Regional Differences in Lung Function during Anaesthesia and Intensive Care: Clinical Implications

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Abstract
Anesthesia and most frequently acute respiratory failure are accompanied by a lowered functional residual capacity (FRC). This lowering promotes airway closure in dependent lung units and forces ventilation to non-dependent regions. Perfusion, is forced towards dependent lung units. A ventilation-perfusion mismatch is created and hypoxemia may develop. General PEEP [positive end-expiratory pressure] counters airway closure but impedes cardiac output and forces perfusion further to dependent regions. Barotrauma may also occur. Improved matching of ventilation and perfusion can be achieved by positioning the subject in the lateral posture, ventilating each lung separately in proportion to its perfusion (differential ventilation) and applying PEEP only to the dependent lung (selective PEEP). Because of lens overall intrathoracic pressure and lung expansion, interference with the total lung blood flow and the danger of barotrauma should be less than with general PEEP. Improved gas exchange with a 50-100% increase in PaO2 [arterial O2 pressure] was observed in a limited number of patients with acute bilateral lung disease studied during differential ventilation and selective PEEP.