Changes In Positive End-expiratory Pressure Research Articles

Transpulmonary pressure and lung elastance can be estimated by a PEEP-step manoeuvre.

Transpulmonary pressure is a key factor for protective ventilation. This requires measurements of oesophageal pressure that is rarely used clinically. A simple method may be found, if it could be shown that tidal and positive end-expiratory pressure (PEEP) inflation of the lungs with the same volume increases transpulmonary pressure equally. The aim of the present study was to compare tidal and PEEP inflation of the respiratory system. A total of 12 patients with acute respiratory failure were subjected to PEEP trials of 0-4-8-12-16 cmH2O. Changes in end-expiratory lung volume (ΔEELV) following a PEEP step were determined from cumulative differences in inspiratory-expiratory tidal volumes. Oesophageal pressure was measured with a balloon catheter. Following a PEEP increase from 0 to 16 cmH2O end-expiratory oesophageal pressure did not increase (0.5 ± 4.0 cmH2O). Average increase in EELV following a PEEP step of 4 cmH2O was 230 ± 132 ml. The increase in EELV was related to the change in PEEP divided by lung elastance (El) derived from oesophageal pressure as ΔPEEP/El. There was a good correlation between transpulmonary pressure by oesophageal pressure and transpulmonary pressure based on El determined as ΔPEEP/ΔEELV, r(2) = 0.80, y = 0.96x, mean bias -0.4 ± 3.0 cmH2 O with limits of agreement from 5.4 to -6.2 cmH2O (2 standard deviations). PEEP inflation of the respiratory system is extremely slow, and allows the chest wall complex, especially the abdomen, to yield and adapt to intrusion of the diaphragm. As a consequence a change in transpulmonary pressure is equal to the change in PEEP and transpulmonary pressure can be determined without oesophageal pressure measurements.

Oxygenation response to positive end-expiratory pressure predicts mortality in acute respiratory distress syndrome. A secondary analysis of the LOVS and ExPress trials.

Previous trials of higher positive end-expiratory pressure (PEEP) for acute respiratory distress syndrome (ARDS) failed to demonstrate mortality benefit, possibly because of differences in lung recruitability among patients with ARDS. To determine whether the physiological response to increased PEEP is associated with mortality. In a secondary analysis of the Lung Open Ventilation Study (LOVS, n = 983), we examined the relationship between the initial response to changes in PEEP after randomization and mortality. We sought to corroborate our findings using data from a different trial of higher PEEP (ExPress, n = 749). The oxygenation response (change in ratio of arterial partial pressure of oxygen to fraction of inspired oxygen: P/F) after the initial change in PEEP after randomization varied widely (median, 9.5 mm Hg; interquartile range, -16 to 47) and was only weakly related to baseline P/F or the magnitude of PEEP change. Among patients in whom PEEP was increased after randomization, an increase in P/F was associated with reduced mortality (multivariable logistic regression; adjusted odds ratio, 0.80 [95% confidence interval, 0.72-0.89] per 25-mm Hg increase in P/F), particularly in patients with severe disease (baseline P/F [less-than-or-equal-to] 150 mm Hg). Changes in compliance and dead space were not associated with mortality. These findings were confirmed by a similar analysis of data from the ExPress trial. Patients with ARDS who respond to increased PEEP by improved oxygenation have a lower risk of death. The oxygenation response to PEEP might be used to predict whether patients will benefit from higher versus lower PEEP.

Changes in Positive End-Expiratory Pressure Alter the Distribution of Ventilation within the Lung Immediately after Birth in Newborn Rabbits

Current recommendations suggest the use of positive end-expiratory pressures (PEEP) to assist very preterm infants to develop a functional residual capacity (FRC) and establish gas exchange at birth. However, maintaining a consistent PEEP is difficult and so the lungs are exposed to changing distending pressures after birth, which can affect respiratory function. Our aim was to determine how changing PEEP levels alters the distribution of ventilation within the lung. Preterm rabbit pups (28 days gestation) were delivered and mechanically ventilated with one of three strategies, whereby PEEP was changed in sequence; 0-5-10-5-0 cmH2O, 5-10-0-5-0 cmH2O or 10-5-0-10-0 cmH2O. Phase contrast X-ray imaging was used to analyse the distribution of ventilation in the upper left (UL), upper right (UR), lower left (LL) and lower right (LR) quadrants of the lung. Initiating ventilation with 10PEEP resulted in a uniform increase in FRC throughout the lung whereas initiating ventilation with 5PEEP or 0PEEP preferentially aerated the UR than both lower quadrants (p<0.05). Consequently, the relative distribution of incoming VT was preferentially directed into the lower lobes at low PEEP, primarily due to the loss of FRC in those lobes. Following ventilation at 10PEEP, the distribution of air at end-inflation was uniform across all quadrants and remained so regardless of the PEEP level. Uniform distribution of ventilation can be achieved by initiating ventilation with a high PEEP. After the lungs have aerated, small and stepped reductions in PEEP result in more uniform changes in ventilation.

Effects of Prone Positioning on Lung Protection in Patients with Acute Respiratory Distress Syndrome

Positive end-expiratory pressure (PEEP) and prone positioning may induce lung recruitment and affect alveolar dynamics in acute respiratory distress syndrome (ARDS). Whether there is interdependence between the effects of PEEP and prone positioning on these variables is unknown. To determine the effects of high PEEP and prone positioning on lung recruitment, cyclic recruitment/derecruitment, and tidal hyperinflation and how these effects are influenced by lung recruitability. Mechanically ventilated patients (Vt 6 ml/kg ideal body weight) underwent whole-lung computed tomography (CT) during breath-holding sessions at airway pressures of 5, 15, and 45 cm H2O and Cine-CTs on a fixed thoracic transverse slice at PEEP 5 and 15 cm H2O. CT images were repeated in supine and prone positioning. A recruitment maneuver at 45 cm H2O was performed before each PEEP change. Lung recruitability was defined as the difference in percentage of nonaerated tissue between 5 and 45 cm H2O. Cyclic recruitment/de-recruitment and tidal hyperinflation were determined as tidal changes in percentage of nonaerated and hyperinflated tissue, respectively. Twenty-four patients with ARDS were included. Increasing PEEP from 5 to 15 cm H2O decreased nonaerated tissue (501 ± 201 to 322 ± 132 grams; P < 0.001) and increased tidal-hyperinflation (0.41 ± 0.26 to 0.57 ± 0.30%; P = 0.004) in supine. Prone positioning further decreased nonaerated tissue (322 ± 132 to 290 ± 141 grams; P = 0.028) and reduced tidal hyperinflation observed at PEEP 15 in supine patients (0.57 ± 0.30 to 0.41 ± 0.22%). Cyclic recruitment/de-recruitment only decreased when high PEEP and prone positioning were applied together (4.1 ± 1.9 to 2.9 ± 0.9%; P = 0.003), particularly in patients with high lung recruitability. Prone positioning enhances lung recruitment and decreases alveolar instability and hyperinflation observed at high PEEP in patients with ARDS.

Pleural Pressure and Optimal Positive End-Expiratory Pressure Based on Esophageal Pressure Versus Chest Wall Elastance

1) To compare two published methods for estimating pleural pressure, one based on directly measured esophageal pressure and the other based on chest wall elastance. 2) To evaluate the agreement between two published positive end-expiratory pressure optimization strategies based on these methods, one targeting an end-expiratory esophageal pressure-based transpulmonary pressure of 0 cm H2O and the other targeting an end-inspiratory elastance-based transpulmonary pressure of 26 cm H2O.Retrospective study using clinical data.Medical and surgical ICUs.Sixty-four patients mechanically ventilated for acute respiratory failure with esophageal balloons placed for clinical management.Esophageal pressure and chest wall elastance-based methods for estimating pleural pressure and setting positive end-expiratory pressure were retrospectively applied to each of the 64 patients. In patients who were ventilated at two positive end-expiratory pressure levels, chest wall and respiratory system elastances were calculated at each positive end-expiratory pressure level.The pleural pressure estimates using both methods were discordant and differed by as much as 10 cm H2O for a given patient. The two positive end-expiratory pressure optimization strategies recommended positive end-expiratory pressure changes in opposite directions in 33% of patients. The ideal positive end-expiratory pressure levels recommended by the two methods for each patient were discordant and uncorrelated (R = 0.05). Chest wall and respiratory system elastances grew with increases in positive end-expiratory pressure in patients with positive end-expiratory esophageal pressure-based transpulmonary pressures (p < 0.05).Esophageal pressure and chest wall elastance-based methods for estimating pleural pressure do not yield similar results. The strategies of targeting an end-expiratory esophageal pressure-based transpulmonary pressure of 0 cm H2O and targeting an end-inspiratory elastance-based transpulmonary pressure of 26 cm H2O cannot be considered interchangeable. Finally, chest wall and respiratory system elastances may vary unpredictably with changes in positive end-expiratory pressure.

Time to reach a new steady state after changes of positive end expiratory pressure

To assess the time interval required to reach a new steady state of oxygenation-, ventilation-, respiratory mechanics- and hemodynamics-related variables after decreasing/increasing positive end expiratory pressure (PEEP). In 23 patients (group 1) with acute respiratory distress syndrome (ARDS), PEEP was decreased from 10 to 5cmH2O and, after 60', it was increased from 5 to 15cmH2O. In 21 other ARDS patients (group 2), PEEP was increased from 10 to 15cmH2O and, after 60', decreased from 15 to 5cmH2O. Oxygenation, ventilation, respiratory mechanics and hemodynamic variables were recorded at time 5', 15', 30' and 60' after each PEEP change. When PEEP was decreased, PaO2, PaO2/FiO2, venous admixture and arterial oxygen saturation reached their equilibrium after 5'. In contrast, when PEEP was increased, the equilibrium was not reached even after 60'. The ventilation-related variables did not change significantly with PEEP. The respiratory system compliance, when PEEP was decreased, significantly worsened only after 60'. Hemodynamics did not change significantly with PEEP. In the individual patients the change of oxygenation-related variables and of respiratory system compliance observed after 5' could predict the changes recorded after 60'. This was not possible for PaCO2. We could not find a unique equilibration time for all the considered variables. However, in general, a decremental PEEP test requires far lower equilibrium time than an incremental PEEP test, suggesting a different time course for derecruitment and recruitment patterns.

Time required for gas exchange equilibration after a change of positive end-expiratory pressure in acute respiratory distress syndrome

Positive end-expiratory pressure (PEEP) is fundamental to prevent lung collapse in ARDS patients. A common method to titrate PEEP is to perform a PEEP test, recording the variation of cardiorespiratory parameters after a PEEP change [1]. The aim of this study was to evaluate the time-course changes of gas exchanges following a PEEP test.

Elucidating the fuzziness in physician decision making in ARDS

The current standard of care for patients suffering from acute respiratory distress syndrome (ARDS) is ventilation with a tidal volume of 6 ml/kg predicted body weight (PBW), but variability remains in the tidal volumes that are actually used. This study aims to identify patient scenarios for which there is discordance between physicians in choice of tidal volume and positive end-expiratory pressure (PEEP) in ARDS patients. We developed an algorithm based on fuzzy logic for encapsulating the expertise of individual physicians regarding their use of tidal volume and PEEP in ARDS patients. The algorithm uses three input measurements: (1) peak airway pressure (PAP), (2) PEEP, and (3) arterial oxygen saturation (SaO₂). It then generates two output parameters: (1) the deviation of tidal volume from 6 ml/kg PBW, and (2) the change in PEEP from its current value. We captured 6 realizations of intensivist expertise in this algorithm and assessed their degree of concordance using a Monte Carlo simulation. Variability in the tidal volume recommended by the algorithm increased for PAP > 30 cmH₂O and PEEP > 5 cmH₂O. Tidal volume variability decreased for SaO₂ > 90 %. Variability in the recommended change in PEEP increased for PEEP > 5 cmH₂O and for SaO₂ near 90 %. Intensivists vary in their management of ARDS patients when peak airway pressures and PEEP are high, suggesting that the current goal of 6 ml/kg PBW may need to be revisited under these conditions.

Prolonged Injury and Altered Lung Function after Ozone Inhalation in Mice with Chronic Lung Inflammation

Surfactant protein-D (Sftpd) is a pulmonary collectin important in down-regulating macrophage inflammatory responses. In these experiments, we analyzed the effects of chronic macrophage inflammation attributable to loss of Sftpd on the persistence of ozone-induced injury, macrophage activation, and altered functioning in the lung. Wild-type (Sftpd(+/+)) and Sftpd(-/-) mice (aged 8 wk) were exposed to air or ozone (0.8 parts per million, 3 h). Bronchoalveolar lavage (BAL) fluid and tissue were collected 72 hours later. In Sftpd(-/-) mice, but not Sftpd(+/+) mice, increased BAL protein and nitrogen oxides were observed after ozone inhalation, indicating prolonged lung injury and oxidative stress. Increased numbers of macrophages were also present in BAL fluid and in histologic sections from Sftpd(-/-) mice. These cells were enlarged and foamy, suggesting that they were activated. This conclusion was supported by findings of increased BAL chemotactic activity, and increased expression of inducible nitric oxide synthase in lung macrophages. In both Sftpd(+/+) and Sftpd(-/-) mice, inhalation of ozone was associated with functional alterations in the lung. Although these alterations were limited to central airway mechanics in Sftpd(+/+) mice, both central airway and parenchymal mechanics were modified by ozone exposure in Sftpd(-/-) mice. The most notable changes were evident in resistance and elastance spectra and baseline lung function, and in lung responsiveness to changes in positive end-expiratory pressure. These data demonstrate that a loss of Sftpd is associated with prolonged lung injury, oxidative stress, and macrophage accumulation and activation in response to ozone, and with more extensive functional changes consistent with the loss of parenchymal integrity.

Model-Based Approach to Estimate dFRC in the ICU Using Measured Lung Dynamics

Acute Respiratory Distress Syndrome (ARDS) is characterized by inflammation, filling of the lung with fluid and collapsed lung unit. Mechanical ventilation (MV) is used to treat ARDS/ALI using positive end expiratory pressure (PEEP) to recruit and retain lung units, thus increasing pulmonary volume and dynamic functional residual capacity (dFRC) at the end of expiration. However, simple methods to measure dFRC at the bedside currently do not exist and other methods are invasive and impractical to carry out on a regular basis. Stress-strain theory is used to estimate ΔdFRC, which represents the extra pulmonary volume due to PEEP, utilizing readily available patient data from a single breath. The model uses commonly controlled or measured parameters (lung compliance, plateau airway pressure, PV data) to identify a parameter β 1 as a function of PEEP and tidal volume. A median β 1 value is calculated for each PEEP level over a cohort and is hypothesised as a constant throughout the population for the particular PEEP. Estimated ΔdFRC values are then compared to measured values to assess accuracy of the model. ΔFRC was calculated for 9 patients and compared to the measured values. The median percentage error was 40.29% [IQR: 14.20-55.39] for PEEP = 5cmH 2 O, 31.12% [IQR: 10.53-192.71] for PEEP = 10cmH 2 O, 20.8% [IQR: 7.51-81.06] for PEEP = 15cmH 2 O, 15.44% [IQR: 11.92-36.18] for PEEP = 20cmH 2 O, 19.7% [IQR: 4.79-20.76] for PEEP = 25cmH 2 O and 11.78% [IQR: 2.99-27.5] for PEEP = 30cmH 2 O. Linear regression between estimated and measured ΔdFRC produced R 2 = 0.862. The model-based approach offers a simple and non-invasive method which does not require interruption of MV to estimate dFRC. The clinical accuracy of the model is limited but was able to track the impact of changes in PEEP and tidal volume on dFRC, on a breath-by-breath basis for each PEEP.

The Effect of Changing Positive End-Expiratory Pressures on the Spatial Pattern of Lung Ventilation in Newborn Rabbits Ventilated from Birth

Background: Positive end-expiratory pressure (PEEP) helps very preterm newborns develop functional residual capacity (FRC) and establish adequate gas exchange at birth. In the immediate newborn period, lungs commonly experience changes in PEEP, particularly when the face mask is removed and replaced. This study aimed to determine the distribution of lung ventilation at different PEEPs and in response to changes in PEEP.

Positive end-expiratory pressure-induced changes in end-expiratory lung volume measured by spirometry and electric impedance tomography

A bedside tool for monitoring changes in end-expiratory lung volume (ΔEELV) would be helpful to set optimal positive end-expiratory pressure (PEEP) in acute lung injury/acute respiratory distress syndrome patients. The hypothesis of this study was that the cumulative difference of the inspiratory and expiratory tidal volumes of the first 10 breaths after a PEEP change accurately reflects the change in lung volume following a PEEP alteration. Changing PEEP induces lung volume changes, which are reflected in differences between inspiratory and expiratory tidal volumes measured by spirometry. By adding these differences with correction for offset, for the first 10 breaths after PEEP change, cumulative tidal volume difference was calculated to estimate ΔEELV(VT) ((i-e)) . This method was evaluated in a lung model and in patients with acute respiratory failure during a PEEP trial. In patients, ΔEELV(VT) ((i-e)) were compared with simultaneously measured changes in lung impedance, by electric impedance tomography (EIT), using calibration vs. tidal volume to estimate changes in ΔEELV(EIT) . In the lung model, there was close correlation (R(2) = 0.99) between ΔEELV(VT) ((i-e)) and known lung model volume difference, with a bias of -4 ml and limits of agreement of 42 and -50 ml. In 12 patients, ΔEELV(EIT) was closely correlated to ΔEELV(VT) ((i-e)) (R(2) = 0.92), with mean bias of 50 ml and limits of agreement of 131 and -31 ml. Changes in EELV estimated by EIT (ΔEELV(EIT) ) exceeded measurements by spirometry (ΔEELV(VT) ((i-e)) ), with 15 (±15)%. We conclude that spirometric measurements of inspiratory-expiratory tidal volumes agree well with impedance changes monitored by EIT and can be used bedside to estimate PEEP-induced changes in EELV.

Data-driven classification of ventilated lung tissues using electrical impedance tomography

Current methods for identifying ventilated lung regions utilizing electrical impedance tomography images rely on dividing the image into arbitrary regions of interest (ROI), manually delineating ROI, or forming ROI with pixels whose signal properties surpass an arbitrary threshold. In this paper, we propose a novel application of a data-driven classification method to identify ventilated lung ROI based on forming k clusters from pixels with correlated signals. A standard first-order model for lung mechanics is then applied to determine which ROI correspond to ventilated lung tissue. We applied the method in an experimental study of 16 mechanically ventilated swine in the supine position, which underwent changes in positive end-expiratory pressure (PEEP) and fraction of inspired oxygen (FIO2). In each stage of the experimental protocol, the method performed best with k = 4 and consistently identified 3 lung tissue ROI and 1 boundary tissue ROI in 15 of the 16 subjects. When testing for changes from baseline in lung position, tidal volume, and respiratory system compliance, we found that PEEP displaced the ventilated lung region dorsally by 2 cm, decreased tidal volume by 1.3%, and increased the respiratory system compliance time constant by 0.3 s. FIO2 decreased tidal volume by 0.7%. All effects were tested at p < 0.05 with n = 16. These findings suggest that the proposed ROI detection method is robust and sensitive to ventilation dynamics in the experimental setting.

Unintentional variation in positive end expiratory pressure during resuscitation with a T-piece resuscitator

Introduction The ability of T-piece resuscitators to deliver consistent peak inspiratory pressure (PIP) and positive end expiratory pressure (PEEP) during real and simulated neonatal resuscitation has been well described. The Neopuff (Fisher & Paykel Healthcare, Auckland, New Zealand) has been the device used for nearly all of these comparisons. Methods All high risk resuscitations were carried out in our dedicated resuscitation room, and were recorded on video tape for quality assurance purposes. 1 In addition to the audio and video recording, physiologic signals and resuscitation parameters, including oxygen saturation, pulse rate, airway pressure, FiO 2, and others signals as appropriate were also captured. These recordings were reviewed on a biweekly basis as part of a continuing quality review process. Resuscitations were graded for standard of care and the resuscitation checklist was reviewed to determine if the team had any unresolved issues that needed to be addressed. Results In the year between April of 2009 and March 2010, a period when we fully reviewed approximately 120 videos, we recognized visually obvious PEEP changes on 8 different occasions in ELBW infants. Our target PEEP was 5 cm H20. We observed rapid changes in the PEEP to as high as 15 cm H20 during resuscitation. Conclusion Based on our single-center experience, a T-piece resuscitation device which has the adjustment for the PEEP level and the orifice which is occluded to deliver a breath in the same location has the potential to cause an inadvertent and potentially toxic increase of PEEP which might not be noticed by the operator.

Physiologic Response to Changing Positive End-Expiratory Pressure During Neurally Adjusted Ventilatory Assist in Sedated, Critically Ill Adults

Neurally adjusted ventilatory assist (NAVA) delivers airway pressure (Paw) in proportion to neural inspiratory drive as reflected by electrical activity of the diaphragm (EAdi). Changing positive end-expiratory pressure (PEEP) impacts respiratory muscle load and function and, hence, EAdi. We aimed to evaluate how PEEP affects the breathing pattern and neuroventilatory efficiency during NAVA. In 20 adult patients, adequate assist (NAVAal) was first identified based on Paw and tidal volume (Vt) responses to systematic increases in NAVA level while using preset PEEP (PEEPbl). Thereafter, using NAVAal, PEEP was increased to 20 cm water (H(2)O) (PEEPhigh) and then lowered stepwise to 1 cm H(2)O (PEEP1). EAdi, Paw, and Vt were recorded. Median NAVAal was 2.7 (interquartile range, 2.3-3.5) cm H(2)O/muV and was similar to NAVAal identified post hoc by 17 independent physicians (2.5 [2.0-3.4] cm H(2)O/muV; P = NS). Reducing PEEPhigh to PEEP1 increased inspiratory EAdi by 34% (2-67; P = .046) and was associated with an increase in mean Paw above PEEP from 8.5 (6.7-11.4) cm H(2)O to 12.2 (8.8-16.7) cm H(2)O (P = .008), whereas Vt and respiratory rate remained unchanged. The response pattern in Vt/EAdi, indicating changes in neuroventilatory efficiency, differed among patients. Tidal breathing occurred at the lowest EAdi cost in seven patients with PEEP1 or half PEEPbl, in six patients with PEEPbl, and in seven patients with PEEPhigh. During NAVAal, increasing PEEP reduces respiratory drive. Patients adapt their neuroventilatory efficiency such that the individual ventilatory pattern is preserved over a wide range of PEEP levels. Monitoring Vt/EAdi during PEEP changes allows identification of a PEEP level at which tidal breathing occurs at minimal EAdi cost. clinicaltrials.gov; Identifier: NCT00529347.

Dynamic functional residual capacity can be estimated using a stress–strain approach

Background Acute Respiratory Distress Syndrome (ARDS) results in collapse of alveolar units and loss of lung volume at the end of expiration. Mechanical ventilation is used to treat patients with ARDS or Acute Lung Injury (ALI), with the end objective being to increase the dynamic functional residual capacity (dFRC), and thus increasing overall functional residual capacity (FRC). Simple methods to estimate dFRC at a given positive end expiratory pressure (PEEP) level in patients with ARDS/ALI currently does not exist. Current viable methods are time-consuming and relatively invasive. Methods Previous studies have found a constant linear relationship between the global stress and strain in the lung independent of lung condition. This study utilizes the constant stress–strain ratio and an individual patient's volume responsiveness to PEEP to estimate dFRC at any level of PEEP. The estimation model identifies two global parameters to estimate a patient specific dFRC, β and mβ. The parameter β captures physiological parameters of FRC, lung and respiratory elastance and varies depending on the PEEP level used, and mβ is the gradient of β vs. PEEP. Results dFRC was estimated at different PEEP values and compared to the measured dFRC using retrospective data from 12 different patients with different levels of lung injury. The median percentage error is 18% (IQR: 6.49) for PEEP = 5 cm H 2O, 10% (IQR: 9.18) for PEEP = 7 cm H 2O, 28% (IQR: 12.33) for PEEP = 10 cm H 2O, 3% (IQR: 2.10) for PEEP = 12 cm H 2O and 10% (IQR: 9.11) for PEEP = 15 cm H 2O. The results were further validated using a cross-correlation ( N = 100,000). Linear regression between the estimated and measured dFRC with a median R 2 of 0.948 (IQR: 0.915, 0.968; 90% CI: 0.814, 0.984) over the N = 100,000 cross-validation tests. Conclusions The results suggest that a model based approach to estimating dFRC may be viable in a clinical scenario without any interruption to ventilation and can thus provide an alternative to measuring dFRC by disconnecting the patient from the ventilator or by using advanced ventilators. The overall results provide a means of estimating dFRC at any PEEP levels. Although reasonable clinical accuracy is limited to the linear region of the static PV curve, the model can evaluate the impact of changes in PEEP or other mechanical ventilation settings.

Regional aeration and perfusion distribution in a sheep model of endotoxemic acute lung injury characterized by functional computed tomography imaging

Sepsis-related lung injury is the most common and morbid form of acute lung injury. The objective of this study was to develop an ovine model of septic acute lung injury and characterize its pathophysiology regarding its recruitability and changes in regional aeration and perfusion distributions at injury and during injury evolution. Experimental animal study. University hospital research laboratory. Adult sheep. Twenty-one anesthetized and mechanically ventilated sheep received intravenous Escherichia coli endotoxin infusion until severe hypoxemia was obtained. Inspiratory- and expiratory-gated computed tomography images of the entire lung were acquired in six subjects at baseline, during endotoxin infusion, and at injury. Perfusion images were obtained at apex and base locations at baseline and injury. Computed tomography images were analyzed for total, air, and tissue lung volumes and axial and vertical aeration and perfusion gradients. Lung recruitability was studied in a subgroup of subjects after injury. Computed tomography imaging showed a patchy, progressive decrease in air volume as injury evolved, partially replaced by an increase in tissue volume. Perfusion showed a nondependent-to-dependent gradient at baseline that remained relatively unchanged with injury. Perfusion to poorly aerated lung regions was unchanged or increased after injury. Aeration and perfusion distributions at baseline were primarily dorsal or dependent. After injury, the heterogeneity of perfusion and aeration increased and the effect of gravity decreased. Recruitment maneuvers and changes in positive end-expiratory pressure resulted in no improvement in aeration or oxygenation. The severe hypoxemia, moderate volume loss, and perfusion patterns are consistent with an injury model in which hypoxemia is exacerbated by endotoxin-mediated failure of hypoxic pulmonary vasoconstriction.

A minimal model of lung mechanics and model-based markers for optimizing ventilator treatment in ARDS patients

A majority of patients admitted to the Intensive Care Unit (ICU) require some form of respiratory support. In the case of Acute Respiratory Distress Syndrome (ARDS), the patient often requires full intervention from a mechanical ventilator. ARDS is also associated with mortality rate as high as 70%. Despite many recent studies on ventilator treatment of the disease, there are no well established methods to determine the optimal Positive End-Expiratory Pressure (PEEP) or other critical ventilator settings for individual patients. A model of fundamental lung mechanics is developed based on capturing the recruitment status of lung units. The main objective of this research is to develop a minimal model that is clinically effective in determining PEEP. The model was identified for a variety of different ventilator settings using clinical data. The fitting error was between 0.1% and 4% over the inflation limb and between 0.3% and 13% over the deflation limb at different PEEP settings. The model produces good correlation with clinical data, and is clinically applicable due to the minimal number of patient specific parameters to identify. The ability to use this identified patient specific model to optimize ventilator management is demonstrated by its ability to predict the patient specific response of PEEP changes before clinically applying them. Predictions of recruited lung volume change with change in PEEP have a median absolute error of 1.87% (IQR: 0.93–4.80%; 90% CI: 0.16–11.98%) for inflation and a median of 5.76% (IQR: 2.71–10.50%; 90% CI: 0.43–17.04%) for deflation, across all data sets and PEEP values ( N = 34 predictions). This minimal model thus provides a clinically useful and relatively simple platform for continuous patient specific monitoring of lung unit recruitment for a patient.

Effect of positive end-expiratory pressure on regional ventilation monitored by electrical impedance tomography in mechanically ventilated ICU patients

The optimal positive end-expiratory pressure (PEEP) is a balance between the prevention of overdistention in the nondependent part and alveolar collapse in the dependent part. Electrical impedance tomography (EIT) has been introduced to monitor regional change of ventilation at the bedside. We evaluated the effect of changes in PEEP on regional ventilation in mechanically ventilated patients with or without lung disorders.

The relationship between arterial Po2 and mixed venous Po2 in response to changes in positive end-expiratory pressure in ventilated patients

The relationship between arterial Po2 and mixed venous Po2 in response to changes in positive end-expiratory pressure in ventilated patients