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We have performed experiments in 26 dogs anesthetized with pentobarbital and fitted with an endotracheal tube. The inflatable cuff of this tube was positioned either at the level of the cricoid cartilage or at the thoracic inlet. In this latter situation the extrathoracic trachea (E.T.T.) is not subjected to any change in transmural pressure both during breathing and airway occlusion. We have compared the inspiratory output in term of the integrated phrenic discharge during airway occlusion at FRC with the tracheal tube positioned at either one of the two levels. In most of the experiments (16 out of 26) the inspiratory output during airway occlusion is significantly greater (157%) when the E.T.T. is not by-passed and this difference disappears after bilateral vagotomy. We interpret these results by the asymmetrical response of the tracheal stretch receptors to positive and negative transmural pressure (Pt); most of these receptors are active at FRC and decrease their activity at low negative Pt, as that attained in the first occluded breath. These results seem to suggest that the reflex influences from the extrathoracic tracheal receptors on the inspiratory output are similar to those originating from the intrathoracic airway stretch receptors.

Seven kittens age 5 to 8 days were anaesthetized with ketamine, tracheotomized, cannulated just below the larynx, paralyzed, and ventilated. The thorax was widely opened and an expiratory load equal to the transpulmonary pressure at functional residual capacity (PLFRC) added. Single vagal fibers were dissected from the peripheral cut end of the right vagus nerve. Thirty-eight receptor discharges modulated during the respiratory cycle (pulmonary stretch receptors, PSR) were studied; 4 (10.5%) were tonically active at PLFRC while the remaining 34 had a mean threshold at 3.2 cmH2O. All the receptors progressively increased their discharge frequency with higher pressures reaching a plateau between 8-10 cmH2O. By occluding the airways at different levels of the tracheobronchial tree 32 PSR were functionally localized: none were found in the extrathoracic trachea; 3 (9.5%) were located in the intrathoracic trachea, 12 (37.5%) at the carina, main bronchi, and lobar bronchi, and 17 (53%) inside the lobes. All three tracheal receptors were tonic PSR. Previously obtained data from adult mammals indicate that 27-60% of PSR are tonically active and most of these are located in the trachea. The low incidence of tonically active PSR in the kitten may suggest a delayed functional maturation of the tracheal receptors.

We have studied the innervation of the stretch receptors in the extra-thoracic trachea by recording their unitary action potentials from the vagus nerve in the neck before and after blocking the nervous conduction in the recurrent laryngeal nerves (RLNs). On the left side all the receptors had their activity going through the ipsilateral RLN:78% of the stretch receptors had their fibres joining the RLN in the neck whilst the remaining receptors join this nerve within the thorax. These latter receptors were located mainly in the lower extra-thoracic trachea. On the right side only 45% of the receptors were found to follow the right RLN whilst the others reach the vagus nerve through a different pathway. These latter receptors were located in the lower part of the extra-thoracic trachea. Therefore sectioning both the RLNs would interrupt about 3/4 of the fibres coming from the extra-thoracic trachea if done just before their entrance into the main vagus nerves. When the section is done exclusively outside the thorax, at the level of the thoracic inlet, the de-afferentation would be about 60% of the total supply.

Because tracheal pressure measurement generally involves the use of a cannula or an endotracheal tube, fluid dynamic factors may cause a considerable artifact. We present a theoretical explanation of the observed apparent paradox in which the resistance of a tracheal cannula or an endotracheal tube is isolation was found to exceed the resistance of the airways plus the cannula or the tube in situ. By estimating the viscous dissipation and the kinetic energy change in a conduit with sudden variation of cross-sectional area, a predictive model is derived. The predictions are verified by a series of in vitro experiments with both steady and oscillatory flows. The experiments showed that the pressure recorded from the sidearm of a tracheal cannula or endotracheal tube contains an error which, in general, increased with the mean Reynolds' number of the through flow and also depends on the diameter ratio between the trachea and the tube or cannula, the position of the pressure tap, and the frequency of ventilation. When feasible, direct measurement with a needle in the trachea is suggested as a way to avoid the possible artifacts arising from the use fo a side tap of the cannula. Theoretical considerations, as well as in vitro and animal experiments, indicate that adding a properly chosen expansion to the tracheal cannula makes it possible to alter inspiratory and expiratory pressures selectively. This device may prove useful in control of breathing studies.

The postnatal development of the mammalian lung, which at birth has only a fraction of the adult number of alveoli, would suggest a mismatch (dysanapsis) of the changes in airway size and lung size with growth. This may have implications on the efficiency of breathing because for any given lung size the dimensions of the airways are the determinant of the amount of dead space (VD) ventilation and of airflow resistance. A comparison of the allometric functions of tracheal dimensions previously published suggests that the tracheal volume, taken as representative of VD, is smaller in newborns than in adults. This difference becomes more apparent when examined per unit of functional residual capacity (VD/FRC). The relatively smaller tracheal volume is the result of both a shorter and narrower trachea in the newborn. This latter difference implies a slightly but significantly higher resistance of the lower airways, as also demonstrated by experimental measurements of peak expiratory flows at a constant driving pressure in rats of different ages. Because the slightly higher resistance of the lower airways is probably compensated by the smaller resistance of the upper airways (J. Appl. Physiol.: Respirat. Environ. Exercise Physiol. 51: 641-645, 1981), it would seem that the structure of the newborn mammalian lung favors the alveolar ventilatory function without a substantial increase of the energetic losses.

We have studied the response of 40 slowly adapting stretch receptors located in the extrathoracic trachea to maintained positive and negative pressures together with their behavior during the breathing cycle and inspiratory efforts against closed airways in anesthetized dogs. Like the other tracheal stretch receptors they are located in the smooth muscle of the posterior wall and are stimulated either by inflation or deflation. Most of these receptors are active at zero transmural pressure and increase their response with positive pressure; with negative pressure their resting discharge decreases at first (-5 to -10 cm H2O) and then increases, but it is generally lower than that of the corresponding positive pressure value. As predicted by their static response, the activity of these receptors decreases during inspiration and increases during expiration and therefore it is out of phase with the discharge of the other airway stretch receptors. Their activity is related to the corresponding transmural pressure and therefore to airflow and upper airway resistance. During inspiratory efforts against the closed airways their activity decreases, unlike that of the intrathoracic airway receptors, which retain essentially their FRC activity.

Tracheal dimensions have been measured in a variety of newborn mammals ranging in body mass (M) from mouse to dolphin. Both tracheal diameter and tracheal length are nearly proportional to the cube root of M (M0.36 and M0.34, respectively). Tracheal volume is proportional to M1.07, and assuming that dead space volume (VD) is proportional to tracheal volume, VD is found to be proportional to FRC. Resting breathing frequency is proportional to M-0.21, therefore dead space ventilation (VD) can be calculated to be proportional to M0.86. This value is very close to the proportionality between oxygen consumption (VO2) and body weight reported in newborns (M0.88, Bartlett and Areson, 1977); since alveolar ventilation (VA) is proportional to VO2, VD is proportional to VA. From this, it can also be inferred that alveolar PCO2 and PO2 are nearly the same in newborns of different species.

We have investigated the possibility of using a two-sidearm tracheal cannula for measurements of respiratory airflow. Measurements of mean flow and tidal volume using four such cannulas (0.7, 1.5, 1.8, and 2.5 mm ID) are described. They have a linear response up to flows of 6, 10, 12, and 28 ml X s-1, respectively, that correspond to 100 ml X s-1 X kg-1 or more, depending on the body size of the animals (15-350 g) in which they could be used. These characteristics, coupled with small volume and flow resistance, make the use of these cannulas as pneumotachographs a suitable and very practical way of measuring mean airflow and tidal volume in anesthetized small animals, ranging in size from newborn rats to newborn puppies.

1. The trachea is constructed by a series of U-shaped cartilaginous rings supporting a membranous posterior wall. We have studied separately the pressure-volume relationships of the two components. 2. The motion of the membranous posterior wall contributes most to the tracheal volume change caused by any given transmural pressure change; the cartilaginous rings provide a semi-rigid support to the posterior wall and have a far greater compliance with negative than positive transmural pressure. 3. The response of tracheal stretch receptors to transmural pressure can be explained by the mechanical coupling between cartilages and posterior wall. They respond both to positive and negative transmural pressure, they are active at zero transmural pressure and have a point of least activity with small negative transmural pressures. 4. The stress-strain relationship of the posterior wall has been studied in static and dynamic conditions in control situations and after removal of either the tunica fibrosa or the trachealis muscle. Each of these two components contributes to the stiffness of the posterior wall, with the trachealis muscle providing most of its viscosity. 5. The response of tracheal stretch receptors to transverse traction of the posterior membranous wall has been studied in both static and dynamic conditions before and after removal of the tunica fibrosa. The behaviour of these receptors reflects the visco-elastic properties of the trachealis muscle in which they have been localized.

During breathing and various respiratory maneuvers the trachea sustains significant longitudinal stresses which could be sensed by its mechanoreceptors. In this study we have measured the longitudinal motion and elongation of the trachea in supine, anesthetized dogs during quiet breathing and with occlusion of the upper airways and recorded the response of tracheal stretch receptors in each of the two conditions. The respiratory modulation of intrathoracic tracheal stretch receptors is essentially absent during airway occlusion when the elongation of the trachea becomes greater than in a quiet breath. We have also measured the response of tracheal stretch receptors to transmural pressure at different tracheal length, both statically and dynamically. When the trachea is elongated the receptors decreased their activity at higher positive pressures and remained unaffected with lower distending pressures, at zero transmural pressure and with collapsing pressures. These results indicate that the longitudinal forces applied to the trachea are not capable of changing the transducing properties of its stretch receptors over most of its pressure-volume range, only toward its TLC position their response appear to become attenuated.

In dogs the authors have recorded from single fibres of the vagus nerve the action potentials originating from rapidly adapting receptors (irritant receptors) of the tracheo-bronchial tree activated both by inflation and deflation and by i.v. injection of histamine. A gross localization between extrapulmonary airways was established for 196 such receptors by probing the structures containing them. For 82 receptors a precise localization was determined within the extrapulmonary airways and for 51 receptors, localized in the lung parenchyma, a further precise localization was established along the intrapulmonary airways be dissecting the lung tissue. 66.9% of these receptors were found to be in the intrapulmonary airways and 33.1% in the extrapulmonary airways. Their distribution along the tracheo-bronchial tree resulted as follows: 4.75% in the higher 2/3 of the trachea, 9.25% in the lower 1/3 of the trachea and the carina, 19.0% in the main bronchus, 25.0% in the lobar bronchi, 36.8% in bronchi having a diameter larger than 1 mm and 5.2% in bronchi having a diameter smaller than 1 mm. Therefore the concentration of irritant receptors increases going from the upper trachea to the lobar bronchi and then sharply decreases reaching the smaller airways.

Two types of sensory receptors were individually identified in dogs on the exposed mucosa of the extrathoracic trachea: slowly adapting stretch receptors (SAR) and rapidly adapting receptors (RAR). Increasing concentrations of lidocaine (L), bupivacaine (B), and tetracaine (T) solutions were topically applied on the mucosa over the sensory fields of the receptors, while their neural activity in response to an appropriate stimulus was recorded. Action potentials from SARS were blocked by the anesthetic at a much higher concentration and with larger exposure times than potentials evoked from RARs (mean values: L = 51.1 mM for 7.4 min and 1.48 mM for 1.03 min, respectively; B = 9.4 mM for 9.4 min and 0.13 mM for 0.23 min; and T = 4.9 mM for 9.8 min and 0.06 mM for 2 min). The minimal blocking concentration of the topical anesthetics varied widely (L = 20 times, B = 5, and T = 5) among SARs and less (L = 3.2, B = 2, and T = 4) among RARs. These results could be explained by a different location in the mucosa of the two types of receptors and suggest the feasibility of a differential blockade by topical anesthetics.

1. During respiratory efforts against a closed airway, the afferent activity of vagal fibres from pulmonary stretch receptors does not appreciably increase during the inspiratory phase because the lung is prevented from expanding.2. Occlusion at different levels of the airways allows the localization of pulmonary stretch receptors in the tracheo-bronchial tree.3. 144 fibres from pulmonary stretch receptors on the left side of the tracheo-bronchial tree have been studied in eleven dogs and their localization was as follows: 17.4% in the upper half of the intrathoracic trachea, 27.1% in the lower half of the intrathoracic trachea and the carina, 11.1% in the main bronchus, 13.9% in the upper lobe and 30.5% in the lower lobe.4. From the surface area of the tracheo-bronchial tree at different levels on the assumption of a total of 2000 stretch receptors on each side, their average concentration was as follows: 34.8% receptors/cm(2) in the upper half of the intrathoracic trachea, 54.2/cm(2) in the lower half of the intrathoracic trachea, 56.8/cm(2) in the main bronchus, 0.37/cm(2) in the intrapulmonary airways.5. Occlusion of the main bronchus caused an increase of the eupnoeic oesophageal pressure swing by about 75% whereas occlusion of the inferior lobar bronchus led to an increase of only 20%. Therefore the reflex effects induced on the respiratory activity by occluding the airways at various levels show the greatest importance of the hilar portions of the airways where the concentration of pulmonary stretch receptors has been found to be greater.