Scientific Principles |
|Part A:||Inhaled Anesthetics|
|Chapter 5B:||Pulmonary Pharmacology|
Hypoxic Pulmonary Vasoconstriction
HPV (Ch. 15). is locally mediated; it occurs in isolated perfused lungs, and a similar response may be elicited in animals following autonomic nervous system blockade or catecholamine depletion. The sympathetic nervous system may, however, play a role in augmenting the response in certain circumstances, particularly in the presence of systemic hypoxemia. HPV in normal lungs appears when alveolar PO2 is less than 100 mm Hg and is maximal when PO2 is approximately 30 mm Hg. 74, 75 Recently, HPV has been investigated in humans with healthy lungs. 76 Unilateral graded hypoxia and unilateral single-step hypoxia produced similar reductions in the perfusion of the hypoxic lung to approximately 30 percent (at an inspired O2 concentration of 0.05). Mixed venous PO2 (PvO2 ) may influence HPV in the atelectatic lung because PO2 in the collapsed segment approaches that of PvO2 . 77, 78 Acidosis is also a pulmonary vasoconstrictor both in intact animals and in isolated perfused lungs. Changes in PVR in response to acidosis are small at normal alveolar oxygen tensions, but in the presence of alveolar hypoxia they are dramatically enhanced. Thus, elevations in arterial hydrogen ion concentration, alveolar PCO2, or both may augment pulmonary vasoconstriction.
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