ECG Interpretation

Preliminary 6-lead ECG identified bradycardia (45 to 49 beats/min) characterized by a narrow-complex ventricular rhythm, absence of distinct P waves, and rapid, prominent, baseline undulations (Figure 1) Echocardiography revealed myxomatous atrioventricular valvular disease with severe mitral and tricuspid valve regurgitation, moderate dilation of all cardiac chambers, mild pericardial effusion, and pleural effusion. Also, prominent undulations were noted on both mitral valve leaflet traces (Figure 2) Presumptive diagnoses included atrial flutter or atrial fibrillation with complete atrioventricular nodal block. The ECG finding of baseline undulations (coarse fibrillatory waves) with characteristic variable morphology, erratic timing, and high cycle-interval rates (between 470 and 575 intervals/min) supported a diagnosis of atrial fibrillation. The slow escape rhythm was attributed to complete atrioventricular block.

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Six-lead ECG recordings obtained from a 12-year-old American Pit Bull Terrier that was referred for evaluation of acute weakness and collapse. The dog was found to have pericardial and abdominal effusions. A—At the initial examination, there is a slow, narrow-complex escape rhythm (QRS duration, 45 milliseconds). Coarse baseline undulations, which are most apparent in leads II, III, and aVF, resemble sawtooth flutter waves (F waves). However, these may appear similar to F waves because this tracing was recorded at double the typical sensitivity. Upon closer inspection, the undulations have variable morphology and timing and occur 470 to 575 times/min, a rate that is too rapid for canine atrial F waves. Consequently, the ECG findings are indicative of coarse atrial fibrillation with complete atrioventricular block and a slow, narrow-complex escape rhythm. Paper speed = 25 mm/s; 2 cm = 1 mV. B—Six-lead ECG recordings obtained from the dog 1 week later. Notice that the baseline undulations are small and more irregular than those in the previous recordings and are consistent with fine atrial fibrillation. A pacemaker-initiated ventricular rhythm with left bundle branch block is present, consistent with ventricular pacing as a result of right ventricular placement of the pacemaker electrode. A small pacing spike precedes each QRS complex, most noticeable in leads II, III, and aVF. The heart rate (107 beats/min) is driven by pacemaker-generated impulses at a higher rate than the initial, slow escape rhythm. Paper speed = 50 mm/s; 1 cm = 1 mV.

Citation: Journal of the American Veterinary Medical Association 257, 1; 10.2460/javma.257.1.41

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M-mode echocardiograms (with the ultrasound probe directed through the right and left ventricles at the mitral valve level) obtained from the dog in Figure 1. A—At the initial examination, undulations are evident on anterior and posterior mitral valve leaflet traces during diastole and on the simultaneous ECG baseline trace. Sweep speed = 33.33 mm/s. Notice the slower sweep speed, compared with that used in panel B, which may make the appearance of these deflections more pronounced. B—One week later, the coarse diastolic undulations on the mitral valve leaflet trace and on the simultaneous ECG baseline trace are absent. The faster sweep speed used in this examination, compared with that used in panel A, may have made the appearance of these deflections less pronounced. Sweep speed = 100 mm/s.

Citation: Journal of the American Veterinary Medical Association 257, 1; 10.2460/javma.257.1.41

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The dog's clinical signs were attributed to volume overload from atrioventricular valve regurgitation, complicated by severe bradycardia. A permanent, artificial, single-chamber pacemaker was implanted, programmed to ventricle-paced, ventricle-sensed, inhibited (pulse generator inhibits a pacing stimulus if a sensed event is detected), and R-rate adaptive (in response to patient activity [VVIR]) mode. Furosemide (2.2 mg/kg [1 mg/lb], PO, q 12 h) and pimobendan (0.3 mg/kg [0.136 mg/lb], PO, q 12 h) were prescribed for the duration of this animal's care. The dog's alertness and activity improved by the time it was rechecked 1 week later. An ECG recording obtained at this visit reflected a paced ventricular rhythm (107 beats/min) with low-amplitude, fine, unorganized, baseline waveforms consistent with fibrillation waves (Figure 1). Echocardiographic examination revealed that diastolic mitral valve leaflet undulations were also absent (Figure 2). The cardiac chambers were comparatively less dilated. Two months later, ascites recurred. Echocardiography revealed pericardial effusion and an intramural right atrial wall mass. The dog was subsequently lost to follow-up.

Discussion

Atrial fibrillation is a common arrhythmia in dogs.^1,2 Although atrial fibrillation is generally characterized by rapid, irregular, atrial and ventricular waveforms, it may be confused with other rhythm disorders, depending on the morphology and timing of atrial deflections and the ventricular response rate.^1–5 Treatment for atrial fibrillation and atrial flutter may differ substantially, which emphasizes the importance of a correct diagnosis. Diagnostic uncertainty can occur when an atrial flutter circuit is rapid with variable atrioventricular nodal conduction, the macro-reentry circuit of atrial flutter is the source of fibrillatory conduction through the atria, multifocal atrial tachycardia is present, or atrial fibrillation and atrial flutter transform into one another.^3–9 Advanced techniques, including frequency distribution analysis of atrial and ventricular deflections, have been promoted to clarify the diagnosis of atrial fibrillation versus atrial flutter.⁵

For the dog of the present report, portions of the initial ECG recording had coarse undulations without an isoelectric baseline, thus simulating sawtooth waveforms characteristic of typical, counterclockwise atrial flutter. In such conditions, conduction involves the cavotricuspid isthmus and an isoelectric baseline is absent, making differentiation of atrial flutter and atrial fibrillation difficult.⁶ Whereas the flutter wave rates determined in experimental studies of dogs vary depending on experimental methods,^4,5,10 the rate of atrial flutter undulations does not exceed 450 undulations/min in clinical situations.¹ In the case described in the present report, the rate of undulations exceeded this limit; moreover, the undulations had variable morphology and timing, consistent with atrial fibrillation waves.

When the dog was reexamined 1 week after pacemaker implantation and initiation of drug administration, the arrhythmia had transformed from coarse to fine atrial fibrillation. Coarse undulations on mitral valve leaflet traces were also absent; this finding is similar to that for humans in whom diastolic mitral valve leaflet undulations occurred with atrial flutter fibrillation or atrial flutter, but not with fine atrial fibrillation.¹¹ In the dog of the present report, atrial fibrillation may have resulted from altered conduction as a result of atrial dilation that was caused by severe atrioventricular valve regurgitation. The slow ventricular rate was attributed to complete heart block caused by the atrioventricular nodal conduction abnormality.

In the case described in the present report, differentiation of atrial fibrillation from atrial flutter and the clinical impact of coexisting bradycardia and atrioventricular valve regurgitation posed challenges for effective management. Several related points are noteworthy. Close observation of ECG waveforms should include evaluation of the variability of morphology, timing, and rate of baseline undulations. Extended recordings of multiple ECG leads improve assessment. Effects of coarse atrial fibrillation can be identified by echocardiography and ECG, and coarse atrial fibrillation may transform to fine atrial fibrillation. It must be realized that doubling ECG sensitivity to improve visualization of baseline undulations may overaccentuate coarse atrial fibrillation deflections, mimicking features consistent with atrial flutter. A slow ventricular response rate can occur with atrial fibrillation when atrioventricular nodal block is also present. Lastly, pacemaker implantation should be considered in selected cases when there are concurrent clinical signs associated with severe volume overload and pronounced bradycardia.