The Performance of Using the Parasympathetic Tone Activity (PTA) Index to Assess Intraoperative Nociception in Cats

Background: The monitoring of nociception/antinociception poses a significant challenge during anesthesia, making the incorporation of new tools like the Parasympathetic Tone Activity (PTA) monitor an added value in feline anesthesia. Objectives: To compare the effectiveness and speed of PTA monitoring when compared to heart rate (HR) in detecting surgical stimuli (SS) during the intraoperative period in 49 female cats undergoing ovariectomy (OV). Methods: Instantaneous Parasympathetic Tone Activity (PTAi) values, HR, fR, and non-invasive SAP and MAP were continuously monitored and systematically assessed. The time required for HR (HR time) and PTAi (PTAi time) to reach their minimum peak values following each surgical stimulus was collected at five points for each anaesthetized cat. Each collected surgical stimulus was categorized into 3 groups for statistical analysis: no nociception detection, no hemodynamic reaction and a PTAi > 50 (Nhre); no hemodynamic reaction and a PTAi < 50 (Nhre < 50); and hemodynamic reaction and PTAi < 50 (Hre < 50). Results: PTAi response demonstrated effectiveness in detecting nociception compared to HR. The SS were categorized as 36.1% in the Nhre group, 50% in the Nhre < 50 group, and only 13.9% in the Hre < 50 group. In the Hre < 50 group, PTAi time and HR time had similar speeds in detecting the SS (p = 0.821); however, PTA time was significantly slower in the Nhre < 50 group when compared to the Hre < 50 group (p = 0.001). Conclusions: PTA monitoring may be a useful tool to complement HR monitoring for detecting nociception. PTA monitoring demonstrated a superior diagnostic value compared to HR for detecting nociception in cats undergoing OV and a similar speed to HR in detecting SS when HR increases above 20%. Future studies are needed to understand in a clinical setting the meaning of sympathetic activation/nociception detected using the PTA monitor when the HR increase is not clinically relevant.