Contribution of hypercapnic stimuli and of vagal afferents to the timing of breathing in anesthetized cats

Functional morphology and physiology of slowly adapting pulmonary stretch receptors

Since the original work by Hering and Breuer (1868) on slowly adapting pulmonary stretch receptors (SARs), numerous studies have demonstrated that these receptors are the lung vagal afferents responsible for eliciting the reflexes evoked by moderate lung inflation. SARs play a role in controlling breathing pattern, airway smooth muscle tone, systemic vascular resistance, and heart rate. Both anatomical and physiological studies support the contention that SARs, by their close association with airway smooth muscle, continuously sense the tension within the myoelastic components of the airways caused by lung inflation, smooth muscle contraction, and/or tethering of small intrapulmonary airways to the lung parenchyma. As a result, the receptor field location within the tracheobronchial tree of a SAR plays an important role in its discharge pattern, with variations in airway transluminal pressure and airway smooth muscle orientation being important modulating factors. The disruption of airway myoelastic components in various pulmonary diseases would be expected to alter the discharge pattern of SARs, and contribute to changes in breathing pattern and airway smooth muscle tone.

An overview of the anatomy and physiology of slowly adapting pulmonary stretch receptors

Since the original work of by Hering and Breuer in 1868 numerous studies have demonstrated that slowly adapting pulmonary stretch receptors (SARs) are the lung vagal afferents responsible for eliciting the reflexes evoked by moderate lung inflation. SARs play a role in controlling breathing pattern, airway smooth muscle tone, systemic vascular resistance and heart rate. Both anatomical and physiological studies support the contention that SARs, by their close association with airway smooth muscle, continuously sense the tension within the myoelastic components of the airways caused by lung inflation, smooth muscle contraction and/or tethering of small intrapulmonary airways to the lung parenchyma. In addition, intrapulmonary SAR discharge activity is sensitive to changes in P(CO2) within the physiological range. Despite this extensive characterization of SARs, their role in determining breathing pattern and airway tone in individuals with respiratory diseases is only recently being appreciated.

High frequency ventilation and afferent vagal activity

Recordings were made from pulmonary afferent fibres in the vagus nerves of anaesthetised dogs during conventional and high frequency ventilation. In single and multiple fibre preparations, the mean spike counts per minute at 14 breaths/minute were in the ranges 36.9-155.3 and 755-1921, respectively. These counts decreased by up to 61 and 44% respectively at 100 breaths/minute, and by 27-89% and 22-51% at 200 breaths/minute. At this frequency there was a further decrease of between 19 and 65% when the positive end expiratory pressure was removed. The findings of this study are intuitively acceptable, since pulmonary stretch receptor activity is proportional to tidal volume, and are in keeping with the clinical impression that high frequency ventilation per se does not eliminate respiratory drive.

Pulmonary receptor chemosensitivity and the ventilatory response to inhaled CO2 in the turtle

Ventilatory responses of unanesthetized turtles to changes in the intrapulmonary CO2 content of a vascularly isolated and an intact lung were measured during spontaneous breathing. The hyperpnea associated with inhalation of CO2 by the vascularly isolated lung was 19% of that associated with inhalation of CO2 by the intact lung. Transection of the vagus nerve supplying the isolated lung abolished this response. We conclude that both inhibition of pulmonary stretch receptor discharge with increasing levels of FICO2 and a functional increase in central inspiratory volume threshold during hypercapnia contribute to tidal volume increases following CO2 inhalation in normal animals. The major component of the ventilatory response of intact turtles to increasing levels of FICO2, however, was an increase in respiratory frequency. When CO2 was inspired only by the vascularly isolated lung the increase in respiratory frequency was only 21% of that recorded when the same levels of CO2 were inspired by the intact lung. Thus the ventilatory response of turtles to increasing levels of FICO2 is primarily dependent upon concomitant hypercapnia.

Behavior of expiratory neurons in response to mechanical and chemical loading

The response of medullary expiratory neurons to added mechanical and chemical loads was studied in anesthetized cats. Alterations in burst characteristics and central timing were compared in the intact and bilaterally vagotomized cat. The following results were obtained: (1) Graded expiratory airflow resistances caused progressive increases in burst duration, spikes per burst and firing rate; similar effects were noted for end-inspiratory tracheal occlusions and continuous positive breathing; all facilitation was eliminated by vagotomy. (2) Graded inspiratory airflow resistances delayed the onset of an expiratory burst but did not change the overall burst characteristics. (3) Acute hypercapnia increased ventilation without noticeable changes in expiratory burst characteristics; acute hypoxia produced a reduction in burst duration concomitant with changes in ventilation. It is concluded that (1) expiratory neurons are responsive to vagally mediated volume information and (2) transient hypoxia and hypercapnia sufficient to increase ventilation does not increase the firing rate of expiratory neurons but exerts differential effects with respect to timing. It is suggested that expiratory duration is related to the time integral of expired volume and that the increase in FRC imposed by expiratory loads does not alter the central timing of the next inspiration.

Histamine and phenyldiguanide induced tachypnea in hypercapnia and hypoxia

The rapid shallow breathing of pulmonary vagal origin following administration of histamine (H) and phenyldiguanide (PDG) was studied at different levels of hypercapnic and hypoxic stimulation. At all levels of chemical drive H and PDG caused an excitatory effect on timing of breathing and an inhibitory effect on the respiratory output. The latter was evaluated from the mean inspiratory flow rate and the mean rate of change of pressure developed in the lungs during an inspiratory effort against closed airways. The timing effect was greater at low than at high PaCO2 while the opposite was true for the output effect. At all PaCO2, H and PDG decreased the volume-threshold for termination of inspiration (leftward displacement of the VT vs TI relationship) of the VT Vs TI relationship). Hypoxia increased the respiratory output in control as much as with drug stimulation. Moreover, hypoxia did not affect the volume-threshold curve both in control and with H and PDG. We concluded that vagal afferents stimulated by H and PDG (irritant and/or J receptors) interfere with the timing and output response to central chemoreceptors stimulation (CO2 sensitivity) without affecting the response to peripheral chemoreceptors stimulation (mainly hypoxic chemosensitivity).

The mechanism of rapid shallow breathing due to histamine and phenyldiguanide in cats and rabbits

In anaesthetized cats and rabbits we analyzed the rapid shallow breathing following exposure to histamine aerosol (mainly an irritant receptor stimulant) and i.v. injection of phenyldiguanide (mainly a J receptor stimulant). Both drugs caused a marked leftward displacement of the tidal volume (VT) vs inspiratory time (TI) relationship (Hering-Breuer threshold curve) without a corresponding increase in inspiratory flow rate so that inspiration was cut off at a lower VT and TI. The leftward displacement of the VT vs TI relationship occurred with a great shortening of the duration of inspiration during occluded breaths (T0I) accompanied by a shortening of the expiratory phase (T0E). These parameters monitored the central respiratory rhythm in absence of the phasic lung volume related vagal loop. It is suggested that the increased central respiratory frequency was due to the augmented firing of fibers from stimulated irritant and J receptors. Stimulation of these endings also caused the TE vs TI relationship to become steeper in cats and to be displaced downwards in rabbits.

The effect of limb movements on the regulation of depth and rate of breathing

In anesthetized dogs and rabbits passive or active limb movements (1) shifted to the left the relationship between tidal volume (Vt) and inspiratory time (Ti), (2) lowered the relationship between expiratory time (Te) and Ti and decreased its slope, and (3) increased the output to the inspiratory muscles (Vt/Ti). These effects increased with increasing the frequency of movements. Similar effects were obtained after vagotomy. When the stimulus was started during expiration. Te was shorted in spite of the previous unaffected Ti, Arterial PCO2 during exercise was similar (active movements) or below (passive movements) control value. Since other chemical and physical humoral factors do not seem involved, the whole increase of ventilation should be produced by neurogenic stimuli. The time course of Te, Ti and VT/Ti at the onset and at the offset of limb movements indicates an abrupt and a slow component in the neurogenic drive. A single contraction of the limb during expiration of inspiration affected the timing and the VT/Ti of 3-8 breaths.

Role of peripheral and central chemosensitive afferents in the control of depth and frequency of breathing

In anaesthetized cats the response to hypercapnia was studied during normoxia, hypoxia (increased peripheral chemosensitive afferents, IPA) and after carotid sinus denervation (decreased peripheral chemosensitive afferents, DPA) in terms of: (a) central output to the inspiratory muscles, (b) bulbopontine respiratory activity and (c) threshold-inhibition curve for termination of inspiration (Vr/Ti relationship). IPA increased, whereas DPA decreased the central output, as estimated from the rate of change of the subatmospheric pressure developed in the trachea during occlusion of the airways. IPA and DPA did not modify the bulbo-pontine rhythm, as estimated from the timing of occluded breaths (phasic vagal afferents nil), nor the Vt/Ti relationship, both of which have been shown to vary as a function of PACO2. It is concluded that peripheral chemosensitive afferents influence only the output of the respiratory centres, whereas central chemosensitive afferents influence also the bulbo-pontine respiratory rhythm and the threshold inhibition curve for termination of inspiration.