Biomedical Engineering Reference
In-Depth Information
Ap
Vp
As
Vp
Ap
Vs
Vs
As
EKG
Fig. 3.5
Surface ECG at different paced and intrinsic heart beat combinations
Fig. 3.6
Intracardial electrogram
about 100 ms and is followed by the ST segment, which is an
isoelectric line lasting about 120 ms and corresponding to the
plateau phase of the membrane potential. The final T wave
lasts about 160 ms and represents repolarization of the ven-
tricular myocardium proceeding in the opposite direction.
The T wave is sometimes followed by a U wave, probably
caused by repolarization of the Purkinje fibers.
With respect to the use of the pacing method, the vulner-
able period of the ventricular myocardium needs to be dis-
cussed. It occurs before the end of the T wave. If an
above-threshold impulse acts on the ventricular myocardium
in this phase of repolarization, the development of ventricu-
lar fibrillation is nearly certain. During ECG evaluation,
fusion or pseudofusion heart beats can also be encountered.
They occur when intrinsic cardiac activity and pacing pulse
occur at the same or nearly the same time. If intrinsic activity
is delayed, the resulting QRS complex will resemble a paced
beat, and vice versa - if intrinsic activity precedes the pacing
pulse, the QRS complex will look like an intrinsic rhythm. If
the pacing pulse has no effect on the intrinsic QRS complex
or T wave, the beat is referred to as pseudofusion. An occur-
rence of fused beats can be mistaken for a system failure.
However, as the sensing of intrinsic activity with sensing cir-
cuits can take place only during the late QRS complex, a
pacing pulse can be delivered in the meantime [94].
In terms of the pacing method, what is commonly encoun-
tered is not a physiological shape of an ECG curve, but rather
a shape of paced heart rhythm, intracardial electrograms, or
both. Figure
3.5
shows typical ECG waveforms with various
combinations of intrinsic and paced cardiac activity in the
right atrium and ventricle. The shape of paced right ventricu-
lar activity is a markedly negative wave. From the left to the
right, it shows intrinsic P and R waves (a physiological
shape), an intrinsic P wave and pacing in the ventricle, pac-
ing in the atrium and an intrinsic R wave, and pacing in both
the atrium and the ventricle.
When implanted devices are followed up, it is often nec-
essary to interpret electrograms sensed by those devices by
means of electrodes of the implanted leads. The curves of
these intracardial electrograms are rather different from those
of the superficial ones because of their placement as well as
different input filter settings and, in particular, a short dis-
tance between the sensing electrodes. Nevertheless, it is
sufficient for distinguishing between paced and intrinsic car-
diac activity. In some implantable defibrillators, it is possible
to observe an electrogram sensed by defibrillation lead elec-
trodes. There is a longer distance between them, which
results in a recording that has superior informative value than
in the case of sensing by means of bipolar pacing leads
(Fig.
3.6
) .
