1.8 Electrocardiographic Physiology Foundation

Electrocardiographic Physiology Foundation is the body of fundamental principles describing how the summed electrical activity of the heart, generated by the coordinated depolarization and repolarization of cardiac tissue, produces a signal detectable at the surface of the body, forming the physiological basis for the electrocardiogram. It establishes the relationship between cellular electrical events and their recordable manifestation, including the origin of characteristic waveforms and the significance of their timing and morphology.

Basis of the Recordable Electrical Signal

Summation of Cellular Electrical Activity

The electrocardiographic signal arises from the summation of electrical activity generated by millions of individual cardiomyocytes, whose combined depolarization and repolarization produce a net electrical vector detectable externally.

Volume Conduction Through Body Tissues

Electrical currents generated within the heart spread through the surrounding body tissues via volume conduction, allowing the summed cardiac electrical activity to be detected at the surface of the skin at a distance from the heart itself.

Correspondence Between Electrical Events and Surface Waveforms

Each phase of cardiac electrical activity, including atrial depolarization, ventricular depolarization, and ventricular repolarization, produces a corresponding deflection on the surface recording, linking specific waveforms to specific underlying events.

Characteristic Waveforms and Their Origins

The P Wave and Atrial Depolarization

The P wave corresponds to the spread of depolarization across the atria, originating from the sinoatrial node and reflecting the electrical activation of atrial myocardium prior to atrial contraction.

The QRS Complex and Ventricular Depolarization

The QRS complex reflects the rapid depolarization of the ventricular myocardium, its distinctive shape arising from the sequence and direction of activation as impulses spread through the His-Purkinje system and surrounding tissue.

The T Wave and Ventricular Repolarization

The T wave corresponds to ventricular repolarization, representing the return of ventricular cardiomyocyte membrane potentials to their resting state following the plateau phase of the action potential.

Physiological Significance of Timing and Morphology

Intervals as Reflections of Conduction Timing

The intervals between waveforms, including the interval from atrial to ventricular depolarization, reflect the underlying timing of impulse conduction through the atrioventricular node and conduction system.

Sensitivity to Underlying Electrical Abnormalities

Because the recorded signal directly reflects underlying cellular electrical events, alterations in impulse generation, conduction, or repolarization produce corresponding changes in waveform timing, shape, or sequence.

Relationship to Mechanical Cardiac Events

The electrical events recorded precede and initiate their corresponding mechanical events, such that atrial depolarization precedes atrial contraction and ventricular depolarization precedes ventricular contraction, linking electrical timing to the mechanical cardiac cycle.