Changes in breathing when switching from nares to tracheostomy breathing in awake ponies

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Abstract
We assessed the consequences of respiratory unloading associated with tracheostomy breathing (TBr). Three normal and three carotid body-denervated (CBD) ponies were prepared with chronic tracheostomies that at rest reduced physiological dead space (VD) from 483 .+-. 60 to 255 .+-. 30 ml and lung resistance from 1.5 .+-. 0.14 to 0.5 .+-. 0.07 cmH2O .cntdot. l-1 .cntdot. s. At rest and during steady-state mild-to-heavy exercise arterial PCO2 (PaCO2) was .apprx. 1 Torr higher during nares breathing (NBr) than during TBr. Pulmonary ventilation and tidal volume (VT) were greater and alveolar ventilation was less during NBr than TBr. Breathing frequency (f) did not differ between NBr and TBr at rest, but f during exercise was greater during TBr than during NBr. These responses did not differ between normal and CBD ponies. We also assessed the consequences of increasing external VD (300 ml) and resistance (R, 0.3 cmH2O .cntdot. l-1 .cntdot. s) by breathing through a tube. At rest and during mild exercise tube breathing caused PaCO2 to transiently increase 2-3 Torr, but 3-5 min later PaCO2 usually was within 1 Torr of control. Tube breathing did not cause f to change. When external R was increased 1 cmH2O .cntdot. 1-1 .cntdot. s by breathing through a conventional air collection system, f did not change at rest, but during exercise f was lower than during unencumbered breathing. These responses did not differ between normal, CBD, and hilar nerve-denervated ponies, and they did not differ when external VD or R were added at either the nares or tracheostomy. We conclude (1) small changes in internal or external VD are compensated primarily by changes in VT to minimize disruption of PaCO2, and (2) small changes in internal or external R result in changes in f and VT that probably minimize the work of breathing or the generation of force by respiratory muscles. These changes are not critically dependent on upper airway, hilar nerve-innervated, and carotid sensory mechanisms.

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