Fig. 2. (  A ) Composite left pulmonary vascular pressure–flow (LPQ) plots in seven dogs in the conscious state during normoxia, normoxia while breathing through a face mask, and hypoxia with systemic arterial P^O2of 60 mmHg and 50 mmHg. Compared with normoxia, there was no change in the LPQ relationship during normoxia  via mask. Hypoxia caused a leftward shift (*  P < 0.01) in the LPQ relationship, which indicates pulmonary vasoconstriction. There was a further shift (†  P < 0.01) in the LPQ relationship as systemic arterial P^O2decreased from 60 mmHg to 50 mmHg. (  B ) Composite hypoxic pulmonary vasoconstriction (HPV) response (increase in pulmonary arterial pressure − left atrial pressure from normoxia to hypoxia) as a function of left pulmonary flow. The magnitude of HPV was stimulus-dependent (†  P < 0.01).