The following are key points to remember from part 2 of a 2-part series on advances and future directions in cardiac pacemakers:

Cardiac Resynchronization Therapy (CRT)

1. The presence of left bundle branch block (LBBB) pattern remains the most powerful predictor of CRT response. The wider the QRS complex, the greater the likelihood of response. A very wide right bundle branch block (RBBB) pattern (i.e., >150 ms) often reflects a delay in both bundles, and CRT may be effective.
2. Women are more likely to benefit from CRT than men, particularly when the QRS duration is <150 ms. When patients with depressed ventricular function and a pacemaker manifest an LBBB that is caused by frequent right ventricular (RV) pacing, upgrading to a CRT system often improves ventricular function.
3. There is strong evidence to support CRT use in patients with NYHA class II heart failure (HF) and higher.
4. Biventricular pacing can reasonably be considered in patients who are anticipated to require a high percentage of ventricular pacing and have ejection fraction ≤50% with mild HF symptoms.
5. Three multicenter trials failed to show substantial improvement in CRT response with dyssynchrony assessment by echocardiography, and the EchoCRT study found increased mortality in patients with a QRS complex <130 ms and echocardiographic dyssynchrony.
6. In terms of coronary sinus lead location, posterior and lateral positions are generally preferred, and apical positions should be avoided. Maximizing the distance between the RV and LV electrodes is also associated with better CRT response. The site of latest electrical local left ventricular (LV) activation also constitutes a preferred pacing site.
7. Endocardial LV pacing, as opposed to LV epicardial pacing, may be preferred due to greater flexibility in selecting the site of lead implantation, absence of phrenic nerve stimulation, and more physiological LV activation. Endocardial LV pacing using conventional pacing leads placed transapically, or through the interatrial or interventricular septum, has been described. This technique is limited by a prohibitive risk of systemic thromboembolism.
8. If pacing occurs between an LV electrode (cathode) and RV ring electrode (anode), and myocardial stimulation occurs only at the RV anode, effective CRT is not delivered. This scenario is known as anodal-only capture.
9. Frequent premature ventricular contractions (PVCs) interfere with CRT and may independently worsen HF due to dyssynchrony. Treatment with beta-blockers, membrane-active antiarrhythmic drugs, and catheter ablation of PVCs may improve CRT response. In patients with permanent atrial fibrillation (AF) who need CRT, a reasonable approach is to start with pharmacological rate control and rapidly escalate to atrioventricular (AV) node ablation if >99% biventricular pacing is not achieved with medications alone.
10. Trials of routine CRT optimization using echocardiography and device-based AV and interventricular (VV) interval optimization have not been shown to improve outcomes.

Para-Hisian Pacing

1. His-bundle capture enables rapid activation of the ventricles by engaging the Purkinje network and results in a narrow QRS complex. This can be achieved with a small-caliber pacing lead (Select Secure Model 3830, Medtronic) delivered through specially designed sheaths (C 315 HIS).

Remote Monitoring

1. Remote monitoring checks patient and device status on a daily basis, is fully automatic, and can verify transmissions and generate alerts. Detection of asymptomatic AF using remote monitoring permits timely therapy, including the introduction of anticoagulation therapy to prevent stroke in at-risk individuals. The use of remote monitoring has been associated with a lower mortality in pacemaker, implantable cardioverter-defibrillator (ICD), and CRT recipients.
2. Current ICD and CRT devices can monitor for HF using thoracic impedance as a surrogate for pulmonary congestion. Although these programs can provide early warning of worsening HF, their use has not been shown to improve HF clinical outcomes and survival.

Leadless Pacing

1. Two leadless pacemakers have been widely tested in humans: the Nanostim (St. Jude Medical) and the MICRA (Medtronic). It is anticipated that the leadless pacemakers may reduce infections and pocket complications, and most certainly lead complications. The major limitation of current-generation leadless pacemakers is their ability to only perform single-chamber ventricular pacing.
2. Leadless endocardial LV pacing holds promise in that it may be more physiological, afford greater opportunities for LV pacing site selection, lead to a greater CRT response with lower risk of proarrhythmia, eliminate phrenic nerve stimulation, and mitigate against the risks of mitral regurgitation and lead-related thrombus.
3. The WiSE-CRT study tested the feasibility of LV endocardial pacing with a tiny receiver electrode implanted in the LV and triggered by a subcutaneous pulse generator emitting ultrasound pulses. While the study was terminated early, it demonstrated the proof of concept.

Keywords: Arrhythmias, Cardiac, Heart Failure, Secondary Prevention, Anti-Arrhythmia Agents, Atrial Fibrillation, Bundle-Branch Block, Cardiac Resynchronization Therapy, Cardiac Resynchronization Therapy Devices, Catheter Ablation, Defibrillators, Implantable, Echocardiography, Mitral Valve Insufficiency, Pacemaker, Artificial, Primary Prevention, Stroke, Thromboembolism, Thrombosis, Ventricular Function, Ventricular Premature Complexes

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