Single-chamber ventricular pacing

If the procedure is to proceed in a smooth and expeditious fashion, careful preoperative planning is essential. The first such decision is whether the patient requires a single-chamber or dual-chamber pacemaker. As a rule, if the patient has intact atrial function, every effort is made to preserve atrial and ventricular relationships. Single-chamber ventricular pacing is usually reserved for the patient with chronic atrial fibrillation or atrial paralysis. A device is selected with acceptable size, longevity, and progranunability. If the heart is chronotropically incompetent, a device that offers some form of rate adaptation... 

Hesselson AB, Parsonnet V, Bernstein AD, Bonavita GJ. Deleterious effects of long-term single-chamber ventricular pacing in patients with sick sinus syndrome the hidden benefits of dual-chamber pacing. J Am Coll Cardiol. 1992 19 1542-1549. 

Fig. 8.4 Single chamber ventricular pacing (spikes before QRS complex)...

Fig. 18.28 Posteroanterior chest radiograph (A) and close-up view (B) from a patient with congenital heart disease and an abandoned dual-chamber endocardial pacing system. Two ventricular epicardial leads are connected with a Y connector to a single chamber ventricular pacemaker. On the close-up view (B) the arrow notes a defect in the lead adaptor just as it exits the connector block.

Nielsen JC, Andersen HR, Thomsen PE, et al. Heart failure and echocardiographic changes during longterm follow-up of patients with sick sinus syndrome randomized to single-chamber atrial or ventricular pacing. Circulation 1998 97 987-95. 

The thresholds of right and left ventricular pacing leads can be difficult to assess in ICDs that do not have separate programmability for each output. This is because the morphology change of the QRS complex on a single surface ECG rhythm strip can be very subtle when capture is lost in a ventricular chamber. Also, since many of the patients for which a CRT device has been implanted have a native left bundle branch block (LBBB) pattern on their surface ECG, the RV capture and native QRS complexes can look remarkably similar. 

The location of the spikes helps to identify the type of pacemaker system in situ. A spike before a P wave denotes atrial pacing, whereas one before the QRS complex identifies ventricular pacing. Spikes before both P waves and QRS complexes can be either dual chamber (atria and ventricle) or Bi-v pacing. As both chambers are synchronized in the Bi-v system only one spike is usually visible. Figures 8.4 and 8.5 show single and dual chamber pacemaker systems. 

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