Single chamber cardiac pacers

Single chamber cardiac pacemakers are cardiac conduction devices with one lead terminating in (most commonly) the right ventricular apex or the right atrium. 

Components

Includes one of the following:

•  lead in the right atrium

•  lead in the right ventricle

Function

Single chamber atrial pacemakers are uncommonly placed, as they require intact atrioventricular conduction and do not safeguard against its future deterioration. Indications include severely symptomatic sinus bradycardia and the sick sinus syndrome (with the tachycardia-bradycardia syndrome). Lead placement is typically in the right atrial appendage.

Single chamber ventricular pacemakers have also fallen out of favor, eclipsed largely by dual-chamber pacemakers which have leads in both the right atrium and right ventricle. Lead placement should be in the right ventricular apex. Indications for permanent single-chamber ventricular pacing include ³:

• sinus node dysfunction

• advanced atrioventricular (AV) block

• neurocardiogenic syncope

• cardiac transplant

• hypertrophic cardiomyopathy

  • with left ventricular outflow tract obstruction

• cardiac sarcoidosis

  • with atrioventricular block

Temporary transvenous pacing commonly relies on single-chamber pacing, however, with the intended location of the lead tip in the right ventricular apex. Indications for temporary single-chamber ventricular pacing often referred to as "transvenous pacing" include ¹:

• bradycardia

  • when associated with myocardial infarction (MI)

  • not associated with MI

    • ventricular dysrhythmias

    • high grade or complete AV block

  • risk of post-procedural bradycardia

• tachycardia (overdrive suppression)

  • supraventricular tachycardia (SVT)

  • ventricular tachycardia (VT)

ECG

On ECG, atrial pacing will result in a sharp, vertical pacemaker spike followed by an ectopic P wave with a location-dependent morphology; when implanted in proximity to the sinoatrial (SA) node, it will appear upright (positive polarity) in the limb leads, with the exception of aVR.

Ventricular pacing from the right ventricular apex will result in an abnormal right-to-left depolarization of the ventricular myocardium, resulting in a left bundle branch-like morphology of the QRS complex. Depolarization will also occur from apex-to-base, resulting in an "extreme" frontal axis deviation (with negative QRS complexes in the inferior leads) ².

Modes

The modes of pacemakers are based on generic code known as NBG ( combined from NASPE/BPEG) and typically consist of 5 letters.

• 1^st letter:

  • area being paced

    • A = atria

    • V = ventricle

    • D = dual

    • O = none

• 2^nd letter

  • area which is sensed

    • A = atria

    • V = ventricle

    • D = dual

    • O = none

• 3^rd letter

  • response of the pacemaker to sensing

    • O = none

    • I = inhibiting

    • T = triggering

    • D = dual

• 4^th letter

  • rate adaptiveness

    • O = none

    • R = rate adaptiveness

Single chamber modes

• VOO: pacing in the ventricle; sensing is off; response to sensing is off

• VVI: pacing in the ventricle; sensing in the ventricle; inhibit

• AOO: pacing in the atrium; sensing is off; response to sensing is off

• AAI: pacing in the atrium; sensing in the atrium; inhibit