The junctional replacement rhythm of the heart sets in as soon as the normal rhythm generator, the sinoatrial node in the right atrium, fails or the frequency input falls below about 60 Hz. Stimulus formation occurs in the junctional zone of the atrioventricular (AV) node, His bundle, and right atrium because the AV node itself has no automaticity for spontaneous depolarization. The junctional replacement rhythm shows a typical rhythm of 40 to 60 Hz.
What is junctional replacement rhythm?
The junctional replacement rhythm of the heart sets in as soon as the normal rhythm generator, the sinus node in the right atrium, fails or the frequency requirement falls below about 60 Hz. The primary excitation of the heart originates from the sinus node, which is located in the wall of the right atrium at the junction of the superior vena cava. The junctional replacement rhythm serves as a secondary cardiac pacemaker. It originates from the junctional region of the AV node, His bundle, and right atrium because the AV node itself does not exhibit spontaneous polarizations and is therefore unsuitable as a pacemaker. The junctional replacement rhythm automatically steps in as a backup whenever the electrical stimuli of the sinus node are absent for a period of slightly more than one second. Due to its typical natural frequency of 40 to 60 discharges per second, the substitute rhythm takes command not only in the event of a total failure of the sinus node, but also in cases where the given frequency falls below the natural frequency of the junctional substitute rhythm. When the replacement rhythm is activated, the atria are usually not excited or are excited only in a backward direction (retrograde). In the ECG, this is noticeable by the absence of the P wave or by a negative P wave. The P wave represents the excitation pattern of the atria and appears on the ECG before the QRS complex with the prominent R wave in each case in people with normal sinus rhythm.
Function and Purpose
The junctional replacement rhythm of the heart has a tremendously important and, in certain situations, even life-saving function. There are a variety of causes that can lead to dysfunction of the sinus node. The causes are located in or around the heart (cardiac) or completely outside the heart (extracardiac). Circulatory disturbances due to coronary artery disease, valvular heart disease, inflammatory and degenerative processes within the heart, and myocardial diseases are typical cardiac conditions that can trigger a manifest cardiac arrhythmia that initially originates from the sinus node. All circumstances and diseases that can affect the sinus rhythm are summarized under the term sick sinus syndrome. The “kick-in” of the junctional replacement rhythm can be life-saving in these situations under certain circumstances. Typical causes of sinus rhythm disturbances that lie outside the heart may include thyroid disease, hormonal imbalances, febrile illness, and pulmonary embolism. Although the heart’s excitation formation and conduction are largely autonomous, the heart’s beat frequency or the amount of blood pumped per unit of time in particular must be able to adapt to the requirements,
This is why, for example, messenger substances of the sympathetic and parasympathetic nervous systems as well as some hormones can influence the control of the heart rhythm and blood pressure. This means that not only hormonal disorders can have a negative influence, but also drugs (in the form of undesirable side effects) and neurotoxins can lead to significant cardiac arrhythmias and functional impairment of the sinus node. The category of extracardiac disorders also includes electrolyte disturbances, especially potassium deficiency, which can disorganize the primary cardiac rhythm. A special situation exists with electrical accidents, since electrical accidents do not occur in nature on land, except for very rare lightning accidents. Within evolution, therefore, no protective mechanism has evolved that could provide appropriate protection for the cardiovascular system. Even in these cases, the junctional replacement rhythm is available as a backup for the primary pacemaker sinus node and may be life-saving in some circumstances.
Diseases and medical conditions
The junctional replacement rhythm does not normally go into action because it is overridden by the faster sinus rhythm.The electrical impulses of the sinus node reach the AV node before the intrinsic depolarization of the connecting regions to the His bundle can set its own electrical impulse. If the sinus node is functioning normally, the junctional replacement rhythm does not cause any discomfort or danger. However, hazards associated with junctional replacement rhythm may arise in the presence of Wolf-Parkinson-White syndrome (WPW syndrome), which is usually genetic, or in the presence of so-called AV block. Symptomatic of WPW syndrome are electrical excitations that circle between the atria and the ventricles. The cause of the circling currents is one, or in very rare cases several, additional conduction pathways that directly connect the atria to the ventricles, thus electrically bypassing the AV node. The creation of the additional conduction pathway between the atria and ventricles is genetic, but does not necessarily trigger WPW syndrome. It is most common in twenty- to thirty-year-olds. Symptomatic of the syndrome is intermittent palpitations (tachycardia). AV block, on the other hand, involves a delayed, a temporary, or a permanent interruption in the conduction of electrical impulses from the atria to the ventricles. AV block can occur due to congenital abnormalities of the heart or be acquired later. Causes include inflammatory processes in the heart, autoimmune diseases, or side effects of medications. In particular, antiarrhythmic drugs, cardiac glycosides, and beta blockers can cause AV block. However, in the case of drug side effects, AV block is reversible in most cases. AV block is classified into severity grades I, II, and III. AV block of I. Grade is simply a delay in signal propagation to more than 200 milliseconds, which can be seen on the ECG by the time interval between termination of the P wave (contraction of the atria) to the onset of the QRS complex. AV block of II. Grade is characterized by the failure to transmit atrial contractions at regular or irregular intervals, and the eventual failure of ventricular contraction. In III degree block, the contraction signal to the ventricles may fail completely, and ventricular replacement rhythm usually kicks in as a second endogenous safeguard.
