What is Heart Block or Conduction Delay (Left Anterior)?
Contractions in the heart are controlled by electrical impulses. A network of nodes and bundles of specialized muscle tissue coordinate the contraction timing among the four different chambers to maximize the efficiency of the heart. The atria at the top of the heart contract first, sending an electrical impulse to the atrioventricular (AV) node. From the AV node, the impulse travels down through the bundle of His and divides into the right and left bundle branches. Proper functioning in the bundle branches ensures the simultaneous contraction of both ventricles. The left bundle branch, located on the interventricular septum, is the more important of the two since it regulates the larger more powerful left ventricle which pumps oxygenated blood throughout the body. After passing through the left bundle branch, the signal is divided further into the left anterior and left posterior fascicles which control a network of Purkinje fibers in the left ventricle. The division is more pronounced in humans, where a block in the left anterior fascicle causes a marked left shift on an electrocardiogram. It is also much more common in cats. Left anterior fascicular block (LAFB) does also occur in dogs, although it is quite rare. The left ventricle will begin to contract normally, but will then become uneven as the signal is delayed in the anterior fascicle. This will be evident on an electrocardiogram as a left shift of less than 40 degrees with tall R waves and deep S waves. Since it is so rare, veterinarians are not entirely sure of the clinical significance of LAFB in dogs but it can be a sign of heart disease especially when combined with another condition like a right bundle branch block. It could also be related to hyperkalemia, an electrolyte imbalance that occurs with kidney failure or with Addison’s disease.
Electrical impulses in the heart pass through a number of specialized areas that coordinate contraction throughout the different heart chambers. A conduction delay in the left anterior fascicle means that part of the left ventricle will not contract normally. This is called a left anterior fascicular block.
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Symptoms of Heart Block or Conduction Delay (Left Anterior) in Dogs
In many cases, LAFB is only evident on an electrocardiogram. If there is an underlying disease associated with the condition these are some of the symptoms you might notice in your dog.
- Exercise intolerance
- Fainting (syncope)
- Increased breathing rate (tachypnea)
- Increased drinking and urination (hyperkalemia from kidney disease or Addison’s disease)
There are many different types of conduction delay of which LAFB is one of the least significant since it occurs so far down the transmission chain. These are some of the results your veterinarian might discuss after an electrocardiogram.
AV Block (Complete Block)
- The impulse will be stopped or delayed in the AV node before it is transmitted to either ventricle, so this is the most serious life-threatening type of block
- LAFB combined with a right bundle branch block can eventually lead to a complete block
Left Bundle Block
- The entire left ventricle is delayed
Left Anterior Fascicular Block
- The signal is blocked in one part of the left ventricle
Right Bundle Block
The right ventricle is delayed
Causes of Heart Block or Conduction Delay (Left Anterior) in Dogs
These are some of the conditions that could be associated with LAFB in dogs.
- Left ventricular hypertrophy
- Hyperkalemia – could be related to renal failure or Addison’s disease
- Ischemia – reduced blood supply to the heart
- Post-cardiac surgery
Diagnosis of Heart Block or Conduction Delay (Left Anterior) in Dogs
LAFB will be diagnosed with an electrocardiogram that measures and records the rhythms and timing between each of the different heart chambers. Your veterinarian may notice the abnormality on an electrocardiogram performed for another purpose, such as a surgical evaluation. If your dog is showing symptoms of heart disease or other serious illness an electrocardiogram will likely be one of the tests ordered by the veterinarian. LAFB is a symptom rather than a definitive diagnosis so further tests will likely be ordered, including x rays and an echocardiogram to evaluate heart conditions, as well as blood and urine tests which can check for kidney failure or hyperkalemia.
Treatment of Heart Block or Conduction Delay (Left Anterior) in Dogs
If LAFB is asymptomatic and there is no underlying disease noted, no treatment is necessary since a delay in the left anterior fascicle does not usually disrupt heart function. Other treatments will focus on the underlying cause of LAFB. If your dog is very ill, this could include immediate stabilization for heart failure or Addison’s crisis. Long term medication might be prescribed to rectify an endocrine imbalance or minimize the symptoms of congestive heart failure. If the condition is due to an operable tumor or congenital heart defect, surgery could be recommended.
Recovery of Heart Block or Conduction Delay (Left Anterior) in Dogs
LAFB alone may be an inconsequential finding on an electrocardiogram which will not shorten or affect the length of your dog’s life. It can also be a sign of a serious heart or kidney problem. Individual prognosis for this condition can vary widely, so your dog’s chances will depend on the diagnosis of a veterinarian.