Pacemaker Changes Flashcard

B. Rationale: Undersensing produces multiple small spikes on ECG during motion or ambulation while recording the ECG. These spikes are produced by movement of the muscles. There are also absent pacemaker spikes, and ventricular asystole may be produced. Oversensing is produced during strenuous exercise, especially in younger individuals.

C. Rationale: In a one-chamber pacemaker, the artificial impulse is stimulated over the right ventricle. So the right ventricle is activated first, then the left ventricle over the septum. There will be a delayed impulse over the left septum, and this will produce a left bundle-branch block pattern on ECG. If the electrode was placed on the left ventricle, then a right bundle-branch pattern will be seen on ECG.

Answer choice A – Oversensing would result in an inadequate number of pacer spikes despite the patient’s low intrinsic heart rate.
Answer choice B – Undersensing would result in multiple unnecessary pacer spikes despite the patient’s intrinsic heart rate.
Answer choice D – If the pacemaker electrode were placed over the left atrium, a right bundle branch pattern would result.

C. Rationale: The ECG spike at the beginning of the ECG tracing refers to the stimulus from a one-chamber pacemaker. This spike is a must-know configuration and can only be produced by a pacemaker. The pattern will also depend on the impulses influenced by the pacemaker and the spontaneous impulses produced in the heart.

Answer choice A – Myocardial ischemia would manifest as ST depression.
Answer choice B – Myocardial infarction would manifest as ST elevation.
Answer choice D – Wolf-Parkinson-White syndrome is a rapid dysrhythmia that displays a characteristic delta wave on ECG.

C. Rationale: Cardiac pacing is indicated for patients with medically refractory and symptomatic hypertrophic obstructive cardiomyopathy. Dual-chamber pacing improves the symptoms in these patients by lowering the left ventricular outflow tract gradients. This is done by forcing the right ventricular pacing to move the ventricular septum to the right ventricle during systole, which results in a widening of the left ventricular outflow tract.

D. Rationale: Complete atrioventricular blocks and diseases involving sinus node dysfunction are the most frequent reasons for providing patients with chronic pacing. This patient population is responsible for about 40% of all pacemaker implantations. Other indications for pacemaker placement include bradycardia, atrial fibrillation, bifascicular blocks, and incomplete trifascicular blocks. Second-degree AV block, sick sinus syndrome, long QT interval syndromes, sleep apnea, hypertrophic obstructive cardiomyopathy, and heart failure with wide QRS complexes are also indications for pacemaker implantation.

Answer choice A – Occlusion of the left anterior descending artery will not affect the SA node and lead to the need for a pacemaker.
Answer choice B – Paroxysmal supraventricular tachycardia is not an indication for a pacemaker. It begins abruptly and ends without intervention.
Answer choice C – A history of atrial fibrillation is not an indication for a pacemaker.

B. Rationale: Undersensing is dangerous. A spike that falls on a vulnerable period, such as on a T phenomenon, can induce ventricular fibrillation. This has been documented, although very rarely, and is usually seen in patients with severe ischemia. Undersensing can be avoided if the threshold is set at a lower voltage, typically 1.5–2.5mV.