Pulmonary afferent activity during high-frequency ventilation at constant mean lung volume

Ventilation induced apnea and its effect on dorsal brainstem inspiratory neurones in the rat

The purpose of this study was to examine the effect of mechanical ventilation (MV) on inherent breathing and on dorsal brainstem nucleus tractus solitarius (NTS) respiratory cell function. In pentobarbitone-anaesthetised rats, application of MV at combined high frequencies and volumes (representing threshold levels) produced apnea. The apnea persisted as long as MV was maintained at or above the threshold frequency and volume. Following removal of MV, inherent breathing did not resume immediately, with the diaphragm exhibiting post-mechanical ventilation apnea. The fall in arterial P(CO2) (Pa(CO2)) levels evoked by MV-engendered hyperventilation was shown not to be the trigger for initiation of apnea. MV-induced apnea was immediately reversed by bilateral vagotomy. Further, MV-induced apnea could not be evoked in bilaterally vagotomized animals suggesting that vagal feedback is the critical pathway for its initiation. NTS inspiratory neurones were inhibited during both MV-induced apnea and post-mechanical ventilation apnea, implying the involvement of central neural mechanisms in mediating this effect.

Discharge of pulmonary rapidly adapting stretch receptors during HFO ventilation

High-frequency oscillatory ventilation (HFOV) has been shown to stimulate slowly adapting pulmonary stretch receptors (PSR) and thereby to inhibit spontaneous breathing, i.e. HFOV prolongs expiration or even elicits normocapnic apnea. However, during HFOV respiratory effects possibly mediated by pulmonary rapidly adapting receptors (RAR) have also been observed, e.g., diaphragmatic activation or augmented breaths. Therefore, we analyzed HFOV-induced changes in RAR activity in anaesthetized rabbits by mean of single fibre preparations of vagal RAR afferents. HFOV was applied in several combinations of airway pressure (Paw) and oscillation frequency (fOsc). In the sample of 60 RAR fibres prepared in 20 rabbits we found a wide spectrum of discharge patterns during HFOV. The inspiratory discharge rate during HFOV was increased in 38, decreased in 10, and unchanged in 12 RAR. The expiratory discharge rate was increased in 34, decreased in 17, and unchanged in 9 RAR. The effects of gradually changing Paw or of fOsc during HFOV were different in different fibres. In 17 fibres both inspiratory and expiratory discharge rates rose with increasing Paw during HFOV, whereas 19 fibres were not affected by increasing Paw. In some fibres either the inspiratory (12) or the expiratory (9) activity was inhibited in proportion to increasing Paw. From these results we conclude, that (a) the changes of RAR activity during HFOV are heterogeneous and the reflex effects of RAR stimulation may be balanced by RAR with decreased activity; (b) this heterogeneity of RAR discharge patterns explains the dominancy in the control of breathing during HFOV of the homogeneously stimulated PSR; and (c) depending on HFOV ventilatory parameters used the overall RAR stimulation may be strong enough to overrule the inspiration-inhibiting effects of PSR.

Reflex apnea induced by high-frequency oscillatory ventilation in rabbits

In rabbits with intact vagus nerves, HFOV applied for 10-20 s caused apnea (i.e., respiratory arrest for as long as HFOV lasted) accompanied by tonic discharges of the diaphragm. To identify the vagal mechanisms involved in this type of apnea, the vagus nerves of anaesthetized rabbits were gradually cooled from 37 degrees C to 0 degree C, i.e., the vagal fibres were, corresponding to their diameter, successively blocked. At each temperature, the effects of HFOV on spontaneous breathing were compared with those of static lung inflation and deflation: Between 20 degrees C and 14 degrees C, the lung inflation reflex (mediated by pulmonary slowly adapting stretch receptors = PSR) was weakened or abolished, whereas the lung deflation reflex (mediated by rapidly adapting stretch receptors = RAR) was reinforced; the HFOV-induced apnea occurred less frequently, however, the accompanying diaphragmatic activity was enhanced. Between 14 degrees C and 5 degrees C, both HFOV and large static inflation caused a slight increase of breathing frequency in the majority of animals. Some animals, however, responded even below 14 degrees C by apnea to both HFOV and inflation, and, under these conditions, both HFOV- and inflation-induced apnea were accompanied by a pronounced tonic diaphragmatic activity. At 5 degrees C, the effects of HFOV as well as of inflation (except in two animals) and deflation were abolished. From the results we conclude that in rabbits the apnea during HFOV is mainly mediated by stimulation of PSR, and the concomitant tonic activity of the diaphragm is mainly due to stimulation of RAR, as it is reinforced with gradual blockade of PSR fibres and abolished when only non-myelinated fibres are intact.

The role of chemical (CO2) drive in the apnea induced by high frequency ventilation in the cat

The purpose of the present study was to compare the relative roles of PaCO2 and pulmonary mechanoreceptors in the generation of apnea during high frequency ventilation (HFV) and conventional mechanical ventilation (CMV) in the anesthetized cat. Our hypothesis was that PaCO2 primarily determines the appearance of apnea, while mechanoreceptor input plays an important but secondary role. We calculated the tidal volume which would have a 50% chance of inducing apnea (VT50) for each combination of ventilator settings, and used it as a standard for comparison of mechanical dynamic inputs. When either 2% or 5% CO2 was added to the bias flow during HFV, the VT50 was significantly increased over that seen with a room air bias flow. Apnea was observed during either mode of ventilation only when PaCO2 was reduced below normocapnic levels. The mean tidal volume associated with each apnea group was larger than that for the corresponding nonapnea group. There was no difference in the PaCO2 for apnea between HFV and CMV, but the EMG of the diaphragm did differ. With HFV-apnea tonic activity was observed, while CMV-apnea showed abolition of all activity. We conclude that both HFV- and CMV-induced apnea depend primarily on lowering the chemical drive (PaCO2), and secondarily on inhibitory mechanical input (VT). The excitation of rapidly adapting receptors during HFV could explain the tonic EMG activity.

Blockade of pulmonary stretch receptors reinforces diaphragmatic activity during high-frequency oscillatory ventilation

During apneic periods elicited by high-frequency oscillatory ventilation (HFOV) a tonic diaphragmatic activity was observed, contrasting with the absence of diaphragmatic activity during apnea induced by lung inflation. To clarify the mechanism underlying the persistence of the diaphragmatic activity during HFOV-induced arrest of breathing the reflex responses to short periods of HFOV, and to periods of lung inflation with airway pressure (Paw) equal to the mean Paw and/or to maximal Paw during HFOV were examined both before and after the blockade of slowly adapting stretch receptors (SR) by inhalation of sulphur dioxide (SO2) in anaesthetized rabbits. In animals with intact SR, the HFOV-induced reflex apnea lasted longer than that induced by lung inflation, the associated diaphragmatic activity being in the most cases higher than the diaphragmatic activity during quiet expiration; inflation, however, completely inhibited diaphragmatic activity. After blockade of SR, spontaneous breathing continued during periods of lung inflation, i.e., the Hering-Breuer inflation reflex was abolished, whereas HFOV still led to a cessation of spontaneous breathing, the associated diaphragmatic activity even exceeding the level observed during quiet inspiration. From these results we conclude that only one part of the reflex response to HFOV is due to SR-stimulation and that in addition other vagal pulmonary receptors (irritant- and/or C-fibre-receptors) are involved. The stimulation of the latter counterbalances the concomitant stimulation of SR, giving rise to the tonic activity of the diaphragm.

Suppression of spontaneous breathing during high-frequency jet ventilation. Separate effects of lung volume and jet frequency

The effect of ventilatory frequency of high-frequency jet ventilation (HFJV) from 1 to 5 Hz, apart from changes in thoracic volume, on spontaneous breathing activity was studied in Yorkshire piglets under pentobarbital anesthesia. The highest PaCO2 at which the animals did not breathe against the ventilator (apnea point) was established either by changing minute volume of ventilation or by adding CO2 to the respiratory gas. The higher the apnea point, the higher the suppression of spontaneous breathing activity was assumed to be. If the apnea point was searched for by changing minute volume a progressive increase of suppression of spontaneous respiratory activity was found at ventilatory rates of 3 Hz or more, concomitantly with a rise in end-expiratory pressure (PEE). In case the tidal volume was kept constant, increase of ventilatory rate resulted in a tremendous increase of lung volume, together with considerably higher levels of PEE. When under these conditions the apnea point was searched for by adding CO2 to the respiratory gas a much higher CO2-drive was needed for spontaneous breathing and therefore a much stronger inhibition of spontaneous breathing was concluded. By placing the animals in a body box in which pressure could be varied, thoracic volume could be kept constant during HFJV. When thoracic volume was kept constant in this way a constant tidal volume at increasing jet frequencies resulted in only a slight increase in suppression of spontaneous breathing. We conclude that the increase in lung volume is a major factor in suppressing central respiratory activity during HFJV. Jet frequency by itself might be an additional suppressive factor.(ABSTRACT TRUNCATED AT 250 WORDS)