dosing meds for obese pt

air · Sep 30, 2004

Usually when "...Calculations are based on lean body mass rather than total body weight, dosage adjustments for age, sex or obesity are not necessary" Barash, Fourth edition, (P. 332)

hope that helps some.

ok, here is the problem...instructor gives info as...dose all meds on IBW except benzo's and barb's.. lipophilic drugs exhibit increased volume of distribution, whereas hydrophilic drugs exhibit decreased volume of distribution...obesity increases GFR and elimination of renally eliminated drugs... phase II (conjugation) increased...so increased metabolism of these drugs...increased plasma cholinesterases - so increase sux/remi doses are possibly needed...
now...there is a disagreement to whether propofol is based on IBW or actual body weight...in my lit review - i must say i could find many recent (within the past 5 yrs) of how meds should be dosed...and in five different anesthesia texts i have found that all drugs should be based on IBW and more may be titrated in if needed...
so - is there any(other) way to determine if drugs should be based on IBW or actual body weight???

deepz · Oct 1, 2004

Not to be flippant about it, but, how much do we give? -- we give enough. Empirical titration is the basic process of anesthesia care for all patients.

Obese individuals can be very fast to awaken from inhalational anesthesia, as the less soluble modern lipophilic agents rapidly disperse into the adipose and blood levels plunge quicker than with a lean person. Narcotics OTOH are known to have a potentially troublesome re-distribution phase later on for obese folks.

deepz

"The usual dose of any drug we'll use in anesthesia is ... one amp."

--- long-ago anesthesia guru

charles-thor · Oct 1, 2004

Since adipose tissue has a low blood flow, it is possible that calculation based on true body wt might actually cause higher than expected plasma concentrations in obese patients. Miller and Stoelting go on to further suggest that when dosing drugs, you should base your initial calculations on ideal body wt and probably not go higher than 100kg for men and 80kg for women.

Basics of Anesthesia - p.317-8

I'd have to agree with Deepz, however, in that individual patients will very and the ultimate decision should be based on patient needs.

athomas91 · Oct 1, 2004

i agree completely and from all my readings I found exactly the information you all posted ...lean body mass and titrate more if needed based on patient response to initial dose...

i guess my question is why and where the info that certain drugs should/shouldn't be based on ideal/actual body weight...

gaspassah · Oct 1, 2004

half the big stick all little stick. :rotfl:

Passin' Gas · Oct 1, 2004

Obese individuals can be very fast to awaken from inhalational anesthesia, as the less soluble modern lipophilic agents rapidly disperse into the adipose and blood levels plunge quicker than with a lean person.

Help. I got lost with this statement. For starters, I'm assuming you are referring to sevo and des as the more modern agents. These have LOW blood:gas and brain:blood partition coefficients. Hence, they are LESS soluble in fat, so when the gas is turned off they are rapidly excreted via the lungs.

On the other hand, halothane has a high blood:gas partition coefficient, a high brain:blood partition coefficient, so when this gas is turned off it is continually taken up by the fat PLUS undergoes liver metabolism, therefore leading to faster than expected awakening with such a fat soluble agent. Sevo and des do not undergo significant metabolism, the rapid awakening is accredited to their low solubilities.

Numbers from Barash: (I couldn't post the table as I desired, the numbers are in order of the agents listed first)

sevoflurane

desflurane

isoflurane

halothane

oil:gas partition coefficient

47

19

91

224

blood:gas part coefficient

0.65

0.42

1.46

2.5

brain:blood part coefficient

1.7

1.3

1.6

1.9

fat:blood part coefficient

47.5

27.2

44.9

51.1

Passin' Gas · Oct 1, 2004

half the big stick all little stick. :rotfl:

Aaahhh, grasshopper, (rather, gaspassah), you learn well...Next, we catch flies with chopsticks....

PG

Passin' Gas · Oct 1, 2004

Not to be flippant about it, but, how much do we give? -- we give enough. Empirical titration is the basic process of anesthesia care for all patients.

'Tis the ART of anesthesia...Give them what they need, not necessarily what the books say. This skill takes time to hone and read patient's responses to different anesthetic agents and techniques. Good point.

PG

TraumaNurse · Oct 1, 2004

TTE: Titrate to Effect. Calculate based on IBW and go from there!

athomas91 · Oct 1, 2004

of course clinically i dose on IBW and titrate to effect...but when you are looking for TESTing purposes...

from what i read in miller, barash, stoelting and artusio...sevo and des show no difference in obese individuals and should be the choices for inhalational anesthetic.

kmchugh · Oct 5, 2004

half the big stick all little stick. :rotfl:

Ya beat me to it!

In truth, most dosing guidelines are just that: Guidelines. We dose drugs to effect, not to books. We have all cared for the big strapping guy, who weighs 250 lbs, all muscle, who needed only look at the syringe of propofol to fall asleep, while the next patient, a frail 82 year old lady who weighs 93 lbs could take 100 mg and keep up with the conversation.

Kevin McHugh