Control of tidal volume and respiratory frequency in anesthetized cats

Excitability changes of the inspiratory "off-switch" mechanism tested by electrical stimulation in nucleus parabrachialis in the cat

The time course of the excitability of the inspiratory "off-switch" mechanism with and without phasic vagal stretch receptor feedback has been studied in cats under light pentobarbitone anesthesia by electrical stimulation in the rostral pons using brief tetanic stimulation (300 Hz for 0.2 s). The threshold strength required just to elicit inspiratory "off-switch" was high early in inspiration and fell steeply with time. The threshold curves were steeper with than without phasic vagal feedback, and the difference reflects the phasic vagal contribution to the excitability of the inspiratory "off-switch" the absence of phasic vagal vagal feedback the time course of this threshold curve usually corresponded closely to that of the "integrated" phrenic activity at all PCO2 levels and body temperatures tested indicating that the "integrated" phrenic activity can be used as an index of the centrally generated inspiratory activity. In response to a rise in PCO2 both the rate of change of excitability of the inspiratory "off-switch" mechanism and its initial threshold level was increased. Changes in body temperature caused no change in the initial threshold but produced marked changes in the rate of rise of the "off-switch" excitability; Following an "augmented breath" the inspiratory "off-switch" threshold was markedly reduced

Effects of lesions in the parabrachial nucleus on the mechanisms for central and reflex termination of inspiration in the cat

The effects of PCO2 and body temperature on the time course and peak amplitude of the central inspiratory activity (CIA) and the inspiratory "off-switch" threshold was studied in apneustic and non-apneustic cats. Apneusis resulted from lesions of the inspiratory inhibiting structures of the medial parabrachial nucleus (NPBM) and by interrupting vagal volume feedback. The cats were paralyzed and ventilated either proportionally to their phrenic output or at predetermined rate and volume. The dependence of the rate of rise and maximal amplitude of phrenic activity on PCO2 and body temperature were comparable in apneustic and non-lesioned animals. The Hering-Breuer volume threshold for inspiratory termination was increased following the rostral pontine lesions. Both hyperthermia and hypercapnia caused augmentation of the absolute rate of rise of inspiratory activity but hypercapnia, in contrast to hyperthermia, caused virtually no change in the fractional increment per unit time. With hypercapnia the inspiratory "off-switch" threshold was raised in the apneustic animals in intact ones, whereas hyperthermia did not seem to influence this threshold. In apneustic conditions expiratory duration remained constant, independent of the large variations in the inspiratory durations. Our results suggest that the NPBM merely provides an excitatory, threshold-lowering input to the inspiratory "off-switch" mechanism.

Temperature effects on the inflation reflex during expiratory time in the cat

In order to obtain further information on the mechanisms controlling expiratory duration (TE) we have studied the effects of changes in body temperature on reflex characteristics of the TE-prolonging inflation reflex. In cats under light pentobarbitone anesthesia controlled volume changes were applied at various times in expiration and the changes in TE were measured from records of phrenic activity. Inflations falling in the last 20% of the control expiratory time were without effect. This reflex insensitive phase remained a constant fraction of expiratory time when respiratory rate was changed with temperature. The relative reflex responsiveness increased, and the volume threshold was reduced with increasing respiratory rate. The inflation reflex characteristics showed integration of the incoming pulmonary stretch receptor activity. The integrating mechanism exhibited a "leaky" character, the decay rate of which changed with body temperature. Functionally the duration of the inspiration-inhibiting activity controlling expiratory duration can be regarded as dependent on 3 main factors 1) the initial peak level of the inhibitory activity, 2) the amount and timing of the vagal afferent activity which adds to this inhibition, and 3) its rate of decay. The results suggest that all these factors are influenced by changes in temperature.

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.

Respiratory regulation after elastic loading and CO2 rebreathing in normal term infants

Studies of airway pressure, tidal volume, respiratory duration, and total breath duration before and after elastic loading airway occlusions were carried out on ten full-term, normal infants on two occassions in the first week of life. Using these noninvasive techniques we infer that static compliance and the Hering-Breuer reflex are unchanged during the first week; that infants may increase sensitivity to chemical drive toward the end of the first week; and that there may be a volume related threshold for vagal inhibition of inspiration in some infants.

Immediate response to expiratory threshold load

In anesthetized rabbits, cats and dogs the end-inspiratory volume referred to normal FRC (deltaVe.i.) of the first breath during expiratory threshold load (ETL) was greater than control deltaVe.i. The increase of deltaVe.i. was roughly proportional to the increase of end-expiratory volume relative to normal FRC (deltaVe.e.) caused by ETL. During squeezing of the rib cage or of the abdomen or after cordotomy at T1 the phenomenon occurred nearly to the same extent. This suggests that afferent impulses from the chest wall are not essential in determining the phenomenon. The first inspiration during ETL occurred after a delay that was roughly proportional to the increase of deltaVe.e: the arterial Pco2, however, was not significantly increased at the time of the first inspiration during ETL. After bilateral vagotomy the first inspiration during ETL was no longer increased. An explantation of the increased deltaVe.i. of the first inspiration during ETL is provided on the basis of the vagal dependent tidal volume-inspiratory duration characteristic of the respiratory center.

The effect of varying tidal volume on the associated phrenic motoneurone output:studies of vagal and chemical feedback

Two groups of dogs were anaesthetised, paralysed and artificially ventilated using a respirator driven by the phrenic motoneurone output, electrically processed to resemble transpulmonary pressure. In one group, blood gases were maintained constant with closed-chest cardiopulmonary bypass; the second group were studied without "bypass". Therefore it was possible to determine the relative contributions of vagal and chemial feedback to the effect of altering the depth of an inspiration on the associated phrenic motoneurone output. Mono-exponential regressions between change of respirator gain and changes in both inspiratory time and peak amplitude of the processed phrenic signal were found for all dogs. The rate of rise of the processed phrenic signal usually changed in the direction of the change in respirator gain, suggesting the presence of vagal positive feedback during eupnoeic breathing. After vagotomy, all responses were absent in the "bypass" group and small in the "non-bypass" group. These experiments quantitate the role of pulmonary vagal afferent discharge, in phase with inspiration, in the regulation of phrenic motoneurone output in a closed-loop situation.

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

In tracheotomized, anesthetized cats, the authors studied the role of phasic and tonic vagal discharge and of hypercapnic stimuli on the timing of breathing. The effect of tonic vagal discharge was separated from that of the other two parameters by comparing the duration of inspiration and of expiration of control breaths with those obtained during occlusions of the airways at FRC (bulbo-pontine activity). The effect of tonic vagal discharge was then separated from that of hypercapnic stimuli by comparing the bulbo-pontine activity before and after vagotomy. Hypercapnia caused shortening of inspiratory and expiratory duration set by the bulbo-pontine pacemaker and increased sensitivity of the respiratory centers for a given phasic vagal input (displacement to the left of the tidal volume vs inspiratory duration relationship). Vagotomy did not modify the bulbo-pontine duration of inspiration nor its possibility to shorten in hypercapnia; by contrast it caused a lengthening of the expiratory time which did not shorten any more in hypercapnia, suggesting that tonic vagal discharge mainly influences the expiratory duration.

Tonic vagal influences on inspiratory duration

The tidal volume (VT), inspiratory (Ti) and expiratory (Te) durations were determined in anesthetized rabbits, cats and dogs under normal conditions, during continuous positive or negative pressure breathing. Elastic loads up to complete occlusion of the airways were applied in order to determine the VT-Ti and Te-Ti relationships. The larger the end-expiratory lung volume the steeper the VT-Ti relationship, the smaller the increase of Ti and the greater the increase of Te produced by the load. Hence, the larger the end-expiratory lung volume the steeper the Te-Ti relationship. In vagotomized animals airways occlusion did not affect Ti and Te. Ti of occluded breaths in vagotomized animals was longer than that of occluded breaths in intact animals, whereas Te was similar at the resting volume of the respiratory system (FRC), shorter above FRC and longer below FRC. These results show that both phasic and tonic vagal discharges determine Ti and Te.

Occlusion pressure as a measure of respiratory center output in conscious man

The output of the "respiratory centers" has been estimated by measuring ventilation, inspiratory muscle power, EMG of the diaphragm, and by various other means, each of which has serious disadvantages. The static pressure generated by the inspiratory muscles at FRC against an obstructed airway is here suggested as a useful alternative. Ten conscious, normal, sitting human subjects were subjected to CO2 rebreathing (Read, 1967) and their airways were occluded at end-expiration at intervals without the subjects being aware in advance. The inspiratory pressure waves so generated were found to be distorted by conscious or unconscious responses to the occlusion which had a minimum latency of 0.15 sec. The pressure generated at 0.1 sec after the onset of inspiration (P0.1) was nevertheless easy to measure and was reproducible in each subject. The CO2 response obtained by plotting P0.1 against PCO2, was curvilinear, the P0.1 increasing more rapidly at high PCO2. The P0.1 is independent of pulmonary mechanics. Since it measures the rate of rise of inspiratory activity and not the peak activity it is also independent of mechanisms that alter the respiratory pattern by affecting inspiratory duration, in particular the vagal volume-related inspiratory-inhibitory reflex. It is concluded that measurements of P0.1 represent a useful index of the output of the respiratory centers.

A short-term memory in the respiratory centres: statistical analysis

It has been observed previously that the successive values of depth and duration of respiratory cycles recorded in man are not independent random variable. The present study has been performed in order to test whether a serial dependence of the characteristics of the respiratory cycles results from an inherent property of the central respiratory oscillator. In the "isolated respiratory centre" preparation of the cat (spinalized, bivagotomized, curarized and artificially ventilated) the following respiratory data have been studied: the value of integrated phrenic discharge per breath (S), inspiratory duration (TI), expiratory duration (TE) and total cycle duration (T), at rest and during hypercapnia and hyperthermia. Each recording in a steady-state condition provided a set of four series of data; using statistical analysis of time series the following results were obtained: 1. In each series the successive values are not independent random variables. This suggests that a cycle may be considered as an event whose characteristics condition those of the following cycle. 2. TE is not independent of the preceding TI, and TI is not independent of the immediately preceding TE. However, this last relation cannot be asserted in hypercapnia and hyperthermia. 3. A positive correlation exists between S and TI. This result indicates that a simultaneous adjustment of S and TI, in terms of one another, does exist in absence of the vagal mechanism.

Respiratory effects of pneumotaxic center lesions and subsequent vagotomy in chronic cats

Ventilation was monitored in awake and anesthetized cats breathing air or CO2 mixtures, both before and after lesions in the pneumotaxic center region and after subsequent bilateral vagotomy. Cats with pontile lesions had, relative to intact state, a prolonged inspiratory duration and an increase in tidal volume and expiration duration. The changes seen on the spirogram differed quantitatively according to the region of the pneumotaxic system which was destroyed. After subsequent bilateral vagotomy, respiratory phases and tidal volume were greatly increased. These results indicate that the pneumotaxic center would control the sensitivity of Breuer-Hering reflex and that the inspiratory activity is released by the successive suppression of the two inspiratory-inhibitory mechanisms, i.e., the pneumotaxic center and the vagal pulmonary afferents.