Cardiovascular and haemodynamic responses to tilting and to standing in tetraplegic patients: a review.

This paper has reviewed the acute and long-term responses to changes in vertical posture in normal and tetraplegic subjects. It has discussed physiological mechanisms causing orthostatic hypotension in acute cervical spinal cord injured patients, and subsequent factors contributing to its amelioration over time. The long-term adaptive mechanisms are still controversial, probably involving multiple neurological, endocrine, renal, cardiovascular and haemodynamic factors. These factors include inhibition of vagal tone, plasma catecholamine levels, sensitivity of vascular beds to catecholamines, stretch reflexes in blood vessels, spinal BP reflexes, renin-angiotensin system, aldosterone and plasma volume changes. Individual differences may also interact with these various mechanisms, further complicating the issues. Although the fact that most tetraplegics do improve their orthostatic tolerance over time with repeated tilting is manifest, the precise mechanisms allowing this improvement are not. Research is needed to clarify these adaptive mechanisms, as well as to investigate the physiological effects of long-term therapeutic standing in devices such as standing frames.