Weight-based dosing of opioids is a commonly used approach for managing patients who present to the ED with more severe pain. “Many patients who present to the ED with pain are obese or morbidly obese,” says Asad E. Patanwala, PharmD. “Heavier patients often receive larger total doses of opioids when compared with normal weight individuals. This can potentially increase the risk of serious adverse events.” He adds that morphine is one of the most commonly used opioids in EDs, but data on morphine dosing are limited among obese individuals. Studies are needed to evaluate the analgesic response to morphine, especially in patients with very high BMIs.
Comparing Analgesic Responses
In a study published in the Emergency Medicine Journal, Dr. Patanwala and colleagues retrospectively reviewed 300 consecutive patients who received intravenous morphine (4 mg) for pain. Patients were categorized into three groups based on their BMI: non-obese, obese, and morbidly obese. The authors then compared analgesic responses to morphine in the three groups. “Our primary goal was to see if patient weight really matters with regard to analgesic response to morphine,” Dr. Patanwala says.
Using a scale of 0 being no pain and 10 being worst possible pain, the median baseline pain scores were 8.5, 8.0, and 8.5 in the non-obese, obese, and morbidly obese groups, respectively. The median analgesic response after morphine administration was 2.0, 3.0, and 2.0 in the non-obese, obese, and morbidly obese groups, respectively. In a linear regression analysis, BMI was not predictive of analgesic response. The analgesic response to a fixed dose of morphine did not appear to change as a function of BMI, says Dr. Patanwala. Morbidly obese patients had similar responses to non-obese patients.
Consider Fixed Doses
Dr. Patanwala and colleagues noted that findings from the study are consistent with other research showing that obesity does not appear to influence analgesic responses, even at the upper extreme of BMI. “It may be preferable to use fixed doses of morphine in a rapid titration strategy when managing obese patients,” Dr. Patanwala says. With this approach, clinicians can reduce the need to predict responses to single doses of morphine.
There are several barriers to rapidly titrating morphine in the ED, such as nursing time required to accomplish the task. This can lead to delays in drug re-dosing or pain re-assessments. “It’s important to find ways to ensure that patients are re-dosed with morphine in a timely manner when needed,” says Dr. Patanwala. “Our results indicate that using fixed morphine doses—regardless of patient weight—appears to be an appropriate strategy, even in the heaviest of patients. The key is to titrate doses appropriately based on how patients respond to morphine rather than base treatment approaches on weight.”
Readings & Resources (click to view)
Patanwala AE, Holmes KL, Erstad BL. Analgesic response to morphine in obese and morbidly obese patients in the emergency department. Emerg Med J. 2014;31:139-142. Available at: http://www.medscape.com/viewarticle/819264 or http://emj.bmj.com/content/31/2/139.abstract.
Patanwala AE, Keim SM, Erstad BL. Intravenous opioids for severe acute pain in the emergency department. Ann Pharmacother. 2010;44:1800-1809.
Patanwala AE, Biggs AD, Erstad BL. Patient weight as a predictor of pain response to morphine in the emergency department. J Pharm Pract. 2011;24:109-113.
Patanwala AE, Edwards CJ, Stolz L, et al. Should morphine dosing be weight based for analgesia in the emergency department? J Opioid Manag. 2012;8:51-55.
Lloret LC, Decleves X, Oppert JM, et al. Pharmacology of morphine in obese patients: clinical implications. Clin Pharmacokinet. 2009;48:635-651.
Todd KH, Ducharme J, Choiniere M, et al. Pain in the emergency department: results of the pain and emergency medicine initiative (PEMI) multicenter study. J Pain. 2007;8:460-466.