Strength of pulmonary vascular response to regional alveolar hypoxia

Abstract

Regional alveolar hypoxia in the lung induces regional pulmonary vasoconstriction which diverts blood flow from the hypoxic area. However, the predominant determinant of the distribution of perfusion in the normal erect lung is gravity so that more perfusion occurs at the base than at the apex. To determine the strength of the regional alveolar hypoxic response in diverting flow with or against the gravity gradient a divided tracheal cannula was placed in anesthetized dogs and unilateral alveolar hypoxia created by venilating one lung with nitrogen while ventilating the other lung with oxygen to preserve normal systemic oxygentation. Scintigrams of the distribution of perfusion obtained with intravenous 13-N and the MGH positron camera revealed a 34 and 32 per cent decrease in perfusion to the hypoxic lung in the supine and erect positions and a 26 per cent decrease in the decubitus position with the hypoxic lung dependent (P equal to 0.94 from supine shift), indicating nearly equal vasoconstriction with shift of perfusion away from the hypoxic lung in all positions. Analysis of regional shifts in perfusion revealed an equal vasoconstrictor response from apex to base in the supine position but a greater response in the lower lung zones in the erect position where perfusion was also greatest.

Hypoxic Pulmonary Vasoconstriction

“…It is interesting to note that oxygen want and carbon dioxide ... call forth a contraction of the lung vessels, thereby increasing the blood flow to better aerated lung areas, which leads to improved conditions for the utilization of alveolar air." Consistent with this proposal, numerous reports subsequently demonstrated that ventilating a sublobar lung region (653,1012), lung lobe (103,141,143,422,1192,1645,1761,1764,1852), or an individual lung (355,380,415,421,473,717,719,778,1192,1435,1579,1823,1825,1999) with an hypoxic gas mixture (F I O 2 ϭ 0 -13%) while ventilating the remaining lung with a normoxic or hyperoxic gas mixture decreased blood flow in the hypoxic region by 20 -100%. Since total flow was unaltered, these decreases represent diversion of flow from hypoxic to nonhypoxic lung due to HPV in hypoxic lung.…”

Effects of pulmonary gas embolism on circulation and respiration in the dog

“…During venous gas infusion, however, hyperoxia impairs CO2 elimination as seen from the increased values of Рзк,_ and (a-A)D(->Q as compared with normoxia (Table 2), which are indicative of an increased alveolar and therefore physiological dead space. It is generally accepted that oxygen plays a major role in the regulation of the ventilation-perfusion ratio of lung units (Duke and Lee, 1963;Fishman, 1961;Hales et al, 1975;Severinghaus et al, 1961;West, 1969). During pulmonary embolism there already is a shift of blood flow to lung units with low ventilation-perfusion ratios as described above.…”

Effects of pulmonary gas embolism on circulation and respiration in the dog

“…During venous gas infusion, however, hyperoxia impairs CO2 elimination as seen from the increased values of Рзк,_ and (a-A)D(->Q as compared with normoxia (Table 2), which are indicative of an increased alveolar and therefore physiological dead space. It is generally accepted that oxygen plays a major role in the regulation of the ventilation-perfusion ratio of lung units (Duke and Lee, 1963;Fishman, 1961;Hales et al, 1975;Severinghaus et al, 1961;West, 1969). During pulmonary embolism there already is a shift of blood flow to lung units with low ventilation-perfusion ratios as described above.…”