HYPOXAEMIA ASSOCIATED WITH ANAESTHESIA AND CONTROLLED RESPIRATION

Arterial oxygen saturation following premedication in children with cyanotic congenital heart disease

To determine the effects of premedication on arterial oxygen saturation (SaO2) and heart rate (HR), 11 children (ages three to seven years) scheduled for elective repair of cyanotic congenital heart defects were studied. Patients were premedicated with oral or rectal pentobarbitone 2 mg.kg-1 90 minutes prior to induction of anaesthesia followed by intramuscular morphine 0.2 mg.kg-1 and atropine 0.02 mg.kg-1 60 minutes prior to induction. The SaO2 and HR of each child were monitored continuously using a Nellcor pulse oximeter during two 90 minute periods: a control period commencing 25.5 hours preoperatively (day 1) and a post premedication period commencing 1.5 hours preoperatively (day 2). Data were compared at time 0 (corresponding to the time of administration of pentobarbitone on day 2), 30 (corresponding to the administration of intramuscular morphine and atropine on day 2), 60 and 90 minutes (the latter corresponding to the time of induction on day 2) after the administration of pentobarbitone. There were no significant differences in SaO2 or HR between day 1 and day 2 at time 0, 60, and 90 minutes. The SaO2 (mean +/- SD) decreased significantly immediately following intramuscular premedication at time 30 minutes on day 2 (72.7 +/- 5.9 per cent) compared to the corresponding time on day 1 (83.9 +/- 2.9 per cent) (p less than 0.05). The duration of this desaturation was 2.5 +/- 1.9 minutes. Heart rate (mean +/- SD) increased from 109.2 +/- 21.3 beats.min-1 at time 30 minutes on day 1 to 142 +/- 20.4 beats.min-1 on day 2 (p less than 0.05). We conclude that administration of intramuscular premedication preceded by oral or rectal pentobarbitone causes transient arterial desaturation and tachycardia in children with cyanotic congenital heart disease.

The mechanism of hypoxaemia after laparotomy

In 18 patients who were undergoing upper laparotomies the alveolo-arterial oxygen difference and right-to-left shunt were measured before the operation, on the first post-operative day, and in some cases also later in the post-operative period. The arterial oxygen tension was found to be considerably reduced post-operatively. The right-to-left shunt estimated by the hydrogen isotope technique was moderately increased in most instances, but the increase was far from large enough to account for the observed hypoxaemia. It is concluded that uneven distribution of ventilation relative to perfusion is the main cause of post-operative hypoxaemia, whereas veno-arterial shunt through atelectatic areas is of minor importance.