What is an ECG?
ECG stands for electrocardiogram. It is a measure of the electrical activity of the heart, measured using 10 electrodes/ leads placed at various points on the body. You will hear the term ’12 lead ECG’, as some of the leads are virtual leads, which are produced using information from multiple leads.
The trace produced by an ECG shows the direction of electrical activity relative to the lead. An upwards movement on the trace indicates an electrical wave moving toward the lead whilst similarly a downward trace indicates an electrical wave moving away from the lead.
In a normal healthy heart, electrical activity starts at the pacemaker node (SAN/ Sinoatrial node) and moves through the atria to the atrioventricular node (AVN), indicating atrial contraction. It then moves along the bundles of his to the purkinje fibres, causing the ventricles to contract. We can work out the net direction of this electrical activity to give us an idea of the cardiac axis, as certain pathologies will result in an abnormal net direction of electrical activity and thus an abnormal cardiac axis..
An ECG is printed on paper with multiple small (1mm x 1mm) and big squares (5mm x 5mm). Each of the small squares typically represents 0.04 seconds, and thus each of the big squares represents 0.20 seconds. 5 big squares therefore is 1 second, and 300 is 1 minute. This is important to remember as it will allow us to work out the heart rate.
Shape of the ECG
Below is an example ECG waveform. First we see the P wave, the P wave demonstrates the wave of electrical activity as it moves from the SAN to the AVN, and thus the contraction of the atria. Typically the P wave should be around 0.12 seconds (3 small squares), and between 2/3 mm high. Changes in P wave morphology could indicate an underlying pathology.
Next we have the PR Interval, this represents conduction through the AVN, and thus the brief time between atrial contraction and ventricular contraction. In an normal heart, the PR interval should be between 0.12 and 0.20 seconds (3 to 5 small squares). Shorter than 3 small squares could indicate the presence of an accessory pathway between the atria and ventricles. Longer than 5 squares could be indicative of a number of types of heart block.
Following the PR interval we have the Q wave. The Q wave is the negative deflection that is just before the R wave. Usually small (septal) Q waves are seen in leads 1, AVL and V5-V6. They are however not usually seen in leads V1-V3, and there presence may be indicative of pathology such as a current or previous MI (Myocardial Infarction).
Next we see the R wave, the R wave represents ventricular depolarization/ contraction, and is usually demonstrable as an upwards movement on the trace after the P wave. High voltage R waves (tall QRS complexes) are often indicative of ventricular hypertrophy, a larger than normal wall thickness in the ventricle. The broadness of the QRS complexes allows us to work out whether the electrical activity of the ventricle has initiated in the atria as should normally happen (narrow complex), or in the ventricle (broad complex) as is the case in ventricular ectopics and VT for example.
Following on from the R wave is the S wave. The S wave is a small deflection shown after the R wave. As has been previously mentioned the Q, R and S waves together form the QRS complex.
The final part of the ECG wave form is the T wave, and this represents ventricular re-polarization, the return of the electrical activity of the cells to their resting potential. The interval between the S and T waves, the ST segment is an highly important in the measure of cardiac ischemia and thus important for diagnosing a Myocardial Infarction.