We discussed first degree, second degree—Mobitz type I (Wenkenback) & Mobitz type II, third degree (complete), left and right bundle branch blocks.
A block is a disturbance, or slowing, of electrical activity and is seen as an elongation of a portion of the ECG i.e. long PR, wide QRS, long QT.
It is important to remember that ECGs are the electrical impulse of the heart and that a mechanical response is not guaranteed.
The ECG above shows sinus rhythm, or pulseless electrical activity (PEA). A pulse check can identify second degree heart block—as this will be a regularly irregular rhythm. It also allows for quick identification of critically unwell patients—no palpable radial pulse, or those with AF—irregularly irregular pulse.
Causes and treatment of first degree heart blockFirst degree heart block may be iatrogenic or organic in cause and may be considered a “normal” variant.
Generally there is little clinical significance to a first degree heart block. It is possible that it may be accompanied by bradycardia and it is more likely that this bradycardia is the problem rather than the heart block.
Athletic training and increased vagal tone may cause a first degree block and there is little to be done. Iatrogenic causes include beta and calcium channel blockade, digoxin, amiodarone and medication that reduces potassium—such as furosemide. This may require adjustment of medication. Mitral valve surgery and myocarditis may result in a first degree block. With the minimal clinical risk of the block it is unlikely that this would be viewed as a reason to withhold mitral valve surgery for those patients who require it. Myocarditis requires treatment and once treated hopefully will resolve a first degree block.
Mobitz I block results from a gradual increase in PR interval until a QRS complex is dropped.
Mobitz II block results from a dropped QRS in the presence of a constant PP and PR interval.
Causes and treatment of Mobitz I—Wenkenbach block
Causes and treatment of Mobitz II block
There is a significant risk of progression to complete heart block requiring careful management. The risk of asystole is approximately 35%. Management is likely to require hospital admission onto a ward offering a monitored bed—or telemetry, temporary pacing and a permanent pace maker.
Causes and treatment of third degree—complete—heart blockComplete heart block is caused by progressive failure of the cardiac conduction system. This is the end point for a second degree block that progresses. There is a high risk of ventricular standstill resulting in sudden cardiac arrest or syncope. Meaning these patients require a permanent pacemaker as soon as possible.
This produces a wide QRS and deep S waves in the right precordial leads and a tall R wave in the lateral leads.
Causes and treatment of left bundle branch blockA LBBB likely has an organic cause, such as: aortic stenosis, ischaemic heart disease, hypertension or fibrosis of the conducting system (Lenegre disease) or a previous MI—particularly anterior.
Current NICE guidelines suggest that an adult with chest pain and a presumed new LBBB should be considered for pPCI. There is some debate in more recent research as to the benefit of PCI for new LBBB, however given the risk of an anterior MI being the cause I support NICEs view that pPCI is worth considering.
Causes and treatment of right bundle branch block
RBBB tends to be caused by chronic conditions, although it may be temporarily caused by myocarditis or pulmonary embolus. Other causes are: ischaemic, congenital and rheumatic heart disease; cor pulmonale, cardiomyopathy and degeneration of the conduction system.
As RBBB tends to be the result of a chronic condition, meaning the condition tends to gradually become worse, a RBBB is less likely to compromise the patient. In terms of treatment that means many people will not require treatment for the RBBB, they may need treatment for the underlying condition.