The importance of assessing the fidelity of electrocardiographs by using a simple calibration signal

ECG distortion, mimicking changes seen in acute myocardial infarction, occurred in traces transmitted by the South East Coast NHS Ambulance Trust. This was due to low frequency phase nonlinearity produced by baseline filtering. The very narrow calibration pulses made the detection of this distortion difficult. A 1 mV square wave signal revealed the cause. Adequate calibration pulses 0.3 s wide should be recorded with all ECGs, to show that the signal is being reproduced reliably.

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