PULMONARY HYDRAULIC IMPEDANCE RESPONSES TO HYPOXIA AND HYPERCAPNIA INNEWBORN PIGS

The purpose of this study was to determine the cumulative effects of b rief intervals of hypoxia and hypercapnia on the pulsatile characteris tics of the pulmonary arterial circulation of 48-h-old compared with 2 -wk-old open-chest Yorkshire pigs while using two different anesthetic regimens: 1) azaperone and ketamine (4 and 12 mg/kg im, respectively) and 2) thiopental sodium (25 mg/kg iv). Animals 48 h old were randoml y allocated to undergo mild hypoxia (inspired O-2 fraction = 0.15), se vere hypoxia (inspired O-2 fraction = 0.05), or hypercapnia (inspired CO2 fraction = 0.20), whereas animals 2 wk old underwent severe hypoxi a or hypercapnia. With use of Fourier analysis, characteristic impedan ce (Zo), mean input impedance (Zm), impedance moduli, and phase angles were determined. In 48-h-old pigs anesthetized with azaperone-ketamin e, neither mild nor severe hypoxia altered Zo, Zm, or pulmonary vascul ar resistance (PVR), whereas hypercapnia increased Zo by 22% (P < 0.00 1), which persisted despite a return to normocapnia. In 48-h-old anima ls anesthetized with thiopental, baseline control Zo and Zm were lower than those in same-age pigs anesthetized with azaperone-ketamine. In thiopental-anesthetized 48-h-old pigs, both severe hypoxia and hyperca pnia increased Zm and PVR but Zo was unaltered. In 2-wk-old pigs anest hetized with thiopental, severe hypoxia but not hypercapnia elevated Z m and PVR, whereas Zo was not changed with either stress. Results indi cate age- and anesthetic-dependent responses of Zo, Zm, and PVR to sev ere hypoxia and hypercapnia. The persistent elevation in Zo caused by hypercapnia indicates a prolonged decrease in arterial compliance of a reduction in effective proximal pulmonary arterial radius.