Holding your breath after breathing in causes the heart rate to slightly decrease as it stimulates the parasympathetic nervous system, explains Ricky Cheng for CurioCity. However without breathing in, holding your breath has very little effect on your heart rate.
The human heart is triggered by the parasympathetic nervous system. This system keeps a healthy human heart beating at a steady pace of 75 beats per minute on average. When you breathe in oxygen and then hold your breath, you stimulate the parasympathetic nervous system even more, causing the heart rate to decrease. This process is known as bradycardia, notes Cheng.
When you continue to hold your breath, organs expand with air and negative suction causes pressure on the thorax. This process slows down the blood flow getting to the heart. It takes longer for the heart to fill with blood, which in turn slows down the heart rate. People who are trying to hold their breath for extremely long periods of time, such as David Blaine's attempt at the world record, use this inhalation technique to slow their heart rate down, as a slower beating heart needs less oxygen, according to Cheng. This means you can hold your breath for longer.