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Actual vs ideal body weight: compare which one sizes adult tidal volume on the vent

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Actual vs ideal body weight for tidal volume comes down to one rule on the adult vent: tidal volume is sized from ideal body weight, calculated from height and sex, not actual weight. Normal lung volumes are predicted from sex and height, which is why the ARDS Network trial calculated its predicted body weight from those two inputs. Actual weight only enters the vent math by mistake, and in obesity that mistake can double the breath. This page covers adult ventilation only; every number below is a starting-point estimate, verified against the patient and your protocol.

Same multiplier, two weights: tidal volume on IBW vs actual weight
AdultActual weightIdeal body weight (Devine)Vt at 6 mL/kg of IBWSame 6 mL/kg on actual weightWhat that breath is in mL/kg of IBW
5’5” male70 kg61.5 kg369 mL420 mL6.8
5’5” male200 kg61.5 kg369 mL1,200 mL19.5
5’2” female110 kg50.1 kg300 mL660 mL13.2
6’0” male60 kg77.6 kg465 mL360 mL4.6

The fifth column is what the same 6 mL/kg multiplier returns if actual weight is used instead of ideal body weight; it is not a setting, only a comparison. Ideal body weight is by the Devine formula, and tidal volume is truncated to the whole milliliter, matching the app. The two 5’5” rows are StatPearls’ own example: a 65-inch male weighing 200 kg has a similar-sized thoracic cavity to another 65-inch male weighing 70 kg, and a similar tidal volume.

Should tidal volume be based on ideal body weight?

Yes. Ventilation references size tidal volume from ideal body weight, calculated from height and sex, never from the weight on the stretcher report. StatPearls gives the reason: “IBW should be used instead of actual weight because lung volume does not change with body mass.” Its companion chapter agrees that tidal volume “is usually determined based on ideal or predicted body weight (PBW) rather than actual weight.” The ARDS Network trial calculated predicted body weight the same way, from sex and height.

Ideal body weight itself is the Devine formula: 50 kg for men or 45.5 kg for women, plus 2.3 kg per inch over 5 feet. That equation feeds directly into the ventilator settings that build on ideal body weight, with the Devine formula’s limits under 5 feet and at the tall end covered on its own page.

Are PBW and IBW the same?

The arithmetic is identical: the ARDSNet predicted body weight equation and the Devine ideal body weight equation return the same number for the same height and sex. Some sources reserve “ideal body weight” for a different clinical idea; AHRQ’s guide says predicted body weight is “not based on their actual body weight, nor even an ideal body weight.”

Compare tidal volume on ideal vs actual body weight, in both directions

At a build close to ideal, the two weights land close together: the 5’5”, 70 kg man in the table gets a breath under 1 mL/kg over target on actual weight. In obesity, the actual-weight breath runs more than twice the ideal-body-weight breath: the 200 kg man at the same height would get 1,200 mL on actual weight, 19.5 mL/kg of his own ideal body weight, against 369 mL calculated correctly. For an adult lighter than ideal, the error reverses: 360 mL against 465 for the table’s 6-foot man. Martin and Richards cover both directions: actual weight might lead to volutrauma in an obese patient, or inadequate ventilation in an underweight one, and height may offer a better basis.

Tidal volume on ideal body weight vs actual weight for four adultsFor a 5 foot 5 inch male at 70 kilograms, ideal body weight tidal volume is 369 mL versus 420 mL on actual weight. For the same height male at 200 kilograms, 369 mL versus 1,200 mL. For a 5 foot 2 inch female at 110 kilograms, 300 mL versus 660 mL. For a 6 foot male at 60 kilograms, 465 mL versus 360 mL.6 mL/kg of IBW6 mL/kg of actual weight5’5” M, 70 kg3694205’5” M, 200 kg3691,2005’2” F, 110 kg3006606’0” M, 60 kg46536004008001,200Tidal volume (mL)
Same height, similar lungs, same breath on ideal body weight. On the scale weight, the 200 kg patient’s breath is 3.25 times larger, and the underweight patient’s is smaller.

What if the patient weighs less than their ideal body weight?

Tidal volume still comes off ideal body weight. Using actual weight instead produces a smaller breath than the multiplier intends: the table’s 6-foot, 60 kg man gets 360 mL on actual weight against the 465 mL his height calls for. Martin and Richards’ rationale covers this direction too: actual weight might lead to inadequate ventilation in an underweight patient.

Why not adjusted body weight for tidal volume?

Adjusted body weight, ideal body weight plus 40% of the excess over ideal in Erstad and Barletta’s definition, is a drug-dosing descriptor, not a ventilation number. A 2015 review of the weight debate calls its use “limited to drug dosing.” Current ARDS ventilation guidelines size tidal volume on predicted body weight instead: both the 2017 ATS/ESICM/SCCM guideline and the 2023 ESICM guideline frame the target in mL/kg predicted body weight.

Recheck the adults most likely to get an oversized breath

Patients documented as receiving larger-than-protective breaths are disproportionately obese, female, and short. Kalra and colleagues, in 613 adults with ARDS, found 53% of those with class III obesity had tidal volumes over 8 mL/kg predicted body weight on day 1, against 26% at normal weight (odds ratio 3.14 after adjustment), with no difference in adjusted mortality across the four weight groups. Self and colleagues, in three emergency departments, found obesity, female sex, and first-quartile height independently associated with breaths outside lung-protective range, short stature carrying the largest odds. Malnoske and colleagues, across 59 US ICUs, found 27% of patients set above 8 mL/kg predicted body weight, 40% of women against 17% of men, a gap largely but not completely explained by height.

Verify the height: the one input that moves the breath

Once actual weight is out of the calculation, height and sex are the only inputs left, so a height error is now the error. Every inch of height error moves ideal body weight by 2.3 kg, about 14 mL of tidal volume at 6 mL/kg (2.3 x 6 = 13.8). O’Brien and colleagues show height errors are “multiplied by 5.5” per centimeter at 6 mL/kg, worse in shorter patients. Sasko and colleagues found estimated heights left just over half of estimation-based volumes outside lung protection. StatPearls calls an accurate height crucial and notes tape measures in many resuscitation bays.

Rapid Vent asks for height, sex, and weight, but only height and sex reach tidal volume, the minute-ventilation target, and the derived rate; the weight typed in feeds only the RSI dose estimates. Type 140 kg or 70 kg and the vent numbers do not move; type the wrong height and they all do. The app cannot see the patient, and calculating tidal volume by height walks the arithmetic step by step.

After the weight basis: the mL/kg target, obesity settings, and RSI doses

Once the weight is right, a few things follow it. The minute-ventilation target also rides on ideal body weight (Mireles-Cabodevila and colleagues’ normal of 100 mL/kg/min), so the derived respiratory rate comes off the same number. The multiplier is covered under whether 6 or 8 mL/kg is the right multiplier. The rest of an obese patient’s settings, PEEP, pressures, and positioning, are covered under ventilator settings for the obese patient beyond tidal volume. The next step in the sequence is initial ventilator settings after RSI, where the weight basis flips.

When to use ideal body weight vs actual body weight?

On the vent, ideal body weight. For RSI drug doses, the app’s dose estimates run on actual weight, published sources split by agent in obesity, and both practices are in use. Rapid Vent’s own results screen shows both as separate labelled rows, “Actual Weight” and “Ideal Body Wt”. Which weight RSI drug doses start from is the drug-dosing half of this same question.

Sources

  1. Kuhl EA, Perera TB. Prehospital Mechanical Ventilation. StatPearls, NCBI Bookshelf NBK537072. Updated 8 March 2024. https://www.ncbi.nlm.nih.gov/books/NBK537072/
  2. Hickey SM, Sankari A, Giwa AO. Invasive Mechanical Ventilation. StatPearls, NCBI Bookshelf NBK539742. Updated 30 March 2024. https://www.ncbi.nlm.nih.gov/books/NBK539742/
  3. The Acute Respiratory Distress Syndrome Network (Brower RG, et al). Ventilation with lower tidal volumes as compared with traditional tidal volumes for acute lung injury and the acute respiratory distress syndrome. New England Journal of Medicine. 2000;342(18):1301-1308. https://pubmed.ncbi.nlm.nih.gov/10793162/
  4. Martin DC, Richards GN. Predicted body weight relationships for protective ventilation: unisex proposals from pre-term through to adult. BMC Pulmonary Medicine. 2017;17:85. https://pmc.ncbi.nlm.nih.gov/articles/PMC5442651/
  5. Agency for Healthcare Research and Quality. Low Tidal Volume Ventilation facilitator guide. https://www.ahrq.gov/hai/tools/mvp/modules/technical/ltvv-intro-fac-guide.html
  6. Fan E, Del Sorbo L, Goligher EC, et al. An Official American Thoracic Society/European Society of Intensive Care Medicine/Society of Critical Care Medicine Clinical Practice Guideline: Mechanical Ventilation in Adult Patients with Acute Respiratory Distress Syndrome. American Journal of Respiratory and Critical Care Medicine. 2017;195(9):1253-1263. https://pubmed.ncbi.nlm.nih.gov/28459336/
  7. Grasselli G, Calfee CS, Camporota L, et al. ESICM guidelines on acute respiratory distress syndrome: definition, phenotyping and respiratory support strategies. Intensive Care Medicine. 2023;49(7):727-759. https://pmc.ncbi.nlm.nih.gov/articles/PMC10354163/
  8. 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/
  9. MacDonald JJ, Moore J, Davey V, Pickering S, Dunne T. The weight debate. Journal of the Intensive Care Society. 2015;16(3):234-238. https://pmc.ncbi.nlm.nih.gov/articles/PMC5606431/
  10. Kalra SS, Siuba M, Panitchote A, et al. Higher Class of Obesity Is Associated With Delivery of Higher Tidal Volumes in Subjects With ARDS. Respiratory Care. 2020;65(10):1519-1526. https://pubmed.ncbi.nlm.nih.gov/32209706/
  11. Self M, Kennis B, Lafree A, et al. Disparities Exist in the Application of Low Tidal-volume Ventilation in the Emergency Department. Western Journal of Emergency Medicine. 2023;24(3):502-510. https://pubmed.ncbi.nlm.nih.gov/37278778/
  12. Malnoske ML, Quill CM, Barwise AK, Pietropaoli AP. Disparities in Lung-Protective Ventilation in the United States. Cureus. 2022;14(10):e29834. https://pmc.ncbi.nlm.nih.gov/articles/PMC9625078/
  13. O’Brien ID, Shacklock E, Middleditch A, Bigham C. Inaccuracies in calculating predicted body weight and its impact on safe ventilator settings. Journal of the Intensive Care Society. 2016;17(3):191-195. https://pmc.ncbi.nlm.nih.gov/articles/PMC5606514/
  14. Sasko B, Thiem U, Christ M, et al. Size matters: an observational study investigating estimated height as a reference size for calculating tidal volumes if low tidal volume ventilation is required. PLoS ONE. 2018;13(6):e0199917. https://pmc.ncbi.nlm.nih.gov/articles/PMC6025863/
  15. Mireles-Cabodevila E, Diaz-Guzman E, Arroliga AC, Chatburn RL. Frequently misinterpreted clinical parameters and terms in mechanical ventilation. Critical Care Research and Practice. 2012;2012:204314. https://pmc.ncbi.nlm.nih.gov/articles/PMC3478732/