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Accident and Emergency Medicine, Ninewells Hospital, Dundee

Evaluation of a rhythm strip

Charge Nurse Jenny Fitzpatrick, Accident and Emergency Department.

ECG Paper

An ECG tracing is a graphic recording of the heart’s electrical activity.

The paper consists of horizontal and vertical lines, each 1mm apart. The horizontal lines denote time measurements. When the paper is run at a sweep of 25mm/second, each small square is equal to 0.04 seconds, whereas a large square [five small squares] equals 0.02 seconds. Height or voltage is measured by counting the lines vertically. Most ECG paper is also marked at 3 second intervals along the top or bottom for rate calculation.

Calculation of Heart Rate

Although monitors provide a continuous display of the heart rate, it is usually necessary also to determine atrial and ventricular rates on the rhythm strip. To calculate the heart rate, the following methods can be used;

¨ Identify a 6 second strip of ECG paper. Count the number of complexes within the 6 seconds and multiply by ten.

¨ Calculate the number of large squares between two QRS complexes and divide into 300.

These methods can be used for regular as well as irregular rhythms and is accurate within +/- 5 beats per minute.

Waveforms and Intervals

During the cardiac cycle, the following waveforms and intervals are produced on the ECG tracing.

P Wave : A small deflection representing depolarisation of the atria. It is

normally seen before the QRS complex at a consistent

interval.

PR interval : The interval representing conduction time through the

atria, AV node, bundle of His, and bundle branches, up to the

point of activation of the ventricular muscle tissue.

The interval is measured from the beginning of the P wave, to

the beginning of the QRS complex. A normal PR interval is

0.12 - 0.20 seconds.

QRS Complex : A large waveform representing ventricular

depolarisation. Each component of the waveform has a specific

connotation. The initial negative deflection is a Q wave, the

initial positive deflection is an R wave and the negative

deflection after the R wave is an S wave.

Not all QRS complexes have all three components, even

though the complex is commonly referred to as the QRS

complex.

ST Segment : The segment between the end of the QRS complex and the

beginning of the T wave. Normally it is isoelectric at baseline,

but may be elevated or depressed in a variety of conditions.

T Wave : A deflection that represents ventricular repolarisation or

recovery. It appears after the QRS complex.

QT Interval : The interval representing total duration of ventricular

electrical systole. The QT interval is measured from the

beginning of the QRS complex to the end of the T wave. A

normal QT does not usually exceed 0.42 seconds.

Steps in assessing a Rhythm Strip

1. Identify the QRS complexes.

2. Look for the P waves.

If present, are they all the same configuration ?

Is there a P wave for every QRS complex ?

3. Measure the PR interval.

Is it normal ?

Is it the same throughout the strip, or does it vary ?

4. Evaluate the QRS complex.

Is it normal in width, or is it wide ?

Are all the complexes of the same configuration ?

Is there a QRS complex for every P wave?

5. Determine the atrial and ventricular rates.

Are they within normal limits ?

Is the atrial rate the same as the ventricular rate ? If not, is there

any relationship between the two ? [i.e. is one a multiple of the

other]

6. Identify the rhythm and determine it’s clinical significance.

Is the patient symptomatic ? [Check skin, neurological status,

Coronary circulation and haemodynamic status / blood pressure]

Is the arrhythmia acute or chronic ?

Is the arrhythmia life threatening ?

Rhythm strip examples will be added in the near future.