The role of spinal cord transmission in the ventilatory response to exercise in man

Abstract

The ventilatory response to electrically induced exercise was studied in thirteen patients with traumatic spinal cord transection at or about the level of T6. The steady-state and on-transient responses to this exercise were compared with those obtained in eighteen normal subjects (Adams, Garlick, Guz, Murphy & Semple, 1984). Exercise was produced by surface electrode stimulation of the quadriceps and hamstring muscles so as to produce a pushing movement at 1 HZ against a spring load. At rest there was no significant difference between normals and patients, except that the patients had a lower CO2 elimination (VCO2) and end-tidal PCO2 (PET,CO2) and a higher heart rate. On exercise the mean rise in VCO2 for the patients was 172 ml min-1 (S.D. 72), and for the normals was 287 ml min-1 (S.D. 143). The corresponding mean changes in ventilation (VI) were 4.4 l min-1 (S.D. 2.2) and 7.6 l min-1 (S.D. 3.2). However, the ventilatory equivalent for CO2 (delta VI/delta VCO2) in the steady state was not significantly different between patients (26.0, S.D. 5.9) and normals (28.5, S.D. 7.4). In the steady state there was a mean rise in PET,CO2 of 0.9 mmHg (S.D. 1.4) in the normals, and 3.2 mmHg (S.D. 2.7) in the patients, but there was overlap between the two groups. In many experimental runs in both groups, PET,CO2 did not rise, and sometimes fell. Where PCO2 did rise, the ventilatory response to exercise could not be accounted for on the basis of the ventilatory sensitivity to CO2 inhalation. From arterial sampling in three of the patients it was found that when PET,CO2 rose, the corresponding change in Pa,CO2 was less. During the on transient, there was a significant rise in both VCO2 and VI by the second breath in both groups. At the end of the on transient the normal subjects had achieved 84% (S.D. 40) of the steady-state increase in VCO2 and 88% (S.D. 24) of the increase in VI. The corresponding values for the patients were 67% (S.D. 17) and 77% (S.D. 16) respectively; these differences between normals and patients are significant. The increase of VI during the on transient in the patients was achieved almost entirely by an increase in tidal volume whereas in normals, an increase in respiratory rate was a more important component. We conclude therefore that in man, spinal cord transection with a presumed loss of muscle afferents allows a ventilatory response to electrically induced exercise that cannot be explained by classical chemoreception.(ABSTRACT TRUNCATED AT 400 WORDS)