TSRA Content:
Author: Benjamin Kramer, MD
Definition and Function
A pacemaker (PM) is a temporary or permanent system used to pace the heart using electrical impulses in the setting of an inadequate, or absent, native rate and/or rhythm. This may be combined with an implantable automatic cardioverter-defibrillator (AICD) to deliver corrective defibrillation for unstable cardiac rhythms (ventricular tachycardia or fibrillation). The main goal of a temporary pacemaker is to support the patient’s heart to provide a perfusing rhythm until native activity/rhythm/rate return, or until a permanent pacemaker can be implanted.
Device Structure
Figure 1 Pacemaker structure. A. Output Dial – control the amount of energy being delivered, B. Rate Dial – controls the rate at which electrical impulses are delivered, C. Sensitivity Dial – Controls the ability of the generator to sense native electrical cardiac activity, D. Power Control – Turns device on and off. Left image provided by https://thoracickey.com/pacemakers. Right image of Medtronic Dual Channel Temporary External Pacemaker.
Moving external to internally, a temporary pacemaker is fundamentally two parts:
- Power Source/Pulse Generator. This can be external or internal and is commonly housed within a plastic box or sometimes a metal/titanium shell. The pacer settings are determined by the input selections. There are also outlets for the pacing leads that represent the chambers being sensed and paced.
- Leads. This are insulated wires that connect the generator to the epicardium (when leads are placed during open surgery) or endocardium (when leads are placed endovascularly).
The pacemaker will discharge a certain amount of electrical energy (V, Volts) at certain intervals, known as the pulse width (ms, milliseconds), in order to produce an electrical response by the heart. The minimum amount of energy required for the pacemaker to produce an electrical response is known as the “capture” threshold. Lower capture thresholds are desired, ideally < 0.7V to obtain an R-wave amplitude >5mV.
The ”sensing” threshold is defined as the minimum amount of electrical activity the heart must produce for the pacemaker to detect it. Depending on the lead placement, atrium vs. ventricle, sensing thresholds may vary. Lower sensing thresholds are desired. Normal thresholds are 0.4-10mV for atria and 0.8-20mV for the ventricles.
Indications for Temporary Pacemaker Placement
Guidelines for the placement of pacemaker were updated by the AHA in collaboration with the AATS and STS in 2018
- Post cardiotomy with concern for transient, or permanent, conduction injury
- Elevated Risk of important bradyarrhythmia
- Bradycardia secondary to an acute and reversible cause (acute MI, electrolyte imbalance, drug toxicities, infectious causes)
- Symptomatic sinus bradycardia with sinus pauses >3 sec or heart rate < 40 bpm
- Ventricular Asystole
- Alternative left and right bundle branch block
Pacing Modes
Pacing modes are denoted according to a 5-letter international code defined by the North American Society of Pacing and Electrophysiology (NASPE) and the British Pacing and Electrophysiology Group (BPEG). The positions define the following and can be found in Table 1.
I. Chamber(s) Paced – Which chamber will be stimulated: Atria (A), Ventricle (V), Dual (D), or none (O).
II. Chamber(s) Sensed – Which chamber is being senses/monitor for electrical activity: Atria (A), Ventricle (V), Dual (D), or none (O).
III. Pacemaker Response to Sensing – What will the device do if it senses an electrical signal (native activity): Triggered (T), Inhibited (I), Dual (D), or none (O)
IV. Rate Modulation – Will the rate of the pacer change based on environmental and patient demands: Rate modulation (R), or none (O). Permanent pacemakers are commonly set to R.
V. Multisite Pacing – Indicated the location of multisite pacing: Atria (A), Ventricle (V), Dual (D), or none (O).
Although all five positions may be needed for completeness in some circumstances, the first three positions are always required, providing the most critical information for pacemaker activity.
Table 1. NASPE/BPEG Generic Codes for Pacemaker Settings, Updated 2021.
Temporary epicardial pacemakers placed following cardiac surgery historically were limited in their settings. However, today’s devices can be set to most combinations of the above settings to appropriately respond and support patients postoperatively.
Common Settings Include: Atrial pacing (AOOO), Ventricular pacing (VOOO), Dual Pacing (DOOO), Atrial Pacing supported by native rhythm (AAIO), Ventricular Pacing supported by native rhythm VVI(O), or Dual pacing, dual sensing (DDDO). In the setting of complete heart block, either postoperatively or secondary to cardiomyopathy, the settings of DDD(R) or VVI(O) (if concomitant atrial fibrillation) are commonly used. For patients with comprised AV-node conduction or a history of Wolff- Parkinson-White syndrome, careful consideration should be made if using atrial pacing settings.
Pacemakers equipped with AICDs can detect tachyarrhythmias, administer treatment, and assess the response. Based on the rhythm detected by the pacemaker the device with deliver either set pacing, low- energy synchronized cardioversion, or high-energy defibrillation.
Surgical Considerations
Patients with implanted pacemakers undergoing surgery should have their device augmented prior to surgery to prevent erroneous activity from electrocautery. A specialized magnet can be placed over the device to alter the settings to inactivate the sensing function, changing or mode to AOO, VOO, or DOO until removed.
Procedural Technique
When considering pacemaker placement, preoperative evaluation should always include assessment of the venous system to ensure patency, history of radiation therapy to upper extremities and/or the chest, and handedness of the patient. Temporary pacemakers can be placed transcutaneously, transvenously, or epicardially during surgery.
Atrial leads are usually placed in/on the right atrium, ideally close to the appendage. Ventricular lead position varies by operative approach. Epicardial leads are placed on the inferior aspect of the left ventricle away from coronary vessels and in a well exposed area of epicardium. Transvenous leads are typically placed in the ventricular septum or the interior apical aspect of the right ventricle, avoiding the free wall reducing the risk of rupture.
Pacing Spikes
Pacing spikes may be identified on EKG tracings to help determine and assess pacer settings and activity. These vertical spikes are usually of short duration (~2 ms) and may be difficult to see in all leads. The amplitude of the spikes depends on the position and type of lead. Epicardially placed leads result in smaller pacing spikes than endocardially placed leads.
Atrial Pacing
- Pacing spikes precede the p wave.
- Morphology of p wave is dependent on lead placement but may appear normal.
Ventricular Pacing
- Pacing spikes precede the QRS complex.
- Right ventricle lead placement results in a QRS morphology similar to LBBB.
- Left epicardial pacing lead placement results in a QRS morphology similar to RBBB.
Dual Chamber Pacing
- Dependent on areas being paced.
- May exhibit features of atrial pacing, ventricular pacing, or both.
- Pacing spikes may precede only p wave, only QRS complex, or both.
Figure 2 Assessing pacing modes based on EKG. Atrial and ventricular pacing spikes are visible before each QRS complex. There is 100% atrial capture — small P waves are seen following each atrial pacing spike. There is 100% ventricular capture — a QRS complex follows each ventricular pacing spike. QRS complexes are broad with a LBBB morphology, indicating the presence of a ventricular pacing electrode in the right ventricle. EKG provided by Life in the Fast Lane (https://litfl.com/pacemaker-rh... normal-patterns/)
Complications
The incidence of significant complications is low (~2%) for transvenous devices.
Early complications are typically identified to be injuries to systemic veins (superior vena cava, inferior vena cava) or right sided cardiac structures (right atrial perforation, traumatic tricuspid regurgitation, or tamponade due to perforation). Documented noncardiac injuries include pneumothorax, hemothorax, air embolus, venous thrombus, and arrhythmias. For surgically placed epicardial leads, when leads are being removed careful attention to the risk of bleeding should be noted. If a mediastinal chest tube remains, when feasible, leads should be pulled before the chest tube is removed.
The most common late complication for implanted devices is infection (up to ~3%). This can involve the implanted leads or power source and usually requires removal of the infected material and replacement, bridged with temporary pacemaker support until the infection is cleared.
References/Suggested Reading:
- Bernstein, Alan D., et al. "The revised NASPE/BPEG generic code for antibradycardia, adaptive‐ rate, and multisite pacing." Pacing and clinical electrophysiology 25.2 (2002): 260-264.
- Kusumoto, Fred M., et al. "2018 ACC/AHA/HRS guideline on the evaluation and management of patients with bradycardia and cardiac conduction delay: executive summary: a report of the American College of Cardiology/American Heart Association Task Force on Clinical Practice Guidelines, and the Heart Rhythm Society." Journal of the American College of Cardiology 74.7 (2019): 932-987.
- https://litfl.com/pacemaker-rhythms-normal-patterns/
- Bojar, Robert M. Manual of perioperative care in adult cardiac surgery Chapter 11 Cardiovascular Management. John Wiley & Sons, 2020: 517-528.