EKG reading made easy

Rates

• Normal: 60-100 bpm
• Tachycardia: > 100 bpm
• Bradycardia: < 60 bpm

Readings

• Count the number of large squares present within one R-R interval.
• Divide 300 by this number to calculate heart rate.

OR

• Count the number of complexes on the rhythm strip (each rhythm strip is typically 10 seconds long).
• Multiply the number of complexes by 6 (giving you the average number of complexes in 1 minute).

Note:

• A patient’s heart rhythm can be regular or irregular.
• Irregular rhythms can be either:
• Regularly irregular (i.e. a recurrent pattern of irregularity)
• Irregularly irregular (i.e. completely disorganized)

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 determine the cardiac axis you need to look at leads I, II, and III.

Right Axis deviation

• Right axis deviation (RAD) involves the direction of depolarisation distorted to the right (between +90º and +180º).
• The most common cause of RAD is right ventricular hypertrophy. Extra right ventricular tissue results in a more robust electrical signal being generated by the right side of the heart. This causes the deflection in lead I to become negative and the deflection in lead aVF/III to be more positive.
• RAD is commonly associated with conditions such as pulmonary hypertension, as they cause right ventricular hypertrophy. RAD can, however, be a normal finding in very tall individuals.

Left Axis deviation

• Left axis deviation (LAD) involves the direction of depolarisation distorted to the left (between -30° and -90°).
• This results in the deflection of lead III becoming negative (this is only considered significant if the deflection of lead II also becomes negative).
• Conduction abnormalities usually cause LAD.

P Wave

The next step is to look at the P waves

• Sawtooth baseline → flutter waves
• Chaotic baseline → fibrillation waves
• Flatline → no atrial activity at all
• The PR interval should be between 120-200 ms (3-5 small squares).

Note: A prolonged PR interval suggests the presence of atrioventricular delay (AV block).

If the PR interval is shortened, this can mean one of two things:

• the SA node is not in a fixed place and some people’s atria are smaller than others.
• an accessory pathway and can be associated with a delta wave (see below which demonstrates an ECG of a patient with Wolff Parkinson White syndrome).

QRS Complex

When assessing a QRS complex, you need to pay attention to the following characteristics:

• Width
• Height
• Morphology

Width can be described as NARROW (< 0.12 seconds) or BROAD (> 0.12 seconds)

Height can be described as either SMALL or TALL:

• Smallcomplexesare is defined as < 5mm in the limb leads or < 10 mm in the chest leads.
• Tallcomplexesimply ventricular hypertrophy (although can be due to body habitus e.g. tall slim people).

To assess morphology, you need to assess the individual waves of the QRS complex.

Note – the presence of a delta wave does NOT diagnose Wolff-Parkinson-White syndrome. This requires evidence of tachyarrhythmias AND a delta wave.

Q Wave

Isolated Q waves can be normal.

A pathological Q wave is > 25% the size of the R wave that follows it or > 2mm in height and > 40ms in width.

R Wave

Assess the R wave progression across the chest leads (from small in V1 to large in V6).

The transition from the S > R wave to the R > S wave should occur in V3 or V4.

Poor progression (i.e. S > R through to leads V5 and V6) can be a sign of previous MI but can also occur in very large people due to poor lead position.

ST-Segment

The ST segment is the part of the ECG between the end of the S wave and the start of the T wave.

In a healthy individual, it should be an isoelectric line (neither elevated nor depressed).

ST-elevation is significant when it is greater than 1 mm (1 small square) in 2 or more contiguous limb leads or >2mm in 2 or more chest leads. It is most commonly caused by acute full-thickness myocardial infarction.

ST depression ≥ 0.5 mm in ≥ 2 contiguous leads indicates myocardial ischemia.

T Waves

T waves are considered tall if they are:

• > 5mm in the limb leadsAND
• > 10mmin the chest leads (the same criteria as ‘small’ QRS complexes)

Tall T waves can be associated with:

• Hyperkalaemia(“tall tented T waves”)
• HyperacuteSTEMI

Inverted T waves in other leads are a nonspecific sign of a wide variety of conditions:

• Ischaemia
• Bundle branch blocks (V4-6 in LBBB and V1-V3 in RBBB)
• Pulmonary embolism
• Left ventricular hypertrophy (in the lateral leads)
• Hypertrophic cardiomyopathy (widespread)
• General illness

Flattened T waves are a non-specific sign, that may represent ischemia or electrolyte imbalance.

Biphasic T waves have two peaks and can be indicative of ischemia and hypokalaemia.

U Waves

U waves are not a common finding.

The U wave is a > 0.5mm deflection after the T wave best seen in V2 or V3.

These become larger the slower the bradycardia – classically U waves are seen in various electrolyte imbalances, hypothermia and secondary to antiarrhythmic therapy (such as digoxin, procainamide, or amiodarone).

References

1. James Heilman, MD. Fast atrial fibrillation. Licence:CC BY-SA 3.0.
2. Michael Rosengarten BEng, MD.McGill. Right axis deviation. Licence:CC BY-SA 3.0.
3. James Heilman, MD. Mobitz type 2 AV block. Licence:CC BY-SA 3.0.
4. James Heilman, MD. Complete heart block. Licence:CC BY-SA 3.0.
5. James Heilman, MD. Delta wave. Licence:CC BY-SA 3.0.
6. Michael Rosengarten BEng, MD.McGill. Q-waves. Licence: CC BY-SA 3.0.
7. Michael Rosengarten BEng, MD.McGill. Poor R-wave progression. Licence:CC BY-SA 3.0.
8. Michael Rosengarten BEng, MD.McGill. Tall tented T-waves. Licence: CC BY-SA 3.0.
9. T-wave morphology. Licence:CC BY-SA 3.0.
10. James Heilman, MD. U-wave. Licence:CC BY-SA 3.0.
11. Michael Rosengarten BEng, MD.McGill. Left axis deviation. Licence:CC BY-SA 3.0.