Variable ventilation improves perioperative lung function in patients undergoing abdominal aortic aneurysmectomy

Comparative effects of variable versus conventional volume-controlled one-lung ventilation on gas exchange and respiratory system mechanics in thoracic surgery patients: A randomized controlled clinical trial

BACKGROUND

Mechanical ventilation with variable tidal volumes (V-VCV) has the potential to improve lung function during general anesthesia. We tested the hypothesis that V-VCV compared to conventional volume-controlled ventilation (C-VCV) would improve intraoperative arterial oxygenation and respiratory system mechanics in patients undergoing thoracic surgery under one-lung ventilation (OLV).

METHODS

Patients were randomized to V-VCV (n = 39) or C-VCV (n = 39). During OLV tidal volume of 5 mL/kg predicted body weight (PBW) was used. Both groups were ventilated with a positive end-expiratory pressure (PEEP) of 5 cm H2O, inspiration to expiration ratio (I:E) of 1:1 (during OLV) and 1:2 during two-lung ventilation, the respiratory rate (RR) titrated to arterial pH, inspiratory peak-pressure ≤ 40 cm H2O and an inspiratory oxygen fraction of 1.0.

RESULTS

Seventy-five out of 78 Patients completed the trial and were analyzed (dropouts were excluded). The partial pressure of arterial oxygen (PaO2) 20 min after the start of OLV did not differ among groups (V-VCV: 25.8 ± 14.6 kPa vs C-VCV: 27.2 ± 15.3 kPa; mean difference [95% CI]: 1.3 [-8.2, 5.5], P = 0.700). Furthermore, intraoperative gas exchange, intraoperative adverse events, need for rescue maneuvers due to desaturation and hypercapnia, incidence of postoperative pulmonary and extra-pulmonary complications, and hospital free days at day 30 after surgery did not differ between groups.

CONCLUSIONS

In thoracic surgery patients under OLV, V-VCV did not improve oxygenation or respiratory system mechanics compared to C-VCV. Ethical Committee: EK 420092019.

TRIAL REGISTRATION

at the German Clinical Trials Register: DRKS00022202 (16.06.2020).

One-lung ventilation with fixed and variable tidal volumes on oxygenation and pulmonary outcomes: A randomized trial

OBJECTIVE

Test the hypothesis that one-lung ventilation with variable tidal volume improves intraoperative oxygenation and reduces postoperative pulmonary complications after lung resection.

BACKGROUND

Constant tidal volume and respiratory rate ventilation can lead to atelectasis. Animal and human ARDS studies indicate that oxygenation improves with variable tidal volumes. Since one-lung ventilation shares characteristics with ARDS, we tested the hypothesis that one-lung ventilation with variable tidal volume improves intraoperative oxygenation and reduces postoperative pulmonary complications after lung resection.

DESIGN

Randomized trial.

SETTING

Operating rooms and a post-anesthesia care unit.

PATIENTS

Adults having elective open or video-assisted thoracoscopic lung resection surgery with general anesthesia were randomly assigned to intraoperative ventilation with fixed (n = 70) or with variable (n = 70) tidal volumes.

INTERVENTIONS

Patients assigned to fixed ventilation had a tidal volume of 6 ml/kgPBW, whereas those assigned to variable ventilation had tidal volumes ranging from 6 ml/kg PBW ± 33% which varied randomly at 5-min intervals.

MEASUREMENTS

The primary outcome was intraoperative oxygenation; secondary outcomes were postoperative pulmonary complications, mortality within 90 days of surgery, heart rate, and SpO2/FiO2 ratio.

RESULTS

Data from 128 patients were analyzed with 65 assigned to fixed-tidal volume ventilation and 63 to variable-tidal volume ventilation. The time-weighted average PaO2 during one-lung ventilation was 176 (86) mmHg in patients ventilated with fixed-tidal volume and 147 (72) mmHg in the patients ventilated with variable-tidal volume, a difference that was statistically significant (p < 0.01) but less than our pre-defined clinically meaningful threshold of 50 mmHg. At least one composite complication occurred in 11 (17%) of patients ventilated with variable-tidal volume and in 17 (26%) of patients assigned to fixed-tidal volume ventilation, with a relative risk of 0.67 (95% CI 0.34-1.31, p = 0.24). Atelectasis in the ventilated lung was less common with variable-tidal volumes (4.7%) than fixed-tidal volumes (20%) in the initial three postoperative days, with a relative risk of 0.24 (95% CI 0.01-0.8, p = 0.02), but there were no significant late postoperative differences. No other secondary outcomes were both statistically significant and clinically meaningful.

CONCLUSION

One-lung ventilation with variable tidal volume does not meaningfully improve intraoperative oxygenation, and does not reduce postoperative pulmonary complications.

The Constrained Disorder Principle Accounts for the Variability That Characterizes Breathing: A Method for Treating Chronic Respiratory Diseases and Improving Mechanical Ventilation

Variability characterizes breathing, cellular respiration, and the underlying quantum effects. Variability serves as a mechanism for coping with changing environments; however, this hypothesis does not explain why many of the variable phenomena of respiration manifest randomness. According to the constrained disorder principle (CDP), living organisms are defined by their inherent disorder bounded by variable boundaries. The present paper describes the mechanisms of breathing and cellular respiration, focusing on their inherent variability. It defines how the CDP accounts for the variability and randomness in breathing and respiration. It also provides a scheme for the potential role of respiration variability in the energy balance in biological systems. The paper describes the option of using CDP-based artificial intelligence platforms to augment the respiratory process's efficiency, correct malfunctions, and treat disorders associated with the respiratory system.

Novel ventilation techniques in children

Extraordinary progress has been made during the past few decades in the development of anesthesia machines and ventilation techniques. With unprecedented precision and performance, modern machines for pediatric anesthesia can deliver appropriate mechanical ventilation for children and infants of all sizes and with ongoing respiratory diseases, ensuring very small volume delivery and compensating for circuit compliance. Along with highly accurate monitoring of the delivered ventilation, modern ventilators for pediatric anesthesia also have a broad choice of ventilation modalities, including synchronized and assisted ventilation modes, which were initially conceived for ventilation weaning in the intensive care setting. Despite these technical advances, there is still room for improvement in pediatric mechanical ventilation. There is a growing effort to minimize the harm of intraoperative mechanical ventilation of children by adopting the protective ventilation strategies that were previously employed only for prolonged mechanical ventilation. More than ever, the pediatric anesthesiologist should now recognize that positive-pressure ventilation is potentially a harmful procedure, even in healthy children, as it can contribute to both ventilator-induced lung injury and ventilator-induced diaphragmatic dysfunction. Therefore, careful choice of the ventilation modality and its parameters is of paramount importance to optimize gas exchange and to protect the lungs from injury during general anesthesia. The present report reviews the novel ventilation techniques used for children, discussing the advantages and pitfalls of the ventilation modalities available in modern anesthesia machines, as well as innovative ventilation modes currently under development or research. Several innovative strategies and devices are discussed. These novel modalities are likely to become part of the armamentarium of the pediatric anesthesiologist in the near future and are particularly relevant for challenging ventilation scenarios.

Tracking respiratory mechanics around natural breathing rates via variable ventilation

Measuring respiratory resistance and elastance as a function of time, tidal volume, respiratory rate, and positive end-expiratory pressure can guide mechanical ventilation. However, current measurement techniques are limited since they are assessed intermittently at non-physiological frequencies or involve specialized equipment. To this end, we introduce ZVV, a practical approach to continuously track resistance and elastance during Variable Ventilation (VV), in which frequency and tidal volume vary from breath-to-breath. ZVV segments airway pressure and flow recordings into individual breaths, calculates resistance and elastance for each breath, bins them according to frequency or tidal volume and plots the results against bin means. ZVV's feasibility was assessed clinically in five human patients with acute lung injury, experimentally in five mice ventilated before and after lavage injury, and computationally using a viscoelastic respiratory model. ZVV provided continuous measurements in both settings, while the computational study revealed <2% estimation errors. Our findings support ZVV as a feasible technique to assess respiratory mechanics under physiological conditions. Additionally, in humans, ZVV detected a decrease in resistance and elastance with time by 12.8% and 6.2%, respectively, suggesting that VV can improve lung recruitment in some patients and can therefore potentially serve both as a dual diagnostic and therapeutic tool.

Temporal Changes in Ventilator Settings in Patients With Uninjured Lungs: A Systematic Review

In patients with uninjured lungs, increasing evidence indicates that tidal volume (VT) reduction improves outcomes in the intensive care unit (ICU) and in the operating room (OR). However, the degree to which this evidence has translated to clinical changes in ventilator settings for patients with uninjured lungs is unknown. To clarify whether ventilator settings have changed, we searched MEDLINE, Cochrane Central Register of Controlled Trials, and Web of Science for publications on invasive ventilation in ICUs or ORs, excluding those on patients <18 years of age or those with >25% of patients with acute respiratory distress syndrome (ARDS). Our primary end point was temporal change in VT over time. Secondary end points were changes in maximum airway pressure, mean airway pressure, positive end-expiratory pressure, inspiratory oxygen fraction, development of ARDS (ICU studies only), and postoperative pulmonary complications (OR studies only) determined using correlation analysis and linear regression. We identified 96 ICU and 96 OR studies comprising 130,316 patients from 1975 to 2014 and observed that in the ICU, VT size decreased annually by 0.16 mL/kg (-0.19 to -0.12 mL/kg) (P < .001), while positive end-expiratory pressure increased by an average of 0.1 mbar/y (0.02-0.17 mbar/y) (P = .017). In the OR, VT size decreased by 0.09 mL/kg per year (-0.14 to -0.04 mL/kg per year) (P < .001). The change in VTs leveled off in 1995. Other intraoperative ventilator settings did not change in the study period. Incidences of ARDS (ICU studies) and postoperative pulmonary complications (OR studies) also did not change over time. We found that, during a 39-year period, from 1975 to 2014, VTs in clinical studies on mechanical ventilation have decreased significantly in the ICU and in the OR.

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.

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.

Positive end-expiratory pressure and variable ventilation in lung-healthy rats under general anesthesia

Objectives

Variable ventilation (VV) seems to improve respiratory function in acute lung injury and may be combined with positive end-expiratory pressure (PEEP) in order to protect the lungs even in healthy subjects. We hypothesized that VV in combination with moderate levels of PEEP reduce the deterioration of pulmonary function related to general anesthesia. Hence, we aimed at evaluating the alveolar stability and lung protection of the combination of VV at different PEEP levels.

Design

Randomized experimental study.

Setting

Animal research facility.

Subjects

Forty-nine male Wistar rats (200–270 g).

Interventions

Animals were ventilated during 2 hours with protective low tidal volume (V_T) in volume control ventilation (VCV) or VV and PEEP adjusted at the level of minimum respiratory system elastance (Ers), obtained during a decremental PEEP trial subsequent to a recruitment maneuver, and 2 cmH₂O above or below of this level.

Measurements and Main Results

Ers, gas exchange and hemodynamic variables were measured. Cytokines were determined in lung homogenate and plasma samples and left lung was used for histologic analysis and diffuse alveolar damage scoring. A progressive time-dependent increase in Ers was observed independent on ventilatory mode or PEEP level. Despite of that, the rate of increase of Ers and lung tissue IL-1 beta concentration were significantly lower in VV than in VCV at the level of the PEEP of minimum Ers. A significant increase in lung tissue cytokines (IL-6, IL-1 beta, CINC-1 and TNF-alpha) as well as a ventral to dorsal and cranial to caudal reduction in aeration was observed in all ventilated rats with no significant differences among groups.

Conclusions

VV combined with PEEP adjusted at the level of the PEEP of minimal Ers seemed to better prevent anesthesia-induced atelectasis and might improve lung protection throughout general anesthesia.

Variable versus conventional lung protective mechanical ventilation during open abdominal surgery: study protocol for a randomized controlled trial

BACKGROUND

General anesthesia usually requires mechanical ventilation, which is traditionally accomplished with constant tidal volumes in volume- or pressure-controlled modes. Experimental studies suggest that the use of variable tidal volumes (variable ventilation) recruits lung tissue, improves pulmonary function and reduces systemic inflammatory response. However, it is currently not known whether patients undergoing open abdominal surgery might benefit from intraoperative variable ventilation.

METHODS/DESIGN

The PROtective VARiable ventilation trial ('PROVAR') is a single center, randomized controlled trial enrolling 50 patients who are planning for open abdominal surgery expected to last longer than 3 hours. PROVAR compares conventional (non-variable) lung protective ventilation (CV) with variable lung protective ventilation (VV) regarding pulmonary function and inflammatory response. The primary endpoint of the study is the forced vital capacity on the first postoperative day. Secondary endpoints include further lung function tests, plasma cytokine levels, spatial distribution of ventilation assessed by means of electrical impedance tomography and postoperative pulmonary complications.

DISCUSSION

We hypothesize that VV improves lung function and reduces systemic inflammatory response compared to CV in patients receiving mechanical ventilation during general anesthesia for open abdominal surgery longer than 3 hours. PROVAR is the first randomized controlled trial aiming at intra- and postoperative effects of VV on lung function. This study may help to define the role of VV during general anesthesia requiring mechanical ventilation.

TRIAL REGISTRATION

Clinicaltrials.gov NCT01683578 (registered on September 3 3012).

Variable Ventilation as a Diagnostic Tool for the Injured Lung

Mechanical ventilation of patients with acute respiratory distress syndrome (ARDS) is a necessary life support measure which may lead to ventilator-induced lung injury, a complication that can be reduced or ameliorated by using appropriate tidal volumes and positive end-expiratory pressures. However, the optimal mechanical ventilation parameters are almost certainly different for each patient, and will vary with time as the injury status of the lung changes. In order to optimize mechanical ventilation in an individual ARDS patient, therefore, it is necessary to track the manner in which injury status is reflected in the mechanical properties of the lungs. Accordingly, we developed an algorithm for assessing the time-dependent manner in which different lung regions open (recruit) and close (derecruit) as a function of the pressure waveform that is applied to the airways during mechanical ventilation. We used this algorithm to test the notion that variable ventilation provides the dynamic perturbations in lung volume necessary to accurately identify recruitment/derecruitment dynamics in the injured lung. We performed this test on synthetic pressure and flow data generated with established numerical models of lung function corresponding to both healthy mice and mice with lung injury. The data were generated by subjecting the models to a variety of mechanical ventilation regimens including variable ventilation. Our results support the hypothesis that variable ventilation can be used as a diagnostic tool to identify the injury status of the lung in ARDS.

Complexity and emergent phenomena

Complex biological systems operate under non-equilibrium conditions and exhibit emergent properties associated with correlated spatial and temporal structures. These properties may be individually unpredictable, but tend to be governed by power-law probability distributions and/or correlation. This article reviews the concepts that are invoked in the treatment of complex systems through a wide range of respiratory-related examples. Following a brief historical overview, some of the tools to characterize structural variabilities and temporal fluctuations associated with complex systems are introduced. By invoking the concept of percolation, the notion of multiscale behavior and related modeling issues are discussed. Spatial complexity is then examined in the airway and parenchymal structures with implications for gas exchange followed by a short glimpse of complexity at the cellular and subcellular network levels. Variability and complexity in the time domain are then reviewed in relation to temporal fluctuations in airway function. Next, an attempt is given to link spatial and temporal complexities through examples of airway opening and lung tissue viscoelasticity. Specific examples of possible and more direct clinical implications are also offered through examples of optimal future treatment of fibrosis, exacerbation risk prediction in asthma, and a novel method in mechanical ventilation. Finally, the potential role of the science of complexity in the future of physiology, biology, and medicine is discussed.

Rationale and study design of ViPS - variable pressure support for weaning from mechanical ventilation: study protocol for an international multicenter randomized controlled open trial

Background

In pressure support ventilation (PSV), a non-variable level of pressure support is delivered by the ventilator when triggered by the patient. In contrast, variable PSV delivers a level of pressure support that varies in a random fashion, introducing more physiological variability to the respiratory pattern. Experimental studies show that variable PSV improves gas exchange, reduces lung inflammation and the mean pressure support, compared to non-variable PSV. Thus, it can theoretically shorten weaning from the mechanical ventilator.

Methods/design

The ViPS (variable pressure support) trial is an international investigator-initiated multicenter randomized controlled open trial comparing variable vs. non-variable PSV. Adult patients on controlled mechanical ventilation for more than 24 hours who are ready to be weaned are eligible for the study. The randomization sequence is blocked per center and performed using a web-based platform. Patients are randomly assigned to one of the two groups: variable PSV or non-variable PSV. In non-variable PSV, breath-by-breath pressure support is kept constant and targeted to achieve a tidal volume of 6 to 8 ml/kg. In variable PSV, the mean pressure support level over a specific time period is targeted at the same mean tidal volume as non-variable PSV, but individual levels vary randomly breath-by-breath. The primary endpoint of the trial is the time to successful weaning, defined as the time from randomization to successful extubation.

Discussion

ViPS is the first randomized controlled trial investigating whether variable, compared to non-variable PSV, shortens the duration of weaning from mechanical ventilation in a mixed population of critically ill patients. This trial aims to determine the role of variable PSV in the intensive care unit.

Trial registration

clinicaltrials.gov NCT01769053

Impact of different breathing protocols on multiple-breath washout outcomes in children

BACKGROUND

To standardize multiple-breath washout (MBW) measurements, 1L tidal volume (VT) protocols were suggested. The effect on MBW derived ventilation inhomogeneity (VI) indices is unclear.

METHODS

We compared VI indices from free breathing MBW at baseline to 1L VT MBW performed in triplicates in 35 children (20 with CF). Mean (range) age was 12.8 (7.0-16.7) years, weight 42 (20-64) kg and height 151 (117-170) cm.

RESULTS

Baseline lung clearance index (LCI) increased from mean (SD) 11.0 (2.2) to 13.0 (2.6), p = 0.011, in CF and from 6.8 (0.5) to 7.7 (1.4), p = 0.004, in controls. Moment ratio and Scond similarly increased. While change in VI indices was heterogeneous in individuals, decrease in functional residual capacity was most strongly associated with LCI increase.

CONCLUSION

MBW protocols strongly influence measures of VI. The 1L VT MBW protocol leads to overestimation of VI and is not recommended in children.

Perioperative lung protection strategies in cardiothoracic anesthesia: are they useful?

Patients are at risk for several types of lung injury in the perioperative period. These injuries include atelectasis, pneumonia, pneumothorax, bronchopleural fistula, acute lung injury, and acute respiratory distress syndrome. Anesthetic management can cause, exacerbate, or ameliorate most of these injuries. Lung-protective ventilation strategies using more physiologic tidal volumes and appropriate levels of positive end-expiratory pressure can decrease the extent of this injury. This review discusses the effects of mechanical ventilation and its role in ventilator-induced lung injury with specific reference to cardiothoracic anesthesia.

Respiratory pattern during neurally adjusted ventilatory assist in acute respiratory failure patients

PURPOSE

To investigate the effect of a wide range of assistance levels during neurally adjusted ventilatory assist (NAVA) and pressure support ventilation (PSV) on respiratory pattern, breathing variability, and incidence of tidal volumes (V (T)) above 8 and 10 ml/kg in acute respiratory failure patients.

METHODS

Eight increasing NAVA levels (0.5, 1, 1.5, 2, 2.5, 3, 4, and 5 cmH(2)O/μV) and four increasing pressure support (PSV) levels (4, 8, 12, and 16 cmH(2)O) were applied to obtain 10 min of stable recordings in 15 patients.

RESULTS

One out of 15 patients did not sustain the NAVA levels of 3, 4, and 5 cmH(2)O/μV and was excluded. The 5 cmH(2)O/μV NAVA level was not tolerated by three patients and it was excluded. Increasing NAVA levels were associated with decreased diaphragm electrical activity (EAdi), and, at variance with PSV, with small changes in V (T), no changes in respiratory rate (RR), and increases in V (T) and EAdi variability. At high NAVA levels, an increase in V (T) variability was associated with increased incidence of V (T) above 8 and 10 ml/kg and an uncomfortable respiratory pattern in some patients.

CONCLUSIONS

Increasing NAVA levels were associated with no effect on RR, small increase in V (T), and increase in V (T) and EAdi variability. Effective decrease in EAdi occurred at NAVA levels below 2-2.5 cmH(2)O/μV, while preserving respiratory variability and low risks of V (T) above 8 or 10 ml/kg.

Recurrent recruitment manoeuvres improve lung mechanics and minimize lung injury during mechanical ventilation of healthy mice

INTRODUCTION

Mechanical ventilation (MV) of mice is increasingly required in experimental studies, but the conditions that allow stable ventilation of mice over several hours have not yet been fully defined. In addition, most previous studies documented vital parameters and lung mechanics only incompletely. The aim of the present study was to establish experimental conditions that keep these parameters within their physiological range over a period of 6 h. For this purpose, we also examined the effects of frequent short recruitment manoeuvres (RM) in healthy mice.

METHODS

Mice were ventilated at low tidal volume V(T) = 8 mL/kg or high tidal volume V(T) = 16 mL/kg and a positive end-expiratory pressure (PEEP) of 2 or 6 cm H(2)O. RM were performed every 5 min, 60 min or not at all. Lung mechanics were followed by the forced oscillation technique. Blood pressure (BP), electrocardiogram (ECG), heart frequency (HF), oxygen saturation and body temperature were monitored. Blood gases, neutrophil-recruitment, microvascular permeability and pro-inflammatory cytokines in bronchoalveolar lavage (BAL) and blood serum as well as histopathology of the lung were examined.

RESULTS

MV with repetitive RM every 5 min resulted in stable respiratory mechanics. Ventilation without RM worsened lung mechanics due to alveolar collapse, leading to impaired gas exchange. HF and BP were affected by anaesthesia, but not by ventilation. Microvascular permeability was highest in atelectatic lungs, whereas neutrophil-recruitment and structural changes were strongest in lungs ventilated with high tidal volume. The cytokines IL-6 and KC, but neither TNF nor IP-10, were elevated in the BAL and serum of all ventilated mice and were reduced by recurrent RM. Lung mechanics, oxygenation and pulmonary inflammation were improved by increased PEEP.

CONCLUSIONS

Recurrent RM maintain lung mechanics in their physiological range during low tidal volume ventilation of healthy mice by preventing atelectasis and reduce the development of pulmonary inflammation.

The emergence of modularity in biological systems

In this review, we discuss modularity and hierarchy in biological systems. We review examples from protein structure, genetics, and biological networks of modular partitioning of the geometry of biological space. We review theories to explain modular organization of biology, with a focus on explaining how biology may spontaneously organize to a structured form. That is, we seek to explain how biology nucleated from among the many possibilities in chemistry. The emergence of modular organization of biological structure will be described as a symmetry-breaking phase transition, with modularity as the order parameter. Experimental support for this description will be reviewed. Examples will be presented from pathogen structure, metabolic networks, gene networks, and protein-protein interaction networks. Additional examples will be presented from ecological food networks, developmental pathways, physiology, and social networks.

Lung protective strategies in anaesthesia

Patients are at risk for several types of lung injury in the perioperative period including atelectasis, pneumonia, pneumothorax, acute lung injury, and acute respiratory distress syndrome. Anaesthetic management can cause, exacerbate, or ameliorate these injuries. This review examines the effects of perioperative mechanical ventilation and its role in ventilator-induced lung injury. Lung protective ventilatory strategies to specific clinical situations such as cardiopulmonary bypass and one-lung ventilation along with newer novel lung protective strategies are discussed.

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.

Effects of different peep levels on mesenteric leukocyte-endothelial interactions in rats during mechanical ventilation

INTRODUCTION

Mechanical ventilation with positive end expiratory pressure (PEEP) improves oxygenation and treats acute pulmonary failure. However, increased intrathoracic pressure may cause regional blood flow alterations that may contribute to mesenteric ischemia and gastrointestinal failure. We investigated the effects of different PEEP levels on mesenteric leukocyte-endothelial interactions.

METHODS

Forty-four male Wistar rats were initially anesthetized (Pentobarbital I.P. 50 mg/kg) and randomly assigned to one of the following groups: 1) NAIVE (only anesthesia; n=9), 2) PEEP 0 (PEEP of 0 cmH2O, n=13), 3) PEEP 5 (PEEP of 5 cmH2O, n=12), and 4) PEEP 10 (PEEP of 10 cmH2O, n=13). Positive end expiratory pressure groups were tracheostomized and mechanically ventilated with a tidal volume of 10 mL/kg, respiratory rate of 70 rpm, and inspired oxygen fraction of 1. Animals were maintained under isoflurane anesthesia. After two hours, laparotomy was performed, and leukocyte-endothelial interactions were evaluated by intravital microscopy.

RESULTS

No significant changes were observed in mean arterial blood pressure among groups during the study. Tracheal peak pressure was smaller in PEEP 5 compared with PEEP 0 and PEEP 10 groups (11, 15, and 16 cmH2O, respectively; p<0.05). After two hours of MV, there were no differences among NAIVE, PEEP 0 and PEEP 5 groups in the number of rollers (118+/-9,127+/-14 and 147+/-26 cells/10 minutes, respectively), adherent leukocytes (3+/-1,3+/-1 and 4+/-2 cells/100 microm venule length, respectively), and migrated leukocytes (2+/-1,2+/-1 and 2+/-1 cells/5,000 microm(2), respectively) at the mesentery. However, the PEEP 10 group exhibited an increase in the number of rolling, adherent and migrated leukocytes (188+/-15 cells / 10 min, 8+/-1 cells / 100 microm and 12+/-1 cells / 5,000 microm(2), respectively; p<0.05).

CONCLUSIONS

High intrathoracic pressure was harmful to mesenteric microcirculation in the experimental model of rats with normal lungs and stable systemic blood pressure, a finding that may have relevance for complications related to mechanical ventilation.

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.

Variable stretch pattern enhances surfactant secretion in alveolar type II cells in culture

Secretion of pulmonary surfactant that maintains low surface tension within the lung is primarily mediated by mechanical stretching of alveolar epithelial type II (AEII) cells. We have shown that guinea pigs ventilated with random variations in frequency and tidal volume had significantly larger pools of surfactant in the lung than animals ventilated in a monotonous manner. Here, we test the hypothesis that variable stretch patterns imparted on the AEII cells results in enhanced surfactant secretion. AEII cells isolated from rat lungs were exposed to equibiaxial strains of 12.5, 25, or 50% change in surface area (DeltaSA) at 3 cycles/min for 15, 30, or 60 min. (3)H-labeled phosphatidylcholine release and cell viability were measured 60 min following the onset of stretch. Whereas secretion increased following 15-min stretch at 50% DeltaSA and 30-min stretch at 12.5% DeltaSA, 60 min of cyclic stretch diminished surfactant secretion regardless of strain. When cells were stretched using a variable strain profile in which the amplitude of each stretch was randomly pulled from a uniform distribution, surfactant secretion was enhanced both at 25 and 50% mean DeltaSA with no additional cell injury. Furthermore, at 50% mean DeltaSA, there was an optimum level of variability that maximized secretion implying that mechanotransduction in these cells exhibits a phenomenon similar to stochastic resonance. These results suggest that application of variable stretch may enhance surfactant secretion, possibly reducing the risk of ventilator-induced lung injury. Variable stretch-induced mechanotransduction may also have implications for other areas of mechanobiology.

Positive pressure ventilation: what is the real cost?

Positive pressure ventilation is a radical departure from the physiology of breathing spontaneously. The immediate physiological consequences of positive pressure ventilation such as haemodynamic changes are recognized, studied, and understood. There are other significant physiological interactions which are less obvious, more insidious, and may only produce complications if ventilation is prolonged. The interaction of positive pressure with airway resistance and alveolar compliance affects distribution of gas flow within the lung. The result is a wide range of ventilation efficacy throughout different areas of the lung, but the pressure differentials between alveolus and interstitium also influence capillary perfusion. The hydrostatic forces across the capillaries associated with the effects of raised venous pressures compound these changes resulting in interstitial fluid sequestration. This is increased by impaired lymphatic drainage which is secondary to raised intrathoracic pressure but also influenced by raised central venous pressure. Ventilation and PEEP promulgate further physiological derangement. In theory, avoiding these physiological disturbances in a rested lung may be better for the lung and other organs. An alternative to positive pressure ventilation might be to investigate oxygen supplementation of a physiologically neutral and rested lung. Abandoning heroic ventilation would be a massive departure from current practice but might be a more rationale approach to future practice.

Breathing cardiovascular variability and baroreflex in mechanically ventilated patients

Heart rate and blood pressure variations during spontaneous ventilation are related to the negative airway pressure during inspiration. Inspiratory airway pressure is positive during mechanical ventilation, suggesting that reversal of the normal baroreflex-mediated pattern of variability may occur. We investigated heart rate and blood pressure variability and baroreflex sensitivity in 17 mechanically ventilated patients. ECG (RR intervals), invasive systolic blood pressure (SBP), and respiratory flow signals were recorded. High-frequency (HF) amplitude of RR and SBP time series and HF phase differences between RR, SBP, and ventilatory signals were continuously computed by Complex DeModulation (CDM). Cross-spectral analysis was used to assess the coherence and the gain functions between RR and SBP, yielding baroreflex sensitivity indices. The HF phase difference between SBP and ventilatory signals was nearly constant in all patients with inversion of SBP variability during the ventilator cycle compared with cycling with negative inspiratory pressure to replicate spontaneous breathing. In 12 patients (group 1), the phase difference between RR and ventilatory signals changed over time and the HF-RR amplitude varied. In the remaining five patients (group 2), RR-ventilatory signal phase and HF-RR amplitude showed little change; however, only one of these patients exhibited a RR-ventilatory signal phase difference mimicking the normal pattern of respiratory sinus arrhythmia. Spectral coherence between RR and SBP was lower in the group with phase difference changes. Positive pressure ventilation exerts mainly a mechanical effect on SBP, whereas its influence on HR variability seems more complex, suggesting a role for neural influences.

Design of a new variable-ventilation method optimized for lung recruitment in mice

Variable ventilation (VV), characterized by breath-to-breath variation of tidal volume (Vt) and breathing rate (f), has been shown to improve lung mechanics and blood oxygenation during acute lung injury in many species compared with conventional ventilation (CV), characterized by constant Vt and f. During CV as well as VV, the lungs of mice tend to collapse over time; therefore, the goal of this study was to develop a new VV mode (VVN) with an optimized distribution of Vt to maximize recruitment. Groups of normal and HCl-injured mice were subjected to 1 h of CV, original VV (VVO), CV with periodic large breaths (CVLB), and VVN, and the effects of ventilation modes on respiratory mechanics, airway pressure, blood oxygenation, and IL-1β were assessed. During CV and VVO, normal and injured mice showed regional lung collapse with increased airway pressures and poor oxygenation. CVLB and VVN resulted in a stable dynamic equilibrium with significantly improved respiratory mechanics and oxygenation. Nevertheless, VVN provided a consistently better physiological response. In injured mice, VVO and VVN, but not CVLB, were able to reduce the IL-1β-related inflammatory response compared with CV. In conclusion, our results suggest that application of higher Vt values than the single Vt currently used in clinical situations helps stabilize lung function. In addition, variable stretch patterns delivered to the lung by VV can reduce the progression of lung injury due to ventilation in injured mice.

Hemodynamics in experimental gastric juice induced aspiration pneumonitis

OBJECTIVE

To characterize hemodynamic changes during experimental aspiration pneumonitis, paying special attention to echocardiographic assessment.

DESIGN AND SETTING

Animal study in a university-based research laboratory.

SUBJECTS

Fourteen mechanically ventilated New Zealand white rabbits

INTERVENTIONS

We instilled 1 ml/kg human gastric juice (mean pH: 4.1+0.2) intratracheally. Hemodynamic and respiratory parameters were measured every hour for 4 h, associated with a transthoracic echocardiography.

MEASUREMENTS AND RESULTS

Lung injury occurred within 1 hour with a marked decrease in PaO(2)/FIO(2) and an increase in plateau pressure; after this initial drop the ratio remained stable throughout the experiment. Seven rabbits experienced only a mild to moderate alteration in lung oxygenation function as defined by a PaO(2)/FIO(2) ratio above 200 (group A), while the other seven developed a severe alteration with a ratio below 200 (group B). At the end of the experiment pH and cardiac output were lower in group B than in group A. Using a PaO(2)/FIO(2) threshold value of 150, pH, mean arterial pressure, and cardiac output were lower in the animals with the more severe hypoxemia. Neither left nor right ventricular dysfunction occurred during the experiment, and no animal experienced circulatory failure

CONCLUSION

Experimental aspiration pneumonitis after intratracheal infusion of human gastric juice is characterized by a stable fall in PaO(2)/FIO(2). Animals with the most severe lung injury experienced a lower systemic arterial pressure, cardiac output, and metabolic acidosis without circulatory failure or cardiac dysfunction.

Ischemia and Reperfusion Increases Susceptibility to Ventilator-induced Lung Injury in Rats

OBJECTIVES

Hemorrhagic shock followed by resuscitation (HSR) commonly triggers an inflammatory response that leads to acute respiratory distress syndrome.

HYPOTHESIS

HSR exacerbates mechanical stress-induced lung injury by rendering the lung more susceptible to ventilator-induced lung injury.

METHODS

Rats were subjected to HSR, and were randomized into an HSR + high tidal volume and zero positive end-expiratory pressure (PEEP) or a HSR + low tidal volume with 5 cm H(2)O PEEP. A sham-operated rat + high tidal volume and zero PEEP served as a control.

RESULTS

HSR increased susceptibility to ventilator-induced lung injury as evidenced by an increase in lung elastance and the wet/dry ratio and a reduction in Pa(O(2)) as compared with the other groups. The lung injury observed in the HSR + high tidal volume group was associated with a higher level of interleukin 6 in the lung and blood, increased epithelial cell apoptosis in the kidney and small intestine villi, and a tendency toward high levels of alanine aminotransferase, aspartate aminotransferase, lactate dehydrogenase, and creatinine in plasma.

CONCLUSIONS

HSR priming renders the lung and kidney more susceptible to mechanical ventilation-induced organ injury.

Linear Model and Algorithm to Automatically Estimate the Pressure Limit of Pressure Controlled Ventilation for Delivering a Target Tidal Volume

OBJECTIVE

To theoretically assess the viability of an automatic procedure to support the anesthesiologist in properly setting mechanical ventilators when the operating conditions are switched from volume controlled to pressure controlled ventilation whilst maintaining the preset tidal volume. The procedure is based on a simple linear model of the ventilator breathing system with constant parameters and utilizes the signals gathered by the ventilator without the need to add further equipment. After a short period of stable volume controlled ventilation with the desired tidal volume, the herewith described algorithm allows the calculation of the value of pressure limit to set in pressure controlled mode which assures the previously settled tidal volume with the same breathing frequency and inspiratory-expiratory time ratio.

METHODS

The algorithm allows the online identification of the four parameters necessary for the mathematical model that are obtained by means of a direct comparison between the pressure, flow and volume waveforms generated by the model and the analog signals provided by the ventilator. The theoretical approach was validated by two different ventilators, various settings, two breathing circuits, endotracheal tubes of various sizes and two mechanical simulators of the respiratory system operating in various conditions.

RESULTS

Errors usually less than 5% (p < 0.05) on the target tidal volume were obtained for various settings typically used for adult ventilation in less than 10 s. The theoretical approach shows its limitations (errors of 10+/- 5%, p < 0.05) at high breathing frequencies (30-40 bpm) and low tidal volumes (200-300 ml).

CONCLUSIONS

The proposed theoretical approach shows the viability, for adult settings, of one of the simplest mathematical model for mechanical ventilation in order to quickly and safely switch from volume controlled to pressure controlled ventilation. The algorithm could easily be in perspective implemented in the software of the ventilator providing the anesthesiologist with an indication on the value of pressure limit to set in order to safely switch ventilation mode.

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.

Decreased physiologic variability as a generalized response to human endotoxemia*

OBJECTIVE

To test the effect in normal human volunteers of transient systemic inflammation on the variability in time-series behaviors of widely divergent physiologic measures of the human inflammatory response.

DESIGN

Prospective study of human volunteers who were tested on 2 consecutive days, a control day and a treatment day. Each participant served as his or her own control.

SETTING

Critical care facility of a university medical center.

SUBJECTS

Subjects were eight healthy human volunteers.

INTERVENTIONS

Participant subjects were tested on both a baseline day with no intervention and on a treatment day when they received 4 ng/kg intravenous Escherichia coli endotoxin.

MEASUREMENTS AND MAIN RESULTS

Continuous electrocardiographic recordings and serial blood sampling (performed every 5 mins) were used to create time-series of heart rate (R-R intervals), neutrophil function (phagocytosis), and plasma cortisol concentrations. For each primary measure, we recorded a significant increase in the regularity (decreased variability) of the functional measurement as assessed by the statistical entity, approximate entropy.

CONCLUSIONS

Increased regularity, or decreased variability, of organ functions is a generalized response to systemic inflammation that occurs in widely divergent systems during endotoxemia.

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

Background

Biologically variable ventilation (return of physiological variability in rate and tidal volume using a computer-controller) was compared to control mode ventilation with and without a recruitment manoeuvre – 40 cm H₂O for 40 sec performed hourly; in a porcine oleic acid acute lung injury model.

Methods

We compared gas exchange, respiratory mechanics, and measured bronchoalveolar fluid for inflammatory cytokines, cell counts and surfactant function. Lung injury was scored by light microscopy. Pigs received mechanical ventilation (F_IO₂ = 0.3; PEEP 5 cm H₂O) in control mode until PaO₂ decreased to 60 mm Hg with oleic acid infusion (PaO₂/F_IO₂ <200 mm Hg). Additional PEEP to 10 cm H₂O was added after injury. Animals were randomized to one of the 3 modes of ventilation and followed for 5 hr after injury.

Results

PaO₂ and respiratory system compliance was significantly greater with biologically variable ventilation compared to the other 2 groups. Mean and mean peak airway pressures were also lower. There were no differences in cell counts in bronchoalveolar fluid by flow cytometry, or interleukin-8 and -10 levels between groups. Lung injury scoring revealed no difference between groups in the regions examined. No differences in surfactant function were seen between groups by capillary surfactometry.

Conclusions

In this porcine model of acute lung injury, various indices to measure injury or inflammation did not differ between the 3 approaches to ventilation. However, when using a low tidal volume strategy with moderate levels of PEEP, sustained improvements in arterial oxygen tension and respiratory system compliance were only seen with BVV when compared to CMV or CMV with a recruitment manoeuvre.

Nonlinear dynamics, complex systems, and the pathobiology of critical illness

PURPOSE OF REVIEW

The review considers problems in critical illness and critical care in the context of complex systems science. Normal physiology is characterized by nonlinear dynamics, and it appears that the pathophysiology of critical illness alters those dynamics.

RECENT FINDINGS

Recent evidence confirms and extends the observation that the rich variability that characterizes normal physiology "decomplexifies" with critical illness. Experimental data in animals and now in humans suggests that physiologic support that mimics normal variability may reduce the severity and/or duration of the illness.

SUMMARY

Physiologic dynamics in health and in critical illness appear to reflect complex, interconnected systems biology. Alterations in illness and during recovery may provide important clues to the underlying structure of the system. With knowledge of the structure, therapy could be better focused toward supporting both function and dynamics, offering hope for improved outcomes.