Can dehydration cause cardiac arrest?

Not directly and: Not unless it is really severe. If you have severe dehydration, you will not have enough blood to circulate, so your heart can stop. Also, your blood might get too thick to pass through your coronary arteries, which in turn can cause cardiac arrest. But the dehydration would need to be severe, enough to cause blood pressure so low that it can cause shock.

Dehydration: Severe dehydration and increase the viscosity of the blood (make it more sluggish). The average normal person will tolerate this. If there is underlying limitation of blood flow to the heart (blockage in the arteries), problems could arise.