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°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/1) were observed when calves breathed He‐O2. The effect of the He‐O2 on RL and on Wrm/1 suggests that, in the bovine species, airflow turbulences in the upper part of the respiratory tract play a significant role in the total airways resistance. © 1990 Blackwell Verlag GmbH