Actual vs ideal body weight in RSI: compare the weight behind each adult drug dose

Actual vs ideal body weight for RSI drug doses is a question of which number sits behind the mg/kg. Actual body weight is the number on the scale. How much it differs from calculated ideal weight depends on the individual adult. In obesity the two numbers pull apart, and the published sources split agent by agent. Actual weight is well supported for succinylcholine and stated on the rocuronium label; Erstad and Barletta’s reviews favor ideal or adjusted weight for ketamine, propofol, and the non-depolarizing paralytics. Adult patients only: your protocol decides which weight you’re authorized to use.

Total body weight and actual body weight name the same measured weight (Radkowski et al., 2024). “None located” means no source was found in the research behind this page, not that none exists. Your protocol governs which basis applies.

Compare the three weights: actual, ideal and adjusted

Three different numbers can sit behind one mg/kg dose. Actual body weight, sometimes called total body weight, is the measured or scale weight. Ideal body weight is calculated from height and sex with the Devine formula that turns height into ideal body weight. Adjusted body weight sits between the two: ideal body weight plus 40% of the amount actual weight exceeds it (Erstad and Barletta, 2020). Erstad and Barletta’s 2021 review also names a fourth descriptor, lean body weight. The first example below has similar actual, ideal and adjusted weights; the second shows how far apart they can be in obesity.

What is adjusted body weight?

Adjusted body weight is ideal body weight plus 40% of however far actual weight sits above it (Erstad and Barletta, 2020). It’s a middle figure some references reach for in obesity. No drug on this page is said to require it.

Two adults, three weights: where the gap opens

Why RSI dosing weight differs by drug: lean tissue, fat and clearance

The weight basis question comes down to where a drug ends up in the body. Rocuronium’s low lipophilicity and small volume of distribution let ideal body weight reach therapeutic levels without accumulation expected in fat, as one ED study’s authors explain (McDowell et al., 2024). Succinylcholine runs the other way: a randomized trial of 45 adults with a BMI over 40 scheduled for gastric bypass surgery found poor intubating conditions in a third of patients in its ideal-weight group and in none in its total-weight group (Lemmens and Brodsky, 2006); a 2024 review ties this to higher pseudocholinesterase levels in morbid obesity (Radkowski et al., 2024). Propofol is highly lipophilic, yet a review of critically ill obese patients’ sedation dosing prefers ideal or adjusted weight because clearance doesn’t scale linearly with weight, an extrapolation from ICU sedation dosing rather than an RSI trial (Erstad and Barletta, 2020).

Paralytics and weight basis: succinylcholine, rocuronium, vecuronium

The three paralytics among the six RSI medications and their adult starting doses don’t share one answer. Succinylcholine is where the sources agree: a 2021 review and the Lemmens and Brodsky trial point to actual weight, and Bhat and colleagues’ emergency-department chart review measured dosing against it (Erstad and Barletta, 2021; Lemmens and Brodsky, 2006; Bhat et al., 2016). Vecuronium sits with the non-depolarizing agents on ideal or adjusted weight in that same 2021 review, and no source located here backs dosing it on actual weight. Rocuronium is the real split, covered below.

Is rocuronium dosed on actual or ideal body weight?

Its FDA label says actual body weight in obese patients; a 2021 review says ideal or adjusted; an ED study comparing them didn’t settle it. The label states obese patients should be dosed on actual body weight, and that obese patients in its studies dosed by ideal weight reached maximum block more slowly and without comparable intubating conditions. Erstad and Barletta (2021) put rocuronium with the non-depolarizing agents on ideal or adjusted weight instead. McDowell and colleagues (2024) studied 96 adults with obesity in a single-center, prospective observational ED study. Physicians selected the weight basis, and both groups had a 100 mg dose cap. Excellent intubating conditions occurred in 68.5% on total weight versus 73.8% on ideal weight; the study did not establish non-inferiority. Median time to observed muscle recovery was 71 minutes versus 43. These results do not isolate weight basis from dose selection or establish equivalent performance. The rocuronium RSI dose and its label get their own page.

Induction agents in obesity: ketamine, etomidate, propofol

Etomidate leans toward actual weight; ketamine and propofol lean toward ideal or adjusted weight in one review, and none of the three labels names a descriptor. For etomidate, a review of critically ill obese patients’ dosing suggests actual weight below a BMI of 40 and adjusted or actual weight at or above it, because underdosing a heavier patient risks awareness during paralysis (Erstad and Barletta, 2020). Propofol’s clearance argument is above. Ketamine gets its own question below.

Should ketamine be dosed on ideal body weight?

One review says yes in obesity, and the FDA label names no descriptor either way. A review of critically ill obese patients’ sedation dosing suggests ideal or adjusted weight, particularly at a BMI of 40 or above, reasoning that ketamine’s clearance tracks lean body mass rather than fat (Erstad and Barletta, 2020). The same review calls actual weight appealing for loading doses; its suggestion is extrapolated from ICU sedation dosing, not an RSI trial. The ketamine RSI dose on actual body weight has its own page.

Both practices are in use: what Rapid Vent displays, and what decides it

Rapid Vent Calculator’s medications card computes all six doses from the actual weight you enter and says so underneath them: “Displayed doses are calculated from entered actual body weight.” Right below that: “Weight scalar varies by medication, body habitus, and local protocol. In obesity, some references use IBW or adjusted body weight for selected agents, especially nondepolarizing paralytics.” The card shows one basis, labeled; it never shows a second, ideal-weight set of doses beside it.

Both practices are genuinely in use. The rocuronium label backs actual weight, and Bhat and colleagues’ emergency-department chart review measured dosing accuracy against total weight. In McDowell and colleagues’ ED study, physicians chose either weight for rocuronium, while the reviews above favor ideal or adjusted weight for several agents in obesity. A 2023 review of RSI pharmacotherapy found the evidence still isn’t enough to change current dosing practice in obese patients (Engstrom et al., 2023). The arithmetic is only as good as the weight it’s given, whether charted, bed-scale or estimated (McDowell et al., 2024). The calculator can’t see the patient or the protocol, so that call belongs to the crew.

Which weight do RSI doses start from?

There’s no single answer: in obesity the sources split by agent, as the table above lays out. Succinylcholine and rocuronium’s label point to actual weight; several non-depolarizing agents and ketamine point to ideal or adjusted weight in the reviews. Your protocol decides which one you’re authorized to use.

After the drugs: the ventilator runs on ideal body weight instead

The vent side runs the opposite way. Tidal volume uses ideal body weight because lung volume tracks the size of the chest, not body mass: “IBW should be used instead of actual weight because lung volume does not change with body mass” (StatPearls NBK537072). Why tidal volume runs on ideal body weight instead is its own comparison. The app’s results screen labels both rows, “Actual Weight” and “Ideal Body Wt”: one for the drugs already given, one for the initial vent settings that switch to ideal body weight next.

Sources

1. Erstad BL, Barletta JF. Drug dosing in the critically ill obese patient: a focus on sedation, analgesia, and delirium. Critical Care. 2020;24:315. https://pmc.ncbi.nlm.nih.gov/articles/PMC7282067/
2. Erstad BL, Barletta JF. Dosing of neuromuscular blocking agents in patients with obesity: a narrative review. Anaesthesia and Intensive Care. 2021;49(2):98-104. PMID: 33906465. https://pubmed.ncbi.nlm.nih.gov/33906465/
3. US Food and Drug Administration. Rocuronium bromide injection prescribing information (Hospira), sections 2.6 and 14.1. DailyMed, revised June 2025. https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=309aeb1b-8022-4df7-8194-ae6529e3395c
4. Lemmens HJ, Brodsky JB. The dose of succinylcholine in morbid obesity. Anesthesia and Analgesia. 2006;102(2):438-442. PMID: 16428539. https://pubmed.ncbi.nlm.nih.gov/16428539/
5. McDowell M, Lewandowski A, Desai D, et al. Rocuronium dosing by ideal vs total body weight in obesity: a prospective, observational non-inferiority study. Western Journal of Emergency Medicine. 2024;25(1):22-27. https://pmc.ncbi.nlm.nih.gov/articles/PMC10777192/
6. Bhat R, Mazer-Amirshahi M, Sun C, et al. Accuracy of rapid sequence intubation medication dosing in obese patients intubated in the ED. American Journal of Emergency Medicine. 2016;34(12):2423-2425. PMID: 27727068. https://pubmed.ncbi.nlm.nih.gov/27727068/
7. Radkowski P, Derkaczew MA, Jacewicz MA, Onichimowski D. The Dosage of Muscle Relaxants in Morbidly Obese Patients in Daily Practice: A Narrative Review. International Journal of General Medicine. 2024;17:4055-4060. https://pmc.ncbi.nlm.nih.gov/articles/PMC11410373/
8. Engstrom K, Brown CS, Mattson AE, Lyons N, Rech MA. Pharmacotherapy optimization for rapid sequence intubation in the emergency department. American Journal of Emergency Medicine. 2023;70:19-29. PMID: 37196592. https://pubmed.ncbi.nlm.nih.gov/37196592/
9. Kuhl EA, Perera TB. Prehospital Mechanical Ventilation. In: StatPearls. StatPearls Publishing; updated March 8, 2024. NCBI Bookshelf NBK537072. https://www.ncbi.nlm.nih.gov/books/NBK537072/
10. US Food and Drug Administration. Prescribing information on DailyMed, checked for obesity or weight-descriptor text: ketamine hydrochloride injection (Civica, revised June 2026), https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=58c86d9b-8694-4bd8-8254-f19ee3ad60b0; AMIDATE etomidate injection (Hospira, revised April 2022), https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=b7ed5bf8-ba75-44dc-8f81-96b4ad5766be; propofol injectable emulsion (Dr. Reddy’s, revised September 2022), https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=800646b8-83a8-01d9-3dc8-bb2ddcb8570c; vecuronium bromide for injection (Hikma, revised May 2022), https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=dceeec01-2910-4489-8d70-9c0c711d17ab