What it is
The tricuspid valve has three thin flaps that close each time the right side of the heart squeezes, stopping blood from slipping back into the upper chamber. In this defect the flaps, or the cords that hold them, are malformed: too thick, too short or stuck to the heart wall. The valve leaks, the right atrium has to take in extra blood with every beat, and it gradually stretches. In some dogs the valve is also narrowed, making it harder for blood to get through.
It is present from birth and can be inherited. When the right side of the heart can no longer keep up, fluid tends to gather in the belly rather than the lungs, and some dogs also develop an irregular heartbeat.
Your main job is to notice gradual change: how far your dog walks happily, how quickly it tires, whether its belly looks rounder. Let it set its own pace, and avoid hard exertion in the heat.
Signs you may notice
A vet may hear a heart murmur at a puppy check while the dog itself seems perfectly healthy. Later the dog may tire more quickly, breathe with effort, seem weak or develop a swollen belly from fluid build-up.
When to see a vet
Book a visit if your dog tires sooner, its belly looks swollen or it seems less keen on walks. Fainting, collapse or a struggle to breathe mean a vet is needed immediately.
This is general information, not veterinary advice. For your own dog's health, tests and treatment, see a vet.