Predicted body weight underestimates delivered tidal volume, especially in women

Adam M Deane, Anthony E Tobin, David A Reid

St Vincent’s Hospital, Melbourne, VIC

Introduction

Recent research suggests an association between the development of acute lung injury (ALI) and ventilation with tidal volumes greater than 6mL/kg.1,2 We conducted a prospective audit comparing recorded body weight to predicted body weight. We then compared delivered tidal volumes calculated as mL/kg from recorded body weight with volumes calculated from predicted body weight.

Subjects and settings

Patients requiring controlled mechanical ventilation admitted to our mixed intensive care unit in October 2006 were eligible. Those who had ALI on admission or had no recorded body weight were excluded. Recorded body weight was defined as an actual or dietitian-estimated weight recorded for the current admission.

Methods

Height calculated from demispan was used to derive predicted body weight from the ARDSNet formula. Hourly Day 1 tidal volumes were downloaded from the medical record, and the mean was calculated for each patient. Volumes (mL/kg) were calculated using predicted and recorded body weight. Data are presented as mean (SD) or median(interquartile range, IQR) depending on normality. The Mann– Whitney rank sum test was used for comparisons, with P< 0.05 taken to signify statistical significance.

Results

Thirty-four patients were studied (20 male), with mean age, 60.6 (SD, 13.3) years, and mean APACHE II score, 19.5 (SD,6.1). Predicted body weight was lower than recorded weight (69 [IQR, 61–74.8] v 75 [65–85] kg; P< 0.05). Median tidal volumes were higher for men than women(552 [IQR, 530–586] v 474 [IQR, 424–500] mL; P< 0.01). Tidal volume (mL/kg) was higher when calculated from predicted than from recorded body weight (7.8 [IQR, 7.3–8.3] v 7.2 [IQR, 6.3–7.9] mL/kg; P< 0.05). Volumes calculated from predicted body weight were higher among women than men (8.2 [IQR, 7.8–8.7] v 7.5 [IQR, 6.8–8] mL/kg; P < 0.05). The difference in volumes between the sexes using recorded weight was not significant (7.5 [IQR, 6.6–8.6] v 6.9 [IQR, 6.2–7.8] mL/kg; P = 0.42).

Conclusions

Predicted body weight was significantly less than recorded body weight. Consequently, in retrospective calculations,larger tidal volumes on a mL/kg basis were delivered when calculated using predicted compared with recorded body weight. In addition, when retrospectively calculating tidal volume as mL/kg using predicted body weight, women received larger tidal volumes than men. Calculating predicted body weight using demispan as a surrogate marker of height is easy and non-invasive and may benefit ventilation of patients without ALI if clinicians plan for low-volume ventilation.

References

1 Gajic O, Dara SI, Mendez JL, et al. Ventilator-associated lung injury in patients without acute lung injury at the onset of mechanical ventilation.
Crit Care Med 2004; 32: 1817-24.

2 Gajic O, Frutos-Vivar F, Esteban A, et al. Ventilator settings as a risk factor for acute respiratory distress syndrome in mechanically ventilated patients.
Intensive Care Med2005; 31: 922-6.