Effects of induced hypotension on arterial blood-gases under spontaneous breathing

The effects of deliberate hypotension on both PaO2 and PaCO2 were investigated under isoflurane anaesthesia with spontaneous breathing from a laryngeal mask. Lumbar epidural block was introduced; anaesthesia was induced with thiamylal (4 mg.kg-1) and maintained with 0.5% isoflurane in nitrous oxide (4 l.min-1) and oxygen (2 l.min-1) under spontaneous breathing. After that nitroglycerin, trimetaphan or prostaglandin E1 were used to induce a hypotension of 70% of control. All three drugs significantly decreased PaO2, from 19.9 +/- 3.3, 19.2 +/- 2.7, and 19.6 +/- 3.1 kPa to 14.6 +/- 1.9, 16.6 +/- 2.2, and 16.2 +/- 2.4 kPa (mean +/- s.d.), respectively; none of them increased PaCO2. In spite of the sparing of functional residual capacity under spontaneous breathing, the levels of reduction of PaO2 were the same as levels reported in paralyzed and mechanically ventilated subjects. In conclusion, under deliberate hypotension PaO2 decreases to a considerable degree, even under spontaneous breathing, presumably not because of alveolar hypoventilation, but because of the suppression of hypoxic pulmonary vasoconstriction by the drugs used in this study.