Every time there is electrical activity from the SA node, it induces an electrical signal that moves in a waveform to the AV node and then on to the right and left ventricles. The electrical activity results in contraction of the myocardium. If for any reason there is interruption of the electrical activity, a heart block can result.
In almost all cases of first degree AV nodal block there are no symptoms. The majority of cases are identified incidentally on a 12 lead ECG.
Second degree Mobitz type 1 heart block is also without symptoms in most patients. In a few rare cases, the patient may complain of syncope, dizziness, or irregular heartbeat. In second degree Mobitz type 2 block, symptoms are usually present. Patients who are on beta blockers, digoxin or calcium channel blockers generally tend to be symptomatic compared to those who are not on these medications.
Third degree heart block is usually associated with symptoms like dizziness, fatigue, lightheadedness and syncope. When an individual develops a very slow heart rate, syncopal episodes that occur are known as Morgagni Adams stokes episodes. Patients who have a 3rd degree heart block often also have an associated acute MI or some degree of coronary artery disease.
General physical exam is not revealing in patients with a first degree heart block. However, in second and third degree heart block the heart rate may be slow. Individuals with ischemic heart disease who develop 3rd degree heart block may have signs of heart failure, pulmonary edema, elevated JVP and pedal edema. In a patient with third degree heart block one may even see cannon waves which are an indication of complete atrial-ventricular dissociation.
Laboratory studies indicated for patients with heart block include:
Routine imaging is not necessary in evaluating patients with heart block. However, echocardiography is needed to assess valvular and overall heart function. Echo may reveal aortic valve calcification, wall motion abnormalities, congenital heart disorders or cardiomyopathy.
In some cases of second degree heart block, it may be difficult to differentiate mobitz 1 from mobitz 2 heart block. In such cases, exercise can be used to differentiate between the two. In mobitz type 2 block the block may become obvious and the individual maybe symptomatic.
The treatment of heart block depends on the type of heart block. First degree heart block never needs any type of treatment except for observation .
In acute situations, one may use temporary transvenous or transcutaneous pacing for bradycardia or a heart block that is causing symptoms. Once the patient is stabilized, the patient should be transferred to a tertiary care center which deals with arrhythmias. If a patient with advanced degree heart block is admitted and awaiting pacemaker implantation, a temporary pacemaker and atropine should be available at the bedside .
However, in the long run, medical therapy is not indicated. Permanent pacing is the treatment of choice for advanced degrees of heart block. Once permanent pacemaker has been inserted, no other medication is needed. All medications that have propensity to cause a heart block should be discontinued if no longer necessary.
Insertion of a pacemaker may be done by the thoracic surgeon or the cardiologist. It is usually performed under IV sedation supplemented with local anesthesia. Most patients require an overnight stay in hospital to ensure that the pacemaker is working well.
Before a pacemaker is inserted, it is necessary to ensure that all reversible causes of heart block have been treated. Some disorders like Lyme induced heart block are temporary and can be managed with close observation. Other reversible causes of heart block include hypothermia, myocarditis or post-operative heart surgery. In most of these scenarios observation is recommended as full recovery occurs within days or weeks.
On the other hand, there are other disorders that are not reversible and no time should be wasted with insertion of a pacemaker. These conditions include amyloidosis, sarcoidosis and other neuromuscular disorders. After open heart surgery, heart block can occur but in most cases it is transient and due to the edema and inflammation. This edema often takes a few days or weeks to subside and there is no need to intervene as the heart rate does return back to normal.
There are many types of pacemaker used to treat heart block. The simplest one includes the ventricular pacing (VVI), and the latest more complex devices are the dual chamber pacers. The cardiologist usually selects the type of pacemaker depending on patient status, comorbidity and longevity of life. In general, biventricular pacing is far more effective and superior to single chamber pacing.
Treatment guidelines by the American College of Cardiology
Complications of pacing
Insertion of a pacemaker is not without complications. Common complications include a pneumothorax, tamponade or hemothorax. Late complications include lead fracture, pacemaker dysfunction, and inappropriate sensing or capture of electrical signals. Even though infection is rare, when it occurs, it usually requires removal of all hardware.
Patients with first degree or mobitz type 1 second degree heart block do not need hospital admission. However, patients with type 2 mobitz or 3rd degree heart block must be admitted and continuously monitored.
The prognosis of first degree heart block is excellent. Most patients need no treatment and are asymptomatic. For patients with Mobitz type 1 who have no symptoms, the prognosis is also excellent. However, patients who are symptomatic do need treatment with a permanent pacemaker. All patients with second degree mobitz Type 2 and third degree heart block require treatment with pacemaker. The prognosis after insertion of a pacemaker is good. If second or third degree heart block is untreated it can lead to syncope, dizziness, heart failure and exacerbation of ischemic heart disease. If a fall occurs during a syncopal attack, it can lead to head and skeletal injuries. Sudden death can occur with third degree heart block.
There are many causes of heart block which include the following:
The exact number of people who develop heart block is not known but the numbers are not small. Because many patients with first and second degree heart block remain asymptomatic, often these patients do not always come to medical attention. Heart block occurs in all races, genders and in both children and adults. Heart block is most common in elderly individuals. In infants, heart block appears to be most common after congenital heart surgery. There appears to be an increase in the number of pacemaker inserted in the USA but these devices are also inserted for other cardiac disorders.
The AV node plays a central role in conduction of impulses from the atria to the ventricles. In general the function of the nodal portion of the AV node is to slow the conduction to the ventricles. The blood supply to the AV node is from the right coronary artery in 90% of cases and by the circumflex artery in 10%. Thus any obstruction to these blood vessels can lead to a heart block. The AV node also receives innervation from both the parasympathetic and sympathetic innervation. With impulses coming to the AV node from the various fibers, the AV node gradually slows then down in a decremental fashion. If there is any infiltrative or infectious process in the vicinity of the AV node this can affect conduction to the ventricles .
In a person with first degree heart block and second degree mobitz type 1 block there is usually a delay at the level of the AV node, whereas in second degree mobitz type 2 block, there is blockage in the lower regions of the AV node and Bundle of His. In a third degree AV block, there is block at the AV node, Bundle of His and the purkinje fibers. In most cases where complete heart block occurs, the ventricle may originate a beat via an escape rhythm but the rate is often slow and hence treatment is necessary.
Heart block is not always preventable. However, patients who develop syncope, dizziness or palpitations should seek medical assistance. In some cases, discontinuation of drugs like beta blocker, calcium channel blockers or digoxin may help worsening of the heart block. It is also important to ensure that levels of electrolytes and drug levels of certain medication are therapeutic. Once a pacemaker is inserted the individual should avoid heavy lifting on the ipsilateral side until the wound has completely healed (about 4-6 weeks). Individuals should avoid contact sports or wear a protective shield when performing exercise. Individuals with pacemaker should avoid power lines and other sources of electromagnetic waves as it may interfere with activity of the pacemaker.
The electrical impulses initiate at the sinus node and move across the atria before reaching the AV node. The AV node acts like a filter/gatekeeper and after a time lag, sends the electrical signals on to the ventricles. The reason for this short and deliberate delay at the AV node is to allow the atria to contact and also fill the ventricles with blood   .
The term heart block is usually used to describe a number of heart disorders where the electrical signal is slowed or even blocked at the AV node and unable to reach the ventricles. Heart blocks can be caused by medications, ischemia, infarction, trauma, congenital disorders, certain electrolyte disorders, high vagal tone and advanced age. There are three basic types of heart blocks.
First degree heart block
In a first degree heart block, the electrical signal from the sinus node takes a little longer to pass through the AV node. In a healthy individual, the delay between the atria and ventricles contraction ranges from 120 ms to 200 ms (this is the PR interval). In a person with a first degree heart block, the PR interval is prolonged and usually more than 200 ms.
The majority of people with a first degree heart block have no symptoms because the heart rate is not affected by the condition. However, there is a slight risk that people with a first degree heart block may develop atrial fibrillation in future. In addition, research also shows that people with a first degree heart block tend to be at risk for requiring a pacemaker in future. In almost all cases of first degree heart block, no treatment is required except for correction for any metabolic or electrolyte abnormality. Observation is sufficient.
Second degree heart block
In second degree heart block, some of the electrical signals from the SA node to AV node go through to the vernicles. Because of the failure of electrical stimulus to reach the ventricles, the heart rate can be slow and the ventricles fail to contract. As a result some patients with a second degree heart block will be symptomatic and present with fainting spells, lightheadedness and dizziness.
There are two subtypes of second degree heart block:
Third degree heart block (Complete heart block)
The most serious type of heart block is third degree. In this case, the ECG reveals dissociation between the atria and ventricle. There are a few beats conducted to the ventricle and the heart rate is profoundly slow. This is a dire situation which can quickly lead to cardiac arrest. The moment third degree heart block is diagnosed the patient should be administered IV atropine and have a permanent pacemaker inserted. There should be no delay in the procedure.
Heart block is a general term used to describe blockage of electrical conduction in the heart. There are essential three types of heart block. The first degree heart block is benign, rarely requires treatment and does not present with symptoms. The second degree heart block depends on the subtype and may present with symptoms. The advanced degree of second degree heart block is serious and almost always presents with symptoms and the need for a permanent pacemaker. The most serious type of heart block is the third degree which can be life threatening and requires a permanent pacemaker. All heart blocks can be detected on an ECG.