the risks can outweigh the benefits. The box on p. 214 lists the more commonly observed complications of intubation and mechanical ventilation. These complications may occur in any mode of intermittent positive pressure ventilation (IPPV). In addition, positive end expiratory pressure (PEEP) increases the likelihood of complications that arise from increased airway pressure.

Fig. 10-10. Airway pressure during normal quiet breathing, and during continuous positive airway pressure (CPAP),

Barotrauma manifests as subcutaneous emphysema, pneumothorax, or pneumomediastinum. A more serious problem is decreased cardiac output. The elevated intrathoracic pressure arising from IPPV can cause a decrease in venous return and hence a decrease in cardiac output. The problem is accentuated when PEEP is used. In fact patients may show a simultaneous increase in arterial partial pressure of oxygen (PaO2) and decrease in cardiac output, with the net result of overall decrease in oxygen transport (Fig. 10-11). This decrease in oxygen transport is one reason why patients receiving PEEP are often monitored with a right-sided heart catheter (see Chapter 8).

STEPS FOR WEANING PATIENT FROM THE VENTILATOR


1.	OPTIMIZE THE PATIENT. Conditions requiring specific 
attention or correction before weaning is initiated include the following 
(in no particular order):


Anemia 
Acid-base abnormalities, especially

	Shock metabolic alkalosis and acidosis

Starvation

Electrolyte imbalance

Thoracic pain

Sleep deprivation

Fever

Reduced cardiac output

Infection

Copious airway secretions

2. ASSESS ABILITY OF PATIENT TO OXYGENATE AND TO VENTILATE WITHOUT THE MACHINE. Physiologic criteria that have been used for this purpose include the following:


Tests of mechanics				Criterion

Tidal volume					At least 4 to 5 cc/kg

Vital capacity At least 10 to 15 cc/kg

Peak inspiratory pressure At least ­ 20 to ­ 30 cm H2O*

Resting minute ventilation (VE) Less than 10 L/min, with ability to at least double VE voluntarily Tests of oxygenation and ventilation

FIO2 that provides adequate PaO2 0.40 or less, without PEEP

P(A-a)O2 while breathing 100% oxygen Less than 350 mm Hg

Shunt fraction while breathing 100% Less than 20% oxygen

Dead space/tidal volume ratio Less than 0.60

l3. WEAN THE PATIENT. The two methods of weaning are:

A. Trial and error method. Disconnect endotracheal tube from ventilator and connect to a T­piece containing humidified oxygen; repeat blood gas analysis in approximately 30 minutes and again before extubation. Carefully observe patient's respiratory rate and effort, blood pressure, and pulse .


	B.	Intermittent mandatory ventilation (IMV). 
Gradually decrease number of IMV breaths per minute; monitor patient's 
respiratory rate and arterial blood gas values with each change in IMV.

 *The more negative the number the better, i.e., 
-40 cm H2O is more desirable than -30 cm H2O2

The point at which PEEP maximally improves PaO2 and oxygen transport is sometimes called ''optimal'' PEEP. However, this level of PEEP may not be optimal if accompanied by other problems, e.g., barotrauma. In truth, there is no consensus on what criteria constitute optimal PEEP. Determining optimal PEEP should probably not depend on sophisticated hemodynamic measurements, especially if the patient is receiving PEEP for a long period. In practice, optimal PEEP is usually the lowest level of PEEP that will give an adequate PaO2 on an FIO2 of 0.6 or less.

Effect of PEEP on cardiac output and oxygen delivery. The PaO2 can increase while cardiac output and arterial oxygen delivery are decreasing. Optimal PEEP for a given patient can only be found by trial and error. See text for discussion.

Machine malfunction varies with the complexity and variety of machines in use. In practice, each machine is checked regularly (usually every hour) along protocols established by the manufacturer and each hospital's respiratory therapy department. Although today's volume ventilators are highly reliable, there is no substitute for constant surveillance by competent personnel. Quite literally, the patient's life depends on it.

The complications of PEEP are the same, qualitatively, as they are with any form of IPPV. Because mean airway pressure is higher with, than without, PEEP, barotrauma and decreased cardiac output tend to occur more commonly when PEEP is used. PEEP can be thought of as an exaggeration of conventional IPPV; it is not unique in causing complications of mechanical ventilation.

POTENTIAL COMPLICATIONS OF INTUBATION AND mechanical VENTILATION


From intubation

Damage to teeth, mouth, and upper airways

Intubation of esophagus

Sedation­related complications (arrhythmia, further depression of ventilation, cardiac arrest)

From the tracheal tube erosion of upper airways, e.g., pressure sores and tracheomalacia

Accidental slippage of tube into a main stem bronchus

Plugged tube, e.g., from mucus or secretions

From the increased airway pressure Barotrauma, e.g.,

pneumomediastinum and pneumothorax

Decreased venous return and decreased cardiac output

Increased physiologic dead space

Oxygen toxicity (only with high FIO2)

Muscle atrophy from muscle disuse

Starvation from inadequate nutritional support

Gastric distention from air entering gastrointestinal tract

Accidents, e.g., disconnection from ventilator

Nosocomial infection, e.g., from machine or humidifier

Machine malfunction3


Clinical problem 8

A patient with severe emphysema is intubated because of progressive ventilatory failure. During his course of treatment the following blood gas values are obtained, all on an FIO2. of 0.40 (V r, tidal volume in cc; RR, respiratory rate; VE, minute ventilation in L/min; PP, peak ventilator pressure in cm H2O; PaCO2 and PaO2 are in mm Hg.):

Time VT RR VE PP pH PaCO2 PaO2 4:20 PM 500 12 6.0 30 7.35 48 76

5:30 PM 600 12 7.2 35 7.34 47 78

7:05 PM 700 12 8.4 38 7.31 54 75

Assuming no change has occurred in the clinical picture, how would you explain the rise in PaCO2?

VENTILATOR WEANING

If all patients undergoing general anesthesia are included, the vast majority of patients who receive mechanical ventilation do not need to be weaned from the machine. When the underlying problem is rapidly corrected (as in recovery from anesthesia), the patient can simply be disconnected from the ventilator and extubated. For many chronically ill patients the procedure is not so simple. They often require prolonged mechanical ventilation and a more gradual weaning from the machine.

Three steps are involved in weaning a patient from the ventilator (see box on p. 216). First, and most important, is to optimize the patient. The patient must be in as stable a condition as is feasible, which means improving or correcting conditions such as hypoxemia, anemia, fever, and metabolic alkalosis, as well as unstable cardiac conditions.

Second, some assessment must be made showing that the patient can comfortably and adequately maintain oxygenation and ventilation without the machine.

Third, the patient has to be removed (weaned) from the ventilator.

The physiologic parameters outlined in the box refer to the second step, assessing the patient's ability to oxygenate and ventilate without the machine. There has been an unfortunate tendency to view these parameters as rigid criteria for extubation. In fact these ''weaning criteria" are merely physiologic guidelines and are not meant to predict who can or cannot be weaned. It is never necessary to perform all or even most of these measurements to wean a patient, and some measurements are totally inappropriate in certain situations.

For example, measurement of the alveolar­arterial oxygen pressure difference (P(A-a)O2) while the patient is breathing 100% oxygen would be ridiculous if the patient never required that fraction of inspired oxygen (FIO2); this measurement and the shunt fraction measurement were at one time advocated for patients with severe adult respiratory distress syndrome, but they have no role in weaning a patient who was intubated primarily for ventilatory failure.

The other weaning measurements can give a useful index of the patient's ability to sustain ventilation. However, experienced physicians successfully extubate many patients without performing any of these tests. No good studies exist on the criteria for weaning chronically diseased patients. Indeed, some patients who are ambulatory and functioning at home would fail these tests. Certainly a patient who exceeds these weaning criteria should have little trouble sustaining spontaneous ventilation.

However, for the vast majority of patients, it is not necessary to measure anything more than arterial blood gases and the patient's respiratory rate; these measurements and simple observation are usually sufficient for ventilator weaning.

Clinical problem 9

A 59­year­old nonsmoking woman with no prior lung disease develops ARDS following a drug overdose. After several weeks her oxygenation has improved, but she remains ventilator­dependent because of hypercapnia and severe restrictive impairment. She is alert and responsive. On an assist­control mode, the patient is initiating 24 breaths/min. The following measurements are obtained: PaO2, 75 mm Hg; PaCO2, 65 mm Hg; pH, 7.34: and mean expired PCO2, 28 mm Hg.

What is the patient's ratio of dead space to tidal volume (VD/VT), and what does this ratio predict about weaning her from the ventilator?

Clinical problem 10

A 65­year­old man with a history of severe emphysema (FEV, of 900 cc, 40% of predicted; baseline arterial blood gas [ABG] analysis # I, below) is admitted to the intensive care unit because of acute pneumonia. Admission ABG analysis is #2. Despite the administration of appropriate antibiotics and low­supplemental oxygen therapy, the patient decompensates the night of admission (ABG analysis #3), and he is intubated. Over the next several days he gradually improves. On the fifth day he is receiving IMV at a rate of 8/min (ABG analysis #4) and appears alert and comfortable. He has a spontaneous (spont.) respiratory rate (in addition to the machine- delivered breaths) of 6/min. How would you proceed with ventilator weaning at this point?

ABG analysis pH PaCO2 PaO2 FIO2 Mode #1 (baseline) 7.37 51 65 0.21 Spont.

#2 (admission) 7.35 54 45 0.28 Spont.

#3 (before intubation) 7.33 58 39 0.35 Spont.

#4 (5 days later) 7.37 47 78 0.30 IMV, 8/min; spont., 6 breaths/min

SUMMARY

Mechanical ventilation is indicated for patients with life-threatening impairment of alveolar ventilation and/or oxygenation. Airway pressures with mechanical ventilation differ fundamentally from normal breathing; in normal breathing airway pressure alternates between negative pressure (on inspiration) and positive pressure (on expiration). In the most common mode of mechanical ventilation used today (intermittent positive pressure ventilation [IPPV]), airway pressure is positive on both inspiration and expiration.

For mechanical ventilation a fraction of inspired oxygen (FIO2) and a mode of ventilation must be chosen. The FIO2, may range from 0.21 to 1.00. The most commonly employed modes of ventilation include control ventilation (machine initiates and delivers each breath), assist-control ventilation (patient initiates or triggers a machine-delivered breath), and intermittent mandatory ventilation (IMV, the patient can breathe spontaneously between mandatory ventilator breaths). Both control and assist­control modes are used for full­ventilatory support. IMV at 8 or more ventilator breaths/ min is tantamount to full-ventilatory support; IMV at 7 or less breaths/min is considered partial ventilatory support and is commonly used as a weaning mode.

Positive end­expiratory pressure (PEEP) is a method of improving oxygenation that can be used in any ventilatory mode. PEEP can be applied to an intubated, mechanically ventilated patient or to a nonintubated patient through a tight­fitting face mask, in which case the technique is called CPAP. All forms of mechanical ventilation have potential complications, which include barotrauma and reduction of cardiac output; these two problems are more commonly seen with the addition of high levels of PEEP.

Ventilator weaning includes three steps: (1) optimizing the patient, (2) assessing the patient's ability to oxygenate and ventilate without the machine, and (3) weaning the patient. There are two methods of weaning­­removing the patient from the ventilator for short periods of time ("trial and error'') and progressive reduction of IMV breaths/minute. Either method is adequate as long as the patient is carefully observed.


REVIEW QUESTIONS

State whether each of the following is true or false .

1. Mechanical ventilation is indicated for any patient with a PaCO2 above 50 mm Hg and a pH less than 7.30.

2. Airway pressures found in normal breathing can be duplicated by mechanical jet ventilation.

3. During controlled positive pressure ventilation, each breath is initiated by the patient.

4. During ventilation with positive end­expiratory pressure (PEEP), the pressure in the upper airways is always above atmospheric pressure.

5. A patient receiving intermittent mandatory ventilation (IMV) is able to alternate spontaneous breathing with machine breaths.

6. Continuous positive airway pressure (CPAP) is defined as a PEEP pressure maintained above 10 cm H2O.

7. The appropriate FIO2 during the initial stages of mechanical ventilation is always 1.00 (100%).

8. With PEEP, a patient's PaO2 may improve while the arterial oxygen delivery is decreasing.

9. Compared with conventional positive pressure ventilation, jet ventilation provides a lower peak airway pressure.

10. Successful ventilatory weaning requires the patient to have a VD/VT of less than 0.45.


References

Ashbaugh, D.B., Bigelow, D.B., Petty, T.L., et al.: Acute respiratory distress in adults, Lancet 2:319, 1967.

Shapiro, B.A., and Cane, R.D. : IMV­AM controversy: a plea for clarification and redirection, Crit. Care Med 12:472, 1984.

Suggested readings Cane, R.D., and Shapiro, B.A.: Mechanical ventilatory support, JAMA 254:87, 1985.

Downs, J.B., Klein, E.F., Desautels, D., et al.: Intermittent mandatory ventilation: a new approach to weaning patients from mechanical ventilators, Chest 64:331, 1973.

Feely, R.W., and Hedley­Whyte, J.: Weaning from controlled ventilation and oxygen, N. Engl. J. Med. 292:903, 1975

Hodgkin, J.E., Bowser, M.A., and Burton, G.G.: Respirator weaning, Crit. Care Med. 2:96, 1974.

McPherson, S.P., and Spearman, D.B.: Respiratory therapy equipment, St. Louis, 1983, The C.V. Mosby Co.

Mushin, W.W., Rendell­Baker, L., Thompson, P.W., et al.: Automatic ventilation of the lungs, Oxford, 1980, Blackwell Scientific Publications, Ltd.

Pepe, P.E., Hudson, L.D., and Camco, C.J.: Early application of positive end expiratory pressure in patients at risk for the adult respiratory distress syndrome, N. Engl. J. Med. 311:281, 1984.

Rounds, S., and Brody, J.S.: Putting PEEP in perspective, N. Engl. J. Med. 311:323, 1984.

Sahn, S.A., Lakshminarayan, S., and Petty, T.L.: Weaning from mechanical ventilation, JAMA 235:2208, 1976.

Suter, P.M., Fairley, H.B., and Isenberg, M.D.: Optimum end-expiratory pressure in patients with acute pulmonary failure. N. Engl. J. Med. 292:284, 1975.


from Pulmonary Physiology in Clinical Practice, copyright 1999 by
Lawrence Martin, M.D.