Time course of respiratory mechanics and pulmonary structural remodelling in acute lung injury

The aim of this study was to evaluate the time course of in vivo and in vitro respiratory mechanics and examine whether these parameters could reflect the temporal changes in lung parenchyma remodelling in paraquat (PQ)-induced lung injury. Measurements were done 1, 3 and 8 weeks after the intraperitoneal (i.p.) injection of saline (control) or paraquat (7mgkg(-1)) in rats. Airway and tissue resistances increased from control in PQ1 and PQ3 and returned to control values in PQ8, in accordance with the magnitude of bronchoconstriction. Viscoelastic/inhomogeneous pressure, tissue elastance, the number of polymorphonuclear cells, and collagen fibre content in lung parenchyma increased in PQ1 and remained elevated in PQ3 and PQ8. Static elastance increased in PQ1, returned to control values after 3 weeks, and was correlated with the volume fraction of collapsed alveoli. In conclusion, there is a restoration of normal alveolar-capillary lung units with a gradual improvement in airway and tissue resistances and static elastance. However, the on-going fibrotic process kept elevated tissue elastance and viscoelastic/inhomogeneous pressure.     

Evaluation of respiratory mechanics and lung histology in a model of atelectasis

To develop a reproducible model of atelectasis, 15 mechanically ventilated Wistar rats were wrapped around the thorax/abdomen with a sphygmomanometer. The cuff was inflated to transpulmonary pressures (PL) of -4 cmH2O (group A) and -8 cmH2O (group B) for 5 sec. Group C was not compressed. Airflow, volume, tracheal and oesophageal pressures were registered. Respiratory system (rs), lung (L), and chest wall resistive (DeltaP1), viscoelastic/inhomogeneous pressures (DeltaP2), DeltaPtot (=DeltaP1 + DeltaP2), static (Est) and dynamic (Edyn) elastances, and DeltaE (=Edyn - Est) were determined before and after compression. In A, respiratory mechanics remained unaltered. In B, Est,rs (+99%), Est,L (+111%), DeltaE,rs (+41%), DeltaE,L (+73%), DeltaP1,rs (+45%), DeltaP1,L (+44%), DeltaP2,rs (+41%), DeltaP2,L (+69%), DeltaPtot,rs (+40%), and DeltaPtot,L (+58%) increased after compression. Mean alveolar diameter and bronchiolar lumen decreased in A, and were even smaller in B. In conclusion, chest wall compression with PL of -8 cmH2O yielded a reproducible alveolar collapse, which resulted in increased elastic, resistive and viscoelastic/inhomogeneous pressures.     

On the preparation of lung strip for tissue mechanics measurement

It is widely believed that it is fundamental to degas and/or rinse the lung prior to the measurement of the tissue mechanics, so that the undesirable effects of surfactant and localized gas trapping are eliminated. However, one could hypothesize that these mechanisms are bound to disappear in the in vitro preparation since the small tissue sample remains suspended oscillating in an organ bath. To investigate the real necessity to follow these procedures, dynamic mechanical properties were studied in strips of lungs previously rinsed with saline, degassed by ventilation with 100% O(2), or without any of these prior procedures. Resistance, elastance, hysteresivity, and the amounts of airway, blood vessel, and alveolar wall were computed. There was no difference in either tissue mechanics or morphology among the groups. In conclusion, the time-consuming degassing and rinsing steps are not necessary to adequately prepare lung tissue for in vitro mechanical analysis, and eliminating these steps potentially helps preserving the intact microstructure of the tissue.     

Effects of dexmedetomidine on respiratory mechanics and control of breathing in normal rats

Dexmedetomidine is a highly selective and specific alpha(2)-adrenergic agonist, with sedative, analgesic, and sympatholytic activities. The aim of the present study was to define the effects of DMED in respiratory mechanics in normal rats. In addition, lung morphometry was studied to determine whether the physiological changes reflected underlying morphological changes defining the sites of action of dexmedetomidine. Arterial blood gases were also determined. Twelve adult Wistar rats were randomly assigned to two groups of six animals each: PENTO and DMED. In PENTO group animals were sedated (diazepam, 5mg, i.p.) and anaesthetised with pentobarbital sodium (20mgkg(-1) i.p.). The rats of the DMED group received dexmedetomidine (250mugkg(-1) i.p. followed by intravenous infusion of 0.5mugkg(-1)h(-1)). In spontaneously breathing rats, minute ventilation, respiratory frequency, and neuromuscular inspiratory drive were lower in dexmedetomidine group, which also presented hypercapnia, whereas tidal volume, inspiratory, expiratory, and total respiratory cycle times were higher in dexmedetomidine group compared to the PENTO group. During mechanical ventilation, respiratory mechanical parameters were similar in both groups. These findings were supported by the absence of histological changes. In conclusion, under the conditions studied, dexmedetomidine did not change respiratory mechanical parameters and lung histology, but induced ventilatory depression.     

Pulmonary mechanics and lung histology in acute lung injury induced by Bothrops jararaca venom

Pulmonary mechanics [static (Est) and dynamic (Edyn) elastances, resistive (DeltaP1) and viscoelastic pressures (DeltaP2)], histology, and bronchoalveolar lavage fluid (BALF) from BALB/c mice were analysed 1, 24, 48 and 72 h after intravenous injection of saline or Bothrops jararaca crude venom [0.3 (V0.3) or 1 (V1) microg.g(-1)]. Est, Edyn, and DeltaP2 increased at 1 h in both V groups, being significantly higher in V1 than in V0.3, decreasing progressively, reaching control values at 48 h in V0.3, but remaining altered in V1 at 72 h. DeltaP1 augmented in V1 at 1 h, returning to normal at 72 h. Histological changes in V0.3 group included interstitial oedema, alveolar collapse, and increased cellularity, which returned to normal at 48 h. These changes were more intense in V1 group, with alveolar oedema and haemorrhage. BALF showed time-dependent neutrophil influx in V0.3. In conclusion, venom led to time- and dose-dependent pulmonary mechanical changes, together with moderate inflammation in V0.3 and acute lung injury in V1.     

Respiratory pattern modulation by the pedunculopontine tegmental nucleus

This study demonstrates respiratory modulation caused by stimulation of the pedunculopontine tegmental nucleus (PPT), a structure not classically included in the pontine respiratory neuronal network. The long-lasting increase in variability of respiratory parameters following glutamate microinjection into PPT in anesthetized, spontaneously breathing Sprague Dawley rats was more pronounced under ketamine than nembutal anesthesia. The induced respiratory perturbations were characterized by intermittent apneas and increased variability of expiratory (TE) and total (TT) breath durations in all animals. Although the baseline spontaneous breathing patterns (mean values of all respiratory parameters and their variabilities) were equivalent under ketamine and nembutal anesthesia, different anesthetic agents did affect respiratory responses to PPT stimulation by glutamate in terms of latency, duration, and structure. We conclude that glutamatergic stimulation of PPT has a significant impact on the brainstem respiratory pattern generator.     

Cardiac activity during airway resistance alterations with intravenous and inhaled methacholine

Measurement of airway contractile responses to methacholine (MCh) is an important investigational tool in humans and mice. However, i.v. administration of MCh in murine models may suffer from potential cardiac sequelae produced by stimulation of cholinergic receptors within cardiac muscle. Therefore, we studied the i.v. (0-10,000 microg/kg) and aerosolized (inhaled; 0-25 mg/ml) administration of MCh, to determine their effects on pulmonary resistance (RL) and cardiac muscle activity (as heart rate; HR) in anesthetized, mechanically-ventilated C57Bl6 mice. MCh, i.v., increased RL but produced: (1) prolonged asystole (29-47 sec); (2) subsequent overshoot of baseline HR; and (3) prolonged HR recovery times (7-25 min), suggestive of sympathetic modulation after cholinergic stimulation. In contrast, inhaled MCh aerosol produced no change in HR, while increasing RL similar to i.v. MCh. These results suggest that, for specific instances utilizing the plethysmographic technique, inhaled MCh aerosol may be superior to i.v. administration, due to the avoidance of potential bouts of asystole that can confound experimental results and lead to premature death of mice.     

Fractal characteristics of breath to breath timing in sleeping infants

We examined interbreath interval (IBI) time series of 19 term infants during active and quiet sleep for fractal properties using Fano factor analysis. For each time series we calculated the fractal exponent (alpha), comparing alpha for the original time series with two forms of surrogate data, a temporally independent surrogate set and an autoregressive surrogate set. alpha values were normally distributed between 0.79 and -0.22, and did not differ with sleep state. The fractal characteristics of the original time series were not retained in the temporally independent surrogate time series indicating that the distribution of intervals alone was not fractal, but were retained using autoregressive surrogates with an order of 10, suggesting that the fractal properties of the IBI time series were related to correlations between successive breaths. These observations suggest that some of the respiratory variability that occurs during sleep in infants, which in the past has been regarded as stochastic noise, may be the product of deterministic processes.     

Relative motion of lung and chest wall promotes uniform pleural space thickness

The pleural space is modeled in two dimensions as a thin layer of fluid separating a deformable membrane and a rigid surface containing a bump. We computed the steady-state membrane configuration and fluid pressure distribution during relative sliding of the two surfaces. For physiologically relevant values of membrane tension, shear flow-induced pressures near the bump and far-field pressure gradients are similar to those measured in vivo within the pleural space (e.g. Lai-Fook et al.) [J. Appl. Physiol.: Respirat. Environ. Exercise Physiol. 56 (1984) 1633-1639]. Deformation of the membrane over the bump suggests that the pressure field generated by the sliding motion promotes an even layer of fluid in the pleural space, preventing asperities from touching. Results also suggest a possible mechanism for pleural fluid redistribution during breathing, whereby irreversible fluid motion is associated with the deformability of the membrane.     

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.     

Improvement after lung volume reduction surgery: a role for inspiratory muscle adaptation

In severe emphysema, lung volume reduction surgery (LVRS) can improve lung function and exercise tolerance. The maximal changes of forced expiratory volume in 1s (FEV(1)) and lung volume occur early after surgery, whereas maximal improvement of exercise tolerance occurs later. We tested the hypothesis that secondary adaptation of inspiratory muscles could explain this delayed clinical improvement. In that purpose, we evaluated nine consecutive patients before LVRS and up to 9 months post-operatively. Six weeks after LVRS, we observed an increase in FEV(1) and 6 min walk distance (6MWD). The gain in sniff nasal inspiratory pressure (SNIP) was inversely proportional to lung volume loss. Values of FEV(1) and lung volume were maintained throughout follow-up whereas SNIP values significantly increased from 6 weeks to 6 months post-LVRS. In the meantime, we observed an increase in 6MWD correlated with the SNIP increase. This suggests that in patients undergoing LVRS, early improvement of SNIP is proportional to decrease in lung volume whereas the further delayed improvement may be due, at least in part, to adaptation of the inspiratory muscles.     

Volume and flow dependence of respiratory mechanics in mechanically ventilated COPD patients

Volume and flow dependencies of respiratory mechanics are examined in 10 COPD patients under mechanical ventilation (MV) at 3 levels of externally applied PEEP (PEEPe). Airways pressure (Paw), flow (V') and volume (V) data are analyzed according to (1) the linear and (2) a non-linear model, accounting for volume dependence of elastance and for flow and volume dependence of resistance. The models' fitness to data is assessed by the regression errors. Non-linear modelling fits significantly better to data, while the difference of fitness decreases with PEEPe. Linear mechanics are not significantly different between the 3 levels of PEEPe. A positive volume dependence of elastance observed at 0, decreases at 5 and increases again at 10 hPa of PEEPe. A seriously negative volume dependence of resistance at 0 turned to positive with PEEPe. These dependencies of respiratory mechanics during COPD under MV, show that the present non-linear respiratory mechanical monitoring may help for better and less risky adjustment of PEEPe.     

Hypothermia-induced respiratory arrest and recovery in neonatal rats

To examine the changes in breathing that occur during progressive hypothermia and rewarming in neonatal rats, we cooled and rewarmed rat pups during the first 6 days of life. During cooling, breathing stopped when rectal temperature (Tr) fell below 10.7+/-0.24 degrees C, and recovered spontaneously during rewarming when Tr reached 13.3+/-0.38 degrees C, regardless of age. During cooling, breathing frequency declined progressively, whereas tidal volume increased until Tr fell below 15 degrees C whence it declined to, but never below, normothermic levels. These data support suggestions that failure occurs at the level of the central rhythm generator for breathing and is not due to an inability to sustain the level of motor output. During rewarming, following respiratory arrest, the pattern of change was reversed, but with a significant thermal hysteresis, resulting in slower breathing and cardiac frequencies at any given rectal temperature during rewarming. There were no effects of age observed over the range studied on the changes in respiratory variables associated with hypothermia or rewarming. Breathing restarted spontaneously on rewarming with no evidence that gasping was required to initiate this process. The overall breathing pattern was episodic during the early stages of rewarming, however, suggesting that the respiratory rhythm is only periodically expressed during the initial stages of recovery from hypothermia.     

Ventilatory pattern and chemosensitivity in M1 and M3 muscarinic receptor knockout mice

Acetylcholine (ACh) acting through muscarinic receptors is thought to be involved in the control of breathing, notably in central and peripheral chemosensory afferents and in regulations related to sleep-wake states. By using whole-body plethysmography, we compared baseline breathing at rest and ventilatory responses to acute exposure (5 min) to moderate hypoxia (10% O(2)) and hypercapnia (3 and 5% CO(2)) in mice lacking either the M(1) or the M(3) muscarinic receptor, and in wild-type matched controls. M(1) knockout mice showed normal minute ventilation (V(E)) but elevated tidal volume (V(T)) at rest, and normal chemosensory ventilatory responses to hypoxia and hypercapnia. M(3) knockout mice had elevated V(E) and V(T) at rest, a reduced V(T) response slope to hypercapnia, and blunted V(E) and frequency responses to hypoxia. The results suggest that M(1) and M(3) muscarinic receptors play significant roles in the regulation of tidal volume at rest and that the afferent pathway originating from peripheral chemoreceptors involves M(3) receptors.     

Endogenous 5-HT(1/2) systems and the newborn rat respiratory control. A comparative in vivo and in vitro study

Consequences of 5-HT(1/2) systems blockade by methysergide on newborn rats respiratory drive were evaluated in vivo with unrestrained animals and in vitro using brainstem-spinal cord preparations. A decrease in respiratory frequency until a plateau level was observed under both in vivo (82.8 +/- 0.6% of control values) and in vitro (76.8 +/- 0.8% of control values) conditions whereas an increase in inspiratory amplitude (135.1 +/- 2.1% of control values) was only retrieved in vivo. By the use of the c-fos expression analysis, we correlated these effects with neuronal activity changes, particularly, in vivo in two key structures between the respiratory ponto-medullary network and the peripheral or suprapontine afferences, namely the commissural subnucleus of the nucleus of the solitary tract and the lateral parabrachial nucleus. Thus, peripheral and suprapontine inputs seem to be of a primeval importance in the respiratory influence of endogenous 5-HT. Besides, as 5-HT is involved in the respiratory perturbations that occur in sudden infant death syndrome (SIDS), our results suggest a participation of peripheral and suprapontine inputs in these disorders.     

A theoretical study of the effect of circadian rhythms on sleep-induced periodic breathing and apnoea

This study employed a mathematical model of the respiratory control system to test the plausibility of the hypothesis that circadian rhythms in respiratory control can significantly influence respiratory stability at sleep onset. Computer simulations utilized a standardized "normal" sleep onset effect, superimposed upon systematic changes in chemoreflex parameters that mimicked the peaks and troughs of normal and high amplitude circadian rhythms. The analysis predicted that circadian influences may augment sleep-induced periodic breathing in nocturnal sleep compared with daytime naps. Furthermore, increased circadian amplitude of chemoreflex threshold, or absence of a circadian rhythm in peripheral chemosensitivity, each acted to stabilize respiration during daytime sleep onset and promote periodic breathing during nocturnal sleep onset. High amplitude circadian rhythms in respiratory control were predicted to cause an increasing number and duration of obstructive apnoeas from early to late night. It is suggested that the circadian timing system creates a nocturnal window of respiratory vulnerability and that abnormal circadian rhythms could potentially induce nocturnal sleep apnoea, even in individuals with normal sleep mechanisms.     

Distribution of ventilation in young and elderly adults determined by electrical impedance tomography

To determine the effect of age and posture on regional lung ventilation, eight young (26 +/- 1 years, mean +/- S.D.) and eight old (73 +/- 5 years) healthy men were studied by electrical impedance tomography in four body positions (sitting, supine, right and left lateral). The distribution of gas into the right and left lung regions was determined in the chest cross-section during tidal breathing at the resting lung volume, near residual volume and total lung capacity, as well as forced and slow vital capacity maneuvers. In the young, significant posture-dependent changes in gas distribution occurred during resting tidal breathing whereas they were absent in the elderly. In the older subjects, the contribution of the right lung to global ventilation fell with the transition from sitting to supine posture during both full expiration maneuvers. During forced vital capacity, the high flow rate and early airway closure in the dependent lung, occurring at higher volumes in the elderly, minimized the posture-dependency in gas distribution which was present during the slow maneuver. Our study revealed the significant effect of age on posture-dependent changes in ventilation distribution.     

Lung-deflating ability of rib cage and abdominal muscles in rabbits

Anesthetized, apneic, mechanically ventilated rabbits were placed into a tilting plethysmograph that a rubber diaphragm, tightly fitting the animal's body just below the xiphoid process, separated into a rib cage and abdominal chamber. Expired volumes (DeltaV) and abdominal pressure changes (DeltaPab) were assessed in supine and upright posture during maximal rib cage (RCC) and/or abdominal compression (ABC) by pressurizing either or both chambers, and during maximal stimulations of abdominal muscles (ABS). With RCC, DeltaV supine and upright amounted to 16+/-4.9 (mean+/-S.D.) and 20.9+/-7% of the vital capacity in supine posture (VCs) and to 75.8+/-14.5 and 44.8+/-13.9% of the expiratory reserve volume (ERV) in corresponding posture, DeltaPab being negligible. With ABC, DeltaV was 13.7+/-2 and 38.9+/-7.3% VCs and 68.4+/-14.8 and 84.4+/-10.5% ERV, respectively. Both DeltaV and DeltaPab were similar with ABC and ABS, independent of posture. If this applies also to RCC and expiratory rib cage muscle contraction, maximal expiratory effects of the latter (a) are larger in upright than supine posture; (b) contribute to ERV more in supine than upright posture; and (c) are similar to those caused by ABS in supine, but substantially smaller in upright posture.     

Frequency responses to hypoxia and hypercapnia in carotid body-denervated conscious rats

The ventilatory response to brief, severe hypoxia is biphasic consisting of an initial facilitation followed by a slowing of breathing frequency (fR). After the hypoxic stimulus is removed, fR drops below baseline levels. This phenomenon is called the post-hypoxic frequency decline (phfd). These fR changes are due to reciprocal changes in expiratory time (TE), mediated by the ventrolateral pontine A5 region (J. Physiol. (London) 497 (1996) 79; Am. J. Physiol. 274 (1998) R1546). The purpose of this study was to determine if carotid body input is required for full manifestation of phfd by quantifying ventilation in intact and carotid sinus denervated rats in response to hypoxic, and contrasted with hypercapnic stimuli. Following carotid denervation the initial facilitation of fR was eliminated in response to hypoxia, but the phfd remained. In contrast the pattern in response to increased CO2 remained constant before and after carotid denervation. These results suggest that phfd is not dependent upon carotid body stimulation, but is mediated centrally.     

Oxidative damage to DNA, p53 gene expression and p53 protein level in the process of aging in rat brain

Levels of 8-oxo2'dG (HPLC), p53 mRNA (PCR) and p53 protein (Western Blot) were estimated in four structures of rat brain, including grey matter (GM) of cerebral cortex, cerebral white matter (WM), cerebellum (C) and medulla oblongata (MO) of control (3.0-3.5-month-old) rats, 12- and 24-month-old rats. The level of oxidative DNA was statistically significantly higher in C of 24-month-old animals. Expression of p53 gene increased in C and also in the all other investigated brain parts, while the protein level of p53 was enhanced only in GM of 24-month-old rats. These data indicated that DNA oxidative damage and p53 gene expression increased significantly in aged brain. The higher expression of p53 gene in aged brain may suggest the activation of DNA repair processes.