J.E. Remmers

Laryngeal regulation of respiratory airflow.

Abstract We measured laryngeal resistance in spontaneously breathing anesthetized cats and upper airway resistance in unanesthetized, unrestrained cats. In anesthetized cats breathing air, the vocal cords were abducted during inspiration and moved toward the midline during expiration, thus increasing laryngeal resistance. With CO2 administration the inspiratory abduction of the cords was more pronounced, and the cords remained open throughout most of expiration as well. Thus hypercapnia decreased laryngeal resistance, especially during expiration. Electromyographic (EMG) studies indicated that the respiratory movements of the cords are caused by phasic contractions of the posterior cricoarytenoid (PCA) muscles. The responses to hypercapnia and lung inflation suggested that PCA motoneurones have central connections with both inspiratory and expiratory neurones. In unanesthetized cats, upper airway resistance was higher during expiration than during inspiration. CO2 administration reduced upper airway resistance, especially during expiration. These findings, together with those reported in the accompanying paper, indicate that the larynx is a significant respiratory effector organ, which provides fine regulation of respiratory airflow, particularly during expiration.

Control of the duration of expiration

Abstract We recorded tidal volume, inspiratory and expiratory durations, upper airway resistance, abdominal pressure and diaphragmatic electromyograms in resting, unanesthetized cats. During air breathing expiratory duration (spontaneous te) exceeded the time required for passive collapse of the intubated respiratory system (passive te). This difference between spontaneous and passive te values resulted from the influence of two factors that retard expiratory flow: post-inspiratory activity of the diaphragm and the resistance of the upper airway. Hypercapnia consistently led to a decrease in spontaneous te, accompanied by a decrease in upper airway resistance, an increase in abdominal expiratory muscle activity and, in some animals, a decrease in post-inspiratory activity of the diaphragm. In cats studied 1–4 weeks after bilateral cervical vagotomy, te was generally increased, a change that is consistent with the operation of a volume-related feedback system controlling te in the intact animal. Our analysis of the factors that regulate expiratory flow indicates the importance of variations in upper airway resistance brought about by movements of the vocal cords. We propose that by this mechanism, via the operation of volume-related feedback, the larynx is a significant determinant of te and the respiratory frequency.

Inhibition of inspiratory activity by intercostal muscle afferents

Thoracic mechanoreceptors inhibit phrenic and inspiratory intercostal discharge in anesthetized, vagotomized cats and dogs. Each of three mechanical stimuli, chest compression, intercostal muscle stretch and rib vibration, inhibits phrenic discharge and each provides evidence that intercostal muscle spindles contribute to this reflex. Phrenic inhibition by airway occlusion is probably accounted for by this reflex. Alterations of respiratory rhythm indicate that supra-spinal reflexes are initiated by the mechanical stimuli. An inspiratory inhibition reflex from intercostal muscle spindles might act to increase the mechanical efficiency of breathing.

Effects of high altitude exposure on the lungs of young rats.

Abstract Month-old male rats were exposed in a hypobaric chamber to a simulated altitude of 4200 m (Pb = 450 mm Hg). After 20–21 days, exposed animals had significantly greater lung volumes and alveolar surface areas than controls. Animals sacrificed after 7 days of exposure had lungs that were abnormally heavy relative to their volume; this increase in lung density was due to an increase in non-blood lung water, presumably as edema fluid. After 20–21 days, lung density and water content had returned essentially to normal. The volume of blood remaining in the lungs after removal from the animal was not influenced by high altitude exposure. The findings indicate that young rats exposed to high altitude undergo an initial period of mild pulmonary edema; during this period the rate of alveolar development is probably not increased above normal. With continued exposure, the edema subsides and alveolar proliferation occurs at a faster rate than in control animals. This results eventually in increased lung volumes and alveolar surface areas in high altitude animals, a change which probably has adaptive significance.

Neural and mechanical mechanisms of feline purring

Abstract Purring results from the intermittent activation of intrinsic laryngeal muscles as manifest by a very regular, stereotyped pattern of EMG bursts occurring 20–30 times per second. Each burst of muscle discharge causes glottal closure and the development of a trans-glottal pressure which, when dissipated by glottal opening, generates sound. During inspiration, the diaphragmatic discharge is also chopped, and the diaphragmatic and laryngeal bursts occur dissynchronously. This alternating activation of the two muscles serves to limit the negative swings in tracheal pressure and promotes inspiratory flow during the period of minimal glottal resistance. Interruption of the afferent pathways for a variety of respiratory mechanoreceptors failed to eliminate the neural oscillation characteristic of purring, suggesting the existence of a high frequency oscillatory mechanism within the central nervous system.

The carbon monoxide diffusing capacity in permanent residents at high altitudes

Abstract D L , D M and V C were determined by the steady-state carbon monoxide method in five residents of 3700 m and in four residents of 4500 m. Observed D L (Pa O 2 ; 100–150 mm Hg) and derived D M were greater than normal in residents of 3700 m but were nearly equal to sea level normal values in residents of 4500 m. Derived V C was near normal in the first group, but was greatly increased in the second group. These differences most likely result from differences in true D M and V C , with the exception that apparent D L and D M for the residents of 4500 m may be spuriously low. The changes in V C agree well with the differences in predicted central blood volume, but the greater D M in residents of high altitude is difficult to reconcile with anatomical measurements in other high altitude species. It is concluded that in permanent residents at high altitude the diffusion and reaction components of the total resistance to oxygen uptake in the lung are decreased, an adaptation not present in acclimatized low-landers.

Effect of controlled ventilation on the tolerable limit of hypercapnia.

Abstract The effect of consciously controlled ventilation on the tolerable limits of the CO2 stimulus was systematically investigated in normal human subjects. Subjects maintained a fixed breathing regime while faco2 was slowly increased to the highest values that could be tolerated. By comparing the results of various breathing patterns, the independent effects of the components of the respiratory cycle could be deduced. Increases in mean lung volume ( L.V. ) and vt separately augmented tolerance of the chemical stimulus. Over a range, CO2 tolerance was directly proportional to f for a constant L.V. and vt, and different proportionalities obtained for different values of L.V. and vt. For wide variations in f and vt, CO2 tolerance was a single, linear function of ve. It is concluded that mechanoreflexes generated by the act of breathing inhibit the conscious perception of a chemical stimulus and, presumably, inhibit respiratory drive. Also, this negative feedback involves both adaptive and nonadaptive mechanisms. Results of experiments with resistive loading and with partial muscular paralysis by gallamine suggest that activation afferent discharge from spindles of respiratory muscles increases central inhibition.

Mechanics of the respiratory cycle in the green turtle (Chelonia mydas)

Abstract Inspiration in the green turtle is, from a mechanical standpoint, quite comparable to the same event in mammals, but expiration is the event of special interest, because there is no well-developed conducting system in the lungs. Nonetheless, peak expiratory flow velocities in this turtle are very high and are not different from those of man with maximal exertion. The trachea is large, and apparently the intrapulmonary airway system, in spite of having no highly evolved bronchial channels, does not present the anticipated problem of collapsing airways during expiration. The avoidance of extrapulmonary positivity, except at the beginning of expiration, helps to circumvent the problem, but in addition the large bores of the airflow pathways keep the resistance low even as the lung volume shrinks, and probably the smooth muscle widely distributed throughout the lung plays a role in generating the force for expiration as well as providing structural rigidity for the airway system.

Pulmonary gas exchange in andean natives at high altitude

Abstract A-a DO2 at 3 levels of oxygenation and the a-A DCO2 and Vd Vt ratios during ambient air breathing were measured in 11 Andean natives at 3700 m. Six were permanent residents of this altitude, 5 had recently come from permanent residence at 4500 m. Breathing ambient air: the A-a DO2 was equal to values at sea level when PIO2 was lowered to that of 3700 m; the a-A DCO2 and Vd Vt ratio was increased indicating increased distribution of high Va/Q; there was no evidence of increased venous admixture. At high inspired PO2 (445 mm Hg) there was no evidence of increased true venous shunt. However, when PIO2 was increased to 160 mm Hg the A-a DO2 was significantly larger than at sea level. These findings differ from those previously reported. The relatively low A-a DO2 under ambient conditions, in the face of increased Vd Vt (high a/) results from the absence of significant venous admixture and the effect of PIO2 on A-a DO2 in the presence of Va/Q abnormality. The significance of these findings in terms of acclimatization is discussed.