How is someone's heart rate affected by a spinal cord injury?

Spinal Cord Injury: When the spinal cord is injured, the sympathetic and parasympathetic nervous system can also be damaged. This segment of the nervous system is assigned to control heart rate and pressure. There is also no neural feedback or control from the rest of the circulatory system resulting in wide swings, especially drops, in blood pressure.

Lots of changes: Lots of changes occur in the human body as a result of spinal cord injury especially as you get higher in the spinal cord in terms of auto regulation of temperature and blood pressure which can affect heart rate as can the spinal cord injury itself. Again, this depends on the completeness of the cord injury and its level in the spinal column.