More than other disciplines electrophysiology (EP) requires the synthesis of different types of information to develop understanding. Despite years working in the environment some individuals fail to develop an interest in EP because they fail to appreciate and combine essential elements.

These elements include:

• The surface ECG

• Detailed cardiac anatomy

• Anatomical description and fluoroscopic views

• Intra-cardiac electrograms

• Cellular electrophysiology and mechanisms of arrhythmia
  

I have previously introduced some of these areas – intra-cardiac electrograms and mechanisms of arrhythmias (re-entry and focal) for example. An understanding of the surface ECG is often assumed but is lacking in some staff groups. Another neglected area is the three dimensional structure of the heart and its relation to two dimensional fluoroscopy. In this article I hope to give a flavour of this 3D element.

Many staff approach EP having had some experience of angiography and percutaneous coronary intervention (PCI) where the use of fluoroscopy is quite different. This difference is not usually explored.

Fluoroscopy in coronary work and EP

In coronary work fluoroscopy is used to produce diagnostic images and to facilitate movement of wires and equipment along the coronary arteries. In diagnostic work the image intensifier is moved in the cranio-cordal plane in addition to the right anterior oblique (RAO) – left anterior oblique (LAO) plane to produce a great variety of views - ensuring lesions are not missed. In PCI the movement of the wire, balloon or stent is constrained by the vessel lumen (hopefully!) – movements being restricted to distal and proximal within the confines of a vessel’s course. A working view is chosen that gives the operator the best information for performing the intervention.

In EP the fluoroscopy is used for “diagnosing” the position of the catheters within the cardiac chambers as the signals can only be understood when the anatomical positions of the catheters are known. Two views - RAO and LAO are usually adequate. In addition an operator is often guided in moving the catheters by instructions from a senior colleague – this requires a mutually intelligible language for describing three dimensional space (see anatomical description).

Anatomical description

For the purposes of anatomical description the body is viewed in the upright position and has three orthogonal (90 degree) axes: superior–inferior, posterior–anterior, and right–left (Fig 1).

Figures 1 - 2
1	2

Anatomical landmarks can be described in terms of these co-ordinates and the same axes are used to describe the ECG and also in fluoroscopic projections. The fluoroscopic screen presents the thorax in an upright image even though the patient is lying down. Superior structures (such as the SVC) are shown in the upper part of the screen and inferior ones in the lower part (such as the IVC). The posterior–anterior direction can be identified because the spine and the sternum are clearly recognisable as reference markers. In RAO the spine which is posterior appears on the left side of the image, in LAO the spine appears at the right. In these oblique views right and left on the screen represents a combination of the posterior–anterior and right–left axes.

However in respect to the human heart this is complicated by the fact that the heart does not sit squarely within these orthogonal axes. The axis of the ventricles tilts laterally to the left from base to apex, extending anteriorly and slightly inferiorly - with tricuspid and mitral valves following the orientation of the ventricles (fig 2). The so-called right atrium is in fact more accurately described as anterior to the left atrium and the right ventricle is anterior to the left ventricle.

Figure 3

In EP it is often essential to know whether the catheter is ventricular or atrial and if it is in the right or the left side of the heart (or if it is septal). The anterior oblique views can provide this information. These tilted views line up with the rotated heart so that right and left on the screen become meaningful with respect to the cardiac anatomy. In RAO the beam aligns with the AV valves such that left on the screen is atrial and right is ventricular. In LAO the beam is aligned with the apex to base axis of the heart such that left on the screen is the right heart while right on the screen is the left heart. Each view is lacking in information about one cardiac plane – in RAO the right and left atrio-ventricular junctions are superimposed and it is difficult to determine if a catheter (for example the coronary sinus electrode) is on the right or left side of the heart. Using the LAO view allows recognition of the right and left free walls and the distinction of these zones from the septal area but lacks information about whether a catheter is atrial or ventricular. RAO for example is useful when positioning the high right atrial (HRA), HIS and right ventricular apex (RVA) catheters. LAO is useful when positioning the coronary sinus (CS) catheter as this structure extends leftwards (with respect to the heart) around the left AV groove. Swapping between the two oblique views allows the position of a catheter to be assessed within the geometry of the heart.

The RAO View

Figure 4

The LAO View

Figure 5

The locations of accessory pathways

The locations of accessory pathways are made as seen in the LAO projection and the terminology commonly used to describe their positions cause some confusion. The descriptive terms used are anatomically inaccurate having been derived from a surgically distorted view (surgeons tended to move the heart from its natural position). The superior aspect of the heart is described as being anterior, while the anterior and posterior aspects are described as right and left lateral (see fig 6). An attempt was made ten years ago to replace the descriptions with a more anatomically correct system but with little success. 1

Fig 6. The commonly used but anatomically incorrect description of the positions of accessory pathways (in the LAO view). (Image courtesy of St. Jude Medical)

References

1. Cosio FG et al. Living anatomy of the atrioventricular junctions: A guide to electrophysiological mapping. A Consensus Statement from the Cardiac Nomenclature Study Group, Working Group of Arrhythmias, European Society of Cardiology, and the Task Force on Cardiac Nomenclature from NASPE. European Heart Journal 1999; 20: 1068–1075