Pulmonary hemodynamic effects of lung inflation and graded hypoxia in conscious sheep

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Abstract
To determine the separate pulmonary hemodynamic effects of lung inflation and graded hypoxia, pulmonary arterial pressure (Ppa), pulmonary arterial wedge pressure, pulmonary blood flow, pleural pressure (Ppl), and alveolar oxygen tension (PAO2) in unsedated, spontaneously breathing sheep was studied. These measurements were obtained during room air breathing, and sequentially while the sheep breathed mixtures of O2 balance N2 with O2 concentrations of 16.7, 13.4, 9.3, 13.4 and 16.7% for 15 min each, followed by room air breathing. There was an inverse relationship between mean Ppa and PAO2. Pulmonary vascular resistance (PVR) showed an early maximal increase with progressive hypoxia and remained above base line at the lowest PAO2. Mean PVR for a given inspired O2 concentration tended to be greater during progressive hypoxia than during gradual return to normoxia. On a subsequent day, the animals breathed room air, 16.7% and 9.3% O2 balance N2 at functional residual capacity (mean transpulmonary pressure 4.7 cmH2O) and at an increased lung volume (mean transpulmonary pressure [Ptp] 9.0 cmH2O) produced by constant positive airway pressure. Lung inflation resulted in upward displacement of the curves of Ppa-Ppl vs. PAO2. During brief exposures to progressive hypoxia and gradual return to normoxia, the pulmonary vascular response tends to show hysteresis and the combined effects of lung inflation and hypoxia on Ppa-Ppl and PVR are additive for moderate increases in Ptp.

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