External Pacemaker

Anatomy and Physiology

Normal electrical conduction through the heart originates in the sinoatrial node. This is located in the superior aspect of the right atrium. Conduction then spreads through the atrium to the atrioventricular node at the inferior portion of the right atria. From there, it travels down the His bundle, followed by the right and left bundle branches in the interventricular septum. Lastly, it spreads across the ventricles via the Purkinje fibers. While in external cardiac pacing, pacer pads activate a focal point in the left ventricle. Electricity then spreads to the rest of the ventricular myocardium from that focal point. Direct capture of the atria is very difficult to obtain regardless of pad placement. Indirect capture can occasionally be obtained via retrograde conduction through the atrioventricular node. Compared to normal atrioventricular pacing, transcutaneous cardiac pacing provides greater cardiac output due to strong diaphragmatic and skeletal muscle contractions despite decreased left ventricular systolic pressure and a reduced stroke index.[4]

Indications

External pacing is the fastest way to synchronize cardiac rhythm in all bradyarrhythmias, as the American Heart Association (AHA) indicates.[5] This temporary method maintains cardiac output in all those irreversible conditions requiring a permanent pacemaker or those reversible situations where permanent pacing is contraindicated (see Image. Pacemaker Table).[6] Bradycardia is a heart rate of less than 60 beats per minute. However, not all cases of bradycardia require pacing. Rather, transcutaneous pacing should only be applied in situations that include systolic blood pressure less than 90, heart rate less than 40, or if an arrhythmia compromises organ perfusion. Before the initiation of pacing, atropine can be given to improve or reverse the bradycardia. Standard dosing is 0.5 milligrams intravenously. However, larger doses can be given to a patient who shows no improvement with the standard dose.

External pacing can be done in patients having Atrioventricular Nodal dysfunction caused by an acute myocardial infarction, chest or cardiac trauma, infections (Lyme disease or bacterial endocarditis), sepsis, etc. Bradycardic patients secondary to electrolyte disturbances, metabolic abnormalities, drugs (such as beta-blockers), or hypothermia are also strong candidates for this pacing method.[7] It is important to emphasize that external pacing never replaces permanent invasive transvenous pacing. Third-degree heart block, a particular type of AV block characterized by an unstable rhythm with a high risk of degeneration to ventricular tachycardia or ventricular fibrillation, is also an indication of transcutaneous pacing.

Certain cases of ventricular tachycardia such as postoperative atrial flutter, Torsades de Pointes may also be improved with external pacing after all other options have been exhausted. Recurrence of the arrhythmia was prevented from recurring in 14 of 16 cases using transcutaneous overdrive pacing.[8][9][10]

Contraindications

Generally, cardiac pacing should not be considered in asymptomatic hemodynamically stable patients to treat bradycardia. No definitive contraindications exist for external pacing when clinically indicated.[11] Some negative sequelae associated with cardioversion have been seen. These outcomes include failure of an implanted ventricular lead and an implanted atrial lead. While these adverse reactions are from a very limited number of cases secondary to cardioversion, it is possible to extrapolate that external pacing may result in the same complications.

Preparation

If possible, the skin should be prepared before placing the pacer pads. Hair should be removed but not shaved, as this can create a nidus for infection. The skin should be cleansed with an alcohol wipe to remove dirt, debris, or sweat whenever possible to ensure maximum conduction between the pads and the skin.

Technique or Treatment

Pacing pads are positioned on the patient's chest in either anterolateral or anteroposterior. Multiple variations of pacer pad placement are equally effective. "Negative electrode in the left parasternal region, the positive electrode in the right subscapular region; a negative electrode in the left parasternal region, a positive electrode in the left subscapular region; or negative electrode at the cardiac apex or the position of lead V3, a positive electrode in the right parasternal region." As long as the apical/anterior pad is negative, precise electrode placement is not essential. The rate should be set between 60 to 90 beats per minute with the electrical output, also known as the current, set to its lowest setting. The rate should slowly increase until a pacer spike is seen on the monitor. Continue to increase the rate until a QRS complex follows each spike. This indicates electrical capture. Confirmation of mechanical capture should be done by feeling for a pulse. Mechanical capture can also be confirmed using ultrasound and observing ventricular contraction. If the patient is conscious, little sedation can reduce discomfort associated with cardiac pacing.[12]

Complications

Pacing may significantly reduce stroke volume in subjects with normal left ventricular function. This change was also seen in patients with heart failure. Both external and internal pacing reduced ejection fraction. There were no significant differences between the reduction caused by external pacing and the internal reduction caused by pacing. The rate of pacing can be increased to maintain approximate cardiac output. 

Side effects of transcutaneous pacing include cutaneous burns and muscle contractions. Superficial injury from the pacing pads resembles mild folliculitis. It is well known that muscle contractions can be extremely painful. To the point that sedation/anesthesia may be required. While the muscle contractions can be painful, no skeletal or myocardial muscle injury (as measured by CK, CK-MB, and troponin blood levels) developed after 30 minutes of pacing at 38 to 70mA with rates 10-20% above resting heart rate. Most cases of cutaneous burns are mild. However, many cases of third-degree burns have been reported in neonates, children, and geriatric age groups after cardiac pacing.[13][14] The risk of burns likely increases when pacer pads are used against manufacture instructions and are reapplied for multiple uses.

Two promising new ways of non-invasive pacing are currently being studied to mitigate the side effects of transcutaneous pacing techniques. The first uses high-intensity frequency ultrasound to stimulate pacing. The second uses iron microparticles injected intravenously and a generated magnetic field to pace in a concept known as mechano-electrical feedback.[10][15]

Clinical Significance

External pacing is 1 of the quickest and easiest methods for treating multiple conduction abnormalities. Everything from bradycardia to ventricular tachycardia can be managed with external pacing until definitive treatment can be implemented. Furthermore, it can be used under various circumstances, ranging from drug-induced electrical disturbances to conduction system inhibition secondary to myocardial infarction. It can be applied in the hospital setting or used by emergency medical service (EMS) providers when transporting critically ill patients to maintain an adequate cardiac output to ensure enough organ perfusion.[6]

Enhancing Healthcare Team Outcomes

External pacing is a procedure that is sometimes required on the medical and surgical floors. Clinicians must closely monitor the patients; to do so, they must be familiar with external pacing and its indications. Before the pacing is initiated, the patient should be educated about potential complications such as burns or failure to capture. The patient should also be informed about the potential discomfort of tingling or small shock-like nerve impulses. The clinician has to be fully aware that transcutaneous pacing may not always work and must be prepared to have a transvenous pacer system kit and atropine at the bedside. Finally, the clinician should ensure that no person may use their mobile phones or other electronic devices near the patient as this may interfere with the pacing unit. An appropriate warning sign needs to be placed on the door.[16][17][18] Studies have shown that the hemodynamic effects are similar between external and internal right ventricular pacing. However, currently available studies do not show that transcutaneous pacing improves patient outcomes. The data available is very limited. Most studies available have sample sizes of less than 150 patients and the time to apply the pacing was over 20 minutes. Larger studies with earlier application times are needed to confirm the effect of transcutaneous pacing on patient outcomes.[19] Healthcare outcomes can also be improved by good coordination and communication between the health professional team. A strong doctor-patient relationship must be there.