G. Miserocchi

Transient responses of pulmonary stretch receptors in the dog to inhalation of carbon dioxide

Abstract The transient response to hypercapnia (8% CO2 in O2) has been studied for 22 pulmonary stretch receptors in 12 dogs, anaesthetized and artificially ventilated, in which the end-tidal CO2 was maintained at 15 mm Hg. Hypercapnia inhibited the activity of the stretch receptors ; the time course of this inhibition was slower for stretch receptors localized in the larger extrapulmonary airways than for those localized in the smaller intrapulmonary airways. The maximum inhibition occurred after 16 sec and 90 sec for receptors placed in the intrapulmonary and extrapulmonary airways, respectively; the mean frequency of discharge was reduced in both eases to 75% of that of the control breaths. The inhibitory effect of CO2 was found to be significantly greater in the intrapulmonary receptors after ligature of the pulmonary artery. These effects do not seem to be due to a variation of the bronchomotor tone since they were not influenced by the administration of isoprenaline. The results are interpreted rather as responses to local variation of H+ concentration since the administration of acetazolamide was found to block the eflects of CO2 on the discharge of stretch receptors. These results might suggest the existence of a mechanism partially responsible for the ventilatory response to carbon dioxide.

Forces deforming the rib cage

Abstract The relaxed rib cage has been squeezed along the lateral diameter at different lung volumes with closed airways. The force applied, the decrease of the lateral diameter, the increase of the dorsoventral one, the esophageal and gastric pressures have been measured. During a 10 kg squeeze the increase of the esophageal pressure is negligible notwithstanding the decrease of the rib cage cross section and the lack of muscle contraction. The diaphragm is lowered, probably through the lowering of the ribs. A marked decrease of the rib cage size without a parallel change of the esophageal pressure occurs also when shifting from sitting to standing at functional residual capacity. The work done by the equipment to reduce the lateral diameter as much as during expiratory efforts is about 1 3 of the compressive work done by the muscles during maximum static efforts performed at functional residual capacity and at 60 % of the vital capacity. During the breathing cycle the deformation work may become appreciable when the flow-resistances are high.

Effect of rib cage or abdomen compression at iso-lung volume on breathing pattern

In anesthetized rabbits a small compression of the rib cage (RC) caused a marked increase of breathing frequency (f), a marked reduction of expiratory duration (Te), a reduction of inspiratory duration (Ti) and tidal volume (VT), while an equal decrease of lung volume produced by abdominal compression(AC) caused a smaller increase of f and no change of Ti, and a slight increase of VT. Similar results occurred when normal end-expiratory volume was maintained during compression. The phenomena disappeared after bilateral vagotomy, whereas persisted after cordotomy in T1. Similar results occurred in dogs (with the exception that VT decreased during AC), whereas in cats only RC increased f by shortening Te. RC caused a marked increase of discharge in 11 out of 19 irritant receptor fibers, whereas AC caused an increase only in 2. In most of the pulmonary stretch receptor fibers the change of activity produced by compression paralleled the lung volume change. The effects of vagal cooling on Ti, Te and the VT-Ti relationship with and without compression suggest irritant receptors as responsible for the different ventilatory pattern during RC and AC.

Local transpulmonary pressure after lobar occlusion

Abstract In the breathing dog occlusion of the right lung or of the right inferior lobe was carried out at FRC. With the occlusion of the right lung transpulmonary pressure (Pl) on the costal side of the occluded lobes did not change significantly at end inspiration except in the inferior lobe. With the occlusion of the right inferior lobe Pl on the costal surface of the occluded lobe increased significantly and that of the freely expanding lobes did not increase uniformly as in normal breathing animals. When the respiratory system was passively inflated, Pl increased on the occluded inferior lobe. From the values of alveolar and pleural surface pressure it was deduced that the occluded and inflated lobes were deformed. The same occurred when only the right inferior lobe was passively inflated. The vertical gradient of P1 on the costal side tended to disappear when pleural surface pressure became positive, as previously observed during passive inflation of the whole respiratory system.

Change pattern of pleural deformation pressure on varying lung height and volume.

Simultaneous measurements of pleural surface (Ppl) and liquid (Pliq) pressures were made at various lung heights in lateral and supine dogs on increasing lung volume from FRC to 80% TLC by decreasing abdominal pressure. The difference between Pliq and Ppl, /Pdef liq/, i.e. the pressure elicited by the deformation forces of the contacting pleurae over the areas of intervening liquid, was greater in the superior parts at all lung volumes. Previous data on pleural liquid thickness (9) and the present measurements show that an increase of /Pdef liq/ may occur either with a decrease or an increase of 9. delta Pdef liq/delta Ppl over a given range of Ppl, increases progressively from bottom up. For a given height the greater the initial /Pdef liq/, the greater delta Pdef liq/delta Ppl. When /Pdef liq/ was decreased by injecting liquid into the pleural space at FRC, delta Pdef liq/delta Ppl decreased. Hence, delta Pdef liq/delta Ppl depends on the initial Pdef liq, i.e. upon the initial tightness of fitting of the pleural membranes. A model interpretation of this relationship is provided.

Kinetics of the intrapleural distribution of a radioactive bolus.

Through the use of a gamma-camera placed horizontally over the chest of 5 supine, anesthetized and spontaneously breathing dogs we studied the kinetics of the intrapleural spreading of 1 ml saline containing 2 mg of 99mTc labelled albumin (250 microCi/kg) injected at various points on the costal side. From the postinjection sequence of gamma-camera images, we defined, along the intrapleural spreading pathways, some regions of interest (ROI) displaying a preferential label accumulation. In all the animals the ROI roughly corresponded to the apical, mediastinal and latero-diaphragmatic regions. The kinetics of the intrapleural label distribution, estimated from the time course of the activity under the ROI, displayed an initial rapid phase, lasting a few minutes, followed by a marked slowing down of the exchange processes. At 30 min regional counts reached an apparent steady state, the activity values greatly varying (even by 10 fold) among the animals, for the ROI corresponding to the same anatomical region, independently on the site of injection. The mathematical modelling of the regional activity curves required the assumption of a progressive decrease in the exchange coefficients to account for a marked slowing down of the exchange processes with increasing time.