The Sigh’ as a Recruitment Maneuver in ARDS?
Abstract & Commentary
Now that the use of small tidal volume ventilation (3-6 mL/kg predicted body weight) is the accepted method of managing patients with the acute respiratory distress syndrome (ARDS), prevention of atelectasis and improvement of gas exchange by alveolar recruitment is a topic of much study. This report evaluates the effect of a single ventilator-delivered sigh breath once a minute on patients receiving pressure support ventilation (PSV) during treatment for ARDS. Thirteen intubated patients were studied within 7 days of the onset of symptoms and the diagnosis of ARDS. Patients were lightly sedated and managed with PSV and positive end-expiratory pressure (PEEP) levels set by the primary clinicians, based on individual patient needs. Average tidal volume was about 400 mL and PEEP about 11 cm H2O during the control study period. Control PaO2 was 91.4 ± 27.4 mm Hg.
Sighs were produced by using the "BiPAP" mode on the Dräger Evita-4 ventilator. This mode consists of continuous positive airway pressure (CPAP) delivered in alternating periods of high and low levels. Spontaneous ventilation is possible during both periods, but no pressure support is applied during the high CPAP period. The high CPAP during the single sigh period was set at 20% greater than the peak airway pressure during spontaneous breathing, or at 35 mm Hg, whichever was higher. The sigh duration was 3-5 seconds depending on patient tolerance. "Intolerance" was determined individually if the patient showed hemodynamic compromise or involuntary cough. Patroniti and colleagues used a lower PEEP during the sigh period to maintain mean airway pressure the same as during spontaneous ventilation. The sigh was delivered once each minute for at least 1 hour. At the conclusion of this period gas exchange, respiratory system mechanics, lung volumes (determined by helium dilution), and drive to breathe (airway occlusion pressure, P0.1) were measured and compared to the control period. After the experimental period another hour at baseline conditions was applied and the studied parameters repeated.
During the sigh period, PaO2 increased (91.4 vs 133 mm Hg), PaCO2 was unchanged (45.8 vs 47.3 mm Hg), and total respiratory system compliance improved (40 ± 12 vs 45 ± 15 mL/cm H2O). Tidal volume (418 ± 57 vs 370 ± 81) declined, as did P0.1 (2.1 ± 1.5 vs 1.2 ± 0.7 cm H2O). Lung volume increased during the hour when sighs were delivered (1242 vs 1378 mL). All changes returned to baseline within an hour after removal of the sigh. (Patroniti N, et al. Anesthesiology. 2002;96:788-794.)
Comment by Charles G. Durbin, Jr., MD
This is one of a number of studies that have attempted to describe the effects of alveolar recruitment maneuvers in patients with ARDS who are managed with small tidal volume ventilation as part of a lung-protective therapy. It demonstrates that rapid improvements in gas exchange occur when a 3-5 second period of CPAP breathing (greater than 35 cm H2O) is delivered in patients breathing spontaneously with PSV. While the observations of this study are interesting (and predictable) it is not clear what the implications are for ARDS treatment. While all the studied patients met the criteria for ARDS, most had relatively good pulmonary compliance and probably were in an early stage of the disease. Only 13 patients were studied, and no attempt was made to systematically study all patients with this disorder. Thus, the patients represent a highly selected population, although the selection criteria were not stated. This weakens the study’s conclusions.
The baseline pulmonary treatment was not standardized. No attempt was made to optimize PEEP, and no pressure-volume curve information was determined. The relationship of the chosen PEEP to the lower inflection point was not determined. Rather low PEEP levels were used (about 10 cm H2O) considering the current understanding of derecruitment. Tidal volume was measured during spontaneous ventilation and undoubtedly varied substantially from breath to breath. The tidal volume was not controlled by protocol, but was determined by the primary clinician. The study conditions are relevant in that spontaneous ventilation is often desirable to optimize ventilation to perfusion matching.
There are significant limits to the usefulness of this study. Pressures were measured at the airway. Without knowing intrathoracic pressures, the lung and chest wall contributions cannot be separated. The recruitment maneuver was only applied for the hour of the study, and was not used in comparison to another approach or in a therapeutic way throughout the course of ARDS. The way the "sigh" was created is specific to the ventilator used, and may not apply to other types of "sighs." The duration of the sigh was determined by "patient tolerance," a subjective and not easily reproducible approach. This paper should help stimulate more studies of sighs for longer periods of time. It is essential to show that these short-term improvements translate into a better long-term outcome.
Dr. Durbin is Professor of Anesthesiology, Medical Director of Respiratory Care, University of Virginia.
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