San Diego—The classical view of lung recruitment holds that some overdistention will occur when positive end-expiratory pressure (PEEP) is administered because each lung area behaves differently. “This view is wrong,” asserts Luciano Gattinoni, MD, a Professor of Anesthesia and Critical Care at the University of Milan in Italy.

Recruitment (the aeration of gasless alveoli) is an inspiratory phenomenon unrelated to PEEP, which is an expiratory maneuver, said Dr. Gattinoni at the Society of Critical Care Medicine’s annual meeting in San Diego.[1] During recruitment, lung opening occurs along the entire pressure-volume curve without significant overdistention; however, various levels of pressure are needed to open different areas of the lung. PEEP simply keeps recruited lung areas open, he stated.

FACTORS AFFECTING RECRUITMENT

In patients with the primary form of the acute respiratory distress syndrome (ARDS), the prevalent consolidation limits—but does not entirely eliminate—the potential for recruitment. In contrast, there is a “huge” recruitment potential in patients with secondary ARDS because that condition typically involves widespread lung collapse, Dr. Gattinoni pointed out.

Although usually ignored in the intensive care unit, intra-abdominal pressure (which can be measured through the bladder) appears to strongly influence recruitment potential. Indeed, said Dr. Gattinoni, it has been shown to have a direct relationship with chest wall compliance in patients with primary or secondary ARDS.

Also, studies in pigs have demonstrated that increased intra-abdominal pressure (20 cm H2O) doubles the extent of pulmonary edema. This finding prompted the hypothesis that a rise in intra-abdominal pressure inhibits lung fluid clearance by unfavorably altering pleural pressure. Although this hypothesis has not yet been fully proved in humans, it has been confirmed in animal studies. “This is for sure in animals,” Dr. Gattinoni emphasized.

RECRUITMENT MANEUVERS

Sangeeta Mehta, MD, Research Director at Mount Sinai Hospital in Toronto, Canada, summarized recent animal and human data on lung recruitment. In sheep, she reported, a strategy that included a short period of PEEP (plateau pressure: 60 cm H2O) seemed to markedly improve oxygenation and reduce the shunt fraction, but only if the procedure was repeated several times. In contrast, brief administration of continuous positive airway pressure (40 cm H2O) produces no recruitment and transiently decreases cardiac output.

Rabbit studies suggest that conventional mechanical ventilation protects the lung as well as high-frequency oscillation does when both are applied with sustained inflation. Furthermore, rat studies suggest that recruitment maneuvers that include both 8 cm H2O of PEEP and sustained inflation are better able to expand the lung than is application of 8 cm H2O of PEEP alone.

Pelosi et al[2] assessed the effect of three consecutive “sighs” (plateau pressure: 45 cm H2O) delivered per minute for one hour in patients with ARDS of pulmonary or extrapulmonary origin. This recruitment maneuver reduced shunting and increased oxygenation and expiratory lung volume in both groups; however, the extent of the improvements was much greater in the patients with extrapulmonary ARDS than in those patients with strictly pulmonary disease.

In studies that Dr. Mehta helped conduct, oxygen saturation rose considerably in adults with early hypoxic respiratory failure after a sustained inflation (30 to 45 cm H2O for 20 seconds) was administered.[3] Improvement was seen within 10 minutes and, in some patients, it persisted for up to four hours. When the recruitment maneuver was repeated with higher PEEP levels, the duration of the increase in oxygen saturation was lengthened.

Foti and coworkers[4] assessed the combination of low PEEP levels (about 9 cm H2O) and volume recruitment in ARDS patients who had previously been shown to respond to PEEP. The patients’ lungs were recruited by raising PEEP from low to high (about 16 cm H2O) for two breaths every minute. The effects of this maneuver were compared to those achieved with low or high PEEP alone.

The combination of low PEEP levels and volume recruitment produced a lower shunt fraction and greater increases in oxygenation and end-expiratory lung volume than did low PEEP alone, but its effects were not as great as those resulting from high PEEP alone. However, high PEEP’s superior performance came at the expense of greater peak inspiratory pressure and lower chest wall compliance, Dr. Mehta pointed out.

It has become increasingly common, she noted, to ventilate ARDS patients at a tidal volume of 6 mL/kg. “However, this seems to promote derecruitment or atelectasis when compared to the conventional level of 10 mL/kg,” she cautioned. Derecruitment may also occur with low PEEP settings (0 to 15 cm H2O).

ASSESSING RECRUITMENT

Most practitioners do not assess alveolar recruitment using the dynamic pressure-volume curve because there is no simple, rapid, safe, and reproducible way to measure the curve at the bedside. However, recruitment can be estimated in other ways, suggested V. Marco Ranieri, MD, a Professor of Anesthesiology at the University of Turin in Italy.

Computed tomography (CT) is one option, he said, because it can detect findings that correlate with changes in the pressure-volume curve. Specifically, the curve’s lower and upper inflection points correspond to CT evidence of atelectasis and overdistention. Dr. Gattinoni added that he considers CT the best imaging tool for assessing recruitment because other methods do not discern as well between recruited and unrecruited lung areas.

Recruitment can also be estimated, noted Dr. Ranieri, by measuring the static pressure-volume curve with and without PEEP administration. A rise in lung volume for the same amount of static pressure indicates recruitment, he explained.

It is important to account for chest wall stiffness when estimating recruitment, he stressed; the tidal volume that can be used without risking overdistention is much higher when the chest wall is stiff. Nearly 50% of patients with respiratory failure exhibit such stiffness.

Like Dr. Gattinoni, Dr. Ranieri emphasized the importance of high intra-abdominal pressure (above 12 mm Hg) in mechanically ventilated patients. “The higher the intra-abdominal pressure, the higher the incidence of organ failure” and the risk of death, he warned.

High intra-abdominal pressure occurs in nearly half of mechanically ventilated patients and is particularly likely to develop in those with medical admissions, Dr. Ranieri explained. Because of the risks associated with increased intra-abdominal pressure, measuring it is probably more relevant to intensive care than is assessing the static pressure-volume curve, he concluded.

—Timothy Begany

References
1. Gattinoni L, Mehta S, Ranieri M, Levy M. How I titrate PEEP and recruit the lung. Presented at: Annual Meeting of the Society of Critical Care Medicine; January 27, 2002; San Diego, Calif.
2. Pelosi P, Cadringher P, Bottino N, et al. Sigh in acute respiratory distress syndrome. Am J Respir Crit Care Med. 1999;159:872-880.
3. Lapinsky SE, Aubin M, Mehta S, et al. Safety and efficacy of a sustained inflation for alveolar recruitment in adults with respiratory failure. Intensive Care Med. 1999;25:1297-1301.
4. Foti G, Cereda M, Sparacino ME, et al. Effect of periodic lung recruitment maneuvers on gas exchange and respiratory mechanics in mechanically ventilated acute respiratory distress syndrome (ARDS) patients. Intensive Care Med. 2000;26:501-507.

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