The common atrial flutter corresponds to a tachycardia associated with a macro-reentry of the right atrium. Although its incidence is approximately 10-times less than that of atrial fibrillation, the occurrence of atrial flutter is relatively common especially in obese patients or in patients with right atrial cardiac disease (inter-atrial communication, Ebstein disease, chronic cor pulmonale, surgical incision of the right atrium, etc.). It should be noted that atrial fibrillation and flutter can coexist or succeed one another in the same patient. With the ablation of the cavo-tricuspid isthmus allowing a radical and effective treatment in the majority of cases, it is important to know the pathophysiological mechanisms as well as the main electrocardiographic characteristics in order to make the diagnosis.
The majority of atrial flutters occurring in an unscarred healthy heart correspond to right atrial macro-reentries which are dependent on the cavo-tricuspid isthmus with a counterclockwise rotation. The reentry circuit is confined to the right atrium with a depolarization front descending in the lateral wall and ascending in the septum. The circuit is delimited anteriorly by the tricuspid valve and posteriorly by a certain number of anatomical obstacles (orifice of the superior vena cava and of the inferior vena cava, Eustachian valve) and by anatomico-functional barriers (crista terminalis, sinus venosus region). The activation rises along the inter-atrial septum in its anterior part, is reflected on the roof of the atrium and then descends along its anterolateral wall in front of the crista terminalis, ultimately reaching the lower part of the septum by passing through a slow conduction isthmus delimited anteriorly by the tricuspid ring and posteriorly by the ostium of the inferior vena cava.
Since the path of the circuit is remarkably reproducible from one patient to another, certain electrocardiographic characteristics are systematically found, often making the diagnosis obvious if the atrial activity is visible (ventricular rhythm not overly rapid). The analysis of the morphology of the flutter waves can be enhanced by performing a sino-carotid massage which does not modify the atrial cycle but slows the conduction and suppresses the superposition of the ventriculograms. The diagnosis of flutter is therefore made with the highlighting of remarkably uniform flutter waves having the following characteristics:
- the atrial rate is typically between 250 and 300 bpm. There is, however, a certain correlation between the degree of dilatation of the right atrium and the flutter cycle, the latter being longer (slower atrial pace) with increasing dilatation of the atrium. Similarly, antiarrhythmic drugs can prolong the cycle and alter the morphology of flutter waves. In contrast, the atrial rate can be faster and reach 350-400 bpm in the neonatal period and in the infant.
- inferior leads (II, III, aVF): in these leads, we find the typical sawtooth, factory-roof pattern; the f waves appear biphasic, with a predominant negative deflection (higher voltage and wider) followed by a reduced positive deflection with a steeper ascending slope comparatively to the descending phase without ever returning to the isoelectric line. A plateau pattern can be observed during the descending phase.
- leads I and aVL: these leads are oriented perpendicularly to the activation vector of the flutter and only record low amplitude signals; atrial activity in this instance is therefore of low voltage.
- right precordial leads (V1): it is common to observe a 2-component atrial depolarization with an initially isoelectric activity followed by a predominant positive wave with a pointed peak.
- left precordial leads (V6): the positive component observed in V1 is reversed with the precordial leads, and in V6 the electrical activation is usually negative and of small amplitude.
- the most common ventricular response is of the 2:1 type; the finding of a tachycardia of 150 bpm should evoke an atrial flutter. Atrioventricular conduction may be altered with a 3:1, 4:1 block pattern or even a complete atrioventricular block pattern. One of the ensuing example tracings illustrates the case where, on the contrary, conduction is improved with a 1:1 ratio. Conduction can sometimes be variable in which a seemingly irregular ventricular rate is observed. The examination of the RR cycles however allows finding repetitive sequences with a characteristic alternating Wenckebach pattern.