"Channelopathies" group involving some ion channels of the myocardium .In developed countries, the prevalence of genetic origin long QT syndrome is estimated at 1/1,100 to 1/3,300. In approximately 30% of patients with a congenital long QT syndrome, QT is normal at ECG and the diagnosis is unknown until an event such as a loss of consciousness, a convulsive episode or a sudden death. In infants, a long congenital QT can occur by a sinus bradycardia, AV block (typically 2/1) or episodes of ventricular tachycardia and torsades de pointes.

The QT interval of ECG is the distance between the start of the Q and the end of the T wave; it is better visible and measured in D2 or V5. It is important to make the measurement over a period of stability of cardiac rhthym and 5 to 6 successive beats on a recording at a 50 mm/sec speed. In addition to heart rate and autonomic tone, age, sex, and electrolyte disturbances alter QT. 

The QT (QTc) can be corrected for heart rate using Bazett's formula: 

                                     QTc = QTmes / √RR in sec.

A QTc > 440 ms QTc is considered to be prolonged but does not necessarily imply the long QT syndrome diagnosis. The criteria vary however according to age and sex:

-        > 450 ms in prepubescent children 

-        > 460 msec in males 

-        > 470 msec in females  

QT prolongation is the witness of a severe defect of ventricular repolarization which exposes the patient to serious ventricular rhythm disorders such as torsade de pointes or ventricular fibrillation.

Another way to assess the risk of ventricular arrhythmias is the measure of the transmural dispersion of repolarization (Tp-e) which is measured on a precordial ECG derivation: it is the distance between the peak of the T wave and its end. The normal value for adults would be 40 to 50 msec with a maximum of 65 msec. Some medicines arrhythmogenic potential is evaluated by measuring the QT prolongation after administration in healthy volunteers or on animal models of LQTS: in reality, it is the extension of the transmural dispersion of repolarization which is the real factor of risk of arrhythmia in case of congenital long QT. in case of congenital long QT syndrome,  administration of drugs that prolong the QT interval should be absolutely avoided: a regularly updated list is available on the website www.qtdrugs.org or www.crediblemeds.org

The syndrome is clinically and genetically heterogeneous: 12 genes are involved and more than 500 mutations have been described. The modified genes code for subunits of potassium or sodium channels. Children suffering from a congenital long QT often have a slower than normal heart rate for their age.

Congenital long QT syndrome is only symptomatic in 60 % of patients: it is manifested by syncope, seizures or cardiac arrest triggered by an emotion (while awakening: LQT3, following an auditory stimulation: LQT2), stress or physical activity (swimming, sport: LQT1) or anaesthesia ! Congenital long QT syndrome is the cause of about 19 % of the cases of sudden death in the child aged of 1 to 13 years and 30% of the cases of those 14 to 21 years old. It is likely that part of sudden infant deaths is due to a unknown long QT.

The form without deafness and dominant transmission (Romano-Ward syndrome) represents more than 90 % of cases. Some cases are associated with a dilated cardiomyopathy. The recessive form with deafness (Jervell-Lange-Nielsen syndrome) represents less than 5% of cases of long QT and its incidence is 0.3% in the deaf persons.

A rarer form, the Andersen syndrome (LQT7), includes a cardiac and muscle phenotype: it combines a long QT and clinical signs of periodic paralysis with micrognathia and clinodactyly.Another form, the  Timothy syndrome (LQT8), associating a long QT with a syndactyly, a heart malformation and autistic-like behavior is due to a mutation on the short arm of chromosome 12 that causes dysfunction of the CAC-NA1C calcium channel.

The main features of these syndromes are summarized in table 1:

Congenital long QT syndrome: genotypes and phenotypes

Table: Acquired Causes of prolongation of the QT segment

Treatment: 

The medical treament aims at preventing the torsades de pointes and sudden death: the risk of mortality for the untreated symptomatic forms is about 20 % in the next 12 months after diagnosis.

1.        β-blocker (propranolol or nadolol) except in case of LQT3 (less efficient)

2.        if failure: placement of an implantable defibrillator

3.        in certain cases of LQT3: flecainamid

4.        in case of unsatisfactory effect of those treatments: stellate ganglion block or ablation of the sympathetic fibers from D2 to D5 and of the lower part of the left stellate ganglion (videoscopy)

List of specific drugs effects on QT space:  www.crediblemeds.org

Anesthetic implications: 

absolutely avoid the administration of drugs that prolong the QT interval. Medical treatment is intended to prevent torsades de pointes and sudden death: the risk of mortality in untreated symptomatic forms is 20% in the year following the diagnosis.

The basis of the treatment is the long-term administration of a β-blocking agent. It may include a pacemaker to prevent significant bradycardia or, in case of LQT3 form where the β-blockers are contraindicated. If syncope or rhythm disorders persist, an implantable defibrillator is implemented.

For asthma in a child carrying a long QT, the cardiologist and the pulmonologist advices must be gathered, especially in the LQT1 and LQT2 forms the basic treatment of which is the administration of β- blockers. in case of mild asthma, first try the ipratropium; add if necessary a leukotrienes antagonist (montelukast). In case of failure, carefully add salbutamol with an inhalator (avoid aerosols), an inhaled corticosteroid (increases the risk of rhythm disorders). In case of asthma crisis, sulfate Mg is useful; avoid theophylline and IV salbutamol .

A salvo of torsade de pointes usually ends spontaneously. However, in case of prolonged, or repeated bursts that are hemodynamically badly tolerated and carry a risk of ventricular fibrillation, a cardioversion should be planned or the administration of IV MgSO4 (30 mg/kg in 2-3 minutes, to be repeated after 15 minutes if the access of torsade de pointes persists).

The patient with a unknown congenital long QT  presents a high risk of ventricular arrhythmia during anesthesia and surgery: keep that in mind to explain any ventricular disturbances the origin of which is inexplicable. Similarly, the patient with a known congenital long QT syndrome and treated by β-blockers is not immune of complications. In a series of 46 children with a congenital long QT and under β-blockers that had undergone 69 anaesthesia, 3 cases of rhythm disorders were observed in the awakening phase shortly after the administration of neostigmine, atropine and ondansetron: although it is difficult to evaluate the exact role of increased sympathetic tone that is brought by the awakening phase and the concomitant administration of these drugs, it is ise to avoid them. The sugammadex has been used without problem.

The main principles of the management of these patients are to avoid factors that may prolong the QT interval and increase sympathetic tone (see table). The best ECG derivations to monitor the QT interval during anesthesia are D2 or V3.

A retrospective study has shown that well conducted anesthesia (see table below) in a patient with a known long QT and which is under  β - blocking treatment does not entail a major risk of torsade de pointes. However, there is a risk in case of surgery directly linked to syndrome (pacemaker or internal defibrillator placement) especially in infants who responds badly to medical treatment.

Anesthetic precautions in case of a congenital long QT syndrome

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Updated: June 2023