HUMAN AUTONOMIC RHYTHMS - VAGAL CARDIAC MECHANISMS IN TETRAPLEGIC SUBJECTS

1. We studied eight young men (age range: 20-37 years) with chronic, c linically complete high cervical spinal cord injuries and ten age-matc hed healthy men to determine how interruption of connections between t he central nervous system and spinal sympathetic motoneurones affects autonomic cardiovascular control. 2. Baseline diastolic pressures and R-R intervals (heart periods) were similar in the two groups. Slopes o f R-R interval responses to brief neck pressure changes were significa ntly lower in tetraplegic than in healthy subjects, but slopes of R-R interval responses to steady-state arterial pressure reductions and in creases were comparable. Plasma noradrenaline levels did not change si gnificantly during steady-state arterial pressure reductions in tetrap legic patients, but rose sharply in healthy subjects. The range of art erial pressure and R-R interval responses to vasoactive drugs (nitropr usside and phenylephrine) was significantly greater in tetraplegic tha n healthy subjects. 3. Resting R-R interval spectral power at respirat ory and low frequencies R-R as similar in the two groups. During infus ions of vasoactive drugs, low-frequency R-R interval spectral power wa s directly proportional to arterial pressure in tetraplegic patients, but was unrelated to arterial pressure in healthy subjects. Vagolytic doses of atropine nearly abolished both low- and respiratory-frequency R-R interval spectral power in both groups. 4. Our conclusions are as follows. First, since tetraplegic patients have significant levels of low-frequency arterial pressure and R-R interval spectral power, huma n Mayer arterial pressure waves may result from mechanisms that do not involve stimulation of spinal sympathetic motoneurones by brainstem n eurones. Second, since in tetraplegic patients, low-frequency R-R inte rval spectral power is proportional to arterial pressure, it is likely to be mediated by a baroreflex mechanism. Third, since low-frequency R-R interval rhythms were nearly abolished by atropine in both tetrapl egic and healthy subjects, these rhythms reflect in an important way r hythmic firing of vagal cardiac motoneurones.