Biomedical Engineering Reference
In-Depth Information
er circuit. The ventricu-
lar electrogram signal detected through the electrodes attached to the ventricle is attenuated
by the voltage divider formed by resistor R7 and one of the resistors among R15, R16, R17,
R18, or R19. Selection is performed via
Figure 8.11 presents the schematic diagram of the sense ampli
fi
fi
five-position switch SW1, which sets the sensitiv-
ity of the sense ampli
fi
er. Past the variable attenuator, the electrogram signal is ampli
fi
ed by
op-amp IC1. The bandpass of this ampli
fi
er is 88 to 100 Hz. The high-pass cutoff
ff
is set by
the
RC
fi
filter formed by R8 and C5. The low-pass characteristics are given by the bandwidth
of the
PC4250 op-amp, which is a function of its bias set through resistor R12.
Comparators IC2 and IC3 detect whether or not the ampli
µ
ed and bandpassed signal
has an amplitude with absolute value above 15 mV. The comparison range is de
fi
ned by the
divider formed by resistors R4, R6, R9, and R11. At rest, the outputs of both comparators
remain at logic low. If the signal exceeds the positive threshold, the output of IC2 goes
high. If the negative threshold is crossed, the output of IC3 goes high. The comparator out-
puts are ORed via the two diodes inside D1. A high signal at the output of the wired-OR
is interpreted by microcontroller IC4 as a sensed event.
Figure 8.12 presents the schematic diagram of the of the pacing pulse generator circuit.
This circuit is able to generate pacing pulses with an amplitude of 3 or 6 V, depending on
the state of the pacing amplitude selector switch. When inactive, the microcontroller
sets the HIGH AMPLITUDE PACING line low, which charges “tank” capacitor C2 to
VDD. The PACING signal is maintained low to keep transistor Q2 open, and line
ACTIVE_DISCHARGE is maintained high to keep switch Q1 open. Coupling capacitor
C1 slowly discharges by way of resistor R1 (100 k
fi
Ω
) through the heart's tissues and elec-
trodes connected to terminals V
.
When a stimulus is to be generated, and if the amplitude selected is 6 V, IC4 sets HIGH
AMPLITUDE PACING line high, which closes Q4 and opens Q3. This causes the positive
terminal of capacitor C2 to be connected with the battery's negative terminal. When pac-
ing at 3 V is desired, HIGH AMPLITUDE PACING is set low, which connects the C2's
positive terminal to the battery's positive terminal (VDD). In the
and V
fi
first case, the potential
di
erence between the negative terminal of C2 and VDD is 6 V, while in the second case
the potential di
ff
erence is 3 V.
To deliver the stimulus to the tissue, microcontroller IC4 sets the PACING line high,
which closes Q2 and connects the negative terminal of C2 to C1 (which is discharged). As
such, the leading-edge voltage of the pulse appearing across electrode terminals V
ff
and
V
is equal to the selected voltage (3 or 6 V). This voltage decays throughout the pacing
pulse as C2 discharges and C1 charges. To terminate current delivery to the tissue, IC4
places all stimulus-related lines back to their rest condition.
Once the pulse has been delivered, coupling capacitor C1 remains charged. The delivery
of a new pacing pulse will require this capacitor to be discharged, a procedure that is done
by delivering the energy stored in this capacitor through the tissue. A net-zero current
ow
through the tissue is accomplished by passing the same amount of charge (albeit not within
the same amount of time) through the tissue as was delivered during the stimulus pulse, but
in the opposite direction. Not doing so would cause electrochemical imbalance, which can
result in electrode corrosion and tissue damage. In this pacemaker, charge balancing is
accomplished during the refractory period by taking line ACTIVE_DISCHARGE low,
which closes Q1 allowing the charge in coupling capacitor C1 to
fl
fl
flow through the tissue via
resistors R2 and R1 (100
). Any remaining charge after the fast
discharge time is delivered at a slower rate through R1 (100 k
Ω
in parallel with 100 k
Ω
).
Pacing parameter selection in this demonstration external pacemaker is done through
switches. As shown in Figure 8.10, parameters with only two possible values (i.e., mode,
pacing pulse amplitude, and refractory period) use SPDT switches to deliver a logic high
or a logic low directly to an input pin of the microcontroller. The rotary switch used for
sensing sensitivity selection acts directly on the sense ampli
Ω
fi
er circuit. Rate and pacing
Search WWH ::
Custom Search