Partitioning of pulmonary resistance in calves

Nine right apical lobes of healthy Friesian calves and 10 right apical lobes of double-muscled calves of Belgian White and Blue (BWB) breed were suspended in an airtight box, inflated at a constant transpulmonary pressure (Ptp), and subjected to quasi-sinusoidal pressure changes (amplitude: 0.5 kPa) at a frequency of 30 cycles/min. Lobar resistance (RL) was partitioned at six different lung volumes into three components: central airway resistance (Rc), small airway resistance (Rp), and tissue resistance (Rt). Pressure in small airways (2-3 mm ID) was measured with a retrograde catheter. Alveolar pressure was sampled in capsules glued onto the punctured pleural surface. RL was minimal at values of Ptp comprised between 0.5 and 0.7 kPa and increased at higher and lower values of Ptp. At a Ptp of 0.5 kPa, Rc, Rp, and Rt represented 30, 15, and 55% of RL, respectively, in Friesian calves and 25, 25, and 50% in BWB calves. Rp increased markedly at low lung volumes. Rt was responsible for the increase of RL at high Ptp. Rc tended to decrease at high Ptp. The significantly higher values of Rp in BWB calves (P less than 0.05) might explain the sensitivity of this breed to severe bronchopneumonia.