Plateau Baseline Voltage in Cardiac Action Potential
Yes, you are correct—the plateau phase of the ventricular cardiac action potential maintains a voltage of approximately +10 mV (ranging from +10 to -10 mV). 1
Electrophysiological Basis
The plateau phase (Phase 2) of the ventricular action potential corresponds to the ST segment on the surface ECG and maintains this positive voltage through a delicate balance of ionic currents:
The transmembrane voltage during the plateau remains at approximately +10 to -10 mV, which is markedly positive compared to the resting membrane potential of approximately -85 mV 1
This plateau voltage is maintained by slow inward calcium currents that balance the outward potassium currents, keeping the membrane depolarized for an extended period 1
The voltage changes slowly during this phase and remains at approximately the same level across all ventricular myocardial cells, which explains why the ST segment appears isoelectric on the ECG 1
Repolarization Sequence
Following the plateau phase, rapid repolarization (Phase 3) occurs:
The membrane potential repolarizes from the plateau voltage of approximately +10 to -10 mV down to the resting level of approximately -85 mV 1
This rapid repolarization phase corresponds to the T wave on the surface ECG and is mediated primarily by time- and voltage-dependent potassium currents 1
Clinical Relevance
Understanding this plateau voltage is essential for interpreting:
ST segment abnormalities, which reflect alterations in the plateau phase voltage gradients between different regions of myocardium 1
Drug effects and electrolyte disturbances that can alter the duration and voltage of the plateau phase, affecting both contractility and arrhythmia susceptibility 1
The mechanism of calcium entry during the plateau, which is critical for excitation-contraction coupling and determines the strength of myocardial contraction 1