The inspiratory efficiency of the diaphragm during unilateral and bilateral phrenic stimulation (UEPS and BEPS) with constant stimulus was studied in seven dogs from FRC to 120% TLC. Alveolar pressures (PAl) were recorded during relaxation, BEPS and UEPS at each lung volume in the closed respiratory system. From the PAl-lung volume curves, tidal volume (VT), and pressure developed by the diaphragm (Pmus) were derived. Results are summarized below. a) Hyperinflation impaired the inspiratory efficiency of the diaphragm which behaved as an expiratory muscle beyond the lung volume of 103.7% TLC (Vinef). b) The diaphragm during UEPS became expiratory at the same Vinef as during (BEPS. C) The VT-lung volume relationship was linear during BEPS, allowing simple quantitation of VT loss with hyperinflation and prediction of Vinef. d) With only one phrenic nerve stimulated, the functional loss is less pronounced in VT than in Pmus, as compared to BEPS, indicating that the respiratory system was more compliant during UEPS than BEPS. This compliance difference from UEPS to BEPS diminished with severe hyperinflation.

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We studied four supine dogs that were anesthetized with pentobarbital, intubated, and ventilated with a piston pump. The dimensional response of central (CAW) (greater than 2 mm diam) and peripheral airways (PAW) (smaller than 2 mm diam) to changes in transpulmonary pressure (Ptp) was determined by progressive increments in tidal volume (VT). A specially designed electronics relay circuit permitted this relationship to be obtained for points of no flow during tidal volume breathing: i.e., preinspiration (FRC); end inspiration (FRC + VT). The airways were dusted with powdered tantalum. Six airway divisions were identified: four CAW: trachea, main stem, lobar, segmental; and two PAW: subsegmental, and lobular. AP and lateral roentgenograms were obtained by standard technics and primary magnification (mag factor 2). Airway diameters were plotted as a function of transpulmonary pressure between 3 and 26 cmH2O with the diameter at total lung capacity expressed as 100%. The data show that: 1) there is significant distensibility above 5 cmH2O for all airways from the trachea to the lobular airways; 2) that the pressure-diameter plot is a linear plot for each airway from 3 to 26 cmH2O with R values between 0.846 and 0.957; 3) the peripheral lobular airways are more distensible than the central airways (P smaller than 0.05). We attribute the difference in distensibility of the peripheral lobular airways to their lack of cartilaginous support, and their decreased muscular support when compared to the CAW.

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