Methemoglobin Formation and Oxygen Transport Following Intravenous Regional Anesthesia Using Prilocaine

Methemoglobin levels, changes in oxygen content of blood, and appearance of cyanosis were studied in 58 subjects following induction of regional anesthesia with 5 mg./kg. of prilocaine intravenously. The average maximum rise in methemoglobin was 1.02 Gm./100 ml. blood, 155 minutes after release of the tourniquet. Cyanosis was not detected. A significant shift in. the hemoglobin dissociation curve occurred at Po2 levels below 30 mm. Hg. When methemoglobin levels were above 0.8 Gm./100 ml. blood and the Po2 levels were below 30 mm. Hg, this shift resulted in a mean increase in oxygen content of 2.64 ± 3.40 ml. O2/100 ml. blood above the calculated normal. Assuming constant flow, less oxygen would be available to the tissues under these conditions. The use of prilocaine by this technique at this dose level in patients with circulatory compromise of heart or brain cannot be recommended until the physiologic importance of these changes is determined. In healthy patients the rise in methemoglobin levels and decrease in oxygen availability are probably within acceptable limits.