Ventilatory Mechanics in Healthy Calves During Helium-Oxygen Breathing

The mechanics of breathing was investigated in six healthy calves breathing either air or a mixture of 79% helium and 21% oxygen (He-O2). Intrapleural pressure (Ppl) was measured with an esophageal balloon catheter and transpulmonary pressure was obtained by subtracting the mouth pressure (Pm) from Ppl. Respiratory airflow (V) was measured using a Fleisch pneumotachograph N degrees 3 and tidal volume (Vt) was electronically derived by integrating V with respect to time. Respiratory airflow, Vt, Ppl and Pm were simultaneously recorded while the calves breathed first air (1 min), secondly He-O2 (1 min) and finally air again (1 min). The pulmonary function values were calculated on the tracings and the He-O2 values were compared to the corresponding "air" values. The ventilatory response to resistance unloading was characterized by a significant increase in minute volume exclusively due to an increase in respiratory frequency, while Vt remained unchanged. A decrease of 32% in the total pulmonary resistance (RL) and 16% in the work per litre of ventilation (Wrm/l) were observed when calves breathed He-O2. The effect of the He-O2 on RL and on Wrm/l suggests that, in the bovine species, airflow turbulances in the upper part of the respiratory tract play a significant role in the total airways resistance.