Effects of low flow on pulmonary vascular flow-pressure curves and pulmonary vascular impedance

Objective: Flow-pressure curves and vascular impedance are commonly used to investigate pulmonary circulation, but they may be affected at low flow by reflex neurohumoral activation. We therefore investigated the mechanical e ffects and the reflex effects of decreased flow on pulmonary vascular resis tance and impedance, Methods: In ten anaesthetized dogs, we compared flow-p ressure curves generated in less than 10 s to prevent sympathetic activatio n (fast curves), or generated over 20-30 min to allow neurohumoral equilibr ation (slow curves), in hyperoxia (inspired oxygen, 40%) and in hypoxia (in spired oxygen, 10%), before and after adrenergic blockade by phentolamine a nd propranolol. Resistance was assessed from the flow-pressure relationship . Impedance was computed from instantaneous flow and pressure obtained with an ultrasonic flowmeter and a micromanometer-tipped catheter. Results: At baseline, fast flow-pressure curves were steeper and had a lower pressure i ntercept. Transient low flow did not affect heart rate or pulmonary arteria l elastance. Sustained low flow increased heart rate, resistance and elasta nce, suggesting baroreceptor-induced sympathetic stimulation. After adrener gic blockade, no difference persisted between effects of transient and sust ained low flow. In hypoxia, slow and fast flow-pressure curves were similar . Hypoxia increased heart rate and resistance but did not decrease elastanc e, suggesting chemoreceptor-induced sympathetic stimulation, In hypoxia, di fferences between transient and sustained low flow were no longer significa nt, and were completely suppressed by adrenergic blockade. In two additiona l dogs, epinephrine infusion increased pulmonary vascular resistance and el astance. Conclusions: We conclude that (1) compared to transient low flow, sustained low flow is associated with increases in distal resistance and pr oximal elastance due to sympathetic stimulation and (2) these differences b etween the effects of transient and sustained low flow do not persist in hy poxia, because of an already present chemoreceptor-induced sympathetic stim ulation. (C) 1999 Elsevier Science BN. All rights reserved.