Changes in transmyocardial impedance during prolonged ventricular fibrillation. Implications for current flow and...

Transthoracic resistance (TTR) and transmyocardial resistance (TMR) were measured during 10 minutes of uninterrupted ventricular fibrillation (VF) in a canine model. TMR was measured at 10- to 50-second intervals with two wire-mesh patch electrodes in 16 dogs. TTR was measured through two identical low-impedance electrodes. A monophasic exponentially truncated pulse with a duration of 5 msec was used for measurement of TMR as well as TTR. Low-energy pulses of 100 V were used for TMR measurements and pulses of 300 V for TTR measurements. TMR showed an increase of 22.8 +/- 5.14 omega (from 96.2 +/- 12.3 omega) after 600 seconds of uninterrupted VF (p less than 0.0006). TTR showed a change of 2.4 +/- 1.94 omega, which was not statistically significant in comparison with the initial value of 69.0 +/- 11.4 omega. A mathematical model was developed to predict energy delivered to the heart after a transthoracic shock. Observed values of TMR and TTR were used in this model. With the use of this model, the predicted fall in transmyocardial current after 600 seconds of uninterrupted VF and 19.3%, and the fall in energy delivered to the heart was 14%. Our study suggests that increase in TMR may contribute to the observed lack of successful defibrillation during prolonged VF.