THE NBG CODE
Mode(s) of Response
NASPE is the North American Society of Pacing
is the British Pacing and Electrophysiology Group
The five position NBG Pacemaker Code is laid out in a simple, easy to
follow, five position format.
Position I of the code indicates the chamber (or chambers) paced. Since
pacing the heart is the primary function of the pacemaker, this function is given position
1. A device used to pace in only one chamber will be represented by either the
letter A (atrial) or V (ventricular), devices that are capable of pacing in both chambers
are represented by the letter D (dual). Some pacemakers allow pacing to be turned
OFF for diagnostic purposes, and, while turned off, the position 1 coding is O
(off). There is a code letter S that identifies the pacemaker as a single chamber
device. This is only used as a manufacturers designation and is not valid once the
pacemaker is attached to a lead. Pacemaker using the manufacturers designation
"S" in the first position may be attached to a lead that has been placed either
in the atrium or ventricle.
Position II of the represents the chamber used for the second function of a
pacemaker, namely, sensing for intrinsic signals. The letter designations for
position 2 are the same as the designations for position 1. It must be noted,
however, that a device may not be represented by the same letter in both positions.
An example of different letter codes in the first two positions is VDD. In this
example, the ventricle is the chamber paced but the pacemaker is capable of sensing in
both the atrium and the ventricle. Just as in position 1, there is a code letter S that
identifies the pacemaker as a single chamber device. This is only used as a manufacturers
designation and is not valid once the pacemaker is attached to a lead. Pacemaker
using the manufacturers designation "S" in the second position may be attached
to a lead that has been placed either in the atrium or ventricle.
Position III is tells us the mode of response to sensing. Position 3 is directly tied into position 2. Without
sensing, there can be no mode of response to sensing.
When position 3 is identified as I (inhibited), the mode of response
is to withhold a pacemaker output in the presence of a sensed event. In a VVI
pacemaker, the pacemaker senses a ventricular event and withholds the ventricular
output. If the pacemaker is programmed to the DDI mode, the pacemaker simply
inhibits the output of the device in the chamber where any signal is sensed. In the
presence of fast atrial rates and heart block, the DDI pacemaker rhythm resembles a VVI
The letter D (dual) in the third position indicates that the device
will respond to the sensed signal by either inhibiting the pacemaker response, tracking
the sensed event, or inhibiting the output on the sensed channel and triggering an output
to maintain AV synchrony. The most common example of the letter D in the 3rd
position can be seen with DDD pacemakers. A sensed atrial signal will cause the
device to inhibit the atrial output, a timer then starts that will cause a triggered
ventricular output after a certain interval. If the patient has an intrinsic R wave
during the triggering interval, the pacemaker will inhibit the ventricular output.
During magnet application to most pacemakers, the sensing function
is temporarily turned OFF and the pacemaker operates in an asynchronous mode.
Examples of asynchronous modes are, AOO, VOO, DOO. Notice that the 2nd and 3rd
positions are "tied" together - no sensing, no mode of response. Many
pacemakers can be permanently programmed to the ?OO mode, but few are.
The letter T in the third position is rarely encountered. It
is, however, an excellent way to observe the location of sensing of intrinsic
events. If a sensing problem is suspected, programming to a ??T mode may allow the
clinician to observe exactly where in the timing of the device, the sensing abnormality is
occurring. ??T pacing will produce pacemaker output spikes concurrently with
intrinsic sensed events (essentially pseudo fusion beats).
Position IV tells us something about the programmable parameters of the
device. Does it have a sensor to regulate the rate during periods of physical
activity, can it be programmed, can it communicate. In the real world, the only
letter usually spoken is the letter R (rate response). A patient with a rate
responsive DDD pacemaker is said to have a DDDR pacemaker. It is unusual to see a
pacemaker described as a DDDC (telemetry available/communicating) except in the
R (rate response) in the 4th position only tells us that the device is capable of (and
programmed to) a rate responsive function. It does not describe the type of sensor
available in the pacemaker.
C (communicating) tells us that the pacemaker is capable of transmitting and/or receiving
data for informational or programming purposes. Most, if not all, devices currently
manufactured have a communicating ability.
M (multiprogrammable) indicates that the device can be programmed in more than 3
parameters. All DDD pacemakers are, also, multiprogrammable. Typical
programmable parameters include: rate, sensing, output, refractory periods, mode and
hysteresis. Dual chamber pacemaker usually have many more programmable parameters
P (simple programmable) usually indicates that the pacemaker is limited to 3 or fewer
programmable parameters. This letter in the 4th position is limited to single
chamber devices. Typical simple programmable parameters include: rate, output and
O (none) is rarely encountered and indicates that the device has no programmable
parameters. This is most commonly found on devices manufactured before the
It must be noted that the fourth position is hierarchical in nature. For
example, a VVIR pacemaker can be assumed to be, in addition to rate responsive,
communicating (we have to be able to program the sensor) and multiprogrammable (there are
always numerous things to programmed on sensor driven devices). Likewise, a DDD pacemaker
can be assumed to be communicating (many different parameters may have to be programmed)
and multiprogrammable. Any device that is labeled with a P or an M in the 4th
position must also be communicating (or else how could it be programmed).
Position V tells us whether or not the device has any antitachycardia
features. Most bradycardia devices do not and are automatically assumed to be ????O
devices. Some brady devices do have limited antitachycardia capabilities, however,
and this position allows us to identify them.
In the pacemaker only population the fifth position can only be represented by O or
P. The device either has no antitachy feature or it can attempt to pace the patient
out of a tachycardia episode.
The fifth position is used fully by implantable cardioverter defibrillators and their
ability to pace or shock patients out of tachyarrhythmias. Most current generation
ICD's will be represented by a D (dual - shocks and paces) in the fifth position.