Interpreting a rhythm strip is a skill. Rhythm strip analysis requires a sequential and systematic approach the eight steps outlined here.
Step 1: Determine the rhythm:
To determine the heart’s atrial and ventricular rhythms, use either the paper-and-pencil methodor the caliper method.
For atrial rhythm, measure the P-P intervals-the intervals between consecutive P waves. These intervals should occur regularly.
Then compare the P-P intervals in several cycles. Consistently similar P-P intervals indicate regular atrial rhythm; dissimilar P-P intervals indicate irregular atrial rhythm.
To determine the ventricular rhythm, measure the intervals between two consecutive R waves in the QRS complexes. The R-R intervals should occur regularly.
Keep in mind that variations of up to 0.04 second are considered normal.
Step 2: Determine the rate
To figure the atrial rate, obtain a 6-second strip, count the number of P waves, and multiply by 10. Ten 6-second strips represent 1 minute.
Calculate ventricular rate the same way, using the R waves.
1,500 method: If the heart rhythm is regular, use the 1,500 method – so named, because 1,500 small squares represent 1 minute. Count the small squares between identical points on two consecutive P waves and then divide 1,500 by that number to get the atrial rate. To obtain the ventricular rate, use the same method with two consecutive R waves.
Step 3: Evaluate the P wave
When examining a rhythm strip for P waves, ask yourself
➤ Are P waves present? Do they all have normal configurations?
➤ Do they all have similar size and shape?
➤ Is there one P wave for every QRS complex?
Step 4: Determine the duration of the PR interval
To measure the PR interval, count the small squares between the start of the P wave and the start of the QRS complex; then multiply the number of squares by 0.04 second.
Now ask yourself:
➤ Is the duration a normal 0.12 to 0.20 second?
➤ Is the PR interval constant?
Step 5: Determine the duration of the QRS complex
When determining QRS duration, be sure to measure straight across from the end of the PR interval to the end of the S wave, not just to the peak.
Remember, the QRS has no horizontal components. To calculate duration, count the number of small squares between the beginning and end of the QRS complex and multiply this number by 0.04 second.
Then ask yourself:
➤ Is the duration a normal 0.06 to 0.10sec.?
➤ Are all QRS complexes the same size and shape?
➤ Does a QRS complex appear after every P wave?
Step 6: Evaluate the T waves
Examine the strip for T waves. Then ask yourself:
➤ Are T wavespresent?
➤ Do they all have a normal shape?
➤ a normal amplitude?
➤ same amplitude?
➤ same deflection as the QRS complexes?
Step 7: Determine the duration of the QT interval
Count the number of small squares between the beginning of the QRS complex and the end of the T wave, where the T wave returns to the baseline.
Multiply this number by 0.04 second.
➤ Is the duration a normal 0.36 to 0.44 second?
Step 8: Evaluate any other components
Check for ectopic beats and other abnormalities. Also check the ST segment for abnormalities, and look for the presence of a U wave. Note your findings, and then interpret them by naming the rhythm strip according to one or all of these findings:
➤ origin of the rhythm (for example, sinus node, atria, AV node, or ventricles)
➤ rate characteristics (for example, bradycardia or tachycardia)
rhythm abnormalities (for example, flutter, fibrillation, heart block, escape rhythm, or other arrhythmias).
Correcting the QT interval
The QT interval is affected by the patient’s heart rate. As the heart rate increases, the QT interval decreases.
as the heart rate decreases, the QT interval increases. For this reason, evaluating the QT interval based on a standard heart rate of 60 is recommended.
This corrected QT interval is known as QTc.
- The normal QTc for women is less than 0.46sec
- The normal QTc for men is less than 0.45sec.
When the QTc is longer than 0.50 second in men or women, torsades de pointes is more likely to develop.
What makes for normal?
Using the 8-step method previously described, these are the characteristics
of normal sinus rhythm:
Atrial and ventricular rhythms are regular.
Atrial and ventricular rates fall between 60 and 100 beats/minute,
[the SA node’s normal firing rate, and all impulses are conducted to the ventricles.]
Normal sinus rhythm
Normal sinus rhythm, shown below, represents normal impulse conduction through the heart.
Characteristics of normal sinus rhythm:
➤ Regular rhythm
➤ Normal rate
➤ A P wave for every QRS complex; all P waves similar in size and shape
➤ All QRS complexes similar in size and shape
➤ Normal PR and QT intervals
➤ Normal (upright and round) T waves
The atrial and ventricular A P wave precedes each QRS complex.
rhythms are regular. Each component of the ECG complex is present.
- P waves are rounded, smooth, and upright in lead II, signaling that a sinus impulse has reached the atria.
- The PR interval is normal (0.12 to 0.20 second), indicating that the impulse is following normal conduction pathways.
- The QRS complex is of normal duration (less than 0.12 second), representing normal ventricular impulse conduction and recovery.
- The T wave is upright in lead II, confirming that normal repolarization has taken place.
- The QT interval is within normal limits (0.36 to 0.44 second).
- No ectopic or aberrant beats occur.
Each component features
Normal P wave
➤ Location-before the QRS complex
➤ Amplitude-2 to 3 mm high
➤ Duration-0.06 to 0.12 second
➤ Configuration-usually rounded and upright
➤ Deflection-positive or upright in leads I, II, aVF, and V2 to V6;
usually positive but may vary in leads III and aVL;
negative or inverted in lead aVR; biphasic or variable in lead V1
Normal PR interval
➤ Location-from the beginning of the P wave to the beginning of the QRS complex
➤ Duration-0.12 to 0.20 second
Normal QRS complex
➤ Location-follows the PR interval
➤ Amplitude-5 to 30 mm high but differs for each lead used
➤ Duration-0.06 to 0.10 second, or half the PR interval
➤ Configuration-consists of the Q wave, the R wave, and the S wave
➤ Deflection-positive in leads I, II, III, aVL, aVF, and V4 to V6 and negative in leads aVR and V1 to V3 Normal ST segment
➤ Location-from the S wave to the beginning of the T wave
➤ Deflection-usually isoelectric; may vary from – 0.5 to + 1 mm in some precordial leads
Normal T wave
➤ Location-after the S wave
➤ Amplitude-0.5 mm in leads I, II, and III and up to 10 mm in the precordial leads
➤ Configuration-typically round and smooth
➤ Deflection-usually upright in leads I, II, and V3 to V6; inverted in lead aVR; variable in all other leads Normal QT interval
➤ Location-from the beginning of the QRS complex to the end of the T wave
➤ Duration-varies; usually lasts from 0.36 to 0.44 second
Normal U wave
➤ Location-after T wave
➤ Configuration-typically upright and rounded
Interpreting a rhythm strip: 8-step method
➤ Step 1: Determine the rhythm
➤ Step 2: Determine the rate
➤ Step 3: Evaluate the P wave
➤ Step 4: Measure the PR interval
➤ Step 5: Determine the QRS complex duration
➤ Step 6: Examine the T waves
➤ Step 7: Measure the QT interval duration
➤ Step 8: Check for ectopic beats and other abnormalities
Normal sinus rhythm: is the standard against which all other rhythms are compared