We will include all study types with primary data available (no review articles) published in a peer-reviewed journal. To minimise the risk of publication bias (small study bias),51 any studies with less than 10 participants will be excluded.

We will include studies involving adults aged 18 years and over who are undergoing non-cardiac surgery, regardless of the presence or absence of cardiovascular risk factors. Studies must include patients who have had HRV measured and who have undergone assessment for postsurgical pain (ie, using a validated measure of pain intensity or change in pain intensity (pain relief)) within the postoperative period (up to 30 days after surgery).

Studies of any design that include measures of pain intensity or pain relief within the first 30 days after non-cardiac surgery.

Pain intensity or pain relief quantified using a validated measurement instrument (eg, 0–10 numerical rating scale or 0–100 mm visual analogue scale for pain intensity; category scale for pain relief).

HRV measurements such as frequency bands, ratios of frequency bands, time indices of HRV and total power. Frequency bands include LF power (0.04–0.015 Hz), HF power (0.15–0.45 Hz) or ratios of LF/HF or HF/LF. Time domain indices of HRV include SD of NN intervals, SD of the averages of NN intervals, square root of the mean of the sum of the squares of differences between adjacent NN intervals and SD of differences between adjacent NN intervals.

Animal studies (no human data).

Review papers (no primary data).

Cardiac surgery.

Studies not written in the English language.

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