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IRREGULAR HEARTBEATS

Irregular heartbeats (arrhythmias)

Many heart cases have irregular heart rhythms (arrhythmias) which vary in frequency. An individual extra heartbeat is called an ectopic beat. If they are over 20 per minute, or the dog is showing symptoms, we start medication to try and prevent them.

Ectopic beats are called:

• Atrial Premature Contractions (APC's)- if they originate in the atria
  
• Ventricular  Premature Contractions (VPC's)- if they originate in the ventricles

Ectopic heartbeats result in reduced pumping of blood out of the ventricles.

One can hear and feel the ectopic heartbeat, but there is no pulse associated with it.

The heart is being "ordered" to contract during its rest period (diastole) where it normally fills up with blood. The ventricles may only be 1/3 or 1/4 full when they suddenly are "told" to contract. As a result, not much blood gets pumped out of the heart and there is not enough blood to create a pulse in arteries. It's a "wasted" contraction and means extra work for an already failing heart.

If you count the heartbeats for one minute and compare it to the pulse rate for a minute, there will be a difference in the number of beats. This is called a pulse deficit and is something owners can do at home to monitor their dog's illness.

For example, the heartbeat is 220 per minute. but the pulse rate is 180. This means there are approx. 40 ectopic heartbeats each minute.

APC's

If the left atria is enlarged due to a leaking mitral valve, a little pacemaker in the walls of the atria gets damaged- the atrio-ventricular node (AVN). It normally controls and co-ordinates heart contractions by sending out tiny electrical impulses at regular intervals. One electrical impulse equals one heartbeat.

When the AVN is damaged, the regularity of the impulses is lost, and irregular heartbeats (ectopics) result. These ectopic beats are called Atrial Premature Contractions (APC's)

APC's are very common in dogs with mitral valve disease.

If the left atria is damaged further, the APC's become more frequent and may eventually take over the entire "running" of the heart. These cases have very rapid heart rates over 200 beats per minute, which are very irregular in timing. This is called Atrial Fibrillation.

Atrial fibrillation (an arrhythmia)

Many heart cases go into atrial fibrillation when the left atrium starts contracting very fast and irregularly (over 200 beat a min).

This in itself is not a problem, but it does cause the bigger chambers below (the ventricles) to follow suite. When they start contracting at these speeds, the following can happen:

1. The ventricles are being told to contract when only 1/4 or 1/2 full- meaning less blood being ejected from the heart with each beat i.e. poor circulation. They are not given enough time to relax and fill up completely before being told to contract again.
   
2. The ventricles are told to work harder, and if they are already diseased, this increases their oxygen demand. It's like an asthma sufferer being told to run a marathon.
   
   
3. The heart's own blood supply (via the coronary arteries) is reduced from the above complications. This means even less oxygen to the heart muscles and a worsening of the condition.
   
4. Because the heart is not pumping blood efficiently to and from the lungs, the blood pressure builds up in the lungs. this eventually leads to fluid forming in the lungs (pulmonary oedema).

Many case are able to cope with a dilated heart not contracting too well until they go into atrial fibrillation. This can happen very suddenly and is life threatening.

Treatment of Atrial Fibrillation

Drugs are given to slow down the heart rate, giving the heart time to relax in between contractions (diastole), and fill-up during these intervals before being "ordered" to pump again. In other words, the ventricles fill up better and when told to contract, they are nearly full, so more blood is pumped out of the heart.

Common drugs include Lanoxin and if not fully effective, it is combined with either a beta-blocker e.g. Inderal or a calcium channel blocker e.g. Diltiazem.

VPC's

VPC's are sometimes seen in dogs with valve disease, but are more common in cases of dilated cardiomyopathy.

VPC's originate from damaged areas in the ventricle walls. This damage is usually due to reduced oxygen-rich blood supply to the very active cardiac muscles.

Approx. 20% of the total oxygen-rich blood pumped out of the heart goes straight into the heart muscles themselves. In other words, it looks after itself first. This blood is transported to the heart muscles by the coronary arteries.

If the left ventricle is not pumping blood out as efficiently as it should be, it means less oxygen-rich blood for the heart muscles. It's like a mini- heart attack in humans with blocked coronary arteries.

Areas of the cardiac muscle die, and start to act as pacemakers, sending off extra electrical impulses making the adjacent healthy ventricle wall contract ( a VPC).

VPC's can occur singly, in groups or as runs (ventricular tachycardia).

If they occur at over 30 per minute, we give drugs to try and prevent them. Obviously, if we can improve the flow of oxygen-rich blood from the heart in the first place, VPC's are less likely to develop.

Treatment of VPC's

Boxers, as a rule, are placed on either Sotalol or Mexiletine to prevent sudden death/feinting during prolonged runs of extra beats.