Is a unipolar
or bipolar pacemaker more sensitive to electrocautery interference?
A unipolar pacemaker lead system positions the
negative electrode (cathode) in the heart and the positive electrode (anode) in
the casing of the subcutaneously placed pace-maker generator. It is unipolar in
the sense that only one pole of the electrical circuit is physically in the
myocardium. A bipolar pacemaker lead system has two electrodes: the negative
(distal) electrode is in the myocardium, and the positive (proximal) electrode
is several millimeters proxi-mal but still within the paced cardiac chamber.
Unipolar electrodes are differentiated from
bipolar elec-trodes in several ways. Unipolar electrodes produce a larger
“spike” on the surface electrocardiogram because of the greater distance
between the positive and negative elec-trodes. This distance between electrodes
produces a large electrical field change on the body surface. The bipolar
pacemaker lead often appears as a coaxial cable and has a characteristic ridge
between the two electrical poles near the end of the lead. This can be seen on
the chest radiograph.
Electrocautery induces voltage (potential)
differences across tissues. Pacemakers can be inhibited by these voltage
changes, which mimic QRS complexes within the pace-maker circuitry. A unipolar
pacemaker lead system is more likely to have this problem because the greater
distance between the positive and negative poles will sense voltage changes
from electrocautery. Conversely, bipolar lead sys-tems are less likely to be
effected by electrocautery, because it is difficult to induce a large enough
voltage change over the small space between the poles of the pacemaker leads to
simulate QRS complexes within the pacemaker circuitry.