Inappropriate sinus tachycardia
Inappropriate sinus tachycardia (IST), first described in 1979 (1), is a fast heart rhythm arising from the sinus node, the normal primary pacemaker of the heart. That is, the heart rhythm is arising from the normal location but at an inappropriately high rate. Usually patients with IST are young women employed in the healthcare field. The exact reason for this is unknown. Usually, patients with IST come to medical attention first in their teens, twenties, or thirties. Patients usually have symptoms of palpitations and/or out-right heart racing. Associated symptoms may include chest pain, pulsations in the neck, shortness of breath, light-headedness, fatigue, sweating, etc. They typically feel their heart racing throughout the day. In some patients, antibodies are present which bind to the cardiac beta-receptors activating them (2).
Diagnosis of IST
IST is a diagnosis of exclusion. Almost always, sinus tachycardia is appropriate. Therefore causes of appropriate sinus tachycardia must first be excluded:
- Volume Depletion
- Anxiety Disorder
If these causes have been excluded, then IST can be evaluated with a 24-hour holter monitor. Patients with IST usually have average daily heart rates in excess of 100 BPM. More significantly, patients with IST do not realize any significant heart rate slowing when they lie down. This distinguishes them from patients with related disorders of orthostatic hypotension or postural orthostatic tachycardia syndrome. IST patients also realize little heart rate slowing during sleep either. On a Bruce protocol treadmill stress test, IST patients usually get their heart rate above 130 within Stage I of the test. Patients with IST may have abnormalities of either the sinus node itself or the autonomic nerves going to the sinus node or both (3,4).
Reversible causes of IST
Patients frequently ask what they can do to help their heart racing. Some benefit can be realized by avoiding alcohol, caffeine, and decongestants, but this seldom suffices to significantly reduce the symptoms. Occasionally medications such as sympathomimetics or stimulants can be stopped and the heart rate will become normal.
Work up of IST
Patients with a history of IST should be seen by a cardiologist or even better by an electrophysiologist. The typical workup of patients with IST involves having an echocardiogram done to exclude structural disease – this is virtually normal. If a cardiomyopathy is present, then the fast heart rhythm is necessary to supply cardiac output to the body. Patients who are being considered for IST must have blood or urine tests to exclude anemia, hyperthyroidism, and pheochromocytoma. Medications can also cause heart racing (hydralazine or other vasodilators, too much thyroid hormone replacement, sympathomimetics for asthma or COPD, anticholinergic medications for irritable bowel syndrome, stimulants prescribed for ADD). A tilt table test is then required to exclude autonomic disorders that can mimic IST: postural orthostatic tachycardia syndrome (POTS) or orthostatic hypotension. This disorder involves abnormal control of blood pressure, and the fast heart rhythm is necessary because of the hypotension that occurs when the patient stands and blood pools in the veins of the legs. Eventually, an EP study may be required to exclude tachycardias arising from near the sinus node, but not from the sinus node itself (5).
Prognosis of IST
The prognosis of patients with IST is excellent: it does not shorten life or cause death, stroke or myocardial infarction. It virtually never leads to tachycardiomyopathy – so if it occurs, the likelihood is that the tachycardia is arising from an atrial focus, not the sinus node. However, IST usually continues to bother the patient until something is done. Many patients initially take a wait and see approach to their IST. However as months and years pass, nearly every patient experiences more symptoms which then begin to interfere with work and other activities necessary for normal quality of life in these young patients. These patients almost always seek help for this condition.
Treatment options for IST
Because IST does not shorten life, the treatment of it does not prevent a catastrophe and prolong life. The only reason therefore to treat IST then is to alleviate symptoms. Patients with IST have four general options for therapy: doing nothing to prevent or cure IST, taking medications, having an ablation procedure or having open-heart surgery. Practically, patients needing open-heart surgery for another reason or those with sever symptoms who have failed all other therapies may have a surgical approach to their IST, but this is quite rare. For the remainder it is living with IST, taking heart rhythm medications, or having an ablation for cure.
Living with IST (no therapy)
This is a good option for patients with only minor symptoms. However for most patients, their symptoms eventually become bothersome enough that they seek some form of therapy.
Heart rhythm medications
Heart rhythm medications must be given continuously to prevent the symptoms of IST. About half of patient with IST respond to beta-blockers. Others may respond to diltiazem or verapamil. Traditional antiarrhythmic drugs such as flecainide seldom work. A new medication, Ivabradine is available in Europe and Canada but not in the US. When the medications work, the patient takes them for several years until the symptoms are entirely gone. Then therapy can be weaned off to see if the IST is still present. Little is known about the natural history of this disease.
Ablation for IST
Ablation is the option selected by the majority of patients with IST. Because sinus node ablation/modification usually worsens symptoms in patients with POTS, these patients are not good candidates (6). Curative ablation for IST is rarely performed even in most major medical centers. It is important, therefore, to find an electrophysiologist who is experienced in this delicate type of ablation. It is imperative to have detailed and accurate mapping of the origin of the tachycardia. Sometimes this mapping is done by finding the earliest site of atrial activation (7). More commonly, intracardiac mapping procedure is performed using a 3-dimensional mapping system (8). Such a system such as NavX, Ensite or Biosense is invaluable to accurately locate the site of the atrial focus causing the tachycardia. In some fortunate patients, there is an irritable spot close to the sinus node that can be ablated without risk to the normal sinus node (5). For most patients, however, the tachycardia is arising from the sinus node itself. In most people, the tachycardia responsible for the IST arises from the part of the sinus node located more cranial, anterior and medial. In these patients, this part of the sinus node which is responsible for the faster sinus tachycardias is ablated, without affecting the part of the sinus node responsible for normal sinus rhythm. These patients usually have the very fast tachycardias improved or eliminated without damage to the main part of the sinus node. In other patients, the origin of the sinus tachycardia arises from the main part of the sinus node. These patients are at risk for destruction/damage of the main sinus node producing sick sinus syndrome requiring a pacemaker if aggressive ablation is performed. However, if too definitive ablation does not occur, then the tachycardia will recur. Therefore it is important that the patient and the electrophysiologist communicate accurately and thoroughly before the procedure about the risk of either a pacemaker or on the other hand the chance of requiring a second ablation procedure. This will allow the electrophysiologist to tailor the procedure to the wishes of the patient.
Chance of success with IST ablation
The chance of ending the procedure with permanently slower heart rate is difficult to estimate. If slowing is the only goal then over 90% are successful in experienced EP labs; however, the risk of a pacemaker is high (over 30%). If on the other hand, the patient really does not want to have a pacemaker, then the chance of success is lower (50%) but so is the risk of a pacemaker (less than 5%). The risk of the procedure, excluding a pacemaker, in experienced hands is about 1%. Possible risks include minor events such as bleeding, infection, pain, nausea/ vomiting etc…. More serious side effects include stroke, perforation of the heart, myocardial infarction, damage to blood vessels requiring an extended hospital stay, open-heart surgery, or even death. There is also the risk of injuring the right phrenic nerve, which runs along the outside of the right atrium near where ablation must be performed. Finally, there is risk of narrowing of the superior vena cava causing SVC syndrome. Even with ablation, 50% or so of patients feel better with or without a pacemaker.
Cryoablation for IST
Cryoablation offers several advantages over RF ablation near the sinus node. First of all, the phrenic nerve is usually close to the site where ablation is required. With RF ablation, there is a risk of injuring or destroying the phrenic nerve. Since the right phrenic nerve is responsible for 55% of pulmonary function, this produces significant impairment to the patient. In some cases, the nerve will recover, but in others, it does not. The other problem with RF ablation near the sinus node is that the effect of the burn cannot be determined without performing it. If while an RF lesion is being made, the sinus node is injured and the heart rate plummets, the RF energy can immediately be stopped. However the temperatures in the lesion may continue to be high and the lesion continues to grow for up to 12 seconds. This may be too late for recovery of some sinus node function, and a permanent pacemaker required.
With cryoablation, the risk of the irreversibly damaging the phrenic nerve is less. Also if cryoablation is used, the site of the tachycardia can be cooled to see what the effect on the sinus node itself before a complete lesion is made. If sudden bradycardia occurs with cooling, the area can be rewarmed and the heart rate returns to its previous rate.
Lee R and Shinbane J. Inappropriate sinus tachycardia: diagnosis and treatment. Cardiol Clinics 1997; 15: 599-605
A good general review of IST from the center that pioneered sinus node modification
Shen W. How to manage patient with inappropriate sinus tachycardia. Heart Rhythm 2005; 2:1016-1019
A review of the treatment of IST from an author who does not believe in sinus node modification
- Bauernfeind R, Amat-Y-Leon F, Dhingra R et.al. Chronic nonparoxysmal sinus tachycardia in otherwise healthy persons. Annals of Internal Medicine 1979; 91: 702-710
- Chiale P, Garro H, Schmidberg J et. al. Inappropriate sinus tachycardia may be related to an immunologic disorder involving cardiac beta adrenergic receptors. Heart Rhythm 2006; 3:1182-1186.
- Leon H, Guzman J, Kuusela T, teal. Impaired baroreflex gain in patients with inappropriate sinus tachycardia. J Cardiovasc Electrophysiol 2005; 16: 64-68.
- Morillo C, Klein G, Thakur R, teal. Mechanism of inappropriate sinus tachycardia- role of sympathovagal balance. Circulation 1994; 90: 873-877.
- Cossu S, and Steinberg J. Supraventricular tachyarrhythmias involving the sinus node: clinical and electrophysiologic characteristics. Progress in Cardiovascular Diseases 1998; 41: 51-63.
- Shen W, Low P, Jahangir A, teal. Is sinus node modification appropriate for inappropriate sinus tachycardia with features of postural orthostatic syndrome? PACE 2001; 24: 217-230.
- Man K, Knight B, Tse H, etal. Radiofrequency catheter ablation of inappropriate sinus tachycardia guided by activation mapping. J Am Coll Cardiol 200; 35: 451-457
- Marrouche N, Beheiry S, Tomassoni G, et.al. Three-dimensional nonfluroscopic mapping and ablation of inappropriate sinus tachycardia. J Am Coll Cardiol 2002; 39: 1046-54