Low Tidal Volume Ventilation: Trust but Verify.

Despite affecting an estimated 4 –7% of patients in the ICU and having a high mortality rate, there are currently a limited number of therapeutic options to treat patients with ARDS.1,2 ARMA, the landmark ARDSnet trial in 2000, established the concept of lungprotective ventilation, where subjects with ARDS receiving mechanical ventilation were randomly assigned to a set tidal volume (VT) of either 6 or 12 mL/kg predicted body weight, calculated from measured height.3 This trial demonstrated a significant reduction in mortality in the low VT arm. As a consequence, it provided strong evidence for abandoning the conventional wisdom of setting ventilation to normalize pH and PCO2 in favor of the emerging view that protective low VT ventilation and permissive hypercapnia could avoid pulmonary mechanical stress and ventilator-induced lung injury (VILI). Although not explicitly stated in the original publication, the ARMA trial used heelto-crown height measured in supine subjects.4 Although clinical practice has shifted toward targeting lower VT levels estimated from height, height measurements have not been standardized; in the real world, height is often obtained from patients or family members, or visually estimated by staff. Subsequent observational trials have demonstrated that height is often overestimated to the detriment of some patients, especially in the case of short and obese females.5-7 The low frequency of heelto-crown height measurements in patients with ARDS is probably due to multiple factors, including time, perceived inaccuracy of height measurements in a critically ill supine patient, and a lack of appreciation that accurate height measurements can alter mortality. The study by Jurecki et al8 explores the complexity of setting low VT for the purpose of lung-protective ventilation; the variation in estimated set VT depends on height estimates which may vary considerably depending on the source used to estimate height. The authors found that, for the study population, the mean height obtained from the electronic health record is similar to the mean predicted height calculated from ulnar length. However, for individuals, differences in height between the 2 sources can be large, leading to large differences in predicted body weight and resultant VT set in mL/kg. The authors did not obtain the accepted standard heel-to-crown height measurements