Isoelectric line
Also calledisoelectric baseline · baseline · isoelectric line
Definition
The flat baseline of the ECG, recorded when there are no significant voltage gradients in the ventricles, against which wave amplitudes and ST-segment deviations are measured. The TP segment (from the end of the T wave to the next P wave) and the ST segment normally lie nearly flat at about this level, and ST- and T-wave amplitudes are measured against the TP or PR segment.
Clinical noteBecause ECG machines filter out slow baseline drift, the true zero-voltage level cannot be determined, so an apparent ST shift can partly reflect a shift of the PR or TP segment.
Sources
- Rautaharju PM, Surawicz B, Gettes LS. AHA/ACCF/HRS recommendations for the standardization and interpretation of the electrocardiogram: part IV: the ST segment, T and U waves, and the QT interval. Circulation. 2009;119(10):e241-e250. doi.org/10.1161/CIRCULATIONAHA.108.191096
- Samesima N, God EG, Kruse JCL, et al. Brazilian Society of Cardiology guidelines on the analysis and issuance of electrocardiographic reports - 2022. Arq Bras Cardiol. 2022;119(4):638-680. doi.org/10.36660/abc.20220623
Last reviewed 2 October 2026. Every definition in the EKG Dictionary cites the guideline, consensus statement or original paper it comes from.