Ventilatory responses to intravenous and airway CO2 administration in anesthetized dogs

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Abstract
Ventilatory responses to steady-state venous CO2 loading (i.v. CO2) and CO2 inhalation were observed in chloralose-urethan-anesthetized dogs. I.v. CO2 was administered by increasing the CO2 fraction of gas ventilating a membrane gas exchanger in an arteriovenous bypass; blood flow rate was fixed at 300 ml/min. A time-dependent hyperventilation was identified in all 14 experimentally treated dogs and in 4 additional sham-treated dogs. In 8 animals tested with small progressive increments in i.v. CO2 loading rate, a response approaching isocapnia occurred during i.v. CO2 and a large hypocapnia occurred when control conditions returned. The use of a randomized protocol in 6 animals demonstrated the necessity of accounting for this systematic base-line shift, because before doing so the response depended more on the passage of time than on the nature of the CO2 load. After this analytical adjustment was made, there was no significant difference between the respiratory controller gains (.DELTA..ovrhdot.VE/.DELTA.PaCO2) for inhaled and i.v. CO2.