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).

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.

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.