We’re about to enter the wonderful and sometimes mysterious world of cardiac arrhythmias. This chapter will provide you with a technique to help remove some of the mystery. Ladder diagrams, also known as laddergrams, graphically represent the heart’s electrical activity, showing the transmission throughout the cardiac chambers and conduction system. Do you remember when you first studied the rules of grammar, learning the technique of diagramming sentences to match the subject with the verb? Laddergrams are the ECG equivalent, using a diagram to illustrate the transmission of the electrical impulse from one chamber to another.
FOR HISTORY BUFFS:
Ladder diagrams actually predate the discovery of the ECG. Scientists August Chauveau in France, Theodore Engelmann in the Netherlands, Karl Wenckebach in Germany, and Thomas Mackenzie in England all used this technique to depict their clinical observations of cardiac arrhythmias. Thomas Lewis is credited with popularizing ladder diagrams in his classic, The Mechanism of the Heart Beat, published in 1911. Accordingly, some references use the term “Lewis diagram,” or “Lewis lines” for this form of illustration.
REVIEW OF ELECTROPHYSIOLOGY
Let’s take a moment to review some of the material we described in Chapter 4 as it pertains to the construction of laddergrams. The normal cardiac impulse starts in the sinoatrial (SA) node and spreads through the atrial tissue via the interatrial and internodal bundles to the AV node. Transmission follows through the three regions of the AV node (atrionodal, nodal, nodal-His), continuing through the common bundle of His, the bundle branches, and the Purkinje network. These events comprise the PR interval. The electrical impulse now activates the ventricular fibers, which forms the basis of the QRS complex (Figure 12-1).
The specialized conduction system. Note that the surface ECG records only depolarization of the atria (P wave) and the ventricles (QRS complex). The tracing remains at the baseline during SA node depolarization and transmission through the AV node, bundle of His, bundle branches, and Purkinje network. (The AV node and bundle of His comprise the AV junction.)
Remember that only some of the steps we just reviewed are visible on the surface ECG. Only the P wave, which represents depolarization of the atria, and the QRS complex, which reflects the depolarization and early repolarization of the ventricular myocardium, are actually recorded by the ECG. Depolarization of the SA node is too small to be detected and the tracing remains at the baseline. Similarly, the ECG cannot record depolarization of the AV node, the His bundle, the bundle branches, and the Purkinje network, so these events can only be inferred. If that’s the case, how do we really know what’s going on in these regions of the cardiac conduction system?
Fortunately, we have techniques that allow us to accurately measure ...