part0013

The method of the decremental PEEP trial is that the patient's lungs should be recruited as fully as possible using a CPAP recruitment maneuver, followed by a stepwise gradual reduction in expiratory pressure until there is a drop-off in oxygenation, or compliance, or both. This has the advantage of being easy to perform at the bedside; additionally, monitoring oxygenation is easily done with a pulse oximeter, and most ventilators will display static and dynamic respiratory system compliance. *

• After the recruitment maneuver, change the ventilator mode to either Volume Control with a tidal volume of 6 mL/kg PBW, or Pressure Control with a driving pressure of 15 cm H 2 O. Set the PEEP at 20 cm. Note the patient's compliance .

• Once the FiO 2 has been reduced, begin dropping the PEEP in 2 cm increments every 5-10 minutes until the SpO 2 falls below 88%, or until there's a notable drop in compliance. Either of these would indicate alveolar derecruitment.

The disadvantages of a decremental PEEP trial include the time required to properly perform the trial, the need for deep sedation, and the possibility of hemodynamic or respiratory compromise during the recruitment maneuver. Clinical trials examining the decremental PEEP strategy have found that it may improve oxygenation and respiratory compliance, but have not proven any benefit toward survival. It may not be reasonable to perform a trial on every ventilated patient in the ICU. For those with moderate to severe ARDS, however, this can be a useful tool for finding an appropriate level of PEEP.

Using a dynamic pressure-volume loop to determine the optimal level of PEEP is appealing. Many ventilators can produce the P-V loop for review, and it seems intuitive that setting the PEEP at or above the point where pulmonary compliance falls would be useful.

The inspiratory limb of the P-V curve is thought to represent the change in compliance as the lungs fill with gas. Initially, the compliance (the slope of the curve) is poor, reflecting the significant inspiratory pressure needed to open collapsed lung units. Once these lung units open up, they inflate rapidly and much more easily. This is the steeper part of the inspiratory P-V curve, and it indicates that the compliance of the respiratory system has improved. The point where the compliance changes (in other words, where the slope of the curve changes) is known as the lower inflection point (LIP).

As the lungs continue to fill with gas, they reach a point where further application of pressure doesn't expand the lungs much at all—this occurs at the upper inflection point (UIP), and inspiratory pressures beyond this point are thought to contribute to alveolar overdistension and potential barotrauma.

Theoretically, using the inspiratory P-V curve should tell the clinician everything he needs to know regarding the PEEP and driving pressure. The PEEP should be set at or just above the lower inflection point to keep the alveoli from collapsing during expiration, and the plateau pressure (that is, the alveolar pressure at end-inspiration) should be kept at or just below the upper inflection point to minimize overdistension and barotrauma. This would keep the patient ventilating along the steep part of the compliance curve.

Unfortunately, it's not that easy. To begin with, establishing a true pressure-volume curve is difficult. The patient can't be breathing spontaneously, because patient-initiated breathing alters intra- and extrathoracic mechanics. Neuromuscular blockade and deep sedation are often necessary. Second, the inspiratory flow must be constant and relatively low—using a decelerating inspiratory flow, which is the case in pressure control ventilation and pressure-regulated volume control ventilation, will produce an inaccurate curve. Third, the PEEP must be at zero during the maneuver, which can be risky in a severely hypoxemic patient. Fourth, and perhaps most important, is the argument that it makes little sense to set an expiratory pressure based on inspiratory pulmonary mechanics.

Clinical data in humans has shown that while there is some rationale for the lower inflection point, alveolar recruitment tends to continue during the entire inspiratory cycle. Additionally, the upper inflection point may represent the end of the recruitment process but not necessarily alveolar overdistension. During expiration, which is largely passive, an expiratory inflection point occurs at a pressure much higher than the inspiratory lower inflection point. This would suggest that alveolar derecruitment begins at a much higher pressure than the LIP, and that in ARDS this may be as high as 20-22 cm H 2 O. 13 Additionally, derecruitment is affected by gravity and the position of the patient. The heterogeneous nature of both ARDS and alveolar recruitment/derecruitment makes the use of a single pressure-volume relationship difficult when it comes to setting the PEEP .

The plateau pressure (P PLAT ) is the pressure measured at the end of inspiration when inspiratory flow is held at zero. This pressure reflects equilibration of pressures throughout the respiratory tree, and presumably is the end-inspiratory alveolar pressure. In general, clinicians should aim to keep the plateau pressure less than 30-35 cm H 2 O, as this is felt to be the upper limit of alveolar pressure before lung injury occurs * . In the ExPress trial, the tidal volume was set at 6 mL/kg predicted body weight, and the PEEP was increased until the P PLAT was 28-30 cm H 2 O. 14 The control group had a PEEP of 5-9 cm H 2 O. The hypothesis was that this would lead to full alveolar recruitment while preventing lung injury. The trial did demonstrate an improvement in oxygenation in the group receiving this intervention; however, there was no difference in survival.

One drawback of this approach is that patients with less severe ARDS may actually receive higher levels of PEEP. Take two patients with ARDS who each have a predicted body weight of 67 kg. For both, the tidal volume should be 400 mL. If one has less severe ARDS and a respiratory system compliance of 40 mL/cm H 2 O, then it will take an inspiratory driving pressure of 10 cm to deliver the tidal volume. Addition of 18 cm PEEP would bring the plateau pressure up to 28.

In the case of the second patient, assume that his condition is worse and that his respiratory compliance is 20 mL/cm H 2 O. This requires a driving pressure of 20 cm to get the tidal volume, and by following this protocol, he would only get 8-10 cm PEEP to bring the P PLAT up to 28-30 .

This example is simplistic and purposefully ignores the fact that compliance would change (for either better or worse) with the application of PEEP, but the point is that targeting one specific number in all patients could be harmful. It is also worth considering that this method of setting PEEP did not improve survival when compared with the control group.

The transmural, or transpulmonary, pressure in the lung is defined as the difference between the pressure inside the alveoli and the pleural pressure. In other words, Pressure (in) – Pressure (out). Under normal conditions, this value is quite small—the alveolar pressure is atmospheric, or zero, while breathing through an open glottis, and the pleural pressure ranges from around - 3 cm H 2 O at end-expiration to -8 cm at end-inspiration. Since the transpulmonary pressure is the difference between the two, it ranges from 3 (o - - 3) to 8 (o - - 8) cm H 2 O. This is what keeps the lungs open and acts as a counterbalance to the elastic recoil of the lung.

During positive pressure ventilation, the alveolar pressure becomes positive and ranges between the plateau pressure at end-inspiration and the end-expiratory pressure (PEEP). The pleural pressure, if unchanged, remains slightly negative. Under certain conditions, however, the pleural pressure may become positive. This usually occurs when there is a reduction in chest wall compliance either due to primary pleural disease or extrinsic compression (increased abdominal pressure, volume overload, morbid obesity, or a circumferential burn of the torso). When this occurs, the transpulmonary pressure is reduced .

Take two patients with ARDS who have a PEEP set at 15 cm. The first patient has no extrinsic chest wall restriction and a pleural pressure of -5 cm. His transpulmonary pressure at end-expiration is 20 (15 - - 5), which serves to maintain alveolar recruitment in the setting of lung inflammation and edema.

The second patient, in addition to having ARDS, also has reduced chest wall compliance due to morbid obesity (BMI 52). His pleural pressure is +18 cm, which means that his transpulmonary pressure at end-expiration is -3 (15 - +18 ). The net effect is alveolar collapse at the end of each respiratory cycle.

Direct measurement of the intrapleural pressure in ICU patients isn't possible, so the esophageal pressure is used as a surrogate. This is by no means exact—pleural pressure itself varies from the base of the lung to the apex and is affected by supine or prone positioning, and esophageal pressure is subjected to the weight of the mediastinal contents. 15 It is, however, useful for titrating PEEP in patients in whom there is considerable extrinsic reduction of chest compliance .

In order to measure esophageal pressure (Peso), an air-filled esophageal balloon catheter must be inserted. These are commercially available 16 and can be connected to a standard pressure monitoring system. The CareFusion Avea ® ventilator has a port to connect an esophageal pressure probe and can display the esophageal pressure as well.

Insertion of the esophageal balloon catheter should be done by a qualified practitioner in accordance with the manufacturer's instructions. The depth to which the catheter should be inserted can be estimated by multiplying the patient's height in centimeters by 0.288. This should, in most people, position the balloon in the lower third of the esophagus. Partial inflation of the balloon with 1 mL of air will allow changes in the esophageal pressure to be reflected on the monitor. The esophageal pressure waveform should slightly increase during ventilator-delivered breaths and have a negative deflection during patient-initiated breaths. Gentle pressure applied to the abdomen that leads to an increase in the pressure reading suggests gastric placement of the balloon, and it should be withdrawn.

Once the esophageal balloon catheter is in proper position, the end-expiratory transpulmonary pressure can be calculated:

For a patient with a PEEP of 15 cm and a PESO of 22, his end-expiratory transpulmonary pressure is -2 cm H2 O. In other words, at end expiration the compression of his lungs by the increased pleural pressure is leading to alveolar collapse. In this situation, the PEEP should be increased to a minimum level of 17 to keep the end-expiratory transpulmonary pressure at zero .

One clinical trial examining the effect of transpulmonary pressure monitoring in patients with ARDS demonstrated a significant improvement in oxygenation but did not show a survival benefit. 17 As such, this technique is not recommended for routine use. It may be helpful, though, in determining the appropriate level of PEEP for patients with intra-abdominal hypertension or morbid obesity.

In a trial of 51 patients with ARDS, Chiumello and colleagues examined different methods of setting PEEP (ARDSNet tables, targeting a plateau pressure a la the ExPress trial, using the time-pressure stress index, and via transpulmonary pressure using esophageal pressure measurement). 18 All methods were assessed using CT scanning to determine the change in recruitability of the lung. Their findings suggested that the only method that correlated with the degree of whole-lung recruitability and the severity of ARDS was the use of the PEEP-FiO2 table. The other methods were associated with more hyperexpansion of normal lung units without a commensurate benefit in recruitment of collapsed alveoli.

The multiple methods of determining the best, or optimal, level of PEEP have a few things in common. They tend to be laborious. They tend to make significant physiologic assumptions that may not be valid—for example, the assumption that the pressure in the lower esophagus accurately reflects pleural pressure throughout the patient, or the assumption that lung recruitment is complete by the lower inflection point of the inspiratory pressure-volume curve. Lastly, they most often focus on surrogate endpoints that may not be meaningful. Clinical trials of different maneuvers designed to find optimal PEEP often report improved oxygenation or compliance when compared with controls, but none have shown a survival benefit .

Perhaps we need to stop searching for optimal PEEP. The history of critical care medicine has consistently shown that attempts by clinicians to optimize different physiologic parameters are often unnecessary and occasionally harmful. * This may be no different. Luciano Gattinoni, one of the foremost researchers in the field, has suggested this very thing. A "good enough" PEEP maintains oxygenation and lung recruitment without compromising hemodynamic function, and can be based on a combination of the severity of ARDS and the good sense of the treating clinician.

* It is important to keep in mind that no one has established a truly "safe" level of plateau pressure, above which lung injury is present and below which no injury occurs. Most experts, however, advise keeping the plateau pressure at or below this range.

* Perioperative hemodynamic optimization using the pulmonary artery catheter; ScvO2 monitoring in septic shock; aggressive transfusion strategies in penetrating trauma, GI hemorrhage, and critical illness; decompressive craniectomy to treat intracranial hypertension; intra-aortic balloon counterpulsation for cardiogenic shock; high-frequency oscillatory ventilation for ARDS. The beat goes on....

FiO 2	PEEP
30%	5
40%	5
40%	8
50%	8
50%	10
60%	10
70%	10
70%	12
70%	14
80%	14
90%	14
90%	16
90%	18
100%	18
100%	20
100%	22
100%	24

FiO 2	PEEP
30%	5
30%	8
30%	10
30%	12
30%	14
40%	14
40%	16
50%	16
50%	18
50%	20
60%	20
70%	20
80%	20
80%	22
90%	22
100%	22
100%	2 4

Degree of ARDS	PaO 2 /FiO 2 Ratio	PEEP
Mild	201-300	5-10 cm H2 O
Moderate	101-200	10-15 cm H2 O
Severe	≤ 100	15-20 cm H2 O