Biomedical Engineering Reference
In-Depth Information
7.1.1
Fixation Mechanisms
considerably bigger area than the electrodes that are intended
for different methods of fixation. Therefore the pacing
threshold is lower.
Active fixation uses a principle of a helix that is extended
into the endocardium (see Fig.
7.2
). The helix can be retracted
and the active fixation is released by that. The helix is
extended/retracted by means of a mechanism that is joined to
the lead conductor with a connector. The helix is extended by
clockwise rotation of the connector pin and is retracted by
counterclockwise rotation. Rotation is performed using a
special fixation tool in the shape of a pair of small pliers. In
the past, the helix was extended permanently and covered by,
for example, polyethylene glycol so that it did not damage
venous structures. This coating was dissolved in the blood
bed after several minutes and the entire lead was screwed to
affix the helix into the tissue. Construction of the extendable/
retractable fixation helix anchors the distal electrode into the
endocardium/myocardium without the support of the trabe-
cular structures and offers various possibilities for lead posi-
tioning. If the extendable/retractable helix is electrically
conductive and connected with the lead conductor, it serves
as the cathode during pacing and sensing. There are also
designs with a nonconductive helix and the electrode placed
at the total distal end of the lead body. The leads are provided
with fluorescence markers near the distal end. These markers
do not transmit X-rays and they can be observed under skias-
copy. They indicate when the helix is completely extended or
completely retracted.
The left ventricular leads intended for pacing in the coro-
nary bed are fixed in a different way. Because the pacing
electrodes remain in the coronary veins, different invasive
fixation cannot be considered. Therefore, the distal ends of
the left ventricular leads are preformed either to a spiral
curve (a pigtail) or to a suitable angled curve (
J
-curve); they
are equipped with two very thin tines so that they generate a
slight force on the narrow wall of the coronary vein to avoid
return movement (see Fig.
7.3
). During implantation of the
lead, the preformed shape is restraightened by means of a
guidewire or a stylet; not until after its extraction does the
distal end of the lead recover its preformed shape.
The long-term operation of cardiac pacing is dependent on
the stabile positioning of the pacing electrodes. Historically,
the first way to fix the transvenous electrodes on the endocar-
dial surface was called passive fixation. It dealt with various
methods of attaching the fixation mechanisms, in the shape
of small fillers or fixation tines (see Fig.
7.1
), into the trabe-
culae of the right atrium or ventricle [4]. During implanta-
tion of the lead with passive fixation, its tines are captured
immediately by the trabeculae; this can be confirmed by
carefully pulling the lead back. Passive fixation is not techni-
cally demanding with regard to the implantation, especially
into the apex. However, the tines of the passive fixation
mechanism are covered quickly by fibrous tissue that makes
later electrode repositioning or extraction more difficult or
even impossible approximately 6 months after its implanta-
tion. The tines also increase the external diameter of the lead
body. The porous electrode with passive fixation tines, which
serves as a cathode for sensing and pacing, might have a
Fig. 7.1
Passive lead fi xation
Fig. 7.2
Active lead fi xation
Fig. 7.3
Various left ventricular lead fi xations
