Is the isoelectrical baseline the same as the resting state of myocardial cells?

Isoelectrical Baseline and Resting Myocardial Cells: Not the Same

The isoelectrical baseline on the ECG (specifically the TP segment) corresponds to the resting state of myocardial cells at approximately -85 mV, while the ST segment is also isoelectric but represents the plateau phase at approximately +10 to -10 mV—these are fundamentally different cellular states that both appear flat on the surface ECG due to absence of voltage gradients, not because the cells are in the same electrical state. 1

Understanding the Cellular Basis

The confusion arises because two different phases of the cardiac action potential both produce isoelectric (flat) segments on the ECG, but for entirely different electrophysiological reasons:

The TP Segment (True Resting State)

• The TP segment represents electric diastole, when ventricular myocardial cells are at their resting transmembrane potential of approximately -85 mV. 1
• During this phase, all ventricular cells have completed repolarization and are at rest, creating minimal voltage gradients between cells. 1
• This is the true baseline reference point, representing cells in their polarized, resting state ready for the next depolarization. 1

The ST Segment (Plateau Phase)

• The ST segment corresponds to the plateau phase (phase 2) of the ventricular action potential, where transmembrane voltage is approximately +10 to -10 mV—markedly positive compared to resting potential. 1, 2
• During this plateau phase, the transmembrane voltage changes slowly and remains at approximately the same level in all ventricular myocardial cells. 1
• The plateau voltage is maintained by slow inward calcium currents that balance outward potassium currents, keeping the membrane depolarized for an extended period. 2

Why Both Appear Isoelectric

The critical insight is that both segments appear flat on the surface ECG because of the absence of significant voltage gradients between different regions of myocardium, not because the cells are in the same electrical state. 1

• When all cells are at the same voltage level (whether at -85 mV during TP or at +10 mV during ST), there are no voltage differences to generate deflections on the body surface ECG. 1
• The surface ECG detects voltage gradients between different regions, not absolute transmembrane potentials. 1

Clinical Implications for ECG Interpretation

Baseline Reference Selection

• The PR segment (end of PR interval) is used as the compromise isoelectric baseline for measuring ST-segment deviations, particularly during exercise when the TP segment becomes unusable due to tachycardia and merging of T and P waves. 1
• The TP segment is the ideal isoelectric reference when identifiable, as it represents true electrical diastole. 1, 3

ST Segment Deviation Measurement

• ST-segment elevation or depression is measured relative to the PR junction (or TP segment when available), representing deviation from the expected isoelectric plateau phase. 1
• Current-of-injury patterns are represented on surface ECG by deviations of the ST segment from the isoelectric baseline, reflecting alterations in either the resting membrane potential or the plateau phase voltage. 4

Common Pitfall to Avoid

Do not assume that because the ST segment appears isoelectric (flat) on the ECG, the myocardial cells are in a "resting" state. During the ST segment, cells are actually in a depolarized plateau phase at approximately +10 mV, actively maintaining calcium influx for contraction—this is metabolically and electrically active, not resting. 1, 2 The cells only return to their true resting state of -85 mV during the TP segment after complete repolarization. 1