ECG signal

Dr Diana R Holdright

Consultant Cardiologist

Dr Diana Holdright

Cardiac Conditions - Other Arrhythmias

For the heart to beat effectively the heart muscle cells must contract in an organised and predetermined way. The heart has its own electrical wiring system, which transmits an electrical pulse along predefined pathways, causing timely contraction of the upper chambers, the atria, which squeeze blood into the lower chambers, the ventricles. These then contract, ejecting blood through the main arteries to the body and lungs. The electrical impulse originates in the heart’s own pacemaker, the sinoatrial (SA) node. This emits an electrical pulse at a rate determined by the body’s requirements; the pulse slows whilst sleeping, but increases with exercise, fever and emotional stress, for example. The electrical pulse wave spreads from the right to the left atrium, triggering muscular contraction of these structures and passage of blood into the ventricles. The electrical impulse then reaches the centre of the heart at a structure called the atrioventricular (AV) node, which acts like a junction box to regulate the flow of electrical impulses from the atria to the ventricles. The impulse then passes into a structure called the Bundle of His, which divides into left and right branches to the left and right ventricles. Specialised conduction fibres called Purkinje fibres conduct the impulse rapidly to the ventricular muscle, causing contracting of the muscle, which ejects the blood into the main arteries, the aorta and pulmonary artery. This complex sequence, called sinus rhythm, creates a highly efficient pumping action by the heart.

atrial fibrillation

Abnormalities in heart rhythm can cause a change in heart rate, either making it faster or slower, and rhythm, which may or may not cause symptoms. Symptoms vary depending on the type of arrhythmia and include breathlessness, fatigue, palpitations, dizzy spells and blackouts. There are many different types of heart irregularity, each with its own mechanism, and the treatment required will be specific to the precise heart rhythm causing the problem. In general terms, very slow heart rates may need treatment with a pacemaker; fast rhythms may need treatment with anti-arrhythmic drugs (chemical cardioversion), electrical cardioversion or ablation, and some patients need the protection of an implantable defibrillator-cardioverter (ICD).

Some fast heart rhythms occur because of the presence of an extra electrical pathway called an accessory pathway, which is present from birth; these are particularly amenable to ablation. The accessory pathway allows the elctrical impulse to pass directly from the atria to the ventricles, bypassing the AV node which normally regulates the speed of electrical conduction; as such it is possible for the electrical impulse to create a loop or circuit passing down the AV node, back up the accessory pathway, down the AV node again, and so on. This can create extremely rapid heart rates which may be poorly tolerated.

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