What Is a Sinus Arrest?
Sinus arrest occurs due to incompetent electrical discharge from the sinoatrial node. Therefore, the ECG (electrocardiogram) temporarily lacks sinus P waves. This can endure for a few seconds and precisely up to several minutes. The sinus arrest develops because the sinus node ceases layoff and can initiate backup at any point. There is usually no connection between earlier P waves and those that pursue, that is, non-compensatory. Likewise, the sinus pause manages to authorize sufficient time for pulling out beats and rhythms to heed. A sinus pause of an irregular beat is not constantly pathologic and is also noticed in non-diseased hearts. However, if a sinus pause and arrest persists for an extended period, patients become symptomatic, undergoing lightheadedness, dizziness, presyncope, syncope, and perhaps even death.
What Is a SA Node?
The SA node characterizes a crescent-like collection of myocytes split up by connective tissue. It is found at the intersection of the crista terminals on the upper border of the right atrium and the entrance of the superior vena cava. These cells possess the capacity to spontaneously generate an electrical impulse. It is the integrated movement of such cells to construct the SA node. This electrical stimulant is then transferred by perinodal cells to the right atrium and then through the remainder of the heart's electrical conduction system, ultimately leading to myocardial contraction and blood diffusion to the remnant of the body. The sinus node constantly elicits electrical impulses, thereby establishing a healthy heart's typical rhythm and acceleration. Therefore, the SA node is also known as the biological pacemaker of the heart.
What Are the Causes of Sinus Arrest?
The following are the primary causes of sinus arrest:
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Sinus Node Dysfunction: Sinus node dysfunction is usually due to irregular impulses stimulated by the pacemaker cells and anomaly in conduction across the perinodal cells. It is of two types: acquired and inherited. The acquired form is more typical. Patients can be symptomatic. There are numerous kinds and interpretations of sinus node dysfunction. These comprise sinus pause, arrest, exit block, arrhythmia, and wandering atrial pacemaker (WAP). The signals delivered by the sinus node are not directly imprinted on the electrocardiogram. But the activity of the sinoatrial nodal pacemaker is speculated from the P waves of atrial depolarization. Therefore, sinus node dysfunction is usually remarked with a problematic sinoatrial nodal reaction to the body's metabolic demands and the scarcity of P waves.
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Perimyocarditis: Perimyocarditis is an acute inflammation of the pericardium (outermost layer of the heart) and the underlying myocardium leading to myocellular damage. It is acute pericarditis with raised cardiac biomarkers. The co-occurrence of acute myocarditis and pericarditis is not unusual, as cardiotropic viruses generally induce both. The symptoms include the following:
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Acute Myocardial Infarction: Acute myocardial infarction is also a heart attack. A life-threatening disorder arises when blood flow to the heart muscle is suddenly reduced. It induces tissue damage. This is generally the consequence of a stoppage in any of the coronary arteries. A blockage evolves due to an accumulation of plaque, a component mostly produced of fat, cholesterol, and cellular waste derivatives, or a sudden blood clot that originates on the blockage. The coronary arteries are significant blood vessels in the body, providing blood to the heart. They ensure the heart rhythms properly and pump blood throughout the body.
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SA Nodal Exit Block: SA nodal exit block arises when the sinus node discharges the impulse, but the electrical impulses fail to surpass the neighboring atrial tissue. It also involves the perinodal (T) cells. The atria do not obtain the acceptable signal to contract; therefore, the electrocardiogram demonstrates a lack of P waves. The three degrees of SA nodal exit block are described below:
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First- Degree: First-degree sinoatrial exit block arises when there is a pause from when the SA node develops an action potential and the period the atrium is depolarized. This is not evident on the 12-lead electrocardiogram because the moment that the sinoatrial node discharges are not competent to be specified. The electrocardiogram entirely indicates sinus rhythm and sinus bradycardia. The first-degree SA nodal exit block shows an impulse withdrawal diverting with normal 1:1 conduction.
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Second- Degree: There are two kinds of second-degree SA nodal exit blocks – type I (Wenckebach) and type II.
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Type I (Wenckebach) Block: The P-P intervals successively shorten in the period until a dropped P wave arises. The dropped P wave consequences in a halt that is less than two P-P intervals in time.
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Type II Block: It also has a halt from a dropped P wave, a multiple of the SA nodal pacemaker input. Hence, the P-P intervals should stay consistent and compensatory.
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Third- Degree: The SA node impulse is incapable of passing the right atrium. Therefore, the atrial block does not depolarize, and there is no evident P wave.
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Sinus Arrhythmia: Sinus arrhythmia characterizes slight deviations in the sinus cycle measurement. It is specified as a deviation in the P-P interval of 120 milliseconds and even additional in the existence of typical P waves. P wave morphology stays moderately unaffected, but there are short divergences in the PR interval. Sinus arrhythmias are more generally noticed in young individuals and those disclosed to morphine and digoxin.
Conclusion
The major function of the SA node is to process as the heart's regular pacemaker. It instigates an action potential that leads to an electrical impulse transiting through the heart's electrical conduction system to induce myocardial contraction. The pacemaker cells in the sinus node do not retain a resting phase. Rather, these cells keep pacemaker potential, which depolarizes automatically after an action potential ceases. Sinus node dysfunction arises from ischemia and necrosis of pacemaker cells because of a reduction in arterial blood secondary to exacerbating coronary artery disease and myocardial infarction. The article reviews the causes, characteristics, and types of sinus arrest in detail.
