IS ST SEGMENT DEPRESSION OF THE ELECTROCARDIOGRAM DURING CESAREAN-SECTION MERELY DUE TO CARDIAC SYMPATHETIC BLOCK

ST segment depression frequently is observed in women undergoing cesar ean section with regional anesthesia, yet myocardial ischemia as a cau se of such depression seems unlikely in this healthy population. To te st whether ST segment depression in this setting is due to decreased s ympathetic outflow to the heart, we monitored electrocardiograph (ECG) leads II and V-5 in 15 healthy women undergoing cesarean section with spinal anesthesia. We estimated cardiac sympathetic tone by spectral analysis of heart rate variability, measuring the area under the power frequency curve of the fast Fourier transform of heart beat intervals . The area under this curve in the low frequency range (0.02-0.15 Hz) has been described recently and validated in humans and animals as a s emiquantitative measure of cardiac sympathetic tone. Significant, >1 m m horizontal or downsloping depression of the ST segment occurred in f ive women (33%), beginning 5-30 min after spinal bupivacaine injection . Those with ST segment depression had increased heart rate (117 +/- 7 beats/ min) compared to those without (94 +/- 6 beats/min; P < 0.05), and had received more ephedrine (25 +/- 3 mg vs 7 +/- 3 mg; P < 0.05) . Before spinal bupivacaine injection and after block resolution the a rea in the low frequency range of the heart rate power spectrum was si milar in all women. During spinal anesthesia, those with ST segment de pression had less power in this low frequency range (0.0018 +/- 0.0010 ms(2)/Hz) than those without ST depression (0.0028 +/- 0.0006 ms(2)/H z). Despite similar levels of sensory block to pin testing (median T3) , these data suggest that the density of cardiac sympathetic block var ies considerably during high spinal anesthesia. We propose that ST seg ment depression during cesarean section with regional anesthesia is mo st often due to diminished cardiac sympathetic tone, likely with diffe ring densities of right and left cardiac sympathetic neural block. Ina smuch as myocardial ischemia causes an increase, not decrease in cardi ac sympathetic tone as determined by spectral analysis of heart rate, these data provide confirming evidence that ST segment depression duri ng cesarean section is not due to myocardial ischemia.