Establishing the Input Impedance Requirements of ECG Recording Amplifiers.

The transient response of electrocardiogram (ECG) amplifiers is investigated in the light of the IEC 60601 performance standard for electrocardiographs. Simulations and bench tests carried out on models of several electrodes reveal that the undershoot limit of $100~\mu \text{V}$ and the recovery slope limit of 300 $\mu $ Vs−1 in response to a narrow 3-mV, 100-ms rectangular pulse are violated when the recommended input impedance of 10 $\text{M}\Omega $ is used in the amplifier. Analyses of dc and ac coupling of the electrodes to the amplifier are carried out, and the constraints are established for the amplifier input impedance based on the electrical properties of the electrodes. We recommend that the amplifier input resistance needs to be determined as $\text{R}_{\mathrm {in}} >60\text{R}_{P}$ or $>25/{\text{C}}_{P}$ , whichever is greater, where ${\text{R}}_{P}$ and $\text{C}_{P}$ are the resistance and capacitance of a simple electrical model of the electrode. In the case of ac coupling of the electrode to the amplifier, $\text{C}_{C}\text{R}_{\mathrm {in}} > 6$ s, where ${\text{C}}_{C}$ is the coupling capacitor. The results of simulations and tests suggest that an amplifier input impedance exceeding 3 $\text{G}\Omega $ and a cutoff frequency no higher than 0.05 Hz are necessary in order to meet the IEC 60601 performance criteria and avoid distortion of the recorded ECG signal. [ABSTRACT FROM AUTHOR]