SA node (and atria): 60-100 times/min
AV node and bundle of His (can take over to initiate impulse if needed): 40-60 times/min
bundle branches and Purkinje fibers (can take over if AV node fails): 20-40 times/min
When assessing a cardiac rhythm, use a consistent and systematic approach. One such approach includes the following:
A. Look for the presence of the P wave. Is it upright or inverted? Is there one for every QRS complex or more than one?
Are there atrial fibrillatory or flutter waves present?
B. Evaluate the atrial rhythm. Is it regular or irregular?
C. Calculate the atrial rate (count the P waves).
D. Measure the duration of the PR interval. Is it normal duration (0.12 – 0.20/3-5 boxes) or prolonged?
E. Evaluate the ventricular rhythm. Is it regular or irregular?
F. Calculate the ventricular rate (count the QRS complexes).
G. Measure the duration of the QRS complex. Is it normal duration (<0.12/< 3 boxes) or prolonged?
H. Assess the ST segment. Is it isoelectric (flat), elevated, or depressed?
I. Measure the duration of the QT interval. Is it normal duration (0.34 – 0.43/8.5-10.75 boxes) or prolonged?
J. Note the T wave. Is it upright or inverted?
Additional questions to consider include the following:
1. What is the dominant or underlying rhythm and/or dysrhythmia?
2. What is the clinical significance of your findings?
3. What is the treatment for the particular rhythm?
PQRST interpretation
P wave – [0.06 – 0.12] – represents atrial depolarization or contraction of atria
PR segment – the flat line following the P wave which demonstrates delay of AV node allows atria to dump blood into
ventricles)
PR interval – [0.12 – 0.20] – starts at beginning of P wave, ends at beginning of QRS complex; demonstrates amount of
time for electrical signal to go from atria to AV node (Note if this takes too long it could indicate heart block)
QRS complex – [<0.12] – represents ventricular depolarization leading to ventricle contraction (systole); atrial
repolarization occurs here but is not visible (overshadowed by ventricle depolarization)
J point – point where QRS complex meets S-T segment
ST segment – [0.12] – represents completion of ventricular depolarization and beginning of ventricular repolarization;
should be flat (isoelectric)
T wave – represents beginning of ventricular repolarization (shows because ventricles are so big they cause activity even
when they relax); the flat line that follows the T wave means ventricle repolarization is complete and the cycle is ready
to start again
QT interval – [0.34 – 0.43] – starts at beginning of QRS complex and ends after T wave; represents time for electrical
signals to cause the ventricles to contract and relax