Anthony J. Hackett, DO

Given the increasing use of pacemakers and AICD’s in medicine, it is not uncommon to encounter these devices in clinical practice. Familiarity with common errors is paramount especially given their approximately 6% per year chance of malfunction. This brief review serves to help make sense of the letters, numbers and symptoms associated with pacemakers and their function or at times, malfunction.

Pacemaker Types and Lead Structure
There are essentially 3 types of pacemakers now in use:
Single chamber: Composed of a single pacing and sensing lead in either the RA or RV although RV is more often seen

Single chamber atrial pacer

Single Chamber Pacer

Dual chamber: Two paired pacing and sensing leads: one in the atrial appendage with the other in the ventricle (RV/LV)

Dual chamber_arrows

Dual Chamber Pacer












Biventricular: essentially a dual chamber pacer with an extra lead in the LV that is placed by threading the wire through the coronary sinus. These pacers thus have an RA and RV lead and a third LV lead. These units are important for treating ventricular dysynchrony resulting from advanced CHF.
Pacemakers made after 1990 now have bipolar leads meaning the positive terminal and negative terminal are on the same lead 1 cm apart. In the past, leads were unipolar and positive and negative terminals were located a great distance away (between the atria and ventricles). This was associated with frequent symptomatic bradycardic events secondary to oversensing of skeletal muscle contractions (explained more below).

Bivent pacer

Biventricular Pacer


What about ICD’s?

ICD’s currently in use have long battery life (~8 years) and are quite advanced possessing multiple functions to include:
• Tachycardia pacing
• Cardioversion
• Defibrillation
• Dual chamber back-up pacing if primary pacer fails (advanced models)

ICD’s generally function as follows when they detect an aberrant rhythm. If the rhythm is a non-VF rhythm, that cannot be immediately shocked then anti-tachycardia pacing (ATP) is employed. ATP is similar to intermittent overdrive pacing in that the ICD essentially fires short bursts of pacing approximately 10 beats faster than the tachycardia in an effort to reset the intrinsic cycle of repolarization. If this is not successful then the defibrillator fires. In case of ICD malfunction as above, magnet application will terminate ICD tachycardia and ability to shock patients. ICD’s can be differentiated on a CXR based their lead structure which is flat, long and paddle-like as compared to the short leads of a pacer.

AICD pacer

AICD Pacer: Note the paddle like ICD electrodes


Interpreting Pacer type:
Pacemakers are described by five letter/position codes although commonly four are used and are explained in depth below. The first letter indicates the chambers that are Paced, the second the chambers Sensed, the third letter is the Response to sensing, and the fourth is an adaptive rate mechanism. The fifth letter is rarely used in Emergency medicine but represents whether or not advanced multisite pacing is used. Below the essential “positions” of the pacer itself are outlined further:
I: Pacing: Can be either Atrial (A), Ventricular (V) or Dual (D)
II: Sensing occurs with the same codes as pacing except the additional modifier “O” which represents absence of sensing.
• III: Responses to sensing of an intrinsic cardiac impulse include Inhibition (I) of pacer, Triggering (T), Dual or advanced mode (D) indicates that atrial and ventricular responses can be generated and modulated separately. Finally, “O” indicates no response to sensing akin to a “magnet response” and is generally used when position II is “O” as well
IV: rate modulation, indicates how the pacer responds to increases in rate with exercise and can either be (R) for response or “O” for no response
• V: rarely used except in multisite Biventricular pacers for heart failure management
This generally results in pacer codes like “DDI” and “VVI” which are the most common types of pacemakers.

For discussion regarding malfunctions and troubleshooting please see part II the related post

• Tintinalli’s Emergency Medicine: A Comprehensive Study Guide, 7e. Judith E. Tintinalli et. al
• CURRENT Diagnosis and Treatment Emergency Medicine, Seventh Edition / Edition 7. C. Keith Stone and Roger Humphries
• Special thanks for images uploaded from