A novel biometric approach to estimating tidal volume.

Background: Low tidal volume ventilation (LTVV) of 4-8 mL/kg of ideal body weight (IBW) reduces mortality in patients with acute respiratory distress syndrome, and, more recently, it has been recommended as the default therapy for all controlled ventilation. However, adherence to LTVV is poor. Barriers to adherence include not having height measurements taken or IBW calculated during admission. The aim of our project was to develop and validate a simple one step biometric measuring tool to directly estimate tidal volume (V) in ventilated patients based on their demispan.

Objectives: To validate our novel biometric approach for the estimation of V in mechanically ventilated patients by demonstrating its accuracy as a simple reliable alternative to IBW derived from measured height.

Design And Setting: A simple computer program was written based on regression equations for demispan, height and IBW which used simple substitution to produce a vector graphic scale with markings in millilitres of 6 mL/kg IBW V printed onto a paper tape. We performed an observational validation study on ventilated patients after cardiac surgery comparing the V derived from demispan measurements using our tape with the V based on IBW calculated from pre-operative vertical height.

Main Outcome Measure: We compared compliance with a target V ≤ 6.5 mL/kg for V derived using our demispan method and with V based on IBW calculated from vertical height.

Results: Eighty-two patients were studied. The mean age was 65.7 years (SD, 11.4) and 61 patients (74%) were male. Mean height was 170.4 cm (SD, 9.5) and mean body mass index for the group was 28.6 kg/m (SD, 5.5). The V based on 6 mL/kg IBW estimated by traditional height method and using our biometric tape method correlated well (r = 0.8) and was not statistically different, with a mean difference of -7.5 mL (SEM, 8.8). Bland-Altman plot showed 95% limits of agreement from -64 mL to 79 mL around the mean difference of 7.5 mL, with 4 points (4.9%) outside the limits of agreement. Fifty-one of the initial V (62%) were compliant, with a target of ≤ 6.5 mL/kg IBW using volumes determined from measured height, while 66 of the tape volumes (80%) would have been compliant at a target of ≤ 6.5 mL/kg IBW.

Conclusion: Estimating V using of our biometric one step approach based on demispan correlates well with V derived from vertical height. The simplicity of its use and accuracy could lead to improved adherence in a large cohort of patients who currently do not receive the recommended V restriction.