At 22:00 he was ordered vancomycin 3g as a one-time dose, which nursing ran over 5 hours as the patient had a peripheral line which was already looking red. The level right after the infusion ended was 20mg/L, however this does not represent steady state.
(Aside about infusion levels: Usually, if you want to run an infusion, a bolus of 30mg/kg is given, then the patient receives about 35mg/kg per day as a continuous infusion. Target steady state infusion levels are between 20-30mg/L for isolates with MICs of 0.5 or less. If you don’t bolus, you can check the infusion level after 24 hours, but if you bolus you can check it earlier. Therefore, in this patient, having an infusion level of 20 after 5 hours is actually quite high. If we continued to run at this rate, which equals 0.6L/h, his steady state infusion level would probably be about 75-100% higher than what was observed – high 30s or low 40s.)
Dosing in the super-obese is a big challenge. Studies show that the most important PK parameter in these patients is the clearance, which is much higher than normal. It’s estimated that the clearance of vanco is very tightly correlated with weight in a linear fashion, and for this patient the estimated half-life of vanco is probably about 3-4 hours. This is postulated to be secondary to a much larger kidney size and more nephrons. The interesting thing is that the although the total volume of distribution in these patients is increased, it’s not quadruple or quintuple that of normal, as vancomycin is quite hydrophilic. In this patient, his Vd is probably about 70L, as compared to 40-50L in a normal patient. One particularly useful reference (attached) determined that super obese patients require about 30mg/kg/d of vanco based on TBW, and usually divided into q8, if not q6 intervals, albeit only a few patients weighted upwards of 300kgs.
Based on what I read, I dosed the patient using his total body weight at about 30mg/kg/d and chose to go with 2g q6h for convenience (rounding, peripheral line etc). Based on PK parameters determined from case series (Vd of 70L, half-life of about 3-4h), this dose would yield a peak of 30 and a trough of 8, but seeing how the infusion of 0.6g/hr already resulted in a level of 20, I’m confident that the trough will come back over 10. In addition, this represents the 30mg/kg/d recommended by the article I attached. I ordered a trough prior to tomorrow morning’s dose. It would be great if someone could follow up. The target trough is in the lower teens. If it’s less than 10, please increase his vanco to 2.5g IV q6. If it’s over 20, please adjust accordingly, probably to 2g q8.
For more information see this excellent paper: Vancomycin dosing in the morbidly obese