Normal Components
of the ECG Waveform
P
wave
- Indicates atrial depolarization,
or contraction of the atrium.
- Normal duration is not
longer than 0.11 seconds (less than 3 small squares)
- Amplitude (height) is
no more than 3 mm
- No notching or peaking
QRS complex
- Indicates ventricular
depolarization, or contraction of the ventricles.
- Normally not longer than
.10 seconds in duration
- Amplitude is not less
than 5 mm in lead II or 9 mm in V3 and V4
- R waves are deflected
positively and the Q and S waves are negative
T wave
- Indicates ventricular
repolarization
- Not more that 5 mm in
amplitude in standard leads and 10 mm in precordial leads
- Rounded and asymmetrical
ST segment
- Indicates early ventricular
repolarization
- Normally not depressed
more than 0.5 mm
- May be elevated slightly
in some leads (no more than 1 mm)
PR interval
- Indicates AV conduction
time
- Duration time is 0.12
to 0.20 seconds
QT interval
- Measured
from the Q to the end of the T.
- Represents ventricular
depolarization and repolarization (sodium influx and potassium efflux)
- V3,
V4 or lead II optimize the T-wave.
- QT
usually less than half the R-R interval
(0.32-0.40 seconds when rate is
65-90/minute)
- QT varies with
rate. Correct for rate by dividing QT by the square root of the
RR interval.
- http://www.qtsyndrome.ch/qtc.html
- Normal corrected
is < 0.46 for women and < 0.45
for men.
- Prolonged QT may be inherited or acquired
(predisposes
to long QT syndrome and torsades
de pointe)
- Inherited - defective
sodium or potassium channels
- Acquired - drugs,
electrolyte imbalance or MI
- Atleast, 50
drugs known to affect QT (including: quinidine,
amiodarone and dofetilide)
Instant
Feedback:
Normal
QRS duration is 0.15 - 0.25 seconds.
The American Heart Association website is an excellent source
of cardiac information.
RnCeus.com