Respiration and medullary blood flow during sinusoidal hypoxia in the peripherally chemodenervated cat

The hypothesis that hypoxic respiratory depression is mediated by changes in medullary blood flow (MBF) was assessed in 18 anesthetized, paralyzed, vagotomized, peripherally chemodenervated, ventilated cats exposed to sinusoidal hypoxic hypoxia. In nine cats, the dynamic response of the central respiratory controller to hypoxia was studied by varying the cycle time of sinusoidal hypoxia (cycle time = 2.5, 4, 6, 10, and 15 min). Peak phrenic neurogram amplitude (PNA) followed sinusoidal oscillations in the hypoxic input [arterial O₂ saturation (S(a)O₂)] at all cycle times. The relationship between PNA and S(a)O₂ was expressed as the transfer function of the system and was approximated as a first-order differential equation with a time constant of 78 ± 1 s, a value consistent with a previous measurement of the time constant of the change in respiration following a change in brain blood flow. In a separate study, MBF was continuously measured during sinusoidal hypoxia (cycle time = 6 min; n = 9) with a laser- Doppler flow probe to directly assess the role of MBF in production of hypoxic respiratory depression. PNA and MBF followed S(a)O₂ oscillations during sinusoidal hypoxia. Infusion of sodium nitroprusside (20 μg · kg^{- 1} · min^-1 iv) increased MBF by 3040% and abolished MBF oscillations during subsequent sinusoidal hypoxia but had no effect on PNA oscillations. We conclude that the increase in brain blood flow seen during sinusoidal hypoxia is not the primary cause of the accompanying central hypoxic respiratory depression.