Biomedical Engineering Reference
In-Depth Information
During routine follow-up, increased attention should be given
to the pacing history in the device's memory.
As in other systems, information on the detection and
therapy for tachycardia episodes is recorded automatically.
Then the data can be searched at various levels of detail using
a programmer. The stored data include episode details with
electrograms and annotated markers from all active leads. As
soon as the memory capacity available for storing episode
data is filled, the device attempts to assign priority to types of
stored episodes and rewrites the stored episodes according to
specific rules. Depending on the type of device and the man-
ufacturer, the trend function can provide, for example, a
graphic representation of atrial and ventricular arrhythmias,
the trend of a patient's intrinsic heart rate, the rate of a
patient's daily activity measured by an accelerometer, the
time of atrial tachy response mode switching, and the trend
of a patient's daily respiratory activity, as well as data con-
cerning the long-term trends of sensed intrinsic heart rate
and impedance amplitudes.
Heart rate variability (HRV) is the measure of changes in
a patient's intrinsic heart rate intervals for a specific period
and can help evaluate the clinical condition of patients with
heart failure. Lower variability of a patient's heart rate can
facilitate recognition of the decompensation condition dur-
ing heart failure. The HRV monitoring function provides
information using the data of sensed intrinsic cardiac inter-
vals. The device can gather information on the intervals for
HRV only as a tracking mode without an adaptable pacing
rate (VDD or DDD) being programmed. Certain special data
useful for detailed evaluation are recorded, and only those
RR intervals meeting set heart failure data collection criteria
are evaluated. The analysis of RR interval variability can be
conducted using two methods: either a spectral analysis in
the frequency domain or by parameters in the time domain.
Spectral analysis is most often carried out by means of
fast Fourier transform, where sensed RR intervals are divided
into characteristic frequency bands. They are classified as
high (0.15-0.40 Hz), low (0.04-0.15 Hz), very low (0.0033-
0.04 Hz), and ultra low (below 0.0033 Hz). Spectrum sam-
ples are recorded from different time intervals according to
the frequency band examined. The proportion of low-fre-
quency and high-frequency components of heart rhythm
spectral analysis is considered the measure of sympathova-
gal balance and expresses modulation of sympathetic ner-
vous system activity. Parasympathetic nervous system tone
is shown primarily in the high-frequency component of the
spectral analysis. The low-frequency component is affected
by the sympathetic as well as parasympathetic nervous sys-
tems. The calculation, which is carried out by devices with
this capability based on a measured RR interval, serves as an
approximation of the actual low-frequency and high-fre-
quency component proportions.
Time parameters include the calculation of indices having
no relation to a specific cardiac cycle value. These indices
represent a simple method of identifying patients with a
decrease in heart rate variability. It is, for example, a stan-
dard deviation of averaged normal R to R intervals in 5-min
periods of time, a standard deviation of all normal R to R
intervals, a percentage of adjacent RR intervals that vary by
more than 50 ms, or the root mean square of the difference
between the coupling intervals of adjacent RR intervals. In
addition, weighted or geometrical means of RR intervals can
be calculated [ 83 ] .
As mentioned earlier in this chapter, 100 % biventricular
pacing must be ensured for good efficiency of CRT systems.
Therefore, it is important to monitor the event counters for
individual leads and the proportion of paced and sensed
events during follow-up. This is expressed as a percentage of
atrial, LV, and RV pacing. Optimally, all events in the atrium
should be sensed (or controlled by a sensor), and all events in
both ventricles should be paced. Each counter shows the total
number of paced events since the reset.
 
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