When conduction through the electrical system of the heart is impaired, this can result in various different types of heart block. In normal hearts, the electrical signalling begins from the sinoatrial node located in the right atrium. From here, the impulse spreads through the atria, reaching over to the left atrium via Bachmann's bundle, causing the atria to contract. It then reaches the atrioventricular node or the AV node where the impulse is held momentarily. From here it travels down to the bundle of His which then divides into the left and right bundle branches followed by the Purkinje fibres on both sides, ultimately causing contraction of the ventricles.
Remember that the left bundle divides into the anterior, posterior and in some instances the septal fascicles, whereas the right bundle does not have other fascicles. Depending on where the impulse is delayed, different forms of heart block are seen. First degree AV block is a delay in conduction through the AV node and is defined when the PR interval, meaning from the beginning of the P wave to the start of the QRS complexes, is longer than 200 milliseconds. First degree AV block can be commonly caused by increased vagal tone in younger patients or athletes especially, and fibrosis in more elderly patients.
Other causes include AV node blocking agents like beta blockers, calcium channel blockers or digoxin, and it may also be a normal variant. In some instances, it can be the result of coronary artery disease, mitral valve surgery, or electrolyte imbalances like hypokalemia and hypomagnesemia. Usually, first degree AV block is asymptomatic and does not need treatment, but can occasionally progress to higher grade AV blocks, and patients with a first degree AV block have three times the risk of developing atrial fibrillation, which is why follow-up is recommended in these patients. In some patients that are symptomatic with PR intervals above 300 milliseconds, pacemakers can be considered.
Second degree AV block has two types, Mobitz 1, also known as Wenkebach, or Mobitz 2, known as Hay block. Mobitz 1 or Wenkebach features a progressively lengthening PR interval followed by a non-conducted ventricular beat. due to progressive fatigue of AV nodal cells. Mobitz 1 is cyclical and typically features a ratio of P waves to QRS complexes per cycle. For example, 4 to 3 if there are 4 P waves and only 3 QRS complexes in a cycle.
Causes of Mobitz 1 or the Wenckebach phenomenon again include high vagal tone and it can again be a normal variant. But it can also come from inferior myocardial ischemia, atrioventricular nodal blocking agents like we mentioned before, mitral valve surgery and hyperkalemia. Often treatment is not necessary but in some instances it can cause bradycardia and hypotension in which case atropine is used. AV blocking medications should also be reduced to the minimum required dose and if atropine is not effective then pacing may be required. On the other hand in MOBITS2 or hay blocks there is no progressively increasing PR interval, only an intermittent P wave that is not conducted.
There is sometimes a fixed ratio between the number of P waves for every elicited QRS, such as 2 to 1 in this example, but they can be higher. Mobitz 2 will usually have structural heart disease, often myocardial ischemia or fibrosis, and in 75% of cases, the anomaly is found below the bundle of his, which means broad QRS complexes are generated. However, the defects are in the bundle in 25% of cases, which means a narrow QRS. Mobitz type 2 are often symptomatic.
as a result of the reduced cardiac output with syncope, fatigue, chest pain and in some cases sudden cardiac death. It also has a relatively high progression rate to complete heart block. There is a risk of around 35% per year of asystole with Mobitz 2, and it will need temporary pacing up until a permanent pacemaker is placed.
Importantly to note is that atropine can precipitate 3rd degree heart block in Mobitz type 2. 3rd degree AV block is also known as complete heart block where there is no association between atrial and ventricular contractions. This means that the rhythm causing blood to leave the heart is either from a junctional or ventricular escape rhythm. This appears on the ECG as P waves that do not lead to a QRS complex and more P waves than QRS complexes indicating a higher atrial rate than ventricular rate.
This is the end stage of second degree heart block and therefore similar causes are present. The most common being inferior myocardial infarction, AV node blocking agents and degeneration of the conduction system. Patients are often symptomatic with syncope, fatigue, chest pain, shortness of breath and are at risk of sudden cardiac death.
Treatment can involve atropine, however it is rarely effective, and dopamine and adrenaline are other medical options. However, transcutaneous pacing or transvenous pacing may be required before ultimately putting in a permanent pacemaker.