Cardiac Pacemakers: Part 1
- Mulpuru SK, Madhavan M, McLeod C, Cha YM, Friedman PA.
- Cardiac Pacemakers: Function, Troubleshooting, and Management: Part 1 of a 2-Part Series. J Am Coll Cardiol 2017;69:189-210.
The following are key points to remember from part 1 of a 2-part series about cardiac pacemakers:
- Up to 50% of the current drain from the battery is used for pacing, whereas the other half is used for sensing and housekeeping functions. The relationship between voltage, current, and resistance is defined by Ohm’s Law (V = IR), where V = voltage, I = current, and R = resistance. Because permanent pacemakers generate a constant voltage, the higher the pacing resistance, the lower the current drain, and the lower the rate of battery depletion.
- In response to a sensed intracardiac signal, a pacemaker may inhibit output, trigger output, or pace in a different chamber after a timed delay. In a five-letter designation, first position identifies the chamber paced (A for atrium, V for ventricle, D for dual/both), the second position indicates the chamber sensed, the third position denotes the device response to sensed events (Inhibit, Trigger, or Dual [both]), the fourth indicates whether rate response is on, and the fifth position (when used), indicates whether multisite pacing is employed in the atrium (A), ventricle (V), or both (D). With triggered pacing, a sensed event may trigger pacing in the same chamber or, typically after a programmed delay, in the other chamber.
- DDD is used when the sinus mode is intact, but atrioventricular (AV) conduction is impaired. Sinus activity is sensed and triggers ventricular pacing following a programmed AV delay. VOO/DOO are asynchronous modes, which are used to prevent oversensing during episodes of electromagnetic interference (EMI), such as during magnetic resonance imaging (MRI) or electrocautery. In dual-chamber systems, an atrial tachyarrhythmia sensed in the DDD mode can lead to ventricular pacing at rates up to the upper rate limit because atrial events are tracked in the ventricle. Mode-switching algorithms switch to a nontracking mode (VVI, DVI, or DDI). Mode-switch events indicate the potential need for anticoagulation.
- Right ventricular (RV) pacing is associated with cardiomyopathy in up to 20% of patients with frequent RV pacing among patients with preserved ejection fraction. Male sex, wide native QRS complexes, and frequent RV pacing (>20%) are predictors of RV pacing-associated cardiomyopathy. Algorithms to avoid or minimize RV pacing include use of an atrial pacing mode (AAI), with an automatic switch to ventricular pacing (DDD) if AV conduction fails, and AV interval prolongation to supra-physiological values, with shortening if intrinsic conduction does not manifest.
- Rate response (also called rate-adaptive pacing) refers to an increase in the pacing rate in response to physical, mental, or emotional exertion. Rate-adaptive pacing improves exercise capacity in patients with chronotropic incompetence. The most common sensor used to drive rate response is the accelerometer. Minute ventilation sensors transmit a low-energy current from the lead tip to the pulse generator to measure the variations in pulmonary impedance that occur with respiration.
- Conduction system disease is not uncommon in muscular dystrophies. Given the unpredictable disease course, a Class I indication for pacing is present when second- or third-degree AV block is seen, irrespective of symptoms.
- Uninterrupted anticoagulation with warfarin lowers the rate of hematoma formation compared with perioperative heparin bridging. The optimal use of periprocedural novel oral anticoagulants remains unresolved and, when possible, they are discontinued 2-3 days before pacemaker placement.
- The lead is the most common pacemaker system component to fail. Conductor fracture typically results in nonphysiological noise caused by the lead itself and can be associated with high lead impedance. An insulation break results in low impedance and oversensing of signals generated by surrounding structures (e.g., muscles) as conductors are exposed. Acute venous entry angle, medial venous access near the costoclavicular ligament, sharp turns in the pocket, young age, subpectoral placement of device, tight sutures, and silicone insulation are risk factors associated with lead fracture and insulation break. When bipolar thresholds are elevated or sensing is impaired (for example, due to a damaged conductor to the ring electrode), unipolar pacing or sensing may be used to avoid re-operation.
- The risk of pacemaker infection is higher for generator changes than for the initial implant procedure. Diabetes, heart failure, renal failure, corticosteroid use, postoperative hematoma, lack of antibiotic prophylaxis, oral anticoagulation, previous infection, and use of a temporary pacemaker are known risk factors for device infection.
- Pacemaker-mediated tachycardia (PMT) is usually caused by premature ectopic beats or paced beats, which when conducted retrograde to the atria set up a circuitous cycle of ventricular pacing at the upper tracking rate. PMT is terminated by magnet application or by use of PMT algorithms.
- In dual-chamber pacing mode, the upper rate is determined by the total atrial refractory period (TARP), which is a combination of AV delay and the PVARP. If properly programmed, the device tracks in a 1:1 fashion until reaching the programmed upper rate. As the atrial rate increases, ventricular pacing cannot violate the upper rate limit, resulting in progressively longer AV intervals. This is referred to as pacemaker Wenckebach. However, if the TARP is excessively long, abrupt 2:1 block may develop with a sudden slowing of the ventricular rate, which may cause symptoms. Undesirable upper rate behavior is avoided by programming to allow a Wenckebach interval between the maximum tracking rate and the 2:1 block rate, and by allowing rate-adaptive shortening of the AV and PVARP intervals.
- The risk of electrosurgical EMI depends on the surgical site and dispersive pad location, with highest risk for surgery of the heart and chest, followed by the head and neck, shoulder/upper extremity, and abdomen-pelvis. When surgery is performed below the umbilicus in pacemaker-dependent patients, or when significant pacing is anticipated during the procedure, the dispersive pad is placed on a lower extremity and pacemaker reprogramming is unnecessary. When the surgical site is above the umbilicus, preoperative pacemaker interrogation is performed, and the device reprogrammed to an asynchronous mode for the surgery (VOO, AOO, or DOO); magnet application (which causes the pacemaker to enter an asynchronous mode while the magnet is applied) is an alternative.
- While historically pacemakers and implantable cardioverter-defibrillators (ICDs) were a contraindication to MRI, all major manufacturers have developed MRI-conditional pacemakers and ICDs. MRI-conditional devices have minimal ferromagnetic material, altered filtering, and have re-designed lead conductors to minimize current induction and heating of tissue. Device requirements include: MRI conditional device paired with MRI-conditional leads, and absence of abandoned, fractured, or epicardial leads. The device must be implanted in the pectoral region. Continuous monitoring during the scan (electrocardiogram, hemodynamics, and symptoms) by an advanced cardiac life support-trained provider, and interrogation before and after scanning are required.
Clinical Topics: Anticoagulation Management, Arrhythmias and Clinical EP, Cardiac Surgery, Heart Failure and Cardiomyopathies, Invasive Cardiovascular Angiography and Intervention, Noninvasive Imaging, Prevention, Implantable Devices, EP Basic Science, SCD/Ventricular Arrhythmias, Atrial Fibrillation/Supraventricular Arrhythmias, Cardiac Surgery and Arrhythmias, Cardiac Surgery and Heart Failure, Acute Heart Failure, Interventions and Imaging, Magnetic Resonance Imaging
Keywords: Anticoagulants, Arrhythmias, Cardiac, Atrioventricular Block, Cardiac Surgical Procedures, Cardiomyopathies, Defibrillators, Implantable, Electric Impedance, Heart Conduction System, Heart Failure, Hematoma, Magnetic Resonance Imaging, Pacemaker, Artificial, Risk Factors, Secondary Prevention, Tachycardia
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