Hypoxic pulmonary vasoconstriction and the pharmacologically denervated lung

The roles of the autonomic nervous system and cardiac output on hypoxic pulmonary vasoconstriction were studied in 15 mongrel dogs anaesthetised with intravenous pentoharbitone (30 mg/kg) and the lungs mechanically ventilated to maintain normal arterial blood gases. After a hypoxic challenge in Group I (n =6) and Group II (n =3) animals, autonomic denervation was achieved by total spinal block with tetracaine (20 mg) injected into the cisterna magna. Group I animals received a large volume of intravenous fluid (80 ml/kg normal saline) before the block while Group II animals were given minimum fluid. When Group I animals were exposed to 10% inspired oxygen, mean pulmonary arterial pressure increased by 88 and by 72% before and after the block, respectively. The cardiac output increased by 27% with hypoxia before the block while it did not change significantly with hypoxia after the block. The pulmonary vascular resistance increased by 65 and by 152% with hypoxia before and after the block. Group II animals were also exposed to 10% inspired oxygen. They showed a similar response to Group I animals before the block. However, after the block irreversible hypotension developed with hypoxia.