Research Article: Size matters: An observational study investigating estimated height as a reference size for calculating tidal volumes if low tidal volume ventilation is required

Date Published: June 29, 2018

Publisher: Public Library of Science

Author(s): Benjamin Sasko, Ulrich Thiem, Martin Christ, Hans-Joachim Trappe, Oliver Ritter, Nikolaos Pagonas, Yu Ru Kou.


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.

Partial Text

Acute lung injury (ALI) is a life-threatening condition with severe hypoxaemia that typically necessitates treatment in the intensive care unit (ICU) and mechanical ventilation. Lung-protective ventilation with low tidal volumes (6 ml/kg predicted body weight [PBW]) and plateau pressures of ≤30 cmH2O have been shown to decrease mortality and are, therefore, part of a ventilation strategy [1]. A prospective cohort study published in 2012 evaluated the association of ventilator tidal volume with two-year survival in patients with ALI [2]. In this study, a three-level categorical model was used to compare three different mean tidal volumes, <6.5, 6.5–8.5, and >8.5 ml/kg PBW, and their impact upon survival. Compared with a mean tidal volume <6.5 ml/kg/PBW, the adjusted hazard ratio for two-year mortality were 1.59 for a mean tidal volume of 6.5–8.5 ml/kg/PBW and 1.97 for >8.5 ml/kg/PBW. The same study also demonstrated an 18% relative increase in mortality for each 1-ml/kg/PBW increase in mean tidal volume [2]. Clearly, a precise definition of tidal volumes is crucial to achieve a respiratory support with low tidal ventilation (LTV) of <6.5ml/kg/PBW. The current recommended ARDSnet formula uses body height as the only changeable variable in a mathematical equation to calculate the PBW, upon which tidal volumes are finally defined [1]. Therefore, the prerequisite for calculation of the PBW is the knowledge of an exact body height. Hence, an accurate measurement of height is necessary to provide an optimal ventilation strategy in ALI. This non-interventional, observational study aimed to investigate the practice of visual estimation of body height, followed by a simulation of ventilator settings with low tidal volumes. Between February and April 2016, 28 patients receiving mechanical ventilation treated in the ICU of a tertiary centre (Marienhospital Herne, University of Bochum, Germany) were enrolled prospectively into this study. Inclusion criteria were age ≥18 years and mechanical ventilation on a pressure-controlled mode in the supine position. Exclusion criteria were body height <150 cm or > 200 cm, amputation of lower limbs and colonisation with multi-resistant pathogens. The study was registered and approved by the institutional review board (Ethics Committee of Ruhr-Universität Bochum, 15-5414-BR) and performed in accordance with the Declaration of Helsinki and its later amendments. It was retrospectively registered in the German Clinical Trial Register (DRKS), trial number DRKS00010899. Informed consent was obtained from all patients post hoc after extubation or from relatives.

In the 3 months of data collection, 28 patients were enrolled prospectively. During this period, 1033 estimations were made by 153 assessors, resulting in mean of 6.7 estimations per assessor.

In our observational study which evaluated 1033 visual estimates of body height of 28 mechanically ventilated patients we identified such practice of body-size definition as being imprecise and potentially harmful. Most estimates were significantly inaccurate and deviated from measured results.

The common clinical practice in most ICUs is to use visual estimates to calculate tidal volumes for ventilation therapy. According to our data, most estimates were significantly inaccurate in comparison to precisely measured results. Our study demonstrates that this practice of body size definition is imprecise and possibly harmful.