ECG or Electrocardiograph is one of the most useful diagnostic tool that helps establish cardiology problems in a patient. However, it takes a lifetime and yet, one simply cannot “master” everything of it. For the start, here are some basics which might help.
Let’s cut out the ECG basics into:
- The ECG “PQRST” wave
- Leads used in ECG
- Rate of ECG
- Rhythm of ECG
- Axis of ECG
Following is a normal typical wave seen on ECG. But what does it mean in relation to heart? Let’s find out.
- P wave- It is result of Atrial Depolarization, initiated by the SA node
- PQ segment- With Atrial depolarization complete, the impulse is delayed at the AV node
- QRS complex- Ventricular depolarization begins at apex, causing the QRS complex. Atrial repolarization occurs
- ST segment- Ventricular depolarization is complete
- T wave- Ventricular repolarization begins at apex, causing T wave
- Ventricular repolarization is complete, after which new wave follows with atrial depolarization (P wave)
There are 12 leads used in ECG. We are not going in details of them, rather name them out as follows:
- 6 Limb leads: Lead I, Lead II, Lead III, Lead AVL, Lead AVR, Lead AVF
- 6 Chest leads (Precordial leads): V1, V2, V3, V4, V5, V6
There are small boxes and big boxes on ECG. 1 big box = 5 small boxes.
1 small box=1mm=0.4 sec, 1 big box=5mm=2 sec
So, the rate of ECG is calculated as:
Rate = 1500 / Number of small boxes between R-R interval
Rate = 300 / Number of big boxes between R-R interval
To find out the rhythm, simply place a ruler between R-R interval. Note the gap and place the same on consecutive waves. If the gap is same in all waves, it is regular, otherwise, it is irregular.
- Patterns of waves could be:
- Regular: RR interval constant
- Basically regular:
- >>Premature ectopic beat
- >>Escape ectopic beat
- Regularly irregular: RR interval variable but with a pattern. Normal and ectopic beats grouped together and repeating over and over.
- Irregularly irregular. RR interval variable with no pattern, totally irregular.
Defining Axis on ECG can be difficult to others, but it’s simple if you have read it once.
Remember this in short for quick recalling:
Lead I- 0
Lead aVF- 90
We have to observe QRS complex in these leads and check whether the deflection is positive or negative. Therefore, axis can be given as:
- Lead I- positive, Lead aVF- positive = 0 to +90 = Normal
- Lead I- positive, Lead aVF- negative = 0 to -90 = Left Axis Deviation
- Lead I- negative, Lead aVF- positive = 90 to 90 = Right Axis Deviation
- Lead I- negative, Lead aVF- negative = -90 to 180 = Extremely Right Axis Deviation