2 Heart Rate and Rhythm Assessment

Paper speed and square timing

Use adult ECG conventions and assume a standard paper speed of $25\ \mathrm{mm/s}$, unless stated otherwise. At this speed, each small square represents $0.04\ \mathrm{s}$, and each large square represents $0.20\ \mathrm{s}$. Measure intervals from the beginning of one waveform to the beginning or end specified.

Estimating ventricular rate

Regular rhythms

For a regular rhythm at $25\ \mathrm{mm/s}$, estimate ventricular rate from the spacing between consecutive R waves. Divide $300$ by the number of large squares, or divide $1{,}500$ by the number of small squares.

For example, $4$ large squares between R waves gives $300 \div 4 = 75$ beats/min. The same spacing is $20$ small squares, giving $1{,}500 \div 20 = 75$ beats/min. These are shortcuts for regular rhythms at the standard paper speed.

Irregular rhythms

When R–R intervals vary, a single interval may not represent the average rate. Instead, count the QRS complexes in a timed strip. For example, $13$ QRS complexes in $10$ seconds gives an estimated average rate of $13 \times 6 = 78$ beats/min.

Assessing rhythm regularity

Assess regularity by comparing interval spacing. Equal R–R intervals indicate regular ventricular timing; visibly varying intervals indicate irregular timing. For example, R–R intervals of $4$, $4$, $4$, and $4$ large squares are regular.

Regularity can also be described by assessing P–P intervals. Consider both atrial and ventricular timing when reviewing a rhythm strip.

Waveforms and atrial–ventricular relationships

The P wave represents atrial depolarization, the QRS complex represents ventricular depolarization, and the T wave represents ventricular repolarization. A pattern of similar upright P waves, each followed by a QRS complex and then a T wave, shows a consistent relationship in which each observed atrial depolarization is followed by ventricular depolarization.

If some P waves are not followed by QRS complexes, report that observation and describe that atrial and ventricular events do not have a one-to-one relationship. Do not assume that every atrial impulse is conducted to the ventricles. Describe the observed pattern and escalate findings according to clinical context and local protocol.

Measuring ECG intervals

PR interval

Measure from the start of the P wave to the start of the following QRS complex. The usual adult reference range is $0.12$–$0.20\ \mathrm{s}$. For example, $4$ small squares represent $4 \times 0.04 = 0.16\ \mathrm{s}$, which is within that range.

QRS duration

Measure the width of the QRS complex. A duration under $0.12\ \mathrm{s}$ is commonly described as narrow; a duration of $0.12\ \mathrm{s}$ or longer is wide. For example, $2$ small squares represent $2 \times 0.04 = 0.08\ \mathrm{s}$, which is narrow by this cutoff.

QT interval

For example, $10$ small squares represent a measured QT interval of $10 \times 0.04 = 0.40\ \mathrm{s}$. QT varies with heart rate, so interpretation should account for rate, typically by considering a corrected QT (QTc), rather than judging the measured QT alone.

A structured rhythm-strip assessment

In addition to calculating rate, describe the rhythm strip by reviewing its regularity, waveforms, and intervals. A structured review can help prevent important features from being overlooked.

• Regularity of the P–P and R–R intervals.

• Presence and consistency of P waves.

• Whether each P wave is followed by a QRS complex.

• PR interval.

• QRS width and shape.

• Sequence and appearance of the T wave.

• R–R interval