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Canadian Journal of Anesthesia, Vol 28, 248-252, Copyright © 1981 by Canadian Anesthesiologists' Society
1 Laboratoire d'anesthésiologie. Hôpital Rothschild - 33, bd de Picpus 75012 Paris - Département d'anesthésiologie Hôpital St Antoine (Professeur Claude Guilmet)
A few effects of carbon dioxide on pain threshold and acid-base balance are known. The purpose of this study was to investigate specifically the variations of analgesia in relation to hypercapnia during general anaesthesia and the respective roles played by carbon dioxide and [H+].
The nociceptive jaw opening reflex was studied on five beagle dogs anaesthetized with alfathesin administered at constant rate under acute hypercapnic conditions and acute metabolic acidosis. Acute hypercapnia did not decrease the jaw opening reflex significantly until a level was reached where Pacoco2 values modified blood [H+] (pH) significantly (10 ± 1.04 kPa corresponding to [H+] 91.5 ± 13.24nmol/l (pH 7.04 ± 0.06) p < 0.05)). At [H+] 176.2 ± 42.77 nmol/1 (pH 6.7 ± 0.13) (p < 0.01) the reflex was only 9.3 ± 3.9 per cent (p < 0.001) of its initial value. The infusion of decinormal solution of HCl during constant capnia caused an abrupt drop of the reflex. There was a correlation between reflex and metabolic acidosis (p < 0.05). The authors conclude that modification of the jaw opening reflex occurs with extreme values of arterial [H+] incompatible with safe anaesthesia and they discuss the mechanisms involved.
Key Words: Anesthésie générale • Hypercapnie • Acidose • Réflexes • Douleurs
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