Introduction. Artifactual variations in the ST segment may lead to confusion with acute coronary syndromes. Objective. To evaluate how the technical characteristics of the recording mode may distort the ST segment. Material and Method. We made a series of electrocardiograms using different filter configurations in 45 asymptomatic patients. A spectral analysis of the electrocardiograms was made by discrete Fourier transforms, and an accurate recomposition of the ECG signal was obtained from the addition of successive harmonics. Digital high-pass filters of 0.05 and 0.5？Hz were used, and the resulting shapes were compared with the originals. Results. In 42 patients (93%) clinically significant alterations in ST segment level were detected. These changes were only seen in “real time mode” with high-pass filter of 0.5？Hz. Conclusions. Interpretation of the ST segment in “real time mode” should only be carried out using high-pass filters of 0.05？Hz. 1. Introduction The existence of electrocardiographic changes in ST segment level is frequently due to acute coronary syndromes. Elevation of the ST segment above 0.1？mV on two adjacent leads, as well as the appearance of complete left bundle block (CLBB), in the presence of angina pain lasting more than 30 minutes within the first 12 hours of the development of symptoms is classical indications of urgent reperfusion therapy [1]. Most biological signals must be processed for adequate recording [2]. Signal acquisition, signal processing, and high-pass filters may distort the shape of the recorded signal [3, 4] and sometimes may cause electrocardiographic changes simulating myocardial ischemia [5–9]. In this paper the frequently described, but not so well-known, distortion of the ST segment due to the technical recording characteristics of the electrocardiography system is profusely analyzed. The objective is to give a clear and graphic explanation of this aspect for a better understanding from a clinical point of view. 2. Material and Method We have analyzed 45 consecutive patients seen in the Cardiology Department for scheduled electrocardiograms (EGG); 19 patients presented for clinical cardiologic followup while the rest were referred to preoperative ECG for various noncardiac pathologies. All were asymptomatic, and none had a history of acute coronary syndrome in the previous six months. The low-frequency cutoff (high-pass filter) was set at 0.05 and 0.5？Hz. Low-pass filters were set at 40, 100, and 150？Hz (high-frequency cutoff). In this way, twelve ECG traces per patient were made, using “auto” and “manual”
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