Reading an EKG Strip

The paper, the waves, and a 5-step routine to use on every strip

Cardiac & EKG

The paper

Standard speed 25 mm/sec.
1 small box = 0.04 sec (1 mm). 1 large box = 0.20 sec (5 small boxes).
5 large boxes = 1 second; 30 large boxes = 6 seconds.
Height: 10 small boxes = 1 mV (amplitude, not time).

Heart rate: three methods

6-second method (any rhythm, best for irregular): count QRS complexes in 6 sec × 10.
300 method (regular only): 300 ÷ number of large boxes between two R waves.
1500 method (regular, most precise): 1500 ÷ number of small boxes between R waves.

300 sequence: 1 box = 300, 2 = 150, 3 = 100, 4 = 75, 5 = 60, 6 = 50.

Normal sinus rhythm (lead II)

P wave = atrial depolarization. Upright and rounded in lead II; one before every QRS.

QRS = ventricular depolarization. (Atrial repolarization is hidden inside it.)

T wave = ventricular repolarization. Peaked T → think hyperkalemia; flat T or U wave → hypokalemia.

Normal intervals (adult)

Measure	Normal	What it means
Heart rate	60–100/min	<60 brady, >100 tachy
PR interval	0.12–0.20 s	3–5 small boxes; AV conduction time
QRS	0.06–0.10 s	≥0.12 s = wide (ventricular origin or bundle branch block)
QTc	<0.45 s (men) <0.46 s (women)	QTc >0.50 s = high risk of torsades

5-step routine

Look at the patient first. Pulse, BP, LOC, symptoms. Treat the patient, not the monitor.
Rate: atrial and ventricular.
Rhythm: regular or irregular? (march out the R–R intervals).
P waves: present? upright? one per QRS? any P without a QRS?
PR and QRS: measure. Constant or changing PR? Narrow or wide QRS?

5-lead placement

White: right arm (below right clavicle). Black: left arm.
Green: right lower chest/abdomen. Red: left lower chest/abdomen.
Brown (chest): V1, 4th intercostal space, right sternal border (or as ordered).

Memory hook "White on the right, smoke (black) over fire (red), snow (white) over grass (green)."
Artifact vs. real VF? Check the patient. A talking patient with a "VF" strip has loose leads or is moving.