Size matters: An observational study investigating estimated height as a reference size for calculating tidal volumes...

Acute lung injury is a life threatening condition often requiring mechanical ventilation. Lung-protective ventilation with tidal volumes of 6 mL/kg predicted body weight (PBW, calculated on the basis of a patient's sex and height), is part of current recommended ventilation strategy. Hence, an exact height is necessary to provide optimal mechanical ventilation. However, it is a common practice to visually estimate the body height of mechanically ventilated patients and use these estimates as a reference size for ventilator settings. We aimed to determine if the common practice of estimating visual height to define tidal volume reduces the possibility of receiving lung-protective ventilation.

In this prospective observational study, 28 mechanically ventilated patients had their heights visually estimated by 20 nurses and 20 physicians. All medical professionals calculated the PBW and a corresponding tidal volume with 6 ml/kg/PBW on the basis of their visual estimation. The patients' true heights were measured and the true PBW with a corresponding tidal volume was calculated. Finally, estimates and measurements were compared.

1033 estimations were undertaken by 153 medical professionals. The majority of the estimates were imprecise and resulting data comprised taller body heights, higher PBW and higher tidal volumes (all p≤0.01). When estimates of patients´ heights are used as a reference for tidal-volume definition, patients are exposed to mean tidal volumes of 6.5 ± 0.4 ml/kg/PBW. 526 estimation-based tidal volumes (51.1%) did not provide lung-protective ventilation. Shorter subjects (<175cm) were a specific risk group with an increased risk of not receiving lung protective ventilation (OR 6.6; 95%CI 1.2-35.4; p = 0.02), while taller subjects had a smaller risk of being exposed to inadequately high tidal volumes (OR 0.15; 95%CI 0.02-0.8; p = 0.02). Furthermore, we found an increased risk of overestimating if the assessor was a female (OR 1.74; 95%CI 1.14-2.65; p = 0.01).

The common practice of visually estimating body height and using these estimates for ventilator settings is imprecise and potentially harmful because it reduces the chance of receiving lung-protective ventilation. Avoiding this practice increases the patient safety. Instead, height should be measured as a standard procedure.