Background:
During cardiopulmonary resuscitation (CPR), pulse detection can be challenging. Invasive blood pressure measurements (IBP) can help monitoring patient hemodynamics, but arterial catheter placement is difficult. Transthoracic impedance (TI) measured between the defibrillator pads can detect circulation activity. We hypothesized that TI changes can predict the corresponding IBP, and potentially be used to non-invasively detect pulse during CPR.
Materials and methods:
We included 28 out of hospital cardiac arrest patients receiving CPR by the Oslo Emergency Service who had concurrent recordings of IBP (radial artery, BD, 20G, US) and TI (via defibrillator pads, LP15, Stryker, US). 5-second segments with stable and CPR artefact free signals were extracted (Figure). The circulation component of the TI signal (Figure, red line) was extracted using a Kalman smoother. Ten waveform features were computed per segment and fed into a random forest regressor to predict systolic and diastolic arterial pressures (SAP, DAP), their difference (DifAP) and area of the IBP signal (ArAP). Pearson correlation coefficients between the regression model and the IBP metrics were computed. Data were divided by patient into training/test sets to fit and evaluate the model, respectively, and the process was repeated 500 times.
Results:
235 minutes (2261 segments) were extracted with median (Q1-Q3) values of 71.3(39.2-88.1) mmHg for SAP, 44.2(30.0-50.0) mmHg for DAP, 25.6(7.1-38.8) mmHg for DifAP and 63.4(17.0-85.9) mmHg*sec for ArAP. The correlation coefficients between TI-predicted and IBP-measured SAP, DAP, DifAP and ArAP were 0.62 (0.49-0.72), 0.36 (0.22-0.49), 0.69 (0.57-0.76) and 0.64 (0.50-0.73), respectively.
Conclusions:
Different hemodynamic phases can be observed in both TI and IBP (Figure). TI-based predictions showed good correlation with IBP measures. This could lead to new non-invasive methods to monitor different phases of circulation based on the TI.
A randomized trial of continuous versus interrupted chest compressions in out-of-hospital cardiac arrest: Rationale for and design of the Resuscitation Outcomes Consortium Continuous Chest Compressions Trial