Recruitment Maneuvers May Help Early in ARDS but not Later
Abstract & Commentary
Low tidal volume ventilation, as a way of reducing alveolar stretch injury in patients with the acute respiratory distress syndrome (ARDS), may result in alveolar and segmental lung collapse with a decline in oxygenation. Alveolar recruitment maneuvers are used to help reverse this problem. It is well recognized that some patients fail to respond to these interventions. This cohort studied the response of a group of 22 patients to 40 seconds of continuous positive airway pressure (CPAP) at 40 cm H2O and compared the responders to the nonresponders. Patients were managed according to the ARDS Net protocol (The Acute Respiratory Distress Syndrome Network. N Engl J Med. 2000;342:1301-1308), using 6 mL/kg tidal volume and setting positive end-expiratory pressure (PEEP) above the lower inflection point, using static pressure-volume curves determined during paralysis. Lung and chest wall compliance and elastance were separated using airway and esophageal pressure measurements. Responders (R) were defined as those who demonstrated an increase in PaO2/FiO2 of greater than 50% when assessed 2 minutes following the application of CPAP, and nonresponders (NR) as those who showed less than this degree of increase.
The 11 nonresponders exhibited a 20 ± 3% increase in oxygenation ratio while the 11 responders increased this ratio by 175 ± 23%. There was no difference in initial oxygenation ratio, tidal volume (6.1 ± 0.1 mL/kg, R vs 6.0 ± 0.2 mL/kg, NR) or PEEP (9.4 ± 2.2 cm H2O in R vs 9.1 ± 2.7 cm H2O in NR) between the groups prior to the recruitment maneuver. The groups were similar in age, sex distribution and cause of ARDS. The time on mechanical ventilation was longer in the NR group, 7.1 ± 1.5 days (NR) vs 1.0 ± 0.3 days (R); P < .001. Both lower and upper inflection points were lower in the NR group, although only by about 1-2 cm H2O. Respiratory system, lung, and chest wall elastance were all higher in the NR group. The most dramatic difference was in chest wall elastance, 10.4 ± 1.8 (NR) vs. 5.6 ± 0.8 (R) cm H2O/L.
Two minutes following the recruitment maneuver, the R group showed an oxygenation ratio of 440 ± 60 mm Hg while the NR group rose to only 180 ± 46 mm Hg. Significant cardiac effects occurred only in the NR group where a fall of 20-30% in blood pressure, stroke volume, and cardiac output occurred. These changes returned to baseline within 30 seconds following release of CPAP. The effects on oxygenation decreased rapidly following the maneuver, retuning to baseline in 20-30 minutes in both groups.
The major findings in this study were that nonresponders had much stiffer chest walls, only slightly stiffer lungs, and had received mechanical ventilation for a significantly longer period of time. The non-responders experienced greater, transient cardiovascular compromise, possibly due to greater transmission of the recruitment maneuver pressure to the thoracic cavity (Grasso S, et al. Anesthesiology. 2002;96:795-802).
Comment by Charles G. Durbin, Jr., MD
This is a well-done study of the acute effects of a standardized recruitment maneuver in patients with ARDS in whom small tidal volume ventilation was being used. Careful determination of the static pressure volume curves assured similar ventilator treatment in these patients at baseline. There was a wide split in the responding and nonresponding groups based on changes in the oxygenation ratio following the recruitment maneuver, and this difference was used to define the 2 study groups. The study is unique in that the chest wall and lung compliance contributions were carefully separated and hemodynamics meticulously monitored. While lung stiffness was slightly higher in the nonresponders, chest wall elastance was nearly double (compliance less than half).
From this study, patients with early and late ARDS appear to differ primarily in chest wall stiffness. This results in the difference in response in oxygenation to an alveolar recruitment maneuver and as well as the cardiovascular consequences. Results of this study should be applied with caution in the clinical care of patients. This was not a randomized study; the patients were carefully selected, although the selection criteria were not explicitly detailed in the "methods." The results should only be used to help understand the pathophysiology of ARDS.
An interpretation of this study could be to suggest only using alveolar recruitment maneuvers early in the course of ARDS. However, both groups did respond, although the later group had a smaller response. Only one standard maneuver was used and only for a single application. Different maneuvers applied more often might demonstrate different acute effects. More importantly, no attempt was made to assess whether an outcome difference was achieved by application of a recruitment maneuver. It has yet to be established whether any long-term benefits accrue from recruitment maneuvers. The ARDS Net management strategy is also undergoing revision and testing. It is likely that more PEEP should be used to prevent alveolar derecruitment than was originally allowed in the ARDS Net protocol. A change in optimal PEEP may reduce the need for recruitment maneuvers altogether.
While this study is interesting and may lead to improved understanding of the changes that occur in patients with ARDS, its findings have no immediate therapeutic applications. More studies of the effects of recruitment maneuvers will be published soon.
Dr. Durbin is Professor of Anesthesiology, Medical Director of Respiratory Care, University of Virginia.
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