Defective exercise-related expiratory muscle recruitment in patients with PHOX2B mutations: A clue to neural determinants of the congenital central hypoventilation syndrome

INTRODUCTION AND OBJECTIVES

The human congenital central hypoventilation syndrome (CCHS) is caused by mutations in the PHOX2B (paired-like homeobox 2B) gene. Genetically engineered PHOX2B rodents exhibit defective development of the brainstem retrotrapezoid nucleus (RTN), a carbon dioxide sensitive structure that critically controls expiratory muscle recruitment. This has been linked to a blunted exercise ventilatory response. Whether this can be extrapolated to human CCHS is unknown and represents the objective of this study.

MATERIALS AND METHODS

Thirteen adult CCHS patients and 13 healthy participants performed an incremental symptom-limited cycle cardiopulmonary exercise test. Responses were analyzed using guideline approaches (ventilation V'E, tidal volume VT, breathing frequency, oxygen consumption, carbon dioxide production) complemented by a breathing pattern analysis (i.e. expiratory and inspiratory reserve volume, ERV and IRV).

RESULTS

A ventilatory response occurred in both study groups, as follows: V'E and VT increased in CCHS patients until 40 W and then decreased, which was not observed in the healthy participants (p<0.001). In the latter, exercise-related ERV and IRV decreases attested to concomitant expiratory and inspiratory recruitment. In the CCHS patients, inspiratory recruitment occurred but there was no evidence of expiratory recruitment (absence of any ERV decrease, p<0.001).

CONCLUSIONS

Assuming a similar organization of respiratory rhythmogenesis in humans and rodents, the lack of exercise-related expiratory recruitment observed in our CCHS patients is compatible with a PHOX2B-related defect of a neural structure that would be analogous to the rodents' RTN. Provided corroboration, ERV recruitment could serve as a physiological outcome in studies aiming at correcting breathing control in CCHS.

L’atteinte de structures neurologiques centrales explique-elle le défaut de perception des sensations respiratoires constaté au cours de l’infection à SARS CoV 2 ?

Introduction

Méthodes

Résultats et Conclusion

Neuroergonomic and psychometric evaluation of full-face crew oxygen masks respiratory tolerance: a proof-of-concept study

INTRODUCTION

Preventing in-flight hypoxia in pilots is typically achieved by wearing oxygen masks. These masks must be as comfortable as possible to allow prolonged and repeated use. The consequences of mask-induced facial contact pressure have been extensively studied, but little is known about mask-induced breathing discomfort. Because breathlessness is a strong distractor and engages cerebral resources, it could negatively impact flying performances.

METHODS

Seventeen volunteers (age 20-32) rated respiratory discomfort while breathing with no mask and with two models of quick-donning full-face crew oxygen masks with regulators (mask A, mask B). Electroencephalographic recordings were performed to detect a putative respiratory-related cortical activation in response to inspiratory constraint (experiment 1, n=10). Oxygen consumption was measured using indirect calorimetry (experiment 2, n=10).

RESULTS

With mask B, mild respiratory discomfort was reported significantly more frequently than with no mask or mask A (experiment 1: median respiratory discomfort on visual analogue scale 0.9 cm (0.5-1.4), experiment 1; experiment 2: 2 cm (1.7-2.9)). Respiratory-related cortical activation was present in 1/10 subjects with no mask, 1/10 with mask A and 6/10 with mask B (significantly more frequently with mask B). Breathing pattern, sigh frequency and oxygen consumption were not different.

CONCLUSIONS

In a laboratory setting, breathing through high-end aeronautical full-face crew oxygen masks can induce mild breathing discomfort and activate respiratory-related cortical networks. Whether or not this can occur in real-life conditions and have operational consequences remains to be investigated. Meanwhile, respiratory psychometric and neuroergonomic approaches could be worth integrating to masks development and evaluation processes.

Respiratory neuromodulation in patients with neurological pathologies: for whom and how?

Implanted phrenic nerve stimulation is a technique restoring spontaneous breathing in patients with respiratory control failure, leading to being dependent on mechanical ventilation. This is the case for quadriplegic patients with a high spinal cord injury level and for patients with congenital central hypoventilation syndrome. The electrophysiological diaphragm explorations permits better patient selection, confirming on the one hand a definite issue with central respiratory command and on the other hand the integrity of diaphragmatic phrenic nerves. Today there are two different phrenic stimulation techniques: the quadripolar intrathoracic stimulation and the bipolar intradiaphragmatic stimulation. Both techniques allow patients to be weaned off their mechanical ventilator, improving dramatically their quality of life. In fact, one of the systems (phrenic intradiaphragmatic stimulation) was granted social security reimbursement in 2009, and now both are reimbursed. In the future, phrenic intradiaphragmatic stimulation may find its place in the intensive care unit, for patients needing it temporarily, for example, after certain surgeries with respiratory complications as well as diaphragmatic atrophies induced by prolonged mechanical ventilation.

Effects of muscle group recruitment on sniff transdiaphragmatic pressure and its components

Measuring maximal sniff pressures is an easy way of assessing inspiratory muscle strength. During a static manoeuvre, the pattern of inspiratory muscle recruitment during a sniff can vary from one individual to another. We therefore assessed how voluntarily changing muscle recruitment would affect sniff oesophageal, gastric and transdiaphragmatic pressures (Pes,sn, Pga,sn and Pdi,sn, respectively). Ten normal subjects (age 27-38 years) performed natural sniff manoeuvres ("nat"), and preferentially diaphragmatic ("dia") or extradiaphragmatic ("extradia") sniff manoeuvres, after having learnt to dissociate between the inspiratory muscle groups. Abdominal displacements were monitored using a belt-mounted strain gauge. Natural patterns of muscle recruitment varied among subjects. On average, Pes,sn,nat was [median (range)] 81 (21-105) cmH2O. All of the subjects were able to modify inspiratory muscle recruitment voluntarily. Pes,sn was not significantly affected by the type of manoeuvre performed, as opposed to Pdi,sn, which, as expected, increased with both the diaphragmatic and extradiaphragmatic manoeuvres [Pdi,sn,dia 132 (99-157) cmH2O, Pdi,sn,extradia 96 (50-146) cmH2O, P<0.05]. Whatever the manoeuvre, there was no correlation between Pes and Pdi, but Pga and Pdi were correlated during both the diaphragmatic (r = 0.82, P < 0.05) and the extradiaphragmatic manoeuvre (r = 0.70, P < 0.05). Pes,sn may have limitations as an index of diaphragm function, but by showing its independence from inspiratory muscle recruitment, this study contributes to its validation as a robust index of global inspiratory muscle strength that is particularly well suited for follow-up studies. This should extend to Pes,sn substitutes measured at the airway opening.