ECG Basics Flashcards
ECG paper speed is ordinarily…
25 mm/sec
small box = ___ sec
0.04 sec
5 small boxes = 1 large box = ____ sec
0.20 sec
P wave represents
atrial depolarization
P wave duration is generally
<0.12 sec (3 small boxes)
P wave amplitude
<0.25 mv (2.5 small boxes)
PR interval includes
P wave + PR segment
PR segment is
- measured from the begining of the P wave to the 1st part of the QRS complex (which may be a Q wave or R wave)
PR interval includes…
time for atrial depolarization (P wave) & conduction through the AV nodes & the His-Purkinje system (which constitutes the PR segment).
PR interval duration
0.12 - 0.20 sec (3 to 5 small boxes)
Length of the PR interval changes with?
Heart rate
PR interval is shorter at…
faster heart rates due to sympathetically mediated enhancement of atrioventricular (AV) nodal conduction
PR interval is longer when…
the rate is slowed as a consequence of slower AV nodal conduction resulting from withdrawal of sympathetic tone or an increase in vagal input.
QRS complex represents
ventricular depolarization
Small Q waves are often seen in leads _______ as a result of initial septal depolarization.
I, aVL & V4 - V6
Q wave in the QRS complex is…
the initial negative deflection.
1st positive deflection of the QRS complex is
R wave
R wave in the QRS complex represents
depolarization of the left ventricular myocardium.
NOTE: right ventricular depolarization is obscured because the left ventricular myocardial mass is much greater than that of the right ventricle.
The small R wave in lead V1 represents
intial septal depolarization
S wave in the QRS complex is the
negative deflection following the R wave
S wave represents
terminal depolarization of the high lateral wall
Lower case letters (q, r, or s) are used for…
relatively small amplitude waves of <0.5mV (<5mm with standard calibration)
An entirely negative QRS complex is called a
QS wave
QRS complex duration
0.06 - 0.10 sec (1.5 - 2.5 small boxes)
Explain R wave progression & in what leads?
- R wave should progress in size across the precordial leads (V1 - V6).
- Normally there is a small R wave in lead V1 with a deep S wave.
- The R wave amplitude should increase in size until V4 - V6 while the S wave becomes less deep.
ST segment occurs
after ventricular depolarization has ended & before repolarization has begun.
J point
It is the intersection of the end of the QRS complex & the initial part of the ST segment.
T wave represents
ventricular repolarization
Why is the T wave asymmetrical & has a variable amplitude?
(T wave is broad, has slow upstroke & a more rapid downslope to the isoelectric lune following its peak)
This is because:
- the rate of repolarization is slower than depolarization.
Difference in characteristics of T wave vs P wave
- Since the rate of repolarization is slower than depolarization, the T wave is broad, has a slow upstroke, & a more rapid downslope to the isoelectric line following its peak. Thus, the T wave is asymmetric & the amplitude is variable. - - In addition, the T wave is usually smooth up and down.
- If there is any irregularity on the T wave (bump, notch, rippled, etc) a superimposed P wave should be considered.
QT interval consists of
- QRS complex
- ST segment
- T wave
QT interval is primarily a measure of…
ventricular repolarization
If the _____ duration is increased, this will lead to an increase in QT interval but doesn’t a change in ventricular repolarization.
- QRS complex
A widened QRS must be considered if a ____________ is being evaluated.
prolonged QT interval
The QT interval is dependent upon…
heart rate = shorter at faster heart rates & longer when the rate is slower.
Clinicians also need to be mindful of the fact that since the QRS widens in the setting of a bundle branch block, the ____________ will also increase.
QT interval
U wave may be seen in some leads especially the precordial leads…
V2 - V4
U wave is more evident in some circumstances such as…
hypokalemia & bradycardia
Normal QRS axis =
if the QRS is (+) (upright) in both leads I and II, then the axis falls between -30 & 90 degree which is normal.
Left axis deviation =
If the QRS complex is is (+) in lead I but (-) in lead II, the it is left axis deviation (-30 to -90 degree).
Right axis deviation =
If the QRS complexes are (-) in lead I and (+) in aVF, then it is right axis deviation (90 to 180 degree).
Extreme axis deviation =
If the QRS complexes are (-) in both I & aVF, then the axis is extreme (180 to -90 degrees).
8 steps in interpretating an ECG:
- Rate
- Rhythm
- Axis
- Intervals
- P wave
- QRS complex
- ST segment T wave
- Overall interpretation
Short PR intervals may suggest of…
Wolff-Parkinson White syndrome
Long QRS intervals represent…
- bundle branch block
- ventricular pre-excitation
- ventricular pacing
- ventricular tachycardia
