Electrophysiology Equipment, Instrumentation, and Basic Principles
Signal chain and amplification
Intracardiac signals are tiny (microvolts to millivolts) and ride on large common-mode noise, so the recording system uses differential amplifiers with high common-mode rejection. Gain (mm/mV) controls display size, not the underlying signal. Filters shape what you see: surface ECG is typically filtered ~0.05–150 Hz, while intracardiac electrograms use a higher high-pass (~30 Hz) to strip baseline wander and a low-pass around 500 Hz; a notch filter removes 60 Hz line noise. Know that a high-pass filter set too low lets in drift, and one set too high can attenuate slow components such as the His potential.
Digitization, stimulation, and impedance
Digital systems sample at a rate comfortably above twice the highest frequency of interest (Nyquist). The stimulator delivers constant-current pulses, usually 2 ms pulse width, with output in milliamps; threshold testing decrements output until loss of capture. Electrode impedance that is very high suggests fracture or poor contact; very low impedance suggests insulation breach or shunt.
Electrical safety
Patient-connected equipment must be isolated, leakage current kept low, and all equipment referenced to a common ground. Cables should not loop near fluoroscopy or lie across the patient's chest. A ground fault or redundant ground path is a burn risk during ablation.
Rule of thumb
When pacing for threshold or refractory-period work, start at twice diastolic threshold and 2 ms pulse width; capture at 1 mA does not mean the catheter will capture reliably at 1 mA during the case. If the His potential disappears when you raise the high-pass filter, you filtered out the signal, not the conduction.