The importance of understanding the anatomy and physiology of the heart and lungs will go a long way in helping you ascertain if there is a medical emergency at hand.
What you might observe
Signs of a heart problem might include a loss of condition, shortness of breath, slow recovery after exercise, an increased effort to breathe, a shorter period of exercise before development of fatigue, and general weakness. Fluid accumulation in the abdomen, legs, or under the skin surrounding the ribs is another indication of poor heart function.(1)
Some condition a horse owner may see are cardiac dysrythmias,
Action: Call your Vet immediately