ECG Basics

General

• 1 small box = 1mm = 0.04 sec = 40 miliseconds
• 5 small boxes = 1 big box = 5mm = 0.2 sec = 200 miliseconds
• Entire ECG strip is ~10 seconds

Intervals

Interval	Time (s)	Boxes
PR	0.12 - 0.20	3-5
QRS	.06 - 0.10	1.5-2.5
QTc	<0.44	N/A

Axis

• Cannot be measured if BBBs are present
• If up in leads 1 and AVF then normal axis

Q Waves

• Significant if >1 box wide or if is 1/3 of entire QRS amplitude
• Early Repolarization:
  • ST Elevation most prominent in lat precord leads (V4-6) but no reciprocal changs
  • T waves usually broad, tall (>5mm) & upright
  • Limb leads may also have ST elevation, rarely >2 mm

T Waves

• Normally upright in 1, 2, V3-V6
• Negative in AVR
• If is greater than 2/3 height of R wave then is abnormal
• Deep symmetrical inverted T waves:

1. Left ventricle apical hypertrophy
2. Raised ICP (e.g. SAH)
3. Wellen's (MI)
4. Paced rhythm
5. BBB's or WPW
6. Idiopathic

Hypertrophy/Enlargement

• RAE
• LAE
• LVH
• RVH

Blocks

• RBBB
• LBBB
• LAFB
• LPFB

Low Voltage

1. QRS < 5 mm in limb leads, or
2. QRS < 10 mm in chest leads
3. From either low electrical power within the heart (e.g. hypothyroidism), or low conduction within the heart (e.g. sarcoid or scar tissue), or poor conduction from the heart to the ECG lead (e.g. obesity, COPD, pericardial effusion
4. Low voltage + tachycardia = pericardial effusion until proven otherwise

See Also

• ECG (Main)

Source

• Adapted from Niemann, Lampe, Pani, Donaldson, ECGpedia.org
• Journal of Electrocardiology. Vol 43 (2010). 40-42.