Action Potential Amplitude and Upstroke Velocity

Action Potential Amplitude and Upstroke Velocity are critical electrophysiological parameters that characterize the nature and dynamics of the cardiac action potential, reflecting the functional integrity of cardiac myocytes and their ability to propagate electrical impulses.

Action Potential Amplitude

Action Potential Amplitude (APA) refers to the difference in membrane potential between the resting state and the peak of the action potential during the depolarization phase. It quantifies the total voltage change that occurs when a cardiac cell rapidly depolarizes in response to an electrical stimulus.

• Definition: APA = Peak Membrane Potential − Resting Membrane Potential
• The resting membrane potential in ventricular myocytes typically ranges between −85 mV and −90 mV. The peak of the action potential reaches approximately +20 mV to +40 mV, depending on cell type and physiological conditions.
• APA reflects the magnitude of inward ionic current flow, predominantly through voltage-gated sodium channels (Na_v1.5) during phase 0 of the cardiac action potential.
• A high APA indicates robust excitability and effective depolarization, essential for initiating and propagating electrical signals through the myocardium.
• Reduced APA can be indicative of sodium channel dysfunction, ischemic injury, or pharmacological effects that impair excitability.

Upstroke Velocity

Upstroke Velocity, often denoted as dV/dt_max, is the maximal rate of rise of the membrane potential during phase 0 depolarization. It measures how quickly the membrane potential changes from the resting state to its peak during the initial phase of the action potential.

• Definition: Upstroke Velocity = Maximum slope of the voltage-time curve during phase 0, mathematically expressed as the highest value of dV/dt, where V is membrane potential and t is time.
• It serves as a direct indicator of the kinetics and availability of fast voltage-gated sodium channels, which generate the inward sodium current (I_Na) responsible for rapid depolarization.
• Upstroke velocity is influenced by factors such as sodium channel density, channel gating properties, extracellular sodium concentration, and membrane excitability.
• A high dV/dt_max corresponds to rapid conduction velocity through cardiac tissue, facilitating synchronous contraction.
• Decreased upstroke velocity can slow conduction, increasing the risk of arrhythmogenic conduction blocks and reentrant circuits.

Physiological and Clinical Significance

Both action potential amplitude and upstroke velocity are fundamental to cardiac excitability and conduction. They determine how effectively electrical impulses are initiated and propagated through the specialized conduction system and myocardial tissue.

• Alterations in APA or dV/dt_max are often seen in pathological states such as ischemia, infarction, cardiomyopathies, channelopathies (e.g., Brugada syndrome, Long QT syndrome), and under the influence of antiarrhythmic drugs.
• Monitoring these parameters provides insights into the electrophysiological substrate of arrhythmias and guides therapeutic interventions.
• Experimental measurement of APA and upstroke velocity is performed using intracellular microelectrodes or patch-clamp techniques in isolated cardiac cells, or inferred from extracellular recordings like electrograms and ECG.

Mathematical Representation

To quantitatively express the upstroke velocity:

where the maximum value of dV/dt during phase 0 defines the upstroke velocity.

The action potential amplitude is calculated as:

where V_peak is the maximum depolarized membrane potential and V_rest is the resting membrane potential.

Experimental Considerations

• The accuracy of APA and upstroke velocity measurements depends on the quality of voltage recordings and stability of resting membrane potential.
• Temperature, ionic composition of the extracellular environment, and pharmacological agents can modulate these parameters.
• In tissue preparations, cell coupling and intercellular resistance also influence the apparent upstroke velocity recorded extracellularly.

Summary

Action Potential Amplitude and Upstroke Velocity are interrelated electrophysiological metrics that reflect the strength and speed of cardiac cell depolarization. Their evaluation is key to understanding cardiac excitability, conduction velocity, and arrhythmia mechanisms, and they serve as essential parameters in basic cardiac electrophysiology research and clinical cardiology.