Central to diagnosing atrial fibrillation is monitoring the electrical activity of the heart. So, before discussing the diagnosis of AF further, we need to understand in more detail the electrical activity as it passes through the heart and what ‘normal’ looks like. Then, we can look at the traditional, simplest and most convenient way of determining that the heart rhythm is normal or otherwiae, and that is with an ECG.
 
the normal heart

As we have already seen, there are four main chambers within the heart: a right atrium (top)-ventricle (bottom) combination and a left atrium-ventricle combination. Blood flows from the body into the right side of the heart where it is pumped to the lungs for gas exchange before being returned to the left side of the heart to be pumped back around the body.

It is the right atrium (top right of the heart) in which the sinoatrial (SA) node acts as the heart’s pacemaker, setting the beat for the rest of the heart.

All the cells of the heart have an automatic depolarisation system. This is called automaticity. This means that if you were to leave some of the cells out on a table, you could watch the cells contract as the electrical activity across the membrane changes. These cells leak sodium, potassium and calcium slowly and spontaneously until the charge across the membrane hits a threshold. When that threshold is reached, the change in the electrical potential over the membrane sets off an electrical trigger, called the action potential, which is the electrical depolarisation of the cell (lots of sodium and calcium flooding into the cell). This sets off the muscle fibrils and thereby, the contraction within those cells.

This is the ‘electricity’ of the heart. The cells in the sinoatrial node have the fastest tendency for this leakage so they have the fastest depolarisation cycle of any of the cells in the heart. That is why they set the beat.

Another really interesting thing about the muscle cells within the heart, over and above their having leaking membranes, is that they are all connected. Each cell connects to the next, to the next, to the next. This is achieved by special receptors between the cells that interlace and interconnect. This allows the electrical activity to flow rapidly through those cells. Those cells acting as one is called a syncytium.

When the sinoatrial node fires off, it leads to a wave of electrical activity (as we have said, not unlike a Mexican wave). Having started at the SA node, the wave moves from right to left through the atria.

Before flowing into the ventricles, the electrical activity has to pass through a fibrous ring that separates the atria and the ventricles. The fibrous ring does not allow the passage of electricity but has a special point where the electrical activity can travel from the top to the bottom of the heart. This is the atrioventricular (AV)  node.  This  node  acts  as  a gatekeeper and permits the electrical activity to pass from the atria to the ventricles, holding the passage for about one-tenth of a second. This ensures the ventricles beat after the atria.

Helping to distribute the electrical activity from the AV node through the ventricles are more specialised cells, Purkinje fibres, that act like copper wires. The heart contracts and then it goes back to normal. Remember the squid moving through the water: synchronous, coordinated, smooth. This is what the heart should do.
 
the ECG

An ElectroCardioGram (ECG) allows the electrical activity of the heart to be seen from different directions. Twelve electrodes monitor the electrical flow. The result is printed out or viewed on a screen.

The electrical activity in the atria is referred to as a P wave and it reflects atrial depolarisation or the electrical flow. As the electrical impulses pass through the AV node and then on to the special distribution cells, a QRS complex is created, reflecting the depolarisation of the major muscle of the heart. The last part of the electrical heartbeat is the T wave and this is the return of normal repolarisation to the heart muscle, ready for the next beat.

So, in the normal heart we see sinus rhythm, a beautiful synchronous atrial contraction followed by a beautiful synchronous ventricular contraction.

In atrial fibrillation, the atria lose their synchronicity; the beats become chaotic and irregular. The AV node, the gateway to the electrical stimulation of the bottom of the heart, is bombarded by chaotic electrical activity, producing a heartbeat that is referred to as ‘irregularly irregular’.

The atria are not working, the ventricles are beating in an irregular way and, so, the pump can’t work properly.

diagnostic thumbprint

In a normal ECG, a P wave is followed by a QRS and a T. In atrial fibrillation there is no P wave. Instead, an ECG trace baseline shows bumps and irregularities and an occasional QRS complex from depolarisation reaching the ventricle, occurring irregularly and unpredictably.

This is our diagnostic thumbprint. Until we obtain a clear electrical tracing, we can’t be sure of the rhythm.

beyond the ECG
 
irregular wriggly line of AF
 
The importance of obtaining an ECG trace to make the correct diagnosis explains why patients are asked to wear monitors for a time or have devices implanted for longer monitoring. It also helps to explain why the pulse sometimes needs to be checked in different ways.

Holter monitor

An ECG which monitors the heart for just a few minutes might not be long enough to establish if atrial fibrillation is present. If someone seems to be going in and out of atrial fibrillation with some regularity, maybe once or twice every couple of days, then the medical practitioner has the option to ask the patient to wear a monitor.

The monitor is a little box that the person can wear while undertaking normal daily activities such as being at home, at work, shopping, playing sport. It records electrical activity so that the medical practitioner can look for any irregularities that would establish an atrial fibrillation diagnosis. The result is not as detailed as from a 12-lead ECG, but it gives good information over a 24-hour, 48-hour or three-to-five-day period.

The most common monitoring, however, occurs in hospitals and is related to anaesthetic monitoring during surgery or monitoring undertaken in high dependency units. In these situations, finding atrial fibrillation is common and its appropriate care is important for the outcome for the patient.

implanted device

In some patients, there can be the suspicion that atrial fibrillation is occurring, but not frequently. If the cardiologist believes that the importance of making a diagnosis is high enough, then a very small device called a loop recorder (pictured) may be implanted under the skin above the heart.

This really neat device can monitor the heartbeat 24-hours a day, seven days a week, for up to three years. The device is interrogated in a similar manner to a pacemaker, on a regular basis, using a special programmer that reads the data.

Patients can live a normal life while the device is implanted. The device does not go near a major organ; it is very simple to implant and take out and provides an incredible amount of data in relation to a suspected irregular rhythm that occurs very infrequently.

It is also worth remembering that pacemaker interrogation may also confirm the presence of atrial fibrillation.
 
pulse

When a person’s heartbeats are rapid and irregular and two or more beats come close together, the heart may not have had time to fill properly. Because the heartbeat is irregular, it means the filling of the ventricle, which takes time, will be variable; some beats will have more time to fill and some less. This results in more or less blood being pumped. Under these circumstances, the pulse won’t necessarily be effectively felt at the wrist
because the beats with ‘less blood’ may not be strong enough to be felt.

Similarly, it may be hard to measure the blood pressure as the beats will be of variable intensity.

Quite commonly, doctors who frequently deal with AF will measure the heartbeat by listening to the heart sounds at the apex of the heart which is on the left lateral border of the chest wall. However, the most accurate way is through an ECG. That way the medical practitioner can count exactly how many QRS complexes, or how many activations of the ventricle, are occurring in a given period.

ANSWERING AN IMPORTANT QUESTION
ARE ALL PALPITATIONS ATRIAL FIBRILLATION?

The simple answer to this question is, “No; not every palpitation, not every irregular heartbeat, is atrial fibrillation”.
Let’s think of this in terms of palpitations or heartbeats that are either rapid and irregular, or rapid and regular.

rapid and irregular

When it comes to rapid and irregular heartbeats then atrial fibrillation, the chaotic electrical activity of the atrium giving rise to an erratic and irregular beat from the ventricle, is definitely the first possibility to consider. An irregular pulse, however, can be felt at the wrist as a consequence of other things.

We know that extra beats can arise in the top of the heart, from the atria, and also from the bottom of the heart, the ventricles. During normal beats, extra beats can slip in and give the illusion of irregularity. Extra beats arising from the top part of the heart are called atrial ectopic beats, atrial (top of the heart) ectopic (out of place) beat (heartbeat). So, atrial ectopic beats can give rise to irregularity of the pulse.

A similar process can occur in the ventricles. It is then called a ventricular ectopic beat.

Both atrial ectopic beats and ventricular ectopic beats can be felt by the patient because there is a change in rhythm. As the ectopic beat arises from a different location to where the atrium or the ventricle is normally activated, there is a discordant electrical impulse through the heart which results in a discordant contraction. Because of that, the heart moves in a different way from normal and the patient will often feel it.

So, an irregularity can arise from atrial ectopic beats or ventricular ectopic beats even though the patient may be in normal sinus rhythm.

rapid and regular

Rapid heartbeats can often be regular and this can point to a number of conditions.

It can be atrial flutter which is similar to atrial fibrillation except that there is a specific short circuit, called a re-entrant circuit, occurring in the right atrium. This sets a timing for the atrial rhythm at 300 bpm which is transferred to the ventricle at 150 bpm or fewer, due to the slowing down of the transmitted signals from the atria to the ventricles by the ‘protective’ AV node. If the AV node blocks every second impulse, we call this a 2:1 block.

The risks associated with atrial flutter are considered the same as for atrial fibrillation and both are treated similarly: rate control, anticoagulation and the possibility of restoration of normal sinus rhythm.

Another rapid regular rhythm is caused when a different type of re-entrant circuit forms within the atria. This re-entrant circuit, like atrial flutter, is an electrical short circuit, but in a different location. It also keeps going around and around, and firing on itself. This gives rise to a supraventricular tachycardia, supra (above the ventricle) tachy (fast) cardia (pertaining to heart).

Supraventricular tachycardia is a relatively common condition. It can give rise to heart rates of over 150 bpm and it often carries a different set of consequences to atrial fibrillation and atrial flutter.

Rapid rhythms that arise in the ventricle are very serious.

Ventricular tachycardia, that the patient feels, is not common but it is a high-risk rhythm. Once the bottom part of the heart, the main pumping chamber of the heart, is beating rapidly in an abnormal rhythm, potentially we have problems.

Ventricular fibrillation is not felt. This is a rhythm that is not compatible with life. In atrial fibrillation, with the top part of the heart not working properly, the function of the pump is diminished but the ventricle still works. In ventricular fibrillation, when the main pumping chamber of the heart goes into chaotic rhythm, it just doesn’t work at all. This is a cause of sudden cardiac death.