Efficacy of the Parasympathetic Tone Activity monitor to assess nociception in healthy dogs anaesthetized with propofol and sevoflurane.

Objective To compare a Parasympathetic Tone Activity (PTA) monitor with cardiovascular changes in invasive mean arterial pressure (IMAP) and heart rate (HR) when evaluating the response to nociceptive stimuli in anaesthetized dogs. Study design Prospective experimental study. Animals A group of nine (seven male and two female) adult Beagle dogs weighing 13.4 +/- 1.5 kg (mean +/- standard deviation). Methods Anaesthesia was induced with propofol and maintained with sevoflurane in oxygen. Electrical stimuli of different nociceptive intensities were applied for 30 seconds. Stimuli were classified in each patient according to the response obtained (relevant change >= 20%) as low (no response), medium (PTA only) or high (PTA and IMAP/HR). Immediate and averaged values of PTA, IMAP and HR were recorded every second from 60 seconds before to 120 seconds after application of the nociceptive stimulus. Time to nociceptive response and peak response were evaluated with analysis of variance and t test. Results Immediate PTA baseline values did not differ significantly before application of the low, medium and high stimuli (73 +/- 15, p = 0.966). Immediate PTA response was observed with the medium stimulus at 33 +/- 7 seconds with a maximum decrease of 57 +/- 13% at 69 +/- 5 seconds. With the high stimulus, the immediate PTA response was of a similar magnitude to the medium stimulus with a response at 28 +/- 7 seconds (p = 0.221) and a maximum decrease of 68 +/- 15% (p = 0.115) at 72 +/- 7 seconds (p = 0.436). The cardiovascular change occurred (22 +/- 8 seconds) prior to the immediate PTA response (p = 0.032). Conclusions and clinical relevance The PTA monitor detected nociceptive stimuli at lower intensities than those eliciting cardiovascular changes. However, nociceptive stimuli of higher intensities provoked cardiovascular changes that occurred before a PTA response was observed.