Carbon dioxide alters the Hoffmann reflex independent of hydrogen ions

The purpose of this study was to examine the effect of changes in capillary blood pH on the resting soleus Hoffmann (H) reflex in the intact human. H-max size, H-wave at 20% of H-max, M-max and H-reflex latency were recorded in 10 subjects (apparently healthy, ages 19-36) before and after exposure to 3 hours of NaHCO(3), CaCO(3), NH(4)Cl (all at 0.3 g/kg) or 10 minutes 7% Carbon dioxide (CO(2)) administration. NaHCO(3) increased capillary blood pH, CaCO(3) did not change capillary blood pH, and NH(4)Cl and 7% CO(2) decreased capillary blood pH. H-max and H-wave at 20% of M-max size were significantly decreased with no change in M-max, and H-reflex latency significantly increased during 7% CO(2) administration only. No other changes in H-maximum size or H-reflex latency in response to dry chemical administration were observed. Seven percent CO(2) administration reduces the size and increases the latency of the H-maximum size as previously found, but other chemicals which alter capillary blood pH do not. CO(2) modulates afferent nerve function, and does so, it appears, independent of changes in capillary blood pH.