The influence of CSF calcium and magnesium on the ventilatory response to carbon dioxide during hyperoxia

Respiratory responses to inhaled carbon dioxide were measured in anaesthetized cats during perfusion of the ventriculocisternal system with artificial cerebrospinal fluid. A study was performed to evaluate the effect of changes in the magnesium and/or calcium concentration of the CSF on the CO₂ response curve which was described as $\dot V_E = S{\text{ (}}P_{{\text{CSF,CO}}_{\text{2}} } - B)$ . A decrease ofS was observed when the magnesium concentration of the perfusion fluid was increased; theB-value remaining the same. The reverse was true down to magnesium concentrations of 0.6 mmol · l⁻¹. Below this concentrationS remained the same or decreased; theB-value was lowered. When both the calcium and magnesium concentrations of the CSF were changed, the relation betweenS and these concentrations could be described as to be proportional toC _Ca ^a ·C _Mg ^b . The effect of changes in the calcium concentration was much more pronounced than comparable changes of the magnesium concentration as reflected by the magnitude of the exponentsa andb which were found to be −2.80 (S.D. 0.11) and −0.60 (S.D. 0.03) respectively.