Ventricular sensing defects

ECG: The electrocardiogram shows atrial fibrillation; the spontaneous QRS complexes are not detected and do not inhibit ventricular pacing which is effective; ventricular sensing defects;

Comments: This tracing reveals ventricular undersensing, sometimes leading to pacing during a vulnerable period. In the absence of proper sensing, the pacemaker operates in asynchronous mode with no possibility of inhibition by intrinsic ventricles. There is a small  risk of inducing a malignant polymorphic ventricular arrhythmia compromising patient survival. Moreover, this unnecessary pacing leads to energy consumption that is detrimental to the life of the device.

The sensitivity expressed in millivolt (mV) depicts the ability of the pacemaker to properly sense spontaneous cardiac events. A pacemaker is equipped with input filters that specifically detect P waves in the atrium and R waves in the ventricle based on the analysis of three characteristics of these electrical signals: rate spectrum, slope and amplitude.

Adequate programming of the sensitivity level should sense all spontaneous cardiac events occurring in the implanted chamber while not sensing events of a different nature (cross-talk with sensing of cardiac signals from the other chamber, myopotentials, interference, etc.). The programming of a bipolar sensing allows increasing the specificity of the sensing relative to unipolar sensing by limiting the risk of detecting extra-cardiac or crosstalk signals and allows the programming of high sensitivity values. On the other hand, in the unipolar configuration, the risk of crosstalk or the detecting of extra-cardiac signals requires the programming of a lower sensitivity level with an increased risk of undersensing.

Traditionally, unlike defibrillators, pacemakers initially operated with a stable and fixed sensitivity over the entire cardiac cycle. Although the constraints in terms of sensing are dissimilar (a crucial necessity for a defibrillator to sense and treat very rapid, polymorphic and low-voltage ventricular rhythm disorders), modern-day pacemaker increasingly allow an adaptive sensitivity (variable level sensitivity as a function of the amplitude of the R wave or the detected P wave) with a gradual increase in  sensitivity during the cardiac cycle (possibility of detecting small amplitude signals without oversensing the T wave).

The programming of a better tailored adjustable ventricular sensitivity allowed correcting the problem in this patient.

Take-home message: Atrial or ventricular undersensing is reflected on the electrocardiogram by the presence of fixed pacing rate artifacts despite the presence of spontaneous complexes which fail to inhibit pacing.