Pharmacokinetic mass of fentanyl for postoperative analgesia in lean and obese patients
K. Shibutani1, M. A. Inchiosa, Jr1,2,*, K. Sawada3 and M. Bairamian1
1 Department of Anesthesiology and 2 Department of Pharmacology, New York Medical College, Valhalla, New York, USA. 3 Department of Anesthesia and Intensive Care, Nagoya University, Nagoya, Japan
* Corresponding author: Department of Pharmacology, New York Medical College, Valhalla, NY 10595, USA. E-mail: [email protected]
Background. We previously proposed
dosing weights for
fentanyl, termed pharmacokinetic mass, that span the total
body weight (TBW) range from 40 to 210 kg. In this study, we examined the relati
onships am
ong
fentanyl doses needed to achieve postoperative analgesia, corresp
onding plasma
fentanyl c
oncentrati
ons, and pharmacokinetic mass in lean and obese patients undergoing abdominal surgery.
Methods. A total of 69 patients were studied, with TBW ranging from 48 to181 kg.
Fentanyl infusi
on was used during surgery. After surgery,
fentanyl infusi
on rates were titrated to achieve analgesia without significant respiratory depressi
on. Plasma
fentanyl c
oncentrati
ons were measured when an apparent steady analgesic state was obtained. Comparis
ons were made for
dosing requirements and effective plasma c
oncentrati
ons for 37 lean patients (
body mass index <30, TBW <85 kg) and 33 obese patients (
body mass index >30, TBW
85 kg).
Results. The average
fentanyl dose (µg h1) required to achieve and maintain analgesia over the 4 h postoperative period had a n
on-linear relati
onship to TBW; in comparis
on,
fentanyl dose had a str
ong linear relati
onship to pharmacokinetic mass: dose (µg h1)=1.22[FONT=arial,helvetica]x.pharmacokinetic mass7.5;
r = 0.741,
P<0.001.
Based on results from our earlier study, the corresp
onding values of TBW and pharmacokinetic mass are: 52 kg 52 kg; 70 kg 65 kg; 100 kg 83 kg; 120 kg 93 kg; 140 kg 99 kg; 160 kg 104 kg; 180 kg 107 kg; 200 kg 109 kg. In the group comparis
ons, there was no statistically significant difference in the postoperative
fentanyl dose per unit of pharmacokinetic mass between lean and obese patients. The plasma c
oncentrati
on of
fentanyl required for analgesia was approximately 1.5 ng ml1, and was similar in the two groups.
Conclusion. The relati
onship between dose and pharmacokinetic mass, compared with that of dose
vs TBW, may provide c
onfidence for the use of pharmacokinetic mass as a
dosing approximati
on for
fentanyl.
Fentanyl dose
based on TBW may cause over
dosing in obese patients.
Presented in abstract form at the Annual Meeting of the American Society of Anesthesiologists, Las Vegas, October 26, 2004.