RELATIONSHIP BETWEEN SEVERITY OF SPINAL-CORD INJURY AND ABNORMALITIESIN NEUROGENIC CARDIOVASCULAR CONTROL IN CONSCIOUS RATS

Abnormal sympathetic tone after spinal cord injury (SCI) initially res ults in hypotension and is subsequently associated with autonomic dysr eflexia characterized by paroxysmal hypertension and bradycardia in re sponse to noxious or visceral stimuli. To evaluate the effect of a cli nically relevant compression model of SCI on cardiovascular control in the early postinjury period, we monitored arterial pressure (AP) and heart rate under control resting conditions and after visceral stimula tion (colon distension) in conscious rats for 1 week after clip compre ssion injury of the cord at T5. Rats were randomly allocated into 4 gr oups (n = 8 each): sham-operated, 20, 35, and 50 g injuries. Only the 50 g injury was associated with significant hypotension (73 +/- 4 mmHg ) at 1 day post-SCI when compared to sham-injured rats (91 +/- 3 mmHg) . In control rats, colon distention caused a transient presser respons e of 16 +/- 3 mmHg and tachycardia. In rats with 20 g 35 g, and 50 g i njuries, colon distension 1 day after SCI increased AP by 8 +/- 2, 15 +/- 3, and 21 +/- 1 mmHg, respectively. The hypertensive response corr elated with injury severity (r = 0.75; p < 0.0001) and was associated with bradycardia. By 7 days after SCI, only rats with 50 g cord injuri es experienced hypertension with reflex bradycardia with visceral stim ulation. These data show that dysfunctional cardiovascular control aft er SCI is correlated with the severity of injury. Mild and moderate co mpressive SCI result in transient cardiovascular abnormalities which n ormalize by 1 week. In contrast, more severe injuries are associated w ith neurogenic hypotension and autonomic dysreflexia.