Электрофизиология сердца: роль синусового и атриовентрикулярного узлов и синдром Вольфа-Паркинсона-Уайта.

Electrical activity in the normal human heart begins when a cardiac action potential arises in the sinoatrial (SA) node, which is located in the right atrium. From there, the electrical stimulus is transmitted via internodal pathways to the atrioventricular (AV) node. After a brief delay at the AV node, the stimulus travels through the bundle of His to the left and right bundle branches and then to the Purkinje fibers and the endocardium at the apex of the heart, then finally to the ventricular myocardium. The AV node serves an important function as a "gatekeeper", limiting the electrical activity that reaches the ventricles. In situations where the atria generate excessively rapid electrical activity (such as atrial fibrillation or atrial flutter), the AV node limits the number of signals conducted to the ventricles. For example, if the atria are electrically activated at 300 beats per minute, half those electrical impulses may be blocked by the AV node, so that the ventricles are stimulated at only 150 beats per minute – resulting in a pulse of 150 beats per minute. Another important property of the AV node is that it slows down individual electrical impulses. This is manifested on the electrocardiogram as the PR interval (the time from electrical activation of the atria to electrical activation of the ventricles), which is usually shortened to less than 120 milliseconds in duration. Individuals with WPW have an accessory pathway that communicates between the atria and the ventricles, in addition to the AV node.

The bundle of Kent is an abnormal extra or accessory conduction pathway between the atria and ventricles that is present in a small percentage (between 0.1 and 0.3%) of the general population. This pathway may communicate between the left atrium and the left ventricle, in which case it is termed a "type A pre excitation", or between the right atrium and the right ventricle, in which case it is termed a "type B pre excitation" in old, currently abandoned classification. Problems arise when this pathway creates an electrical circuit that bypasses the AV node. The AV node is capable of slowing the rate of conduction of electrical impulses to the ventricles, whereas the bundle of Kent lacks this capability. When an aberrant electrical connection is made via the bundle of Kent, tachydysrhythmias may therefore result.

WPW is commonly diagnosed on the basis of the electrocardiogram in an asymptomatic individual. In this case, it is manifested as a delta wave, which is a slurred upstroke in the QRS complex that is associated with a short PR interval. The short PR interval and slurring of the QRS complex are reflective of the impulse making it to the ventricles early (via the accessory pathway) without the usual delay experienced in the AV node. If a person with WPW experiences episodes of atrial fibrillation, the ECG shows a rapid polymorphic wide complex tachycardia (without torsades de pointes). This combination of atrial fibrillation and WPW is considered dangerous, and most antiarrhythmic drugs are contraindicated. When an individual is in normal sinus rhythm, the ECG characteristics of WPW are a short PR interval (less than 120 milliseconds in duration), widened QRS complex (greater than 120 milliseconds in duration) with slurred upstroke of the QRS complex, and secondary repolarization changes (reflected in ST segment T wave changes). In individuals with WPW, electrical activity that is initiated in the SA node travels through the accessory pathway, as well as through the AV node to activate the ventricles via both pathways. Since the accessory pathway does not have the impulse slowing properties of the AV node, the electrical impulse first activates the ventricles via the accessory pathway, and immediately afterwards via the AV node. This gives the short PR interval and slurred upstroke of the QRS complex known as the delta wave. In case of type A pre excitation (left atrioventricular connections), a positive R wave is seen in V1 ("positive delta") on the precordial leads of the electrocardiogram, while in type B pre excitation (right atrioventricular connections), a predominantly negative delta wave is seen in lead V1 ("negative delta"). Wolff–Parkinson–White syndrome is sometimes associated with Leber's hereditary optic neuropathy, a form of mitochondrial disease.

WPW carries a small risk of sudden death, presumably due to rapidly conducted atrial fibrillation causing ventricular fibrillation. While the overall risk is approximately 2.4 per 1000 person years, the risk in an individual is dependent on the properties of the accessory pathway causing pre excitation. It is unclear whether invasive risk stratification (with PES) is necessary in the asymptomatic individual. While some groups advocate PES for risk stratification in all individuals under 35 years old, others only offer it to individuals who have history suggestive of a tachydysrhythmia, since the incidence of sudden cardiac death is so low (less than 0.6% in some reports). Other methods of risk stratification include observing the ventricular rate during spontaneous atrial fibrillation on a 12 lead ECG. RR intervals of less than 250 ms suggest a higher risk pathway. During exercise testing, abrupt loss of pre excitation as heart rate increases also suggest a lower risk pathway.

According to the ACLS protocol, people with WPW who are experiencing rapid abnormal heart rhythms (tachydysrhythmias) may require synchronized electrical cardioversion if they are demonstrating severe signs or symptoms (for example, low blood pressure or lethargy with altered mental status). If they are relatively stable, medication may be used.

WPW pattern with hemodynamically stability and orthodromic AVRT leading to a regular narrow complex tachycardia may be managed similarly to other regular narrow complex supraventricular tachycardias: first with vagal maneuvers followed by a trial of adenosine (first line therapy). The 2015 ACC/AHA/HRS guidelines recommend beta blockers or calcium channel blockers as second line agents, electric cardioversion is reserved for refractory arrhythmias. However, if there is any doubt about the diagnosis of orthodromic AVRT or if aberrant conduction leading to a wide complex QRS is observed, it may be prudent to manage as undifferentiated wide complex tachycardia. People with atrial fibrillation and rapid ventricular response may be treated with amiodarone or procainamide to stabilize their heart rate. Procainamide and cardioversion are accepted treatments for conversion of tachycardia found with WPW. Amiodarone in atrial fibrillation with WPW, is linked to ventricular fibrillation, and thus may be worse than procainamide. They can exacerbate the syndrome by blocking the heart's normal electrical pathway (therefore favoring 1:1 atrial to ventricle conduction through the pre excitation pathway, potentially leading to unstable ventricular arrhythmias). Findings from 1994 indicate success rates of as high as 95% in people treated with radiofrequency catheter ablation for WPW. If radiofrequency catheter ablation is successfully performed, the condition is generally considered cured. Recurrence rates are typically less than 5% after a successful ablation.

The bundle of Kent is eponymously named for British physiologist Albert Frank Stanley Kent (18631958), who described lateral branches in the atrioventricular groove of the monkey heart (erroneously believing these constituted the normal atrioventricular conduction system). In 1915, Frank Norman Wilson (18901952) became the first to describe the condition later called Wolff–Parkinson–White syndrome. Alfred M. Wedd (18871967) was the next to describe the condition in 1921. Cardiologists Louis Wolff (18981972), John Parkinson (18851976) and Paul Dudley White (18861973) are credited with the definitive description of the disorder in 1930.