1.5.3 The conduction process
The conduction network in horses is broadly similar to that found in other species, but there are important differences. The conduction process follows a predictable pathway in the normal heart, passing along specialised conduction fibres, leading to a co-ordinated contraction of atrial and then ventricular muscle (Figure 1.11). The contractile cells of the heart are found in two syncytia, the atria and the ventricles. These syncytia are normally electrically separated except at the atrioventricular (AV) node.
The SA node governs the rate of the normal heart. The normal rhythm is therefore called sinus rhythm. The SA node is a crescent-shaped structure located at the site where the cranial vena cava enters the right atrium. The impulse is formed in the SA node and spreads across the atria to the AV node. Conduction occurs along specialised fibres; however, the impulse also leads to contraction of atrial muscle. The right and then the left atrium are depolarised. The electrical activity associated with depolarisation of this muscle mass results ina sufficiently large electrical field for it to be detected at the body surface as a P wave.
The AV node is found at the junction of the atria and the ventricles, in the interventricular septum (IVS). When the impulse reaches the atrioventricular junction it finds a barrier to further spread. The specialised cells of the AV node conduct the impulse slowly because of their high resting potential, slow phase 0 depolarisation and poor electrical coupling. When the AV node is depolarised, because only a small number of cells are affected, no deflection is seen on the surface ECG. The delay in conduction is represented on the surface ECG by the interval between the P wave and the onset of the QRS complex. The AV node has a slower natural rate of automaticity than the SA node; however, if the SA node fails to initiate an impulse, it can take over as the pacemaker at a slower rate. In the horse, conduction through the AV node is profoundly affected by vagal tone. It is often sufficiently slowed or reduced in amplitude to result in a marked reduction in the normal rate of conduction, or complete abolition of further spread of the impulse (see section 7.7.2).
Once the impulse has passed through the AV node, conduction spreads via specialised fast-conducting fibres within the bundle of His, the main bundle branches (left and right), and the Purkinje network. The Purkinje network ramifies throughout the myocardium. In the horse and other ungulates it is particularly widespread in comparison with humans and small animals. Depoof the Purkinje fibres activates adjacent myocardial cells. Depolarisaof the ventricles is completed rapidly and results in a co-ordinated contraction. Depolarisation of the bundle of His and the Purkinje network is not detected on the body surface ECG. Intracardiac electrodes are required to detect the relatively small change in potential associated with depolarisation of the small number of cells. However, depolarisation of the myocardium results in subelectrical forces, the net result of which produces the QRS deflection on the surface ECG. The Purkinje tissue has a slower natural rate of automaticity than the SA node or the AV node. If, however, they fail to initiate an impulse, the Purkinje tissues can take over as the pacemaker at a slower rate.
Each cell within the heart repolarises after depolarisation. The sum of the repolarisation processes within the heart can be detected at the body surface in the same way as the electromotive forces of depolarisation. Ventricular repois seen as the T wave. The change in electrical field caused by atrial repolarisation may or may not be seen. It is termed the atrial T wave, or Ta wave.