Re: Why does cardiac muscle not fatigue?

Although skeletal and cardiac muscle are similar in many ways, there are important physiological differences that make cardiac muscle much less prone to fatigue. As you point out in your question, the build-up of lactic acid plays a major role in muscle fatigue. Lactic acid is a by-product of an anaerobic form of muscle metabolism (one lacking oxygen).

Muscle (normally skeletal muscle) starts using this type of metabolism to produce energy (in the form of ATP) when the supply of oxygen via the blood stream is too low. This normally occurs when muscle is very active. Cardiac muscle has the advantage of being supplied with oxygen from the blood stream via the coronary artery. This artery is large, and is one of the first places oxygenated blood travels to after it leaves heart. As a result, the blood supply to cardiac muscle cells is very rich in oxygen, making unnecessary for the heart to switch to anaerobic metabolism to produce ATP.

Another important difference between cardiac and skeletal muscle that is important in preventing cardiac muscle fatigue is the relative number of mitochondria, an organelle important in energy production. In general, cardiac muscle cells have many more mitochondria than skeletal muscle cells. Because it is mitochondria that produce ATP while utilizing oxygen (aerobic metabolism), cardiac muscle cells are able to produce more ATP per cell, when oxygen is in good supply, than skeletal muscle cells can.

I hope these points have answered your excellent question. To be honest, it is not a question that I had personally thought about before you asked it, so I very much enjoyed composing this answer.