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Abstract

A method and apparatus for use with medical electrode systems that sense the integrity of lead connections and patient transthoracic impedance is provided. In an ECG electrode application, a carrier circuit (12) produces two carrier signals (S.sub.C1 and S.sub.C2) that are out of phase with each other. The S.sub.C1 signal is applied to an RA lead through a terminating impedance (Z1). The S.sub.C2 signal is applied to LA, LL and V leads through terminating impedances (Z2, Z3, and Z4). Each of the S.sub.C1 and S.sub.C2 carrier signals comprises a lead impedance frequency component (S.sub.LI) and an impedance respiration frequency component (S.sub.IR). First stage amplifiers (A1, A2, and A3) located in an ECG preamplifier (13) amplify the difference between a lead voltage on the RA lead (V.sub.RA) and lead voltages on the LA, LL, and V leads (V.sub.LA, V.sub.LL and V.sub.V). High pass filters (F1, F2 and F3) remove patient ECG signals from the outputs of A1, A2 and A3 to produce first stage output voltages (V.sub.1, V.sub.2 and V.sub.5). The V.sub.1, V.sub.2 and V.sub.5 voltages are demodulated by a demodulator circuit (18). Lead impedance demodulators (DM1, DM2 and DM3) are clocked by a lead impedance control signal (S.sub.A) that has the same frequency as the S.sub.LI component so that the outputs of DM1, DM2 and DM3 are lead impedance-related voltages (V.sub.L1, V.sub.L2 and V.sub.L5). V.sub.2 is also demodulated by an impedance respiration demodulator (DM4). DM4 is clocked by an impedance respiration control signal (S.sub.B) that has the same frequency as the S.sub.IR component so that the output of DM4 is a transthoracic impedance-related voltage V.sub.T. V.sub.T is amplified by an operational amplifier (OA10) to produce an impedance respiration-related voltage (V.sub.R). The ECG preamplifier (13) and the demodulator circuit (18) include fast DC restoration circuits (66, 68, 70 and 71) that use switched capacitor integrators (76) to restore ECG outputs (V.sub.E1, V.sub.E2, and V.sub.E5) and the V.sub.R voltage to nominal values when a DC offset has occurred.

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