Upper Inflection Point Research Articles

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Figure 1: Pressure-Volume curve. Horizontal axis is airway pressure in cmH2O, vertical axis is resultant tidal volume in ml. LIP: Lower inflection point, HIP: high or upper inflection point, PMC: point of maximum curvature or expiratory inflection point.

An automated (novel) algorithm for estimating green vegetation cover fraction from digital image: UIP-MGMEP.

Green vegetation cover fraction (VCF) is an important indicator of vegetation status in ecology and agronomy. Digital image analysis (DIA) has been widely accepted as a new VCF measurement technique. In this study, we present a novel fully automatic threshold segmentation algorithm for VCF measurements, which is named as upper inflection point plus mean gradient magnitude of edge pixels (UIP-MGMEP). The algorithm performs VCF estimation upon the vegetation index Excess Green (EXG). UIP-MGMEP optimizes the EXG threshold by searching the upper inflection point (UIP) of the M-Et curve (mean gradient magnitude of edge pixels (MGMEP) vs. EXG threshold), based on the assumption that EXG variance of the boundary pixels between vegetation and background is larger than the variance of the background. Five typical sample images are used to illustrate how ground complexity reduces the distinctness of the UIP. Three controlled experiments are illustrated to test the robustness of UIP-MGMEP to resolution, exposure, and ground complexity. The results show that UIP-MGMEP is a promising algorithm for automatic VCF estimation upon digital images. Compared to broad-leaved grass, narrow-leaved grass is more sensitive to resolution and exposure. To reduce ground complexity, smaller footprint size while more images to cover the same area may be better than one image with large footprint size. UIP-MGMEP is fully automatic, making it promising for batch processing of VCF measurements that is very difficult in any wide-range field survey in the past. UIP-MGMEP algorithm can only extract green vegetation and is not suitable for non-green (even grayish-green) vegetation, due to the limits of vegetation index EXG. In addition, UIP-MGMEP is not recommended for images with VCF less than 0.5% or greater than 99.5%.

An experimental study of turbulent boundary layers approaching separation

The present paper deals with the experimental analysis of a strong decelerated turbulent boundary layer developed on a flat plate. The aim of the study was to examine the effects of pressure gradient on a non-equilibrium boundary layer while indicating local areas of equilibrium flow. The effect of the Reynolds number on a turbulent boundary layer developed with matching the external pressure gradient conditions was also analysed. The emphasis was on the analysis of mean flow statistics i.e. mean velocity profiles, streamwise Reynolds stress and the effect of large- and small-scale interactions by analysing the skewness factor and energy isocontours maps. The comparative analysis of the external data indicated that the structure of the turbulent boundary layer depends not only on local effects of pressure gradient but also on the upstream history of the flow. For the same condition of pressure gradient, the increased momentum is observed near the wall with the increase of the Reynolds number at the Incipient Detachment, where increased turbulence production is also observed, leading to the failure of the outer scaling methods. Surprisingly, the effect of the Reynolds number decays at the intermittent transitory detachment where similar profiles were observed. The upper inflection point in the mean profile corresponded well with the outer maximum of the Reynolds stress and zero crossing of skewness factor. Position of this point occurs at different locations, depending on the flow history effects. The last observation demonstrates that the inflection points results from large- and small-scale interactions, which led to the increased convection velocity of small scales near the wall.

Optimal esophageal balloon volume for accurate estimation of pleural pressure at end-expiration and end-inspiration: an in vitro bench experiment

BackgroundEsophageal pressure, used as a surrogate for pleural pressure, is commonly measured by air-filled balloon, and the accuracy of measurement depends on the proper balloon volume. It has been found that larger filling volume is required at higher surrounding pressure. In the present study, we determined the balloon pressure-volume relationship in a bench model simulating the pleural cavity during controlled ventilation. The aim was to confirm whether an optimal balloon volume range existed that could provide accurate measurement at both end-expiration and end-inspiration.MethodsWe investigated three esophageal balloons with different dimensions and materials: Cooper, SmartCath-G, and Microtek catheters. The balloon was introduced into a glass chamber simulating the pleural cavity and volume-controlled ventilation was initiated. The ventilator was set to obtain respective chamber pressures of 5 and 20 cmH2O during end-expiratory and end-inspiratory occlusion. Balloon was progressively inflated, and balloon pressure and chamber pressure were measured. Balloon transmural pressure was defined as the difference between balloon and chamber pressure. The balloon pressure-volume curve was fitted by sigmoid regression, and the minimal and maximal balloon volume accurately reflecting the surrounding pressure was estimated using the lower and upper inflection point of the fitted sigmoid curve. Balloon volumes at end-expiratory and end-inspiratory occlusion were explored, and the balloon volume range that provided accurate measurement at both phases was defined as the optimal filling volume.ResultsSigmoid regression of the balloon pressure-volume curve was justified by the dimensionless variable fitting and residual distribution analysis. All balloon transmural pressures were within ±1.0 cmH2O at the minimal and maximal balloon volumes. The minimal and maximal balloon volumes during end-inspiratory occlusion were significantly larger than those during end-expiratory occlusion, except for the minimal volume in Cooper catheter. Mean (±standard deviation) of optimal filling volume both suitable for end-expiratory and end-inspiratory measurement ranged 0.7 ± 0.0 to 1.7 ± 0.2 ml in Cooper, 1.9 ± 0.2 to 3.6 ± 0.3 ml in SmartCath-G, and 2.2 ± 0.2 to 4.6 ± 0.1 ml in Microtek catheter.ConclusionsIn each of the tested balloon, an optimal filling volume range was found that provided accurate measurement during both end-expiratory and end-inspiratory occlusion.

An experimental investigation of an unsteady adverse pressure gradient turbulent boundary layer: embedded shear layer scaling

An experimental investigation of an unsteady adverse pressure gradient turbulent boundary layer is described. It is demonstrated that the local flow physics is largely dominated by an inflectional instability which gives rise to an embedded shear layer contained within the boundary layer. Experimental measurements are presented which are fully consistent with the presence of clockwise spanwise-oriented coherent vorticity within the embedded shear layer. Using embedded shear layer scaling parameters in the form of the shear layer vorticity thickness and the velocity defect at the upper inflection point, both the mean and the phase-averaged boundary layer velocity profiles exhibit similarity in both space and time over a large wall-normal extent. In a similar manner, the profiles of the streamwise-component turbulence intensity and Reynolds stress also exhibit similarity when scaled with the embedded shear layer parameters. The embedded shear layer scaling of previously published adverse pressure gradient turbulent boundary layer measurements confirms its generic applicability in a wide range of flow-field geometries and extending to high Reynolds numbers.

Individualized lung protective ventilation regimen based on pressure-volume curve for one lung ventilation in patients undergoing thoracic surgery

Objective To evaluate the effectiveness of setting PEEP and tidal volume (VT ) according to pressure-volume (P-V) curve during one lung ventilation (OLV) in patients undergoing thoracic surgery.Methods Twenty-five ASA Ⅰ or Ⅱ patients of both sexes aged 44-64 yr weighing 57-75 kg undergoing lobectomy under general anesthesia were enrolled in this study.Double-lumen tube was inserted.Correct positioning was verified by flberoptic bronchoscopy.The patients were mechanically ventilated.P-V curve was measured by SSS system during OLV.Lower inflection point (LIP)and upper inflection point (UIP) were determined.The pressure at LIP (PLIP) and volume at UIP (VUIP) were measured.Bilateral lungs were ventilated for 30 min (T0) at first before OLV was started.PEEP was set at PLIP + 0.196 kPa and VT was set at VUIP,and the patients were ventilated for 30 min (T1).VT was then reduced to 80％ of VUIP.OLV was performed for another 30 min (T2).VT was then further reduced to 60％ of VUIP and the patients were ventilated for 30 min (T3).PEr CO2 was maintained at 4.67-6.00 kPa.Arterial blood and central venous samples were taken at T0-3.Blood gas analysis was performed.Qs/Qt was calculated.MAP,HR,CVP,peak airway pressure (Peak),airway resistance (Rsw) and lung compliance (CL) were measured and recorded at T0-3.Results HR,Ppeakk,Rsw and Qs/Qt were significantly increased while CL and PaO2 decreased at T1-3,CVP was significantly increased at T1.2 and MAP and PaCO2 were increased at T3 as compared with the baseline values at T0.Ppeak and Rsw were significantly decreased at T2.3 and PaO2 was significantly increased while Qs/Qt decreased at T2,CVP was decreased,MAP and PaO2 were increased at T3 as compared with the values at T1.Conclusions Mechanical ventilation with VT set at 80％ of VUIPandPEEPatPUIP+0.196kPa provides best ventilatory efficacy for OLV in terms of PaO2 and hemodynamics. Key words: Positive-pressure respiration; Tidal volume

Changes in Lung Volume and Ventilation during Lung Recruitment in High-Frequency Ventilated Preterm Infants with Respiratory Distress Syndrome

To assess global and regional changes in lung volume and ventilation during lung recruitment in preterm infants with respiratory distress syndrome. Using electrical impedance tomography, changes in lung volume and ventilation were measured in 15 high-frequency oscillatory ventilated preterm infants during oxygenation-guided recruitment maneuvers. The inflation and deflation limbs were mapped, and the lower and upper inflection points were calculated using both oxygenation and impedance data. The impedance data were also used to determine recruitment-related changes in oscillation volume and distribution. During inflation, lower and upper inflection points were identified in the majority of infants. The deflation limb showed clear lung hysteresis in all infants. The upper inflection point was significantly lower when comparing the pressure/oxygenation and pressure/impedance curves. Lung volume changes differed between the ventral and dorsal regions, but did not show a consistent pattern. Optimal recruitment increased the oscillation volume, but the distribution of ventilation was relatively homogeneous along the ventral-dorsal axis. Lung hysteresis is present in preterm infants with respiratory distress syndrome. Regional differences in lung volume changes and ventilation during high-frequency oscillatory ventilation with lung recruitment are relatively modest and do not follow a gravity-dependent distribution.

Comparative study of recruitment maneuver guided by pressure-volume curve on respiratory physiology and lung morphology between acute respiratory distress syndrome of pulmonary and extrapulmonary origin in canine models

To determine effects of recruitment maneuver (RM) guided by pressure-volume (P-V) curve on respiratory physiology and lung morphology in canine models of acute respiratory distress syndrome of pulmonary or extrapulmonary origin (ARDSp and ARDSexp). Twenty-four healthy dogs were randomly divided into two groups with 12 dogs each: ARDSexp and ARDSp. Each dog in ARDSexp group was injected with oleic acid 0.1 ml/kg through femoral vein, and each dog in ARDSp group received hydrochloric acid 2 ml/kg via trachea. Subsequently, dogs with both models were randomly subdivided into lung protective ventilation strategy (LPVS) group and LPVS+RM group, respectively. Dogs in LPVS group were given LPVS only without RM. RM guided by P-V curve was performed in LPVS+RM group followed by LPVS and pressure controlled ventilation (PCV) mode was selected. Phigh was set at upper inflection point (UIP) of the P-V curve, positive end-expiratory pressure (PEEP) was set at lower inflection point (LIP)+2 cm H(2)O (1 cm H(2)O=0.098 kPa), and the duration of RM was 60 seconds. The duration of mechanical ventilation (MV) in both subgroups was 4 hours. The oxygenation index (PaO(2)/FiO(2)), relative lung mechanical indexes were measured in two ARDS models before establishment of ARDS model, and before and after RM. The UIP and LIP were calculated with P-V curve. The percentage of different volume in ventilation of lung accounting for total lung volume was compared by CT scan. The PaO(2)/FiO(2), UIP and LIP did not showed significant differences among all groups before ARDS and before RM. PaO(2)/FiO(2) and respiratory system compliance (Crs) were significantly elevated in LPVS+RM group of both models 4 hours after RM compared with corresponding LPVS group [PaO(2)/FiO(2) (mm Hg, 1 mm Hg=0.133 kPa) of ARDSexp model: 263.9±69.2 vs. 182.8±42.8, Crs (ml/cm H(2)O) of ARDSexp model: 11.3±4.2 vs. 9.7±3.7; PaO(2)/FiO(2) (mm Hg) of ARDSp model: 193.4±33.5 vs. 176.4±40.2, Crs (ml/cm H(2)O) of ARDSp model: 10.1±3.9 vs. 9.0±3.9, P<0.05 or P<0.01], and the airway pressure was significantly declined compared with corresponding LPVS group [peak inspiratory pressure (PIP), cm H(2)O ] of ARDSexp model: 24.1±7.4 vs. 30.2±8.5, plateau pressure (Pplat, cm H(2)O) of ARDSexp model: 19.1±7.3 vs. 25.6±7.7; PIP (cm H(2)O) of ARDSp model: 26.6±8.4 vs. 29.6±10.3, Pplat (cm H(2)O) of ARDSp model: 21.9±7.3 vs. 25.1±8.4, P<0.05 or P<0.01]. Moreover, PaO(2)/FiO(2), Crs, PIP and Pplat were improved better in ARDSexp model than ARDSp model (P<0.05 orP<0.01). Compared with LPVS maneuver, RM plus LPVS maneuver could significantly decrease the proportion of closure and hypoventilation region, and increase the proportion of normal ventilation region in both models [closure region of ARDSexp model: (9.9±3.1)% vs. (16.3±5.2)%, hypoventilation region of ARDSexp model: (10.2±4.2)% vs. (23.4±6.7)%, normal ventilation region of ARDSexp model: (76.2±12.3)% vs. (57.5±10.1)%; closure region of ARDSp model: (14.3±4.8)% vs. (18.2±5.1)%, hypoventilation region of ARDSp model: (17.4±6.3)% vs. (24.1±5.9)%, normal ventilation region of ARDSp model: (63.2±10.7)% vs. (54.6±11.3)%, P<0.05 or P<0.01]. All of the ventilation regions were better improved with ARDSexp model than ARDSp model (all P<0.05). RM guided by P-V curve could help obtain better oxygenation, improve pulmonary compliance and lung ventilation in ARDSexp and ARDSp, and better treatment effects are seen in ARDSexp dogs than ARDSp dogs.

Respiratory System Model for Quasistatic Pulmonary Pressure-Volume (P-V) Curve: Inflation-Deflation Loop Analyses

A respiratory system model (RSM) is developed for the deflation process of a quasistatic pressure-volume (P-V) curve, following the model for the inflation process reported earlier. In the RSM of both the inflation and the deflation limb, a respiratory system consists of a large population of basic alveolar elements, each consisting of a piston-spring-cylinder subsystem. A normal distribution of the basic elements is derived from Boltzmann statistical model with the alveolar closing (opening) pressure as the distribution parameter for the deflation (inflation) process. An error minimization by the method of least squares applied to existing P-V loop data from two different data sources confirms that a simultaneous inflation-deflation analysis is required for an accurate determination of RSM parameters. Commonly used terms such as lower inflection point, upper inflection point, and compliance are examined based on the P-V equations, on the distribution function, as well as on the geometric and physical properties of the basic alveolar element.

Abstract 45: A Mathematical Model of Ventilation, Perfusion, and Oxygenation in Low-Flow States

Background: Recent investigations underscore the critical importance of ventilation strategies on resuscitation outcomes. In low perfusion states, such as cardiac arrest and traumatic shock, the rise in intrathoracic pressure that accompanies positive-pressure ventilation (PPV) can significantly impede venous return and lead to a decrease in cardiac output. The optimal ventilation strategy in these “low-flow” states remains unclear. Objectives: To create a mathematical model of perfusion and oxygenation to predict the effects of PPV with both normotension and hypotension. Methods: The lung pressure-volume relationship was modeled using a novel formula allowing manipulation of various lung characteristics, including vital capacity, compliance, and the upper and lower inflection points. A separate formula was then derived to predict mean intrathoracic pressure for a given minute ventilation using the pressure-volume formula. The addition of positive end-expiratory pressure was also modeled. Finally, a formula was derived to model oxygen absorbance as a function of alveolar surface area and flow based on ventilation rate and mean intrathoracic pressure. Results: Mathematical models of the lung pressure-volume relationship, mean intrathoracic pressure, and absorbance were successfully derived. Manipulation of vital capacity, compliance, upper and lower inflection points, positive end-expiratory pressure, and minute ventilation allowed prediction of optimal ventilation rate and tidal volume for a normal and an ARDS lung. For a normal lung, optimal values for both mean intrathoracic pressure and absorption were achieved with a ventilation rate of 4 breaths/min. A decrease in the upper inflection point or increase in minute ventilation resulted in faster optimal ventilation rates, although none exceeded 14 breaths/min. Conclusions: A mathematical model of ventilation was successfully created allowing manipulation of multiple variables related to lung compliance and ventilation strategy. This model suggests the use of lower ventilation rates with larger tidal volumes to minimize the hemodynamic effects of PPV and maximize oxygen absorbance.

Using pressure–volume curves to set proper PEEP in acute lung injury

The evolution of respiratory care on patients with acute respiratory distress syndrome (ARDS) has been focused on preventing the deleterious effects of mechanical ventilation, termed ventilator-induced lung injury (VILI). Currently, reduced tidal volume is the standard of ventilatory care for patients with ARDS. The current focus, however, has shifted to the proper setting of positive end-expiratory pressure (PEEP). The whole lung pressure-volume (P/V) curve has been used to individualize setting proper PEEP in patients with ARDS, although the physiologic interpretation of the curve remains under debate. The purpose of this review is to present the pros and cons of using P/V curves to set PEEP in patients with ARDS. A systematic analysis of recent and relevant literature was conducted. It has been hypothesized that proper PEEP can be determined by identifying P/V curve inflection points. Acquiring a dynamic curve presents the key to the curve's bedside application. The lower inflection point of the inflation limb has been shown to be the point of massive alveolar recruitment and therefore an option for setting PEEP. However, it is becoming widely accepted that the upper inflection point (UIP) of the deflation limb of the P/V curve represents the point of optimal PEEP. New methods used to identify optimal PEEP, including tomography and active compliance measurements, are currently being investigated. In conclusion, we believe that the most promising method for determining proper PEEP settings is use of the UIP of the deflation limb. However, tomography and dynamic compliance may offer superior bedside availability.

Correlation between alveolar recruitment /derecruitment and inflection points on the pressure-volume curve

To determine whether individual alveolar recruitment/derecruitment (R/D) is correlated with the lower and upper inflections points on the inflation and deflation limb of the whole-lung pressure-volume (P-V) curve. Prospective experimental study in an animal research laboratory. Five anesthetized rats subjected to saline-lavage lung injury. Subpleural alveoli were filmed continuously using an in vivo microscope during the generation of a whole-lung P-V curve using the super syringe technique. Alveolar R/D was correlated to the calculated inflection points on both limbs of the P-V curve. There was continual alveolar recruitment along the entire inflation limb in all animals. There was some correlation (R2=0.898) between the pressure below which microscopic derecruitment was observed and the upper inflection point on the deflation limb. No correlation was observed between this pressure and the lower inflection point on the inflation limb. In this physiological experiment in lungs with pure surfactant deactivation we found that individual alveolar recruitment measured by direct visualization was not correlated with the lower inflection point on inflation whereas alveolar derecruitment was correlated with alveolar derecruitment on deflation. These data suggest that inflection points on the P-V curve do not always represent a change in alveolar number.

Method and Interpretation of the Pressure Volume Curve in Patients With Acute Respiratory Distress Syndrome

Abstract: The pressure volume curve of the respiratory system is a useful tool that has been proposed to assess respiratory mechanics impairment exhibited by patients with the acute respiratory distress syndrome. Acute respiratory distress syndrome is characterized by a significant reduction in lung compliance mainly due to the reduction in healthy lung volume. During mechanical ventilation, lung inflation results in recruitment of previously nonaerated alveoli and a concomitant distension of healthy lung units. This phenomenon has been shown to be the main determinant of ventilatory-induced lung injuries. The lower inflection point, which is present on the lower part of the pressure volume curve, and the upper inflection point have been proposed as an insight to both recruitment and overdistension, respectively. This interpretation of the curve has led to the concept of lung protective ventilation. Nevertheless, specific rules governing recruitment and distension are only now emerging from the abundant literature resulting from more than 30 years of physiological research. The interpretation and the utility of the pressure volume curve should, therefore, be reconsidered in the light of recent knowledge.

Pressure-volume curve variations after a recruitment manoeuvre in acute lung injury/ARDS patients: implications for the understanding of the inflection points of the curve

Background and objective: Although the pressure-volume (P-V) curve has been proposed in the management of mechanically ventilated patients, its interpretation remains unclear. Our aim has been to study the variations of the P-V curve after a recruitment manoeuvre (RM). Our hypothesis was that the lower inflection point (LIP) represents the presence of compressive atelectases, so it should not change after lung recruitment, while the upper inflection point (UIP) reflects reabsorptive atelectases, and an effective recruitment should result in changes at this level. Methods: Two P-V curves (quasi-static method) separated by an RM (40 cmH2O, two consecutive manoeuvres) were plotted in 35 postoperative patients with criteria of acute lung injury/acute respiratory distress syndrome (ARDS). LIP, UIP and expiratory inflection point (EIP) were defined as the first point where the curve consistently starts to separate from the line. Results: One to six measurements were obtained per patient (73 procedures). Neither the lower nor the EIPs varied significantly after the RM (P = 0.11 and 0.35, respectively). An UIP was observed in 18 curves (25%) before the RM and disappeared on nine occasions after the recruitment. Similar results were obtained when first measurements only were analysed, and when the cause (pulmonary vs. extrapulmonary), severity of lung injury or duration of mechanical ventilation at first measurement were studied. Conclusions: An RM does not modify the LIP significantly, but induces the disappearance of the UIP in 50% of the cases in which this point is found.

Pressure-volume curve variations after a recruitment manoeuvre in acute lung injury/ARDS patients

Although the pressure-volume (P-V) curve has been proposed in the management of mechanically ventilated patients, its interpretation remains unclear. Our aim has been to study the variations of the P-V curve after a recruitment manoeuvre (RM). Our hypothesis was that the lower inflection point (LIP) represents the presence of compressive atelectases, so it should not change after lung recruitment, while the upper inflection point (UIP) reflects reabsorptive atelectases, and an effective recruitment should result in changes at this level. Two P-V curves (quasi-static method) separated by an RM (40 cmH2O, two consecutive manoeuvres) were plotted in 35 postoperative patients with criteria of acute lung injury/acute respiratory distress syndrome (ARDS). LIP, UIP and expiratory inflection point (EIP) were defined as the first point where the curve consistently starts to separate from the line. One to six measurements were obtained per patient (73 procedures). Neither the lower nor the EIPs varied significantly after the RM (P = 0.11 and 0.35, respectively). An UIP was observed in 18 curves (25%) before the RM and disappeared on nine occasions after the recruitment. Similar results were obtained when first measurements only were analysed, and when the cause (pulmonary vs. extrapulmonary), severity of lung injury or duration of mechanical ventilation at first measurement were studied. An RM does not modify the LIP significantly, but induces the disappearance of the UIP in 50% of the cases in which this point is found.

Use of a sigmoidal equation to analyze the pressure–volume curve obtained by the low flow method

The adjustment of PEEP and tidal volume to the lower inflection point and upper inflection point of the P–V curve, respectively, has been proposed to optimize mechanical ventilation in ARDS. Usually, the P–V curve is analyzed by eye from a graph, with high variability. Venegas and colleagues [1] developed a sigmoidal equation that fits remarkably well to P–V curves obtained by the supersyringe method. Our objective was to determine whether this equation is able to model P–V curves obtained by the low flow method.

Derecruitment of the lung induced by stepwise lowering of positive end-expiratory pressure in patients with adult respiratory distress syndrome

Background and objective: It has recently been suggested that recruitment proceeds far above the lower inflection point of the elastic pressure-volume (Pel/V) curve of the respiratory system. Accordingly, the value of the lower inflection point as a guide to set the positive end-expiratory pressure (PEEP) has been challenged. Our aim was to evaluate the derecruitment induced by stepwise decreasing PEEP levels. Methods: Seven consecutive sedated and paralysed patients with acute respiratory distress syndrome were studied. Multiple Pel/V curves of the respiratory system were recorded at PEEP levels progressively decreasing in steps of 3.75 cmH2O from +15 to zero according to the principles of the low flow inflation method. Results: Multiple Pel/V curves shifted towards lower volumes at decreasing PEEP. Dynamic compliance was higher for Pel/V curves recorded from lower PEEP levels. A high correlation (r = 0.99) was found between dynamic compliance and PEEP. The lower inflection point was on average 9.2 cmH2O. However, the transition between the lower segment and the linear part of the Pel/V curve was in general smooth to the eye. The upper inflection point was on average 23.8 cmH2O. A high correlation (r = 0.98) between the upper inflection point and PEEP was found. Conclusions: The lower inflection point is a poor indicator of alveolar closure. The evaluation of derecruitment induced by a stepwise reduction in PEEP seems to be more useful than individual titration of PEEP and tidal volume in patients with adult respiratory distress syndrome.

What has been learnt from P/V curves in patients with acute lung injury/acute respiratory distress syndrome.

Mechanical impairment of the respiratory system was recognised soon after the description of acute respiratory distress syndrome. The analysis of the pressure/volume (P/V) curve of the respiratory system contributed a lot to the understanding of the pathophysiology of acute lung injury and formed the basis for lung protection. The lower and upper inflection points were regarded as points of interest to avoid cyclic derecruitment and overdistension and to optimise ventilatory settings. However, because of the heterogeneity of lung injury, reducing the mechanical properties of the whole respiratory system to a single curve is a schematic approach, which makes interpretation difficult. New data suggest that alveolar re-inflation occurs along the whole P/V curve that can, therefore, be considered as a recruitment curve. The lower inflection point has no relationship with alveolar opening and closure and does not indicate the positive end-expiratory pressure needed to prevent alveolar collapse. The shape of the P/V curve gives information about the extension and the homogeneity of lung injury, indicating the possibility of lung recruitment. The upper inflection point, classically seen as the beginning of overdistension, may also indicate the end of recruitment. The pressure/volume curve offers the unique opportunity of evaluating alveolar recruitment/derecruitment at the bedside that can be helpful for the identification of optimal ventilatory settings and makes the curve a valuable tool for the ventilatory management of acute lung injury.

How respiratory system mechanics may help in minimising ventilator-induced lung injury in ARDS patients.

The main supportive therapy in acute respiratory distress syndrome patients is mechanical ventilation. As with any therapy, mechanical ventilation has side-effects, and may induce lung injury (ventilator-induced lung injury (VILI)/ventilator-associated lung injury). The mechanical factors responsible for VILI are thought to be related to tidal recruitment/derecruitment of previously collapsed alveoli and/or pulmonary overdistension. The volume/pressure (V/P) curve of the respiratory system in patients as well as in animal models of acute lung injury (ALI) has a characteristic sigmoid shape, with a lower inflection point (LIP) corresponding to the pressure/end-expiratory volume required to initiate recruitment of collapsed alveoli, and an upper inflection point (UIP) corresponding to the pressure/end inspiratory volume at which alveolar overdistension occurs. "Protective" ventilatory approaches have therefore set out to minimise mechanical injury by using the V/P curve to individualise positive end-expiratory pressure (PEEP) (PEEP above the LIP) and tidal volume (by setting end-inspiratory V/P below the UIP) since a large number of experimental studies correlate P/V curves to histological and biological manifestations of VILI and two randomised trials showed that protective ventilatory strategy individually tailored to the P/V curve minimised pulmonary and systemic inflammation and decreased mortality in patients with ALI. However, despite the fact that several studies have: 1) proposed new techniques to perform pressure/volume curves at the bedside, 2) confirmed that the lower inflection point and upper inflection point correspond to computed tomography scan evidence of atelectasis and overdistension, and 3) demonstrated the ability of the pressure/volume curve to estimate alveolar recruitment with positive end-expiratory pressure, no large studies have assessed whether such measurement can be performed in all intensive care units as a monitoring tool to orient ventilator therapy. Preliminary experimental and clinical studies show that the shape of the dynamic inspiratory pressure/time profile during constant flow inflation (stress index), allows prediction of a ventilatory strategy that minimises the occurrence of ventilator-induced lung injury.

The upper inflection point of the pressure-volume curve. Influence of methodology and of different modes of ventilation.

The pressure-volume (P/V) curve has been proposed as a tool to adjust the ventilatory settings in cases of acute respiratory distress syndrome (ARDS). The aim of this study was to test the influence of P/V tracing methodology on the presence and value of the upper inflection point (UIP). In 13 medical ARDS patients, the interruption and the automated low flow inflation methods were compared while the patients were ventilated at conventional (10-12 ml/kg) and at low (5-6 ml/kg) tidal volume (Vt). Two levels of inspiratory flow and insufflation time were used (3 and 6 s). No significant difference in UIP was found between the static and the dynamic methods, whatever the flow used. At Vt 10-12 ml/kg, the static and dynamic UIPs were 22.4 +/- 4.4 cmH(2)O and 22.1 +/- 4.5 cmH(2)O ( p = 0.86), respectively; at Vt of 5-6 ml/kg, the static and dynamic UIPs were 26.6 +/- 4.1 cmH(2)O and 25.5 +/- 5 cmH(2)O ( p = 0.34), respectively. Significant differences in UIP were found, in the static and dynamic conditions, between the two levels of Vt ( p < 0.005): it was lower with the higher Vt, suggesting that UIP is dependent on previous tidal alveolar recruitment. Interruption and continuous flow techniques gave similar results, but the previous Vt influences the pressure value of the UIP.