Situation: A 58-year-old man with severe community-acquired pneumonia is intubated in the intensive care unit (ICU). On day 2, chest imaging shows new bilateral opacities, an echocardiogram shows normal left ventricular function, and the team diagnoses acute respiratory distress syndrome (ARDS). He is 170 cm tall and weighs 95 kg. The physician orders lung-protective ventilation with a tidal volume of 6 mL/kg of predicted body weight. For a man, predicted body weight (kg) = 50 + 0.91 × (height in cm − 152.4). Round the final tidal volume to the nearest 10 mL. Which tidal volume matches the order?

Predicted body weight = 50 + 0.91 × (170 − 152.4) = 50 + 16.0 = 66.0 kg. Tidal volume = 6 mL/kg × 66.0 kg = 396 mL, rounded to 400 mL. Predicted, not actual, weight is used because lung size follows height and sex, and larger volumes overstretch the injured lung.

Clinical calculation

The order specifies lung-protective ventilation using 6 mL/kg of predicted body weight (PBW), not actual body weight. This distinction is central to ARDS management because the injured lung does not grow larger with increased body mass; its size follows height and sex.

For a man, PBW = 50 + 0.91 × (height in cm − 152.4). Substituting the patient’s height of 170 cm gives PBW = 50 + 0.91 × (170 − 152.4) = 50 + 0.91 × 17.6 = 50 + 16.0 = 66.0 kg. Multiplying by the ordered 6 mL/kg yields 396 mL, which rounds to 400 mL when the final tidal volume is expressed to the nearest 10 mL.

Watch out! If the calculation mistakenly used the patient’s actual weight of 95 kg, the tidal volume would be 570 mL (6 × 95 = 570). That value would overdistend the already injured lung and increase the risk of ventilator-induced lung injury (VILI). The option 660 mL reflects an even larger error, and 530 mL does not correspond to either weight-based calculation. Only 400 mL matches the PBW-based order.

The rationale for using PBW rather than actual weight is supported by the observation that lung size scales with height and sex, not with adiposity or total body mass. In the ARMA trial, tidal volume was normalized to PBW precisely because PBW serves as a proxy for preinjury lung volume ^[1]. Using actual weight would systematically deliver excessive volumes to shorter patients and to patients with higher body mass, which is a recognized source of nonadherence to lung-protective targets ^[2].

Height and sex, not current weight, determine the appropriate tidal volume in ARDS. This principle also explains why recorded height and sex are both needed for accurate PBW calculation. When height is recorded inaccurately or sex is not considered, the resulting tidal volume may fall outside the protective range, particularly for women and shorter patients ^[2]. In this patient, the correct PBW is 66.0 kg, and the corresponding tidal volume is 400 mL.

Key point! For ARDS, calculate tidal volume from predicted body weight, not actual body weight. A 170 cm man has a PBW of 66 kg, and 6 mL/kg gives 396 mL, rounded to 400 mL.