During the Neuroscience course, you completed a module on the effects of alterations in plasma calcium levels on neuronal excitability. Conditions that alter plasma calcium levels also affect the characteristics of the electrocardiogram. To complete the story we started in neuroscience, this module provides information on changes in the ECG when plasma calcium levels are altered.
High Ca2+ levels can block sodium movement through voltage-gated sodium channels, retarding sodium entry into excitable membranes. Thus, generation of action potentials is altered in neurons as well as skeletal and cardiac muscle cells.
The reduced depolarization of cardiac myocytes shortens repolarization time, so the Q-T interval is reduced. In addition, Ca2+ entry during phase 2 of the cardiac action potential is facilitated during hypercalcemia. This affects the closing kinetics of the L-type Ca2+ channel, such that the plateau phase of the cardiac action potential is abbreviated and repolarization occurs earlier.
|Hypercalcemia may also cause a positive deflection in the ECG at the junction between the QRS complex and the ST segment. This deflection is called an Osborn wave, or J wave. The biophysical mechanisms responsible for generation of this wave are unclear.|
Low Ca2+ levels facilitate sodium transport, as the normal inhibition of sodium movement by Ca2+ is lost. Thus, low Ca2+ levels result in hyper-excitability of excitable tissues, including cardiac myocytes. The increased depolarization of cardiac myocytes lengthens repolarization time, so the Q-T interval increases. In addition, Ca2+ entry during phase 2 of the cardiac action potential is reduced during hypercalcemia. This affects the closing kinetics of the L-type Ca2+ channel, such that the plateau phase of the cardiac action potential is lengthened and repolarization occurs later.
The table below summarizes the effects of alterations in plasma Ca2+ on cardiac activity:
|State||Plateau Phase of Cardiac Action Potential||Change in Q-T interval|
|Hypercalcemia||Shorter; May be Followed by Osborn Wave (J Wave)||Reduced|