Purpose: Transthoracic impedance (TTI) waveform provides important information about ventilations during cardiac arrest. The objective was to characterize ventilation caused TTI changes.
Materials and methods: 21 subjects scheduled for abdominal surgery in general anaesthesia were enrolled in the study (12 BMI<25, 9 BMI>25). Written consents were signed by all participants. Subjects were mechanically ventilated with tidal volume control settings (150ml-800ml, 6 modes), ventilation frequency (12min-1-30min-1, 4 modes) and air insufflation time (1s-3.5s, 7 modes). Continuous waveforms were recorded using LP15 defibrillators (Stryker, USA), with its pressure transducer connected to the airway. TTI ventilation waveform was obtained through filtering. Ventilation peak, insufflation onset and expiration offset times for TTI and pressure waveforms were automatically detected, then manually audited. TTI amplitude and duration were computed for each ventilation. Regression analysis was used to assess the influence of volume, frequency and insufflation time on TTI amplitude and duration, controlling for BMI.
Results: Expiration amplitude showed positive relation with volume (slopes 1.59/lt, frequency 12min-1), no relation with frequency (slope 0.003/min-1, p>0.05, volume 400ml) and small negative relation with insufflation time (-0.041/sec, volume 400ml and frequency 10min-1). Slopes for TTI amplitude showed significant differences between normal weight (1.93/l) and overweight (1.31/l) subjects, p<10-9 (figure). Durations had small positive relation with volume (slope 0.48sec/l), strong negative relation with frequency (-0.051sec/min-1) and strong positive relation with insufflation time (0.89) (figure). Slopes for durations showed no significant differences between normal and overweight subjects, p>0.05.
Conclusions: TTI ventilation amplitude is controlled by volume, with significant differences depending on BMI. Duration of TTI fluctuation is controlled by frequency and insufflation time. Quantitative measures were obtained for amplitudes and durations to characterize ventilation in the TTI waveform that can be of use during cardiac arrest.

5 - Characterization of the thoracic impedance waveform fluctuations during controlled ventilation modes