Impulse Generation in the Heart

Gami  NK

Basic Principles

Impulse Generation in the Heart1

INTRODUCTION

The most important feature of heart is to contract. Each contraction maintains the rhythmicity of heart due to excitation wave of electrical activity preceding contraction. The electrocardiograph records this electrical impulse. The electrical impulse starts in the sinus node and spreads from there to the atrio-ventricular node through the specialised tissues in the atrium and then proceeds also through the specialised conducting tissues (Tables 1.1 and 1.2). “Anatomy and Physiology of specialised conducting tissues.”

Table 1.1 Anatomy of specialised tissues

	Site	Blood supply	Nerve supply
1. Sinus node:	Right atrium near junction of superior vena cava with lateral right atrial wall	Right coronary artery	Vagus slows rate of impulse transmission Sympathetic increases it
2. AV node:	Right of upper margin of intervent septum	Right coronary artery	—Do—
3. Other specialised tissues: In atria: 3 internodal conducting pathways which channelise impulses from SA node to AV node activating atria longitudinally In ventricle: Bundle of His and its two branches—right and left bundle branch. The right branch runs right side of interventricular septum and left branch crosses to the left side of septum dividing into anterior fascicle spreading anteriorly and the posterior fascicle spreading inferiorly. Ventricle is activated transversely. Purkinje fibers in LV muscle.

Table 1.2 Physiology of specialised conducting tissues

	Rate of activation	Remarks
Sinus node	60–100/mt	Impulse travels in a wave like fashion and longitudinal way towards AV node. No retrograde conduction
AV node	50–60/mt	Unique capacity to delay passage of impulse to maintain proper sequence of atria followed by ventricular contraction
Ventricle	30–35/mt	Ventricle activated in transverse fashion from endocardium to epicardium

Electrical Activity in Cardiac Muscle

The resting cardiac muscle cell is in a state of electrical equilibrium keeping positive charges on the outer surface of the cell and negative charges on the inner surface and this state is termed polarised state. When the muscle is stimulated the charges are reversed so that the outer surface becomes negative and the inner surface positive and this is termed depolarised state. The current of stimulation will have positive head and a negative tail. A unipolar electrode facing the oncoming head of this current will record a positive or upward deflection and an electrode facing the receding tail will record a negative or downward deflection (Fig. 1.1).

Fig. 1.1: Electrical activity in cardiac muscle

Depolarisation and Repolarisation of Cardiac Muscles

Electrographically the ventricles are composed of three muscle groups—interventricular septum, free wall of right ventricle and free wall of left ventricle. Depolarisation commences in the left side of septum and spreads through the septum from left to right. This is the first stage of depolarisation (indicated in Fig. 1.2). The depolarisation then proceeds outward simultaneously through the free wall of both ventricles from endocardial to epicardial surfaces. This is the second stage of depolarisation (indicated in Figs 1.3 and 1.4). The free wall of left ventricle has a larger muscle mass possessing larger potential electrical force than the free wall of the right ventricle, produces larger deflection in the ECG nullify the smaller force of the right ventricle. An electrode facing the left ventricle therefore writes an initial downward deflection “q” due to spread of the impulse away from the electrode through the septum, followed by a large deflection or “R” wave due to spread of the impulse towards the electrode through the left ventricular free wall producing a “qR” wave called the left ventricular complex (Fig. 1.5) and while the electrode facing the right ventricle will show an initial small upright deflection “r” due to spread of the impulse towards the electrode through the septum followed by a large downward deflection a “S” due to spread of the impulse away from the left ventricular free wall producing an “rS” termed right ventricular complex.3

Fig. 1.2: First stage of depolarisation of ventricles

Fig. 1.3: Second stage of depolarisation of ventricles

Fig. 1.4: Depolarisation of the ventricles in a simplified form

Fig. 1.5: Left ventricular complex

Fig. 1.6: Right ventricular complex

Fig. 1.7: Relation of systole and diastole with ECG waves and heart sounds

See Table 1.3 “Sequence of depolarisation and repolarisation in cardiac cycle”. Figure 1.7 shows relation of systole and diastole with ECG waves and heart sounds.

Table 1.3 Sequence of depolarisation and repolarisation in cardiac cycle

■ At atria
	• Impulse starts at sinoatrial node at 60–100 per minute
	• Passes through atrial muscles and 3 specialized conducting pathways (Internodal bundles) writing P-wave
	↓
■ At junctional tissue
	• Then impulse reaches atrioventricular node, Bundle of His and main left and right bundle branch (Junctional tissues) responsible for P-R segment where impulse is delayed. So rapid is the transmission of impulses that the delay in AV node is necessary to maintain the proper sequence of atrial followed by ventricular, contraction. Grant says: “There is no parallel elsewhere in biology or in physics for an electrical impulse delayed for so long an interval in so small a region without undergoing decrement or becoming extinguished”. P-R interval represents the time taken for atrial activation plus the time taken for the impulse to traverse AVN and Bundle of His
	↓
5
	↓
■ Ventricle activation/and repolarisation
• First stage of ventricular activation: IVS activation:
	– Impulse after passing through junctional tissues activates interventricular septum from left side via bundle branches writing in electrocardiogram normal small q-wave in left sided leads (I, aVL, V₅-V₆) and small r-wave in right sided chest leads (V₁-V₂)
	↓
• Second stage of ventricular activation:
	– Further spread of impulses occur down the bundle branches to simultaneous activation of both ventricles via Purkinje fibers and is responsible for R and S-wave
	↓
• Third stage of ventricular activation:
	– The final stage of activation occurs from endocardium to epicardium of ventricle. The total time taken for ventricular activation is responsible for QRS interval
	↓
• Fourth stage of ventricular repolarisation (recovery) :
	– Following completion of ventricular activation, there is period of electrical inactivity shown by the S-T segment in ECG when all parts of ventricle are in depolarised state. Finally, repolarisation of ventricle occurs writing T-wave

Chapter Notes

Impulse Generation in the Heart1