Recruitment and derecruitment during acute respiratory failure: a clinical study

Protective ventilation in experimental acute respiratory distress syndrome after ventilator-induced lung injury: a randomized controlled trial

Background

Low tidal volume (V_T), PEEP, and low plateau pressure (P_PLAT) are lung protective during acute respiratory distress syndrome (ARDS). This study tested the hypothesis that the aspiration of dead space (ASPIDS) together with computer simulation can help maintain gas exchange at these settings, thus promoting protection of the lungs.

Methods

ARDS was induced in pigs using surfactant perturbation plus an injurious ventilation strategy. One group then underwent 24 h protective ventilation, while control groups were ventilated using a conventional ventilation strategy at either high or low pressure. Pressure–volume curves (P_el/V), blood gases, and haemodynamics were studied at 0, 4, 8, 16, and 24 h after the induction of ARDS and lung histology was evaluated.

Results

The P_el/V curves showed improvements in the protective strategy group and deterioration in both control groups. In the protective group, when respiratory rate (RR) was ≈60 bpm, better oxygenation and reduced shunt were found. Histological damage was significantly more severe in the high-pressure group. There were no differences in venous oxygen saturation and pulmonary vascular resistance between the groups.

Conclusions

The protective ventilation strategy of adequate pH or Pa_CO2 with minimal V_T, and high/safe P_PLAT resulting in high PEEP was based on the avoidance of known lung-damaging phenomena. The approach is based upon the optimization of V_T, RR, PEEP, I/E, and dead space. This study does not lend itself to conclusions about the independent role of each of these features. However, dead space reduction is fundamental for achieving minimal V_T at high RR. Classical physiology is applicable at high RR. Computer simulation optimizes ventilation and limiting of dead space using ASPIDS. Inspiratory P_el/V curves recorded from PEEP or, even better, expiratory P_el/V curves allow monitoring in ARDS.

Physiological relevance and performance of a minimal lung model: an experimental study in healthy and acute respiratory distress syndrome model piglets

Background

Mechanical ventilation (MV) is the primary form of support for acute respiratory distress syndrome (ARDS) patients. However, intra- and inter- patient-variability reduce the efficacy of general protocols. Model-based approaches to guide MV can be patient-specific. A physiological relevant minimal model and its patient-specific performance are tested to see if it meets this objective above.

Methods

Healthy anesthetized piglets weighing 24.0 kg [IQR: 21.0-29.6] underwent a step-wise PEEP increase manoeuvre from 5cmH₂O to 20cmH₂O. They were ventilated under volume control using Engström Care Station (Datex, General Electric, Finland), with pressure, flow and volume profiles recorded. ARDS was then induced using oleic acid. The data were analyzed with a Minimal Model that identifies patient-specific mean threshold opening and closing pressure (TOP and TCP), and standard deviation (SD) of these TOP and TCP distributions. The trial and use of data were approved by the Ethics Committee of the Medical Faculty of the University of Liege, Belgium.

Results and discussions

3 of the 9 healthy piglets developed ARDS, and these data sets were included in this study. Model fitting error during inflation and deflation, in healthy or ARDS state is less than 5.0% across all subjects, indicating that the model captures the fundamental lung mechanics during PEEP increase. Mean TOP was 42.4cmH₂O [IQR: 38.2-44.6] at PEEP = 5cmH₂O and decreased with PEEP to 25.0cmH₂O [IQR: 21.5-27.1] at PEEP = 20cmH₂O. In contrast, TCP sees a reverse trend, increasing from 10.2cmH₂O [IQR: 9.0-10.4] to 19.5cmH₂O [IQR: 19.0-19.7]. Mean TOP increased from average 21.2-37.4cmH₂O to 30.4-55.2cmH₂O between healthy and ARDS subjects, reflecting the higher pressure required to recruit collapsed alveoli. Mean TCP was effectively unchanged.

Conclusion

The minimal model is capable of capturing physiologically relevant TOP, TCP and SD of both healthy and ARDS lungs. The model is able to track disease progression and the response to treatment.

Visual anatomical lung CT scan assessment of lung recruitability

PURPOSE

The computation of lung recruitability in acute respiratory distress syndrome (ARDS) is advocated to set positive end-expiratory pressure (PEEP) for preventing lung collapse. The quantitative lung CT scan, obtained by manual image processing, is the reference method but it is time consuming. The aim of this study was to evaluate the accuracy of a visual anatomical analysis compared with a quantitative lung CT scan analysis in assessing lung recruitability.

METHODS

Fifty sets of two complete lung CT scans of ALI/ARDS patients computing lung recruitment were analyzed. Lung recruitability computed at an airway pressure of 5 and 45 cm H(2)O was defined as the percentage decrease in the collapsed/consolidated lung parenchyma assessed by two expert radiologists using a visual anatomical analysis and as the decrease in not aerated lung regions using a quantitative analysis computed by dedicated software.

RESULTS

Lung recruitability was 11.3 % (interquartile range 7.39-16.41) and 15.5 % (interquartile range 8.18-21.43) with the visual anatomical and quantitative analysis, respectively. In the Bland-Altman analysis, the bias and agreement bands between the visual anatomical and quantitative analysis were -2.9 % (-11.8 to +5.9 %). The ROC curve showed that the optimal cutoff values for the visual anatomical analysis in predicting high versus low lung recruitability was 8.9 % (area under the ROC curve 0.9248, 95 % CI 0.8550-0.9946). Considering this cutoff, the sensitivity, specificity, and diagnostic accuracy were 0.96, 0.76, and 0.86, respectively.

CONCLUSIONS

Visual anatomical analysis can classify patients into those with high and low lung recruitability allowing more intensivists to get access to lung recruitability assessment.

Effect of regional lung inflation on ventilation heterogeneity at different length scales during mechanical ventilation of normal sheep lungs

Heterogeneous, small-airway diameters and alveolar derecruitment in poorly aerated regions of normal lungs could produce ventilation heterogeneity at those anatomic levels. We modeled the washout kinetics of (13)NN with positron emission tomography to examine how specific ventilation (sV) heterogeneity at different length scales is influenced by lung aeration. Three groups of anesthetized, supine sheep were studied: high tidal volume (Vt; 18.4 ± 4.2 ml/kg) and zero end-expiratory pressure (ZEEP) (n = 6); low Vt (9.2 ± 1.0 ml/kg) and ZEEP (n = 6); and low Vt (8.2 ± 0.2 ml/kg) and positive end-expiratory pressure (PEEP; 19 ± 1 cmH(2)O) (n = 4). We quantified fractional gas content with transmission scans, and sV with emission scans of infused (13)NN-saline. Voxel (13)NN-washout curves were fit with one- or two-compartment models to estimate sV. Total heterogeneity, measured as SD[log(10)(sV)], was divided into length-scale ranges by measuring changes in variance of log(10)(sV), resulting from progressive filtering of sV images. High-Vt ZEEP showed higher sV heterogeneity at <12- (P < 0.01), 12- to 36- (P < 0.01), and 36- to 60-mm (P < 0.05) length scales compared with low-Vt PEEP, with low-Vt ZEEP in between. Increased heterogeneity was associated with the emergence of low sV units in poorly aerated regions, with a high correlation (r = 0.95, P < 0.001) between total heterogeneity and the fraction of lung with slow washout. Regional mean fractional gas content was inversely correlated with regional sV heterogeneity at <12- (r = -0.67), 12- to 36- (r = -0.74), and >36-mm (r = -0.72) length scales (P < 0.001). We conclude that sV heterogeneity at length scales <60 mm increases in poorly aerated regions of mechanically ventilated normal lungs, likely due to heterogeneous small-airway narrowing and alveolar derecruitment. PEEP reduces sV heterogeneity by maintaining lung expansion and airway patency at those small length scales.

Structural identifiability and practical applicability of an alveolar recruitment model for ARDS patients

Patient-specific mathematical models of respiratory mechanics can offer substantial insight into patient state and pulmonary dynamics that are not directly measurable. Thus, they offer significant potential to evaluate and guide patient-specific lung protective ventilator strategies for acute respiratory distress syndrome (ARDS) patients. To assure bedside applicability, the model must be computationally efficient and identifiable from the limited available data, while also capturing dominant dynamics and trends observed in ARDS patients. In this study, an existing static recruitment model is enhanced by considering alveolar distension and implemented in a novel time-continuous dynamic respiratory mechanics model. The model was tested for structural identifiability and a hierarchical gradient descent approach was used to fit the model to low-flow test responses of 12 ARDS patients. Finally, a comprehensive practical identifiability analysis was performed to evaluate the impact of data quality on the model parameters. Identified parameter values were physiologically plausible and very accurately reproduced the measured pressure responses. Structural identifiability of the model was proven, but practical identifiability analysis of the results showed a lack of convexity on the error surface indicating that successful parameter identification is currently not assured in all test sets. Overall, the model presented is physiologically and clinically relevant, captures ARDS dynamics, and uses clinically descriptive parameters. The patient-specific models show the ability to capture pulmonary dynamics directly relevant to patient condition and clinical guidance. These characteristics currently cannot be directly measured or established without such a validated model.

[Positive end-expiratory pressure : adjustment in acute lung injury]

Treatment of patients suffering from acute lung injury is a challenge for the treating physician. In recent years ventilation of patients with acute hypoxic lung injury has changed fundamentally. Besides the use of low tidal volumes, the most beneficial setting of positive end-expiratory pressure (PEEP) has been in the focus of researchers. The findings allow adaption of treatment to milder forms of acute lung injury and severe forms. Additionally computed tomography techniques to assess the pulmonary situation and recruitment potential as well as bed-side techniques to adjust PEEP on the ward have been modified and improved. This review gives an outline of recent developments in PEEP adjustment for patients suffering from acute hypoxic and hypercapnic lung injury and explains the fundamental pathophysiology necessary as a basis for correct treatment.

Time-dependent recruitment effects in ventilated healthy and lung-injured rats: "recruitment-memory"

We investigated the sustained effects of recruitment manoeuvres in terms of "recruitment memory" in healthy and lung injured rats. 46 ventilated rats were allocated to either the control (sham) or the lavage group. Two consecutive low-flow manoeuvres were performed before sham/lavage and hourly during a 2-h-observation period. The slopes of the inspiratory limbs of the two resulting pressure-volume loops were translated into compliance-volume curves. The difference between the two compliance curves was smaller after lavage (root-mean-square deviation: 0.065 ml/cm H2O control group, 0.038 ml/cm H2O lavage group; p<0.05) and stayed small during the whole experiment. In the control group, the deviation was small after sham manoeuvre but increased throughout the experiment. Compliance gain after recruitment was higher in the control group (0.1 ml/cm H2O) compared to the lavage group (0.02 ml/cm H2O, p<0.05). We conclude that lung lavage led to alveolar collapse not susceptible to recruitment manoeuvres. On the contrary in healthy lungs recruitment manoeuvres led to persistent lung recruitment which we interpret as recruitment memory.

Analysis of regional compliance in a porcine model of acute lung injury

Lung protective ventilation in acute lung injury (ALI) focuses on using low tidal volumes and adequate levels of positive end-expiratory pressure (PEEP). Identifying optimal pressure is difficult because pressure-volume (PV) relations differ regionally. Precise analysis demands local measurements of pressures and related alveolar morphologies. In a porcine model of surfactant depletion (n=24), we combined measuring static pressures with endoscopic microscopy and electrical impedance tomography (EIT) to examine regional PV loops and morphologic heterogeneities between healthy (control group; CON) and ALI lungs ventilated with low (LVT) or high tidal volumes (HVT). Quantification included indices for microscopy (Volume Air Index (VAI), Heterogeneity and Circularity Index), EIT analysis and calculation of regional compliances due to generated PV loops. We found that: (1) VAI decreased in lower lobe after ALI, (2) electrical impedance decreased in dorsal regions and (3) PV loops differed regionally. Further studies should prove the potentials of these techniques on individual respiratory settings and clinical outcome.

Pulmonary mechanics during mechanical ventilation

The use of mechanical ventilation has become widespread in the management of hypoxic respiratory failure. Investigations of pulmonary mechanics in this clinical scenario have demonstrated that there are significant differences in compliance, resistance and gas flow when compared with normal subjects. This paper will review the mechanisms by which pulmonary mechanics are assessed in mechanically ventilated patients and will review how the data can be used for investigative research purposes as well as to inform rational ventilator management.

Use of computed tomography scanning to guide lung recruitment and adjust positive-end expiratory pressure

PURPOSE OF REVIEW

We discuss the possible role of computed tomography (CT) to guide protective mechanical ventilation in acute lung injury/acute respiratory distress syndrome (ALI/ARDS), especially tidal volume (VT) and positive-end expiratory pressure (PEEP) settings and recruitment manoeuvres.

RECENT FINDINGS

CT should be used as early as possible after the onset of ALI/ARDS and then repeated after 1 week in the absence of clinical improvement. Advantages of CT include: the regional response to recruitment can be determined; it is objective; the morphofunctional correlations obtained are useful for a comprehensive patient evaluation. CT should be performed at different pressure levels to identify potential for recruitment. Initially, one single whole-lung CT scan is performed at end-expiration at PEEP 5-10 cmH2O to evaluate aeration and compute lung weight. Afterwards, two lung CT slices are performed to assess lung recruitability (at PEEP = 5-10 cmH2O; inspiratory plateau pressure of the respiratory system = 45 cmH2O).

SUMMARY

In ALI/ARDS patients, CT reveals discrepancies between bedside chest radiograph and various clinical and physiological parameters, and it is essential to assess lung morphology and recruitability. Specific algorithms, including or not CT, should be used to better identify ALI/ARDS with potential of recruitment and setting of VT and PEEP.

Model-based optimal PEEP in mechanically ventilated ARDS patients in the intensive care unit

BACKGROUND

The optimal level of positive end-expiratory pressure (PEEP) is still widely debated in treating acute respiratory distress syndrome (ARDS) patients. Current methods of selecting PEEP only provide a range of values and do not provide unique patient-specific solutions. Model-based methods offer a novel way of using non-invasive pressure-volume (PV) measurements to estimate patient recruitability. This paper examines the clinical viability of such models in pilot clinical trials to assist therapy, optimise patient-specific PEEP, assess the disease state and response over time.

METHODS

Ten patients with acute lung injury or ARDS underwent incremental PEEP recruitment manoeuvres. PV data was measured at increments of 5 cmH2O and fitted to the recruitment model. Inspiratory and expiratory breath holds were performed to measure airway resistance and auto-PEEP. Three model-based metrics are used to optimise PEEP based on opening pressures, closing pressures and net recruitment. ARDS status was assessed by model parameters capturing recruitment and compliance.

RESULTS

Median model fitting error across all patients for inflation and deflation was 2.8% and 1.02% respectively with all patients experiencing auto-PEEP. In all three metrics' cases, model-based optimal PEEP was higher than clinically selected PEEP. Two patients underwent multiple recruitment manoeuvres over time and model metrics reflected and tracked the state or their ARDS.

CONCLUSIONS

For ARDS patients, the model-based method presented in this paper provides a unique, non-invasive method to select optimal patient-specific PEEP. In addition, the model has the capability to assess disease state over time using these same models and methods.

Prone position and recruitment manoeuvre: the combined effect improves oxygenation

Introduction

Among the various methods for improving oxygenation while decreasing the risk of ventilation-induced lung injury in patients with acute respiratory distress syndrome (ARDS), a ventilation strategy combining prone position (PP) and recruitment manoeuvres (RMs) can be practiced. We studied the effects on oxygenation of both RM and PP applied in early ARDS patients.

Methods

We conducted a prospective study. Sixteen consecutive patients with early ARDS fulfilling our criteria (ratio of arterial oxygen partial pressure to fraction of inspired oxygen (PaO₂/FiO₂) 98.3 ± 28 mmHg; positive end expiratory pressure, 10.7 ± 2.8 cmH₂O) were analysed. Each patient was ventilated in both the supine position (SP) and the PP (six hours in each position). A 45 cmH₂O extended sigh in pressure control mode was performed at the beginning of SP (RM1), one hour after turning to the PP (RM2) and at the end of the six-hour PP period (RM3).

Results

The mean arterial oxygen partial pressure (PaO₂) changes after RM1, RM2 and RM3 were 9.6%, 15% and 19%, respectively. The PaO₂ improvement after a single RM was significant after RM3 only (P < 0.05). Improvements in PaO₂ level and PaO₂/FiO₂ ratio were transient in SP but durable during PP. PaO₂/FiO₂ ratio peaked at 218 mmHg after RM3. PaO₂/FiO₂ changes were significant only after RM3 and in the pulmonary ARDS group (P = 0.008). This global strategy had a benefit with regard to oxygenation: PaO₂/FiO₂ ratio increased from 98.3 mmHg to 165.6 mmHg 13 hours later at the end of the study (P < 0.05). Plateau airway pressures decreased after each RM and over the entire PP period and significantly after RM3 (P = 0.02). Some reversible side effects such as significant blood arterial pressure variations were found when extended sighs were performed.

Conclusions

In our study, interventions such as a 45 cmH₂O extended sigh during PP resulted in marked oxygenation improvement. Combined RM and PP led to the highest increase in PaO₂/FiO₂ ratio without major clinical side effects.

Efficacy and safety of recruitment maneuvers in acute respiratory distress syndrome

Recruitment maneuvers (RM) consist of a ventilatory strategy that increases the transpulmonary pressure transiently to reopen the recruitable lung units in acute respiratory distress syndrome (ARDS). The rationales to use RM in ARDS are that there is a massive loss of aerated lung and that once the end-inspiratory pressure surpasses the regional critical opening pressure of the lung units, those units are likely to reopen. There are different methods to perform RM when using the conventional ICU ventilator. The three RM methods that are mostly used and investigated are sighs, sustained inflation, and extended sigh. There is no standardization of any of the above RM. Meta-analysis recommended not to use RM in routine in stable ARDS patients but to run them in case of life-threatening hypoxemia. There are some concerns regarding the safety of RM in terms of hemodynamics preservation and lung injury as well. The rapid rising in pressure can be a factor that explains the potential harmful effects of the RM. In this review, we describe the balance between the beneficial effects and the harmful consequences of RM. Recent animal studies are discussed.

Quantitative imaging of alveolar recruitment with hyperpolarized gas MRI during mechanical ventilation

The aim of this study was to assess the utility of (3)He MRI to noninvasively probe the effects of positive end-expiratory pressure (PEEP) maneuvers on alveolar recruitment and atelectasis buildup in mechanically ventilated animals. Sprague-Dawley rats (n = 13) were anesthetized, intubated, and ventilated in the supine position ((4)He-to-O(2) ratio: 4:1; tidal volume: 10 ml/kg, 60 breaths/min, and inspiration-to-expiration ratio: 1:2). Recruitment maneuvers consisted of either a stepwise increase of PEEP to 9 cmH(2)O and back to zero end-expiratory pressure or alternating between these two PEEP levels. Diffusion MRI was performed to image (3)He apparent diffusion coefficient (ADC) maps in the middle coronal slices of lungs (n = 10). ADC was measured immediately before and after two recruitment maneuvers, which were separated from each other with a wait period (8-44 min). We detected a statistically significant decrease in mean ADC after each recruitment maneuver. The relative ADC change was -21.2 ± 4.1 % after the first maneuver and -9.7 ± 5.8 % after the second maneuver. A significant relative increase in mean ADC was observed over the wait period between the two recruitment maneuvers. The extent of this ADC buildup was time dependent, as it was significantly related to the duration of the wait period. The two postrecruitment ADC measurements were similar, suggesting that the lungs returned to the same state after the recruitment maneuvers were applied. No significant intrasubject differences in ADC were observed between the corresponding PEEP levels in two rats that underwent three repeat maneuvers. Airway pressure tracings were recorded in separate rats undergoing one PEEP maneuver (n = 3) and showed a significant relative difference in peak inspiratory pressure between pre- and poststates. These observations support the hypothesis of redistribution of alveolar gas due to recruitment of collapsed alveoli in presence of atelectasis, which was also supported by the decrease in peak inspiratory pressure after recruitment maneuvers.

Ventilator-induced lung injury: the anatomical and physiological framework

Since its introduction into the management of the acute respiratory distress syndrome, mechanical ventilation has been so strongly interwoven with its side effects that it came to be considered as invariably dangerous. Over the decades, attention has shifted from gross barotrauma to volutrauma and, more recently, to atelectrauma and biotrauma. In this article, we describe the anatomical and physiologic framework in which ventilator-induced lung injury may occur. We address the concept of lung stress/strain as applied to the whole lung or specific pulmonary regions. We challenge some common beliefs, such as separately studying the dangerous effects of different tidal volumes (end inspiration) and end-expiratory positive pressures. Based on available data, we suggest that stress at rupture is only rarely reached and that high tidal volume induces ventilator-induced lung injury by augmenting the pressure heterogeneity at the interface between open and constantly closed units. We believe that ventilator-induced lung injury occurs only when a given threshold is exceeded; below this limit, mechanical ventilation is likely to be safe.

Cardiac output estimation using pulmonary mechanics in mechanically ventilated patients

The application of positive end expiratory pressure (PEEP) in mechanically ventilated (MV) patients with acute respiratory distress syndrome (ARDS) decreases cardiac output (CO). Accurate measurement of CO is highly invasive and is not ideal for all MV critically ill patients. However, the link between the PEEP used in MV, and CO provides an opportunity to assess CO via MV therapy and other existing measurements, creating a CO measure without further invasiveness.

This paper examines combining models of diffusion resistance and lung mechanics, to help predict CO changes due to PEEP. The CO estimator uses an initial measurement of pulmonary shunt, and estimations of shunt changes due to PEEP to predict CO at different levels of PEEP. Inputs to the cardiac model are the PV loops from the ventilator, as well as the oxygen saturation values using known respiratory inspired oxygen content. The outputs are estimates of pulmonary shunt and CO changes due to changes in applied PEEP. Data from two published studies are used to assess and initially validate this model.

The model shows the effect on oxygenation due to decreased CO and decreased shunt, resulting from increased PEEP. It concludes that there is a trade off on oxygenation parameters. More clinically importantly, the model also examines how the rate of CO drop with increased PEEP can be used as a method to determine optimal PEEP, which may be used to optimise MV therapy with respect to the gas exchange achieved, as well as accounting for the impact on the cardiovascular system and its management.

Pros and cons of recruitment maneuvers in acute lung injury and acute respiratory distress syndrome

In patients with acute lung injury and acute respiratory distress syndrome, a protective mechanical ventilation strategy characterized by low tidal volumes has been associated with reduced mortality. However, such a strategy may result in alveolar collapse, leading to cyclic opening and closing of atelectatic alveoli and distal airways. Thus, recruitment maneuvers (RMs) have been used to open up collapsed lungs, while adequate positive end-expiratory pressure (PEEP) levels may counteract alveolar derecruitment during low tidal volume ventilation, improving respiratory function and minimizing ventilator-associated lung injury. Nevertheless, considerable uncertainty remains regarding the appropriateness of RMs. The most commonly used RM is conventional sustained inflation, associated with respiratory and cardiovascular side effects, which may be minimized by newly proposed strategies: prolonged or incremental PEEP elevation; pressure-controlled ventilation with fixed PEEP and increased driving pressure; pressure-controlled ventilation applied with escalating PEEP and constant driving pressure; and long and slow increase in pressure. The efficiency of RMs may be affected by different factors, including the nature and extent of lung injury, capability of increasing inspiratory transpulmonary pressures, patient positioning and cardiac preload. Current evidence suggests that RMs can be used before setting PEEP, after ventilator circuit disconnection or as a rescue maneuver to overcome severe hypoxemia; however, their routine use does not seem to be justified at present. The development of new lung recruitment strategies that have fewer hemodynamic and biological effects on the lungs, as well as randomized clinical trials analyzing the impact of RMs on morbidity and mortality of acute lung injury/acute respiratory distress syndrome patients, are warranted.

The physical basis of ventilator-induced lung injury

Although mechanical ventilation (MV) is a life-saving intervention for patients with acute respiratory distress syndrome (ARDS), it can aggravate or cause lung injury, known as ventilator-induced lung injury (VILI). The biophysical characteristics of heterogeneously injured ARDS lungs increase the parenchymal stress associated with breathing, which is further aggravated by MV. Cells, in particular those lining the capillaries, airways and alveoli, transform this strain into chemical signals (mechanotransduction). The interaction of reparative and injurious mechanotransductive pathways leads to VILI. Several attempts have been made to identify clinical surrogate measures of lung stress/strain (e.g., density changes in chest computed tomography, lower and upper inflection points of the pressure-volume curve, plateau pressure and inflammatory cytokine levels) that could be used to titrate MV. However, uncertainty about the topographical distribution of stress relative to that of the susceptibility of the cells and tissues to injury makes the existence of a single 'global' stress/strain injury threshold doubtful.

Alveolar recruitment maneuver in patients with subarachnoid hemorrhage and acute respiratory distress syndrome: a comparison of 2 approaches

PURPOSE

The purpose of the study was to compare 2 alveolar recruitment maneuvers (ARMs) approaches in patients with subarachnoid hemorrhage (SAH) and acute respiratory distress syndrome (ARDS).

MATERIAL AND METHODS

Sixteen SAH patients with ARDS were randomized in 2 similar groups. One received ARM with continuous positive airway pressure (CPAP) of 35 cm H(2)O for 40 seconds (CPAP recruitment), whereas the other received pressure control ventilation with positive-end expiratory pressure of 15 cm H(2)O and pressure control above positive end-expiratory pressure of 35 cm H(2)O for 2 minutes (pressure control recruitment maneuver [PCRM]). Intracranial pressure (ICP) and cerebral perfusion pressure (CPP) were measured before and after ARM. The ratio of arterial oxygen tension to fraction of inspired oxygen was measured before and 1 hour after the ARM.

RESULTS

After ARM, ICP was higher in CPAP recruitment (20.50 ± 4.75 vs 13.13 ± 3.56 mm Hg; P = .003); and CPP was lower in CPAP recruitment (62.38 ± 9.81 vs 79.60 ± 6.8 mm Hg; P = .001). One hour after the ARM, the ratio of arterial oxygen tension to fraction of inspired oxygen increased significantly only in PCRM (108.5 to 203.6; P = .0078).

CONCLUSION

In SAH patients with ARDS, PCRM did not affect ICP and decreased CPP in safe levels, besides improving oxygenation.

Modeling the complex dynamics of derecruitment in the lung

Recruitment maneuvers using deep inflations (DI) have long been used clinically with the objective of recruiting collapsed regions of the lung. Considerable uncertainty continues to exist, however, as to how best to employ recruitment maneuvers or even if they should be used routinely at all for patients receiving mechanical ventilation. Much of this uncertainty may arise from a lack of understanding about the dynamic nature of recruitment and derecruitment. To shed some light on this complex issue, we developed a time-dependent computational model of recruitment and derecruitment in the lung based on a symmetrically bifurcating airway tree in which each branch has a critical closing and opening pressure as well as pressure-dependent opening and closing speeds. Starting from the fully open state, the model underwent regular ventilation for 8 min followed by a series of identical DIs separated by 5 min of identical regular ventilation. We found that the geographical nature and extent of derecruitment before and 5 min after each DI were not always the same, demonstrating that the model exhibits multiple stable states. We conclude that the effectiveness of a recruitment maneuver is not only simply a function of the duration and magnitude of a DI, but may also have an unpredictable component arising from the distributed bi-stable nature of the derecruitment process itself.

The physical basis of ventilator-induced lung injury

Although mechanical ventilation (MV) is a life-saving intervention for patients with acute respiratory distress syndrome (ARDS), it can aggravate or cause lung injury, known as ventilator-induced lung injury (VILI). The biophysical characteristics of heterogeneously injured ARDS lungs increase the parenchymal stress associated with breathing, which is further aggravated by MV. Cells, in particular those lining the capillaries, airways and alveoli, transform this strain into chemical signals (mechanotransduction). The interaction of reparative and injurious mechanotransductive pathways leads to VILI. Several attempts have been made to identify clinical surrogate measures of lung stress/strain (e.g., density changes in chest computed tomography, lower and upper inflection points of the pressure-volume curve, plateau pressure and inflammatory cytokine levels) that could be used to titrate MV. However, uncertainty about the topographical distribution of stress relative to that of the susceptibility of the cells and tissues to injury makes the existence of a single 'global' stress/strain injury threshold doubtful.

Absolute electrical impedance tomography (aEIT) guided ventilation therapy in critical care patients: simulations and future trends

Thoracic electrical impedance tomography (EIT) is a noninvasive, radiation-free monitoring technique whose aim is to reconstruct a cross-sectional image of the internal spatial distribution of conductivity from electrical measurements made by injecting small alternating currents via an electrode array placed on the surface of the thorax. The purpose of this paper is to discuss the fundamentals of EIT and demonstrate the principles of mechanical ventilation, lung recruitment, and EIT imaging on a comprehensive physiological model, which combines a model of respiratory mechanics, a model of the human lung absolute resistivity as a function of air content, and a 2-D finite-element mesh of the thorax to simulate EIT image reconstruction during mechanical ventilation. The overall model gives a good understanding of respiratory physiology and EIT monitoring techniques in mechanically ventilated patients. The model proposed here was able to reproduce consistent images of ventilation distribution in simulated acutely injured and collapsed lung conditions. A new advisory system architecture integrating a previously developed data-driven physiological model for continuous and noninvasive predictions of blood gas parameters with the regional lung function data/information generated from absolute EIT (aEIT) is proposed for monitoring and ventilator therapy management of critical care patients.

Ventilator management for hypoxemic respiratory failure attributable to H1N1 novel swine origin influenza virus

Novel H1N1 swine origin influenza virus has led to a worldwide pandemic. During the pandemic, a significant number of patients became critically ill primarily because of respiratory failure. Most of these patients required intubation and mechanical ventilation and were treated with conventional modes of mechanical ventilation using a lung-protective strategy with low tidal volumes, plateau pressures <30 to 35 cm H2O, and optimal positive end-expiratory pressure. In some patients with persistent hypoxemia, alternative modes of ventilation, such as high-frequency oscillatory ventilation and airway pressure release ventilation, were used. We review the ventilatory management, recruitment maneuvers, prone positioning, and goals of ventilatory therapy for hypoxemic respiratory failure in general, as well as lessons learned in the management of H1N1-related respiratory failure.

Positive end-expiratory pressure

PURPOSE OF REVIEW

In the last 2 years, several reports have dealt with recruitment/positive end-expiratory pressure (PEEP) selection. Most of them confirm previous results and few add new information.

RECENT FINDINGS

It has been definitely confirmed that opening pressures are different throughout the acute respiratory distress syndrome lung parenchyma, ranging from 5-10 up to 30-40 cmH2O. The highest opening pressures are required to open the most dependent lung regions. It has been found that in 2 s, most of the recruitable lung regions may be open when a proper pressure is applied. The best way to assess recruitment is computed tomography scanning, whereas lung mechanics are a reasonable bedside surrogate. Impedance tomography has been increasingly tested, whereas gas exchange is the less reliable indicator of recruitment. A large outcome study showed that higher PEEP might provide survival benefit in a subgroup of more severe patients as compared with lower PEEP. To set PEEP in each individual patient, the use of the expiratory limb of the pressure-volume curve has been suggested. Setting PEEP according to transpulmonary pressure has a robust physiological background, although it requires confirmatory study.

SUMMARY

Indiscriminate application of recruitment maneuver in unselected acute respiratory distress syndrome population does not provide benefits. However, in the most severe patients, recruitment maneuver has to be considered and higher PEEP applied. To individualize PEEP, the expiratory phase has to be considered, and the esophageal pressure measurement to compute the transpulmonary pressure should be progressively introduced in clinical practice.

[The basics on mechanical ventilation support in acute respiratory distress syndrome]

Acute Respiratory Distress Syndrome (ARDS) is understood as an inflammation-induced disruption of the alveolar endothelial-epithelial barrier that results in increased permeability and surfactant dysfunction followed by alveolar flooding and collapse. ARDS management relies on mechanical ventilation. The current challenge is to determine the optimal ventilatory strategies that minimize ventilator-induced lung injury (VILI) while providing a reasonable gas exchange. The data support that a tidal volume between 6-8 ml/kg of predicted body weight providing a plateau pressure < 30 cmH₂O should be used. High positive end expiratory pressure (PEEP) has not reduced mortality, nevertheless secondary endpoints are improved. The rationale used for high PEEP argues that it prevents cyclic opening and closing of airspaces, probably the major culprit of development of VILI. Chest computed tomography has contributed to our understanding of anatomic-functional distribution patterns in ARDS. Electric impedance tomography is a technique that is radiation-free, but still under development, that allows dynamic monitoring of ventilation distribution at bedside.

Lung protective ventilation strategies in paediatrics-A review

Ventilator Associated Lung Injury (VALI) is an iatrogenic phenomena that significantly impacts on the morbidity and mortality of critically ill patients. The hazards associated with mechanical ventilation are becoming increasingly understood courtesy of a large body of research. Barotrauma, volutrauma and biotrauma all play a role in VALI. Concomitant to this growth in understanding is the development of strategies to reduce the deleterious impact of mechanical ventilation. The majority of the research is based upon adult populations but with careful extrapolation this review will focus on paediatrics. This review article describes the physiological basis of VALI and discusses the various lung protective strategies that clinicians can employ to minimise its incidence and optimise outcomes for paediatric patients.

Acute lung injury and acute respiratory distress syndrome

Every year, more information accumulates about the possibility of treating patients with acute lung injury or acute respiratory distress syndrome with specially designed mechanical ventilation strategies. Ventilator modes, positive end-expiratory pressure settings, and recruitment maneuvers play a major role in these strategies. However, what can we take from these experimental and clinical data to the clinical practice? In this article, we discuss substantial options of mechanical ventilation together with some adjunctive therapeutic measures, such as prone positioning and inhalation of nitric oxide.

VTCO2 and dynamic compliance-guided lung recruitment in surfactant-depleted piglets: a computed tomography study

OBJECTIVE

Using computed tomography (CT) as reference, our primary objectives were to test if maximal tidal elimination of carbon dioxide (VTCO2) could be used as a marker of "optimal recruitment," indicating maximal available lung tissue for gas exchange and if a decrease in dynamic compliance (Cdyn) indicated the beginning of lung collapse during a downward positive end-expiratory pressure (PEEP) titration.

DESIGN

Prospective laboratory animal investigation.

SETTING

Clinical physiology research laboratory.

SUBJECTS

Six piglets undergoing lung lavage.

INTERVENTIONS

Saline-lavaged piglets were initially ventilated without PEEP at a tidal volume (VT) of 10 mL/kg followed by baseline ventilation at end-inspiratory pressure (EIP) 25 cm H2O and PEEP 6 cm H2O. PEEP was increased to 12 or 15 cm H2O. Then EIP was increased in steps of 5 cm H2O and the EIP where VTCO2 peaked or leveled off was assumed to define optimally recruited lungs. A downward PEEP titration followed from 12 or 15 to 4 cm H2O in steps of 1 cm H2O. First decline of Cdyn was assumed to define onset of lung collapse. VTCO2 and Cdyn were continuously recorded and CT scans iterated for each change of ventilation. "Open-lung PEEP" was set 2 cm H2O above PEEP at the first Cdyn decline and was used for a final period of "open-lung ventilation."

MEASUREMENTS AND MAIN RESULTS

CT images showed recruited lungs at peak VTCO2 and that a minimal amount of normally aerated lung was added by further increase in EIP. Cdyn declined just before CT scans indicated lung collapse. Compared with baseline, the target VT of 10 mL/kg was achieved at lower EIP and pressure amplitude (EIP-PEEP) during the final open-lung ventilation with more normally aerated and fewer collapsed lungs. Cdyn was doubled after recruitment.

CONCLUSIONS

The lung recruitment maneuver was effective and lungs optimally recruited at maximal VTCO2. A fall in Cdyn indicated lung collapse during downward PEEP titration as confirmed by CT.

Acute hemodynamic effects of recruitment maneuvers in patients with acute respiratory distress syndrome

BACKGROUND

The recruitment maneuver (RM) in acute respiratory distress syndrome (ARDS) can cause hemodynamic derangement. We evaluated circulatory and cardiac changes during RMs.

METHODS

We performed sustained inflation (SI) with a pressure of 40 cm H(2)O for 30 seconds as an RM on 22 patients with ARDS. Blood pressure (BP) and heart rate were recorded immediately before, every 10 seconds during, and 30 seconds after the RM. Ventricular dimensions were obtained simultaneously using M-mode echocardiography, and tissue Doppler imaging was performed on the left ventricular wall.

RESULTS

Mean, systolic, and diastolic BP decreased at 20 and 30 seconds during 30-second RMs (mean BP: 92 +/- 12 at baseline to 83 +/- 18 mm Hg at the end of the RM, P < .05) and subsequently recovered. Heart rate decreased at 10 and 20 seconds during the RM, and tended to increase afterward. Both ventricular dimensions decreased significantly during the RM. The left ventricular ejection fraction and peak velocity of the left ventricle during systole remained stable. The fractional changes in mean BP and left ventricular end-diastolic dimension during the RMs were correlated significantly with each other (r(s) = 0.59). Static compliance of the respiratory system (Crs) was lower in patients with mean BP change > or =15% than in patients in whom the change was <15% (P < .05).

CONCLUSIONS

A transient decrease in mean BP was observed during the RM, and its degree was correlated with the preload decrease, while cardiac contractility was maintained.

Prolonged recruitment manoeuvre improves lung function with less ultrastructural damage in experimental mild acute lung injury

The effects of prolonged recruitment manoeuvre (PRM) were compared with sustained inflation (SI) in paraquat-induced mild acute lung injury (ALI) in rats. Twenty-four hours after ALI induction, rats were anesthetized and mechanically ventilated with VT=6 ml/kg and positive end-expiratory pressure (PEEP)=5 cmH(2)O for 1h. SI was performed with an instantaneous pressure increase of 40 cmH(2)O that was sustained for 40s, while PRM was done by a step-wise increase in positive inspiratory pressure (PIP) of 15-20-25 cmH(2)O above a PEEP of 15 cm H(2)O (maximal PIP=40 cmH(2)O), with interposed periods of PIP=10 cmH(2)O above a PEEP=15 cmH(2)O. Lung static elastance and the amount of alveolar collapse were more reduced with PRM than SI, yielding improved oxygenation. Additionally, tumour necrosis factor-alpha, interleukin-6, interferon-gamma, and type III procollagen mRNA expressions in lung tissue and lung epithelial cell apoptosis decreased more in PRM. In conclusion, PRM improved lung function, with less damage to alveolar epithelium, resulting in reduced pulmonary injury.

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=34predictions). This minimal model thus provides a clinically useful and relatively simple platform for continuous patient specific monitoring of lung unit recruitment for a patient.

A physicochemical approach to acid-base balance in critically ill trauma patients minimizes errors and reduces inappropriate plasma volume expansion

BACKGROUND

This study assesses if a physicochemical (PC) approach to acid-base balance improves the accuracy of acid-base diagnosis, and reduces inappropriate fluid loading.

METHODS

Hundred consecutive patients with trauma admitted to a surgical intensive care unit at a level I trauma center were prospectively analyzed. Demographics, acid-base data and diagnoses, and interventions were collected. Patients were cared for by one physician using a PC approach, or four using conventional (CONV) acid-base balance techniques. The diagnoses and interventions made by CONV physicians were reviewed by the PC physician for accuracy and appropriateness using PC techniques. Data are mean +/- SD or percents; p values reflect PC evaluation of CONV analysis.

RESULTS

There were 50 PC patients and 50 CONV. There were no differences in age (p = 0.13), injury severity score (p = 0.21), number of operations (p = 0.87), transfusions (p = 0.87), or survival (p = 0.15). CONV missed 12 diagnoses of metabolic acidosis (p = 0.03), 10 of hyperchloremic metabolic acidosis (p = 0.003), 11 metabolic alkalosis (p = 0.02), and 19 tertiary disorders (p < 0.001). CONV missed 38 diagnoses of increased unmeasured ions (p < 0.001). PC normalized their acid-base balance sooner than CONV (3.3 days +/- 3.4 days vs. 8.3 days +/- 7.4 days, p < 0.01).

CONCLUSIONS

A PC approach improves acid-base diagnosis accuracy. CONV often miss acidosis (particularly those because of hyperchloremia), alkalosis, and tertiary disorders. Inappropriate volume loading follows in the wake of misinterpretation of increased base deficit using CONV and is avoided using PC. PC-directed therapy normalizes acid-base balance more rapidly than CONV.

Oscillation after inhalation: high frequency oscillatory ventilation in burn patients with the acute respiratory distress syndrome and co-existing smoke inhalation injury

The purpose of this study was to evaluate the effectiveness of, and complications associated with High Frequency Oscillatory Ventilation (HFOV) in burn patients with the Acute Respiratory Distress Syndrome (ARDS) who have had a smoke inhalation injury, and to compare with those without an inhalation injury. Burn patients with progressive oxygenation failure from ARDS while on conventional mechanical ventilation were placed on HFOV as a "rescue" ventilation modality. There were 19 patients with burn + inhalation injury and 30 patients with burn only. Burned patients with ARDS but without inhalation injury had significant temporal improvement in the oxygenation index from 27 +/- 8 on conventional mechanical ventilation to 17 +/- 6 within 48 hours of initiating HFOV. However, burned patients with ARDS and smoke inhalation injury did not achieve significant or even eventual improvements in oxygenation index with HFOV. There was also a trend towards higher rates of early HFOV failure and severe hypercapnia while on HFOV among the patients with inhalation injury. Delivery of nebulized bronchodilators, heparin and n-acetyl cysteine, normally mainstays of smoke inhalation therapy, was impossible during HFOV. The presence of a smoke inhalation injury appears to impair the response to HFOV when this ventilation modality is instituted for ARDS-related oxygenation failure. Severe hypercapnia tended to be more frequent during HFOV among patients with smoke inhalation. These findings, combined with the difficulties in delivery of nebulized medications during HFOV suggest that HFOV may not be the optimal "rescue" ventilation modality in cases of ARDS if there has been an inhalation injury.

The role of time and pressure on alveolar recruitment

Inappropriate mechanical ventilation in patients with acute respiratory distress syndrome can lead to ventilator-induced lung injury (VILI) and increase the morbidity and mortality. Reopening collapsed lung units may significantly reduce VILI, but the mechanisms governing lung recruitment are unclear. We thus investigated the dynamics of lung recruitment at the alveolar level. Rats (n = 6) were anesthetized and mechanically ventilated. The lungs were then lavaged with saline to simulate acute respiratory distress syndrome (ARDS). A left thoracotomy was performed, and an in vivo microscope was placed on the lung surface. The lung was recruited to three recruitment pressures (RP) of 20, 30, or 40 cmH(2)O for 40 s while subpleural alveoli were continuously filmed. Following measurement of microscopic alveolar recruitment, the lungs were excised, and macroscopic gross lung recruitment was digitally filmed. Recruitment was quantified by computer image analysis, and data were interpreted using a mathematical model. The majority of alveolar recruitment (78.3 +/- 7.4 and 84.6 +/- 5.1%) occurred in the first 2 s (T2) following application of RP 30 and 40, respectively. Only 51.9 +/- 5.4% of the microscopic field was recruited by T2 with RP 20. There was limited recruitment from T2 to T40 at all RPs. The majority of gross lung recruitment also occurred by T2 with gradual recruitment to T40. The data were accurately predicted by a mathematical model incorporating the effects of both pressure and time. Alveolar recruitment is determined by the magnitude of recruiting pressure and length of time pressure is applied, a concept supported by our mathematical model. Such a temporal dependence of alveolar recruitment needs to be considered when recruitment maneuvers for clinical application are designed.

Modeling the dynamics of recruitment and derecruitment in mice with acute lung injury

Lung recruitment and derecruitment contribute significantly to variations in the elastance of the respiratory system during mechanical ventilation. However, the decreases in elastance that occur with deep inflation are transient, especially in acute lung injury. Bates and Irvin (8) proposed a model of the lung that recreates time-varying changes in elastance as a result of progressive recruitment and derecruitment of lung units. The model is characterized by distributions of critical opening and closing pressures throughout the lung and by distributions of speeds with which the processes of opening and closing take place once the critical pressures have been achieved. In the present study, we adapted this model to represent a mechanically ventilated mouse. We fit the model to data collected in a previous study from control mice and mice in various stages of acid-induced acute lung injury (3). Excellent fits to the data were obtained when the normally distributed critical opening pressures were about 5 cmH(2)O above the closing pressures and when the hyperbolically distributed opening velocities were about an order of magnitude greater than the closing velocities. We also found that, compared with controls, the injured mice had markedly increased opening and closing pressures but no change in the velocities, suggesting that the key biophysical change wrought by acid injury is dysfunction of surface tension at the air-liquid interface. Our computational model of lung recruitment and derecruitment dynamics is thus capable of accurately mimicking data from mice with acute lung injury and may provide insight into the altered biophysics of the injured lung.