24. The patient has unnatural rhythm – analysing the ECG part 3 April 2015
Keywords: prehospital ambulance paramedic ECG
Using the main game plan, the normal heart beat is set in motion by the SA node. This is capable of spontaneous contraction but accepts instructions for rate from the central nervous system. It has a normal rate of control between 60 and 100 per minute. The electrical impulse then moves down to the AV node, moves slowly through there to allow the atria contract. It continues on at top speed through the bundle branches and Purkinje fibres in the ventricles. The T wave completes this as the ventricle repolarises for the next impulse to arrive.
Analysing the ECG is all about identifying the impulse that is running the show and identifying the conduction pathway that is then transmitting that impulse. This approach can be used to divide all of the presentations on the ECG into one of three groups. The first is where there is an abnormal rate of the SA node. The correct pacemaker is working but working outside its expected performance. The second is where the impulse is set in motion by a pacemaker that is not the SA node at all. The third is where there is a fault somewhere in the conduction system.
This second type of problem occurs where the impulse has its origin somewhere lower down the conduction pathway than the SA node. The impulses can originate essentially at any other part of the conduction pathway. The problem that accompanies all of them is that the abnormal impulse origin fires faster than the SA node wants to giving it the ability to take over at least momentarily.
One location where this can happen is below the SA node but still above the AV node. That is, from a focus somewhere in the atria. Where this happens a P wave will appear but it will look very different from the others. The exact shape will depend on where it starts and the direction of impulse flow. It may be flat, part up and down (biphasic) or smaller and peaked. The impulse will still enter the AV node and can still follow the best conduction pathway downward through the ventricles. Hence there will be a PR interval and a normal looking QRS. The atria can in some cases also contract and contribute to ventricular filling.
Impulses can begin in the AV node itself. Where this happens there may be no P wave as the impulse is already past the atria. On some occasions the impulse may start near the atria and be able to pass back upward. In this case a P wave might appear but it will be formed by the impulse going in reverse. Because it is going backward it will appear upside down compared to a normal P wave. Also the PR interval will be all wrong. The upside down P wave may appear very slightly before the QRS or just as likely it may come afterward. In this case it is the QRS that is causing the P wave and not the other way round. Clearly where this happens the atria are not coordinated and helping to fill the ventricles with blood.
Finally, impulses can begin in the ventricles below the AV node. This may be higher up near the AV node in the His bundle, in the right and left bundle branches or further down in the Purkinje fibres. The spread of impulse flow will vary with where the origin is. The origin point will be important in determining how much of the normal conduction pathway will be used. The QRS will be formed depending on this. It may be slightly wider than normal and pointing in the same direction as a normal QRS or it may be much wider and pointing mostly the other direction. Often no P wave is seen. Where there is, it will be after the QRS complex.
In each of these cases the abnormal impulse, or ectopic beat, can be single, or a few of them or even a sustained run. They will come before the next scheduled and SA node impulse is expected. Where one impulse comes before the SA node has a chance to fire it is known as a premature contraction – either atrial, junctional or ventricular. Where a few occur it is a salvo. Where there is a sustained run the rhythm is called an atrial, junctional or ventricular rhythm. Frequently these are faster than the SA rate producing a tachycardia. This is important as it does not allow the SA node to reassert itself and take over again.
Rhythms that originate above the ventricle (AV node or above) are known as supraventricular rhythms. There are numerous different types depending on where the original impulse is. Arguably the most common is atrial fibrillation where there are multiple fast firing foci in the atria. The AV node cannot let them all through so the ventricular rate can vary from slow to very fast. Since the impulses arrive rapidly and irregularly, the QRS response is usually quite irregular without any predictable pattern. Instead of a P wave being present there are multiple little deviations on the base line that look almost like it is trembling.
A similar but far less common rhythm is atrial flutter. Unlike atrial fibrillation this does not have multiple foci for its impulse origin. Instead it has one focus but with a difference. This one foci finds a path in the atria that allows it to perform circles and return back to the same point to set another in motion. A very tall pointed P wave results that when finished has another one immediately appear again. Each of these is so close they take on the look of a toothed saw blade. This allows for a fast firing rate and many of these can be passed by the AV node. It is usually regular but sometimes the AV node varies which impulses it will let through.
Sometimes the focus is just another part of the conduction system in the atria. Where this happens a regular but fast rhythm follows that is faster than the SA node. The P waves will be different to the SA node P wave as said but the QRS will look as expected. This is atrial tachycardia. Sometimes there is more than one focus but not as many as atrial fibrillation. This is multifocal atrial tachycardia.
Finally rhythms that start in the AV node can set up a similar loop to that which occurs in atrial flutter. In this way the impulse keeps looping back in the AV node setting itself up to start the next impulse. This allows for a very fast rhythm to occur, faster than the SA node. The QRS will appear normal since it continues down the bundle branches from then on.
Rhythms that originate below the AV node are ventricular. They can be fast and once greater than 100 per minute are called ventricular tachycardia. The QRS will not appear totally normal as the origin is already part way down one of the bundles or in the Purkinje fibres. Part of the normal conduction will be used and if high enough up one ventricle might receive the impulse by the normal path. The other though will have to wait slightly for the impulse to travel by a less useful pathway. As such, the QRS will appear quite wide and perhaps in two distinct parts. The slower ventricle may make the QRS look like a W or an M. Depending on the origin point, it may appear pointing up or down but will often be a different direction to the normal QRS. There may be a P wave if the impulse is passed back up but it will follow the QRS.
Finally, like the atrium, the ventricle can be overwhelmed by multiple foci causing it to quiver and become completely uncoordinated. The appearance will be a bizarre and irregular series of deviations without pattern ranging from small deviations to very large. There is no discernible P wave, QRS or T wave.
Jeff Kenneally – www.prehemt.com