Comparison of Brody's formula with respiratory gas analysis for estimating oxygen consumption in anaesthetized dogs

OBJECTIVE: To determine agreement in oxygen consumption (V˙O2) values calculated using Sykes' formula V˙O2 = (FiO2 - Fe'O2) * V˙E (where Fi and Fe are the inspired and end-tidal fractional concentrations of O2, respectively, and V˙E is minute volume) with values derived using Brody's formula (V˙O2 = 10 kg3/4). It was hypothesized that the two methods would not yield statistically significant differences in calculated values.

STUDY DESIGN: Prospective, clinical, pilot study.

ANIMALS: A total of 22 client-owned dogs.

METHODS: Dogs undergoing surgery were anaesthetized with either isoflurane or sevoflurane. The V˙E, FiO2 and Fe'O2 were measured during mechanical ventilation of the lungs (tidal volume 10 mL kg-1; respiratory rate: 8-12 breaths minute-1). Oesophageal temperature was maintained between 37.0 °C and 38.5 °C. Values for V˙O2 derived by Sykes' and Brody's methods were compared and agreement was determined using Bland-Altman analysis.

RESULTS: Mean V˙O2 values were 4.67 ± 0.51 mL kg-1 minute-1 and 5.32 ± 1.69 mL kg-1 minute-1 calculated using Brody's formula and Sykes' equation, respectively. There was greater variability in the values obtained from Sykes' equation. The Bland-Altman plot revealed a proportional error with correlation but poor agreement between values.

CONCLUSIONS AND CLINICAL RELEVANCE: Both methods yielded V˙O2 values of approximately 5 mL kg-1minute-1 with no statistically significant differences between the two methods.