Rhythm

Sinus rhythm is the term given to a rhythm that is regular, originates in the sinoatrial node, with a heart rate of 60-99 beats per minute. Sinus bradycardia is similar but the rate is below 60 whereas with sinus tachycardia, the rate is between 100 and 130 bpm.

P wave

T wave

QRS complex Fig. 8.2 The PQRST waves of a sinus rhythm complex.

ST segment

Characteristic features of sinus rhythm

• The P wave - is produced by atrial depolarisation. As the atria possess little muscle, the P wave is small. The P wave is best seen in lead II and VI where it is normally a positive deflection. Its shape is rounded and smooth and is less than 0.12 sec in duration (see Figure 8.2).

• The PR interval - represents the time it takes for the impulse to travel from the SA node to the AV node and the bundle of His and cause ventricular contraction, normally 0.12-0.20sec (see Table 8.1).

• The QRS complex - represents the spread of impulses by the ventricles. The R wave is the first upward deflection after the P wave. The height of the R wave is variable in different leads but the normal QRS width should be less than 0.12 sec or three little squares.

• The ST segment - usually runs fairly flat along the isoelectric line. It signifies the end of ventricular depolarisation and the start of electrical recovery. A rise of 1 mm in either standard or limb leads or a 2 mm rise in the precordial leads is associated with ischaemia or infarction.

• The T wave - confirms that repolarisation has taken place. The T wave should be upright in I, II, AVF, V3-V6, inverted in AVR as well as leads III and VI and variable in the remaining leads. The shape of the T wave is usually smooth and rounded.

Table 8.1 Normal PQRST intervals (Jevon 2003, Meek & Morris 2002b).

Interval

Duration

Measurement

Abnormality

PR interval

0.12-0.20 sec

From the start of the

If prolonged, consider

P wave to the first

delayed or

deflection of QRS

abnormal

conduction

QRS

0.08-0.12 sec

Begins at the Q wave

If QRS is >0.12sec

interval

and ends with the

consider

S wave deflection

abnormalities in

conduction such as

bundle branch

block or ventricular

arrhythmia. A QRS

complex that is

<0.12 sec will be

narrow in shape

and originate

above the

ventricles

QT interval

0.35-0.45 sec

From the beginning of

Prolonged QT interval

QRS until the end

is due to

of the T wave

hypothermia, drugs

and electrolyte

disorders such as

calcium; may need

to be adjusted for

age and gender

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