What is sinoatrial block?
Sinoatrial block is a heart rhythm disorder characterized by blocking electrical impulse conduction from the SA node (the origin of the normal electrical activity) and atria (upper chambers of the heart).
Normal heartbeat is controlled by electrical signals that start in the sinoatrial node. The sinoatrial node is located at the top of the right atrium. The heart is divided into four chambers: two atria and two ventricles top to bottom. The role of the atria is to fill the ventricles with blood, and the ventricles and propel (pump) the blood to the lungs and the body. Normally, the electrical impulse from the sinoatrial node spreads concentrically (in oil stain) into the atria, then it reaches the atrioventricular node from where ventricular it is transmitted to the ventricles through His and Purkinje network.
Sinoatrial block represents the delay or interruption, intermittent or permanent, of the electrical impulse from sinoatrial node to the atria.
What causes sinoatrial block?
• vagotonia (state of hyperexcitability of the vagus nerve, manifested by the predominance of parasympathetic system).
• abdominal or pleural surgery
• brain tumour
• intracranial hypertension (abnormally high pressure in the skull, inside the brain)
• sick sinus syndrome (impaired function of the sinoatrial node)
• ischemic cardiomyopathy (disease of the heart caused by insufficient blood supply, including angina and myocardial infarction).
• myocarditis (inflammation of the heart muscle) or acute pericarditis
• digitalis intoxication (digoxin), quinidine, beta-blockers, calcium channel blockers.
• amyloidosis (amyloid infiltration atrial myocardium)
• old age
• hypothyroidism (decreased thyroid hormone secretion)
• advanced liver disease
• hyperkalemia (high blood potassium)
However, in most cases, a specific cause for the sinoatrial block cannot be determined.
What are the types of sinoatrial block?
• sinoatrial exit block first degree means a delay in the transmission of electrical stimulus, an elongation of the time needed to get from sinoatrial node to the surrounding atrial tissue. It cannot be diagnosed based on the electrocardiogram and requires electrophysiological studies in order to confirm the diagnosis.
• sinoatrial exit block second degree means that the impulse from the sinoatrial node is intermittently blocked in its passage to the atrial tissue. It is manifested by intermittent absence of P waves on the electrocardiogram.
• sinoatrial exit block third degree or complete, is characterized by the absence of atrial activity or the occurrence of another pacemaker (electric stimulator) in the atria. The electrocardiogram shows an electrical break with a length of multiple PP intervals.
What are the clinical manifestations of the sinoatrial block?
Frequently, the sinoatrial block is well tolerated and does not cause clinical manifestations. Clinical manifestations of bradycardia may occur such as fatigue, dizziness, vertigo, syncope (loss of consciousness, complete and sudden, due to a decrease of the level of oxygen in the brain), faintness (malaise accompanied by nausea and sweating) and fainting.
Diagnosis of sinoatrial block is established using:
The resting electrocardiogram
Electrocardiogram (ECG) is a path of the electrical impulses of the heart recorded with small electrodes that are fixed on the chest and connected to an electrocardiograph. ECG characteristics help determine the type of sinoatrial block.
Ambulatory electrocardiogram (Holter monitoring)
Since sinoatrial block may not occur during hospitalization, there are portable ECG devices that record the ECG paths at home. Some devices are turned on continuously for a period of time (often for 24-48 hours), others are activated when the patient experiences symptoms (dizziness).
Electrophysiological studies are complex explorations. Very small probes fitted with electrodes are inserted through a vein (from the forearm or leg) and pushed to the heart. These wells contain electrodes that can detect heart muscle tissue that redirect or block the electrical signal from the sinoatrial node.
Sinoatrial blocks first and second degree do not require treatment.
Treatment of grade III sinoatrial block comprises three elements:
– Treatment of the causes of sinoatrial block
– Drug therapy
– Permanent pacing (cardiac pacemaker)
Atropine is a parasympatholytic drug (inhibits the activity of the parasympathetic nervous system) that causes increased heart rate, cardiac output and blood pressure.
Ephedrine and norepinephrine are drugs that stimulate the sympathetic nervous system through its action on adrenergic receptors. They have stimulation action (heart rate increases), vasoconstrictor, increase blood pressure, coronary and cerebral blood flow.
The pacemaker consists of a pulse generator connected to one or two electric wire introduced, through veins into the cardiac cavities or placed on the outer surface of the heart. When the generator remains outside the patient’s body, it creates a temporary pacemaker.
In case of a permanent pacemaker, the pulse generator is placed in a subcutaneous pocket(under the skin). Electrical impulses emitted by it are led to the heart through either vein catheters to the right cavities of the heart, or through epicardic electrodes implanted in the myocardium. Basically, in the case of a patient with pacemaker, it intervenes when needed and, through electrical shocks, it assures a heart rate similar to the normal one.
Permanent pacing in pacemaker is the main treatment for patients with symptomatic sinoatrial block.