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Automatic mode switching is the ability of the pace-maker to automatically change from one mode to another in response to an inappropriately rapid atrial rhythm. In the DDD or DDDR pacing modes, if a supraventricular tachyarrhythmia occurs and the PMK senses the pathological rhythm, rapid ventricular pacing can occur. Mode switching avoids this limitation by switching from DDD or DDDR during sinus rhythm to a nontracking mode, such as DDI or DDIR, during the pathological atrial rhythm. Sometimes PMKs could generate themselves arrhythmias. For example in patient with AAI/R PMK previously implanted for sick sinus syndrome we can observe AV delays when the atrial rate is increased by rate responsive function. This may be due to a mismatch between the actual heart rate and the autonomic nervous tone (overstimulation) and may lead to a pacemaker syndrome. Sometimes we can observe an atrial retrograde conduction (PMK VVI) that generates a PMK syndrome. This phenomenon can also bring to atrial arrhythmias (atrial flutter or atrial fibrillation (AF)). Arrhythmias on surface ECG can be induced by undersensing, oversensing or failure to capture. Another interesting example could happen when we have an atrial stimulus sensed by the ventricle. In this case, we get a failure to pace with consequent asystolic pause (cross talk) if there is not a spontaneous ventricular depolarization. Sometimes, and it is very important especially in PMK dependent patients, we can see on ECG a ventricular spike within 110 msec of an atrial stimulus. This algorithm (ventricular safety pacing) avoids an asystolic pause in case of ventricular oversensing. In case of appropriate sensing the stimulus, falling during the absolute ventricular refractory period, will not give a ventricular depolarization.
ECG analysis in ICD patients
The ECG analysis can lead ourselves to the diagnosis of ventricular tachyarrhythmias based on the morphology of wide QRS tachycardia, but not with certainty. Obviously, this is a relevant problem in ICD patients. In single chamber ICD it could be difficult to discriminate a supraventricular (SVT) from ventricular tachyarrhythmias so the device can use some algorithms that analyze R wave morphology and the onset of the tachycardia.
• Wavelet: discriminates ventricular tachycardia (VT) from SVT with rapid conduction
• Onset: discriminates VT from sinus tachycardia
• Stability: discriminates atrial fibrillation with rapid conduction from VT