INTENSE SLOW HYPOXIC PULMONARY VASOCONSTRICTION IN GAS-FILLED AND LIQUID-FILLED LUNGS - AN IN-VIVO STUDY IN THE RABBIT

To examine the hypothesis that hypoxic pulmonary vasoconstriction may have a slower time course and greater intensity than is currently reco gnized, experiments were conducted in twelve anaesthetized rabbits sub jected to unilateral lung hypoxia for 6 h. Endobronchial cannulation w as used to maintain apnoea of one lung at constant airway pressure whi lst inflating the lung with nitrogen or liquid. The second lung was ve ntilated with oxygen to maintain normocapnia and oxygen transfer. A pu lmonary artery catheter was introduced non-invasively. Pulmonary shunt was derived from mixed venous and arterial blood gas parameters. Pulm onary artery pressure was monitored continuously and cardiac output wa s estimated from oxygen uptake measurements before and after 6 h unila teral hypoxia. The experiments show that a rapid phase of hypoxic pulm onary vasoconstriction is followed by a slow phase which develops over hours. The slow phase is associated with a massive blood flow diversi on from the hypoxic lung, such that pulmonary shunt after 6 h unilater al hypoxia is indistinguishable from baseline shunt during bilateral v entilation with oxygen. The response is reversible, but with a similar ly slow time course. Results from nitrogen and liquid filling of the l ung are similar. These findings are consistent with early experiments by Dirken and Heemstra in 1948 (Quart J Exp Physiol 34, 193-211), and challenge the prevailing notion that hypoxic pulmonary vasoconstrictio n is always a rapid and relatively weak physiological response to hypo xia.