Biomedical Engineering Reference
In-Depth Information
electrode. After implantation, set the output amplitude and
the width of pacing pulse to the device's nominal values; at a
minimum this should be double the value of the measured
voltage pacing threshold. For the permanent setting, double
the value of the measured voltage pacing threshold or triple
the value of the pulse width at the threshold amplitude is
recommended.
Fig. 12.7
Acute and chronic electrogram record
12.2.4 Slew Rate
while the PSA might measure peak-to-peak amplitude or
vice versa. Various bandpass filters in implantable devices
and PSAs attenuate signals that are not typical of P wave and
R wave frequency. For these reasons, the amplitudes of the
intrinsic intracardial electric signals measured first by the
PSA and then by the device in situ might differ a lot. The
electrogram taken at the time of implantation will show ST
segment elevation due to local myocardial damage (see
Fig.
12.7
). The long-term electrogram will show no further
S-T elevations and lower amplitude of the R wave.
The slew rate parameter shows behavior of the intracardial
signal voltage in time. It deals with the inclination of the
leading edge of the intracardial signal impulse. It is mea-
sured in volts per second or millivolts per millisecond. In
general, the higher the slew rate is, the higher the frequency
of the signal and the more likely it is to be sensed. It may be
used as an adjunct measurement if the intracardial signal
amplitude is on the borderline of acceptable/unacceptable.
The purpose is to ascertain whether the sensing is trouble
free. If the slew rate is low (ventricular slew rate < 0.5 V/s or
atrial slew rate < 0.3 V/s), the frequency is also low and there-
fore a larger amplitude is needed before the signal can be
sensed. Signals with a low slew rate can potentially result in
undersensing.
12.2.2 Pacing Circuit Impedance
Impedance deals with the total resistance to the flow of elec-
tric current through the lead's conductors, electrodes, lead-
to-tissue interface, and body fluids and tissues. It is measured
in Ohms. This measurement requires delivery of a pacing
pulse. The purpose is to verify the integrity of the pacing
circuit at the given position of the lead. Over the long term,
the decrease in impedance might indicate problems with the
lead's insulation, whereas an increase in impedance might
indicate problems with the lead's conductor, lead-to-tissue
touch, or contact of the lead's connector in the device
header.
12.2.5 Retrograde Conduction Time
This parameter deals with the period of time it takes an elec-
tric impulse originating in the ventricles to travel through the
conduction system to the atrium, where it causes atrial depo-
larization (retrograde P wave). Retrograde conduction may
result in endless-loop tachycardia in the atrial-tracking pac-
ing modes when the retrograde P wave is sensed by the atrial
sensing circuit and begins an atrioventricular delay. This
testing may be deferred until the first follow-up after implan-
tation and event markers from the programmer may be used
for the monitoring. Optimal programming of the
Postventricular atrial refractory period parameter to the value
at least 25 ms longer than the measured retrograde conduc-
tion time serves for the prevention of pacemaker-mediated
tachycardia. The retrograde conduction time may vary from
100 to 400 ms.
12.2.3 Pacing Threshold (Voltage/Current)
The pacing threshold is a minimal value of electric pacing
that consistently produces cardiac depolarization. It is stated
as voltage amplitude of rectangular pulse at a defined pulse
width (most often it is 0.4 or 0.5 ms) or, rarely, as current
amplitude or pulse width of a given voltage. The purpose is
to ensure an adequate safety margin between the pacing
threshold and the programmed output of the device at the
given position of lead. Furthermore, it is necessary to assure
the integrity of the pacing circuit. The threshold is measured
by a gradual decrease of the pacing voltage/current/pulse
width at an adequate pacing rate. The last value before the
loss of pacing is the pacing threshold.
Presence of the electrode next to the endocardial surface
causes an inflammatory reaction that heals within several
months, leaving behind a small fibrotic capsule around the
12.2.6 Diaphragm Pacing
Measurement of diaphragm pacing is performed by tempo-
rary pacing at the maximal amplitude (10 V). The purpose is
to verify that phrenic nerve stimulation (in the case of an
atrial lead) and diaphragm stimulation (in the case of ven-
tricular leads) does not occur. This might cause hiccoughing
