LOCAL PULMONARY BLOOD-FLOW - CONTROL AND GAS-EXCHANGE

We studied the local response of the pulmonary vasculature to combined changes in alveolar P(O2) and P(CO2) in the right apical lobe (RAL) o f six conscious sheep. That lobe inspired an O2-CO2-N2 mixture adjuste d to produce one of 12 alveolar gas compositions: end-tidal P(CO2) (PE T(CO2)) of 40, 50, and 60 Torr, each coupled with end-tidal P(O2) (PET (O2)) of 100, 75, 50, and 25 Torr. In addition, at each of the four PE T(O2), the inspired CO2 was set to 0 and PET(CO2) was allowed to vary as RAL perfusion changed. The remainder of the lung, which served as c ontrol (CL) inspired air. Fraction of the total pulmonary blood flow g oing to the RAL (%QRAL) was obtained by comparing the methane eliminat ion from the RAL to that of the whole lung, and expressed as a percent age of that fraction at PET(CO2) = 40, PET(O2) = 100. Cardiac output, pulmonary vascular pressures, and CL gas tensions were unaffected or o nly minimally affected by changes in RAL gas composition. A drop in P( O2) from 100 to 50 Torr decreased local blood flow by 60% in normocapn ia and by 66% at a P(CO2) of 60. At all levels of oxygenation, an incr ease in P(CO2) from 40 to 60 reduced QRAL by nearly 50%. With these st imulus-response data, we developed a model of gas exchange, which take s into account the effects of test segment size on blood flow diversio n. This model predicts that: (1) when the ventilation to one compartme nt of a two compartment lung is progressively decreased, PA(O2) remain s above 60 Torr for up to 60% reductions in alveolar ventilation, irre spective of compartment size; (2) the decrease in PA(O2) that occurs a t altitude is accompanied by a drop in PA(CO2) that limits the decreas e in conductance and minimizes the pulmonary hypertension; and (3) as we stand, local blood flow control by the alveolar gas tensions halves the alveolar-arterial P(O2) and P(CO2) differences imposed by gravity .