Sinus arrhythmias

Normal ECG intervals:

PR interval 200ms (5 small squares)

QRS complex duration 120ms (3 small squares)

QT interval 400ms (2 large squares)

Sinus arrhythmia:

• A normal variation
• During inspiration, parasympathetic tone decreases and the heart rate increases
• On expiration, the heart rate falls

Sinus bradycardia:

• A sinus rate of <60bpm during the day or <50bpm during the night
• Normal in athletes and the elderly

Causes include:

• Hypothermia
• Hypothyroidism
• Cholestatic jaundice
• Raised ICP
• Drugs (e.g. digitalis, ß-blockers)
• Acute ischaemia/infarction of the sinus node

Treatment:

• IV atropine 600μg
• Pacemaker if unsuccessful

Sinus tachycardia:

• Sinus rate acceleration to >100bpm

Causes include:

• Fever
• Exercise
• Emotion
• Pregnancy
• Anaemia
• Thyrotoxicosis
• Catecholamine excess

Treatment:

• Treat underlying cause
• ß-blockers can be used