Question.

Which one of the following statements is true of the spread of electrical activity through the heart?

a. Conduction velocity in atrio-ventricular (AV) node is 5 meters/second
b. Atrioventricular (AV) nodal delay gives ventricles time to fill before ventricular systole
c. Conduction of cardiac impulse is slowest in the bundle of His
d. Purkinje fibres transmit the cardiac impulse from the bundle of his to the left and right bundle branches
e. The right bundle branch is primarily responsible for septal depolarisation

 

 

Answer.

The answer is b.
 

Explanation.

Conduction velocity in cardiac tissues (meter/second):

SA node: 1m/s
Atrial pathway: 1m/s
AV node: 0.5m/s
Bundle of His: 1m/s
Purkinje fibres: 4m/s
Ventricular muscles: 1m/s

 

In sinus rhythm, the cardiac impulse begins at the sinoatrial (SA) node and passes via fast conducting internodal tracts (atrial pathway) to the atrioventricular (AV) node and via Bachmann’s bundle to the right atrium. The cells of both the sinoatrial (SA) and atrioventricular (AV) node are slow conducting. Of note, the time taken for the cardiac impulse to pass across the AV node is around 0.2 seconds (corresponding to the PR interval on the ECG). This built in nodal delay allows atrial contraction to take place (marked by the P-wave on the ECG) and ventricular filling to complete before the next ventricular systole (marked by the RS of the ECG)

From the AV node, the impulse passes across the non-conducting fibrous septum separating the atria and ventricles via the bundle of His and then on into the left and right bundle branches which pass through the interventricular septum. Depolarization of the septum starts on the left side and moves to the right across mid-portion of the septum. This generates a brief rightward and downward movement of the cardiac impulse seen as the physiological Q waves on the normal resting ECG.

Finally, from the bundle branches, the impulse is distributed tom the ventricular myocardium by the fast conducting Purkinje fibres.

 

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