INCREASED LEFT ATRIAL PRESSURE INHIBITS HYPOXIC PULMONARY VASOCONSTRICTION

An increase in left atrial pressure (Pla) has been reported to either inhibit or not affect hypoxic pulmonary vasoconstriction in intact dog s. We investigated mean pulmonary arterial pressure (Ppa)-flow (Q) rel ationships at low and high fixed Pla and Ppa-Pla relationships at fixe d and in piglets, which are known to present with a stronger hypoxic p ulmonary pressor response than dogs. Seven piglets were anesthetized; equipped with balloon catheters in inferior vena cava and left atrium to control and and Pla, respectively; and ventilated alternatively in hyperoxia fractional concn of O-2 in inspired air (FIO2) 0.4 and hyp oxia (FIO2 0.12). In ail experimental conditions, Ppa-Q plots were bes t described by a linear approximation with extrapolated pressure inter cepts (Pi) not different from Pla. Hypoxia increased slope but not Pi of Ppa-Q plots. An increase in Pla from 8 to 17 mmHg induced a paralle l shift of Ppa-Q plots to higher Ppa in hyperoxia but did not affect P pa-Q plots in hypoxia. In hyperoxia, an increase in Pla at constant Q induced an approximately equal increase in Ppa, whereas in hypoxia the re was no effect. The hypoxia-induced increase in Ppa was blunted by i ncreased Pla at all levels of Q studied. We conclude that in anestheti zed piglets at fixed Pla hypoxia increases the slope of Ppa-Q plots wi thout affecting Pi and an increase in Pla inhibits hypoxic pulmonary v asoconstriction. The results suggest that no closing pressure higher t han normal Pla contributes to hyperoxic or hypoxic Ppa in the intact p orcine pulmonary circulation.