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662 PA R T IV / Pathophysiology and Management Disease

■ Figure 28-5 Biventricular pac-
ing. There is a lead in the atrium and
a lead in the right ventricle for dual-
chamber pacing. The left ventricle is
Atrial lead Coronary sinus lead paced via a lead threaded through
the coronary sinus and down a lateral
or posterior cardiac vein in the left
ventricle.

Right ventricular lead

Pacing Modes larization (I in third position). VVI is the most commonly used
mode of pacing with temporary transvenous leads because it is the
The current nomenclature used to describe the expected function quickest and easiest method of pacing in an emergency, and it is
of a pacemaker was established by members of the North Amer- difficult to get a temporary atrial lead to stay in place. VVI is also
ican Society of Pacing and Electrophysiology and the British Pac- often used with epicardial leads after cardiac surgery, especially if
ing and Electrophysiology Group and is designated the NBG third-degree AV block is present, and is the mode that has to be
22
code for pacing nomenclature. The code describes the expected used for permanent pacing in patients with chronic atrial fibrilla-
function of the device according to the site of the pacing elec- tion. The DDD mode means that both atrial and ventricular elec-
trodes and the mode of pacing. The first letter describes the trodes are present and both chambers are paced (first D), both
chamber that is paced: A, atrium; V, ventricle; D, dual (both chambers are sensed (second D), and the device either inhibits or
atrium and ventricle); O, none. The second letter describes the triggers an output in response to sensed intrinsic activity (D in
chamber where intrinsic electrical activity is sensed: A, atrium; V, third position means dual-response to sensing). DDD is the most
ventricle; D, dual (both atrium and ventricle); O, none. The frequently used permanent pacing mode, unless the patient has
third letter describes the pacemaker’s response to sensing of in- chronic atrial fibrillation or flutter. Other pacing modes that are
trinsic electrical activity: I, inhibited; T, triggered; D, dual (in- sometimes used are AOO, AAI, DVI, DDI, and VDD.
hibits or triggers); O, none. The fourth letter denotes the pres-
ence or absence of rate modulation (R, rate modulation and O, Basics of Pacemaker Operation
none). The fifth letter specifies the location or absence of multi-
site pacing, which includes either biatrial or biventricular pacing, Electrical current flows in a closed-loop circuit between two pieces
or more than one stimulation site in a single chamber (e.g. two of metal (poles). For current to flow, there must be conductive
atrial pacing sites in the right atrium). Table 28-1 illustrates the material (i.e. a lead, muscle, or conductive solution) between the
pacemaker code in detail. two poles. In the heart, the pacing lead, cardiac muscle, and body
The most commonly used pacing modes are VVI and DDD. tissues serve as conducting material for the flow of electrical cur-
The VVI mode means that the electrode is in the ventricle and rent in the pacing system. The pacing circuit consists of the pace-
paces the ventricle (first V), senses ventricular activity (second V), maker (the power source), the conducting lead (pacing lead), and
and inhibits its output when it senses intrinsic ventricular depo- the myocardium. The electrical stimulus travels from the pulse

Table 28-1 ■ FIVE-LETTER PACEMAKER CODE
First Letter: Second Letter: Third Letter: Fourth Letter: Fifth Letter:
Chamber Paced Chamber Sensed Response to Sensing Rate Modulation Multisite Pacing
O None O None O None O None O None
A Atrium A Atrium I Inhibited R Rate modulation A Atrial
V Ventricle V Ventricle T Triggered V Ventricle
D Dual (A and V) D Dual (A and V) D Dual (I and T) D Dual