Hypoxic pulmonary vasoconstriction in dogs: effects of lung segment size and oxygen tension

Abstract

Six pentobarbital-anesthetized dogs were prepared with endobronchial tubes and electromagnetic flow probes. The effects of changing inspired oxygen concentrations (FIO2 = 1, 0.21, 0.15, 0.1, 0.075, 0.05, and 0) were tested on test segments of different size corresponding to left lower lobe, left upper lobe-lingula, left lung, right lung, right lung plus left lower lobe, right lung plus left upper lobe-lingula, and whole lung. In each test the rest of the lung received oxygen. Hypoxic pulmonary vasoconstriction is demonstrated by both diversion of blood flow away from hypoxic test segments and by increased perfusion pressure. Flow diversion (FD%) decreases with the size of the hypoxic test segment (%QSN) from a maximum of 75% for very small segments to zero when the whole lung is hypoxic. FD% increases linearly as alveolar oxygen tension (PAO2) of the test segment is decreased in the range of 130--28 Torr. When mixed venous oxygen tension (PVO2) is less than 45 Torr FD% is reduced. These relationships are described by FD% = [74.99 - 0.0778 (%QSN) - 0.00661 (%QSN)2] [1.268 - 0.0096 (PAO2)] [0.47 + 0.012 (PVO2)], with r = 0.92 and standard error for prediction of 8.4%. Pulmonary perfusion pressure changes (PPH/PPN) increase with the size of the hypoxic test segments from 0 with very small segments to approximately 2.2 for the hypoxic whole lung. For all test segments PPH/PPN increases linearly with PAO2. These relationships are described by PPH/PPN = 1 + [0.0043 (%QSN) + 0.000072 (%QSN)2] [1.234 - 0.0096 (PAO2)], with r = 0.91 and standard error for prediction of 0.3 units. Responses to hypoxic pulmonary vasoconstriction in dogs are therefore shown to be predictable and continuous, and the physiological basis for action of each of the variables is discussed.

The relationship between hypoxic pulmonary vasoconstriction and arterial oxygen tension in the intact dog.

“…However they found comparable decreases in the percentage of the control blood flow passing to the hypoxic region of lung (in their studies reducing PaO2 from 500 to 32 Torr resulted in a mean decrease in blood flow of 450 compared with 50 % in the present study during normocapnia). This implies that: (i) the percentage decrease in blood flow to a hypoxic region of lung is independent of the size of that region; this is contrary to the results of Marshall, Marshall, Benumof & Saidman (1981) who showed an inverse correlation between the magnitude of h.p.v. and the size of the lung region made hypoxic; it is still possible that trauma to the lung in the study of Barer et al (1970) tended to diminish the magnitude of h.p.v., while the small size of the region made hypoxic tended to increase its magnitude.…”

Gas exchange and ventilation–perfusion relationships in the lung

“…8). The efficacy of HPV in correcting V9A/Q9 mismatch decreases with the extent of the hypoxic lung region [53,57]. One example is low PAO 2 within the entire lung (e.g.…”

Regulation of hypoxic pulmonary vasoconstriction: basic mechanisms

“…This principle, also known as the von Euler-Liljestrand mechanism, thereby optimises gas exchange at the blood-air interface [1,2]. Impairment of this mechanism during pathological situations in lung or systemic disease (for example, adult respiratory distress syndrome [3] or hepatopulmonary syndrome [4]) or during anaesthesia [5], may result in insufficient oxygenation of arterial blood and poor oxygen supply to the body.…”