The purpose of this study was to determine the cumulative effects of brief intervals of
hypoxia and
hypercapnia on the pulsatile characteristics 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 i.v.). Animals 48 h old were randomly allocated to undergo mild
hypoxia (inspired O2 fraction = 0.15), severe
hypoxia (inspired O2 fraction = 0.05), or
hypercapnia (inspired CO2 fraction = 0.20), whereas animals 2 wk old underwent severe
hypoxia or
hypercapnia. With use of Fourier analysis, characteristic impedance (Zo), mean input impedance (Zm), impedance moduli, and phase angles were determined. In 48-h-old pigs anesthetized with
azaperone-
ketamine, neither mild nor severe
hypoxia altered Zo, Zm, or pulmonary vascular resistance (PVR), whereas
hypercapnia increased Zo by 22% (P < 0.001), which persisted despite a return to normocapnia. In 48-h-old animals 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
hypercapnia increased Zm and PVR but Zo was unaltered. In 2-wk-old pigs anesthetized with
thiopental, severe
hypoxia but not
hypercapnia elevated Zm and PVR, whereas Zo was not changed with either stress. Results indicate age- and
anesthetic-dependent responses of Zo, Zm, and PVR to severe
hypoxia and
hypercapnia. The persistent elevation in Zo caused by
hypercapnia indicates a prolonged decrease in arterial compliance or a reduction in effective proximal pulmonary arterial radius.