Biomedical Engineering Reference
In-Depth Information
•
Lead II:
tracing of the potential di
erence between the left leg (F) and the right arm,
where the left leg is the noninverting input
ff
•
Lead III:
tracing of the potential di
erence between the left leg and the left arm,
where the left leg is the noninverting input
ff
A
unipolar chest lead
is obtained by connecting the chest electrode to the noninverting
input, while the WCT is connected to the inverting input. The speci
fi
c chest lead (V1-V6)
is obtained by placing the chest electrode on speci
fi
c anatomical landmarks according to
international standards:
•
V1
in the fourth intercostal, at the right side of the sternum
•
V2
in the fourth intercostal, at the left side of the sternum
•
V3
halfway between V2 and V4
•
V4
in the
fi
fifth intercostal, left mid clavicular
•
V5
in the same horizontal plane as V4, halfway between V4 and V6 (anterior axially)
•
V6
in the same horizontal plane as V4, left mid axially
V1 and V2 are thus placed above the right ventricle, V3-V6 above the left ventricle.
The output of IC3 contains an ampli
ed version of the true ECG signal and the pacing
pulse artifact. This signal is separated into the two components via the slew-rate limiter
and pacing pulse detector circuits implemented around IC4A and IC4D. Once a certain
slew rate has been exceeded by the output of IC3, the slew-rate limiter limits the excur-
sions of the output signal to a speci
fi
fi
ed rate of change. When the input voltage to IC4A is
zero, the current
flowing through R11 and R12 is the same, and thus the output of the slew-
rate limiter is zero (neglecting possible op-amp o
fl
set). When a positive-going signal
appears at the noninverting input terminal of IC4A, the output of IC4A will go positive,
and current through R12 will be sourced by the op-amp. At the same time, current will be
sourced by capacitor 46. As long as slewing-rate conditions given by the values of
R11
ff
R12 and C10 are not exceeded, the slew-rate limiter's output voltage tracks IC4A's
input voltage. However, if a pacing pulse artifact is presented to the noninverting input of
IC4A, the slew-rate limit imposed by the time constant of R11
R12 and C10 is exceeded
and capacitor C10 will charge (or discharge) at a rate limited by the value of R11
R12.
At the time of the slew rate limiting, the output signal's slew rate will be limited to the
charge or discharge rate of capacitor C10. Soon after the slew rate of the input signal falls
under the slew-rate limit, op-amp IC4A returns from saturation, and the output of the slew-
rate limiter tracks IC4A's noninverting input. Once the ECG signal has been cleaned by the
slew-rate limiter from the pacing pulse artifact, baseline zeroing is accomplished by feed-
ing the inverted baseline level (derived by inverting and heavily low-pass
fi
filtering the
slew-rate limiter's output) to the reference pin of IC3.
Pacing pulse detection exploits the fact that the output of IC4A rails when the slew rate
of its input exceeds the set limit. IC4D recti
es IC4A's output. Whenever the slew-rate
limit is violated (which is assumed to happen only whenever a pacing pulse is present), a
positive pulse appears at the trigger input of nonretriggerable one-shot IC5. The artifact-
free ECG signal is ampli
fi
fi
ed further via IC4C before it is presented to a Burr-Brown
ISO107 isolation ampli
er. Every time a pacing pulse is detected, an optically isolated 5-
ms pulse is added to the isolated and
fi
fi
filtered ECG signal via R27 and R28 before being
bu
ered by IC8B.
The circuits in galvanic contact with the patient are powered via ISO107, which gener-
ates isolated
ff
12 V when powered from
12 V. L2, C24, C25, and C26 form a pi
fi
filter to
clean the isolated
12-V power line generated by ISO107 from switching noise. An iden-
tical network is used to
fi
filter the negative isolated supply rail. A pi
fi
filter formed by L1,
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