Relationship between heart rate and oxygen uptake in thoracic level paraplegics

In able-bodied subjects heart rate and oxygen uptake have a linear relation up to submaximal workloads. Cardiac response to exercise or physical stress is described to be under the control of the sympathetic nervous system. Various workers have claimed that the sympathetic contribution to the cardiac plexus arises between thoracic spinal cord levels T1 and T6. Paraplegics are participating in various sporting activities in increasing numbers. Theoretically, assessing the progress of physical fitness of paraplegics with lesions above T6 by monitoring heart rate alone becomes unreliable because of damage to their sympathetic system. Forty-four paraplegics with lesion levels between T3 and T10 were put through an arm cranking exercise routine with increasing power levels. Their heart rate and oxygen uptake was measured for each power level. For analytical purposes the subjects were grouped into two groups according to their lesion level. All the subjects with the lesion at T6 or above were in one group and the other group consisted of lesion at T7 or below. Almost linear relation was found between the heart rate and oxygen uptake in all subjects of both groups. The findings of this study suggest that either the sympathetic contribution comes from above spinal level T3, or the cardiac response to an increased demand in physical exercise is controlled by some other mechanism.