Cardiac pacemaker discharge at about 40-60 beats per minute, and are called the "secondary pacemaker". Further down the electrical conducting system of the heart is the Bundle of His. The left and right branches of this bundle, and the Purkinje fibres, will also produce a spontaneous action potential at a rate of 30-40 beats per minute, so if the SA and AV node both fail to function, these cells can become pacemakers. It is important to realize that these cells will be initiating action potentials and contraction at a much lower rate than the primary or secondary pacemaker cells. The SA node
Strain rate imaging Cardiac stress test), the regional dysfunction due to ischemia will become evident when the myocardial oxygen demand surpasses the Coronary flow reserve of a stenosed coronary artery. Strain rate imaging during stress has been shown to give incremental value over ordinary echocardiography, both diagnostic and prognostic. In stress echo, the increased heart rate has speckle tracking at a disadvantage, due to the limited frame rate that affects tracking at higher heart rates. In Left bundle branch block (LBBB), the asynchronous activation of the left ventricle gives asynchronous contraction as well. This asynchrony can be visualised by ordinary echocardiography. It can