Introduction to Electromechanical Association

Electromechanical association refers to the direct relationship between the heart’s important mechanical events (to relax and fill, then contract and eject blood) and the electrical events that drive these mechanical events. Each cardiac cycle is driven by a specific sequence of electrical events, that is, movement of specific ions across plasma membranes generating the action potential. The end result is increase in calcium in the cytosol where it may interact with the contractile apparatus and trigger mechanical contraction, followed by removal of calcium from the cytosol enabling myocardial relaxation.

Important definitions related to electrical and mechanical events:

Systole: refers to the contraction phase of the cardiac cycle.

Diastole: refers to the filling phase of the cardiac cycle.

Inotropy: refers to the ability of the heart to contract.

Chronotropy: refers to the ability to increase heart rate.

Lusitropy: refers to the ability of the heart to relax.

Dromotropy: refers to the speed of conduction.

The action potential and Ca²⁺ ion movement

The action potential (described in more detail in the next section Electrical Side of the Heart) results in an influx of Ca²⁺ into the cell.

• Ca enters the cell via the long type (L-type) Ca²⁺ channels during the plateau phase of the action potential (Phase 2).
• The entry of Ca²⁺ into the cell is the first step toward contraction of the cell.
• The entry of Ca²⁺ into the cell occurs as a result of the electrical excitation of the cell (depolarization).