The ECG tracing in our hands is a hard copy of (some of) the electrical activity in the heart. The shape of this tracing is determined by
- The recording site (intracellular, intracardiac, exposed surface of the heart, external surface of the heart)
- The type of (recording) electrode
- The orientation/position of the cardiac lead(s)
The evaluation of the ECG depends on
- Having a basic knowledge of the anatomy of the heart’s conduction system
- Being able to measurerates, intervals, and amplitudes
- Recognizing contours and patterns
- The clinical context/setting
- Knowing the effect of drugs on the ECG
The details of cardiac electrophysiology add little to the bedside evaluation of an ECG, apart from the concepts of depolarization, repolarization, dipoles and vectors.
The laddergram is a method of analysing the origin, conduction and relationship of electrical activity in different parts of the heart. This method was first used in the middle of the nineteenth century, and is particularly useful in the analysis of conduction disorders and arrhythmias.
You & Me & The ECG
Each ECG tracing is the "tip" of an iceberg of knowledge that includes practical issues (the hands-on process of electrodes and leads) and applied knowledge (anatomy, physiology, pathology, radiology etc).
The Anatomy of Cardiac Electrical Activity
Electrical activity in the heart begins in the sinoatrialnode (SAN) and spreads through the atria, the atrioventricular node (AVN), the His-Pukinje system and the ventricles.
The location of the SAN and the AVN in the right atrium, and the course of the Bundle of His, the right bundle branch and the left bundle branch are shown in the following figures.
Some textbooks show specific conduction pathways in the right atrium linking the SAN and the AVN. There is no evidence to support their presence. However studies with intracardiac electrodes have identified two pathways near the AVN that conduct the atrial electrical impulse with different velocities: a slow pathway and a fast pathway. These pathways have an important role in some types of supraventricular tachycardia.
The structure of the heart and the pathways of electrical conduction from the SAN to the ventricles are shown below.
The normal electrical activity of the heart shown as a series of depolarizations that begin in the SAN.
Depolarization and the Parts of the PQRST Complex
Until now we have focused on the structures and pathways involved in the spread of electrical activity from the SAN to the Purkinje system. We have shown a simplified view of this process. We will now show the relation of this process to the PQRST complexes of the ECG.
Drawing a Laddergram
A laddergram shows conduction through the heart. It also allows us to deduce patterns of impulse formation or impulse conduction that are not evident from the surface ECG tracing. A laddergram can have four tiers (one for the SAN, one for the atria, one for the AV junctional area, and one for the ventricles), but usually a three tier laddergram (one for the atria, one for the AV junctional area, and one for the ventricles) is preferred.
Figure_8 shows that SAN activation is not seen on the ECG. Similarly AV nodal activation and conduction through the His-Purkinje system are not visble as separate events on the ECG, but occupy the interval from the crest of the P wave to the start of the QRS complex
We will now draw a laddergram in a person with normal sinus rhythm.
Laddergram 1 has four tiers. A vertical line (1) is drawn from just before the start of the P wave to the upper third of the SAN (sinoatrial node) tier. This marks depolarization of the SAN. A second vertical line (2) is drawn from the start of the P wave to the upper third of the A(atria) tier. This marks the start of atrial depolarization. A third vertical line (3) is drawn from just after the peak of the P wave to the top of the AVN (atrioventricular node) tier. This represents the start of conduction through the AVN (atrioventricular node) and the His-Purkinje system. A fourth vertical line (4) is drawn from the start of the QRS complex to the top of the V (ventricle) tier. This marks the start of ventricular depolarization. A fifth vertical line (5) is drawn from the end of the QRS complex to the bottom of the V (ventricle) tier. This marks the end of ventricular depolarization.
The SAN tier and line 1 have been removed from Laddergram 2 and Laddergram 3.
The vertical lines in Laddergram 2 are lines 2 to 5.
Laddergram 3 has 3 vertical lines (lines 2, 4 and 5).
Laddergram 3 is the preferred one because it is the most simple and because there are no assumptions about where AVN activation begins (as was done in Laddergram 2).