Inertance of the respiratory system in ponies

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Abstract
The purpose of the present work was to measure the pulmonary inertance (IL) in ponies and to analyze its potential influence on the mechanics of breathing and on their aptitude to increase ventilation during exercise. Five healthy ponies 2.4-4 yr old [mean wt 255 .+-. 15 (SE) kg] were used. On the one hand, inertance of the respiratory system (Irs) was computed from the value of the resonant frequency (fr) measured by the forced oscillation technique. On the other hand, respiratory airflow, tidal volume (VT), and transpulmonary pressure (PL) changes were recorded while the ponies were performing a light treadmill exercise, and IL was calculated as the ratio of the associated differences in inertial pressure (.DELTA.Pin) to volume acceleration (V). Respiratory airflow and VT were measured with a Fleisch pneumotachograph (no. 5) and PL with an intraesophageal balloon catheter. First, the protocol was carried out with the ponies breathing air and He-O2, second, while the ponies breathed through two addition tubes (100 cm long, 3 cm ID), then one, and finally none, fixed on the Fleisch pneumotachograph. Finally, the contribution of the extra- vs. the intrathoracic airways to IL was estimated by measuring the lateral midtracheal pressure recorded simultaneously with the aforementioned parameters. The values of Irs calculated with fr andof IL calculated on the basis of the .DELTA.Pin-V ratio were 29.8 .+-. 0.4 and 19.8 .+-. 1.0 .cntdot. 10-4 kPa .cntdot. l-1 .cntdot. s-2, respectively. During He-O2 breathing IL decreased about three times; this result was similar to the predicted decrease based on gas density only. When one tube was added, IL increased by 17.9 .+-. 1.8 .cntdot. 10-4 kPa < l-1 .cntdot. s-2 and, when two tubes were added, by 20.6 .+-. 1.2 .cntdot. 10-4 kPa .cntdot. l-1 .cntdot. s2, which corresponded to the mass of air in the tubes. The value of IL measured in the last protocol was 19.0 .+-. 1.5 .cntdot. 10-4 kPa .cntdot. l-1 .cntdot. s2, the extrathoracic airways inertance amounts to .apprx. 45% of IL. Inertance of the trachea in ponies was estimated to be about two-thirds of the total IL. The collected data show that IL is greater in ponies than in humans and that the trachea contributes greatly to the difference between both species. The value of IL in the equine species, associated with specific characteristics in the respiratory adjustment to exercise (i.e., large VT and high respiratory frequency) implies that the pressure required to produce .DELTA.+V in horses during strenuous exercise may be a limiting or at least a constraining factor in the further increase of ventilation. Furthermore, the fact that fr is close to the observed respiratory frequency of trotting ponies suggests that in some special conditions of exercise that can adopt a respiratory strategy that minimizes the work of breathing.