25. The patient has unnatural rhythm – analysing the ECG part 4 April 2015
Keywords: prehospital ambulance paramedic ECG
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.
The third group of ECG rhythm problems is a fault somewhere in the conduction system other than the SA node. There are several problems that can happen here, some more serious than others.
The first example is where the AV node is functional but there is a fault in the tissue divide between the atria and the ventricles. Normally there is only the one pathway through, the AV node. In some cases there can be a second pathway that can also allow impulses to get through. This accessory pathway does not have the capacity to delay the impulse. As such it can set in motion contraction of part of the ventricle where it first comes into contact. This meets up with the impulse flowing through the AV node so most of the QRS is still normal. The big difference here will be that the early contact with the ventricle will blur the end of the P wave with the start of the QRS. In fact they will appear to touch each other as the QRS curves off the P wave. This is known as the delta wave.
This problem is known as pre-excitation syndrome. Wolf-Parkinson-White is a well known example of this. The other problem this can cause is the impulse coming down the AV node can be transmitted back up the accessory pathway. When this happens this new impulse loose in the atria can find its way back down to the AV node again. This can set up a loop that can run much faster than the SA node and lead to an abnormal tachycardia.
There are problems within the AV node. The AV node is meant to slow impulse conduction on the way through to the ventricles. Sometimes it can be even slower than normal. This is first degree heart block and shows as a longer PR interval. The QRS still gets through only is further behind the P wave than expected. Also, because the rest of the conduction pathway is still normal, the P wave and the QRS look normal.
Sometimes this delay in the AV node can vary. Instead of staying one consistently prolonged time it can instead get progressively longer. When this happens eventually a point will come when one impulse doesn’t get through at all. In this case there will be a P wave not followed by a QRS. This is one type of second degree heart block called Mobitz type one.
In some cases the PR interval may be relatively normal for a number of beats then have one P wave that is not conducted so not followed by a QRS. This is the other type of second degree heart block, Mobitz type two. In both cases the QRS should then go on to be normal as it can follow the rest of the working conduction system. Occasionally the rhythm is one where every second P wave is not followed by a QRS. This is called 2:1 second degree heart block since you cannot work out if it is the type one or the type two.
Finally impulses from the SA node may not get through at all. In such cases the ventricles will have to take over running themselves albeit more slowly. Since the normal conduction pathway may not be totally used given the origin of the ventricular impulse being somewhere in the ventricle a wide QRS complex is likely to be seen. The P waves can still be seen but they will be at a different rate to the QRS complexes. The P waves will not be related to the QRS complexes and the two rhythms will be running independently. The AV node and His bundle can take over at a slower rate of around 40 – 60 per minute. In this way it cannot normally compete with the SA node and is a backup plan. Failing this the Purkinje fibres also have the ability to initiate an impulse. This will be at an even slower rate limited to around 15 – 40 per minute.
Of course the P wave may fail to fire completely leaving some other part of the conduction system to take over. Sinus pause/arrest has been discussed earlier as has the ability of the conduction system to provide an earlier than expected impulse. In this case though the fall back impulse will come later than expected since it is trying to cover for the higher up failure. This fall back impulse can be atrial, AV junctional or ventricular. It is referred to as an escape beat. At times these escape beats may have to be a series of slow beats if the SA node simply won’t come back into action. This is an escape rhythm.
The QRS should normally be fairly narrow since movement through the bundle branches and Purkinje fibres happens quickly. If it was wider it would mean that it took longer. If either of the branches is damaged, the impulse on that side will be blocked. For it to continue, it will have to pass around the block using less efficient pathways. This will take longer and so the QRS will get wider. The higher the block is, the less of the normal pathway that can be used. In turn the delay will be greater and so the wider the QRS.
Also, instead of the QRS forming one peak, the efficient side will peak slightly earlier, the other side later. The QRS will form more of a W or M shape. This is a bundle branch block or intraventricular conduction defect. It can be in the left sided bundle or the right forming identifiable patterns. These blocks may form in lower branches and so not be as disruptive to the conduction path. The critical thing to determine when a wide QRS is found is to determine if there is a P wave related to it or not. A wide QRS with a P wave is SA node in origin but a conduction problem in the ventricles. A wide QRS without a P wave is ventricular in origin. There is a big difference between sinus tachycardia with a conduction defect and ventricular tachycardia!
One final abnormal conduction pattern warrants discussion – the external cardiac pacemaker. Often installed when the underlying cardiac conduction system has shown a tendency to fail the pacemaker has generally two key features. The first is that there is an external impulse origin. This is known as the pacemaker spike and appears as a straight line immediately preceding the P wave or QRS depending on where the spike is placed. It is a straight line as the impulse is very quick. The second feature is that the QRS is often very wide. This is because in many cases the pacemaker wire is embedded in the right ventricle wall after being fed up via a vein. This causes the impulse to start off the normal conduction system just as if it was an impulse forming in the Purkinje network.
Jeff Kenneally – www.prehemt.com