Relative impact of respiratory muscle activity on tidal flow and end expiratory volume in healthy neonates

INTRODUCTION: It has been suggested that infants dynamically regulate their tidal flow and end-expiratory volume level. The interaction between muscle activity, flow and lung volume in spontaneously sleeping neonates is poorly studied, since it requires the assessment of transcutaneous electromyography of respiratory muscles (rEMG) in matched comparison to lung function measurements. METHODS: After determining feasibility and repeatability of rEMG in 20 spontaneously sleeping healthy neonates, we measured the relative impact of intercostal and diaphragmatic EMG activity in direct comparison to the resulting tidal flow and FRC. RESULTS: We found good feasibility, repeatability and correlation of timing indices between rEMG activity and flow. The rEMG amplitude was significantly dependent on the resistive load of the face mask. Diaphragm and intercostal muscle activity commenced prior to the onset of flow and remained active during the expiratory cycle. The relative contribution of intercostal and diaphragmatic activity to flow was variable and changed dynamically. CONCLUSION: Using matched rEMG, air flow and lung volume measurements, we have found good feasibility and repeatability of intercostal and diaphragm rEMG measurements and provide the first quantitative measures of the temporal relationship between muscle activity and flow in spontaneously sleeping healthy neonates. Lung mechanical function is dynamically regulated and adapts on a breath to breath basis. So, non-invasive rEMG measurements alone or in combination with lung function might provide a more comprehensive picture of pulmonary mechanics in future studies. The data describing the timing of EMG and flow may be important for future studies of EMG triggered mechanical ventilation.

Visualization of alveolar recruitment in a porcine model of unilateral lung lavage using 3He-MRI

BACKGROUND: In the acute respiratory distress syndrome potentially recruitable lung volume is currently discussed. (3)He-magnetic resonance imaging ((3)He-MRI) offers the possibility to visualize alveolar recruitment directly. METHODS: With the approval of the state animal care committee, unilateral lung damage was induced in seven anesthetized pigs by saline lavage of the right lungs. The left lung served as an intraindividual control (healthy lung). Unilateral lung damage was confirmed by conventional proton MRI and spiral-CT scanning. The total aerated lung volume was determined both at a positive end-expiratory pressure (PEEP) of 0 and 10 mbar from three-dimensionally reconstructed (3)He images, both for healthy and damaged lungs. The fractional increase of aerated volume in damaged and healthy lungs, followed by a PEEP increase from 0 to 10 mbar, was compared. RESULTS: Aerated gas space was visualized with a high spatial resolution in the three-dimensionally reconstructed (3)He-MR images, and aeration defects in the lavaged lung matched the regional distribution of atelectasis in proton MRI. After recruitment and PEEP increase, the aerated volume increased significantly both in healthy lungs from 415 ml [270-445] (median [min-max]) to 481 ml [347-523] and in lavaged lungs from 264 ml [71-424] to 424 ml [129-520]. The fractional increase in lavaged lungs was significantly larger than that in healthy lungs (healthy: 17% [11-38] vs. lavage: 42% [14-90] (P=0.031). CONCLUSION: The (3)He-MRI signal might offer an experimental approach to discriminate atelectatic vs. poor aerated lung areas in a lung damage animal model. Our results confirm the presence of potential recruitable lung volume by either alveolar collapse or alveolar flooding, in accordance with previous reports by computed tomography.

Effects of mild vitamin a deficiency on lung maturation in newborn rats: a morphometric and morphologic study

OBJECTIVE To evaluate the effects of a 60% vitamin A deficiency (VAD) on the two postnatal stages of lung development: alveolarization and microvascular maturation. Lungs from deficient rats were compared to age-matched controls. STUDY DESIGN Starting at 3 weeks before mating, female rats were maintained under a diet lacking vitamin A. Due to the slow depletion of the vitamin A liver stores the pregnant rats carried to term and delivered pups under mild VAD conditions. Mothers and offspring were then kept under the same diet what resulted in a mean reduction of vitamin A plasma concentration of about 60% vs. controls during the whole experimental period. Pups were sacrificed on days 4, 10 and 21 and their lungs fixed and analyzed by means of a combined morphologic and morphometric investigation at light and electron microscopic levels. RESULTS During the whole experiment, body weights of VAD animals were lower than controls with a significant decrease on day 10. On days 4, 10 and 21 the pulmonary structure was in a comparable gross morphologic state in both groups. Despite this morphologic normality, quantitative alterations in some functional parameters could be detected. On day 4, lung volume and the volume and surface area of air spaces were decreased, while the arithmetic mean barrier thickness and type 2 pneumocyte volume were increased in the VAD group. On day 21, some changes were again manifest mainly consisting in an augmentation of the vascularization and a decrease in interstitial volume in deficient animals. CONCLUSIONS Mild VAD causes no gross disturbances in the postnatal phases of lung development in rats. However, a body weight-related transient retardation of lung maturation was detectable in the first postnatal week. At 3 weeks, the VAD lungs showed a more mature vascular system substantiated by an increase in volume of both capillary volume and the large non-parenchymal vessels. In view of these quantitative alterations, we suspect that mild VAD deregulates the normal phases of body and lung growth, but does not induce serious functional impairments.

Influence of postnatally administered glucocorticoids on rat lung growth

Postnatal formation of alveoli can be largely prevented by glucocorticoid treatment, which accelerates alveolar wall thinning and inhibits outgrowth of new interalveolar septa. Since a double capillary network is a prerequisite for interalveolar wall formation, we hypothesized that glucocorticoid treatment inhibited alveolar formation, indirectly, by inducing precocious microvascular maturation. Between 4 and 60 days we followed up qualitatively and quantitatively the effects of 2 weeks (days 2-15) of daily Decadron (Dexamethasone phosphate) injections on the lung structure. Glucocorticoid induced only small changes in body weight or lung volume. However, during the first 2 weeks, it accelerated alveolar wall thinning and microvascular maturation and partly suppressed the outgrowth of new interalveolar septa. In Decadron-treated rats, the interstitial tissue mass was significantly reduced during the first 2 weeks, and a larger alveolar surface area was endowed with a capillary monolayer on days 10 and 13. One week after drug withdrawal, the trend towards precocious maturation of the lung was reversed. Lipofibroblasts reappeared, and inter-airspace septa regressed towards a more immature state. We found indications of a second burst of alveolization by resumption of secondary septa formation. The late sequelae of Decadron treatment (day 60) were manifested as an 'emphysematous' condition of the lungs, with larger and fewer airspaces, the delayed alveolization being insufficient to compensate for the initial deficit.

Protein deficiency and the growing rat lung. I. Nutritional findings and related lung volumes

We investigated the consequences of early malnutrition on milk production by dams and on body weight and structural lung growth of young rats using two models of protein restriction. Dams of the early restriction group were fed an 8% casein diet starting at parturition. Those of the delayed restriction group received a 12% casein diet from lactation d 8-14 and thereafter the 8% diet. After weaning, early restriction and delayed restriction group rats were maintained on low protein until d 49, then refed the control diet (18% casein) up to d 126. Milk was analyzed on d 12. Animals were killed at d 21, 49, and 126 for lung fixation in situ. In this report, we show that protein restriction lowered milk yield to 38% of normal. Milk lipid per gram of dry weight tended to be increased, whereas lactose and protein were significantly decreased. Pups from protein-restricted dams grew less and had lower lung volumes, effects being more serious at d 49. However, specific lung volumes (in milliliters per 100 g body weight) were constantly increased. This means that lung was either less affected than body mass or overdistended due to less connective tissue. After refeeding, both groups showed a remarkable catch-up in growth with restoration of the normal allometric relationship between lung volume and body weight. Thus, even after an early onset of protein restriction to total body, the lung is still capable to substantially recover from growth retardation.

Protein deficiency and the growing rat lung. II. Morphometric analysis and morphology

Effects of protein deficiency during the whole period of postnatal development and intensive growth were studied in the rat lung parenchyma. Dams received a low protein diet as follows: early restriction, 8% casein diet from parturition, and delayed restriction, 12% then 8% casein diet from lactation d 8. After weaning (d 21), early restriction and delayed restriction group rats were maintained on the 8% casein diet until d 49, wherefore they were returned to normal food (18% casein) for 11 wk. Lungs were processed for light and electron microscopic morphometry on d 21, 49, and 126. The diffusion capacity of the lung for O2 (DLO2) was also determined from the morphologic parameters. Volume and surface densities of the parenchymal components of malnourished rats did not consistently differ from controls. Because of lower lung volumes, absolute values, including DLO2, were all significantly decreased. Further, although lung volume growth was less impaired than body growth and thus deviated from the normal allometric relationship, most morphometric parameters paralleled body weight changes. Visually, we detected minor morphologic alterations at d 21 and 49, not necessarily reflected by morphometric data. But, importantly, lung parenchyma appeared mature at weaning despite the growth retardation. Normal refeeding resulted in a striking regrowth of the lung parenchyma. Although early restriction rats did not fully catch up in lung volume, most parenchymal parameters and DLO2 were largely restored in both refed groups.

Efficient estimation of the total number of acini in adult rat lung

Pulmonary airways are subdivided into conducting and gas-exchanging airways. An acinus is defined as the small tree of gas-exchanging airways, which is fed by the most distal purely conducting airway. Until now a dissector of five consecutive sections or airway casts were used to count acini. We developed a faster method to estimate the number of acini in young adult rats. Right middle lung lobes were critical point dried or paraffin embedded after heavy metal staining and imaged by X-ray micro-CT or synchrotron radiation-based X-rays tomographic microscopy. The entrances of the acini were counted in three-dimensional (3D) stacks of images by scrolling through them and using morphological criteria (airway wall thickness and appearance of alveoli). Segmentation stopper were placed at the acinar entrances for 3D visualizations of the conducting airways. We observed that acinar airways start at various generations and that one transitional bronchiole may serve more than one acinus. A mean of 5612 (±547) acini per lung and a mean airspace volume of 0.907 (±0.108) μL per acinus were estimated. In 60-day-old rats neither the number of acini nor the mean acinar volume did correlate with the body weight or the lung volume.

Pulmonary lymphangioleiomyomatosis: analysis of disease manifestation by region-based quantification of lung parenchyma

PURPOSE Lymphangioleiomyomatosis (LAM) is characterized by proliferation of smooth muscle tissue that causes bronchial obstruction and secondary cystic destruction of lung parenchyma. The aim of this study was to evaluate the typical distribution of cystic defects in LAM with quantitative volumetric chest computed tomography (CT). MATERIALS AND METHODS CT examinations of 20 patients with confirmed LAM were evaluated with region-based quantification of lung parenchyma. Additionally, 10 consecutive patients were identified who had recently undergone CT imaging of the lung at our institution, in which no pathologies of the lung were found, to serve as a control group. Each lung was divided into three regions (upper, middle and lower thirds) with identical number of slices. In addition, we defined a "peel" and "core" of the lung comprising the 2 cm subpleural space and the remaining inner lung area. Computerized detection of lung volume and relative emphysema was performed with the PULMO 3D software (v3.42, Fraunhofer MEVIS, Bremen, Germany). This software package enables the quantification of emphysematous lung parenchyma by calculating the pixel index, which is defined as the ratio of lung voxels with a density <-950HU to the total number of voxels in the lung. RESULTS Cystic changes accounted for 0.1-39.1% of the total lung volume in patients with LAM. Disease manifestation in the central lung was significantly higher than in peripheral areas (peel median: 15.1%, core median: 20.5%; p=0.001). Lower thirds of lung parenchyma showed significantly less cystic changes than upper and middle lung areas combined (lower third: median 13.4, upper and middle thirds: median 19.0, p=0.001). CONCLUSION The distribution of cystic lesions in LAM is significantly more pronounced in the central lung compared to peripheral areas. There is a significant predominance of cystic changes in apical and intermediate lung zones compared to the lung bases.

Correlation between alveolar ventilation and electrical properties of lung parenchyma.

One key problem in modern medical imaging is linking measured data and actual physiological quantities. In this article we derive such a link between the electrical bioimpedance of lung parenchyma, which can be measured by electrical impedance tomography (EIT), and the magnitude of regional ventilation, a key to understanding lung mechanics and developing novel protective ventilation strategies. Two rat-derived three-dimensional alveolar microstructures obtained from synchrotron-based x-ray tomography are each exposed to a constant potential difference for different states of ventilation in a finite element simulation. While the alveolar wall volume remains constant during stretch, the enclosed air volume varies, similar to the lung volume during ventilation. The enclosed air, serving as insulator in the alveolar ensemble, determines the resulting current and accordingly local tissue bioimpedance. From this we can derive a relationship between lung tissue bioimpedance and regional alveolar ventilation. The derived relationship shows a linear dependence between air content and tissue impedance and matches clinical data determined from a ventilated patient at the bedside.

A new approach in virtopsy: Postmortem ventilation in multislice computed tomography

Although postmortem imaging has gained prominence in the field of forensic medicine, evaluation of the postmortem lung remains problematic. Specifically, differentiation of normal postmortem changes and pathological pulmonary changes is challenging and at times impossible. In this study, five corpses were ventilated using a mechanical ventilator with a pressure of 40 mbar (40.8 cm H(2)O). The ventilation was performed via an endotracheal tube, a larynx mask or a continuous positive airway pressure mask. Postmortem computed tomographic images of the lungs before and with a ventilation of 40 mbar (40.8 cm H(2)O) were evaluated and the lung volumes were measured with segmentation software. Postmortem ventilation led to a clearly visible decrease of both the density in the dependant parts of the lungs and ground glass attenuation, whereas consolidated areas remained unchanged. Furthermore, a mean increase in the lung volume of 2.10 l was seen. Pathological changes such as septal thickening or pulmonary nodules in the lung parenchyma became more detectable with postmortem ventilation. Intracorporal postmortem mechanical ventilation of the lungs appears to be an effective method for enhancing detection of small pathologies of the lung parenchyma as well as for discriminating between consolidation, ground glass attenuation and position-dependent density.

Respiratory function during anesthesia: effects on gas exchange

Anaesthesia causes a respiratory impairment, whether the patient is breathing spontaneously or is ventilated mechanically. This impairment impedes the matching of alveolar ventilation and perfusion and thus the oxygenation of arterial blood. A triggering factor is loss of muscle tone that causes a fall in the resting lung volume, functional residual capacity. This fall promotes airway closure and gas adsorption, leading eventually to alveolar collapse, that is, atelectasis. The higher the oxygen concentration, the faster will the gas be adsorbed and the aleveoli collapse. Preoxygenation is a major cause of atelectasis and continuing use of high oxygen concentration maintains or increases the lung collapse, that typically is 10% or more of the lung tissue. It can exceed 25% to 40%. Perfusion of the atelectasis causes shunt and cyclic airway closure causes regions with low ventilation/perfusion ratios, that add to impaired oxygenation. Ventilation with positive end-expiratory pressure reduces the atelectasis but oxygenation need not improve, because of shift of blood flow down the lung to any remaining atelectatic tissue. Inflation of the lung to an airway pressure of 40 cmH2O recruits almost all collapsed lung and the lung remains open if ventilation is with moderate oxygen concentration (< 40%) but recollapses within a few minutes if ventilation is with 100% oxygen. Severe obesity increases the lung collapse and obstructive lung disease and one-lung anesthesia increase the mismatch of ventilation and perfusion. CO2 pneumoperitoneum increases atelectasis formation but not shunt, likely explained by enhanced hypoxic pulmonary vasoconstriction by CO2. Atelectasis may persist in the postoperative period and contribute to pneumonia.

Alternative protocol to initiate high-frequency oscillatory ventilation: an experimental study

INTRODUCTION: The objective was to study the effects of a novel lung volume optimization procedure (LVOP) using high-frequency oscillatory ventilation (HFOV) upon gas exchange, the transpulmonary pressure (TPP), and hemodynamics in a porcine model of surfactant depletion. METHODS: With institutional review board approval, the hemodynamics, blood gas analysis, TPP, and pulmonary shunt fraction were obtained in six anesthetized pigs before and after saline lung lavage. Measurements were acquired during pressure-controlled ventilation (PCV) prior to and after lung damage, and during a LVOP with HFOV. The LVOP comprised a recruitment maneuver with a continuous distending pressure (CDP) of 45 mbar for 2.5 minutes, and a stepwise decrease of the CDP (5 mbar every 5 minute) from 45 to 20 mbar. The TPP level was identified during the decrease in CDP, which assured a change of the PaO2/FIO2 ratio < 25% compared with maximum lung recruitment at CDP of 45 mbar (CDP45). Data are presented as the median (25th-75th percentile); differences between measurements are determined by Friedman repeated-measures analysis on ranks and multiple comparisons (Tukey's test). The level of significance was set at P < 0.05. RESULTS: The PaO2/FiO2 ratio increased from 99.1 (56.2-128) Torr at PCV post-lavage to 621 (619.4-660.3) Torr at CDP45 (CDP45) (P < 0.031). The pulmonary shunt fraction decreased from 51.8% (49-55%) at PCV post-lavage to 1.03% (0.4-3%) at CDP45 (P < 0.05). The cardiac output and stroke volume decreased at CDP45 (P < 0.05) compared with PCV, whereas the heart rate, mean arterial pressure, and intrathoracic blood volume remained unchanged. A TPP of 25.5 (17-32) mbar was required to preserve a difference in PaO2/FIO2 ratio < 25% related to CDP45; this TPP was achieved at a CDP of 35 (25-40) mbar. CONCLUSION: This HFOV protocol is easy to perform, and allows a fast determination of an adequate TPP level that preserves oxygenation. Systemic hemodynamics, as a measure of safety, showed no relevant deterioration throughout the procedure.

Optimized temperature and deadspace correction improve analysis of multiple breath washout measurements by ultrasonic flowmeter in infants

BACKGROUND: Assessment of lung volume (FRC) and ventilation inhomogeneities with ultrasonic flowmeter and multiple breath washout (MBW) has been used to provide important information about lung disease in infants. Sub-optimal adjustment of the mainstream molar mass (MM) signal for temperature and external deadspace may lead to analysis errors in infants with critically small tidal volume changes during breathing. METHODS: We measured expiratory temperature in human infants at 5 weeks of age and examined the influence of temperature and deadspace changes on FRC results with computer simulation modeling. A new analysis method with optimized temperature and deadspace settings was then derived, tested for robustness to analysis errors and compared with the previously used analysis methods. RESULTS: Temperature in the facemask was higher and variations of deadspace volumes larger than previously assumed. Both showed considerable impact upon FRC and LCI results with high variability when obtained with the previously used analysis model. Using the measured temperature we optimized model parameters and tested a newly derived analysis method, which was found to be more robust to variations in deadspace. Comparison between both analysis methods showed systematic differences and a wide scatter. CONCLUSION: Corrected deadspace and more realistic temperature assumptions improved the stability of the analysis of MM measurements obtained by ultrasonic flowmeter in infants. This new analysis method using the only currently available commercial ultrasonic flowmeter in infants may help to improve stability of the analysis and further facilitate assessment of lung volume and ventilation inhomogeneities in infants.

Deadspace estimation from CO2 versus molar mass measurements in infants

BACKGROUND: Estimation of respiratory deadspace is often based on the CO2 expirogram, however presence of the CO2 sensor increases equipment deadspace, which in turn influences breathing pattern and calculation of lung volume. In addition, it is necessary to correct for the delay between the sensor and flow signals. We propose a new method for estimation of effective deadspace using the molar mass (MM) signal from an ultrasonic flowmeter device, which does not require delay correction. We hypothesize that this estimation is correlated with that calculated from the CO2 signal using the Fowler method. METHODS: Breath-by-breath CO2, MM and flow measurements were made in a group of 77 term-born healthy infants. Fowler deadspace (Vd,Fowler) was calculated after correcting for the flow-dependent delay in the CO2 signal. Deadspace estimated from the MM signal (Vd,MM) was defined as the volume passing through the flowhead between start of expiration and the 10% rise point in MM. RESULTS: Correlation (r = 0.456, P < 0.0001) was found between Vd,MM and Vd,Fowler averaged over all measurements, with a mean difference of -1.4% (95% CI -4.1 to 1.3%). Vd,MM ranged from 6.6 to 11.4 ml between subjects, while Vd,Fowler ranged from 5.9 to 12.0 ml. Mean intra-measurement CV over 5-10 breaths was 7.8 +/- 5.6% for Vd,MM and 7.8 +/- 3.7% for Vd,Fowler. Mean intra-subject CV was 6.0 +/- 4.5% for Vd,MM and 8.3 +/- 5.9% for Vd,Fowler. Correcting for the CO2 signal delay resulted in a 12% difference (P = 0.022) in Vd,Fowler. Vd,MM could be obtained more frequently than Vd,Fowler in infants with CLD, with a high variability. CONCLUSIONS: Use of the MM signal provides a feasible estimate of Fowler deadspace without introducing additional equipment deadspace. The simple calculation without need for delay correction makes individual adjustment for deadspace in FRC measurements possible. This is especially important given the relative large range of deadspace seen in this homogeneous group of infants.

Methacholine responsiveness in mice from 2 to 8 wk of age

Many chronic human lung diseases have their origin in early childhood, yet most murine models used to study them utilize adult mice. An important component of the asthma phenotype is exaggerated airway responses, frequently modelled by methacholine (MCh) challenge. The present study was undertaken to characterize MCh responses in mice from 2 to 8 wk of age measuring absolute lung volume and volume-corrected respiratory mechanics as outcome variables. Female BALB/c mice aged 2, 3, 4, 6, and 8 wk were studied during cumulative intravenous MCh challenge. Following each MCh dose, absolute lung volume was measured plethysmographically at functional residual volume and during a slow inflation to 20-hPa transrespiratory pressure. Respiratory system impedance was measured continuously during the inflation maneuver and partitioned into airway and constant-phase parenchymal components by model fitting. Volume-corrected (specific) estimates of respiratory mechanics were calculated. Intravenous MCh challenge induced a predominantly airway response with no evidence of airway closure in any age group. No changes in functional residual volume were seen in mice of any age during the MCh challenge. The specific airway resistance MCh dose response curves did not show significant differences between the age groups. The results from the present study do not show systematic differences in MCh responsiveness in mice from 2 to 8 wk of age.