Beat‐by‐Beat Recording of Low‐Level Potentials. Due to the excessive electrical and magnetic noise in a hospital environment, the utility of low‐amplitude electrocardiographic signals has been greatly impeded. In order to record surface ventricular late potentials and His‐Purkinje potentials on a beat‐by‐beat basis outside a shielded environment, a new system was implemented with a microcomputer. In the case of late potential recording, six bipolar orthogonal leads (two along each axis) were high‐pass filtered, spatially averaged, and finally combined into vectormagnitude complexes on an every beat basis. Bidirectional filters and finite impulse response filters with linear phase response were used to prevent filter ringing in the terminal QRS. Late potentials were recorded on a heat‐by‐beat hasis in patients who exhibited late potentials in the signal‐averaged electrocardiogram (ECG). His‐Purkinje potentials were recorded using spatial averaging and channel selection algorithms that discriminate the signal from noise. All the recordings were performed in a clinical environment without using a shielded room. Magnetic fields of 60 Hz and its harmonics were the major sources of interference, and interference‐free recordings could he obtained if the field was smaller than 6.6 × 10−8Tesla (T) at 60 Hz. The study indicates that with appropriate instrumentation and care during acquisition, it is feasible to acquire high quality data and analyze late potentials and His‐Purkinje potentials on a beat‐hy‐heat hasis in a clinical environment. Interested investigators are cautioned against basing clinical decisions on marginal quality and noisy data. (J Cardiovasc Elec‐trophysio
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