Immobility can lead to pressure sores,  particularly in bony areas, requiring precautions such as extra cushioning and turning in bed every two hours (in the acute setting) to relieve pressure. In the long term, people in wheelchairs must shift periodically to relieve pressure. Another complication is pain, including nociceptive pain (indication of potential or actual tissue damage) and neuropathic pain, when  nerves affected by damage convey erroneous pain signals in the absence of noxious stimuli. Spasticity, the uncontrollable tensing of muscles below the level of injury, occurs in 65–78% of chronic SCI. It results from lack of input from the brain that quells muscle responses to stretch reflexes. It can be treated with drugs and physical therapy. Spasticity increases the risk of contractures (shortening of muscles, tendons, or ligaments that result from lack of use of a limb); this problem can be prevented by moving the limb through its full range of motion multiple times a day. Another problem lack of mobility can cause is loss of bone density and changes in bone structure. Loss of bone density (bone demineralization), thought to be due to lack of input from weakened or paralysed muscles, can increase the risk of fractures. Conversely, a poorly understood phenomenon is the overgrowth of bone tissue in soft tissue areas,  called heterotopic ossification. It occurs below the level of injury, possibly as a result of inflammation, and happens to a clinically significant extent in 27% of people.
The constant interval between the sinus beat and PVC suggests a reentrant etiology rather than spontaneous automaticity of the ventricle.Premature atrial contractions by contrast do not have a compensatory pause, since they reset the sinus node, but atrial or supraventricular bigeminy can occur. If the PACs are very premature, the wavefront can encounter a refractory AV node and not be conducted. This can be mistaken for sinus bradycardia if the PAC is buried in the T wave since the PAC will reset the SA node and lead to a long P–P interval. Diagnosis
Rule of bigeminy
When the atrial rhythm is irregular (as in atrial fibrillation or sinus arrythmia) the presence of bigeminy depends on the length of the P–P interval and happens more frequently with a longer interval. As with post PVC pauses, a longer P–P interval leads to a higher chance of re-entrant circuits and thus PVCs. The term "rule of bigeminy" is used to refer to the dependence of bigeminy on the ventricular cycle length in irregular rhythms. Classification
There can be similar patterns depending on the frequency of abnormal beats. If every other beat is abnormal, it is described as bigeminal. If every third beat is aberrant, it is trigeminal; every fourth would be quadrigeminal. Typically, if every fifth or more beat is abnormal, the aberrant beat would be termed occasional.Bigeminy is contrasted with couplets, which are paired abnormal beats.