Short QT syndrome is a new and still insufficiently studied variant of rhythm disturbance, which today stands out as an important proarrhythmogenic factor and predictor of sudden cardiac death. The development of this condition is associated with several genetic mutations. In most patients, the syndrome proceeds without clinical manifestations, less often it manifests dizziness, pre-fainting and syncopal states. For diagnostic purposes, ECG, daily Holter monitoring, and other instrumental methods (Echo-KG, cardiac EFI) are performed. The only effective treatment option is the installation of a cardioverter defibrillator.
R94.3 Deviations from the norm detected during functional studies of the cardiovascular system
In clinical cardiology, long QT syndrome is better known, which is often associated with the sudden death of young people. The reverse condition, short QT syndrome, was described only in 2000, and detailed clinical observations of patients with this pathology were carried out in 2005. The true frequency of occurrence of this type of rhythm disorders has not been established due to the complexity of diagnosis and the lack of vivid clinical signs, however, scientists assume a greater prevalence of the problem in the population.
Most scientists agree that the development of the short QT syndrome is associated with genetically determined disorders of the excitation process in the conduction system of the heart. This explains the frequent family cases of sudden death in the absence of organic cardiac pathologies. Mutations are inherited by autosomal dominant type. There are the following types of damaged genes responsible for the occurrence of the syndrome:
- KCNH2 gene. The mutation is located on the long arm of the 7th chromosome at the 7q35-q36 locus. This gene is necessary for the production of a protein that controls the work of myocardial potassium channels, the activity of individual nerve cells.
- KCNQ1 gene. The gene is localized on the short arm of chromosome 11 (11p15.5). It encodes a protein that promotes the excretion of potassium from cells into the intercellular space. Protein is found in the tissues of the heart, inner ear, stomach and large intestine.
- KCNJ2 gene. A recently identified mutation is the cause of type 3 short QT syndrome. The disease occurs when the function of one of the protein components of the potassium pump, regulated by the corresponding gene, increases.
In addition to genetic mutations, external factors can contribute to reducing the interval. The most dangerous are heart palpitations, an increase in body temperature, an increase in the content of potassium and calcium in the blood. An independent risk factor is an imbalance of the autonomic nervous system with a predominance of sympathetic nerve tone.
The electrophysiological basis of short QT syndrome is the acceleration of the second phase of the action potential in the myocardium of the right ventricle. Pathology occurs due to a genetically determined defect of potassium channels, which leads to an increased release of potassium ions from the intracellular fluid, an ionic imbalance. Hypersympathicotonia plays a role in the pathogenesis of the syndrome, which inhibits the work of calcium channels and exacerbates existing disorders.
To determine how short the interval is, there is a special formula that looks like “656/(1+HR/100)”. This formula is used to calculate the normal interval in milliseconds (ms) at the appropriate heart rate. The short interval syndrome is established if the indicator is less than 88% of the norm. 2 variants of the disease were identified: permanent idiopathic shortening, paradoxical (bradysponsive) decrease in the QT interval.
The main clinical manifestation of the syndrome is syncopal states — short-term episodes of loss of consciousness that occur in combination with arrhythmia, disorders of blood supply to the brain. Despite the rapid development of symptoms, 3 consecutive phases are distinguished in syncope: presyncopal state, postsyncopal state in syncope itself. Most cases begin at the moment of physical activity or immediately after its termination.
The presyncopal (lipothymic) phase lasts up to several minutes. Patients feel sudden increasing weakness, nausea, dizziness and flashing “flies” in front of their eyes. As a rule, noise and stuffiness in the ears are bothering, a person has a premonition that he will soon lose consciousness. Cardiogenic fainting may be preceded by tightness in the chest, a feeling of lack of air, a feeling of heart sinking.
The second stage is a fainting — loss of consciousness with a sharp decrease in muscle tone, as a result of which a person falls. Objectively, there is a pallor of the skin, a decrease in blood pressure, a weak fullness of the pulse. Cardiogenic syncope lasts for several minutes, after which consciousness returns to the patient. The postsyncopal period lasts from a few minutes to several hours, manifests itself as weakness, increased sweating, uncertainty of movements.
Short QT syndrome is a proven prognostically unfavorable sign, in which sudden cardiac death occurs more often. According to the results of studies, it was found that reducing the interval to 400 ms and below increases the probability of death by 2 times against the background of a sudden arrhythmia. Without treatment, the probability of death by the age of 40 is about 40%. Short QT syndrome is considered one of the main causes of sudden death in children and adolescents.
During physical examination, the cardiologist fails to detect signs pathognomonic for the short QT syndrome. The exception is a combination of pathology with atrial or ventricular fibrillation, which is characterized by pronounced objective symptoms. To diagnose a violation of the interval length, the patient is assigned an extended examination, which includes the following methods:
- ECG. When registering standard ECG leads, a decrease in the duration of the QT interval of less than 320 ms, a shortening or complete absence of the ST segment, the appearance of symmetrical narrow T teeth in the thoracic leads is revealed. For more accurate results, Holter ECG monitoring is recommended for 24 hours.
- Functional tests. An important diagnostic criterion is an ECG with physical activity (treadmill test). In patients with short QT, in response to an increase in heart rate at the time of training, the interval does not change or increases. In healthy people, with an increase in heart rate, the duration of QT decreases.
- EFI hearts. During electrophysiological examination, a short refractory period of the atria and ventricles is determined, while in 90% of patients, ventricular tachycardia or atrial fibrillation are induced during diagnosis.
- Ultrasound of the heart. If organic pathologies are suspected that could provoke arrhythmia, echocardiography is performed. Using a non-invasive method, the main structural and functional parameters of the heart and large vessels are visualized.
- Laboratory methods. To determine the possible reasons for the decrease in the interval, tests of electrolyte composition, acid-base balance of blood, cardiospecific enzymes are prescribed. To exclude hormonal prerequisites of pathology, the levels of catecholamines, thyroid hormones, androgens are measured.
- Genetic research. When sequencing the genome, mutations typical for this syndrome can be detected, however, due to the insufficient amount of data on this disease and the complexity of the study, genetic tests are used only to a limited extent.
With a shortened QT interval, standard antiarrhythmic drugs do not demonstrate proper effectiveness, therefore their use is impractical. Modern scientific data show a positive effect of alkaloids of the bark of the cinchona tree on the prolongation of the refractory period of the ventricles, prevention of ventricular arrhythmias. The main method remains surgical treatment — implantation of a cardioverter defibrillator, which is carried out in the
- presence of a documented episode of cardiac arrest during a life-threatening arrhythmia;
- the presence of episodes of ventricular tachycardia, which are accompanied by a sharp deterioration in health;
- regularly occurring cardiogenic syncope;
- burdened family history of sudden cardiac death.
A cardioverter defibrillator (CVD) is installed by a minimally invasive method using endovascular access. This method shortens the rehabilitation period of patients, reduces the risk of postoperative complications. During the recovery period, patients need to follow the recommendations on the motor regime and physical activity, regularly visit a cardiologist to monitor cardiac activity.
Prognosis and prevention
The prognosis depends on how short QT is in calculations, the presence or absence of clinical symptoms of the disease, the state of the cardiovascular system. With early diagnosis and CVD diagnosis, the prognosis is relatively favorable. To reduce the risk of life-threatening rhythm disturbances, patients with short QT syndrome are recommended to avoid heavy physical exertion, strong emotional stress. Effective primary prevention measures have not yet been developed.