Measurement of factors impairing gas exchange in man with hyperbaric pressure

Abstract

Alveoli with very low ventilation/perfusion ratios (Va/Q) contribute capillary blood with venous saturation to the mixed arterial stream. The portion of the alveolar arterial oxygen difference (A-a Do₂) due to these units with very low Va/Q ratios cannot be differentiated from that due to an anatomical shunt at ambient pressures. By raising the barometric pressure to such a level that venous blood is saturated, the O₂ dissociation curve no longer affects the A-a Do₂ measurement, and the differentiation can be made. In the present experiments 75% O₂ was inspired at sea level and at a depth equivalent to 2.6 atm. The A-aD was measured for O₂, CO₂ and N₂. In seven out of eight subjects A-a Do₂ and A-a Dn₂ varied when the barometric pressure was increased. A theoretical analysis showed that the results can be interpreted on the basis of a bimodal distribution of Va/Q composed of a large group of well-ventilated alveoli and of a small group of units having an undeterminable Va/Q. The shunt calculated from the total A-a Do₂ at sea level was 3.47% of the total cardiac output. When the A-a Do₂ due to Va/Q (i.e., A-a Dco₂ + A-a Dn₂) was subtracted from the total A-a Do₂ the shunt was 1.78%. At 2.6 atm that part of the total A-a Do₂ found to be due to shunt was 0.8%, or 25% of the shunt estimated at sea level from the pure O₂ technique. pulmonary shunts; ventilation/perfusion distribution; diving; hyperbarism Submitted on June 24, 1963