A comparison of biologically variable ventilation to recruitment manoeuvres in a porcine model of acute lung injury

Working with Convex Responses: Antifragility from Finance to Oncology

We extend techniques and learnings about the stochastic properties of nonlinear responses from finance to medicine, particularly oncology, where it can inform dosing and intervention. We define antifragility. We propose uses of risk analysis for medical problems, through the properties of nonlinear responses (convex or concave). We (1) link the convexity/concavity of the dose-response function to the statistical properties of the results; (2) define "antifragility" as a mathematical property for local beneficial convex responses and the generalization of "fragility" as its opposite, locally concave in the tails of the statistical distribution; (3) propose mathematically tractable relations between dosage, severity of conditions, and iatrogenics. In short, we propose a framework to integrate the necessary consequences of nonlinearities in evidence-based oncology and more general clinical risk management.

Constant Tidal Volume Ventilation and Surfactant Dysfunction: An Overlooked Cause of Ventilator-Induced Lung Injury

Ventilator-induced lung injury (VILI) is currently ascribed to volutrauma and/or atelectrauma but the effect of constant tidal volume ventilation (CVTV) has received little attention. This Perspective summarizes the literature documenting that CVTV causes VILI and reviews the mechanisms by which it occurs. Surfactant is continuously inactivated, depleted, displaced or desorbed as a function of the duration of ventilation, the tidal volume, the level of PEEP and possibly the respiratory rate. Accordingly, surfactant must be continuously replenished and secretion primarily depends on intermittent delivery of large ventilatory excursions. The surfactant abnormalities resulting from CVTV result in atelectasis and VILI. While surfactant secretion is reduced by the absence of intermittent deep breaths continuous administration of large tidal volumes depletes surfactant and impairs subsequent secretion. Low or normal lung volumes result in desorption of surfactant. PEEP can be protective by reducing surface film collapse and subsequent film rupture on re-expansion, and/or by reducing surfactant displacement into the airways, but PEEP can also down-regulate surfactant release. Conclusions: The effect of CVTV on surfactant is complex. If attention is not paid to facilitating surfactant secretion and limiting its inactivation, depletion, desorption or displacement surface tension will increase and atelectasis and VILI will occur.

Benefit of Physiologically Variable Over Pressure-Controlled Ventilation in a Model of Chronic Obstructive Pulmonary Disease: A Randomized Study

Introduction

The advantages of physiologically variable ventilation (PVV) based on a spontaneous breathing pattern have been demonstrated in several respiratory conditions. However, its potential benefits in chronic obstructive pulmonary disease (COPD) have not yet been characterized. We used an experimental model of COPD to compare respiratory function outcomes after 6 h of PVV versus conventional pressure-controlled ventilation (PCV).

Materials and Methods

Rabbits received nebulized elastase and lipopolysaccharide throughout 4 weeks. After 30 days, animals were anesthetized, tracheotomized, and randomized to receive 6 h of physiologically variable (n = 8) or conventional PCV (n = 7). Blood gases, respiratory mechanics, and chest fluoroscopy were assessed hourly.

Results

After 6 h of ventilation, animals receiving variable ventilation demonstrated significantly higher oxygenation index (PaO₂/FiO₂ 441 ± 37 (mean ± standard deviation) versus 354 ± 61 mmHg, p < 0.001) and lower respiratory elastance (359 ± 36 versus 463 ± 81 cmH₂O/L, p < 0.01) than animals receiving PCV. Animals ventilated with the variable mode also presented less lung derecruitment (decrease in lung aerated area, –3.4 ± 9.9 versus –17.9 ± 6.7%, p < 0.01) and intrapulmonary shunt fraction (9.6 ± 4.1 versus 17.0 ± 5.8%, p < 0.01).

Conclusion

PVV applied to a model of COPD improved oxygenation, respiratory mechanics, lung aeration, and intrapulmonary shunt fraction compared to conventional ventilation. A reduction in alveolar derecruitment and lung tissue stress leading to better aeration and gas exchange may explain the benefits of PVV.

Brainstem inflammation modulates the ventilatory pattern and its variability after acute lung injury in rodents

KEY POINTS

Compared with sham rats, rats a week after acute lung injury (ALI) express more pro-inflammatory cytokines in their brainstem respiratory control nuclei, exhibit a higher respiratory frequency (fR) and breathe with a more predictable pattern. These characteristics of the respiratory pattern persist in in situ preparations even after minimizing pulmonary and chemo-afferent inputs. Interleukin (IL)-1β microinjected in the nucleus tractus solitarii increases fR and the predictability of the ventilatory pattern similar to rats with ALI. Intracerebroventricular infusion of indomethacin, an anti-inflammatory drug, mitigates the effect of ALI on fR and ventilatory pattern variability. We conclude that changes in the ventilatory pattern after ALI result not only from sensory input due to pulmonary damage and dysfunction but also from neuro-inflammation.

ABSTRACT

Acute lung injury (ALI) increases respiratory rate (fR) and ventilatory pattern variability (VPV), but also evokes peripheral and central inflammation. We hypothesized that central inflammation has a role in determining the ventilatory pattern after ALI. In rat pups, we intratracheally injected either bleomycin to induce ALI or saline as a sham control. One week later, we recorded the ventilatory pattern of the rat pups using flow-through plethysmography, then formed in situ preparations from these pups and recorded their 'fictive' patterns from respiratory motor nerves. Compared with the ventilatory pattern of the sham rat pups, injured rat pups had increased fR and predictability. Surprisingly, the fictive patterns of the in situ preparations from ALI pups retained these characteristics despite removing their lungs to eliminate pulmonary sensory inputs and perfusing them with hyperoxic artificial cerebral spinal fluid to minimize peripheral chemoreceptor input. Histological processing revealed increased immunoreactivity of the pro-inflammatory cytokine Interleukin-1β (IL-1β) in the nucleus tractus solitarii (nTS) from ALI but not sham rats. In subsequent experiments, we microinjected IL-1β in the nTS bilaterally in anaesthetized naïve adult rats, which increased fR and predictability of ventilatory pattern variability (VPV) after 2 h. Finally, we infused indomethacin intracerebroventricularly during the week of survival after ALI. This did not affect sham rats, but mitigated changes in fR and VPV in ALI rats. We conclude that neuro-inflammation has an essential role in determining the ventilatory pattern of ALI rats.

Effects of variable versus nonvariable controlled mechanical ventilation on pulmonary inflammation in experimental acute respiratory distress syndrome in pigs

BACKGROUND

Mechanical ventilation with variable tidal volumes (VT) may improve lung function and reduce ventilator-induced lung injury in experimental acute respiratory distress syndrome (ARDS). However, previous investigations were limited to less than 6 h, and control groups did not follow clinical standards. We hypothesised that 24 h of mechanical ventilation with variable VT reduces pulmonary inflammation (as reflected by neutrophil infiltration), compared with standard protective, nonvariable ventilation.

METHODS

Experimental ARDS was induced in 14 anaesthetised pigs with saline lung lavage followed by injurious mechanical ventilation. Pigs (n=7 per group) were randomly assigned to using variable VT or nonvariable VT modes of mechanical ventilation for 24 h. In both groups, ventilator settings including positive end-expiratory pressure and oxygen inspiratory fraction were adjusted according to the ARDS Network protocol. Pulmonary inflammation (primary endpoint) and perfusion were assessed by positron emission tomography using 2-deoxy-2-[18F]fluoro-d-glucose and 68Gallium (68Ga)-labelled microspheres, respectively. Gas exchange, respiratory mechanics, and haemodynamics were quantified. Lung aeration was determined using CT.

RESULTS

The specific global uptake rate of 18F-FDG increased to a similar extent regardless of mode of mechanical ventilation (median uptake for variable VT=0.016 min-1 [inter-quartile range, 0.012-0.029] compared with median uptake for nonvariable VT=0.037 min-1 [0.008-0.053]; P=0.406). Gas exchange, respiratory mechanics, haemodynamics, and lung aeration and perfusion were similar in both variable and nonvariable VT ventilatory modes.

CONCLUSION

In a porcine model of ARDS, 24 h of mechanical ventilation with variable VT did not attenuate pulmonary inflammation compared with standard protective mechanical ventilation with nonvariable VT.

Variable Ventilation Is Equally Effective as Conventional Pressure Control Ventilation for Optimizing Lung Function in a Rabbit Model of ARDS

Background

Introducing mathematically derived variability (MVV) into the otherwise monotonous conventional mechanical ventilation has been suggested to improve lung recruitment and gas exchange. Although the application of a ventilation pattern based on variations in physiological breathing (PVV) is beneficial for healthy lungs, its value in the presence of acute respiratory distress syndrome (ARDS) has not been characterized. We therefore aimed at comparing conventional pressure-controlled ventilation with (PCS) or without regular sighs (PCV) to MVV and PVV at two levels of positive end-expiratory pressure (PEEP) in a model of severe ARDS.

Methods

Anesthetised rabbits (n = 54) were mechanically ventilated and severe ARDS (PaO₂/FiO₂ ≤ 150 mmHg) was induced by combining whole lung lavage, i.v. endotoxin and injurious ventilation. Rabbits were then randomly assigned to be ventilated with PVV, MVV, PCV, or PCS for 5 h while maintaining either 6 or 9 cmH₂O PEEP. Ventilation parameters, blood gas indices and respiratory mechanics (tissue damping, G, and elastance, H) were recorded hourly. Serum cytokine levels were assessed with ELISA and lung histology was analyzed.

Results

Although no progression of lung injury was observed after 5 h of ventilation at PEEP 6 cmH₂O with PVV and PCV, values for G (58.8 ± 71.1[half-width of 95% CI]% and 40.8 ± 39.0%, respectively), H (54.5 ± 57.2%, 50.7 ± 28.3%), partial pressure of carbon-dioxide (PaCO₂, 43.9 ± 23.8%, 46.2 ± 35.4%) and pH (−4.6 ± 3.3%, −4.6 ± 2.2%) worsened with PCS and MVV. Regardless of ventilation pattern, application of a higher PEEP improved lung function and precluded progression of lung injury and inflammation. Histology lung injury scores were elevated in all groups with no difference between groups at either PEEP level.

Conclusion

At moderate PEEP, variable ventilation based on a pre-recorded physiological breathing pattern protected against progression of lung injury equally to the conventional pressure-controlled mode, whereas mathematical variability or application of regular sighs caused worsening in lung mechanics. This outcome may be related to the excessive increases in peak inspiratory pressure with the latter ventilation modes. However, a greater benefit on respiratory mechanics and gas exchange could be obtained by elevating PEEP, compared to the ventilation mode in severe ARDS.

Biomedical engineer's guide to the clinical aspects of intensive care mechanical ventilation

Background

Mechanical ventilation is an essential therapy to support critically ill respiratory failure patients. Current standards of care consist of generalised approaches, such as the use of positive end expiratory pressure to inspired oxygen fraction (PEEP–FiO₂) tables, which fail to account for the inter- and intra-patient variability between and within patients. The benefits of higher or lower tidal volume, PEEP, and other settings are highly debated and no consensus has been reached. Moreover, clinicians implicitly account for patient-specific factors such as disease condition and progression as they manually titrate ventilator settings. Hence, care is highly variable and potentially often non-optimal. These conditions create a situation that could benefit greatly from an engineered approach. The overall goal is a review of ventilation that is accessible to both clinicians and engineers, to bridge the divide between the two fields and enable collaboration to improve patient care and outcomes. This review does not take the form of a typical systematic review. Instead, it defines the standard terminology and introduces key clinical and biomedical measurements before introducing the key clinical studies and their influence in clinical practice which in turn flows into the needs and requirements around how biomedical engineering research can play a role in improving care. Given the significant clinical research to date and its impact on this complex area of care, this review thus provides a tutorial introduction around the review of the state of the art relevant to a biomedical engineering perspective.

Discussion

This review presents the significant clinical aspects and variables of ventilation management, the potential risks associated with suboptimal ventilation management, and a review of the major recent attempts to improve ventilation in the context of these variables. The unique aspect of this review is a focus on these key elements relevant to engineering new approaches. In particular, the need for ventilation strategies which consider, and directly account for, the significant differences in patient condition, disease etiology, and progression within patients is demonstrated with the subsequent requirement for optimal ventilation strategies to titrate for patient- and time-specific conditions.

Conclusion

Engineered, protective lung strategies that can directly account for and manage inter- and intra-patient variability thus offer great potential to improve both individual care, as well as cohort clinical outcomes.

Periodic Fluctuation of Tidal Volumes Further Improves Variable Ventilation in Experimental Acute Respiratory Distress Syndrome

In experimental acute respiratory distress syndrome (ARDS), random variation of tidal volumes (V_T) during volume controlled ventilation improves gas exchange and respiratory system mechanics (so-called stochastic resonance hypothesis). It is unknown whether those positive effects may be further enhanced by periodic V_T fluctuation at distinct frequencies, also known as deterministic frequency resonance. We hypothesized that the positive effects of variable ventilation on lung function may be further amplified by periodic V_T fluctuation at specific frequencies. In anesthetized and mechanically ventilated pigs, severe ARDS was induced by saline lung lavage and injurious V_T (double-hit model). Animals were then randomly assigned to 6 h of protective ventilation with one of four V_T patterns: (1) random variation of V_T (WN); (2) P₀₄, main V_T frequency of 0.13 Hz; (3) P₁₀, main V_T frequency of 0.05 Hz; (4) VCV, conventional non-variable volume controlled ventilation. In groups with variable V_T, the coefficient of variation was identical (30%). We assessed lung mechanics and gas exchange, and determined lung histology and inflammation. Compared to VCV, WN, P₀₄, and P₁₀ resulted in lower respiratory system elastance (63 ± 13 cm H₂O/L vs. 50 ± 14 cm H₂O/L, 48.4 ± 21 cm H₂O/L, and 45.1 ± 5.9 cm H₂O/L respectively, P < 0.05 all), but only P₁₀ improved PaO₂/F_IO₂ after 6 h of ventilation (318 ± 96 vs. 445 ± 110 mm Hg, P < 0.05). Cycle-by-cycle analysis of lung mechanics suggested intertidal recruitment/de-recruitment in P₁₀. Lung histologic damage and inflammation did not differ among groups. In this experimental model of severe ARDS, periodic V_T fluctuation at a frequency of 0.05 Hz improved oxygenation during variable ventilation, suggesting that deterministic resonance adds further benefit to variable ventilation.

Periodicity: A Characteristic of Heart Rate Variability Modified by the Type of Mechanical Ventilation After Acute Lung Injury

We present a novel approach to quantify heart rate variability (HRV) and the results of applying this approach to synthetic and original data sets. Our approach evaluates the periodicity of heart rate by calculating the transform of Relative Shannon Entropy, the maximum value of the RR interval periodogram, and the maximum, mean values, and sample entropy of the autocorrelation function. Synthetic data were generated using a Van der Pol oscillator; and the original data were electrocardiogram (ECG) recordings from anesthetized rats after acute lung injury while on biologically variable (BVV) or continuous mechanical ventilation (CMV). Analysis of the synthetic data revealed that our measures were correlated highly to the bandwidth of the oscillator and assessed periodicity. Then, applying these analytical tools to the ECGs determined that the heart rate (HR) of BVV group had less periodicity and higher variability than the HR of the CMV group. Quantifying periodicity effectively identified a readily apparent difference in HRV during BVV and CMV that was not identified by power spectral density measures during BVV and CMV. Cardiorespiratory coupling is the probable mechanism for HRV increasing during BVV and becoming periodic during CMV. Thus, the absence or presence of periodicity in ventilation determined HRV, and this mechanism is distinctly different from the cardiorespiratory uncoupling that accounts for the loss of HRV during sepsis.

Variable mechanical ventilation

Objective

To review the literature on the use of variable mechanical ventilation and the main outcomes of this technique.

Methods

Search, selection, and analysis of all original articles on variable ventilation, without restriction on the period of publication and language, available in the electronic databases LILACS, MEDLINE^®, and PubMed, by searching the terms "variable ventilation" OR "noisy ventilation" OR "biologically variable ventilation".

Results

A total of 36 studies were selected. Of these, 24 were original studies, including 21 experimental studies and three clinical studies.

Conclusion

Several experimental studies reported the beneficial effects of distinct variable ventilation strategies on lung function using different models of lung injury and healthy lungs. Variable ventilation seems to be a viable strategy for improving gas exchange and respiratory mechanics and preventing lung injury associated with mechanical ventilation. However, further clinical studies are necessary to assess the potential of variable ventilation strategies for the clinical improvement of patients undergoing mechanical ventilation.

Recruitment manoeuvres for adults with acute respiratory distress syndrome receiving mechanical ventilation

BACKGROUND

Recruitment manoeuvres involve transient elevations in airway pressure applied during mechanical ventilation to open ('recruit') collapsed lung units and increase the number of alveoli participating in tidal ventilation. Recruitment manoeuvres are often used to treat patients in intensive care who have acute respiratory distress syndrome (ARDS), but the effect of this treatment on clinical outcomes has not been well established. This systematic review is an update of a Cochrane review originally published in 2009.

OBJECTIVES

Our primary objective was to determine the effects of recruitment manoeuvres on mortality in adults with acute respiratory distress syndrome.Our secondary objective was to determine, in the same population, the effects of recruitment manoeuvres on oxygenation and adverse events (e.g. rate of barotrauma).

SEARCH METHODS

For this updated review, we searched the Cochrane Central Register of Controlled Trials (CENTRAL), MEDLINE (OVID), Embase (OVID), the Cumulative Index to Nursing and Allied Health Literature (CINAHL, EBSCO), Latin American and Caribbean Health Sciences (LILACS) and the International Standard Randomized Controlled Trial Number (ISRCTN) registry from inception to August 2016.

SELECTION CRITERIA

We included randomized controlled trials (RCTs) of adults who were mechanically ventilated that compared recruitment manoeuvres versus standard care for patients given a diagnosis of ARDS.

DATA COLLECTION AND ANALYSIS

Two review authors independently assessed trial quality and extracted data. We contacted study authors for additional information.

MAIN RESULTS

Ten trials met the inclusion criteria for this review (n = 1658 participants). We found five trials to be at low risk of bias and five to be at moderate risk of bias. Six of the trials included recruitment manoeuvres as part of an open lung ventilation strategy that was different from control ventilation in aspects other than the recruitment manoeuvre (such as mode of ventilation, higher positive end-expiratory pressure (PEEP) titration and lower tidal volume or plateau pressure). Six studies reported mortality outcomes. Pooled data from five trials (1370 participants) showed a reduction in intensive care unit (ICU) mortality (risk ratio (RR) 0.83, 95% confidence interval (CI) 0.72 to 0.97, P = 0.02, low-quality evidence), pooled data from five trials (1450 participants) showed no difference in 28-day mortality (RR 0.86, 95% CI 0.74 to 1.01, P = 0.06, low-quality evidence) and pooled data from four trials (1313 participants) showed no difference in in-hospital mortality (RR 0.88, 95% CI 0.77 to 1.01, P = 0.07, low-quality evidence). Data revealed no differences in risk of barotrauma (RR 1.09, 95% CI 0.78 to 1.53, P = 0.60, seven studies, 1508 participants, moderate-quality evidence).

AUTHORS' CONCLUSIONS

We identified significant clinical heterogeneity in the 10 included trials. Results are based upon the findings of several (five) trials that included an "open lung ventilation strategy", whereby the intervention group differed from the control group in aspects other than the recruitment manoeuvre (including co-interventions such as higher PEEP, different modes of ventilation and higher plateau pressure), making interpretation of the results difficult. A ventilation strategy that included recruitment manoeuvres in participants with ARDS reduced intensive care unit mortality without increasing the risk of barotrauma but had no effect on 28-day and hospital mortality. We downgraded the quality of the evidence to low, as most of the included trials provided co-interventions as part of an open lung ventilation strategy, and this might have influenced results of the outcome.

Variable ventilation from bench to bedside

This article is one of ten reviews selected from the Annual Update in Intensive Care and Emergency medicine 2016. Other selected articles can be found online at http://www.biomedcentral.com/collections/annualupdate2016. Further information about the Annual Update in Intensive Care and Emergency Medicine is available from http://www.springer.com/series/8901.

Ventilator-Induced Lung Injury (VILI) in Acute Respiratory Distress Syndrome (ARDS): Volutrauma and Molecular Effects

Acute Respiratory Distress Syndrome (ARDS) is a clinical condition secondary to a variety of insults leading to a severe acute respiratory failure and high mortality in critically ill patients. Patients with ARDS generally require mechanical ventilation, which is another important factor that may increase the ALI (acute lung injury) by a series of pathophysiological mechanisms, whose common element is the initial volutrauma in the alveolar units, and forming part of an entity known clinically as ventilator-induced lung injury (VILI).

Injured lungs can be partially protected by optimal settings and ventilation modes, using low tidal volume (VT) values and high positive-end expiratory pressure (PEEP). The benefits in ARDS outcomes caused by these interventions have been confirmed by several prospective randomized controlled trials (RCTs) and are attributed to reduction in volutrauma.

The purpose of this article is to present an approach to VILI pathophysiology focused on the effects of volutrauma that lead to lung injury and the ‘mechanotransduction’ mechanism. A more complete understanding about the molecular effects that physical forces could have, is essential for a better assessment of existing strategies as well as the development of new therapeutic strategies to reduce the damage resulting from VILI, and thereby contribute to reducing mortality in ARDS.

Pulmonar recruitment in acute respiratory distress syndrome. What is the best strategy?

Supporting patients with acute respiratory distress syndrome (ARDS), using a protective mechanical ventilation strategy characterized by low tidal volume and limitation of positive end-expiratory pressure (PEEP) is a standard practice in the intensive care unit. However, these strategies can promote lung de-recruitment, leading to the cyclic closing and reopening of collapsed alveoli and small airways. Recruitment maneuvers (RM) can be used to augment other methods, like positive end-expiratory pressure and positioning, to improve aerated lung volume. Clinical practice varies widely, and the optimal method and patient selection for recruitment maneuvers have not been determined, considerable uncertainty remaining regarding the appropriateness of RM. This review aims to discuss recent findings about the available types of RM, and compare the effectiveness, indications and adverse effects among them, as well as their impact on morbidity and mortality in ARDS patients. Recent developments include experimental and clinical evidence that a stepwise extended recruitment maneuver may cause an improvement in aerated lung volume and decrease the biological impact seen with the traditionally used sustained inflation, with less adverse effects. Prone positioning can reduce mortality in severe ARDS patients and may be an useful adjunct to recruitment maneuvers and advanced ventilatory strategies, such noisy ventilation and BIVENT, which have been useful in providing lung recruitment.

Combined effects of ventilation mode and positive end-expiratory pressure on mechanics, gas exchange and the epithelium in mice with acute lung injury

The accepted protocol to ventilate patients with acute lung injury is to use low tidal volume (V(T)) in combination with recruitment maneuvers or positive end-expiratory pressure (PEEP). However, an important aspect of mechanical ventilation has not been considered: the combined effects of PEEP and ventilation modes on the integrity of the epithelium. Additionally, it is implicitly assumed that the best PEEP-V(T) combination also protects the epithelium. We aimed to investigate the effects of ventilation mode and PEEP on respiratory mechanics, peak airway pressures and gas exchange as well as on lung surfactant and epithelial cell integrity in mice with acute lung injury. HCl-injured mice were ventilated at PEEPs of 3 and 6 cmH(2)O with conventional ventilation (CV), CV with intermittent large breaths (CV(LB)) to promote recruitment, and a new mode, variable ventilation, optimized for mice (VV(N)). Mechanics and gas exchange were measured during ventilation and surfactant protein (SP)-B, proSP-B and E-cadherin levels were determined from lavage and lung homogenate. PEEP had a significant effect on mechanics, gas exchange and the epithelium. The higher PEEP reduced lung collapse and improved mechanics and gas exchange but it also down regulated surfactant release and production and increased epithelial cell injury. While CV(LB) was better than CV, VV(N) outperformed CV(LB) in recruitment, reduced epithelial injury and, via a dynamic mechanotransduction, it also triggered increased release and production of surfactant. For long-term outcome, selection of optimal PEEP and ventilation mode may be based on balancing lung physiology with epithelial injury.

Variable ventilation enhances ventilation without exacerbating injury in preterm lambs with respiratory distress syndrome

BACKGROUND

As compared with constant respiratory rate (RR) and tidal volume (V(T)) during controlled conventional mechanical ventilation (CV), variable ventilation (VV) using the same breath-to-breath minute volume but variable V(T) and RRs enhances ventilation efficiency in preterm lambs. We hypothesized that if V(T) was adjusted to target permissive hypercarbia, VV would result in more efficient gas exchange without increasing inflammatory and injurious responses in the lung.

METHODS

Preterm lambs at 129 d gestation were anesthetized, tracheotomized, and randomized to either CV (n = 8) or VV (n = 8) using the same initial average V(T) and RR. Lung mechanics and gas exchange were measured intermittently, and average V(T) was adjusted to target partial pressure of arterial carbon dioxide (PaCO2) of 40-50 mm Hg for 3 h. Lung injury and inflammation were assessed from bronchoalveolar lavage fluid, lung tissue, and peripheral blood.

RESULTS

VV achieved permissive hypercarbia using a lower average V(T), peak inspiratory pressure, and elastance (increased compliance) as compared with CV. Oxygenation and markers of lung tissue inflammation or injury were not different apart from a lower wet:dry tissue ratio in the VV lungs.

CONCLUSIONS

VV improves ventilation efficiency and in vivo lung compliance in the ovine preterm lung without increasing lung inflammation or lung injury.

Effects of recruitment/derecruitment dynamics on the efficacy of variable ventilation

Variable (or noisy) ventilation (VV) has been demonstrated in animal models of acute lung injury to be superior to constant (or conventional) ventilation (CV), in terms of improved gas exchange and mitigation of lung injury, for reasons that are not entirely clear. We hypothesized that the efficacy of VV is related to the fact that recruitment and derecruitment of lung units are dynamic processes. To test this hypothesis, we modeled the lung computationally as a symmetrically bifurcating airway tree terminating in elastic units. Each airway was fully open or completely closed, at any point in time, according to its pressure history. The model is able to accurately mimic previous experimental measurements showing that the lungs of mice injured by acid aspiration are better recruited after 60 min of VV than CV. The model also shows that recruitment/derecruitment dynamics contribute to the relative efficacy of VV, provided lung units open more rapidly than they close once a critical opening or closing pressure threshold has been crossed. We conclude that the dynamics of recruitment and derecruitment in the lung may be important factors responsible for the benefits of VV compared with CV.

Pulmonary toxicity of chronic exposure to tobacco and biomass smoke in rats

OBJECTIVE

The objective of this study was to examine the separate and combined effects of tobacco and biomass smoke exposure on pulmonary histopathology in rats.

INTRODUCTION

In addition to smoking, indoor pollution in developing countries contributes to the development of respiratory diseases.

METHODS

Twenty-eight adult rats were divided into four groups as follows: control group (Group I, no exposure to tobacco or biomass smoke), exposed to tobacco smoke (Group II), exposed to biomass smoke (Group III), and combined exposure to tobacco and biomass smoke (Group IV). After six months the rats in all four groups were sacrificed. Lung tissue samples were examined under light microscopy. The severity of pathological changes was scored.

RESULTS

Group II differed from Group I in all histopathological alterations except intraparenchymal vascular thrombosis. There was no statistically significant difference in histopathological changes between the subjects exposed exclusively to tobacco smoke (Group II) and those with combined exposure to tobacco and biomass smoke (Group IV). The histopathological changes observed in Group IV were found to be more severe than those in subjects exposed exclusively to biomass smoke (Group III).

DISCUSSION

Chronic exposure to tobacco and biomass smoke caused an increase in severity and types of lung injury.

CONCLUSION

Exposure to cigarette smoke caused serious damage to the respiratory system, particularly with concomitant exposure to biomass smoke.

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.

New and conventional strategies for lung recruitment in acute respiratory distress syndrome

This article is one of ten reviews selected from the Yearbook of Intensive Care and Emergency Medicine 2010 (Springer Verlag) and co-published as a series in Critical Care. Other articles in the series can be found online at http://ccforum.com/series/yearbook. Further information about the Yearbook of Intensive Care and Emergency Medicine is available from http://www.springer.com/series/2855.

Recruitment manoeuvres for adults with acute lung injury receiving mechanical ventilation

BACKGROUND

Recruitment manoeuvres are often used to treat patients with acute lung injury (ALI) or acute respiratory distress syndrome (ARDS) but the effect of this treatment on clinical outcomes has not been well established.

OBJECTIVES

The objective of this review was to examine recruitment manoeuvres compared to standard care as therapy for adults with acute lung injury in order to quantify the effects on patient outcomes (mortality, length of ventilation, and other relevant outcomes).

SEARCH STRATEGY

We searched the Cochrane Central Register of Controlled Trials (CENTRAL) (The Cochrane Library 2008, Issue 2); MEDLINE (January 1966 to May 2008); EMBASE (January 1980 to May 2008); LILACS (1982 to May 2008); CINAHL (1982 to May 2008); and Current Controlled Trials (www.controlled-trials.com).

SELECTION CRITERIA

We included randomized controlled trials of adults who were mechanically ventilated comparing recruitment manoeuvres to standard care for those patients diagnosed with ALI or ARDS.

DATA COLLECTION AND ANALYSIS

Two authors independently assessed trial quality and extracted data. We contacted study authors for additional information.

MAIN RESULTS

Seven trials met the inclusion criteria for this review (the total number of included participants was 1170). All trials included a recruitment manoeuvre as part of the treatment strategy for patients on mechanical ventilation for ARDS or ALI. However, two of the trials included a package of ventilation that was different from the control ventilation in aspects other than the recruitment manoeuvre. The intervention group showed no significant difference on 28-day mortality (RR 0.73, 95% CI 0.46 to 1.17, P = 0.2). Similarly there was no statistical difference for risk of barotrauma (RR 0.50, 95% CI 0.07 to 3.52, P = 0.5) or blood pressure (MD 0.9 mm Hg, 95% CI -4.28 to 6.08, P = 0.73). Recruitment manoeuvres significantly increased oxygenation above baseline levels for a short period of time in four of the five studies that measured oxygenation. There were insufficient data on length of ventilation or hospital stay to pool results.

AUTHORS' CONCLUSIONS

There is not evidence to make conclusions on whether recruitment manoeuvres reduce mortality or length of ventilation in patients with ALI or ARDS.

Variable tidal volumes improve lung protective ventilation strategies in experimental lung injury

RATIONALE

Noisy ventilation with variable Vt may improve respiratory function in acute lung injury.

OBJECTIVES

To determine the impact of noisy ventilation on respiratory function and its biological effects on lung parenchyma compared with conventional protective mechanical ventilation strategies.

METHODS

In a porcine surfactant depletion model of lung injury, we randomly combined noisy ventilation with the ARDS Network protocol or the open lung approach (n = 9 per group).

MEASUREMENTS AND MAIN RESULTS

Respiratory mechanics, gas exchange, and distribution of pulmonary blood flow were measured at intervals over a 6-hour period. Postmortem, lung tissue was analyzed to determine histological damage, mechanical stress, and inflammation. We found that, at comparable minute ventilation, noisy ventilation (1) improved arterial oxygenation and reduced mean inspiratory peak airway pressure and elastance of the respiratory system compared with the ARDS Network protocol and the open lung approach, (2) redistributed pulmonary blood flow to caudal zones compared with the ARDS Network protocol and to peripheral ones compared with the open lung approach, (3) reduced histological damage in comparison to both protective ventilation strategies, and (4) did not increase lung inflammation or mechanical stress.

CONCLUSIONS

Noisy ventilation with variable Vt and fixed respiratory frequency improves respiratory function and reduces histological damage compared with standard protective ventilation strategies.

Biologically variable ventilation improves gas exchange and respiratory mechanics in a model of severe bronchospasm*

OBJECTIVE

Mechanical ventilation can be lifesaving for status asthmaticus, but how best to accomplish mechanical ventilation is unclear. Biologically variable ventilation (mechanical ventilation that emulates healthy variation) and conventional control mode ventilation (monotonously regular) were compared in an animal model of bronchospasm to determine which approach yields better gas exchange and respiratory mechanics.

DESIGN

A randomized prospective trial of biologically variable ventilation vs. control mode ventilation in swine.

SETTING

University research laboratory.

SUBJECTS

Eighteen farm-raised pigs.

INTERVENTIONS

Methacholine was administered as a nebulized aerosol to initiate bronchospasm, defined as doubling of peak inspiratory pressure and respiratory system resistance, and then randomized (n = 9 each group) to either continue control mode ventilation or switch to biologically variable ventilation at the same minute ventilation. Over the next 4 hrs, hemodynamics, blood gases, respiratory mechanics, and carbon dioxide expirograms were recorded hourly. At end-experiment, tracheobronchial lavage was undertaken to determine interleukin-6 and -10 concentrations.

MEASUREMENTS AND MAIN RESULTS

Measurements of physiologic variables and inflammatory cytokines showed that biologically variable ventilation significantly improved gas exchange, with greater arterial oxygen tensions (p = .006; group x time interaction), lower arterial carbon dioxide tensions (p = .0003; group effect), lower peak inspiratory pressures (p = .0001; group x time), greater static compliance (p = .0001; group x time), greater dynamic compliance (p = .0001; group x time), and lower total respiratory system resistance (p = .028; group x time), compared with conventional ventilation. The appearance of inflammatory cytokines in bronchoalveolar lavage fluid (interleukin-6 and -10) was not affected by mode of ventilation.

CONCLUSIONS

In this experimental model, biologically variable ventilation was superior to control mode ventilation in terms of gas exchange and respiratory mechanics during severe bronchospasm.

A novel mechanical lung model of pulmonary diseases to assist with teaching and training

BACKGROUND

A design concept of low-cost, simple, fully mechanical model of a mechanically ventilated, passively breathing lung is developed. An example model is built to simulate a patient under mechanical ventilation with accurate volumes and compliances, while connected directly to a ventilator.

METHODS

The lung is modelled with multiple units, represented by rubber bellows, with adjustable weights placed on bellows to simulate compartments of different superimposed pressure and compliance, as well as different levels of lung disease, such as Acute Respiratory Distress Syndrome (ARDS). The model was directly connected to a ventilator and the resulting pressure volume curves recorded.

RESULTS

The model effectively captures the fundamental lung dynamics for a variety of conditions, and showed the effects of different ventilator settings. It was particularly effective at showing the impact of Positive End Expiratory Pressure (PEEP) therapy on lung recruitment to improve oxygenation, a particulary difficult dynamic to capture.

CONCLUSION

Application of PEEP therapy is difficult to teach and demonstrate clearly. Therefore, the model provide opportunity to train, teach, and aid further understanding of lung mechanics and the treatment of lung diseases in critical care, such as ARDS and asthma. Finally, the model's pure mechanical nature and accurate lung volumes mean that all results are both clearly visible and thus intuitively simple to grasp.

Comparison of variable and conventional ventilation in a sheep saline lavage lung injury model*

OBJECTIVE

There has recently been considerable interest in alternative lung-protective ventilation strategies such as variable ventilation (VV). We aimed at testing VV in a large animal lung injury model and exploring the mechanism of improvement in gas exchange seen with VV.

DESIGN

Randomized, controlled comparative ventilation study.

SETTING

Research laboratory at a veterinary hospital.

SUBJECTS

Female sheep weighing 59.8 +/- 10.57 kg and excised calf lungs.

INTERVENTIONS

In a sheep saline lavage model of lung injury, we applied VV, whereby tidal volume (VT) and frequency (f) varied on each breath. Sheep were randomized into one of two groups (VV, n = 7; or control, n = 6) and ventilated for 4 hrs with all mean ventilation settings matched.

MEASUREMENTS AND MAIN RESULTS

Gas exchange, lung mechanics, and hemodynamic measures were recorded over the 4 hrs. VV sheep showed improvement in gas exchange (i.e., oxygenation and carbon dioxide elimination) and ventilation pressures (i.e., reduced mean and peak airway pressures) but control sheep did not. VV sheep also displayed lower-lung elastance and mechanical heterogeneity in comparison with control sheep from 2 to 4 hrs of ventilation. To study the mechanism behind improvements seen with VV, we examined the time course associated with the enhanced recruitment occurring during VV in eight saline-lavaged excised calf lungs. We found that the recruitment associated with a larger VT during VV lasted over 200 secs, nearly an order of magnitude greater than the average time interval between large VT deliveries during VV.

CONCLUSIONS

The application of VV in a large animal model of lung injury results in improved gas exchange and superior lung mechanics in comparison with CV that can be explained at least partially by the long-lasting effects of the recruitments occurring during VV.

Alveolar recruitment in acute lung injury

Alveolar recruitment is one of the primary goals of respiratory care for acute lung injury. It is aimed at improving pulmonary gas exchange and, even more important, at protecting the lungs from ventilator-induced trauma. This review addresses the concept of alveolar recruitment for lung protection in acute lung injury. It provides reasons for why atelectasis and atelectrauma should be avoided; it analyses current and future approaches on how to achieve and preserve alveolar recruitment; and it discusses the possibilities of detecting alveolar recruitment and derecruitment. The latter is of particular clinical relevance because interventions aimed at lung recruitment are often undertaken without simultaneous verification of their effectiveness.

Mathematical modelling to centre low tidal volumes following acute lung injury: a study with biologically variable ventilation

BACKGROUND

With biologically variable ventilation [BVV--using a computer-controller to add breath-to-breath variability to respiratory frequency (f) and tidal volume (VT)] gas exchange and respiratory mechanics were compared using the ARDSNet low VT algorithm (Control) versus an approach using mathematical modelling to individually optimise VT at the point of maximal compliance change on the convex portion of the inspiratory pressure-volume (P-V) curve (Experimental).

METHODS

Pigs (n = 22) received pentothal/midazolam anaesthesia, oleic acid lung injury, then inspiratory P-V curve fitting to the four-parameter logistic Venegas equation F(P) = a + b[1 + e-(P-c)/d]-1 where: a = volume at lower asymptote, b = the vital capacity or the total change in volume between the lower and upper asymptotes, c = pressure at the inflection point and d = index related to linear compliance. Both groups received BVV with gas exchange and respiratory mechanics measured hourly for 5 hrs. Postmortem bronchoalveolar fluid was analysed for interleukin-8 (IL-8).

RESULTS

All P-V curves fit the Venegas equation (R2 > 0.995). Control VT averaged 7.4 +/- 0.4 mL/kg as compared to Experimental 9.5 +/- 1.6 mL/kg (range 6.6 - 10.8 mL/kg; p < 0.05). Variable VTs were within the convex portion of the P-V curve. In such circumstances, Jensen's inequality states "if F(P) is a convex function defined on an interval (r, s), and if P is a random variable taking values in (r, s), then the average or expected value (E) of F(P); E(F(P)) > F(E(P))." In both groups the inequality applied, since F(P) defines volume in the Venegas equation and (P) pressure and the range of VTs varied within the convex interval for individual P-V curves. Over 5 hrs, there were no significant differences between groups in minute ventilation, airway pressure, blood gases, haemodynamics, respiratory compliance or IL-8 concentrations.

CONCLUSION

No difference between groups is a consequence of BVV occurring on the convex interval for individualised Venegas P-V curves in all experiments irrespective of group. Jensen's inequality provides theoretical proof of why a variable ventilatory approach is advantageous under these circumstances. When using BVV, with VT centred by Venegas P-V curve analysis at the point of maximal compliance change, some leeway in low VT settings beyond ARDSNet protocols may be possible in acute lung injury. This study also shows that in this model, the standard ARDSNet algorithm assures ventilation occurs on the convex portion of the P-V curve.