Invasive cardiac electrophysiology is based on the interpretation of intra-cardiac signals (electrograms or EGMs for short) to study arrhythmias and guide ablation.
Why are intra-cardiac signals important?
The surface ECG represents the sum of electrical activity from the heart, presenting lots of information in one easily digestible form. However it does not reveal the timing or sequence of activation of specific locations within the chambers – the distal coronary sinus or the left upper pulmonary vein for example. For this we need local electrograms picked up by endocardial catheters. With these it is also possible to record signals from structures too small to produce sufficient voltage to register on the surface ECG. One such structure is the His bundle - crucial for understanding the heart’s electrical system and many tachycardias.
The His Bundle Electrogram (HBE) (see image right) first recorded endocardially in humans in 1969 provides the key to unlock the mysteries of many tachycardias. It consists of three signals in one – an atrial component, the His “spike” and a ventricular component. The His spike or potential cannot be seen on the surface ECG – it is only by going inside the heart that it can be recorded. The separate components would appear jumbled together and confusing at the paper speed of a standard ECG recording (25mm/sec). As a result EP signals are usually recorded at 100 or 200mm/sec to stretch the signals out which has the side effect of making the ECG look a bit odd at first sight (see Fig 2).
| Fig. 2: The surface ECG during EP studies The figure below shows a surface ECG (V1) recorded at 200mm/sec. The complex is stretched out – an appearance not unlike old analogue Holter recorders with “tape stretch”
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What do the signals reveal and why record so many?
The signals in EP are a lot like floating objects that bob up and down as an ocean wave passes them by. If there are enough floating objects around they may give an impression of the direction of motion of the wave by the order in which they are moved (or activated) by it. Placing catheters in key locations within the heart allows electrophysiologists to follow the propagation of electrical activation by noting the order of the signals in time - the more signals that can be displayed the more of the wave’s propagation through the heart can be understood. So, unlike the ECG, it is the activation sequence which is most important not the morphology of the signals. The electrograms are filtered so that the timing of local activation is clear and repolarisation is largely removed.
Figure 3 demonstrates a simple use of electrograms in the diagnosis of a tachycardia. The ECG shows a regular broad complex tachycardia for which the differential diagnosis is VT or SVT with aberrant conduction. If the rhythm were SVT with aberrant conduction there would be an atrial signal (HRAp) for each ventricular signal (RVAp). In the example there are clearly more ventricular signals than atrial -making this VT. Cardiac physiologists may be familiar with using the electrograms stored by pacemakers and ICDs in this way.
Distinguishing atrial from ventricular signals
Atrial signals occur during the P wave on the ECG. Ventricular signals occur during the QRS complex. In sinus rhythm (figures 4-6) the His spike can be seen on the His channel spanning the gap between the two (when all is quiet on the ECG).
Near and far-field signals
Signals from tissue close to the recording catheter usually appear large and sharp whereas signals from tissue farther away appear somewhat smaller and more rounded. An example of this is seen the in the coronary sinus signals in the sinus rhythm example above. The coronary sinus signal labelled CS 7-8 shows an initial sharp signal from the atrium followed by more rounded one from the ventricle. This is because the proximal coronary sinus is physically closer to the atrium than the ventricle.
Distinguishing between near and far field signals can be an additional way to determine if they originate from atrium or ventricle. This can be useful when the A and V signals become very close together or superimposed - as occurs in patients with accessory pathways or during AVNRT. The example above (figure 7) shows A and V signals almost superimposed during ventricular pacing in a patient with a pathway. The signals are separated in sinus rhythm (right hand side). The mapping catheter is on the atrial side of the valve annulus and the atrial signal is the slightly sharper of the two.
| Bipolar and Unipolar Signals There are two types of signals in EP - bipolar and unipolar. Bipolar signals are produced when the voltages on the two recording electrodes both vary with time - as is the case when each is positioned within the heart. This is equivalent to plotting the height of two floating objects relative to each other as they are moved up and down by passing waves. Unipolar signals in contrast are produced when one varying signal is compared with a constant reference (an indifferent) placed outside the heart. This is equivalent to recording the position of our floating object compared to something fixed on the beach. Most signals used in EP are bipolar because they reduce far-field signals and they are less prone to electrical noise. Unipolar signals are sometimes used - because they reveal when all activation is headed away from an electrode or to see the signals from each pole of a catheter separately. The surface ECG also contains bipolar and unipolar elements – the standard limb leads are bipolar, the chest leads unipolar. |
More about the His Bundle Electrogram (HBE)
The His signal is so important because it reveals information about conduction through the AV node, essential for normal conduction and critical for many arrhythmias. Although it is not possible to record signals directly from the AV node itself we can make inferences about its behaviour by observing the atrial signal as the wave “disappears” into the node and the timing of the His spike as the wave emerges from the node and whizzes down the His bundle. If the time between these two components (the AH interval) increases, conduction through the AV node is said to have slowed. The interval between the His spike and the ventricular component (the HV interval) represents conduction down the His Purkinje system. If AV block occurs it is possible to identify the site of block – whether in the AV node or the distal conduction system (the His Purkinje tissue).
