To examine the effects of subarachnoid tetracaine and epinephrine on spinal cord blood flow (SCBF), lumbar SCBF and cerebral blood flow (CBF) were measured simultaneously by the hydrogen clearance technique in dogs (n = 45) anesthetized with halothane. The lumbar subarachnoid administration of tetracaine, 5 mg dissolved in 1 ml of a 7.5% dextrose solution had no significant effect on either SCBF or CBF for 4 hr even though arterial blood pressure and heart rate decreased significantly. After subarachnoid epinephrine alone (100, 300, and 500 μg), SCBF varied widely but did not change significantly with any of the injections, nor did CBF.
Responses of SCBF to hypercapnia and to acute blood loss during spinal anesthesia with tetracaine were also examined. Increased Paco2 (from 35 to 57 mm Hg) increased both SCBF and CBF similarly before and after subarachnoid tetracaine; SCBF increased from 26.8 ± 9.0 ml≬100 g−1≬min−1 (mean ± SD) before to 34.2 ± 13.6 ml≬100 g−1≬min−1 during hypercapnia during spinal anesthesia, which was almost identical to the increase (from 31.5 ± 8.1 ml≬100 g-1≬min−1 to 39.9 ± 6.0 ml≬100 g−1≬min−1) before spinal anesthesia. Whereas acute blood loss (approximately 20% of estimated blood volume) during spinal anesthesia with tetracaine caused a 23% reduction of SCBF (P<0.05), in the absence of tetracaine SCBF remained unchanged during hemorrhagic hypovolemia. These results indicate that sub-arachnoid epinephrine does not produce a significant reduction in SCBF, though its inclusion with a local anesthetic solution for prolongation of spinal anesthesia has been thought to produce local vasoconstriction. Blood flow to the spinal cord is unlikely to be affected by subarachnoid tetracaine per se under normal conditions. The present study suggests, however, that spinal anesthesia may alter the autoregulatory capacity of SCBF during hypotension due to acute blood loss, but may leave the local blood flow reactivity to respiratory acidosis intact.
Accepted for publication December 11, 1986.
© 1987 International Anesthesia Research Society