Ventilatory Ratio Calculator
Ventilatory Ratio Calculator
Ventilatory ratio compares the minute ventilation and PaCO₂ a patient actually has with the values predicted for their predicted body weight. It is a dead-space surrogate needing no capnography — and a surrogate is what it is.
Ventilatory ratio
Measured over predicted ventilationMinute ventilation 10 L/min, PaCO₂ 45 mmHg, predicted body weight 65 kg
Formula
equivalently VR = (V̇E measured × PaCO₂ measured) ÷ (V̇E predicted × PaCO₂ predicted)
- 100 and 37.5
- the reference minute ventilation in mL per kg of predicted body weight per minute, and the reference PaCO₂ in mmHg. The index was first published with 5 kPa, and 5 kPa IS 37.5 mmHg, so the two statements of the denominator are the same number in two units rather than a disagreement between sources
- predicted body weight
- from height and sex, not measured weight. Sinha and colleagues’ 2019 statement names predicted body weight explicitly; using a measured weight in an obese patient inflates the denominator and understates the ratio, which is the direction that hides a problem
- what it assumes
- that carbon dioxide production is at the reference rate. It is not a measurement of dead space: PaCO₂ depends on CO₂ production as well as on alveolar ventilation, so a hypermetabolic patient and one with a large dead space give the same ratio
- why it exists
- a true dead-space fraction needs a mixed expired CO₂, and so volumetric capnography. This needs a ventilator screen, a blood gas and a height. It correlated with physiological dead-space fraction at r = 0.99 in the ex vivo model that validated it, and the dead space fraction calculator is where the measurement itself lives
Worked example
Minute ventilation 10 L/min, PaCO₂ 45 mmHg, predicted body weight 65 kg
Numerator = 10,000 mL/min × 45 mmHg = 450,000
Denominator = 65 × 100 × 37.5 = 243,750, which is a reference 6.5 L/min at a reference PaCO₂ of 37.5 mmHg
VR = 450,000 ÷ 243,750 = 1.85
Entering the same gas as 6 kPa with the unit set to kPa gives 45.0 mmHg and the same 1.85 — the check that the unit select is doing its job
A patient the same size at 6.5 L/min with a PaCO₂ of 37.5 mmHg returns exactly 1.00, the definition of the reference point
Hold the ventilation at 10 L/min and let the PaCO₂ rise to 60 and the ratio is 2.46; hold the PaCO₂ at 45 and raise the ventilation to 14 L/min and it is 2.58. The index cannot tell those two patients apart
Use a MEASURED weight of 95 kg where the predicted weight is 65 and the ratio falls to 1.26 — a 32% understatement, in the reassuring direction
What has been found, and in whom
| Source | Population | Finding |
|---|---|---|
| Sinha and colleagues, Crit Care 2011 | 122 patients with acute lung injury or ARDS, plus an ex vivo model | VR 1.45 (SD 0.56) in survivors against 1.7 (0.64) in non-survivors, P less than 0.03; VR against physiological dead-space fraction r = 0.99 |
| Sinha and colleagues, AJRCCM 2019 | A prospective ARDS cohort and two NHLBI ARDS Network trials | Independently associated with mortality: odds ratio 1.51 (P = 0.024) adjusted for oxygenation and PEEP, 1.59 (P = 0.04) adjusted for severity |
| Morales-Quinteros and colleagues, Ann Intensive Care 2019 | 940 patients with ARDS, MARS consortium | Day-2 ratio independently associated with 30-day mortality, odds ratio 1.20 |
| Reference point | By construction | VR = 1 at 100 mL/kg predicted body weight per minute and a PaCO₂ of 37.5 mmHg (5 kPa) |
Three things that raise the ventilatory ratio
| Cause | Which term moves | How to tell |
|---|---|---|
| Increased dead space — embolism, ARDS, overdistension, circuit | Ventilation must rise to hold the PaCO₂, or the PaCO₂ rises | Volumetric capnography: the Bohr fraction rises too |
| Increased CO₂ production — fever, agitation, sepsis, overfeeding | The same, and the index cannot distinguish it | Measured VCO₂; the dead-space fraction does not rise |
| Deliberate hypercapnia | The PaCO₂ numerator rises while ventilation is held low on purpose | The ventilator settings: the ratio rises for something that is not a lung finding |
A dead-space surrogate that needs only a gas and a height
A patient’s PaCO₂ is set by how much carbon dioxide they make and how much alveolar ventilation they have to clear it. If dead space grows, more of each breath is wasted, so either the minute ventilation must rise to hold the PaCO₂ or the PaCO₂ must rise. The ventilatory ratio turns that into one number: the product of the measured minute ventilation and the measured PaCO₂, divided by the product of the values predicted for that patient’s predicted body weight — 100 mL per kg per minute of ventilation and a PaCO₂ of 37.5 mmHg, which is the same figure as the 5 kPa the index was first published with. A ratio of 1 is the reference point by construction.
What makes it useful is what it does not need. A measured dead-space fraction requires a mixed expired carbon dioxide, which requires volumetric capnography and a monitor most units do not have at every bed. The ventilatory ratio needs a ventilator screen, an arterial gas and a height. In the ex vivo model that validated it, it tracked physiological dead-space fraction with a correlation of 0.99, and across an ARDS Network reanalysis it was independently associated with mortality after adjustment for oxygenation and for severity scores.
What makes it a surrogate rather than a measurement is also what it does not need: any knowledge of carbon dioxide production. PaCO₂ sits in the numerator, and it rises both when dead space grows and when a patient becomes febrile, agitated, septic or overfed. It also rises when hypercapnia is accepted deliberately to keep tidal volumes small, which is a ventilator decision rather than a lung finding. And the denominator is predicted body weight from height and sex: substituting a measured weight in an obese patient inflates it and understates the ratio, in the direction that conceals a problem. A raised ratio is a reason to ask which term moved, not an answer about dead space.
This supports a clinician’s judgement rather than replacing it.
Frequently asked questions
How is the ventilatory ratio calculated?
Measured minute ventilation in mL/min multiplied by measured PaCO₂ in mmHg, divided by predicted body weight multiplied by 100 and by 37.5. The 100 is the reference minute ventilation in mL per kg per minute and the 37.5 is the reference PaCO₂ in mmHg, the same value as 5 kPa.
What is a normal ventilatory ratio?
One, by construction: a patient ventilated at 100 mL per kg of predicted body weight per minute with a PaCO₂ of 37.5 mmHg returns exactly 1. In ARDS cohorts it averaged about 1.45 in survivors and 1.7 in non-survivors, and values above 2 have been associated with higher mortality.
Does a high ventilatory ratio mean the dead space is large?
Not necessarily. The ratio rises whenever the product of minute ventilation and PaCO₂ rises, and carbon dioxide production is not in the equation. Fever, agitation, shivering, sepsis and overfeeding all raise it without any change in dead space, and so does deliberate hypercapnia.
Should I use measured or predicted body weight?
Predicted, from height and sex — Sinha and colleagues’ 2019 statement names it explicitly. A measured weight inflates the denominator: 95 kg measured against 65 kg predicted understates the ratio by about a third.
Can I enter the PaCO₂ in kPa?
Yes. Set the unit to kPa and the page multiplies by 7.50062 first, because the published constant of 37.5 is in mmHg. The two are the same reference value — 5 kPa is 37.5 mmHg — so the kPa and mmHg statements of this index are not two different definitions.
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References
- Sinha P, Calfee CS, Beitler JR, Soni N, Ho K, Matthay MA, Kallet RH. Physiologic analysis and clinical performance of the ventilatory ratio in acute respiratory distress syndrome. Am J Respir Crit Care Med. 2019;199(3):333–41.
- Sinha P, Corrie K, Bersten A, Hardman JG, Soni N. Ventilatory ratio: validation in an ex vivo model and analysis in ARDS/ALI patients. Crit Care. 2011;15(Suppl 1):P179.
- Cipulli F, et al. Dead space in critical care: a practical approach with clinical scenarios. J Anesth Analg Crit Care. 2026;6:20.
- Morales-Quinteros L, Schultz MJ, Bringué J, et al. Estimated dead space fraction and the ventilatory ratio are associated with mortality in early ARDS. Ann Intensive Care. 2019;9(1).
- Kallet RH, Zhuo H, Ho K, Lipnick MS, Gomez A, Matthay MA. Lung injury etiology and other factors influencing the relationship between dead-space fraction and mortality in ARDS. Respir Care. 2017;62(10):1241–8.
Not medical advice. For healthcare professionals and education. Reference intervals vary by laboratory and assay — always use your own laboratory's. Never base a dose or a treatment decision on this page alone. Full disclaimer at calcengines.com/disclaimer/