Effect of rib cage or abdomen compression at iso-lung volume on breathing pattern

A vagal reflex evoked by airway closure

Airway integrity must be continuously maintained throughout life. Sensory neurons guard against airway obstruction and, on a moment-by-moment basis, enact vital reflexes to maintain respiratory function1,2. Decreased lung capacity is common and life-threatening across many respiratory diseases, and lung collapse can be acutely evoked by chest wall trauma, pneumothorax or airway compression. Here we characterize a neuronal reflex of the vagus nerve evoked by airway closure that leads to gasping. In vivo vagal ganglion imaging revealed dedicated sensory neurons that detect airway compression but not airway stretch. Vagal neurons expressing PVALB mediate airway closure responses and innervate clusters of lung epithelial cells called neuroepithelial bodies (NEBs). Stimulating NEBs or vagal PVALB neurons evoked gasping in the absence of airway threats, whereas ablating NEBs or vagal PVALB neurons eliminated gasping in response to airway closure. Single-cell RNA sequencing revealed that NEBs uniformly express the mechanoreceptor PIEZO2, and targeted knockout of Piezo2 in NEBs eliminated responses to airway closure. NEBs were dispensable for the Hering-Breuer inspiratory reflex, which indicated that discrete terminal structures detect airway closure and inflation. Similar to the involvement of Merkel cells in touch sensation3,4, NEBs are PIEZO2-expressing epithelial cells and, moreover, are crucial for an aspect of lung mechanosensation. These findings expand our understanding of neuronal diversity in the airways and reveal a dedicated vagal pathway that detects airway closure to help preserve respiratory function.

Sex-specific vagal and spinal modulation of breathing with chest compression

Lung volume is modulated by sensory afferent feedback via vagal and spinal pathways. The purpose of this study was to systematically alter afferent feedback with and without a mechanical challenge (chest compression). We hypothesized that manipulation of afferent feedback by nebulization of lidocaine, extra-thoracic vagotomy, or lidocaine administration to the pleural space would produce differential effects on the motor pattern of breathing during chest compression in sodium pentobarbital anesthetized rats (N = 43). Our results suggest that: 1) pulmonary stretch receptors are not the sole contributor to breathing feedback in adult male and female rats; 2) of our manipulations, chest compression had the largest effect on early expiratory diaphragm activity (“yield”); 3) reduction of spinally-mediated afferent feedback modulates breathing patterns most likely via inhibition; and 4) breathing parameters demonstrate large sex differences. Compared to males, female animals had lower respiratory rates (RR), which were further depressed by vagotomy, while chest compression increased RR in males, and decreased yield in females without changing RR. Collectively, our results suggest that balance between tonic vagal inhibition and spinal afferent feedback maintains breathing characteristics, and that it is important to specifically evaluate sex differences when studying control of breathing.

Ventilatory load compensation response to long-term chest compression in rat model

Short-term chest compression has been shown to decrease tidal volume and increase respiratory frequency. The present study was designed to assess and characterize the effect of long-term chest compression on breathing pattern and blood gases in awake rats. Chest compression was carried out by inflating a pneumatic cuff placed around the chest to a pressure of 25 mmHg and the pressure was maintained for 28 days. Respiratory frequency increased progressively until 14 days after chest compression whereas a decrease in tidal volume was stabilized within 3 days after chest compression. Although the changes in minute ventilation were small and no substantial change in Pa(CO2) was observed, an impairment of weight gain and a decrease in body temperature with a concomitant hypoxemia were evident during sustained chest compression. These observations suggest that the ventilatory response to chest compression may involve not only neural reflex mechanisms but also other non-reflex mechanisms. Sustained chest compression possibly impairs growth and metabolism.

Effects of abdominal distension on breathing pattern and respiratory mechanics in rabbits

The effects of acute abdominal distension (AD) on the electromechanical efficiency (Eff) of the inspiratory muscles were investigated in anesthetized rabbits by recording the electrical activity (A), pressure (P) exerted by the diaphragm (di) and parasternal intercostal muscles (ic), and lung volume changes when an abdominal balloon was inflated to various degrees. Eff,ic increased with increasing AD both in supine and upright postures. In upright rabbits Eff,di increased for intermediate but decreased at higher levels of AD, whilst it decreased at all levels of AD in supine rabbits. Tidal volume (VT) response followed that of Eff,di. Tonic Aic and Adi and inspiratory prolongation were elicited by AD. The effects of these neural mechanisms, acting to limit end-expiratory lung volume and VT changes, were however small since vagotomy prevented tonic Adi and inspiratory prolongation and reduced tonic Aic, but changed lung volume responses to AD only little. Hence, reduced respiratory system compliance and changes in inspiratory muscle electromechanical efficiency dominate lung volume responses to acute AD.

Characterisation of the Hering-Breuer deflation reflex in the human neonate

Human infants have been observed making inspiratory efforts in response to chest compression. These may be a manifestation of the Hering-Breuer deflation reflex. We sought to stimulate the reflex in 33 term infants by rapidly reducing lung volume using an inflatable jacket. The effect of altering the timing, magnitude or rate of application of the lung deflation on the strength of the inspiratory response was investigated. Inspiratory effort was quantified by measuring (1) the rate of fall in oesophageal pressure on inspiration; and (2) the mean inspiratory flow (MIF) in response to lung deflation. Variables which significantly affected (1) and resulted in increased inspiratory effort were, in order of importance: larger rises in oesophageal pressure on chest compression (38%) (percentage of variance explained), greater reductions in lung volume below functional residual capacity (FRC) (26%), faster rates of lung deflation (19%) and slower respiratory rates (11%). Increased inspiratory efforts, as assessed by response (2), were generated by greater reductions in FRC (23%), larger rises in oesophageal pressure (11%) and faster rates of lung deflation (10%). Increasing deflation pressures eventually resulted in a plateau in both measures of inspiratory response. These results were consistent with the Hering-Breuer deflation reflex being activated which could have a role in protecting the FRC of the newborn infant.

Correlation structure of end-expiratory lung volume in anesthetized rats with intact upper airway

The correlation structure of breath-to-breath fluctuations of end-expiratory lung volume (EEV) was studied in anesthetized rats with intact airways subjected to positive and negative transrespiratory pressure (i.e., PTRP and NTRP, correspondingly). The Hurst exponent, H, was estimated from EEV fluctuations using modified dispersional analysis. We found that H for EEV was 0.5362 +/- 0.0763 and 0.6403 +/- 0.0561 with PTRP and NTRP, respectively (mean +/- SD). Both H were significantly different from those obtained after random shuffling of the original time series. Also, H with NTRP was significantly greater than that with PTRP (P = 0.029). We conclude that in rats breathing through the upper airway, a positive long-term correlation is present in EEV that is different between PTRP and NTRP.

Respiratory sensation during chest wall restriction and dead space loading in exercising men

We mimicked important mechanical and ventilatory aspects of restrictive lung disorders by employing chest wall strapping (CWS) and dead space loading (DS) in normal subjects to gain mechanistic insights into dyspnea causation and exercise limitation. We hypothesized that thoracic restriction with increased ventilatory stimulation would evoke exertional dyspnea that was similar in nature to that experienced in such disorders. Twelve healthy young men [28 +/- 2 (SE) yr of age] completed pulmonary function tests and maximal cycle exercise tests under four conditions, in randomized order: 1) control, 2) CWS to 60% of vital capacity, 3) added DS of 600 ml, and 4) CWS + DS. Measurements during exercise included cardiorespiratory parameters, esophageal pressure, and Borg scale ratings of dyspnea. Compared with control, CWS significantly reduced the tidal volume response to exercise, increased dyspnea intensity at any given work rate or ventilation, and thus limited exercise performance. DS stimulated ventilation but had minimal effects on dyspnea and exercise performance. Adding DS to CWS further increased dyspnea by 1.7 +/- 0.6 standardized Borg units (P = 0.012) and decreased exercise performance (total work) by 21 +/- 6% (P = 0.003) over CWS alone. Across conditions, increased dyspnea intensity correlated best with decreased resting inspiratory reserve volume (r = -0.63, P < 0.0005). Dyspnea during CWS was described primarily as "inspiratory difficulty" and "unsatisfied inspiration," similar to restrictive disorders. In conclusion, severe dyspnea and exercise intolerance were provoked in healthy normal subjects when tidal volume responses were constrained in the face of increased ventilatory drive during exercise.

Control of ventilation during lung volume changes and permissive hypercapnia in dogs

We investigated the effect changes in end-expiratory lung volume (EEVL) had on the response to progressive hypercapnia (CO2-response curve) in eight open-chest, anesthetized dogs, in order to clarify the role that vagal lung mechanoreceptors have in altered respiratory drive during permissive hypercapnia. The dogs were ventilated using a positive-pressure ventilator driven by phrenic neural activity. Systemic arterial CO2 tension (PaCO2) was elevated by increasing the fraction of CO2 delivered to the ventilator. EEVL was altered from approximated functional residual capacity ("FRC") to 1.5 and 0.5 "FRC" by changing positive end-expiratory pressure. Although the tidal volume (VT)-PaCO2 and inspiratory time (TI)-PaCO2 relationships were not affected, decreasing EEVL from 1.5 "FRC" to "FRC" and then to 0.5 "FRC" caused a significant (p < 0.01) upward shift in the CO2-response curves for minute ventilation (V I) and frequency (f ), and a significant (p < 0.01) downward shift in the CO2- response curve for expiratory time (TE). We conclude that these shifts were explained by a decrease in the inhibitory activity of slowly adapting pulmonary stretch receptors (PSRs) as EEVL was lowered. In addition, increases in EEVL from 0.5 "FRC" to 1.5 "FRC" caused a significant (p < 0.05) increase in the apneic threshold, which we attribute to an inhibitory effect on central drive caused by increased PSR activity.

Control of breathing during acute hemorrhage in anesthetized cats

We studied in anesthetized cats the ventilatory response to acute blood loss under hyperoxic iso-capnic conditions. From a blood pressure, of approximately 150 down to approximately 70 mm Hg ventilation increased on the average by 2.5 times. The ventilatory response was characterized by two phases: there was an initial phase (down to a PB approximately 100 mm Hg) of increase in frequency due to an excitatory effect on bulbo-pontine respiratory timing which was estimated from the duration of breaths following occlusion of the airways at the end expiratory volume. This effect was mainly due to the withdrawal of inhibitory afferents from the baroreceptors of the aortic arch. To this phase corresponded the phase of vasomotor compensation. Subsequently there was a phase of excitatory effect on the respiratory output which was estimated from the rate of change of the pressure developed in the airways during occluded breaths. This effect was mainly due to afferents from carotid sinuses; a minor role was due to the decrease in inhibitory afferents from carotid baroreceptors while the greater part was likely to be due to the hypoxic stimulation of glomus cells due to reduced blood flow. Following vagotomy above the superior laryngeal and sinus denervation the excitatory effect on respiratory timing and output were reduced to about 30% of that observed in intact cats.

Involvement of thoracic nerve afferents in the respiratory response to chest compression

Chest compression elicits extravagal neural reflexes which can alter the respiratory pattern. Experiments were conducted to determine the source of the afferents responsible for the respiratory response to chest compression (CC). The effects of CC on VT, f, TI, TE, blood gases, end-tidal CO2, and blood pressure were studied in anesthetized, vagotomized dogs and cats. In dogs, thoracic wall afferents were eliminated by thoracic dorsal rhizotomies (TDR) and/or spinal blocks (SB). There were two different respiratory responses to CC. In one (I), Tt decreased and TE increased, resulting in a decreased f. The second (II) resulted in a decreased TI and TE. The I response was still present, but weaker, in animals after TDR (1--4), TDR (5--9), TDR (1--9, T5 or T10 SB and absent in those with T1SB. The II response was still present after TDR ()--4), TDR (5--9), TDR (1--9), or T10SB and absent after T5SB. The results indicate that: (1) afferents responsible for the I response to CC arise from the upper, middle and lower thoracic wall, (2) afferents responsible for the II response arise from the middle and lower thoracic wall, and (3) the responses are not due to changes in chemical drive, blood pressure or lung receptors.

Central and direct vagal dependent control of expiratory duration in anaesthetized rabbits

In anaesthetized rabbits, total or partial (only inflation reflex nearly abolished) DC current vagal block was performed during inspiration (ITB and IPB), or expiration (ETB and EPB), or throughout the breathing cycle (CTB and CPB). During CTB inspiratory (Ti) and expiratory duration (Te) increased as after vagotomy. With ITB Ti equally Tivag; Te increased, but remained shorter than Tevag. During ETB, Ti was unchanged, Te increased, but remained shorter than Tevag. The sum of deltaTe during ITB and ETB equalled deltaTevag. During CPB and IPB, Ti and Te behaved as during ITB. With EPB, Ti was unchanged and Te shortened. Preferential stimulation of large myelinated fibers in the central vagal stumps during expiration lengthened Te. Inspiratory stimulation shortened both Ti and Te, restored breath timing of ETB, but not that of pre-vagotomy control. Hence, Te of eupneic breaths should depend on a central mechanism relating Te to preceding Ti and on expiratory vagal discharge, having both a small lengthening (from stretch receptors) and a variable shortening effect (from irritant receptors). Both central and peripheral mechanisms are affected by CO2 breathing.

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.

Effects of single breath lung inflation on the pattern of subsequent breaths

In rabbits, on release of lung inflations 0.4 to 3.3 times control VT and lasting 1 to 30 sec, VT, peak diaphragmatic activity (Ep) and inspiratory duration (Ti) increased, whereas expiratory duration (Te) decreased relative to pre-inflation values. Similar changes occurred between pre- and postinflation occluded breaths. These changes lasted from a few breaths up to 30 sec, and were positively correlated with magnitude and duration of inflations. Postinflation changes of pulmonary stretch receptor activity were relatively small and limited to 1-3 breaths. At chemical drive close to control: (a) postinflation VT vs Ti relationship moved to the right without changing its slope, Ti occluded eventually exceeding Ti after vagotomy; (b) the Te vs Ti relationship moved downwards, its slope being decreased and eventually abolished; (c) the average rate of rise of E was decreased. An increase of VT, Ep and Ti, and a decrease Te also occurred on release of stimulation of the central ends of the cut vagi producing apnea at FRC in mono- and bilateral vagotomized rabbits. Postinflation effects were mainly of central origin and tentatively explained as rebound phenomena within the respiratory center.

Effects of uneven elastic loads on breathing pattern of anesthetized and conscious men

In anesthetized subjects rib cage strapping (RCS) did not change tidal volume (VT) and increased ventilation (V), whereas abdomen strapping (AS) markedly decreased VT and V. Both kinds of strapping decreased expiratory duration (TE), but did not change inspiratory duration (TI) and breathing rate. RCS and AS decreased lung volume by about 200 ml and increased the elastance of the repiratory system by 12 cm H2O/1 and 9 CM H20/l, repectively. The changes produced are mainly due to mechanical factors, although reflexes also seem to be operating in some cases. In conscious subjects RCS decreased VT, TI, TE and did not change V, whereas AS did not change these parameters. The different changes in conscious and anesthetized subjects show the effects of cortical influences, which also partly explain the differen effects elicited in conscious subjects by RCS and AS. The effects produced by RCS are mainly due to the sensation of hindrance to rib cage expansion, rather than to that of rib cage squeezing, as shown by experiments of RCS without reduction of rib cage volume.