Supraventricular Tachycardia (SVT)

How does the normal heart rhythm function?

The normal heart rhythm originates from a natural pacemaker which is located in the right upper chamber of the heart. The natural pacemaker, known as the sinus node, fires off electrical impulses at regular intervals, akin to a ‘switch’ at the top of the heart going on and off at regular intervals. The electrical activity then spreads to the rest of the heart though specialized channels in the heart, which could be thought of as electrical activity being conducted through a system of ‘wires’. These ‘wires’ conduct electricity from the top chambers to the bottom chambers of the heart. In response to the spread of these electrical impulses, the heart beats at regular intervals. The normal heart rate is typically between 60-100 beats/minute.

What is an SVT?

SVT is a heart rhythm abnormality which results in a rapid heart rate. While a number of different electrical abnormalities of the heart can result in SVT (discussed in next section), the end result is that the normal heart rhythm from the natural pacemaker (sinus node) is replaced by rapid electrical activity discharging from a different and abnormal site.

What are the different types of SVT?

SVT is an umbrella term that covers a number of different heart rhythm abnormalities. The heart rhythm abnormality in patients with SVT may either begin in the upper chambers of the heart or may involve the upper heart chambers. The abnormal electrical activity may arise from a focal area which is firing rapidly (akin to an abnormal ‘switch’ going on and off rapidly) or may be the result of an abnormal circuit in the heart (electrical activity continuously and looping around an abnormal electrical circuit).

There are three main types of SVT that account for the majority of cases:

  • Atrioventricular nodal reentrant tachycardia – an abnormal electrical circuit forms a loop in a small area at the junction between the upper and lower chambers of the heart. The abnormal looping circuit arises within the hearts normal ‘wiring’
  • Atrioventricular reentrant tachycardia – an abnormal electrical circuit forms a loop between an extra abnormal pathway (connection) and the normal conduction pathway
  • Atrial tachycardia – the abnormal impulse a focal area which is discharging rapidly in the upper chambers of the heart

What are the symptoms associated with SVT?

While a sensation of the heart racing may occur due to a normal heart rhythm, a sensation of the heart racing without any specific cause may be indicative of a heart rhythm abnormality.

The rapid heart rhythm associated with SVT may result in number of symptoms. Patients most commonly experience palpitations (an uncomfortable sensation of a rapid pounding heartbeat or a fluttering sensation). Patients may also experience difficulty breathing, chest pain, dizziness, and in certain circumstances, loss of consciousness. Generally speaking, SVT is not a life-threatening heart rhythm abnormality.

The pattern of SVT is highly variable between patients. The symptoms can vary from short-lived abnormal heart rhythm episodes that stop spontaneously, to prolonged episodes lasting for many hours and eventually requiring treatment with medications. Amongst patients who have frequent and/or prolonged episodes, the arrhythmia can have a significant impact on quality of life, both from a physical symptom perspective and from a psychological perspective.

How is SVT diagnosed?

Your physician will take a detailed history with particular emphasis on the pattern of the symptoms, the frequency of symptoms and any triggers of your arrhythmia. In the majority of cases, a diagnosis of SVT is made using an electrocardiographic (ECG) heart tracing. If a patient has an ECG during an episode of SVT, the diagnosis is relatively simple as SVT is associated with typical changes on an ECG.

In a proportion of cases, capturing the SVT on an ECG may represent a challenge as it is not practical to perform the ECG during an episode of SVT (for example, because the heart rhythm abnormality stops before a patient reaches the hospital or GP surgery). In these circumstances, your physician may arrange more advanced tests to monitor the heart rhythm over a prolonged period of time (typically one day to one week depending on the frequency of symptoms). Examples of such tests include Holter monitors and event recorders.

In a minority of cases, a specialised test, referred to as an electrophysiological study, maybe necessary to reach a diagnosis. Electrophysiological studies are minimally invasive procedures designed to map the electrical pathways of the heart in detail. Electrophysiological studies are discussed in more detail in the section on catheter ablation procedures.

What are the treatment options for SVT?

The treatment of SVT can be divided into treatment in the acute setting (i.e. what can be done to stop an episode of SVT once it starts) and longer-term treatment (i.e. what can be done to prevent future episodes from occurring).

An episode of SVT may either stop spontaneously or require specific manoeuvres or medications to terminate an episode. Patients can attempt to stop an episode themselves by performing a simple vagal manoeuvre. A typical example of a vagal manoeuvre is the Valsalva manoeuvre, which involves holding the nostrils closed while trying to blow air through the nose. If an SVT episode persists for a prolonged period of time, hospital treatment may be necessary. In the majority of cases, injection of a drug called adenosine is effective for terminating the arrhythmia. Less commonly, an electrical cardioversion maybe necessary. A cardioversion involves delivering an electric shock to return the heart back to a normal rhythm.

Amongst patients who have significant symptoms associated with SVT, treatment to eliminate or suppress the symptoms should be considered. Options for treatment include either a catheter ablation procedure with the aim of eliminating the heart rhythm abnormality or medications to suppress the arrhythmia.

In a large proportion of cases with SVT, patient choice is the major determinant of the best treatment strategy. A patient’s decision to pursue a catheter ablation procedure may be influenced by adverse side effects associated with medications, the inconvenience of having to take regular medications and/or failure of medications to suppress the arrhythmia. Of note, based on current guidelines, catheter ablation can be used as a first-line treatment and a trial of medications is not necessary before deciding to undergo a catheter ablation procedure.

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