Impact of Height Estimation on Tidal Volume Calculation for Protective Ventilation-A Prospective Observational Study

Abstract

Objectives: The current standard of care to deliver invasive mechanical ventilation support is the protective ventilation approach. One pillar of this approach is the limitation of tidal volume to less than 6 mL/Kg of predicted body weight. Predicted body weight is calculated from patient's height. Yet, little is known about the potential impact of errors arising from visual height estimation, a common practice, to calculate tidal volumes. The aim of this study was to evaluate that impact on tidal volume calculation to use during protective ventilation.

Design: Prospective observational study.

Setting: An eight-bed polyvalent ICU.

Patients: Adult patients (≥ 18 yr).

Interventions: None.

Measurements and main results: Tidal volumes were calculated from visual height estimates made by physicians, nurses, and patients themselves and compared with tidal volumes calculated from measured heights. Comparisons were made using the paired t test. Modified Bland-Altman plots were used to assess agreement between height estimates and measurements. One-hundred patients were recruited. Regardless of the height estimator, all the mean tidal volumes would be greater than 6 mL/Kg predicted body weight (all p < 0.001). Additionally, tidal volumes would be greater than or equal to 6.5 mL/Kg predicted body weight in 18% of patients' estimates, 25% of physicians' estimates, and 30% of nurses' estimates. Patients with lower stature (< 165 cm), older age, and surgical typology of admission were at increased risk of being ventilated with tidal volumes above protective threshold.

Conclusions: The clinical benefit of the protective ventilation strategy can be offset by using visual height estimates to calculate tidal volumes. Additionally, this approach can be harmful and potentially increase mortality by exposing patients to tidal volumes greater than or equal to 6.5 mL/Kg predicted body weight. In the interest of patient safety, every ICU patient should have his or her height accurately measured.

Keywords: acute respiratory distress syndrome; body height; critical care; intensive care units; pulmonary ventilation; quality improvement.