Transvenous pacing utilizes a central venous catheter (CVC), usually placed into the right internal jugular or left subclavian vein, which serves as an introducer to “float” a pacing wire through the venous system utilizing a small balloon, terminating into the apex of right ventricle. Transvenous pacers (TVPs) typically have both atrial and ventricular electrodes which permit pacing in a physiological, dual chamber manner to promote AV synchrony, optimizing cardiac output. Transvenous pacing is very well tolerated and comfortable for the patient and does not require sedation, if secured well is less likely to fail or lose capture compared to transcutaneous pacing and has the added benefit of utilizing a pacer box to control pacing output.

TVP placement complications is not without risk. Complications may be related to venous access (e.g. arterial puncture, pneumothorax, etc.), right heart catheterization (e.g. dysrhythmias, atrial or ventricular perforation, misplacement of the wire into the coronary sinus, valvular damage, etc.), the pacing wire itself (e.g. wire fracture, displacement, fibrosis, infection), and the external generator.

Indications for prophylactic TVP placement

If a patient has required transcutaneous pacing and time and resources permit, a TVP should be placed urgently, ideally in the first 24 hours. Certain pathology also predisposes patients to arrythmias, and often a TVP should be place prophylactically prior to transport. Myocardial infarctions (MIs) affecting the inferior or posterior aspects of the myocardium can result in complete (3rd degree) heart block in up to 5% of patients. These patients may also have a high degree AV block (P wave to QRS complex ratio of ≥ 3:1), which is usually transient and less likely to be symptomatic. Right sided MIs often affect the SA node and conduction system, resulting in symptomatic bradycardia requiring pacing. An anterior-lateral MI may also result in high grade AV blocks, new bundle branch block, particularly the left bundle branch, or bi-fascicular blocks. Prior to transport a Cardiology consultation may be helpful to guide prophylactic TVP placement.

Epicardial Pacing

Epicardial wires are placed during cardiothoracic surgery and sutured directly into the epicardium and exit through the skin and are covered by a dressing. By surgical convention, atrial (A) wires emerge on the patient’s right, and ventricular (V) wire emerge on the patient’s left. These wires will connect to the pacemaker box using a special cable with a screw in connector that will accept the wires placed by the surgeon. Patients will always have V wires but will not uncommonly have A wires as well; a patient will never have only A wires. Pacer box usage is the same for both epicardial or transvenous pacing, with the exception being if the patient does not have atrial epicardial wires.

Transvenous Pacer Box nomenclature and modes

1. A generator box is used for both transvenous and epicardial pacing. The first letter defines the area of the heart paced: atrium (A), ventricle (V), or both atrium and ventricle (D).
2. The second letter defines the area of the heart sensed: atrium (A), ventricle (V), both atrium and ventricle (D), or neither (O).
3. The third letter defines what the pacemaker does with the sensing information. Sensing can result in:

- I (Inhibited): The pacemaker being inhibited from firing
- O (None): No effect on how the pacemaker fires
- D (Dual): Both the atria and ventricle are paced
- A (Atrium): Only the atrium is paced
- V (Ventricle): Only the ventricle is paced

Ventricular demand pacing is the mode of choice during patient evacuations because attaining the correct electrode position when “floating” the transvenous pacemaker is relatively simple. Although the loss of AV synchrony can result in about a 20% decrease in cardiac output, it is generally tolerated by most patients, apart from those with significant myocardial depression. It is also the least sensitive to small changes in catheter positions.

- In mode VVI (synchronous mode), the ventricle is paced, as well as sensed. If the pacemaker’s pulse generator senses an impulse created by the heart, the pacemaker’s impulse is not sent, thus the pulse generator is inhibited. This mode is used to prevent R-on-T complications when there is an underlying ventricular rhythm. This is the preferred mode, unless electromagnetic interference (EMI) is present.
- In mode VOO (asynchronous mode), the ventricle is paced at a fixed rate and there is no sensing; electrical impulses are delivered regardless of the underlying ventricular rhythm. This mode is best used if electrocautery is being used (during surgery), and during periods of excessive electrical stimulation from EMI due to aircraft electronics. The rate with VOO must be greater than the patient’s intrinsic heart rate to avoid R-on-T which can result in ventricular tachycardia (VT) or ventricular fibrillation (VF).

Transvenous pacemaker modes to use for conduction pathologies are summarized in Table 1 below: