ECG Quiz

In atrial fibrillation, the atria no longer conduct electricity from the sinoatrial node in an orderly fashion. As a result, P-waves are lost. As a result of disordered atrial activity, only occasional waves of depolarisation pass through to the atrioventricular node and cause ventricular activation. This causes the typical irregular rhythm.

Second-degree heart block (Mobitz type 1) is a disease of the atrioventricular node. Typical ECG findings include progressive prolongation of the PR-interval with associated regular dropping of QRS complexes.

In most healthy individuals, you would expect QRS complexes to be between 70 - 110 milliseconds. If a QRS complex lasts longer it is described as a "wide QRS" and indicates inefficient conduction ventricular conduction (e.g. bundle branch block).

A short PR-interval indicates abnormally short conduction time between the atria and ventricles. This is typically caused by the presence of an accessory pathway between the atria and ventricles. WPW syndrome is an example of this kind of disorder. In WPW syndrome, an accessory pathway known as the bundle of Kent is present. Most individuals are asymptomatic, however, there is a risk of sudden death without treatment.

In healthy individuals, the PR-interval is between 0.12-0.2 seconds. A PR interval longer than 0.2 seconds suggests the presence of heart block. A short PR-interval may suggest the presence of an accessory pathway between the atria and ventricles (e.g. WPW syndrome).

Left axis deviation is rarely the result of left ventricular hypertrophy and more often due to defects in the conduction system of the heart.

To calculate heart rate from an ECG you can count the number of large squares in an R-R interval then divide 300 by this number. Therefore 300/3 = 100 bpm.

Leads II, III and aVF all view the heart in the inferior plane. ST-elevation in only these leads would be suggestive of an inferior myocardial infarction.

In right ventricular hypertrophy, the increased muscle mass of the right ventricle causes an increased signal on the ECG. As a result, the axis of the heart is shifted to the right with lead III becoming more positive and lead I and II becoming less positive.

Leads V3 and V4 represent the anterior portion of the heart. ST elevation in these leads would be suggestive of anterior myocardial infarction. The anterior portion of the heart is supplied primarily by the left anterior descending artery.