The Laddergram

Introduction
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).

Figure_1.  Overview of the many steps that are involved in producing the ECG tracings that we evaluate

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.

Figure_2. Coronal CAT scan through the thorax showing: 1. Superior vena cava; 2. Sinoatrial node, which is located at the junction of the superior vena cava and the auricle of the right atrium; 3. Right atrium; 4. Atrioventricular node, located at the apex of the triangle of Koch in the right atrium; 5. Left ventricle; 6. Aorta; 7. Pulmonary artery

Figure_3

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.

Figure_4

The structure of the heart and the pathways of electrical conduction from the SAN to the ventricles are shown below.

Figure_5.  A. The chambers of the heart are represented as a series of rows placed on top of each other. This is similar to the layout used in constructing a laddergram. B. A simplified view of the pathway taken by the electrical impulse as it travels from the atria to the ventricles.
Abbreviations: SAN. Sinoatrial node; AVN. Atrioventricular node; F. Fast conduction pathway that joins the AVN; S. Slow conduction pathway that joins the AVN; BOH. Bundle of His; RBB. Right bundle branch; LBB. Left bundle branch

The normal electrical activity of the heart shown as a series of depolarizations that begin in the SAN.

Figure_6.   A simplified view of four consecutive depolarization events in a normal heart. Each impulse begins in the SAN and travels through through the atria. Near the AVN the impulse can travel through the fast pathway or the slow pathway. The impulse then travels through the BOH, the RBB and the LBB to reach the ventricles. Ventricular depolarization is not shown. Abbreviations: SAN. Sinoatrial node; AVN. Atrioventricular node; F. Fast conduction pathway that joins the AVN; S. Slowconduction pathway that joins the AVN;  BOH. Bundle of His; RBB. Right bundle branch; LBB. Left bundle branch

Figure_6.   A simplified view of four consecutive depolarization events in a normal heart. Each impulse begins in the SAN and travels through through the atria. Near the AVN the impulse can travel through the fast pathway or the slow pathway. The impulse then travels through the BOH, the RBB and the LBB to reach the ventricles. Ventricular depolarization is not shown.
Abbreviations: SAN. Sinoatrial node; AVN. Atrioventricular node; F. Fast conduction pathway that joins the AVN; S. Slowconduction pathway that joins the AVN;  BOH. Bundle of His; RBB. Right bundle branch; LBB. Left bundle branch

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.

Figure_7. The spread of depolarization from the SAN to the Pukinje system is shown in the left upper part of the figure. The contribution of this process to the normal PQRST complexes is also shown.
Abbreviations: SAN. Sinoatrial node; AVN. Atrioventricular node; F. Fast conduction pathway that joins the AVN; S. Slow conduction pathway that joins the AVN; BOH. Bundle of His; RBB. Right bundle branch; LBB. Left bundle branch;
1. PR interval; 2. PR (PQ) segment; 3. QRS complex (and QRS duration); 4. QT interval; 5. ST segment; 6. R-R interval

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

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.

Figure_9. The top of the figure shows three consecutive PQRST complexes. A slightly different laddergram has been drawn under each complex.

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).

Figure_10. The dotted vertical lines in the laddergrams are the same as those described in the previous figure. The orange circle in Laddergram 1 marks the start of SAN depolarization and the green circle in Laddergram 2 and Laddergram 3 marks the start of atrial depolarization

Figure_11. The bottom of the dotted lines in each laddergram are connected; the resulting solid line (with an arrow at the bottom of the ventricle tier) shows the depolarization sequence in the different parts of the heart.

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).