Tidal volume measurements via transthoracic impedance waveform characteristics: The effect of age, body mass index and gender. A single centre interventional study

Background and aim: Measuring tidal volumes (TV) during bag-valve ventilation is challenging in the clinical setting. The ventilation waveform amplitude of the transthoracic impedance (TTI-amplitude) correlates well with TV for an individual, but poorer between patients. We hypothesized that TV to TTI-amplitude relations could be improved when adjusted for morphometric variables like body mass index (BMI), gender or age, and that TTI-amplitude cut-offs for ventilations with adequate TV (>400ml) could be established. Materials and methods: Twenty-one consenting adults (9 female, and 9 overall overweight) during positive pressure ventilation in anaesthesia before scheduled surgery were included. Seventeen ventilator modes were used (>= five breaths per mode) to adjust different TVs (150-800 ml), ventilation frequencies (10-30 min(-1) ) and insufflation times (0.5-3.5s). TTI from the defibrillation pads was filtered to obtain ventilation TTI-amplitudes. Linear regression models were fitted between target and explanatory variables, and compared (coefficient of determination, R-2 ). Results: The TV to TTI-amplitude slope was 1.39 Omega/l (R-2 =0.52), with significant differences (p<0.05) between male/female (1.04 Omega/l vs 1.84 Omega/l) and normal/overweight subjects (1.65 Omega/l vs 1.04 Omega/l). The median (interquartile range) TTI-amplitude cut-off for adequate TV was 0.51 Omega(0.14-1.20) with significant differences between males and females (0.58 Omega/0.39 Omega), and normal and overweight subjects (0.52 Omega/0.46 Omega). The TV to TTI-amplitude model improved (R-2 =0.66) when BMI, age and gender were included. Conclusions: TTI-amplitude to TV relations were established and cut-offs for ventilations with adequate TV determined. Patient morphometric variables related to gender, age and BMI explain part of the variability in the measurements.