we know The cardiac action potential is about 90mv-100mv
and ECG is the plot of electrical potential caused by irritation of the heart muscle
I want to know why in ECG we get a signal about 1mv while The cardiac action potential is about 90mv-100mv ?
Hello Farzaneh. Please provide more details about your specific query.
Thank you
Dear Farzaneh,
compared to the active electrical behaviour of cardiomyocytes (capable of generating action potentials), the torso is only a passive medium. This means it only "diffuses" the electrical activity generated in the heart, attenuating its magnitude.
Maybe some modelling studies can help you to understand this phenomenon better. See for example:
Article Human ventricular activation sequence and the simulation of ...
The cardiac action potential measures the difference of electric potential between the intracytoplasmic fluid and the extracellular fluid (which is high), while the ECG measures the difference of electric potential between two points outside the cells (for bipolar leads, i.e. Di DII DIII) or between one point outside the cell and a "mean" lead (for unipolar leads, i.e. aVR, aVL, aVF, V1-V6), which is low. Moreover, the tissues interposed between the cardiomiocytes and the lead will attenuate the amplitude of this signal.
I fully agree with the views of respected colleagues Andrei G Dan and Enrico Brunetti.
The surface ECG is present a record of the complex composite electrical signal of the heart. Actually it is the picture of action potentials distorted by their algebraic sum, dispersed in time and space and modified by the capacitances and resistances found in the way. The ECG in all its variations and the intracavitary electrographic recordings, provide a distance view of the cardiac electric phenomen, because it represent the summation of the cellular electric activity of a bigger or smaller cardiac area, depending on the technique used. Electrograms hide cellular electrical phenomen during a great part of the cardiac cycle, when the absence of electrical gradients in the organ creates a real "electric silence" from a distance. Mathematical models with dipoles and multipoles have been developed; however, a well-defined relation between action potentials and electrograms lacks scientific basis. What we have are useful, but not so scientific, didactic attempts to relate action potentials to ECG in some situations. The diagnosis of anatomical, metabolic, ionic, and hemodynamic alterations using of electrograms is only possible due to study of innumerable empiric correlations. However, in regard to the occurrence of electrical events of the cardiac cycle, electrograms provide very precise and useful data, that is fundamental importance in the diagnosis of arrhythmias.
In addition, we usually think about electrical activity of the cardiac cell that triggers the mechanical activity in a single forward direction. However, there is evidence of a reverse effect - the mechanical activity could also cause changes in the electrical potential of the cells. This phenomenon is called excitation-contraction coupling or mechanoelectrical feedback. As a example, the appearance of action potentials with early/late postpotentials due to extrasystoles (corresponding to cardiac beats, where the arterial pressure curve shows absence significant ejection). They can be considered as isovolumetric contractions against an infinite afterload, causing evident changes in the action potential.
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