Pacemaker Basics

Implantation of a permanent pacemaker is indicated for a variety of situations, and is summarised in the 2013 ESC guidelines. View link

Technical Issues related to pacemakers:

Device anatomy

Various forms of conventional pacing are available, and the choice depends predominantly upon the electro-anatomical site and nature of the conduction problem. These devices are delivered trans-venously, so pacing activity originates from the right heart.

Right atrial leads are usually positioned with the tip in the RA appendage. Traditionally, the RV lead is positioned at the RV apex.

Pacemaker nomenclature

Many of the terms used by specialist pacing services are difficult to appreciate, and not relevant to the ED assessment of these patients. However, understanding a limited glossary is useful.

• Pacing: delivery of a programmed electrical stimulus from the pacemaker to the myocardium via the implanted lead
• Capture: successful depolarisation of the myocardium from a pacemaker stimulus. Failure to capture leads to a pacing spike on the ECG, but no depolarisation
• Threshold: the lowest output voltage from the device that will reliably capture
• Oversensing and undersensing: the pacemaker will ‘see’ an enormous amount of electrical activity across a wide range of frequencies. Some will of course be generated by the hearts own electrical cycle, but much of the rest will come from non-cardiac sources such as skeletal muscle activity. If the device is too sensitive, it will see depolarisations that are not really there, and if it is not sensitive enough it will miss normal intrinsic depolarisations. It is helpful to remember these basic concepts:
  • Undersensing leads to overpacing
  • Oversensing leads to underpacing
• Lower Rate Limit (LRL): the slowest rate, or longest cycle length, that the device will allow the heart to drop to
• Upper Rate Limit (URL): the fastest heart rate, or shortest cycle length, that the device will pace at.
• Battery End Of Life (EoL): As the battery is depleted, replacement needs to be planned. With follow-up in specialised pacing clinics, it is rare for a battery to become completely exhausted and the unit to fail
• Hysteresis: This can be a difficult concept to understand, but can lead to a mis-diagnosis of pacemaker failure. In the simplest set-up, a pacemaker with an LRL of 50/min would start pacing as soon as the intrinsic rate fell to 49/min. This is undesirable, partly due to the unnecessary drain on battery life, but also because intrinsic activity is now known to be preferable to paced activity wherever possible. Hysteresis is the separation of the lower pacing rate from the lower sensing rate. In practical terms, this means that the device will pace at 50/min, but only when the intrinsic rate drops to, for example, 40/min. It is easy to see why this might give the impression that the device has malfunctioned, if the patients own rate is perhaps 42/min, but the pacemaker has not stepped in

Pacemaker classification (NBG Codes)

In simple terms, the pacemaker may have leads to either, or both, of the right atrium (RA), or right ventricle (RV). This is the origin of the terms single-chamber and dual chamber systems.

A standardised nomenclature for pacemaker modes explains, using a three-letter code, which chambers of the right heart are involved, as well as the way in which the device interprets electrical signals. Modern pacemakers are able to recognise intrinsic depolarisations (sensing) and provide artificial stimuli (pacing).

1st letter: the chamber paced: A(trium), V(entricle), or D(ual)
2nd letter: the chamber sensed: again, A,V, D, or very occasionally, neither (O)
3rd letter: the way in which the signals determine the activity of the pacemaker itself: I(nhibited), T(riggered), or D(ual). The differences between these are not relevant here.
Common modes of pacing are:
1. VVI: a single lead to the RV provides both sensing information and pacing
2. DDD: leads are positioned in both the RA and RV, and can both sense and pace

This can initially appear complex and confusing, but only if the device set-up is properly understood, can we begin to understand the resulting ECGs. This is particularly true if device dysfunction is suspected.

DDD pacing	AAI pacing	VVI Pacing: underlying atrial flutter
Click the images to see larger versions

Pacing leads

The major distinction is between passive and active fixation.

Passive fixation leads use small tines at the tip to hook into the trabeculations of the right heart, and provide resistance to displacement.

Active fixation uses a corkscrew mechanism to drive into the myocardium itself. Active leads are much less likely to displace. They are often used in the ventricular leads of ICDs.

Both types develop a fibrotic reaction at the tip over time, which increases stability.