5.2 Atrial flutter

Atrial Flutter — the sawtooth rhythm

Atrial flutter is a rapid, organized atrial rhythm produced by a single large re-entrant circuit — most commonly a loop rotating around the tricuspid annulus in the right atrium (typical, cavotricuspid-isthmus-dependent flutter). Instead of discrete upright P waves, the continuous re-entry produces a repetitive, uniform, saw-tooth pattern of flutter waves, labeled F waves, that are best seen in the inferior leads (II, III, aVF) and in V1. The baseline has no flat isoelectric segment between deflections; it undulates continuously like the teeth of a saw or a picket fence.

Rates and the AV "gatekeeper"

The atrial rate in flutter is characteristically 250–350 beats per minute, most classically right around 300/min. The AV node cannot conduct every one of these rapid impulses to the ventricles — it acts as a protective gatekeeper, blocking a fixed fraction. This produces a conduction ratio: for every so many flutter waves, one is passed to the ventricles.

• 2:1 conduction → two F waves per QRS → ventricular rate ≈ 150/min
• 3:1 conduction → ventricular rate ≈ 100/min
• 4:1 conduction → ventricular rate ≈ 75/min

Because the atrial rate is metronome-steady and the block is often fixed, the ventricular rhythm is usually regular. A quick way to estimate the ventricular rate is to divide the atrial rate by the conduction ratio: an atrial rate of 300 with 2:1 block gives 150; with 4:1 block it gives 75.

The "150" clue and hidden flutter waves

A very high-yield CRAT pearl: a regular narrow-complex tachycardia at almost exactly 150/min should make you suspect 2:1 atrial flutter. At that rate, one flutter wave falls right on top of each QRS or T wave and can be buried, so the sawtooth is easy to miss. Techniques to unmask the F waves include examining leads II and V1 carefully and — clinically — vagal maneuvers or adenosine, which transiently increase AV block, drop out several QRS complexes, and reveal the flutter waves marching through. The CRAT technician's job is to recognize the pattern and alert the clinician, not to administer these maneuvers.

QRS and a counting method

Conduction below the AV node is normal, so the QRS is narrow (≤ 0.12 s) unless there is a pre-existing bundle branch block or rate-related aberrancy. To read a flutter strip systematically: (1) identify the sawtooth F waves and confirm they are regular, (2) count the atrial rate, (3) count how many F waves fall between each QRS to establish the conduction ratio, and (4) decide whether the ventricular response is regular or variable.

Variable conduction

Sometimes the AV node does not block a fixed fraction — the ratio alternates (for example 2:1, then 4:1, then 3:1). This variable conduction makes the ventricular rhythm irregular, and such a strip can resemble atrial fibrillation. The distinguishing feature is that flutter still shows regular, identical sawtooth F waves marching through at a constant atrial rate, whereas atrial activity in fibrillation is chaotic and disorganized. Recognizing regular F waves at about 300/min — even with an irregular ventricular response — identifies flutter.

Symptoms and clinical picture

Patients in flutter may report palpitations, a fluttering sensation in the chest, shortness of breath, chest discomfort, lightheadedness, or fatigue; some — especially with a well-controlled 4:1 response — are asymptomatic. Flutter that conducts 1:1 (an atrial rate near 300 passing every beat to the ventricles) is a dangerous emergency, producing a ventricular rate approaching 300/min with hemodynamic collapse; this can occur when AV conduction is enhanced or after certain antiarrhythmic drugs slow the atrial rate enough for 1:1 conduction. Because flutter shares disease substrates and stroke risk with atrial fibrillation, a new flutter finding warrants the same prompt documentation and notification.

Causes and significance

Atrial flutter rarely occurs in a completely normal heart. Common associations include:

• Underlying heart disease — coronary artery disease, heart failure, cardiomyopathy, valvular (especially mitral) disease
• Atrial enlargement or dilation and prior cardiac surgery
• COPD and other pulmonary disease, and pulmonary embolism
• Hyperthyroidism, alcohol ("holiday heart"), and the peri-onset of atrial fibrillation (flutter and fibrillation often coexist and convert into one another)

Clinically, flutter matters for several reasons. A 2:1 conducted flutter at 150/min may not be tolerated — a sustained fast ventricular rate shortens diastolic filling time, drops cardiac output, and can precipitate ischemia, hypotension, or heart failure. Like atrial fibrillation, flutter lets blood stagnate in the atria, creating a risk of thrombus formation and embolic stroke, so anticoagulation is a consideration. Finally, flutter with variable or high-ratio conduction can warn that the rhythm may destabilize or degenerate into atrial fibrillation. For all these reasons the CRAT technician must document the atrial rate, the conduction ratio, and whether the ventricular response is regular or variable.