Supraventricular tachycardia is a general term that describes tachycardias arising from within the atria or the atrioventricular node. It is the preferred term as the exact cause of the tachycardia is not always obvious from reviewing the ECG.
Paroxysmal supraventricular tachycardia defines a type of rapid supraventricular arrhythmia characterised by a sudden onset and often abrupt termination.
Three of the most common supraventricular tachycardias are:
- atrial tachycardia,
- atrioventricular reentrant tachycardia, and
- atrioventricular nodal reentrant tachycardia.
The likeliest cause of atrial tachycardia is rapid and repetitive depolarisations from a focus of cells within the atria that lie outside of the sinus node.
The ECG trace below shows four sinus beats (arrowed), at a rate of 106 bpm (the RR interval is approximately three boxes), followed by a fast, regular heart rate at 147 bpm (the RR interval is approximately two boxes).
Atrioventricular Reentrant Tachycardia
Atrioventricular reentrant tachycardia is a supraventricular rhythm whose conduction pathway is a circuit that includes both the atria and the ventricles, using an additional atrioventricular pathway for at least one limb of that circuit.
As atrial conduction does not precede ventricular conduction, the P wave may appear inverted or even be hidden in the QRS complex.
Orthodromic atrioventricular reentrant tachycardia proceeds antegrade (from the atria to the ventricles) through the atrioventricular node, and retrograde (from the ventricles to the atria) over the additional pathway.
Antedromic atrioventricular reentrant tachycardia proceeds in the reverse direction, appearing on the ECG as a wide QRS complex tachycardia.
The rhythm in the following ECG trace is regular and rapid at 196 bpm. The P waves are not visible as they are hidden in the QRS complex. Note the absence of a P wave in the longer (shaded) RR interval.
Atrioventricular Nodal Reentrant Tachycardia
The most common cause of supraventricular tachycardia is atrioventricular nodal reentrant tachycardia. It is caused by two electrical pathways within the atrioventricular node that create a continuous circuit of conduction.
The tachycardia is normally initiated by an atrial ectopic beat arriving at the atrioventricular node. Depolarisation is conducted towards the ventricles down one pathway and then back to the atria through the other pathway. This sequence can perpetuate, assuming the pathways have repolarised before the next impulse arrives.
In the following example, paroxysmal tachycardia is present for four beats (shaded) at a rate of 180 bpm. The tachycardia is initiated by atrial bigeminy (beats two and four) and reverts to sinus rhythm. Normally conducted (sinus) beats are arrowed.
Atrial fibrillation is a common arrhythmia that is usually due to constant conduction within the atria caused by multiple circuits. Certain parts of the atria are depolarising whilst other parts are repolarising. This causes a loss of atrial contraction due to the ineffective quiver of the muscle.
P waves are absent from the ECG and the baseline consists of irregular waveforms that continuously change in shape, duration, amplitude, and direction. These irregular waveforms are referred to as fibrillatory waves. The resulting ventricular response (seen as QRS complexes) is completely irregular.
In the ECG trace below, the QRS complexes (arrowed) form an irregular rhythm with an average rate of 78 bpm. The ECG baseline (shaded) shows fibrillatory waveforms.
Approximately one in five strokes can be attributed to atrial fibrillation, causing this particular arrhythmia to be a significant public health concern. Prevalence increases with age such that it is present in more than 10% of the population over 75 years old.
Patients are categorised clinically based on whether their atrial fibrillation is paroxysmal (self-terminating), persistent (can be converted to sinus rhythm), or permanent (unable to convert or maintain sinus rhythm).
Atrial ectopic beats are caused by a premature depolarisation of the atria originating from a focus outside of the sinus node. They are common and benign without any additional indication of heart abnormality.
Ectopics may occur in patterns, such as bigeminy (every other beat), trigeminy (every third beat), or quadrigeminy (every fourth beat). They may also occur in sequences, such as couplets, triplets, salvos (four to seven), or episodes (eight or more).
Because these beats are premature, the RR interval between the ectopic beat and the preceding sinus beat is shorter than the RR interval between two sinus beats. Ectopic beats from the same focus have similar, if not identical, P wave morphologies.
If the conduction through the atria does not reach the sinus node before it depolarises normally, then the sinus node continues to provide the underlying rhythm of the heart. The ectopic beat will still occur early, whilst the natural depolarisation of the node will not conduct due to the depolarised state of the atria. The atrial ectopic beat will be followed by a compensatory delay and the rhythm will then continue as before the ectopic beat.
In the ECG trace below, note the difference in the P wave morphologies (shaded) for the sinus beats (one, three, four, and six) and atrial ectopics (two and five). The underlying rhythm is retained (indicated by arrows) by the compensatory delay.
If the ectopic focus causes the sinus node to be depolarised earlier than would be expected, the natural pacemaker is reset and the succeeding RR interval will be as observed from the underlying rhythm. This is termed a noncompensated atrial ectopic.
In the following example, the RR intervals (shaded) following the atrial ectopic beats exhibit no compensatory delay, indicating that the sinus node has been rest.