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

Department of Physiology, University of Kentucky, Lexington, Kentucky 40536 Acute head-down tilt (AHDT, 30°) in humans induces a transient ventilatory augmentation for 1-2 min accompanied by a high venous return. However, the mechanisms underlying this respiratory response remain obscure because of limitations of experiments carried out in human subjects. The present study was undertaken to determine whether AHDT-induced respiratory augmentation exists in the anesthetized, paralyzed, and ventilated cat and, if so, whether this response depends on 1 ) the cerebellum, 2 ) the carotid sinus (CS) and/or vagal afferents, and 3 ) elevation of central venous return. The integrated phrenic neurogram, arterial blood pressure, central venous pressure (CVP), and end-tidal P CO 2 were recorded before, during, and after AHDT. The results showed that AHDT produced a transient (~2 min) enhancement of minute phrenic activity (~30%) primarily via an increase in peak integrated phrenic neurogram amplitude associated with a remarkable elevation of CVP (~3 min). Cerebellectomy, CS denervation, bilateral vagotomy, or clamping CVP did not affect the presence of the AHDT-induced minute phrenic activity response. These findings demonstrate that the anesthetized cat is a suitable model for investigating the mechanisms involved in AHDT-induced respiratory augmentation. Preliminary studies suggest that this response does not require the cerebellum, CS/vagal afferents, or an associated rise in central venous return. cerebellectomy; vagotomy; carotid body denervation; central venous pressure; phrenic efferent activity; vestibular system