Signalment: Tango is a 7-year-old male castrated Chesapeake Bay retriever.
History: Tango recently has become lethargic and has been coughing. This became noticeable in the last three days. The owner has also noticed some weight loss.
T: 98.9 P: 240 R: 48
Tango has lost some muscle mass giving him a body condition score of four out of nine. A gallop rhythm is heard as well as a 2/6 systolic murmur with point of maximal intensity over the mitral valve. The rhythm sounds very irregular. Some fluid is appreciated in the abdomen. Pulses are rapid and irregular. An ECG is obtained.
1. What is the rate and rhythm on this ECG tracing?
2. What are some proposed causes of this ECG change and what effects does it have on the patient’s cardiovascular status?
3. What treatment is recommended for this rhythm?
4. What other diagnostics are appropriate for Tango?
5. What is the prognostic significance of this rhythm?
1. The rate on this tracing is around 260 BPM, given the marked variation in the R-to-R interval it would be important to determine the heart rate over a prolonged period of time to get an accurate reading. Six seconds would be ideal.
The ECG shows a lack of P-waves. There is some inconsistent baseline undulation. The QRS complexes are somewhat prolonged in duration, however not markedly so. This ECG is typical for atrial fibrillation with rapid and highly irregular rhythm, relatively narrow QRS complexes, no visible P-waves.
2. This ECG change is usually associated with increased atrial size or mass. Damage to the atrium by pathologic stretch may also play a role. As a result, this rhythm is most commonly diagnosed in larger breed dogs and is relatively rare in small dogs and cats.
In almost all cases, this rhythm is associated with advanced heart disease such as is seen with dilated cardiomyopathy or valvular insufficiency that has led to marked atrial enlargement. In giant breed dogs, a benign form of atrial fibrillation can occur; the ECG is similar, however the rate is close to normal.
This rhythm will have significant negative effects on this patient. It is thought that many times the initiation of atrial fibrillation leads to acute decompensation in otherwise stable patients. Atrial fibrillation results in a markedly elevated heart rate, which results in poor ventricular filling and therefore decreases cardiac output.
The irregular R-to-R interval also contributes to the decrease in cardiac output. Many of the ventricular contractions result in very little blood movement from the heart into the circulation. The high heart rate also means that myocardial oxygen demand increases at the same time that oxygen delivery is impaired (coronary arteries are only perfused during diastole). With fibrillation of the atrium, the atrium no longer contracts. The atrium, however, plays a vital role in ensuring optimal ventricular filling (increased filling results in increased stroke volume, Frank-Starling mechanism). This effect becomes most pronounced at high heart rates. A loss of 20 percent or more of cardiac output results when atrial function is lost.
3. In humans, atrial fibrillation often occurs without underlying heart disease. It was controversial if rate control or return to a normal rhythm was most desirable in this scenario. Most studies show that neither approach is superior. In almost all cases of atrial fibrillation in dogs, severe cardiac disease is present and a return to a normal rhythm is highly unlikely. As such the goal of therapy in dogs is rate control; an in-house ECG rate of less than 160 BPM is ideal. It is important to remember that in patients with underlying heart disease, too dramatic of a drop in heart rate can lead to adverse effects because this can decrease cardiac output significantly.
Generally digoxin and diltiazem (0.5 to 1.5 mg/kg p.o. TID) are used to slow heart rate in these patients. Given Tango’s heart rate , using both digoxin and diltiazem concurrently would be most likely to achieve the desired decrease in heart rate.
An abstract presented at the European College of Veterinary Internal Medicine in Barcelona by researchers from Cornell dealt with the efficacy of various medications to control heart rate in six dogs with atrial fibrillation.1
The patients in this study received either digoxin (0.005 mg/kg BID) or digoxin and diltiazem (Dilacor XR 3 mg/kg BID). After seven days of therapy, a Holter was obtained at baseline. At this time, the patient also crossed over and obtained the other treatment for seven days. In addition to a Holter recording, a short ECG strip was also run at each visit. Heart rates were calculated from the ECG strip and from the Holter data. Initially heart rate was 173 ± 40 BPM. Digoxin alone decreased this value to 154 ± 38 BPM. The combination of diltiazem and digoxin was much more effective, bringing the rate down to 121 ± 29.
The heart rate on the ECG strips obtained in hospital was on average 15 percent to 25 percent higher than the Holter data. This study shows that rate control with diltiazem and digoxin is superior to digoxin alone. It also points out that the stress of being in clinic can significantly affect various parameters including heart rate.
There are alternate medications that can be considered as well. Beta-blockers would help to slow heart rate. However, in the face of heart failure they can be poorly tolerated. This often means a low dosage needs to be used initially and slowly titrated upward.
A drug commonly used in humans is amiodarone. This has been very promising, resulting in conversion to normal rhythm in some patients. Recently, a retrospective study was published looking at the use of this medication in dogs.2 The results are quite promising with some dogs reverting to normal rhythm even though severe underlying heart disease was present. The one draw back to amiodarone is that adverse side effects such as hepatopathies, cytopenias, thyroid dysfunction and others can occur and are not necessarily a rarity. Close monitoring of biochemical and CBC parameters is important if this medication is to be used longer term.
Treating underlying heart failure (diuretics plus ACE inhibitors, such as enalapril) will also help Tango. With heart failure, sympathetic tone is high. This makes the AV node less functional and more of the fibrillation waves are passed on to the ventricle. Treating the heart failure will decrease sympathetic tone thereby indirectly dropping heart rate with this rhythm.
4. Given the association of this rhythm with heart failure, imaging studies are clearly needed. Radiographs as well as an echocardiogram are indicated. Laboratory tests are essential as well, especially to evaluate renal function. This is very important if digoxin is to be given. Approximately one week after starting therapy, laboratory parameters should be rechecked and a digoxin blood concentration determined. Additionally, an ECG should be obtained to determine if the desired heart rate has been achieved.
5. Atrial fibrillation with heart failure implies a poor prognosis. This may be because the atrial fibrillation has a direct negative effect or it may be because atrial fibrillation is more likely to occur in patients with severe heart disease.
Researchers from the University of Guelph retrospectively studied 275 dogs.3 The dogs were subdivided into giant breed dogs (>40kg, n=44), large breed dogs (>25 kg and <40kg, n=37), spaniels (n=19) and Doberman pinschers (n=175).The mean survival time for all dogs was 284 days, the median was 59. The disparity between these two numbers usually is the result of outliers, in this case, animals that had much longer survival times than others. The mean and median survival times for the Doberman pinschers without AF, 95.71 and 55.5 respectively, was considerably shorter than the other groups. The presence of atrial fibrillation at presentation in Doberman pinschers decreased the mean to 56.32 and the median to merely nine days.
1. Kraus MS, et al. Assessment of antiarrhythmic drug efficacy to control heart rate in dogs with atrial fibrillation using 24-hour ambulatory electrocardiographic (Holter) recordings. JVIM 18; 779: 2004.
2. Saunders AB, Miller MW, et al. Oral amiodarone therapy in dogs with atrial fibrillation. JVIM 20; 921-926: 2006.
3. Bronsoiler J, O’Grady MR, et al. Dilated cardiomyopathy win dogs with congestive heart failure: prognostic factors and influence of atrial fibrillation on survival. JVIM 19:456;2005.