This condition is characterised by feinting (syncope) and sudden death in Boxer dogs. It is unlike the traditional form of cardiomyopathy we see in large dog breeds (most of the time). Boxer cardiomyopathy is also known as "Boxer arrhythmogenic right ventricular cardiomyopathy".
Ventricular Premature Contractions (VPC's)
The problem is due to a run of extra heartbeats interrupting the normal flow of oxygen rich blood to the brain. No oxygen to the brain means a none functioning brain, hence the feinting( syncope).
These extra heartbeats originate in damaged heart muscle in the right ventricle (ventricular premature contractions -VPC's).
A run of 3-4 VPC's, one after the other, overtake the normal heartbeats. VPC's are ineffective contractions- i.e. they don't pump enough blood out of the heart to the brain and body. If they are very rapid, the ventricle does not have time to fill up to the top befoire it is "told" to contract. It ends up only 1/4 or less full when it contracts resulting in only 1/4 or less of the normal flow of oxygen rich blood out of the heart.
A run of several or more VPC's can cause sudden death which is a common consequence of the condition.
By listening to the heart for several minutes with a stethoscope, a vet may detect a skip in the normal heart rhythm although this is not a sure fire way of detecting a problem as affected dogs can go for several hours without as much as one skipped VPC.
Similarly, connecting a dog to an ECG machine to record the heartbeat for a few minutes may miss a run of VPC's. If a vet sees the occasional VPC in a symptomatic dog, its pretty close to a diagnosis.
Listening to the heart or recording an ECG while the dog has a syncopal episode would be ideal way to diagnose the problem, but these events tend to happen away for vet surgeries and owners can be too stressed to think about recording the heartbeat while their unconscious dog lies at their feet. By the time they do have a feel of the heartbeat or pulse, the rhythm may be back to normal as it may have just been a run of several VPC's taking up less than 1-2 seconds of time.
Halter ECG Recording
Specialist cardiologists can loan out 24hr ECG recording machines (Halters) which attach to the trunk of the body in special leather harnesses. A computer runs through the 24hrs of recording in seconds and prints out a report highlighting all the abnormal heartbeats.
Quite often it is astounding to see how many runs of VPC's occur while the dog is asleep. A dog that "passes away in its sleep" is a real possibility here.
The typical boxer cardiomyopathy has normal heart and lungs on chest xrays
Again, most boxer cardiomyopathy cases have normal ultrasound heart measurements.
We still see the occasional classical dilated cardiomyopathy in boxers (see images below).
The drug of choice for suppressing runs of VPC's is Sotalol. It is usually given twice a day.