Less rather than more volume is better when ventilating patients after cardiac surgery
Most would agree that for patients with acute lung injury, tidal volumes should be low since higher tidal volumes can lead to lung trauma. Ventilation with high tidal volumes has also been shown to be a risk for lung injury. Can tidal volume influence outcome for patients who undergo cardiac surgery? Such patients usually have normal lungs before surgery. Respiratory mechanics are probably little affected during the short period that cardiac surgery patients are ventilated, yet systemic inflammation may be secondary to cardiopulmonary bypass and multiple transfusions, and inappropriately high ventilation may make it worse. Low ventilation, however, may result in atelectasis. In the study “High Tidal Volumes in Mechanically Ventilated Patients Increase Organ Dysfunction after Cardiac Surgery,” Dr. Francois Lellouche (Adjunct Professor, Centre de Recherche de l’Institut Universitaire de Cardiologie et de Pneumologie de Québec, Université Laval, Quebec City, Quebec) and colleagues hypothesized that higher tidal volumes, delivered after surgery, would increase organ failure.
The authors studied patients who underwent cardiac surgery procedures that used sternotomy and cardiopulmonary bypass. Upon arrival in the ICU, patients were ventilated using low (< 10 ml/kg predicted body weight), traditional (10-12 ml/kg predicted body weight) or high (> 12 ml/kg) tidal volume. This was an observation study: patients were not randomized to receive different tidal volumes.
Some 3,434 patients were analyzed. Organ failure rates increased as tidal volume increased and in patients who had prolonged intubation, hemodynamic instability and renal failure. Based on multivariate analysis, high tidal volume was an independent risk factor for organ failure and multiple organ failure, mechanical ventilation more than 24 h and hemodynamic instability. Traditional tidal volume was an independent risk factor for hemodynamic instability. BMI > 30 kg/m2 and female gender were risk factors for use of high tidal volumes.
Patients were not randomized, PEEP was not separately analyzed, and the volume groupings were arbitrary. Nonetheless, the study provides evidence to support use of lower tidal volumes. In the accompanying editorial, “Tidal Volumes during General Anesthesia: Size Does Matter!,” Drs. Paolo Pelosi (Department of Surgical Sciences and Integrated Diagnostics, University of Genoa, Genoa, Italy) and Marcelo Gama de Abreu (Department of Anesthesiology and Intensive Care Medicine, Pulmonary Engineering Group, University Hospital Carl Gustav Carus, Dresden University of Technology, Dresden, Germany) note:
“Certainly, clinical trials addressing the use of protective ventilation with low tidal volumes in the general surgical population are required. However, we do believe that until such trials have been conducted, and provided there are no contraindications, tidal volumes less than 10 ml/kg set according to predicted rather than actual body weight should be the standard. For now, there is enough evidence and a strong rationale to support those recommendations. For the future, the questions to be addressed are how low tidal volumes have to be and how much positive end-expiratory pressure is needed during general anesthesia.”