Is “silent ischemia” detectable by endocardial pacemaker leads?

The question arose in a project (Body Worn) aimed at planning a prompt aid system for patients at high risk of acute cardiovascular events, using physiologic intracardiac parameters detected by an implanted pacemaker: in case of dangerous abnormalities, intracardiac electrograms (IEGMs) are transmitted to an emergency unit by means of a mobile phone, the patient is localized with a GPS and the intervention procedure is activated. Life threatening conditions such as tachycardia or myocardial infarction can be easily recognized, nevertheless the prompt detection of parameters that could alert for oncoming worsening conditions would be very useful to prevent an emergency allowing a more effective medical aid. The analysis of standard holter recordings showed that in most patients with heart diseases a state of emergency is often preceded by a “silent ischemia” episode. In current clinical practice the diagnosis of ischemia is based on the standard surface 12-lead ECG, which enables to localize and to quantify the extension of the ischemic area, while the detection of this pathological condition using IEGMs recording is almost unknown. To evaluate the possibility of using IEGMs as trigger signals for the Body Worn intervention algorithm, we compared the electrocardiograms recorded by endocardial and surface electrodes in a group of 10 patients undergoing a PTCA interventional procedure. The PTCA therapeutic procedure was chosen because during the inflation of the coronary balloon an ischemia of non perfused tissue is always detectable. In 10 patients undergoing PTCA a four electrodes catheter has been inserted in the right ventricle to detect 3 bipolar IEGMs at basal and during the PTCA procedure, and, at the same time, a surface 12-lead ECG has been acquired. The signals have been sampled at 500 Hz, with a resolution of 10 bit (Light Workstation, Spark s.r.l Bologna) and the S-T change, the T wave amplitude and polarity, the QTc and pNN50 values have been calculated. Results showed a good agreement between endocardial and surface ECG signals in ischemic events which involved the septal and the anterior ventricular wall, whereas the correlation was poor in ischemic events affecting the posterior ventricular wall. A different number and positioning of the endocardial electrodes may give a better response.