Biomedical Engineering Reference
In-Depth Information
10ms
a)
b)
0.5mV
TR = TF = 20ns
0.5 - 8V
17.5ms
20 ms
0.5 ms
Figure 2.33 A typical pacing pulse consists of a main pulse that stimulates the heart and a discharge phase used to deplete the charge on
the capacitive coupling generated by the delivery of the pacing pulse. Simple biopotential amplifiers unduly distort ECG signals from pace-
maker patients because of the large amplitude difference between (a) pacing pulses and (b) the heart's intrinsic signals.
As shown in Figure 2.33 a , a typical pacing pulse consists of two components, a
main pulse and a discharge phase. The main pulse, which is used to stimulate the heart, is
characterized by its narrow width, sharp rise and fall, and large variation in amplitude. The
actual shape of the pacing pulse depends on the output coupling design of the pacemaker.
The discharge phase is used to deplete the capacitive coupling generated by the delivery of
the pacing pulse charge built up between the heart's tissue and the pacemaker's electrodes.
The shape and size of the discharge phase is a function of the energy content of the pacing
pulse and the amount of capacitive coupling.
The problem with using a simple biopotential ampli
er to diagnose patients with pace-
makers is the large voltage ratio between the artifact caused by the pacing pulse and the
true ECG signal. When this ratio is large, the ECG signal baseline will be shifted to the
discharge baseline of the pacer artifact signal, thus distorting the ECG. A large artifact sig-
nal can also produce ampli
fi
er overloads, preventing observation of the heart's electrical
activity following pacing. To overcome this problem, ECG ampli
fi
ers designed to follow
up on patients receiving pacing therapy have the means to separate and separately process
the ECG and pacing pulse artifact signals, and augment the pacer artifact by providing a
uniform pacing pulse artifact.
Figures 2.34 and 2.35 illustrate a four-lead ECG ampli
fi
er useful for evaluating patients
implanted with a cardiac pacemaker. In this circuit, op-amps IC1B, IC1C, and IC1D bu
fi
er
the biopotential signals detected by electrodes placed in the right arm, left arm, left leg,
and chest of the patient. IC1A bu
ff
ff
ers the ECG signal detected from an electrode placed
on one of the precordial electrodes.
The bu
ered left-arm, right-arm, and left-leg signals are summed by resistors R8, R9,
and R10 prior to being bu
ff
ered by IC2B to derive the Wilson central terminal (WCT)
potential , which is considered to be the reference potential for ECG recording from the
ff
 
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