¹Departments of Anesthesia and Internal Medicine, Javeriana and University, Bogota; Center for Clinical Epidemiology and Biostatistics, Department of Biostatistics and Epidemiology, and Department of Anesthesia, University of Pennsylvania, Philadelphia; Departments of Anesthesia and Medicine, Tufts University School of Medicine and New England Medical Center, Boston.

Correspondence: Brian L. Strom, MD, MPH, Center for Clinical Epidemiology and Biostatistics, University of Pennsylvania School of Medicine, 824 Blockley Hall, 423 Guardian Dr, Philadelphia, PA 19104-6021. E-mail: bstrom@cceb.med.upenn.edu

^*Supported by United States Agency for International Development grant No. 5-35682, Colciencias, and National Institutes of Health KO8 grant No. NS01865.

Received 28 December 2000; Accepted 9 July 2001.

Abstract

Objective: Our objective was to evaluate ethnic differences in response to morphine and to determine whether any detectable differences were pharmacokinetically based.

Methods: This cohort study was carried out in a teaching hospital. Sixty-six young, healthy male subjects from 3 ethnic groups (Caucasians, native Indians, and Latinos; n = 22 in each group) consented to participate. All subjects received an intravenous morphine bolus of 0.08 mg/kg followed by 0.002 mg/kg min infused for 30 minutes. Respiratory response was evaluated with the carbon dioxide rebreathing method before and at 25, 95, 180, and 360 minutes after morphine administration. Vital signs and opioid side effects were recorded, and serial blood samples were analyzed for morphine, morphine-3-glucuronide, and morphine-6-glucuronide (M6G).

Results: All 3 groups had suppression of the ventilatory response to hypercapnia, but the degree of blunting of the ventilatory response differed among groups. Compared with Caucasians, native Indians had an additional 18% reduction in ventilatory response after morphine administration (95% confidence interval, -35% to -2%). The incidence of side effects was similar in all groups (P = .18). Caucasians had higher plasma levels of M6G than did native Indians or Latinos. M6G areas under 6-hour concentration-versus-time curve were as follows: Caucasians, 12,065 4354; native Indians, 8464 4809; and Latinos, 9156 3764 ng min/mL (P = .03).

Conclusions: Ethnicity influences the response to morphine. Native Indians are more susceptible to morphine depression of the ventilatory response than Caucasians, despite the higher serum M6G levels in Caucasians.