Pleural Pressure Gradient Research Articles

Esophageal pressure as estimation of pleural pressure: a study in a model of eviscerated chest.

Transpulmonary pressure is the effective pressure across the lung parenchyma and has been proposed as a guide for mechanical ventilation. The pleural pressure is challenging to directly measure in clinical setting and esophageal manometry using esophageal balloon catheters was suggested for estimation. However, the accuracy of using esophageal pressure to estimate pleural pressure is debated due to variability in the mechanical properties of respiratory system, esophagus and esophageal catheter. Furthermore, while a vertical pleural pressure gradient exists across lung regions, esophageal pressure balloon provides a single value, representing, at most, the pressure surrounding the esophagus. In a swine model with a preserved esophagus and a single homogenous, easily measurable intrathoracic pressure, we evaluated esophageal pressure's agreement with intrathoracic pressure at different positive end-expiratory pressure (PEEP) levels (0, 5, 10, 15 cmH2O). We assessed the improvement of measurement accuracy by correcting absolute esophageal values using a previously described technique, that accounts for the pressure generated by the esophageal wall in response to esophageal balloon inflation. The study involved five swine, wherein two different esophageal catheters were used alongside the four distinct PEEP levels. Swings, uncorrected and corrected absolute esophageal pressures (end-inspiratory, end-expiratory) were compared with their respective intrathoracic pressures. The effect of correction technique was assessed with manual incremental step inflation procedure. We found that both catheters significantly overestimated absolute esophageal pressure compared to intrathoracic pressure (5.01 ± 3.32 and 6.06 ± 5.62 cmH2O at end-expiration and end-inspiration, respectively), with error increasing at higher positive end-expiratory pressure levels (end-expiration: 2.36 ± 2.03, 3.77 ± 1.37, 6.24 ± 2.51 and 7.69 ± 4.02 for each PEEP level, P < 0.0001; end-inspiration: 1.71 ± 2.10, 3.70 ± 1.73, 7.67 ± 3.62 and 11.14 ± 7.60 for each PEEP level, P = 0.0004). Applying the correction technique significantly improved agreement for absolute values (0.82 ± 1.62 and 1.86 ± 3.94 cmH2O at end-expiration and end-inspiration, respectively). Esophageal pressure swings accurately estimated intrathoracic pressure swings at low-medium intrathoracic pressures (-0.64 ± 0.62, -0.07 ± 0.53, 1.43 ± 1.51, and 3.45 ± 3.94 at PEEP 0, 5, 10 and 15 cmH2O, respectively; P = 0.0197). The correction technique, based on the mechanical response of esophageal wall to the balloon inflation, is fundamental for obtaining reliable estimations of absolute intrathoracic pressure values, and for ensuring its correct application in clinical setting.

The pleural gradient does not reflect the superimposed pressure in patients with class III obesity

BackgroundThe superimposed pressure is the primary determinant of the pleural pressure gradient. Obesity is associated with elevated end-expiratory esophageal pressure, regardless of lung disease severity, and the superimposed pressure might not be the only determinant of the pleural pressure gradient. The study aims to measure partitioned respiratory mechanics and superimposed pressure in a cohort of patients admitted to the ICU with and without class III obesity (BMI ≥ 40 kg/m2), and to quantify the amount of thoracic adipose tissue and muscle through advanced imaging techniques.MethodsThis is a single-center observational study including ICU-admitted patients with acute respiratory failure who underwent a chest computed tomography scan within three days before/after esophageal manometry. The superimposed pressure was calculated from lung density and height of the largest axial lung slice. Automated deep-learning pipelines segmented lung parenchyma and quantified thoracic adipose tissue and skeletal muscle.ResultsN = 18 participants (50% female, age 60 [30–66] years), with 9 having BMI < 30 and 9 ≥ 40 kg/m2. Groups showed no significant differences in age, sex, clinical severity scores, or mortality. Patients with BMI ≥ 40 exhibited higher esophageal pressure (15.8 ± 2.6 vs. 8.3 ± 4.9 cmH2O, p = 0.001), higher pleural pressure gradient (11.1 ± 4.5 vs. 6.3 ± 4.9 cmH2O, p = 0.04), while superimposed pressure did not differ (6.8 ± 1.1 vs. 6.5 ± 1.5 cmH2O, p = 0.59). Subcutaneous and intrathoracic adipose tissue were significantly higher in subjects with BMI ≥ 40 and correlated positively with esophageal pressure and pleural pressure gradient (p < 0.05). Muscle areas did not differ between groups.ConclusionsIn patients with class III obesity, the superimposed pressure does not approximate the pleural pressure gradient, which is higher than in patients with lower BMI. The quantity and distribution of subcutaneous and intrathoracic adiposity also contribute to increased pleural pressure gradients in individuals with BMI ≥ 40. This study introduces a novel physiological concept that provides a solid rationale for tailoring mechanical ventilation in patients with high BMI, where specific guidelines recommendations are lacking.

Pleural manometry during thoracocentesis in patients with malignant pleural effusion: A randomized controlled trial.

Large-volume therapeutic thoracocentesis may be associated with pulmonary congestion or a more serious complication; re-expansion pulmonary edema (RPE). We investigated whether monitoring pleural pressure with manometry during thoracocentesis would prevent these pulmonary symptoms/RPE and allow larger volume drainage. We did a randomized controlled trial involving 110 patients with large malignant pleural effusions. Patients were randomly allocated to obtain thoracocentesis with or without pleural manometry. We measured the incidence of pulmonary congestion symptoms, total fluid aspirated, and pleural pressures in both groups. This trial is listed on ClinicalTrials.gov as NCT04420663. The mean amount of total thoracocentesis fluid withdrawn from the control group was 945.4±78.9 (mL) and 1690.9±681.0 (mL) from the intervention group (P<0.001). Clinical symptoms of pulmonary congestion appeared in (n=20) (36.3%) of patients in the intervention group while no symptoms appeared in controls (P<0.001). The difference between opening and closing pressures between the non-symptomatic cluster and the symptomatic cluster was (32.8±15.6 versus 42.2±13) respectively (P=0.02). Total fluid withdrawn from the non-symptomatic cluster was 1828.5±505 mL in comparison to 1,450±875 mL in the symptomatic cluster (P=0.04). Pleural manometry can be used to increase the volume of fluid removed on each occasion in patients with malignant pleural effusion. In our study, pleural manometry was associated with a larger number of pulmonary congestion symptoms/RPE. We believe that manometry may be a useful tool to not exceed a 17 cm H2O gradient in pleural pressure which should be avoided to prevent pulmonary congestion symptoms or RPE. Pulmonary congestion symptoms/RPE are not related to the amount of volume withdrawn but to the gradient of pleural pressure drop. Our conclusion does support the adoption of pleural manometry whenever large-volume thoracocentesis is intended.

Effect Of Eight Hours Per Day Of Intermittent Self Prone Positioning For Seven Days On The Severity Of Covid-19 Pneumonia/ Acute Respiratory Distress Syndrome.

Prone positioning improves ventilation-perfusion mismatch, distribution of gravitational gradient in pleural pressure, and oxygen saturation significantly in patients with Covid pneumonia. We aimed to find out the efficacy of eight hours per day of intermittent selfprone positioning for seven days in patients affected with COVID-19 pneumonia/ ARDS. This Randomized Clinical Trial was conducted in the Covid isolation wards of Ayub Teaching Hospital, Abbottabad. Patients suffering from COVID-19 pneumonia/ ARDS were enrolled with permuted block randomization into a control and an experimental group each consisting of 36 patients. Parameters of Pneumonia Severity Index (PSI) score along with other sociodemographic data was noted on a preformed structured questionnaire. Death was confirmed by requesting the death certificate of patients on the 90th day of enrolment. Data Analysis was done with SPSS Version 25. Tests of significance were applied to calculate the difference in the patients of the two groups with respect to respiratory physiology and survival. The mean age of the patients was 63.79±15.26 years. A total of 25 (32.9%) male and 47 (61.8%) female patients were enrolled. Statistically significant improvement was found in the respiratory physiology of the patients at 7th and 14th DOA between the groups. Pearson Chi-Square test of significance showed a difference in mortality between the two groups at 14th DOA (pvalue=0.011) but not at 90th DOA (p-value=0.478). Log Rank (Mantel-Cox) test of significance, applied on the Kaplan Meier curve and showed no statistically significant difference among the groups based on the survival of the patients. (p-value=0.349). Early transient improvement in respiratory physiology and mortality does occur with 8 hours of self-prone positioning for seven days but there is no effect on the 90-day survival of the patients. Thus, the impact of the manoeuvre on improving survival needs to be explored with studies having an application of the manoeuvre for a longer duration and period.

Modeling the influence of gravity and the mechanical properties of elastin and collagen fibers on alveolar and lung pressure–volume curves

The relationship between pressure (P) and volume (V) in the human lung has been extensively studied. However, the combined effects of gravity and the mechanical properties of elastin and collagen on alveolar and lung P–V curves during breathing are not well understood. Here, we extended a previously established thick-walled spherical model of a single alveolus with wavy collagen fibers during positive pressure inflation. First, we updated the model for negative pressure-driven inflation that allowed incorporation of a gravity-induced pleural pressure gradient to predict how the static alveolar P–V relations vary spatially throughout an upright human lung. Second, by introducing dynamic surface tension and collagen viscoelasticity, we computed the hysteresis loop of the lung P–V curve. The model was tested by comparing its predicted regional ventilation to literature data, which offered insight into the effects of microgravity on ventilation. The model has also produced novel testable predictions for future experiments about the variation of mechanical stresses in the septal walls and the contribution of collagen and elastin fibers to the P–V curve and throughout the lung. The model may help us better understand how mechanical stresses arising from breathing and pleural pressure variations affect regional cellular mechanotransduction in the lung.

Pronation in acute respiratory distress syndrome (ARDS) secondary to COVID-19

: In March 2020, the outbreak of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) was declared a pandemic by the World Health Organization. Patients with SARS-CoV-2 infection can develop coronavirus disease 2019 (COVID-19), the most concerning complication of which is acute hypoxaemic respiratory failure requiring mechanical ventilation and intensive care unit (ICU) admission. In this context prone position ventilation is an established method to improve oxygenation in severe acute respiratory distress syndrome (ARDS), and its application was able to reduce mortality rate. Prone position has been used since the 1970s to treat severe hypoxemia in patients with ARDS because of its effectiveness at improving gas exchange. Compared with the supine position, placing patients in prone position effects a more even tidal volume distribution, in part, by reversing the vertical pleural pressure gradient, which becomes more negative in the dorsal regions. Prone position also improves resting lung volume in the dorsocaudal regions by reducing the superimposed pressure of both the heart and the abdomen. In contrast, pulmonary perfusion remains preferentially distributed to the dorsal lung regions, thus improving overall alveolar ventilation/perfusion relationships. Moreover, the larger tissue mass suspended from a wider dorsal chest wall effects a more homogeneous distribution of pleural pressures throughout the lung that reduces abnormal strain and stress development. This is believed to ameliorate the severity or development of ventilator-induced lung injury and may partly explain why prone position reduces mortality in severe ARDS. In this review we investigate the physiological aspects of the pronation.

External chest-wall compression in prolonged COVID-19 ARDS with low-compliance: a physiological study

BackgroundExternal chest-wall compression (ECC) is sometimes used in ARDS patients despite lack of evidence. It is currently unknown whether this practice has any clinical benefit in patients with COVID-19 ARDS (C-ARDS) characterized by a respiratory system compliance (Crs) < 35 mL/cmH2O.ObjectivesTo test if an ECC with a 5 L-bag in low-compliance C-ARDS can lead to a reduction in driving pressure (DP) and improve gas exchange, and to understand the underlying mechanisms.MethodsEleven patients with low-compliance C-ARDS were enrolled and underwent 4 steps: baseline, ECC for 60 min, ECC discontinuation and PEEP reduction. Respiratory mechanics, gas exchange, hemodynamics and electrical impedance tomography were recorded. Four pigs with acute ARDS were studied with ECC to understand the effect of ECC on pleural pressure gradient using pleural pressure transducers in both non-dependent and dependent lung regions.ResultsFive minutes of ECC reduced DP from baseline 14.2 ± 1.3 to 12.3 ± 1.3 cmH2O (P < 0.001), explained by an improved lung compliance. Changes in DP by ECC were strongly correlated with changes in DP obtained with PEEP reduction (R2 = 0.82, P < 0.001). The initial benefit of ECC decreased over time (DP = 13.3 ± 1.5 cmH2O at 60 min, P = 0.03 vs. baseline). Gas exchange and hemodynamics were unaffected by ECC. In four pigs with lung injury, ECC led to a decrease in the pleural pressure gradient at end-inspiration [2.2 (1.1–3) vs. 3.0 (2.2–4.1) cmH2O, P = 0.035].ConclusionsIn C-ARDS patients with Crs < 35 mL/cmH2O, ECC acutely reduces DP. ECC does not improve oxygenation but it can be used as a simple tool to detect hyperinflation as it improves Crs and reduces Ppl gradient. ECC benefits seem to partially fade over time. ECC produces similar changes compared to PEEP reduction.

Positive End-Expiratory Pressure, Pleural Pressure, and Regional Compliance during Pronation: An Experimental Study.

Rationale: The physiological basis of lung protection and the impact of positive end-expiratory pressure (PEEP) during pronation in acute respiratory distress syndrome are not fully elucidated. Objectives: To compare pleural pressure (Ppl) gradient, ventilation distribution, and regional compliance between dependent and nondependent lungs, and investigate the effect of PEEP during supination and pronation. Methods: We used a two-hit model of lung injury (saline lavage and high-volume ventilation) in 14 mechanically ventilated pigs and studied supine and prone positions. Global and regional lung mechanics including Ppl and distribution of ventilation (electrical impedance tomography) were analyzed across PEEP steps from 20 to 3 cm H2O. Two pigs underwent computed tomography scans: tidal recruitment and hyperinflation were calculated. Measurements and Main Results: Pronation improved oxygenation, increased Ppl, thus decreasing transpulmonary pressure for any PEEP, and reduced the dorsal-ventral pleural pressure gradient at PEEP < 10 cm H2O. The distribution of ventilation was homogenized between dependent and nondependent while prone and was less dependent on the PEEP level than while supine. The highest regional compliance was achieved at different PEEP levels in dependent and nondependent regions in supine position (15 and 8 cm H2O), but for similar values in prone position (13 and 12 cm H2O). Tidal recruitment was more evenly distributed (dependent and nondependent), hyperinflation lower, and lungs cephalocaudally longer in the prone position. Conclusions: In this lung injury model, pronation reduces the vertical pleural pressure gradient and homogenizes regional ventilation and compliance between the dependent and nondependent regions. Homogenization is much less dependent on the PEEP level in prone than in supine positon.

Awake prone positioning in COVID-19

In the absence of effective targeted therapies for COVID-19, optimisation of supportive care is essential. Lung injury with features of acute respiratory distress syndrome (ARDS) appears to be the principal...

Prone positioning in ARDS: physiology, evidence and challenges

Introduction: Prone position has been used since the 1970s as a rescue therapy to treat severe hypoxemia in patients with acute respiratory distress syndrome (ARDS). Despite numerous observational and randomized controlled trials showing the effectiveness of prone position in improving oxygenation, mortality benefit was demonstrated only recently in the PROSEVA study1. Intensivists taking care of patients with ARDS should be aware about the physiological changes during prone ventilation, the latest evidence available and challenges that can be encountered in managing such patients. Physiology of prone position ventilation: When a person is supine, the weight of the ventral lungs, heart, and abdominal viscera increase dorsal pleural pressure. This compression reduces transpulmonary pressure in the dorsal lung regions. The increased mass of the edematous ARDS lung further increases the ventral-dorsal pleural pressure gradient and reduces regional ventilation of dependent dorsal regions. The ventral heart is estimated to contribute approximately an additional 3 to 5 cm of water pressure to the underlying lung tissue. In addition to the weight of the heart, intraabdominal pressure is preferentially transmitted through the diaphragm, further compressing dorsal regions. Although these factors tend to collapse dependent dorsal regions, the gravitational gradient in vascular pressures preferentially perfuses these regions, yielding a region of low ventilation and high perfusion, manifesting clinically as hypoxemia. Placing a person in the prone position reduces the pleural pressure gradient from nondependent to dependent regions, in part through gravitational effects and conformational shape matching of the lung to the chest cavity2[Figure 1]. Clinical evidence: A few large randomized clinical trials, conducted over a period of 15 years, investigated the possible benefit of prone position on ARDS outcome [Table 1]. The improvements in oxygenation apparent in most trials were not associated with improvements in mortality, suggesting that oxygenation is not itself the source of improved survival with prone positioning. Most recently, the PROSEVA study group1 enrolled 466 subjects with moderate-to-severe ARDS. Mortality at 28 and 90 days was significantly lower with prone position versus supine position (16% vs 33%, respectively, p < 0.001, and 24% vs 41%, respectively, p < 0.001). Challenges: There are only a few absolute contraindications to prone positioning, such as unstable vertebral fractures and unmonitored or significantly increased intracranial pressure. Hemodynamic instability and cardiac rhythm disturbances are some of the relative contraindications. The common complications of prone positioning are pressure ulcers, ventilator-associated pneumonia and endotracheal tube obstruction. More serious fatal events such as accidental extubation is rare (zero to 2.4% prevalence). A recent meta-analysis of the safety and efficacy of the maneuver showed that it is safe and inexpensive but requires teamwork and skill. Reports in the literature suggest that the incidence of adverse events is significantly reduced in the presence of trained and experienced staff. Thus, centers with less experience may have difficulty managing complications, but nursing care protocols and guidelines can mitigate this risk4. Conclusion: Prone position ventilation in patients with moderate-to-severe ARDS improves hypoxemia, provides mortality benefit and is relatively safe.

Regional distribution of transpulmonary pressure.

The pressure across the lung, so-called transpulmonary pressure (PL), represents the main force acting toward to provide lung movement. During mechanical ventilation, PL is provided by respiratory system pressurization, using specific ventilator setting settled by the operator, such as: tidal volume (VT), positive end-expiratory pressure (PEEP), respiratory rate (RR), and inspiratory airway flow. Once PL is developed throughout the lungs, its distribution is heterogeneous, being explained by the elastic properties of the lungs and pleural pressure gradient. There are different methods of PL calculation, each one with importance and some limitations. Among the most known, it can be quoted: (I) direct measurement of PL; (II) elastance derived method at end-inspiration of PL; (III) transpulmonary driving pressure. Recent studies using pleural sensors in large animal models as also in human cadaver have added new and important information about PL heterogeneous distribution across the lungs. Due to this heterogeneous distribution, lung damage could happen in specific areas of the lung. In addition, it is widely accepted that high PL can cause lung damage, however the way it is delivered, whether it's compressible or tensile, may also further damage despite the values of PL achieved. According to heterogeneous distribution of PL across the lungs, the interstitium and lymphatic vessels may also interplay to disseminate lung inflammation toward peripheral organs through thoracic lymph tracts. Thus, it is conceivable that juxta-diaphragmatic area associated strong efforts leading to high values of PL may be a source of dissemination of inflammatory cells, large molecules, and plasma contents able to perpetuate inflammation in distal organs.

Continuous negative abdominal pressure: mechanism of action and comparison with prone position.

We recently reported that continuous negative abdominal pressure (CNAP) could recruit dorsal atelectasis in experimental lung injury and that oxygenation improved at different transpulmonary pressure values compared with increases in airway pressure (Yoshida T, Engelberts D, Otulakowski G, Katira BH, Post M, Ferguson ND, Brochard L, Amato MBP, Kavanagh BP. Am J Respir Crit Care Med 197: 534-537, 2018). The mechanism of recruitment with CNAP is uncertain, and its impact compared with a commonly proposed alternative approach to recruitment, prone positioning, is not known. We hypothesized that CNAP recruits lung by decreasing the vertical pleural pressure (Ppl) gradient (i.e., difference between dependent and nondependent Ppl), thought to be one mechanism of action of prone positioning. An established porcine model of lung injury (surfactant depletion followed by ventilator-induced lung injury) was used. CNAP was applied using a plexiglass chamber that completely enclosed the abdomen at a constant negative pressure (-5 cmH2O). Lungs were recruited to maximal positive end-expiratory pressure (PEEP; 25 cmH2O) and deflated in steps of PEEP (2 cmH2O, 10 min each). CNAP lowered the Ppl in dependent but not in nondependent lung, and therefore, in contrast to PEEP, it narrowed the vertical Ppl gradient. CNAP increased respiratory system compliance and oxygenation and appeared to selectively displace the posterior diaphragm caudad (computerized tomography images). Compared with prone position without CNAP, CNAP in the supine position was associated with higher arterial partial pressure of oxygen and compliance, as well as greater homogeneity of ventilation. The mechanism of action of CNAP appears to be via selective narrowing of the vertical gradient of Ppl. CNAP appears to offer physiological advantages over prone positioning. NEW & NOTEWORTHY Continuous negative abdominal pressure reduces the vertical gradient in (dependent vs. nondependent) pleural pressure and increases oxygenation and lung compliance; it is more effective than prone positioning at comparable levels of positive end-expiratory pressure.

Gas Exchange in the Prone Posture.

The prone posture is known to have numerous effects on gas exchange, both under normal conditions and in patients with ARDS. Clinical studies have consistently demonstrated improvements in oxygenation, and a multi-center randomized trial found that, when implemented within 48 h of moderate-to-severe ARDS, placing subjects in the prone posture decreased mortality. Improvements in gas exchange occur via several mechanisms: alterations in the distribution of alveolar ventilation, redistribution of blood flow, improved matching of local ventilation and perfusion, and reduction in regions of low ventilation/perfusion ratios. Ventilation heterogeneity is reduced in the prone posture due to more uniform alveolar size secondary to a more uniform vertical pleural pressure gradient. The prone posture results in more uniform pulmonary blood flow when compared with the supine posture, due to an anatomical bias for greater blood flow to dorsal lung regions. Because both ventilation and perfusion heterogeneity decrease in the prone posture, gas exchange improves. Other benefits include a more uniform distribution of alveolar stress, relief of left-lower-lobe lung compression by the heart, enhanced secretion clearance, and favorable right-ventricular and systemic hemodynamics.

Pleural pressure theory revisited: a role for capillary equilibrium.

Theories elucidating pleural pressures should explain all observations including the equal and opposite recoil of the chest wall and lungs, the less than expected pleural hydrostatic gradient and its variation at lobar margins, why pleural pressures are negative and how pleural fluid circulation functions. A theoretical model describing equilibrium between buoyancy, hydrostatic forces, and capillary forces is proposed. The capillary equilibrium model described depends on control of pleural fluid volume and protein content, powered by an active pleural pump. The interaction between buoyancy forces, hydrostatic pressure and capillary pressure was calculated, and values for pleural thickness and pressure were determined using values for surface tension, contact angle, pleural fluid and lung densities found in the literature. Modelling can explain the issue of the differing hydrostatic vertical pleural pressure gradient at the lobar margins for buoyancy forces between the pleural fluid and the lung floating in the pleural fluid according to Archimedes' hydrostatic paradox. The capillary equilibrium model satisfies all salient requirements for a pleural pressure model, with negative pressures maximal at the apex, equal and opposite forces in the lung and chest wall, and circulatory pump action. This model predicts that pleural effusions cannot occur in emphysema unless concomitant heart failure increases lung density. This model also explains how the non-confluence of the lung with the chest wall (e.g., lobar margins) makes the pleural pressure more negative, and why pleural pressures would be higher after an upper lobectomy compared to a lower lobectomy. Pathological changes in pleural fluid composition and lung density alter the equilibrium between capillarity and buoyancy hydrostatic pressure to promote pleural effusion formation.

A Comprehensive Review of Prone Position in ARDS.

Prone position (PP) has been used since the 1970s to treat severe hypoxemia in patients with ARDS because of its effectiveness at improving gas exchange. Compared with the supine position (SP), placing patients in PP effects a more even tidal volume distribution, in part, by reversing the vertical pleural pressure gradient, which becomes more negative in the dorsal regions. PP also improves resting lung volume in the dorsocaudal regions by reducing the superimposed pressure of both the heart and the abdomen. In contrast, pulmonary perfusion remains preferentially distributed to the dorsal lung regions, thus improving overall alveolar ventilation/perfusion relationships. Moreover, the larger tissue mass suspended from a wider dorsal chest wall effects a more homogeneous distribution of pleural pressures throughout the lung that reduces abnormal strain and stress development. This is believed to ameliorate the severity or development of ventilator-induced lung injury and may partly explain why PP reduces mortality in severe ARDS. Over 40 years of clinical trials have consistently reported improved oxygenation in approximately 70% of subjects with ARDS. Early initiation of PP is more likely to improve oxygenation than initiation during the subacute phase. Maximal oxygenation improvement occurs over a wide time frame ranging from several hours to several days. Meta-analyses of randomized controlled trials suggest that PP provides a survival advantage only in patients with relatively severe ARDS (PaO2 /FIO2 < 150 mm Hg). Moreover, survival is enhanced when patients are managed with a smaller tidal volume (≤ 8 mL/kg), higher PEEP (10-13 cm H2O), and longer duration of PP sessions (> 10-12 h/session). Combining adjunctive therapies (high PEEP, recruitment maneuvers, and inhaled vasodilators) with PP has an additive effect in improving oxygenation and may be particularly helpful in stabilizing gas exchange in very severe ARDS.

Lagerungstherapie beim akuten Lungenversagen

The collapse of lung tissue, edema and intrapulmonary shunt are the main symptoms in patients with acute respiratory insufficiency. The techniques of ventilation in a prone position and continuous lateral rotational therapy (CLRT) are based on these pathophysiological changes. Ventilation in a prone position was found to improve ventilation and perfusion relationships and reduction in the pleural pressure gradient. In hypoxemic lung failure (PaO(2)/FIO(2) <100) a prone position was found to improve oxygenation as a rescue measure and to improve survival. In contrast CLRT is considered to be an early therapeutic or prophylactic measure aimed at prevention of ventilation-associated complications. In trauma patients these beneficial effects were demonstrated in several studies. Positioning therapy can be accompanied by potentially serious complications (e.g. face and skin ulceration, accidental loss of tubes and catheters and cardiac arrhythmias) and its use requires routine management and exact knowledge of indications and risks.

Supine and prone differences in regional lung density and pleural pressure gradients in the human lung with constant shape

The explanation for prone and supine differences in tissue density and pleural pressure gradients in the healthy lung has been inferred from several studies as compression of dependent tissue by the heart in the supine posture; however, this hypothesis has not been directly confirmed. Differences could also arise from change in shape of the chest wall and diaphragm, and because of shape with respect to gravity. The contribution of this third mechanism is explored here. Tissue density and static elastic recoil were estimated in the supine and prone left human lung at functional residual capacity using a finite-element analysis. Supine model geometries were derived from multidetector row computed tomography imaging of two subjects: one normal (subject 1), and one with small airway disease (subject 2). For each subject, the prone model was the supine lung shape with gravity reversed; therefore, the prone model was isolated from the influence of displacement of the diaphragm, chest wall, or heart. Model estimates were validated against multidetector row computed tomography measurement of regional density for each subject supine and an independent study of the prone and supine lung. The magnitude of the gradient in density supine (-4.33%/cm for subject 1, and -4.96%/cm for subject 2) was nearly twice as large as for the prone lung (-2.72%/cm for subject 1, and -2.51%/cm for subject 2), consistent with measurements in dogs. The corresponding pleural pressure gradients were 0.54 cmH(2)O/cm (subject 1) and 0.56 cmH(2)O/cm (subject 2) for supine, and 0.29 cmH(2)O/cm (subject 1) and 0.27 cmH(2)O/cm (subject 2) for prone. A smaller prone gradient was predicted without shape change of the "container" or support of the heart by the lung. The influence of the heart was to constrain the shape in which the lung deformed.

Pleural mechanics and fluid exchange.

The pleural space separating the lung and chest wall of mammals contains a small amount of liquid that lubricates the pleural surfaces during breathing. Recent studies have pointed to a conceptual understanding of the pleural space that is different from the one advocated some 30 years ago in this journal. The fundamental concept is that pleural surface pressure, the result of the opposing recoils of the lung and chest wall, is the major determinant of the pressure in the pleural liquid. Pleural liquid is not in hydrostatic equilibrium because the vertical gradient in pleural liquid pressure, determined by the vertical gradient in pleural surface pressure, does not equal the hydrostatic gradient. As a result, a viscous flow of pleural liquid occurs in the pleural space. Ventilatory and cardiogenic motions serve to redistribute pleural liquid and minimize contact between the pleural surfaces. Pleural liquid is a microvascular filtrate from parietal pleural capillaries in the chest wall. Homeostasis in pleural liquid volume is achieved by an adjustment of the pleural liquid thickness to the filtration rate that is matched by an outflow via lymphatic stomata.

Prone position in acute respiratory distress syndrome.

In the last few years prone positioning has been used increasingly in the treatment of patients with acute respiratory distress syndrome (ARDS) and this manoeuvre is now considered a simple and safe method to improve oxygenation. However, the physiological mechanisms causing respiratory function improvement as well as the real clinical benefit are not yet fully understood. The aim of this review is to discuss the physiological and clinical effects of prone positioning in patients with ARDS. The main physiological aims of prone positioning are: 1) to improve oxygenation; 2) to improve respiratory mechanics; 3) to homogenise the pleural pressure gradient, the alveolar inflation and the ventilation distribution; 4) to increase lung volume and reduce the amount of atelectatic regions; 5) to facilitate the drainage of secretions; and 6) to reduce ventilator-associated lung injury. According to the available data, the authors conclude that: 1) oxygenation improves in approximately 70-80% of patients with early acute respiratory distress syndrome; 2) the beneficial effects of oxygenation reduce after 1 week of mechanical ventilation; 3) the aetiology of acute respiratory distress syndrome may markedly affect the response to prone positioning; 4) extreme care is necessary when the manoeuvre is performed; 5) pressure sores are frequent and related to the number of pronations; 6) the supports used to prone and during positioning are different and nonstandardised among centres; and 7) intensive care unit and hospital stay and mortality still remain high despite prone positioning.

Multibreath washout analysis: modelling the influence of conducting airway asymmetry

The contributions of respiratory and conducting airway asymmetry, gas exchange, and non-uniform ventilation to the sloping alveolar plateau in phase III of the gas washout curve are investigated using mathematical models of the human lung. The models range from fully symmetric, to a detailed asymmetric conducting airway model coupled with 29 445 lumped parameter respiratory airway models. A gas transport equation is solved in the models using a Lagrange-Galerkin method. The alveolar slope is normalised by the mean expired gas concentration to give S n. The model results confirm that first breath S n is influenced mainly by respiratory airway asymmetry, and that at near-normal levels of ventilation the conducting airway asymmetry contributes a significant amount to further increases in S n. Gas exchange moderates the plot of S n against breath number, such that it approaches a plateau in later breaths. Non-uniform flow also alters S n, indicating that an accurate description of the pleural pressure gradient will be necessary for more accurate simulations.