Analgesic response to morphine in obese and morbidly obese patients in the emergency department.

OBJECTIVE

The primary objective of this study was to compare the analgesic response to morphine in non-obese, obese and morbidly obese patients for acute pain.

METHODS

This was a retrospective cohort study conducted in a tertiary care emergency department in the USA. Consecutive adults who received intravenous morphine 4 mg for pain were included. Patients were categorised into three groups based on body mass index (BMI): non-obese (18.5-29.9 kg/m(2)); obese (30.0-39.9 kg/m(2)); and morbidly obese (≥40 kg/m(2)). Baseline and post-dose pain scores were recorded. Pain was measured on a 0-10 numerical rating scale (0=no pain; 10=worst possible pain). Analgesic response was defined as the difference between the initial pain score and post-dose pain score.

RESULTS

300 patients were included in the study (100 in each group). The median baseline pain scores were 8.5, 8 and 8.5 in the non-obese, obese and morbidly obese groups, respectively (p=0.464). The median analgesic response after morphine administration was 2, 3 and 2 in the non-obese, obese and morbidly obese groups, respectively (p=0.160). In the linear regression analysis (R(2)=0.006), BMI was not predictive of analgesic response (coefficient -0.020; p=0.199).

CONCLUSIONS

Obesity status did not influence analgesic response to a fixed dose of morphine. This suggests that obese and morbidly obese patients do not require a higher dose of morphine for acute pain reduction compared to non-obese patients.