A heart beat depends on rhythmical contraction of the heart muscle activated by an electrical impulse originating in a pacemaker at the top of the heart called the sino-atrial node.  This impulse spreads through the upper chambers of the heart (the atria) causing them to contract and pump blood into the lower chambers (the ventricles).  It now passes from the upper to the lower chambers via specialised conducting fibres called the bundle of His, which then divide into two lesser bundles to distribute the impulse throughout the muscle of the ventricles.  The latter now contract, pumping blood into the lungs from the right side and to the rest of the body from the left.

Damage to these ventricular bundles may block the passage of the electrical impulses.  The heart continues to beat, because the impulses spread out into the muscle bypassing the block.  It is called left or right bundle branch block, depending on which bundle is affected.  It usually causes no symptoms, but may appear on a resting ECG tracing.  This is the usual way that bundle branch block is found in pilots/ATCOs.  The problem is that bundle branch block may be the first sign that a serious medical condition exists which could damage other parts of the heart and which, in turn, could lead to incapacitation.  These problems include a lack of blood getting to the heart muscle (coronary artery disease which may lead to a heart attack) or some specific damage to the heart muscle itself (called cardiomyopathy).  If left or right bundle branch block appears on a pilot’s/ATCO's ECG it is necessary, in order to maintain medical certification, to check that no such disease process is occurring.  This requires further investigation including exercise ECG testing, recording the ECG for a 24 hour period, ultrasound scanning of the heart (echocardiography) and sometimes assessment of the heart arteries with a scan (myocardial perfusion scan) or an x-ray (coronary angiography).  Happily, in the majority of cases, no cause for the bundle branch block is found, and pilots/ATCOs can be recertificated, albeit with follow-up checks to make sure that no insidious problem remains.  If further heart disease is found by these investigations, appropriate treatment can be started.

There are more investigations for left-sided block than for right, because the former is more likely to be associated with heart disease.  Once the tests have been done, and the results are satisfactory, the pilot/ATCO can be recertificated subject to the appropriate follow-up.  For a pilot a multi-crew limitation may be imposed for a period of time, to guard against the possibility that heart disease is developing, despite the satisfactory investigations.

The protocols for bundle branch block are given below.

Aircrew

Left Bundle Branch Block

Right Bundle Branch Block 

ATCO

Left Bundle Branch Block

Right Bundle Branch Block