Pupillary. Reflex for Evaluation of Thoracic Paravertebral Block: A Prospective Observational Feasibility Study

BACKGROUND: Although thoracic paravertebral block (TPVB) is recommended in major breast surgery, there is no gold standard to assess the success of TPVB. Pupillary dilation reflex (PDR) is the variation of the pupillary diameter after a noxious stimulus. The objective was to evaluate the feasibility of recording the PDR to assess analgesia in an anesthetized thoracic dermatome after TPVB. METHODS: This prospective, observational, single-center study included 32 patients requiring breast surgery under general anesthesia and TPVB. TPVB was performed before surgery under ultrasound guidance with 20 mL of 0.75% ropivacaine. At the end of the surgery, remifentanil was stopped and the PDR was recorded after a 5-second tetanic stimulation (60 mA, 100 Hz) applied to the anterior chest wall. The PDR was defined as the maximal increase in pupil diameter after a standardized noxious stimulus, expressed as a percentage of the initial pupil diameter. The PDR was recorded twice in the same eye for each patient after a stimulus on both the TPVB and the control sides. Postoperative pain scores were recorded in a postanesthesia care unit. The primary outcome was the difference between the PDR on the TPVB and the control sides. RESULTS: The median (interquartile range) PDR was 9% (4%-13%) on the TPVB side and 41% (27%-66%) on the control side. There was a significant difference in the PDR between the TPVB and the control sides with a Hodges-Lehmann estimate of absolute difference of 37% points (95% confidence interval, 25-52, P < .001). Median postoperative pain scores (interquartile range) in the postanesthesia care unit were 1 (0-3) at rest and 1 (0-3) during mobilization, respectively. There was a linear correlation between maximal postoperative pain scores and the PDR on the TPVB side. with a Pearson's correlation coefficient r = 0.40 (95% confidence interval, 0.06-0.66, P = .02). No correlation was found between the number of blocked dermatomes and maximal postoperative pain scores (P = .06) or between the number of blocked dermatomes and the PDR on the TPVB side (P = .15). CONCLUSIONS: This proof-of-concept trial suggests that the effect of TPVB could be monitored by measuring the PDR after anterior chest wall stimulation in the dermatome of interest.