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Objectives Respirophasic variation of inferior vena cava (IVC) size is affected by large variability with spontaneous breathing. This study aims at characterizing the dependence of IVC size on controlled changes in intrathoracic pressure. Methods Ten healthy subjects, in supine position, performed controlled isovolumetric respiratory efforts at functional residual capacity, attaining positive (5, 10, and 15 mmHg) and negative (−5, −10, and −15 mmHg) alveolar pressure levels. The isovolumetric constraint implies that equivalent changes are exhibited by alveolar and intrathoracic pressures during respiratory tasks. Results The IVC cross-sectional area equal to 2.88 ± 0.43 cm2 at baseline (alveolar pressure = 0 mmHg) was progressively decreased by both expiratory and inspiratory efforts of increasing strength, with diaphragmatic efforts producing larger effects than thoracic ones: −55 ± 15% decrease, at +15 mmHg of alveolar pressure (P

The equal pressure point (EPP) states that expiratory flow limitation occurs at the precise location along the tracheobronchial tree where intraluminal and peribronchial pressures are equal (i.e., where the local transmural pressure is 0 cmH2O). The EPP may be determined numerically via the expiratory isovolume pressure flow (IVPF) curve. For instance, using data from a given isovolume curve, the EPP is determined as the point along the x‐axis at which pressure‐flow data fails to conform to Rohrer’s equation: driving pressure = k1(flow) + k2(flow2). The probability of incurring expiratory flow‐limitation can be greatly determined by the EPP such that a lower EPP at a given lung volume requires only minimal pressure development to create a flow‐limiting segment during expiration. The EPP is affected by two principal factors: lung elastic recoil (PLUNG) & airway geometry. Given recent evidence suggesting that airway geometry is different between men & women, we sought to determine whether the EPP is likewise affected by sex in healthy adults. Purpose Determine whether or not there is an effect of sex on the magnitude of the EPP. Methods To address this question, 11 men (23.1 ± 4.8 years) and 12 women (21.2 ± 1.0 years) completed a study requiring two visits to the laboratory. On the initial visit, we obtained written informed consent and conducted spirometry to ensure subjects met the inclusion criteria of normal (>85% of predicted) lung function. During the second visit, a balloon catheter was passed intranasally to the lower one‐third of the esophagus and remained there for the duration of the visit to measure esophageal pressure (PES). Subjects then performed a minimum of 10–15 graded vital capacity maneuvers from 100‐5% of maximal effort. Data were imported into Matlab and, using custom‐written code, multiple calculations were performed. First, the lowest effort vital capacity efforts were taken as quasi‐static expiratory deflations of the lung and the negative sign of PES was taken to equal PLUNG. Second, alveolar pressure (PALV) was taken to equal PES + PLUNG. Finally, the EPP was determined by iteratively fitting Rohrer’s equation to pressure‐flow data at a given iso‐volume. The pressure at which Rohrer’s equation no longer adequately fit the data (>5% error) was termed the EPP. To test for an effect of sex and lung volume, a two‐way ANOVA was computed. Pairwise comparisons were adjusted for multiple comparisons using the Tukey‐Sidak post hoc test. Results There was no effect of sex on PLUNG, but PALV was greater in men than women throughout the entire VC range. EPP was significantly greater in men than women during the initial half of VC (95‐55% of VC) (p 0.05). Conclusion Because PLUNG did not differ between men and women, the differing location of EPP was the result of a greater PES in men. This suggests that the larger airways present in men are more resistant to compression than those of women.

Background and objective During forced expiration, alveolar pressure (PALV) increases and intrathoracic gas is compressed. Thus, 1-s forced expiratory volume measured by spirometry (FEV1-sp) is smaller than 1-s forced expiratory volume measured by plethysmography (FEV1-pl). Thoracic gas compression volume (TGCV) depends on the amount of gas within the lung when expiratory flow limitation occurs in the airways. We therefore tested the hypothesis that bronchoconstrictor and bronchodilator responses using FEV1-sp are biased by height and gender, which are major determinants of lung volume. Methods We studied 54 asthmatics during methacholine challenge and 55 subjects with airway obstruction (FEV1-sp increase >200 mL and >12% after salbutamol) measuring at the same time FEV1-sp or FEV1-pl. Results During methacholine challenge, TGCV increased more in males than females, correlated with PALV, total lung capacity (TLC) and height, and the provocative dose was lower using FEV1-sp than FEV1-pl. With salbutamol, FEV1-pl increased

Background and objective: The origin of frequency dependence of respiratory resistance has been explained by ventilation inhomogeneity; however, it is unclear which components in the respiratory system generate the frequency dependence. The author constructed a 4D pulmonary lobule model and analysed relationships between airflow rate, pressure and airway resistance by the use of computational fluid dynamics. Methods: The lobule model contained bifurcated bronchioles with two adjacent acini in which deformable inter-acinar septa and alveolar duct walls were designed. Constrictive conditions of respective bronchioles were designed, too. 4D finite element models for computational fluid dynamics were generated and airflow simulations were performed under moving boundary conditions of the arbitrary Lagrangean-Eulerean method. From the simulation results, airway resistances for various conditions were calculated. Results: Tissue resistance emerged under the condition of different acinar pressures caused by unequal airway resistances. If the inter-acinar septum was shifted so as to cancel the pressure difference, the acinar pressures were equal in spite of unequal airway resistances, and hence, tissue resistances did not emerge. Therefore, the tissue resistance in the former case is thought to be an index of alveolar pressure inequality (which could be cancelled by mechanical interaction of lung parenchyma), rather than a material property of the tissue itself. Conclusions: Inequality of alveolar pressure decreases as the input oscillatory frequency increases. Therefore, frequency dependence of the respiratory resistance should be regarded as a conditional index of the alveolar pressure inequality caused by heterogeneous changes in the intra-pulmonary airway and/or the lung parenchyma.

Introduction: Potentially recruitable lung has been assessed previously in patients with acute lung injury (ALI) by computed tomography. A large variability in lung recruitability was observed between patients. In this study, we assess whether a new non-radiological bedside technique could determine potentially recruitable lung volume (PRLV) in ALI patients. Methods: Sixteen mechanically ventilated patients with early ALI/ARDS were subjected to a recruitment manoeuvre and decremental PEEP titration. Electric impedance tomography, together with measurements of end-expiratory lung volume (EELV) and tracheal pressure, were used to determine PRLV. The method defines fully recruited open lung volume (OLV) as the volume reached at the end of two consecutive vital capacity manoeuvres to 40 cmH2O. It also uses extrapolation of the baseline alveolar pressure/volume curve up to 40 cmH2O, the volume reached being the non-recruited lung volume. The difference between the fully recruited and the non-recruited volume was defined as PRLV. Results: We observed a considerable heterogeneity among the patients in lung recruitability, PRLV range 11–47%. In a post hoc analysis, dividing the patients into two groups, a high and a low PRLV group, we found at baseline before the recruitment manoeuvre that the high PRLV group had lower compliance and a lower fraction of EELV/OLV. Conclusions: Using non-invasive radiation-free bedside methods, it may be possible to measure PRLV in ALI/ARDS patients. It is possible that this technique could be used to determine the need for recruitment manoeuvres and to select PEEP level on the basis of lung recruitability.

Background The mechanical in-exsufflator (MIE) is commonly used to augment cough in patients with neuromuscular disease from infancy to adulthood. Little is known about the alveolar pressures, lung volumes, and expiratory flow rates generated by the MIE when used via tracheostomy tube in infants and children. Methods A high-fidelity mechanical lung model was programmed to simulate infants with tracheostomy tubes. Generated pressures, volumes, and expiratory flows using the MIE device at variable insufflation/exsufflation pressures and times were recorded. The primary measure of interest was maximal expiratory flow (MEF). Results Pressure equilibration across the tracheostomy tube did not occur with insufflation time 70% vital capacity attained at insufflation pressures as low as 20 cmH2O. Though higher insufflation pressures resulted in increased expiratory flow rates, more negative exsufflation pressure had a greater absolute impact on MEF. Conclusions Using the MIE via tracheostomy tube in an infant lung model, we found that an insufflation time of >1 sec is required for equilibration of insufflation pressure and alveolar pressure. Longer exsufflation time does not significantly alter MEF. Higher insufflation and exsufflation pressures both increased MEF, but greater exsufflation pressure had more substantial impact. Pediatr Pulmonol. 2011; 46:211–217. © 2011 Wiley-Liss, Inc.

Background and objective: The interrupter resistance (Rint) can be calculated from various estimates of alveolar pressure based on mouth pressure during occlusion. We compared Rint, as measured by the opening interrupter technique (Rint1), and the linear back-extrapolation method (Rint2), with the ‘gold standard’ airway resistance measured by plethysmography (Raw). Methods: The study included 32 asthmatic children and 11 children with cystic fibrosis, aged 5 to 18 years, who were categorized into non-obstructed (NObs) (n = 27) and obstructed (Obs) (n = 16) groups. Spirometry and the three different resistance measurements were performed on all children. Rint1 and Raw were assessed after a bronchodilator (BD) test in 16 and nine children, respectively, in the Obs group. Results: Raw (0.48 ± 0.20 kPa.s/L) was lower than Rint1 (1.04 ± 0.34 kPa.s/L) and Rint2 (0.63 ± 0.18 kPa.s/L) (P

We develop a lung ventilation model based on a continuum poroelastic representation of lung parenchyma that is strongly coupled to a pipe network representation of the airway tree. The continuous system of equations is discretized using a low-order stabilised finite element method. The framework is applied to a realistic lung anatomical model derived from computed tomography data and an artificially generated airway tree to model the conducting airway region. Numerical simulations produce physiologically realistic solutions and demonstrate the effect of airway constriction and reduced tissue elasticity on ventilation, tissue stress and alveolar pressure distribution. The key advantage of the model is the ability to provide insight into the mutual dependence between ventilation and deformation. This is essential when studying lung diseases, such as chronic obstructive pulmonary disease and pulmonary fibrosis. Thus the model can be used to form a better understanding of integrated lung mechanics in both the healthy and diseased states. Copyright © 2015 John Wiley and Sons, Ltd.

Background: Closed system suctioning, CSS, has been advocated to avoid alveolar collapse. However, ventilator manufacturers indicate that extreme negative pressure levels can be obtained during closed system suctioning, impeding the performance of the ventilator. Methods: Suctioning with a 12 or 14 Fr catheter with a vacuum level of −50 kPa was either performed with an open technology or a CSS, where the catheter is introduced through a tight-fitting connection through the endotracheal tube, ETT. The lung model was ventilated with a Servo 900C or 300 ventilator with an I:E ratio of 1:2, 1:1 and 2:1 and extrinsic positive end-expiratory pressure (PEEP) at 0 or 10 cm H2O. Respiratory volumes and alveolar pressure were measured at the lung model alveolus. Results: The initial suctioning flow was >40 l/min with a 14 Fr catheter. When inserting the catheter through a no. 7 ETT, PEEP rose from 11 to 23 cm H2O during volume control ventilation with an I:E ratio 1:1. During suctioning the alveolar pressure fell to 10 cm H2O below the set PEEP level. CSS during pressure control ventilation had fewer effects. Low tidal volumes, inverse I:E ratio and secretions in the tube resulted in pressures down to −92 cm H2O. Conclusion: CSS should not be used in volume control ventilation due to risk of high intrinsic PEEP levels at insertion of the catheter and extreme negative pressures during suctioning. Pressure control ventilation produces less intrinsic PEEP effect. The continuous positive airway pressure (CPAP) mode offers the least intrinsic PEEP during insertion of the catheter and least sub-atmospheric pressure during suctioning.

Background: All monitoring of respiratory mechanics should depend on tracheal pressures (Trach-P) as endotracheal tube resistance (ETT-Res) will otherwise distort them. The aim of this study was to investigate factors that may vary ETT-Res, causing difficulties in ETT-Res estimation clinically, and to evaluate a method for direct Trach-P measurements to obviate these problems. Methods: In a model we studied: 1) The influence on ETT-Res caused by different connectors and secretions; 2) Direct Trach-P measurements with a catheter (o.d. 2 mm, i.d. 0.9 mm) with either end or side hole, filled with either air or liquid, introduced through the ETT lumen and evaluated regarding response time and position. Results: The pressure drop between trachea and Y-piece increased by 15% when respectively a swivel connector and a humidification device were connected to the ETT. When injecting 1 ml and 2 ml gel into the ETT lumen the inspiratory resistance increased 100% and 600% respectively. The response time of all catheters was ≤12 ms. During constant flow in inspiratory and expiratory directions the pressure difference between an end hole catheter positioned from 2 cm above the ETT tip to 4 cm below and a reference pressure in the artificial trachea was less than 1.5 cmH2O. Conclusions: ETT connections and secretions cause a variance in resistance. Tracheal pressure can be measured with high precision with an air- or liquid-filled catheter. An end hole catheter placed within 2 cm above or below the ETT tip will give sufficiently precise measurements for clinical purposes.

We studied respiratory system resistance by the interrupter technique (Rint) in healthy and asthmatic children, paying special attention to the effect of cheek compliance and the effects of supporting the cheeks, the influence of lung volume at which interruption was performed, the effect of direction of flow before occlusion (inspiration vs. expiration), and short-term reproducibility of this method. One hundred and thirty-two children (36 controls and 96 asthmatics) were included in the study (mean, 9.0 ± SD 3.6 years). Rint was calculated from the ratio of the alveolar pressure (estimated from mouth pressure during occlusion) to the flow prior to interruption. We observed that (1) underestimation of airway resistance due to upper airway compliance can be minimized by supporting the cheeks; Rint was significantly lower when measured without supporting the cheeks than with support of the cheeks in controls (3.9 ± 0.9 vs. 4.7 ± 1.2 cmH2O · L−1 · s, respectively) and asthmatics (5.2 ± 1.6 vs 6.9 7 plusmn; 2.0 cmH2O · L−1 · s); the quantitative differences of Rint with and without cheek support was larger in small children and in the most obstructed children; (2) performing occlusion at mid-tidal volume accurately reflected the respiratory system resistance of the whole respiratory cycle since we observed no difference in Rint when performing occlusion at different volumes during quiet respiration or at the middle of tidal volume; (3) Rint measured during expiration was higher than Rint obtained during inspiration in controls and in asthmatics; moreover, the effect of direction of flow before occlusion was greater in the small children; (4) Rint was closely correlated to height in controls (r = −0.82; P < 0.001); and (5) short-term reproducibility (at 15 min intervals) was satisfactory in controls and asthmatics (coefficients of variation were 9% and 7%, respectively). Pediatr. Pulmonol. 1997;24:78–85. © 1997 Wiley-Liss, Inc.

Ventilatory flow rates with either He-O2 (heliox) or N2-O2, (nitrox) mixtures during high frequency jet ventilation (HFJV) were calculated using a mathematical model, and were measured in a physical monoalveolar model of the infant lung. A constant flow was delivered to the model (V,) drawing with it an entrained flow (Ve). When the inspiratory time (Tl) was long, a back flow (Vr) was generated that increased progressively as the alveolar pressure increased. In order to reduce Vr the reflux time (Trc), i.e., the time needed for the injected gas to reflux, was measured (Trm)and calculated (Trc) with the model: The Tr depends on both chest-lung compliance and resistance to reflux as well as on the Ve/Vd ratio. For a given setting of the ventilator, values of Trm and Trc were significantly lower with heliox than nitrox (P < 0.01), and the correlation between Trm and Trc were fair with both gases. Tidal volume (VT) was calculated when the Tl was interrupted at Tr The values of VT were significantly higher with heliox than nitrox (P < 0.05). It is concluded that the evaluation of Tr during HFJV may provide useful information for setting the ventilator. Pediatr Pulmonol. 1994; 17:183–188. © 1994 Wiley-Liss, Inc.

Summary The relationship between peak airway pressure, alveolar pressure and respiratory frequency was calculated for the range of compliances and airway resistances which might be encountered during mechanical ventilation of a 3-kg neonate. The pressure/flow relationships of 2.5, 3.0, 3.5 and 4-mm tracheal tubes were determined at a series of flows from 0.5 to 4 litres/minute. Peak airway and alveolar pressures were then measured at various frequencies and inspiratory: expiratory ratios with the tubes incorporated in a model lung. Large differences between peak airway and alveolar pressures developed when frequency was increased or inspiratory time decreased; the differences were greatest with the smaller tubes. Shortening expiratory time by increasing the frequency or altering the inspiratory: expiratory ratio resulted in increased end-expiratory pressure because of incomplete emptying of the lung.

Summary Ventilatory and metabolic compensation to acid-base disturbances is reviewed. The mechanisms for compensation as well as the values obtained from several studies using normal dogs and dogs with experimentally induced diseases are provided. Compensation is not the same in dogs and human beings. Dogs have a better ability to adapt to most respiratory disorders, and human beings adapt better to metabolic acidosis. In metabolic alkalosis and chronic respiratory acidosis there is no difference in compensation between these species. Ventilatory compensation for metabolic disorders in dogs is the same whether they have metabolic acidosis or metabolic alkalosis, whereas metabolic compensation in respiratory disturbances is less effective in acidosis. Values for the expected changes in PCO2 in dogs with metabolic acidosis and metabolic alkalosis, and for bicarbonate concentration (HCO3-) in dogs with acute and chronic respiratory alkalosis and acidosis are presented.

The advantages of an isolated, perfused lung preparation in studies of transvascular fluid shifts are described. We find the permeability characteristics of the exchange vessels in the rabbit lung preparation to be similar to those obtained in lungs in situ. The filtration coefficient is similar during plasma and during Krebs-Ringer dextran T 70 perfusion. Therefore proteins in the perfusate do not modify the permeability of the exchange vessels in these lungs as had been reported for other vascular beds. Interstitial fluid pressure in both the alveolar walls and the alveolar corner regions increases with a rise in alveolar pressure. The result is a reduction in the fluid filtration rate under both zone-III and zone-I conditions. The resistance of the extravascular pathway and the fluid pressure in the periarterial/perivenous spaces both appear to be reduced when lung volume is increased by a reduction in pleural pressure. The net effect of positive airway pressure on the transvascular fluid exchange will depend on the relative contribution of the alveolar pressure effects and the lung volume effects. Fluid reabsorption can be observed to occur in the isolated, perfused rabbit lung. This reabsorption does not take place at the level of the loose perivascular tissue spaces since the interstitial fluid pressure at these sites is much lower than the microvascular pressure. We suggest that fluid reabsorption takes place at the thick part of the air-blood barrier, which, most likely, has a higher fluid pressure.

The sections in this article are: 1 Lung Volumes 2 Volume-Pressure Relations of Respiratory System During Relaxation 2.1 Total Respiratory System 2.2 Chest Wall and Lung 2.3 Effects of Gravity and Posture 2.4 Changes Throughout Life Span 3 Volume-Pressure Relations of Respiratory System During Static Muscular Efforts 3.1 Alveolar Pressure 3.2 Abdominal and Thoracic Pressures 4 Factors Limiting Volume Extremes 4.1 Upper Volume Extreme 4.2 Lower Volume Extreme

When the pancreatic duct or ducts are obstructed the pressure of pancreatic secretion rises to a height closely approximating that reached by the bile in obstruction of the common bile-duct. In the dog and cat the average maximum pressure attained as the result of intravenous injection of duodenal extract is about 300 mm. water. The highest pressure recorded in our experiments was 381 mm. in a dog; the lowest, also in a dog, was 240 mm. In three small Macacus rhesus monkeys the maximum pressure averaged 277 mm. water. The intravenous injection of duodenal extract brings about a rapid rise of pressure in the pancreas when its secretion is prevented from escaping into the duodenum. The pressure reaches a maximum, and at once begins to fall. Both rise and fall of pancreatic pressure are more rapid than the rise and fall of bile pressure in obstruction of the common bile-duct. Carmine gelatine, when injected into the pancreas through its main duct shortly after the death of the animal, tends to rupture the fine ducts, and escapes into the surrounding connective tissue in the manner described by Heidenhain. When injected at a low pressure during life, rupture of ducts may or may not occur. Apart from any escape in this manner, the carmine gelatine passes into the cells of the acini and of the walls of the small ducts, and from them to the exterior. Pancreatic secretion is absorbed at a low pressure, and it is probable that the secreting cells, when the alveolar pressure has reached a certain point, get rid of their secretion by turning it into the lymph stream. The pancreatic juice follows the lymph paths from the pancreas, and finally enters the blood from the thoracic duct. The mechanism of absorption of pancreatic juice is very similar to the mechanism of absorption of bile in obstruction of the bile-duct. The expenses of this research have been defrayed by a grant from the Earl of Moray Fund for the Prosecution of Research in the University of Edinburgh.