7.8.4 Atrial tachycardia
Atrial tachycardia is a supraventricular tachycardia with a rapid, regular rhythm and a rate which is often 120-220 bpm. It may be paroxysmal (short bursts) or sustained. Four or more consecutive APCs constitute an atrial tachycardia. The ECG will show normal QRS complexes, with regular R-R intervals, but the P' wave may be a different configuration from normal. Often the P' wave is lost in the preceding T wave and cannot be identified. In some cases, intermittent 20AV block is also present and the rhythm may be fast and irregular.
It is important to distinguish atrial tachycardia from sinus tachycardia and ventricular tachycardia. Diagnosis is usually straightforward if a paroxysm of tachycardia starts during the ECG recording. This shows the normal sinus rate and the normal QRS complex configuration, followed by a different configuraP' wave and a shorter P'-P' and R-R interval. However, if the rhythm is persistent the distinction is more difficult. An atrial tachycardia can be disfrom sinus tachycardia because the heart rate is inappropriately high, i.e. there are no other apparent causes for the high rate such as excitement, pyrexia, pain, etc. Ventricular tachycardia can be more difficult to distinguish from atrial tachycardia in horses than in small animals because the QRS comof ventricular origin may not be abnormally wide. However, if a normal QRS complex is seen, comparison allows complexes of ventricular origin to be identified. In addition, in ventricular tachycardia, P waves may be identified and bear no relationship to the QRS complexes.
The presence of atrial tachycardia may indicate underlying atrial myocardial disease. A thorough clinical examination and laboratory evaluation including haematology, plasma fibrinogen and serum biochemistry should be performed in order to try to identify any underlying systemic disease. If the clinical signs at rest are sufficiently severe, antidysrhythmic treatment should be used. However, underlying conditions such as electrolyte imbalance should be corrected before antidysrhythmic treatment is initiated. If clinical signs are severe, digoxin can be used to control the ventricular rate by slowing conduction through the AV node, or quinidine can be used to suppress the ectopic focus. However, quinidine is negatively inotropic, vagolytic and hypotensive, which may all exacerbate the clinical condition, so the drug must be used with caution. Digoxin should also be used with caution (see section 6.9.2).
Atrial tachycardia is occasionally seen during quinidine conversion of AF.