US 4,993,423 AGranted

Method and apparatus for differential lead impedance comparison

AssigneePhysio-Control, Inc.

InventorJohn R. Stice

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Abstract

A differential lead impedance comparison apparatus (10) senses lead impedance and compensates for patient-to-patient and electrode variability. A bridge circuit (12) is connected to one end of electrode conductors (22, 24 and 26) in an ECG Leads I configuration. The other end of the conductors (22, 24 and 26) are connected to a patient (18) via electrodes (RA, LA and LL). Leads formed in part by RA, LA and LL and the respective conductors (22, 24 and 26) have lead impedances (R.sub.b, R.sub.a, and R.sub.c). Constant current sources (11, 12 and 13) are connected to the conductors (22, 24 and 26) and supply constant AC currents (I.sub.1, I.sub.2 and I.sub.3). A first bridge output voltage (V.sub.M) is produced by I.sub.1 and a combination 32 of R.sub.a, R.sub.b, and R.sub.c. A second bridge output voltage (V.sub.P) is produced by I.sub.2 and a combination 34 of R.sub.a, R.sub.b, and R.sub.c. A differential amplifier circuit (14) differentially amplifies the V.sub.M and V.sub.P voltages to produce differential voltages (V.sub.OM and V.sub.OP). Demodulators (DM1 and DM2) demodulate V.sub.OM and V.sub.OP to produce differential impedance voltages (V.sub.1M and V.sub.1P). A first comparator (OA3) changes states and produces a high logic output when V.sub.1M equals or exceeds a first threshold level (V.sub.TH1). A second comparator (OA4) changes states and produces a high logic output when V.sub.1P equals or exceeds a second threshold level (V.sub.TH2). An exclusive OR gate (G1) produces a high logic output (V.sub.OUT) when one and only one of OA3 or OA4 produce a high logic output.

Claims12

Dependent claims

An apparatus, for use with devices employing a first lead having a first lead impedance, a second lead having a second lead impedance and a reference lead having a reference lead impedance, said first, second and reference leads being coupleable to electrodes, that senses the integrity of lead connections comprising:

(a) a current source means coupleable to said first lead, said second lead and said reference lead, wherein said current source means:
(i) produces a first AC current and applies said first AC current to said first lead;
(ii) produces a second AC current and applies said second AC current to said second lead;
and,
(iii) produces a reference AC current and applies said reference AC current to said reference lead;
(b) a receiver coupleable to said first and second leads for receiving a first voltage produced by said first AC current and said first lead impedance and a second voltage produced by said second AC current and said second lead impedance;
(c) differential amplifying means for differentially amplifying said first and second voltages to produce first and second differential impedance voltages, wherein said first differential impedance voltage is proportional to a first difference between said first and second lead impedances and wherein said second differential impedance voltage is proportional to a second difference between said first and second lead impedances;
and
(d) an output means coupled to said differential amplifying means for receiving said first and second differential impedance voltages and producing a leads-off output signal when one and only one of said first and second differential impedance voltages equals or exceeds a corresponding first or second threshold level.

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Dependencies are inferred from the claim text.