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Abstract & Commentary
Low Tidal Volume Ventilation in theAbsence of Acute Lung Injury: A Study in Post-Cardiac Surgery Patients
By David J. Pierson, MD, Editor, Professor, Pulmonary and Critical Care Medicine, Harborview Medical Center, University of Washington, Seattle, is Editor for Critical Care Alert.
SYNOPSIS: This randomized study of low-tidal-volume ventilation during and after cardiac surgery, in comparison to ventilation with tidal volumes of 10 mL/kg predicted body weight, showed no differences in median ventilation time but higher rates of extubation by 6 and 8 hours postoperatively and fewer reintubations in the low-tidal-volume group.
SOURCE: Sundar S, et al. Influence of low tidal volume ventilation on time to extubation in cardiac surgical patients. Anesthesiology 2011;114: 1102-1110.
Supported by robust data from numerous clinical trials, low-tidal-volume ventilation is now standard-of-care in managing patients with acute lung injury (ALI) or the acute respiratory distress syndrome (ARDS). While the tidal volumes used in ventilating non-ALI patients have generally decreased in the last two decades, no clinical studies have directly shown that deliberate low-tidal-volume ventilation to the extent applied in ALI is beneficial in that patient population. To address this issue, investigators at Beth Israel Deaconess Medical Center in Boston assessed time to extubation and several other factors in patients undergoing elective cardiac surgery who were randomized to be ventilated with tidal volumes of either 6 or 10 mL/kg predicted body weight.
Sundar and colleagues evaluated 621 patients who were to undergo cardiac surgery in their institution. They excluded emergent cases, patients on inotropic agents or intra-aortic balloon support, and those with pre-existing pulmonary disease or active infection, enrolling 74 patients in the low-tidal-volume arm and 75 in the control arm of the study. The allocated tidal volume was applied immediately following intubation, during surgery, and throughout the period of postoperative mechanical ventilation in the cardiothoracic ICU. Mechanical ventilation, sedation, and weaning and extubation were applied according to established protocols. The primary end point was time to extubation, with secondary analyses performed on other aspects of intubation time and the rate of reintubation.
Median total ventilation time was 7.5 hours vs 10.7 hours in the 6- and 10-mL/kg tidal volume groups, respectively (P = 0.10). At 6 hours from intubation, more patients in the 6-mL/kg group had been extubated (37%) than in the 10-mL/kg group (20%; P = 0.02). Corresponding proportions at 8 hours were 53% vs 31% (P = 0.0006). More patients in the larger-tidal-volume group required reintubation (9.5%) than in the low-tidal-volume group (1.3%; P = 0.03). There were no differences between the groups with respect to ICU or hospital lengths of stay.
Several studies have suggested that large-tidal-volume mechanical ventilation of critically ill patients without ALI predisposes them to development of that condition. Despite the facts that postoperative ventilation in cardiac surgery patients typically lasts only a few hours, and that this group of patients is generally at low risk for developing ALI or ARDS, there is evidence from studies of blood cytokines and other potential mediators of lung injury that the use of tidal volumes of 10-12 mL/kg predicted body weight may increase risk in this setting. This type of evidence, along with the finding that ALI and ARDS frequently go unrecognized by their physicians, supports arguments by some authorities that virtually all patients subjected to mechanical ventilation in the context of acute illness should be managed with low-tidal-volume, so-called lung-protective ventilation.1
This study was carried out in a single center, and excluded higher-risk cardiac surgery patients, limiting its generalizability even to the management of this restricted patient population of ventilated patients in other institutions. However, regardless of whether one accepts the authors' positive spin on their findings, the fact that no downside to low-tidal-volume ventilation was demonstrated lends strength to the notion that this approach to ventilatory support is reasonable in patient populations well beyond those with ALI and ARDs.