REGIONAL MATCHING OF VENTILATION AND PERFUSION DURING LOBAR BRONCHIALOCCLUSION IN MAN

1. Ventilation-perfusion balance in the presence of airway obstruction will depend on the efficiency of hypoxic pulmonary vasoconstriction b eyond obstructed airways and the matching of redistributed blood flow and ventilation to the rest of the lung, This study investigated the r elative importance of these mechanisms in man during experimental bron chial occlusion. 2. The bronchus to the left lower lobe was temporaril y occluded with a balloon-tipped catheter during fibreoptic bronchosco py in eight supine normal volunteers, Respiratory gas tensions were me asured within the occluded lobe with a respiratory mass spectro-meter, The distribution of ventilation and perfusion was assessed under cont rol conditions and after 5 min of bronchial occlusion by computer anal ysis of tbe regional distribution of radioactivity during inhalation o f Kr-81m gas and following injection of Tc-99m-labelled macroaggregate d albumin respectively. 3. Respiratory gas partial pressures within th e occluded lobes rapidly stabilized at mixed venous gas tensions: PO2 43.4+/-2.2 (SEM) mmHg, Pco(2) 40.2+/-1.8 mmHg, During occlusions the a rterial oxygen saturation fell from a baseline of 96.3+/-0.46% to a na dir of 92.1+/-0.43%. Bronchial occlusion produced underventilation in the left lung relative to perfusion, both in the region of the occlude d lower lobe and at the lung apex, Relative overventilation occurred i n the right lung. 4. It is concluded that arterial hypoxaemia during l obal bronchial occlusion is caused primarily by shunting of mixed veno us blood, though the shunt fraction is reduced by approximately 50% by hypoxic pulmonary vasoconstriction. In lung adjacent to obstructed re gions reduced compliance may impair ventilation more than perfusion to contribute to hypoxaemia. It seems likely that redistribution of vent ilation and perfusion to unobstructed regions during lobar bronchial o cclusion is dependent on mechanical factors rather than O-2- or CO2- d ependent changes in bronchial or vascular tone.