Though caudal block is a relative simple technique, it has not been widely used in adults because of a high failure rate. We assumed that any tests to quantify the changes of sympathetic tone in the affected areas would be excellent indicators of successful block. We tested the usefulness of two candidates (pulse oximetry plethysmographic waveform amplitude measured at 5^th toe and calf minus 5^th toe skin temperature gradient) as indicators of successful caudal block.

In 45 adult patients undergoing anal surgery with caudal block, these two variables were simultaneously measured at 2-min intervals for 20 min. A two-fold increase in the plethysmographic waveform amplitude from baseline and skin temperature gradient of 0℃ were predefined as test criteria of successful block.

While the sensitivity, specificity, positive predictive value, and negative predictive value of the skin temperature gradient test were 45.9%, 100%, 100%, and 9.1%, those of the plethysmographic waveform test were 86.5%, 100%, 100%, and 28.6%. The plethysmographic waveform test showed a significantly higher discriminative capacity than the skin temperature gradient test (94.9% vs. 48.7%, P < 0.05)

Unlike the skin temperature gradient test, the plethysmographic waveform test showed a considerably high validity in detecting successful block. Considering its simple and real time monitoring potentials together with a high failure rate of caudal block in adults, we cautiously recommend it as a supplemental diagnostic tool to predict successful block, especially when verbal communication with patient is difficult.