WARNING:
This post may be meaningless to you if you are not a doctor.
Reading an ECG was not the most pleasant of tasks during the days of my internship (housemanship). At the time, I considered myself lucky that I somehow managed to avoid the Cardiology Unit during my rotation through Internal Medicine.
With the benefit of the gray hairs I have acquired over the several months that have intervened between September 2010 and now, I have come to appreciate the need for the clinician to be able to read an ECG effectively, systematically, accurately, and in a way that ensures that he doesn't miss anything. That is the inspiration behind what follows hereunder.
Step 1 - Calculate the Heart Rate
You can easily calculate the patient's heart rate by:
- working out the number of large squares in one R-R interval
- then dividing 300 by this number (i.e. the number of large squares in one R-R interval)
The answer you get is the patient's heart rate.
In the example illustrated in the graph above, there are four large squares in the R-R interval. The patient's heart rate is therefore calculated as 300/4 = 75bpm.
Step 2 - Determine the Regularity of the Heart Rhythm
Heart rhythm can be either regular or irregular. Again, this can be demonstrated by looking at the R-R interval.
The heart rhythm is classed as irregular if the R-R interval is inconsistent.
Note that an irregular rhythm with no distinct P-waves suggests atrial fibrillation.
Step 3 - Characterize the Cardiac Axis
The cardiac axis describes the overall direction of electrical spread within the heart. In a healthy individual, the axis should spread from 11 o'clock to 5 o'clock.
To figure out the cardiac axis, you need to look at leads I, II, and III.
Normal Cardiac Axis
In a normal cardiac axis, lead II has the most positive deflection compared to leads I and III.
Right Axis Deviation
In right axis deviation, lead III has the most positive deflection and lead I should be in negative territory. This is commonly seen in individuals with right ventricular hypertrophy.
Left Axis Deviation
In left axis deviation, Lead I has the most positive deflection, and leads II and III are negative. This is commonly seen in patients with heart conduction defects.
Step 4 - Look at the P-Waves
When looking at the P-waves, try to answer the following questions:
- Are P-waves present?
- Do they occur regularly?
- Does a P-wave precede each QRS complex? (in other words, is there sinus rhythm?)
- Do the P-waves look normal? (Meaning: are they smooth, rounded, upright?)
Note that if P-waves are absent and there is irregular rhythm, this may suggest atrial fibrillation.
Step 5 - Assess the PR Interval
The P-R interval should be between 0.12 - 0.20second (Each small square corresponds to 0.04second. The P-R interval should therefore be between 3-5 small squares), although it may be longer in elderly people.
The interval shortens with increased heart rate such as may occur following moderate to severe physical exertion.
 |
| Prolonged P-R interval |
A prolonged P-R interval may suggest the presence of heart block, whereas a shortened P-R interval suggests that the patient may have Wolff-Parkinson-White syndrome WPW.
Step 6 - Assess the QRS Complex
The QRS complex should be 0.08-0.12second (2-3 small squares).
Step 7 - Assess the S-T Segment
The S-T segment is that part of the ECG between the end of the S-wave and the start of the T-wave; in healthy individuals, it is usually an isoelectric line (with no elevation or depression). Abnormalities of the ST segment should raise the red flag for investigation so as to rule out any pathology.
S-T elevation is significant when it is > 1mm (> 1 small square) in relation to the baseline. S-T elevation is commonly seen in acute myocardial infarction. The morphology of the elevation differs however depending on how long ago the myocardial infarction occurred.
S-T depression is significant when it is > 1mm (> 1 small square) in relation to the baseline. Because S-T depression lacks specificity, its detection does not give one leave to jump to any diagnostic conclusions. A few of the many different conditions that can produce S-T depression are:
- anxiety
- tachycardia
- digoxin toxicity
- hemorrhage, hypokalemia, myocarditis
- coronary artery insufficiency
- myocardial ischemia
A look at the rather small list above and a consideration of the many different ways in which their modes of management vary, one from another, is enough to convince the reader that an ECG finding of S-T depression has to be interpreted within the context of the health circumstances of the particular patient undergoing review.
Step 8 - Consider the T-Waves
Are the T-Waves Inverted?
Among the most common abnormalities found on the ECG is the inverted T-wave. This happens because T-waves can be altered by many different processes. As a result, the finding of an inverted T-wave is a non-specific finding and is not normally used as a standalone factor in the clinching of a diagnosis. Besides, if the inverted T-waves are seen in V1 and V2, then they are not significant because V1 and V2 inverted T-waves are seen in normal individuals.
Some of the causes of inverted T-waves are:
- smoking
- anxiety
- tachycardia, hemorrhage, and shock
- hypokalemia
- pericarditis
- myocardial ischemia (acute and previous)
- bundle branch block
- Wolff-Parkinson-White sydrome
Are the T-waves Tall (and Tented)?
A T-wave is considered as tall when it is:
- 5mm in the standard leads
- 10mm in the precordial leads
Tall T-waves are caused by:
- hyperkalemia
- in hyperkalemia, the T-waves are described as tall, tented T-waves; this is because, in addition to being tall, they are also very narrow, with a sharp apex.
- myocardial ischemia
- the tall T-waves of (hyperacute) myocardial ischemia are not as narrow as those that are seen in hyperkalemia.
If the clinician can accurately interpret the information presented on the patient's ECG according to the guidelines above, he can be sure that he has captured a large part of the important information displayed on that tracing.
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