Anesthesiology
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When positive end-expiratory pressure (PEEP) is added to intermittent positive pressure ventilation, cardiac output and stroke volume frequently fall despite unchanged or increased transmural left ventricular end-diastolic pressure. To determine whether a part of the fall in stroke volume with PEEP is explained by depressed left ventricular systolic function (increased end-systolic volume at a given end-systolic pressure on PEEP) the authors measured left ventricular end-diastolic volume (EDV), end-systolic volume (ESV), and the corresponding pressures in nine patients with acute hypoxemic respiratory failure. Measurements were made before and after 10 cm H2O PEEP was added to the ventilator. ⋯ Despite reduced EDV, pulmonary wedge pressure increased from 12 to 14 torr on PEEP, indicating reduced diastolic compliance or unstressed volume of the left ventricle in these patients similar to that reported in dogs. The authors conclude that PEEP reduces venous return and cardiac output without depressing left ventricular pumping function because end-systolic volume decreased from 64 to 49 ml on PEEP despite identical blood pressures (78 torr). They speculate that PEEP might improve ventricular performance by increasing intrathoracic pressure and left ventricular pressure relative to systemic blood pressure in extrathoracic vessels.
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Comparative Study
Lung volumes, mechanics, and oxygenation during spontaneous positive-pressure ventilation: the advantage of CPAP over EPAP.
To determine if continuous positive airway pressure (CPAP) or expiratory positive airway pressure (EPAP) is superior for achieving or maintaining effective lung volume in spontaneously breathing critically ill patients in acute respiratory failure, the authors measured functional residual capacity (FRC), airway and esophageal pressures, and arterial oxygen tensions when CPAP and EPAP were 5 and 10 cm H2O. Arterial oxygenation, FRC, and transpulmonary pressure at end-expiration were greatest when CPAP was 10 cm H2O. Lung compliance did not change. The authors conclude that CPAP at 10 cm H2O is the more effective technique, either because it allows relaxation of chest wall musculature on expiration, or because EPAP at 10 cm H2O increases chest wall muscle tone.