By inspecting the 12-lead ECG, the pacing site in the atrium or ventricle can be determined. This is usually poorly understood.There are four identifying ECG features:

• The stimulus artefact gives the clue as to which chamber is being paced.
• The bundle branch block configuration determines which ventricle is being paced first. For right ventricular pacing, the appearance is left (LBBB configuration), whereas for left ventricular pacing it is right (RBBB configuration). Remember with ventricular pacing, depolarization is not via the conducting system, and thus the term “configuration” must be used.
• For right ventricular pacing, the presence or absence of R waves in leads V4 to V6 helps determine the position of the cathode in the chamber.
• The frontal plane P or QRS axis determines the position in the heart. For simplicity, the axis will be referred to as normal, left, right or indeterminate. Leads attached low in the heart have a wave of depolarization propagating superior, resulting in a left or occasionally an indeterminate axis. Leads attached higher have a normal or right axis. Transitional axis appearances occur with pacing leads lying between these positions

Particularly for ventricular pacing, the presence of fusion beats will nullify these features, so there must be “forced pacing” without any intrinsic contribution.Right Atrial Pacing The P wave axis is helpful in determining where the cathode lies in the right atrium.

Left: Lead tip is in the right atrial appendage (red circle) adjacent to the sinus node and thus the axis is normal with upright P waves in leads I, II and III (red highlight). This is the traditional right atrial pacing site.Right: Lead tip is low in the atrium in the triangle of Koch above the coronary sinus ostium (yellow ring). The axis is to the left with inverted P waves in leads II and III (yellow highlight).Right Ventricular PacingThe traditional site for right ventricular pacing is at the apex (red oval).

Features: Left bundle branch block configuration (red highlight)Absence of R waves V4 to V6 (blue highlight)Left or indeterminate axis (yellow highlight) .More recently, for physiologic reasons, the right ventricular outflow tract (red oval) has been preferred.

Features: Left bundle branch block configuration (red highlight)Prominent R waves V4 to V6 (blue highlight)Right axis deviation (yellow highlight)The mid right ventricle (red circle) is now also preferred over the apex.

Features: Left bundle branch block configuration (red highlight)Prominent R waves V6 and maybe V5 (blue highlight)Axis depends on how high the lead is (yellow highlight)Intentional Left Ventricular PacingThe most common reason for intentional left ventricular pacing is biventricular pacing. This will be covered in a future “fun with ECGs”. However, epi-myocardial left ventricular pacing (red arrows) may occasionally be performed, when there is no venous access as in some congenital heart disease anatomies.

Features: Right bundle branch block configuration (red highlight).Right axis deviation (R in III) as lead is high in ventricle (yellow highlight).In about 20% of congenitally corrected L transposition of the great vessels, atrioventricular block occurs, which requires permanent pacing.

A: Anatomy. The ventricles are inverted, so the right atrium drains into a morphologic left ventricle (red arrow) and the left atrium into the morphologic right ventricle (blue arrow). The aortic root is transposed anterior to the pulmonary artery.B: With transvenous pacing the lead passes into a small right sided chamber and may appear malpositioned (red circles).C: The ECG demonstrates a right bundle branch block configuration (red highlight) and a left axis as it is apical (yellow highlight).Non-intentional Left Ventricular Pacing“Oops, I made a mistake”.On occasion, pacemaker leads implanted via the traditional transvenous route with the intention of pacing the right ventricle, demonstrate a right bundle branch block configuration on the ECG.However, not all leads lie in or on the left ventricle.

Despite the lead lying at the apex of the right ventricle (right - pathology specimen), the ECG demonstrates a right bundle branch block configuration (red highlight) and a left axis (blue highlight). An explanation is preferential left ventricular activation via His-purkinje bridges.Pacing leads inadvertently implanted via the coronary sinus into epicardial cardiac veins (red ovals) close to the cardiac apex (PA) are commonly missed at implant, unless the left lateral (LL) view is visualized with the tip lying posterior (red oval).

Lead lies in the middle cardiac vein.Features: Right bundle branch block configuration (red highlight).Left axis as the lead lies at the cardiac apex (yellow highlight).When the lead is higher on the chest radiograph (red ovals, black arrow), then it lies in an epicardial lateral vein.

Features: Right bundle branch block configuration (red highlight).Normal axis as lead is higher on the cardiac silhouette (yellow highlight).On rare occasions, the lead crosses an atrial septal defect or patent foramen ovale and paces the endocardial left ventricular apex (PA). The course through the right atrium and left ventricle takes a characteristic route (red stippled and white lines).

Features: Right bundle branch block configuration (red highlight).Extreme axis with tip at the apex of the left ventricle (yellow highlight).A lead may perforate the right ventricle at the apex and traverse the left ventricular epicardial surface to high on the chamber (red circles).

Features: Right bundle branch block configuration (red highlight).Right axis as the tip lies high on the left ventricle (yellow highlight).And the best for last!Interpret this ECG and determine where the lead lies.

Features: Right bundle branch block configuration (red highlight).Right axis (yellow highlight).Answer:High left ventricular pacing.

The ECG is pacing from a temporary pacemaker lead (T) positioned via the femoral artery into the mid to high left ventricle:Hence right bundle branch block configuration and right axis. The permanent lead (P) is at the apex of the right ventricle.It is not always the timing, but also the position!Harry Mond