• Basic principles
• Waves
• Segments & intervals
• Electrodes & leads
• Standardization

Basic principles

The EKG provides a record of the heart’s electrical activity.

A 12-lead EKG allows us to determine:

• Rate
• Rhythm
• Conduction defects
• Axis
• Hypertrophy
• Ischemia and infarction

At rest, cardiac cells (myocytes) are INSIDE NEGATIVE.

Upon depolarization, cells become INSIDE POSITIVE, causing contraction of the myocardium.

Upon repolarization, cells return to INSIDE NEGATIVE.

Waves:

• P WAVE: First small wave. Required for ‘sinus’ rhythm. Atrial depolarization.
• Q WAVE: First downward deflection of QRS complex. Not always present. Beginning of ventricular depolarization.
• R WAVE: First upward deflection of QRS complex. Ventricular depolarization.
• S WAVE: Downward deflection following R of QRS complex. Ventricular depolarization.
• T WAVE: Follows QRS. Ventricular repolarization.

Segments and Intervals (see below)

Electrodes and leads

Electrodes: You need 10 electrodes for a 12-lead EKG

Limb electrodes: RA, LA, RL, LL

Chest electrodes: V1, V2, V3, V4, V5, V6

Leads:

The limb electrodes form the bipolar and unipolar limb leads

Bipolar leads: I, II, III (Einthoven’s triangle)

Unipolar (augmented voltage) leads: avR, avL, avF

The chest electrodes form the precordial leads

V1, V2, V3, V4, V5, V6

Standardization

Speed: horizontal

Ruled paper travels through the cardiograph @ 25 mm/sec

1 tiny square = 1 mm = 0.04 sec

1 heavy square = 5 mm = 0.20 sec

How many heavy squares in 1.0 second?

5

How many 6.0 seconds?

30

How many in 1 minute?

300

Voltage: vertical

10 mm (2 heavy boxes) of vertical displacement = 1 mV