Weight based medications an ideal body weight

[Handsome Robb]

Alright maybe this is a dumb question and I'm usually not one to start threads, just add my input or cause problems in other threads


When we set ventilator settings they are based off of Ideal Body Weight (IBW) correct? Just because someone gets fatter doesn't make their lungs get any bigger. Super simple explanation of why we use IBW vs. actual weight for ventilator settings. Please correct me if I am wrong!

When it comes to weight based medication dosages I have always assumed that the actual body weight is used rather than IBW, then I got to thinking... uh-oh right? haha. When people get bigger generally it is fat stores correct. Barring lipid soluble medications wouldn't weight based meds theoretically be based off IBW? Does the rate we metabolize medications increase with body weight even if it is mostly all extra fat?

I don't know if this question makes any sense, it was a long day and I've had a couple beers. When I asked my partner the same question earlier, when we were on duty, he didn't want to give me an answer and have it be incorrect.

 

[Smash]

You are correct. With ventilator settings you should use ideal body weight. A 500lb person doesn't have the lungs of a 500lb person.

Generally with meds you should also use ideal body weight. Some things are going to act a little different depending on how lipophillic or otherwise the drug is, but broadly speaking IBW should be fine. Most protocols actually address this, but without putting it explicitly by having a maximum single bolus dose for medications.

IBW can be estimated by height (in cm) minus 100. Although like BMI this isn't exact. For example I am 190cm tall which would make my IBW 90kg. I'm actually 105kg but with 9% body fat that is find by me!

 

[Veneficus]

Generally I go by the actual estimated weight for medications.

I respectfully think that simplifying it to fat percentage does not realistically reflect volume of distribution.

There are also other issues like protein binding, vascular volume, muscle mass etc.

 

[Dwindlin]

Generally I go by the actual estimated weight for medications.

I respectfully think that simplifying it to fat percentage does not realistically reflect volume of distribution.

There are also other issues like protein binding, vascular volume, muscle mass etc.

This. Way more to drug distribution than % body fat.

 

[MSDeltaFlt]

The only thing that teaches ideal body weight that I am aware of is Vt. Simply put a 270# pt that is 6'3" will have different lung sizes than a 270# pt that 5'2".

I know of no drug dosages that say IBW. Very easy to over think situations. For even the obese pt, go with actual body weight and titrate. If you get the desired effect with less drug then stop there. Simple.

If that seems too much to worry with then treat every drug you IVP like phenergan: open the fluids up as you give the drug. Then back the fluids off when you're done. That's what I do and I'm yet to have an issue.