Pulmonary vascular impedance response to hypoxia in dogs and minipigs: effects of inhaled nitric oxide.

The pig has been reported to present with a stronger hypoxic pulmonary vasoconstriction (HPV) than many other species, including dogs. We investigated [pulmonary arterial pressure (Ppa)-pulmonary arterial occluded pressure (Ppao)] vs. pulmonary blood flow (Q) relationships and pulmonary vascular impedance (PVZ) spectra in nine minipigs and nine weight-matched dogs. The animals were anesthetized and ventilated in hyperoxia [inspired O2 fraction 0.4] or hypoxia (inspired O2 fraction 0.12). PVZ was computed from the Fourier series for Ppa and Q. In hyperoxia, the pigs had a higher Ppa (26 +/- 1 vs. 16 +/- 1 mmHg), a higher first-harmonic impedance (Z1), and a more negative low-frequency phase angle but no different characteristic impedance (Zc) compared with the dogs at the same Q. Hypoxia in the dogs increased (Ppa-Ppao) at all levels of Q studied by an average of 2 mmHg but did not affect Z1 or Zc. Hypoxia in the pigs increased (Ppa-Ppao) at all levels of Q by an average of 13 mmHg and increased Z1 and Zc. Inhaled NO (150 ppm) reversed the hypoxia-induced changes in (Ppa-Ppao)/Q plots and PVZ in the dogs and pigs. However, differences in (Ppa-Ppao)/Q plots and PVZ between the dogs and pigs in hyperoxia and hypoxia were not affected by inhaled NO. We conclude 1) that minipigs present with an elevated pulmonary vascular resistance and impedance in hypoxia more than in hyperoxia and 2) that baseline differences in pulmonary hemodynamics between dogs and minipigs are structural rather than functional.