The relationship between the numeric rating scale, as a subjective measure of postoperative pain, and the plasma levels of catecholamines

It is commonly believed that acute postoperative pain can lead to an impaired outcome by means of stress hormone provoked physiological changes (e.g. hyperglycaemia, altered immune response, increased myocardial oxygen consumption). However, it has not yet been investigated whether or not pain that is often measured by means of subjective pain scales (e.g. numeric rating scale, NRS) actually correlates with changes in the plasma level of stress hormones. Aim of this study is to investigate the relationship between the subjective NRS score (0-10; 0= no pain, 10 = worst pain) and the plasma levels of adrenaline and noradrenaline. Furthermore, the correlation between these parameters and other non-invasive measures of pain (heart rate [HR], blood pressure [BP], respiration rate [RR], heart rate variability [HRV]) will be investigated. For this purpose, 85 patients scheduled for non-emergency plastic surgery will be included in the trial. On arrival in the recovery room, 4 ml blood will be taken from a cannula sited during the anaesthetic into the cubital fossa to measure plasma levels of catecholamines. Simultaneously, HR, BP and RR will be recorded as part of the recovery room protocol and the patient will be asked to rate their pain on the NRS. In addition, data from the connected electrocardiogram will be downloaded onto a laptop PC to calculate parameters of HRV by means of the MEMCALC method: total power, low frequency, high frequency, the low: high frequency ratio and a non-linear approach, ultra-short entropy. These HRV parameters are purely mathematical calculations (hence “no touch” technique) and have previously shown correlations with the sympatho-vagal balance. At the time of discharge from the recovery unit, above mentioned procedure will be repeated. In between these 2 time points, the procedure may be repeated up to 3 times if a patient “moves” from one pain category into another (4 categories defined: no [NRS 0], mild [NRS 1-3], moderate [NRS 4-5] and severe [NRS 6-10] pain). To identify changes in pain perception, patients are frequently (approx. 5 minutely) asked to rate their pain on the NRS. This is a standard procedure in the Royal Perth Hospital recovery room. Hence, a minimum of 2 and a maximum of 5 blood samples (=20ml) may be taken of a subject. The study period ends with discharge of a patient from the recovery room.