Heartbeat will typically slow and one's blood pressure increases when the body is held upside down for more than a few minutes. Another concern is retinal pressure increase - the pressure in your eyeballs increases significantly when your head is down. Knowing this, those with high blood pressure, heart conditions or glaucoma should avoid this type of back pain remedy.
It appears that studies evaluating inversion therapy and spinal traction find technique ineffective for long-term relief. It may be helpful, however for temporary relief of back pain.