This patient presented for routine clinic evaluation of a dual chamber pacemaker. Please refer to Figure 1 for initial programming. The patient was in sinus rhythm, had a rate 83 BPM, and was asymptomatic.

Measured p-wave signals were 3-4 mv in amplitude. Atrial sensing was programmed to Auto, thus, the pacemaker sensing level would self-adjust to measure intrinsic atrial activity. This particular device uses a weighted average of the measured p-wave amplitudes to determine the average p-wave amplitude. The sensing level is also programmed to set itself at twice the estimated noise level. This feature provides protection from oversensing potentially resulting from electrical noise on a lead which may result in inappropriate inhibition of pacing. In this case, the three impulses preceeding the non-sensed beat were significantly larger, therefore setting the sensing window too big to see the circled p. The sensing window then became smaller and was able to sense the following impulse. Was this significant in terms of a negative outcome related to the pacemaker response? No, because the patient was in sinus rhythm and maintaining his own rhythm. However, to correct this problem, we took off the Auto sensing, and reprogrammed atrial sensitivity to a fixed 0.5 mv. Previously non-sensed p-waves were all sensed.

There are several timing schemes used by pacemaker devices, depending on the manufacturer. Alternatives are atrial- or ventricular-based and modified atrial or ventricular. The commonality is that after every sensed or paced ventricular impulse, an atrial escape interval (AEI) is initiated. By the end of this interval, if no atrial or ventricular impulses are seen, an atrial pacing stimulus will be delivered. The goal of the timing schemes is to keep the paced ventricular rate as smooth as possible. In this device which uses a form of modified atrial based timing, the AEI consists of the lower rate limit interval minus the programmed AV interval or the intrinsic PR interval (In this case, 1000 ms - 160 ms = AEI of 840 ms.). If no atrial activity is sensed at the end of the AEI timed from the last ventricular impulse, an atrial pacing output will occur. In Figure 2, the R-R interval around the non-sensed p is 720 ms, thus, atrial pacing did not occur because a sensed ventricular impulse occurred before the AEI timed out resetting the AEI timer.

A 55-year-old male received an implantable cardioverter-defibrillator (ICD) 6 months earlier for MADIT II criteria (coronary artery disease, ejection fraction < 30%, no documented arrhythmias). Four months after the implant, he had a shock preceeded by lightheadedness and near syncope. Figure 3 shows the electrograms retrieved from the device. Post-shock, the patient immediately felt fine.

Two months later, the patient was involved in a minor auto accident. While sitting on his car talking to the police, he received four shocks. He was asymptomatic from an arrhythmia standpoint. By the time the paramedics came and obtained a surface ECG recording, the patient was in sinus rhythm. The following electrogram for this event was later retrieved from the device (Figure 4).