Methemoglobin Formation and Oxygen Transport Following Intravenous Regional Anesthesia Using Prilocaine

Abstract

Methemoglobin levels, changes in O2 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 g/100 ml blood, 155 min. 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 g/100 ml blood and the P02 levels were below 30 mm. Hg, this shift resulted in a mean increase in O2 content of 2.64[plus or minus] 3.40 ml O2/IOO ml blood above the calculated normal. Assuming constant flow, less O2 would be available to the tissues under these condition. 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 anddecrease in O2 availability are probably within acceptable limits.