Positive deflection of an ECG
I understand that the voltage will be positive as measured by the ECG but I was wondering why the voltage reaches a maximum exactly at the midpoint. Is there a physiological reason (like sodium channels closing or something else) that causes the wave to increase, reach a maximum then decrease?
Picture source: Harvard Medical School. Pathophysiology of Heart Disease : A Collaborative Project of Medical Students and Faculty. Edited by Leonard S Lilly, Edition 6 ed., Wolters Kluwer, 2016.
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The mechanism is depicted in the diagram you posted, most obvious in "A":
ECG (and any voltage measurement) is based on a potential difference
You don't measure the voltage at one spot, you measure the difference between two spots. Those spots are depicted in the diagram with a (-) and (+) symbol, although the (-) is left out of BCD.
In A and D, the charges are arranged such that the (-) and (+) leads 'see' the same charge, so you measure zero voltage difference.
In B and C, there are more negative charges near the (-) lead and more positive charges near the (+) lead: you measure a positive voltage. The maximum in C is because this is the time when the charge measured at (-) and (+) is most different.
Of course the measured ECG is not from one single cell but rather the entire heart, but the concept is the same.
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