Transthoracic Impedance Measured with Defibrillator Pads-New Interpretations of Signal Change Induced by Ventilations

Abstract

Compressions during the insufflation phase of ventilations may cause severe pulmonary injury during cardiopulmonary resuscitation (CPR). Transthoracic impedance (TTI) could be used to evaluate how chest compressions are aligned with ventilations if the insufflation phase could be identified in the TTI waveform without chest compression artifacts. Therefore, the aim of this study was to determine whether and how the insufflation phase could be precisely identified during TTI. We synchronously measured TTI and airway pressure (Paw) in 21 consenting anaesthetised patients, TTI through the defibrillator pads and Paw by connecting the monitor-defibrillator's pressure-line to the endotracheal tube filter. Volume control mode with seventeen different settings were used (5-10 ventilations/setting): Six volumes (150-800 mL) with 12 min^-1 frequency, four frequencies (10, 12, 22 and 30 min^-1) with 400 mL volume, and seven inspiratory times (0.5-3.5 s ) with 400 mL/10 min^-1 volume/frequency. Median time differences (quartile range) between timing of expiration onset in the Paw-line (Paw_EO) and the TTI peak and TTI maximum downslope were measured. TTI peak and Paw_EO time difference was 579 (432-723) m s for 12 min^-1, independent of volume, with a negative relation to frequency, and it increased linearly with inspiratory time (slope 0.47, R 2 = 0.72). Paw_EO and TTI maximum downslope time difference was between -69 and 84 m s for any ventilation setting (time aligned). It was independent ( R 2 < 0.01) of volume, frequency and inspiratory time, with global median values of -47 (-153-65) m s , -40 (-168-68) m s and 20 (-93-128) m s , for varying volume, frequency and inspiratory time, respectively. The TTI peak is not aligned with the start of exhalation, but the TTI maximum downslope is. This knowledge could help with identifying the ideal ventilation pattern during CPR.

Keywords: cardiopulmonary resuscitation (CPR); peak inspiration pressure; pulmonary barotrauma; pulmonary injury; transthoracic impedance; ventilation; ventilation pattern.

Figure 1

A 25 $\mathrm{s}$ example of the data recorded by the LP15 with the electrocardiogram (ECG) (top channel), raw transthoracic impedance (TTI) (middle channel) and airway pressure (Paw) line (bottom channel). The raw impedance (blue) shows a strong circulation component aligned with the QRS complexes in the ECG. This component was filtered to obtain the ventilatory component in green. The red square highlights a typical ventilation waveform and the fiducial points in the TTI. As reference an interpolated Paw curve (in red) is superposed to the Paw curve (blue) to show how Paw gradually approaches the ${\mathrm{Paw}}_{\mathrm{EO}}$ point. The time intervals $t_1=t_{\mathrm{PawEO}}-t_{\mathrm{TTIpeak}}$ and $t_2=t_{\mathrm{PawEO}}-t_{\mathrm{maxDS}}$ are the ones used in Figure 2, Figure 3 and Figure 4.

Figure 2

Effect of volume on the time differences between the TTI fiducial points (TTI peak, $t_{\mathrm{TTIpeak}}$, and maximum downslope, $t_{\mathrm{maxDS}}$) and the peak pressure time in the Paw line marking exhalation onset ($t_{\mathrm{PawEO}}$). All ventilator settings had a constant frequency of 12 $\min$⁻¹. Data is shown as median with first to third quartile interval. Time differences were independent of volume ($R^2$ < 0.01 for the linear fit), and were around 0.5 $\mathrm{s}$ for TTI peak and around 0 $\mathrm{s}$ for maximum downslope.

Figure 3

Effect of inspiratory time in the time differences between the TTI fiducial points (TTI peak, $t_{\mathrm{TTIpeak}}$, and maximum downslope, $t_{\mathrm{maxDS}}$) and the peak pressure time in the Paw line marking exhalation onset ($t_{\mathrm{PawEO}}$). All ventilator settings had a constant tidal volume of 400 mL and frequency of 10 $\min$⁻¹. Data is shown as median with first to third quartile interval. There was a linear relation with positive slope 0.47 for the TTI peak, and no relation with a median time difference for maximum downslope ($R^2$ < 0.01).

Figure 4

Effect of ventilation frequency in the time differences between the TTI fiducial points (TTI peak, $t_{\mathrm{TTIpeak}}$, and maximum downslope, $t_{\mathrm{maxDS}}$) and the peak pressure time in the Paw line marking exhalation onset ($t_{\mathrm{PawEO}}$). All ventilator settings had a constant tidal volume of 400 mL. The time difference for TTI peak decreased as frequency increased, although not linearly. There was no relation with a median time difference for maximum downslope ($R^2$ < 0.01).