Tension. A tension pneumo causes pressure build up with air and squeezes your heart until it can't beat anymore.
Blood flow. In general, your heart beats until it fails to get adequate oxygen/blood flow. The pressure from a "tension pneumothorax" compresses/prevents blood from entering the right side of the heart. Decreased blood return to the heart results in decreased output (i.e. How much blood it has to pump out). The heart gets its oxygen/circulation from the left side output.
If there is tension. If the pneumothorax creates a lot of pressure/air buildup in the chest, it can compress the veins draining into the heart and cause the heart to stop beating. This is called Tension Pneumothorax, and it is a medical emergency. If not promptly treated, it can cause death.
Blood flow. All blood from the right side of the heart go to the lungs, oxygenate then go to the left side of the heart to be pumped to the rest of the body. In pneumothorax, the collapsed lung has high pressure due to collapsed arteries and put too much strain on the right side of the heart to overcome, less oxygenation, and the heart may not be able to do it and it stops pumping.
Closed space. The chest cavity is a closed space so if there is a puncture in the lung, air escapes through the lining of the lung into the surrounding chest cavity. Since it can't escape, the pressure increases pushing everything to the other side of the chest, including the heart. Decompression with a needle or tube is then necessary to relieve the pressure and allow the heart to shift back into place.
Pressure. The pressure from a "tension pneumothorax" pushes heart towards the opposite side of the chest. This tension/compression can be sufficient to prevent adequate blood from entering the right side of the heart. This can result in rapidly fatal consequences unless decompression is achieved.
Balloon effect. Pneumo=air; thorax=chest This is a situation where air enters between the internal chest wall & the lungs creating a balloon like cavity under pressure. Depending on the amount of air & pressure, it can push the lungs flat & move the heart over. Take out the extra air and pressure & it returns to normal.