Tidal Volume Calculator

Lung volume scales with height and sex, not with fat mass, so ventilator tidal volumes are prescribed against predicted body weight rather than what the patient weighs on the bed. The Devine formulas used here, 50 kg plus 2.3 kg per inch over 5 feet for men and 45.5 kg plus 2.3 kg per inch for women, are the ones written into the ARDS Network protocol. The difference is not academic: a man of 5 feet 8 inches has a predicted weight of 68.4 kg and a 6 mL/kg volume of about 410 mL, whether he weighs 70 kg or 140 kg. Prescribing 6 mL/kg on a 140 kg actual weight would deliver 840 mL into the same lungs, which is frank overdistension. The target of 6 mL/kg is paired with a plateau pressure ceiling of 30 cmH2O, measured on an inspiratory hold, because volume and pressure limitation address the same injury from two directions. Compliance determines how they relate: at a normal 50 to 80 mL/cmH2O a 410 mL breath costs about 6 to 8 cmH2O above PEEP, but in stiff ARDS lungs at 20 to 30 mL/cmH2O the same breath costs 14 to 20. Anatomic dead space of roughly 2.2 mL/kg predicted weight is fixed, so as tidal volume falls it consumes a larger share of each breath and alveolar ventilation drops faster than minute ventilation suggests. Rate is raised to compensate, up to about 35 per minute before gas trapping and intrinsic PEEP appear, and the ARDS Network protocol accepts permissive hypercapnia as long as arterial pH stays above roughly 7.20 to 7.25. The Devine formulas are not validated below 5 feet or in children.

Positive pressure ventilation entered wide clinical use during the Copenhagen poliomyelitis epidemic of 1952, when Bjorn Ibsen persuaded clinicians to treat respiratory failure with tracheostomy and manual bag ventilation, an effort staffed by relays of medical students and generally regarded as the founding moment of intensive care. For the next three decades the prevailing doctrine favoured large breaths of 10 to 15 mL/kg, chosen to prevent atelectasis and keep blood gases normal, and mechanical ventilation was assumed to be supportive rather than injurious. That assumption collapsed through the 1970s and 1980s as laboratory work demonstrated ventilator-induced lung injury, with animal studies showing that high end-inspiratory volume rather than airway pressure itself produced oedema and alveolar damage, a distinction later summarised as volutrauma. Marcelo Amato and colleagues in Brazil reported a survival advantage for a protective strategy in 1998. The decisive evidence came from the ARDS Network ARMA trial published in the New England Journal of Medicine in 2000, which randomised 861 patients to 6 versus 12 mL/kg of predicted body weight and was stopped early when mortality fell from 39.8 to 31.0 percent in the low-volume arm. Its protocol, including the Devine weight formulas published by B. J. Devine in 1974 and the 30 cmH2O plateau limit, became standard practice; later analyses by Amato in 2015 refined the target toward limiting driving pressure.