JOSEPH BRIMACOMBE, CHRISTIAN KELLER AND CHRISTOPH HÖRMANN
Department of Anaesthesia and Intensive Care, Cairns Base Hospital, Cairns, Australia
Anesthesiology, 92: 1 621–1623, 2000
Spontaneous breathing is the most popular mode of ventilation with the laryngeal mask airway (LMA); however, it provides less effective gas exchange than does positive pressure ventilation (PPV). Pressure support ventilation (PSV) is a form of partial ventilatory support in which each spontaneous breath is assisted to an extent that depends on the level of a constant pressure applied during inspiration. This randomized crossover study tested the hypothesis that PSV provides more effective gas exchange than does unassisted ventilation with continuous positive airway pressure (CPAP) in anesthetized adult patients treated using the LMA.
Forty consecutive adult patients undergoing peripheral musculoskeletal surgery in which the LMA was considered suitable were studied. Group 1 patients underwent CPAP, PSV, and CPAP in sequence. Group 2 patients underwent PSV, CPAP, and PSV in sequence. PSV comprised positive end-expiratory pressure set at 5 cm H2O and inspiratory pressure support set at 5 cm H2O above positive end-expiratory pressure. CPAP was set at 5 cm H2O. Each ventilatory mode was maintained for 20 min. End-tidal CO2, oxygen saturation, expired tidal volume, leak fraction, respiratory rate, noninvasive mean arterial pressure, and heart rate were recorded once every minute for the last 5 min for each ventilatory mode.
The two groups showed no demographic differences. In both groups, PSV had lower end-tidal CO2, higher oxygen saturation, and higher expired tidal volume compared with CPAP. In both groups, PSV had similar leak fraction, respiratory rate, mean arterial pressure, and heart rate compared with CPAP. In group 1 patients, measurements for CPAP were similar before and after PSV. In group 2, measurements for PSV were similar before and after CPAP. Data for CPAP and PSV were similar between groups. Gastric insufflation was not detected. These results support the conclusion that PSV provides more effective gas exchange than does unassisted ventilation with CPAP during anesthesia with the LMA while preserving leak fraction and hemodynamic homeostasis.
This study examines two different modes of assisted spontaneous ventilation through an LMA during general anesthesia to determine which results in more effective gas exchange. However, actual change in the effectiveness of blood oxygenation and carbon dioxide removal is never directly measured because blood gas analysis is not performed. The two modes of ventilation compared in this study are 5 cm H2O of CPAP and PSV (which consists of 5 cm H2O PEEP and inspiratory pressure support set at 5 cm H2O above PEEP when inspiratory flow is greater than 3 L/min). Although the CPAP would be fairly easy to administer to the spontaneously ventilating patient, the PSV mode is not readily available on standard anesthesia machines. Therefore, this would necessitate the use of total intravenous anesthesia if one wanted to employ PSV during LMA anesthesia. The authors do not specify the type of ventilator used during the study.
The results of the study indicate that ETCO2 is decreased and oxygen saturation is slightly increased when using PSV as compared with CPAP. However, the clinical significance of these values (O2 saturation 96 to 98% and ETCO2 40 to 50 mm Hg) in the average patient is probably negligible and does not warrant the effort required to provide PSV in the operating room. In addition, although minute ventilation increased by an impressive 50% using PSV, the ETCO2 only decreased by 20%, suggesting that 30% of the additional ventilation was wasted. The authors attribute this to an increase in alveolar dead space that probably is related to the increase in mean airway pressure during PSV.
Roxanne Zarmsky M.D.