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

Use-dependent learning and memory of the Hering-Breuer inflation reflex in rats

The classic Hering-Breuer inflation reflex (HBIR) is a widely held tenet for understanding the lung volume-related vagal control of respiratory rhythm. Recent evidence, however, has revealed that the fictive HBIR elicited by electrical vagal stimulation in rats is not static but may be attenuated centrally by two forms of non-associative learning (habituation and desensitization) that continually mitigate the reflex effects with exponential adaptations like a differentiator or high-pass filter. Desensitization is analogous to habituation but exhibits an explicit short-term memory (STM) in the form of a rebound response with exponential decay during recovery from stimulation. To investigate whether such learning and memory effects are lung volume related and use-dependent (practice makes perfect), we compared the time-dependent changes in inspiratory and expiratory durations (t(I) and t(E)) during and after 1 or 8 min unilateral lung inflation or high-frequency, low-intensity vagal stimulation in anaesthetized, uni- or bi-vagotomized rats. Unilateral lung inflation and vagal stimulation both elicited abrupt shortening of t(I) and lengthening of t(E) (HBIR effects) and gradual habituation and desensitization throughout the 1 or 8 min test period, followed by rebound responses in t(I) and t(E) with exponential recovery (STM effects) in the post-test period. In both cases, the STM time constants for t(I) and t(E) were significantly longer with the 8 min test than with the 1 min test (17-45 versus 4-11 s, P < 0.01). We conclude that the HBIR and its central habituation and desensitization are mediated peripherally by lung volume-related vagal afferents, and that the STM of desensitization is use-dependent. The translational implications of these findings are discussed.

Effect of slow wave and REM sleep on thyropharyngeus and stylopharyngeus activity during induced central apneas

The pharyngeal constrictors have been hypothesized to play an important role in the regulation of upper airway (UAW) patency in patients with sleep apnea. However, little research has focused on the activation and control of muscles that determine the lateral and posterior wall of the retropalatal airway dimensions. Our aim was to investigate the effects of slow wave sleep (SWS) and rapid eye movement (REM) sleep on the activation of pharyngeal constrictor (thyropharyngeus; TP) and dilator (stylopharyngeus; SP) muscles during eupneic breathing and induced central apneas. In nine goats, we found that eupneic TP and SP activity progressively decreased from awake to SWS (57 and 56%, respectively; P<0.01) and further in REM (25.6 and 19.9%, respectively; P<0.01). In contrast, diaphragm activity decreased equally during SWS and REM (89.3 and 87.7%, respectively; P<0.01) compared to awake. Following induced apneas while SP activity was eliminated in every state, maximal TP activity was highest in awake state (318.6% of control; P<0.02), less in SWS (157.6%; P<0.02), and nearly absent in REM (117.3%; P>0.02). During the recovery from an induced apnea when diaphragm activity was at 95% of its' control, awake TP activity remained significantly elevated and SP reduced (P>0.02) while TP activity during SWS was elevated and SP had returned to control level. During REM, TP and SP activity were not different from their reduced controls (P>0.02). The data supports our hypotheses that SWS and REM sleep causes a reduction in the eupneic TP and SP activity, as well as a reduction in TP response to induced apneas. However, the relative imbalance in TP vs SP activity during the recovery from an apnea (awake and SWS) suggest that an imbalance of active neuromuscular forces may contribute to upper airway narrowing in mixed apneas, but not in central apnea during sleep.

Contribution of multiple inert gas elimination technique to pulmonary medicine. 1. Principles and information content of the multiple inert gas elimination technique

This introductory review summarises four different aspects of the multiple inert gas elimination technique (MIGET). Firstly, the historical background that facilitated, in the mid 1970s, the development of the MIGET as a tool to obtain more information about the entire spectrum of VA/Q distribution in the lung by measuring the exchange of six gases of different solubility in trace concentrations. Its principle is based on the observation that the retention (or excretion) of any gas is dependent on the solubility (lambda) of that gas and the VA/Q distribution. A second major aspect is the analysis of the information content and limitations of the technique. During the last 15 years a substantial amount of clinical research using the MIGET has been generated by several groups around the world. The technique has been shown to be adequate in understanding the mechanisms of hypoxaemia in different forms of pulmonary disease and the effects of therapeutic interventions, but also in separately determining the quantitative role of each extrapulmonary factor on systemic arterial PO2 when they change between two conditions of MIGET measurement. This information will be extensively reviewed in the forthcoming articles of this series. Next, the different modalities of the MIGET, practical considerations involved in the measurements and the guidelines for quality control have been indicated. Finally, a section has been devoted to the analysis of available data in healthy subjects under different conditions. The lack of systematic information on the VA/Q distributions of older healthy subjects is emphasised, since it will be required to fully understand the changes brought about by diseases that affect the older population.

Activities of pulmonary stretch receptors during ventilatory cycles without lung inflation

When lung inflation is temporarily withheld in paralyzed, ventilated cats with intact vagi, the activities of inspiratory motor nerves are greater during the second cycle without inflation than during the first. This response is not easily attributable to increasing drive from chemoreceptors as it is abolished by vagotomy. We examined the hypothesis that the increasing inspiratory activity is the result of decreasing inhibitory feedback from pulmonary stretch receptors (PSRs). Decerebrate, paralyzed cats were ventilated by a servo-respirator in accordance with their own phrenic nerve activity. Afferent activities from individual PSRs were recorded from a few cut fibers of one vagus nerve; the vagi were otherwise intact. When lung inflation was withheld, phrenic and hypoglossal nerve activities and the durations of inspiration and expiration all increased and were significantly greater during the second cycle without inflation than during the first. The frequency of PSR discharge was also greater during the second cycle and thus did not account for the responses recorded from the motor nerves. We conclude that the latter responses probably reflect neural processes within the brain stem, involving a persistent inhibitory influence from lung inflation, which outlasts the inflation itself.

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.