Transient respiratory augmentation elicited by acute head-down tilt in the anesthetized cat

Acute head-downtilt (AHDT, 30°) in humans induces a transient ventilatoryaugmentation for 1-2 min accompanied by a high venous return.However, the mechanisms underlying this respiratory response remainobscure because of limitations of experiments carried out in humansubjects. The present study was undertaken to determine whetherAHDT-induced respiratory augmentation exists in the anesthetized,paralyzed, and ventilated cat and, if so, whether this response dependson 1) the cerebellum,2) the carotid sinus (CS)and/or vagal afferents, and3) elevation of central venousreturn. The integrated phrenic neurogram, arterial blood pressure,central venous pressure (CVP), and end-tidalPCO₂ were recorded before, during,and after AHDT. The results showed that AHDT produced a transient (~2min) enhancement of minute phrenic activity (~30%) primarily via anincrease in peak integrated phrenic neurogram amplitude associated witha remarkable elevation of CVP (~3 min). Cerebellectomy, CSdenervation, bilateral vagotomy, or clamping CVP did not affect thepresence of the AHDT-induced minute phrenic activity response. Thesefindings demonstrate that the anesthetized cat is a suitable model forinvestigating the mechanisms involved in AHDT-induced respiratoryaugmentation. Preliminary studies suggest that this response does notrequire the cerebellum, CS/vagal afferents, or an associated rise incentral venous return.