Physiology of Finfish and Shellfish (2+1)

2.2.20. Myocardial Electrical Activity

Typically for a vertebrate, most fish hearts are myogenic (ie. no nervous signal from the brain is necessary for each heart beat) and show a complex electromyogenic wave form. Although, the actual sites of the primary pacemaker nodes are still obscure, evidence gathered for some teleosts suggest islets of pacemaker cells in the sinus venosus and atrium.

A fish electrocardiogram (ECG) is a graphical representation of the electrical activity of the heart during a cardiac cycle. It is obtained by implementation of bipolar electrodes under the skin, commonly spanning the pericardial area. The electrodes are wired to an appropriate amplification and display system consisting of either an oscilloscope or physiological recorder. Fig. shows the ECG of an elasmobranch. Each peak in the ECG is identified with a letter from P to T that corresponds to a specific electrical activity of the heart. The chronological sequence depolarization (contractions) shows through the synchronous movement of blood through the heart. The P-wave represents the electrical excitation (or depolarisation) of the atria, which leads to the contraction of both the atria. The QRS complex represents the depolarisation of the ventricles, which initiates the ventricular contraction. The contraction starts shortly after Q and marks the beginning of the systole. The T-wave represents the return of the ventricles from excited to normal state (repolarisation). The end of the T-wave marks the end of systole. Obviously, by counting the number of QRS complexes that occur in a given time period, one can determine the heart beat rate of an individual.

Wave	Chamber Myocardium Depolarized
V	Sinus Venosus
P	Atrium
QRS	Ventricle
B	Conus ateriosus (elasmobranchs only)