Slightly more detail:

Cocaine strongly stimulates a portion of your nervous system known as the sympathetic nervous system. It is the portion responsible for you “fight or flight” response. i.e. if someone is holding a gun to your head, your heart rate increases, your blood pressure increases, you start shunting blood away from your abdominal organs (digestion is not the priority) and toward skeletal muscle, your pupils dilate, etc…

Let’s focus just on increased heart rate. A fast rate both increases “demand” on the heart and decreases blood “supply” to the heart. Demand is fairly obvious: a heart going at 150 beats per minute requires more oxygen than a heart going 75 beats per minute.

Supply is a little more complicated. To understand this you have to know a little cardiac physiology:

#1. The heart is always in one of two phases: 1. pumping blood (what is called systole) or 2. filling with blood (diastole). The heart can only supply itself with blood (via the coronary arteries) during the relaxed phase. This is because the coronary arteries are located within the wall of the heart muscle. When the heart is contracting, the high pressure of the surrounding muscle almost completely squeezes the arteries shut, minimizing blood flow.

and #2. Systole is a set time period (approximately 0.4 seconds) and changes very little with heart rate, whereas, diastole (filling) varies greatly with heart rate. Thus, the faster your heart beats (the more time in systole) the less time it spends filling and supplying itself with blood.

So now your left with a overworked, under-supplied heart.

You may argue that exercise also increases your heart rate and should also be associated with this dangerous lake of blood/oxygen to the heart muscle, and to a certain degree your correct. People with a profound narrowing of their coronary arteries should avoid exercise until they see a cardiologist and possibly undergo a medical procedure to stent open or bypass the blocked coronary arteries. But for those of us with non-blocked heart vessels, we compensate for increased cardiac demand and decreased cardiac supply by dilating our coronary arteries and thus increasing blood flow/oxygen delivery proportionately. Cocaine, however, inhibits this response. As part of it’s over-stimulation of the sympathetic nervous system, cocaine causes a profound constriction of the vascular system that is refractory to our normal compensatory mechanisms.

Think of doing cocaine as running a marathon without increasing blood flow to your heart (CHEST PAIN). The resulting lack of oxygen to the heart’s electrical conduction system and muscle leads to very fast, uncoordinated rhythms. Once your heart starts beating out of control (a rhythm known as ventricular fibrillation), it no longer fills with blood and doesn’t supply itself/the rest of your body with oxygen (LOSS OF CONSCIOUSENESS). Finally, your heart, completely out of oxygen, can no longer contract (DEATH).