Atrial standstill, also known as silent atrium or atrial arrest was recognized predominantly in young mature Springer Spaniel dogs a number of years ago. The following abstract was submitted to the American Heart Association in 1980 but was not selected for presentation: “Persistent Atrial Standstill in Dogs with Chronic Myocarditis”. Buchanan JW, and Dubielzig RR, School of Veterinary Medicine, University of Pennsylvania.

“Permanent atrial standstill with absence of P waves in electrocardiograms occurred in 6 dogs one to 3 years old. Two were mongrels and 4 were Springer Spaniels. Three of 5 previously reported cases also were Springer Spaniels indicating a breed predisposition. Clinical signs included congestive heart failure, fainting, cardiomegaly, and multifocal ventricular escape rhythms. P waves and complete heart block were present in an earlier ECG of one dog. Atrial activity was not recorded in any of 3 dogs by intracavitary or epicardial recording and pacing. Ventricular responsivness to pacing was normal. All of the hearts had bilateral severe thinning of the atria with a lack of atrial myocardium. Fatty replacement and fibrosis were present in some cases. In one heart, patches of atrial myocardium remained. Foci of chronic inflammatory cells were seen in the atria of most hearts but the extent was quite variable. The ventricular myocardium appeared normal in two hearts. Two hearts had clusters of inflammatory cells in the ventricles but no loss of myocardium. One heart had large foci of intense chronic inflammation with myocardial dropout and one had large areas of myocardial loss affecting the full thickness of the right ventricular myocardium with mild chronic inflammation. The cause of this condition appears to be a chronic myocarditis of unknown etiology”.

My saddest experience with atrial standstill began with an attempt to do electrophysiologic studies on an active 2 year-old female Springer Spaniel used for hunting. She had no signs of heart disease except for bradycardia noted when she was presented to the dermatology clinic for scaly skin and pruritis (Figure 1).

Figure 1

She had a grade 4/6, band shaped, holosystolic murmur at the left apex with a faint precordial thrill and a grade 3/6 systolic murmur over the right apex. Lung sounds were normal. The pulse rate was 56/minute and an ECG revealed idioventricular rhythm with re-entrant ventricular bigeminy, no P waves and possible atrial fibrillation (Figure 2).

Figure 2

Radiographs showed marked generalized cardiac enlargement, dilated pulmonary veins and possible pulmonary edema. Blood analysis was unremarkable except for LDH 759 I.U./L (reference range 0-293) and T4 0.9 ng/ml (reference range 1-4) which indicated borderline hypothyroidism.

Four days later we anesthetized the dog for planned electrophysiologic studies and a pacemaker but things went bad immediately after thiopental anesthesia induction when ventricular fibrillation occurred. After external massage, nonsterile emergency thoracotomy, direct cardiac massage and 2 or 3 DC cardioversion shocks, the heart was defibrillated and an epicardial lead and pacemaker were implanted (Figure 3). Blood pressure rose with the onset of pacing (Figure 4).

Figure 3 Figure 3. Lateral radiograph showing generalized cardiomegaly and the positions of a corkscrew lead near the apex of the left ventricle and a unipolar, non-shielded pulse generator between the lateral abdominal muscles.

Figure 4 Figure 4. Aortic pressure recording before and after attachment of the pacemaker showed increased blood pressure due to increased heart rate.

Before chest closure, the thorax was flushed with broad spectrum antibiotics and the dog was kept on megadoses of antibiotics for 3 weeks and treated with levothyroxine (0.2mg daily). She was discharged with a paced heart rate of 100 bpm. When re-examined one month later her skin condition was improved and the pacemaker was functioning normally with no other ectopic ventricular foci (Figure 5).

Figure 5

Eight months later, the dog was presented in acute heart failure with watery bloody diarrhea.(Figure 6). Abnormal laboratory results and reference ranges were T4 0.2 (1-4), Alkaline phosphatase 606units/L (0-40), LDH 1410 units (0-293), BUN 89 mg/dl (5-25), SGO transaminase 165 iu/L (0-67), Cholesterol 420 mg/dl (82-287), Potassium 3.1 meq/L (3.7-5.3). The ECG showed regular idioventricular rhythm at 48 bpm with no pacemaker stimulus artifact (Figure 7) and she was euthanized.

Figure 6

Figure 6. Photographs of the dog 10 months after cardiac arrest showing open mouth breathing and pale mucous membranes.

Figure 7

Figure 7. Lead II ECG recorded 10 months after pacemaker implantation showing slow, regular, idioventricular rhythm at 48 bpm. Absence of pacemaker stimulus artifact indicated pacemaker failure. It had been a donated pulse generator and usage history was unknown.

Necropsy revealed paper thin, transparent atria (Figure 8) and peculiar areas of fibrosis with focal loss of myocardium in the right ventricle (Figure 9) yielding a translucent area adjacent to the interventricular septum (Figure 10). A focal area of endocardial fibrosis also was present on the interventricular septum opposite the transparent area (Figure 10).

Figure 8 Figure 8. Necropsy photograph of the right atrium with a ruler inside demonstrating transparency of the thin wall.

Figure 9 Figure 9. Necropsy photograph of the heart showing circumscribed area of fibrosis in the wall of the right ventricle adjacent to the interventricular septum.

Figure 10

Figure 10. Necropsy photograph of the opened right ventricle showing the internal aspect of the translucent area of fibrosis (T) evident in Figure 9. The insert photograph made with back lighting demonstrates translucency of the crescent shaped area of fibrosis. A round area of endocardial fibrosis (EF) also was evident in the interventricular septum opposite the translucent area.

The areas of fibrosis had a curved appearance and did not correspond to coronary artery distribution. They had the same radius as the defibrillation paddles used 10 months earlier (Figures. 11-13) and it is likely that all of the ventricular pathology in this case resulted from earlier electrical defibrillation.

Figure 11

Figure 11. Paddles used for DC cardioversion 10 months prior to necropsy

Figure 12 Figure 12. Parallel curved bands of fibrosis on the right ventricle had the same radius of curvature as the defibrillation paddles (Figure 13).

Figure 13

Another Springer Spaniel (Case 2) with atrial standstill had right ventricular wall thinning that was transparent in several areas bordered by pale, parchment like wrinkles in the right ventricle (Figure 14). None of the other dogs had pathologic changes in the ventricles. Additional information on histopathology and medical records is limited because stored paraffin blocks, slides and records prior to 1980 have been discarded due to space limitations.

Figure 14

Figure 14. Gross photographs of the cranial, oblique and caudal aspect of the heart in case 2 showing multiple areas of transparency and pale areas throughout the free wall of the right ventricle.

We did not observe coexistent scapulohumeral muscular dystrophy as was reported by other investigators around that time period. Chronic active myocarditis in a 10 month old Springer Spaniel with atrial standstill was reported by Jeraj et al (Am Heart Jr, vol 99;185-192,1980). Similar myocardial pathology was found in a recent case of atrial standstill in a 2 year-old dog that fibrillated following anesthesia induction for pacemaker implantation and could not be resuscitated (case reported by Kevin Christiansen).

Marked ascites was found in the two mongrel dogs with atrial standstill (Figures 15-18). The atria in both dogs had translucent, paper thin walls.

Figure 15

Figure 15. Two year-old mixbeed dog with ascites and atrial standstill.

Figure 16A Figure 16A. Transparency of the right atrium is demonstrated by a rectangular piece of black x-ray film inside the atrium and a hexagonal piece of film underneath the right atrial appendage.

Figure 16B Figure 16B. Transparency of the left atrium is demonstrated by black x-ray film inside the left atrium and appendage.

Figure 17

Figure 17. Eighteen month-old mixbeed dog with ascites and atrial standstill.

Figure 18A Transparency of the atria is demonstrated by a segment of 35 mm film inside the right atrium.

Figure 18B Transparency of the atria is demonstrated by a segment of 35 mm film inside the left atrium.