Can you stop your heart by breath holding?
Heart is not under voluntary control. So you cant stop it voluntarily. If it was possible, suicide would have been very easy!
When you hold your breath and strain, heart beats slow down transiently. In some individuals this can cause a transient blackout. The heart beats usually spontaneously resume soon after that and the individual recovers consciousness.
When the heart slows during certain procedures like coronary angiography, asking the person to cough repeatedly can increase the heart rate. This is possibly by squeezing out the dye used for coronary angiography from the blood vessels of the heart by increase in the pressure within the chest cavity. The increase in the pressure in the chest cavity is also transmitted to the aorta, the great vessel arising from the heart and supplying oxygenated blood to the whole body.