Sinus tachycardiaAtrial tachycardiaAtrial flutterAtrial fibrillationAtrioventricular re-entry

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When the QRS complex is narrow (as it usually is), excitation or conduction problems are restricted to the atria, since the complex represents ventricular depolarisation. No two EGCs are the same, so there are no truly firm “rules” for differentiating different disturbances. However, there are features that are common enough to help diagnosis in the more typical presentations.

Sinus tachycardia

Normal waveforms at a high rate. “Camel’s hump” (P wave in previous T wave).

Sinus tachycardia is generally due to stimulation of the sympathetic nervous system, such as during exercise or anxiety. Some drugs such as β2-adrenoceptor agonists can also exert a stimulatory effect on the heart. Sinus tachycardia is also a reflex response to hypovolaemia, as in blood loss or sepsis. Sinus tachycardia rarely exceeds 180 bpm.

Atrial tachycardia

Results from ectopic focus. Odd/inverted p waves in inferior leads (II, III, aVF).

In atrial tachycardia, action potentials are generated by one or more ectopic foci (multifocal atrial tachycardia) rather than the SA node. Because the spread of atrial depolarisation is not in the usual direction from the SA node, P waves have an atypical appearance and are often inverted. Atrial tachycardia is typically between 150 and 250 bpm.

Atrial flutter

Flutter waves obvious in most leads. Loss of isoelectric baseline. Block.

The most striking feature of the ECG in atrial flutter is the classic “saw-toothed” baseline. Flutter occurs when a re-entry circuit exists in the atria, allowing action potentials to spread around the atria in cycles. The electrophysiological properties of atrial myocytes can support up to 300 such cycles per minute, but the delaying effect of the AV node prevent most impulses from passing to the ventricles. This produces a continuous series of waves with no isoelectric intervals, with regular QRS complexes occurring every 2 or 3 waves.

Atrial fibrilation

Wobbly baseline (never isoelectric). Irregularly irregular rhythm.

Atrial fibrillation results from many ectopic foci generating action potentials in the atria. The result of this random pattern of activation is a continuously fluctuating baseline without distinct P waves. Some of these impulses reach the AV node and are conducted (they are "blocked") to the ventricles. The most striking feature of the ECG in fibrillation is that QRS complexes are “irregularly irregular” – there is no discernible pattern to when they occur.

AV re-entry tachycardia

No “block”. P waves missing or after QRS complex. Isoelectric baseline.

There are two types of re-entry tachycardia. In atrioventricular nodal re-entry tachycardia (AVNRT), a circuit of action potentials rotates around the AV node. By contrast in atrioventricular re-entry tachycardia (AVRT) impulses re-enter the AV node from an accessory pathway in one of the ventricles. In both cases the P wave is either buried in the QRS complex, or may appear after it.

Adapted from: AR Houghton & D Gray, Making sense of the ECG: a hands-on guide (3rd Ed.)

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