An electrocardiogram (ECG) is an essential tool used in cardiovascular assessment. It is used as the initial diagnosis tool in a variety of ailments of the heart.
The ECG gives the medical officer a quick and easy assessment of the present condition of the heart and also sometimes past damages which have occurred to it.
It helps in the differential diagnosis of chest pains and also management and diagnosis of abnormal cardiac rhythms.
An ECG records the electrical activity of the heart.
To understand how ECG's work, it is important to know that contractions of muscles are associated with electrical changes.
This electrical change is know as depolarisation, and occurs at a cellular level.
These electrical changes are recorded via electrodes placed on the limbs and chest wall, and are traced on to graph paper to produce an ECG.
The heart is divided in to four chambers: at the top the left and right atria, and at the bottom the left and right ventricles.
The electrical discharge for each cardiac cycle normally starts in a special area of the right atrium called the sinoatrial node.
The sinoatrial node is a natural pacemaker as it initiates electrical impulses, which are propagated to the atrioventricular node, which is in another area in the atrium. This electrical discharge thereafter travels rapidly through a specialised conduction pathway : the bundle of His, which then divides in to the left and right branches, supplying the ventricles respectively.
This is the normal electrical conduction pathway of the heart which results in the contractions of the heart muscles, leading to the pumping of blood.
This basic mechanism should be understood to interpret an ECG , so that if there is any deviation from the norm , the medical officer knows exactly at which point the problem is occurring.
Picture of the conduction system of the heart. (http://medical-dictionary.thefreedictionary.com)
A normal ECG is has three main areas of interest, The 'P' wave, 'QRS' complex and 'T' Wave.
P represents the contraction of the atria.
'QRS' complex is associated with the depolarisation of the ventricles.
The 'T' wave represents the ventricles returning to its normal resting electrical state.
A normal ECG. (http://medical-dictionary.thefreedictionary.com)
Variation in any one of these areas, which include rate,change in wave form, additions of extra waves or deletions of specific waves tell us that there is a problem in the conduction system of the heart.
The normal heart beat is 60-100 beats per minute. Any deviation from this rate, be it lower (Bradycardia) or higher (Tachchycardia) is known as an arrhythmia.
Impending heart attacks and damage done to the cardiac muscle after one may also be determined with the use of an ECG recording.In this case the ECG may help us to point to a specific area of damage, represented by changes in the different electrical leads recordings.
There are two other methods of recording ECG's.
+Exercise ECG (exercise tolerance test) - This method records an ECG while the patient is on a treadmill or exercise bike. This helps to diagnose coronary artery disease, check a persons exercise capacity of the heart, and also to identify rhythm disturbances during exercise.
+Twenty four hour ECG - this test requires a small portable ECG recorder to be attached to a person, so that it may record the hearts electrical activity for a twenty four hour period. A trained clinician known as a Cardiac Electrophysiologist may interpret the printed out readings to determine a diagnosis.
Advantages of ECG's are that they are non invasive and there are no side effects.