ABSTRACT

This work presents the results of a study using ventricular electrogram signals to identify the heart's evoked response following an electrical stimulus from a pacing electrode. Acute data were collected in vivo from seven human subjects and analyzed offline. The ability to recognize the evoked response following the pacing pulse was characterized as a function of the parameters of the pacing pulse. Although the characteristics of the evoked response were dependent on the patient, inter-patient data became consistent when time derivative of the electrocardiogram was considered.

INTRODUCTION

Intracardiac electrodes of conventional cardiac pacemakers serve two major functions, namely pacing and sensing. Pacing is achieved by the delivery of a short, intense electrical pulse to the myocardial wall where the distal end of the electrode is attached. The same electrode can be used to detect the intrinsic activity of the heart shown by the intracardiac electrogram. This allows inhibition of the pacing pulse from an artificial pacemaker whenever the heart beats on its own. However, sensing the cardiac activity immediately following the stimulus is complicated due to the strong residual polarization of the surrounding tissue.

The cathodic pacing pulse strongly depolarizes the myocardium around the pacing electrode. Excessive polarization charges left on the surrounding tissue are commonly reduced in two ways. Either a biphasic pacing pulse is used to introduce anodic charges to cancel the negative pacing polarization, or a steroid eluting pacing electrode is used, which allows utilization of lower pacing magnitudes. However, the reduction in polarization is not sufficient to allow the ready measurement of evoked cardiac potentials--especially when pacing leads which cause high polarization are used (the long duration anodic pulses following the cathodic stimulation interfere with the evoked response measurements).

One means of assuring that the polarization caused by the cathodic pacing pulse is adequately reduced to allow the timely sensing of the evoked response is to utilize two anodic pulses, one before and one after the cathodic pulse. Figures 1 and 2 compare the traditional biphasic pulse to the tri-phasic pulse with two anodic segments.

DATA COLLECTION

Data were collected from seven patients, two using bipolar and five using unipolar electrode configurations. These patients were studied during hospitalization for pacemaker implant or replacement, while the pacing leads were accessible. An external pacer capable of delivering tri-phasic stimuli was used acutely. For each subject, as many combinations of the following settings were used as the condition of the patient would allow:

Vpace = 1.5, 3.0, 4.5 Volts

Tstim = 0.25, 0.5, 1.0 msec

Tpre/Tpost = 2/2,2/4,4/4,4/8,6/6,6/12 msec

For each combination, 20 seconds of ventricular electrocardiogram and surface ECG were recorded for offline data analysis.

DATA ANALYSIS

The data were analyzed after classification of each event as intrinic beat , paced beat or a paced non-beat (i.e., delivery of the pacing pulse did not result in systole). Classification was done using the surface ECG and the intrinsic beats were excluded from the analysis. Features of the intracardiac electrogram were studied in a time window from 20 to 100 milliseconds after delivery of the cathodic pulse. Analysis concentrated on recognition of an evoked response following the pacing spike in the ventricular electrocardiogram.

RESULTS

Evoked responses of each patient displayed specific, consistent features. However, inter-patient variations prevented a generalization of the paced evoked response features in the time domain.

Although the use of the tri-phasic pacing pulse helped to reduce residual polarization, the evoked response was not completely revealed after the pace. However, the peak-to-peak values measured from the time derivative (d/dt) of the ventricular electrocardiogram were found to be a reliable indicator of the presence of the evoked response.

In Figure 3, the collective peak-to-peak values of the electrocardiogram of one patient is plotted for various parameters of the pacing pulse. A high peak-to-peak measurement from paced beats definitively indicated the presence of an evoked response. It is also apparent that the peak-to-peak value of the time derivative of the electrocardiogram was fairly independent of the pacing parameters. Therefore, slope domain signals may be most suitable for measuring the evoked response following a tri-phasic pulse.

ACKNOWLEDGEMENTS

Data were collected by Dr. Hardwin Mead from Sequoia Hospital District, Redwood City, CA.