Lung recruitment maneuvers in acute respiratory distress syndrome and facilitating resolution

Correlation between the hysteresis of the pressure–volume curve and the recruitment-to-inflation ratio in patients with coronavirus disease 2019

Background

Since the response to lung recruitment varies greatly among patients receiving mechanical ventilation, lung recruitability should be assessed before recruitment maneuvers. The pressure–volume curve (PV curve) and recruitment-to-inflation ratio (R/I ratio) can be used bedside for evaluating lung recruitability and individualing positive end-expiratory pressure (PEEP). Lung tissue recruitment on computed tomography has been correlated with normalized maximal distance (NMD) of the quasi-static PV curve. NMD is the maximal distance between the inspiratory and expiratory limb of the PV curve normalized to the maximal volume. However, the relationship between the different parameters of hysteresis of the quasi-static PV curve and R/I ratio for recruitability is unknown.

Methods

We analyzed the data of 33 patients with severe coronavirus disease 2019 (COVID-19) who received invasive mechanical ventilation. Respiratory waveform data were collected from the ventilator using proprietary acquisition software. We examined the relationship of the R/I ratio, quasi-static PV curve items such as NMD, and respiratory system compliance (C_rs).

Results

The median R/I ratio was 0.90 [interquartile range (IQR), 0.70–1.15] and median NMD was 41.0 [IQR, 37.1–44.1]. The NMD correlated significantly with the R/I ratio (rho = 0.74, P < 0.001). Sub-analysis showed that the NMD and R/I ratio did not correlate with C_rs at lower PEEP (− 0.057, P = 0.75; and rho = 0.15, P = 0.41, respectively). On the contrary, the ratio of C_rs at higher PEEP to C_rs at lower PEEP (C_rs ratio (higher/lower)) moderately correlated with NMD and R/I ratio (rho = 0.64, P < 0.001; and rho = 0.67, P < 0.001, respectively).

Conclusions

NMD of the quasi-static PV curve and R/I ratio for recruitability assessment are highly correlated. In addition, NMD and R/I ratio correlated with the C_rs ratio (higher/lower). Therefore, NMD and R/I ratio could be potential indicators of recruitability that can be performed at the bedside.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13613-022-01081-x.

Tidal Volume and Positive End-Expiratory Pressure and Postoperative Hypoxemia during General Anesthesia: A Single Center Multiple Cross-over Factorial Cluster Trial

BACKGROUND

Intraoperative mechanical ventilation is a major component of general anesthesia. The extent to which various intraoperative tidal volumes and positive end-expiratory pressures (PEEP) on postoperative hypoxia and lung injury remains unclear. We hypothesized that adult patients having orthopedic surgery, ventilation using different tidal volumes and PEEP levels affects the oxygenation within first hour in the postoperative care unit.

METHODS

We conducted a 2 by 2 factorial cross-over cluster trial at the Cleveland Clinic Main Campus. We enrolled patients having orthopedic surgery with general anesthesia who were assigned to factorial clusters with tidal volumes of 6 or 10 ml/kg of predicted body weight and to PEEP of 5 or 8 cm H20 in one-week clusters The primary outcome was the effect of tidal volume or PEEP on time-weighted average peripheral oxygen saturation divided by the fraction of inspired oxygen (SpO2/FiO2 ratio) during the initial postoperative hour.

RESULTS

We enrolled 2860 patients who had general anesthesia for orthopedic surgery from September 2018 through October 2020. The interaction between tidal volume and PEEP was not significant (p = 0.565). The mean (SD) time-weighted average of SpO2/FiO2 ratio was 353 (47) and not different in patients assigned to high and low tidal volume (estimated effect 3.5% (97.5%CI: -0.4%,7.3%;P=0.042), and for those assigned to high and low PEEP (-0.2% (97.5%CI: -4.0%,3.6%;P=0.906). We did not find significant difference in ward SpO2/FiO2 ratio, pulmonary complications, and duration of hospitalization among patients assigned to various tidal volumes and PEEP levels.

CONCLUSION

Among adults having major orthopedic surgery, pulse oximetry oxygenation is similar with tidal volumes between 6 and 10 ml/kg and PEEP between 5 and 8 cm H20. Our results suggest that any combination of tidal volumes between 6 and 10 ml/kg and PEEP between 5 vs. 8 mL cmH20 can be used safely for orthopedic surgery.Trial Registration ClinicalTrials.gov Identifier: NCT03657368.

The acute respiratory distress syndrome

Acute respiratory distress syndrome (ARDS) is a prevalent cause of acute respiratory failure with high rates of mortality, as well as short- and long-term complications, such as physical and cognitive impairment. Therefore, early recognition of this syndrome and application of well-demonstrated therapeutic interventions are essential to change the natural course of this entity and bring about positive clinical outcomes. In this article, we review updated concepts in ARDS. Specifically, we discuss the current definition of ARDS, its risk factors, and the evidence supporting ventilation management, adjunctive therapies, and interventions required in refractory hypoxemia.

What is the proper ventilation strategy during laparoscopic surgery?

The main stream of intraabdominal surgery has changed from laparotomy to laparoscopy, but anesthetic care for laparoscopic surgery is challenging for clinicians, because pneumoperitoneum might aggravate respiratory mechanics and arterial oxygenation. The authors reviewed the literature regarding ventilation strategies that reduce deleterious pulmonary physiologic changes during pneumoperitoneum for laparoscopic surgery under general anesthesia and make appropriate recommendations.

Ventilation in Trauma Patients: The First 24 h is Different!

INTRODUCTION

Ventilation of major trauma patients is often needed in both the acute (emergency department and early ICU phase) and subsequent phases of trauma care for those who need ICU admission. What is unclear is whether ICU ventilation strategies should be directly extrapolated to the acute phase of treatment.

METHODS

This paper reviews the ARDS.net study, highlights recent developments in ventilation strategies, and provides practical ventilation guidance to the trauma surgeon for acute phase (in the ED or ICU) and the subsequent phase of ICU care.

RESULTS

The acute phase of care in the ED and the ICU is different from the subsequent phases of ICU care as the lung is more recruitable and there are other aspects of resuscitation from metabolic acidosis and traumatic brain injury, which require a different ventilation strategy to the traditional ARDS.net approach.

DISCUSSION AND CONCLUSION

The acute phase is different from the subsequent phase of care and there appears to be some inappropriate extrapolation of ICU practice to the acute phase. Application of the proposed ventilation strategies should ensure an optimal outcome. It is important to treat patients as individuals during assessment and treatment.

Regional tidal lung strain in mechanically ventilated normal lungs

Parenchymal strain is a key determinant of lung injury produced by mechanical ventilation. However, imaging estimates of volumetric tidal strain (ε = regional tidal volume/reference volume) present substantial conceptual differences in reference volume computation and consideration of tidally recruited lung. We compared current and new methods to estimate tidal volumetric strains with computed tomography, and quantified the effect of tidal volume (VT) and positive end-expiratory pressure (PEEP) on strain estimates. Eight supine pigs were ventilated with VT = 6 and 12 ml/kg and PEEP = 0, 6, and 12 cmH2O. End-expiratory and end-inspiratory scans were analyzed in eight regions of interest along the ventral-dorsal axis. Regional reference volumes were computed at end-expiration (with/without correction of regional VT for intratidal recruitment) and at resting lung volume (PEEP = 0) corrected for intratidal and PEEP-derived recruitment. All strain estimates demonstrated vertical heterogeneity with the largest tidal strains in middependent regions (P < 0.01). Maximal strains for distinct estimates occurred at different lung regions and were differently affected by VT-PEEP conditions. Values consistent with lung injury and inflammation were reached regionally, even when global measurements were below critical levels. Strains increased with VT and were larger in middependent than in nondependent lung regions. PEEP reduced tidal-strain estimates referenced to end-expiratory lung volumes, although it did not affect strains referenced to resting lung volume. These estimates of tidal strains in normal lungs point to middependent lung regions as those at risk for ventilator-induced lung injury. The different conditions and topography at which maximal strain estimates occur allow for testing the importance of each estimate for lung injury.

The acute respiratory distress syndrome

The acute respiratory distress syndrome (ARDS) is a major cause of acute respiratory failure. Its development leads to high rates of mortality, as well as short- and long-term complications, such as physical and cognitive impairment. Therefore, early recognition of this syndrome and application of demonstrated therapeutic interventions are essential to change the natural course of this devastating entity. In this review article, we describe updated concepts in ARDS. Specifically, we discuss the new definition of ARDS, its risk factors and pathophysiology, and current evidence regarding ventilation management, adjunctive therapies, and intervention required in refractory hypoxemia.

Effects of ventilation strategy on distribution of lung inflammatory cell activity

Introduction

Leukocyte infiltration is central to the development of acute lung injury, but it is not known how mechanical ventilation strategy alters the distribution or activation of inflammatory cells. We explored how protective (vs. injurious) ventilation alters the magnitude and distribution of lung leukocyte activation following systemic endotoxin administration.

Methods

Anesthetized sheep received intravenous endotoxin (10 ng/kg/min) followed by 2 h of either injurious or protective mechanical ventilation (n = 6 per group). We used positron emission tomography to obtain images of regional perfusion and shunting with infused ¹³N[nitrogen]-saline and images of neutrophilic inflammation with ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG). The Sokoloff model was used to quantify ¹⁸F-FDG uptake (K_i), as well as its components: the phosphorylation rate (k₃, a surrogate of hexokinase activity) and the distribution volume of ¹⁸F-FDG (F_e) as a fraction of lung volume (K_i = F_e × k₃). Regional gas fractions (f_gas) were assessed by examining transmission scans.

Results

Before endotoxin administration, protective (vs. injurious) ventilation was associated with a higher ratio of partial pressure of oxygen in arterial blood to fraction of inspired oxygen (PaO₂/FiO₂) (351 ± 117 vs. 255 ± 74 mmHg; P < 0.01) and higher whole-lung f_gas (0.71 ± 0.12 vs. 0.48 ± 0.08; P = 0.004), as well as, in dependent regions, lower shunt fractions. Following 2 h of endotoxemia, PaO₂/FiO₂ ratios decreased in both groups, but more so with injurious ventilation, which also increased the shunt fraction in dependent lung. Protective ventilation resulted in less nonaerated lung (20-fold; P < 0.01) and more normally aerated lung (14-fold; P < 0.01). K_i was lower during protective (vs. injurious) ventilation, especially in dependent lung regions (0.0075 ± 0.0043/min vs. 0.0157 ± 0.0072/min; P < 0.01). ¹⁸F-FDG phosphorylation rate (k₃) was twofold higher with injurious ventilation and accounted for most of the between-group difference in K_i. Dependent regions of the protective ventilation group exhibited lower k₃ values per neutrophil than those in the injurious ventilation group (P = 0.01). In contrast, F_e was not affected by ventilation strategy (P = 0.52). Lung neutrophil counts were not different between groups, even when regional inflation was accounted for.

Conclusions

During systemic endotoxemia, protective ventilation may reduce the magnitude and heterogeneity of pulmonary inflammatory cell metabolic activity in early lung injury and may improve gas exchange through its effects predominantly in dependent lung regions. Such effects are likely related to a reduction in the metabolic activity, but not in the number, of lung-infiltrating neutrophils.

Role of mechanical stretching and lipopolysaccharide in early apoptosis and IL-8 of alveolar epithelial type II cells A549

OBJECTIVE

To investigate the effects of mechanical stretching and lipopolysaccharide (LPS) on the early apoptosis and IL-8 production of alveolar epithelial type II cells A549.

METHODS

The experimental matrix consisted of three integrated studies. In the first study, A549 cells were subjected to different stretching strain frequency and duration time to see the effects on the early apoptosis. In the second study, A549 cells were subjected to mechanical stretch (15% 4 h, 0.5 Hz) and LPS (1 or 100 ng/mL) to see whether mechanical strain and LPS also have an addictive effect on the early apoptosis. In the third study to investigate whether this addictive effect could be induced by LPS and mechanical stretch on IL-8 production, A549 cells were subjected to LPS (100 ng/mL) and mechanical strain (15%, 0.5 Hz, 4 h). Real time PCR and enzyme linked immunosorbent assay were used to measure mRNA and protein level of IL-8. The early apoptosis was detected by flow cytometry.

RESULTS

Mechanical stretch induced the early apoptosis in a force and frequency and time-dependent manner. In the presence of LPS, mechanical stretch enhanced LPS-induced early apoptosis, especially in 100 ng/mL LPS group compared with 1 ng/mL LPS and the control group. Mechanical stretch increased IL-8 production and enhanced LPS-induced IL-8 screation both in mRNA and protein levels.

CONCLUSIONS

Mechanical stretch can induce the early apoptosis and IL-8 secretion. Mechanical stretch and LPS have an addictive effect on the early apoptosis and IL-8 production in alveolar type 2 cells, which is one of the mechanisms of ventilator-induced lung injury.

Mechanical ventilation during anaesthesia: challenges and opportunities for investigating the respiration-related cardiovascular oscillations

The vast majority of the available literature regarding cardiovascular oscillations refers to spontaneously breathing subjects. Only a few studies investigated cardiovascular oscillations, and especially respiration-related ones (RCVO), during intermittent positive pressure mechanical ventilation (IPPV) under anaesthesia. Only a handful considered assisted IPPV, in which spontaneous breathing activity is supported, rather than replaced as in controlled IPPV. In this paper, we review the current understanding of RCVO physiology during IPPV, from literature retrieved through PubMed website. In particular, we describe how during controlled IPPV under anaesthesia respiratory sinus arrhythmia appears to be generated by non-neural mechano-electric feedback in the heart (indirectly influenced by tonic sympathetic regulation of vascular tone and heart contractility) and not by phasic vagal modulation of central origin and/or baroreflex mechanisms. Furthermore, assisted IPPV differs from controlled IPPV in terms of RCVO, reintroducing significant central respiratory vagal modulation of respiratory sinus arrhythmia. This evidence indicates against applying to IPPV interpretative paradigms of RCVO derived from spontaneously breathing subjects, and against considering together IPPV and spontaneously breathing subjects for RCVO-based risk assessment. Finally, we highlight the opportunities that IPPV offers for future investigations of RCVO genesis and interactions, and we indicate several possibilities for clinical applications of RCVO during IPPV.

Extracorporeal life support for management of refractory cardiac or respiratory failure: initial experience in a tertiary centre

Introduction

Extracorporeal Life Support (ECLS) and extracorporeal membrane oxygenation (ECMO) have been indicated as treatment for acute respiratory and/or cardiac failure. Here we describe our first year experience of in-hospital ECLS activity, the operative algorithm and the protocol for centralization of adult patients from district hospitals.

Methods

At a tertiary referral trauma center (Careggi Teaching Hospital, Florence, Italy), an ECLS program was developed from 2008 by the Emergency Department and Heart and Vessel Department ICUs. The ECLS team consists of an intensivist, a cardiac surgeon, a cardiologist and a perfusionist, all trained in ECLS technique. ECMO support was applied in case of severe acute respiratory distress syndrome (ARDS) not responsive to conventional treatments. The use of veno-arterial (V-A) ECLS for cardiac support was reserved for cases of cardiac shock refractory to standard treatment and cardiac arrests not responding to conventional resuscitation.

Results

A total of 21 patients were treated with ECLS during the first year of activity. Among them, 13 received ECMO for ARDS (5 H1N1-virus related), with a 62% survival. In one case of post-traumatic ARDS, V-A ECLS support permitted multiple organ donation after cerebral death was confirmed. Patients treated with V-A ECLS due to cardiogenic shock (N = 4) had a survival rate of 50%. No patients on V-A ECLS support after cardiac arrest survived (N = 4).

Conclusions

In our centre, an ECLS Service was instituted over a relatively limited period of time. A strict collaboration between different specialists can be regarded as a key feature to efficiently implement the process.

A recruitment maneuver increases oxygenation after intubation of hypoxemic intensive care unit patients: a randomized controlled study

Introduction

Tracheal intubation and anaesthesia promotes lung collapse and hypoxemia. In acute lung injury patients, recruitment maneuvers (RMs) increase lung volume and oxygenation, and decrease atelectasis. The aim of this study was to evaluate the efficacy and safety of RMs performed immediately after intubation.

Methods

This randomized controlled study was conducted in two 16-bed medical-surgical intensive care units within the same university hospital. Consecutive patients requiring intubation for acute hypoxemic respiratory failure were included. Patients were randomized to undergo a RM immediately (within 2 minutes) after intubation, consisting of a continuous positive airway pressure (CPAP) of 40 cmH₂O over 30 seconds (RM group), or not (control group). Blood gases were sampled and blood samples taken for culture before, within 2 minutes, 5 minutes, and 30 minutes after intubation. Haemodynamic and respiratory parameters were continuously recorded throughout the study. Positive end expiratory pressure (PEEP) was set at 5 cmH₂O throughout.

Results

The control (n = 20) and RM (n = 20) groups were similar in terms of age, disease severity, diagnosis at time of admission, and PaO₂ obtained under 10-15 L/min oxygen flow immediately before (81 ± 15 vs 83 ± 35 mmHg, P = 0.9), and within 2 minutes after, intubation under 100% FiO₂ (81 ± 15 vs 83 ± 35 mmHg, P = 0.9). Five minutes after intubation, PaO₂ obtained under 100% FiO₂ was significantly higher in the RM group compared with the control group (93 ± 36 vs 236 ± 117 mmHg, P = 0.008). The difference remained significant at 30 minutes with 110 ± 39 and 180 ± 79 mmHg, respectively, for the control and RM groups. No significant difference in haemodynamic conditions was observed between groups at any time. Following tracheal intubation, 15 patients had positive blood cultures, showing microorganisms shared with tracheal aspirates, with no significant difference in the incidence of culture positivity between groups.

Conclusions

Recruitment maneuver following intubation in hypoxemic patients improved short-term oxygenation, and was not associated with increased adverse effects.

Trial registration

NCT01014299

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.

A recruitment manoeuvre performed after endotracheal suction does not increase dynamic compliance in ventilated paediatric patients: a randomised controlled trial

QUESTION

Does a recruitment manoeuvre after suctioning have any immediate or short-term effect on ventilation and gas exchange in mechanically-ventilated paediatric patients?

DESIGN

Randomised controlled trial with concealed allocation, assessor blinding, and intention-to-treat analysis.

PARTICIPANTS

Forty-eight paediatric patients with heterogeneous lung pathology. Fourteen patients were subsequently excluded from analysis due to large leaks around the endotracheal tube.

INTERVENTION

The experimental group received a single standardised suctioning procedure followed five minutes later by a standardised recruitment manoeuvre. The control group received only the single suctioning procedure.

OUTCOME MEASURES

Measurements of ventilation (dynamic lung compliance, expiratory airway resistance, mechanical and spontaneous expired tidal volume, respiratory rate) and gas exchange (transcutaneous oxygen saturation) were recorded, on three occasions before and on two occasions after the recruitment manoeuvre, using a respiratory profile monitor.

RESULTS

There was no difference between the experimental and the control group in dynamic compliance, expired airway resistance, or oxygen saturation either immediately after the recruitment manoeuvre, or after 25 minutes. The experimental group decreased mechanical expired tidal volume by 0.3 ml/kg (95% CI 0.1 to 0.6), increased spontaneous expired tidal volume by 0.3 ml/kg (95% CI 0.0 to 0.6), and increased total respiratory rate by 3 bpm (95% CI 1 to 4) immediately after the recruitment manoeuvre compared with the control group, but these differences disappeared after 25 minutes.

CONCLUSION

There is insufficient evidence to support performing recruitment manoeuvres after suctioning infants and children.

Response of Alveolar Type II Epithelial Cells to Mechanical Stretch and Lipopolysaccharide

BACKGROUND

Although many therapeutic strategies have been developed clinically, the mortality associated with acute respiratory distress syndrome remains very high.

OBJECTIVES

In this research, we used a cytomechanical method to elucidate the reason for this.

METHODS

A549 cells were stimulated with lipopolysaccharide (LPS; 1 or 100 ng/ml) and/or mechanical stretch (5, 15, 30%) in varying frequency (0.2, 0.5, 1 Hz) at indicated time (1, 2, 4 h). Real time PCR and enzyme-linked immunosorbent assay were used to measure mRNA and protein levels of IL-8.

RESULTS

In the presence of mechanical stretch, 100 ng/ml LPS significantly increased IL-8 production after 4 h of 5% stretch (p < 0.05). In the presence of LPS, stretch enhanced LPS-induced IL-8 protein production in a force-, time- and frequency-dependent manner. At both the 1- and 4-hour time points, mechanical stretch and LPS increased IL-8 mRNA levels, respectively, and stretch enhanced LPS-induced IL-8 mRNA levels (p < 0.05).

CONCLUSIONS

Using cytomechanic methods, we found a synergistic effect of LPS and mechanical stretch on IL-8 production. The response of alveolar type II cells to mechanical stretch depends on their different pathologic states and the applied mechanical stretch, which may reversely influence the outcome of patients with acute respiratory distress syndrome.

Mechanical ventilation in acute respiratory failure: recruitment and high positive end-expiratory pressure are necessary

PURPOSE OF REVIEW

To review as best the critical care clinicians can recruit the acute respiratory distress syndrome (ARDS) lungs and keep the lungs opened, assuring homogeneous ventilation, and to present the experimental and clinical results of these mechanical ventilation strategies, along with possible improvements in patient outcome based on selected published medical literature from 1972 to 2004 (highlighting the period from June 2003 to June 2004 and recent results of the authors' group research).

RECENT FINDINGS

In the experimental setting, repeated derecruitments accentuate lung injury during mechanical ventilation, whereas open lung concept strategies can attenuate lung injury. In the clinical setting, recruitment maneuvers improve short-term oxygenation in ARDS patients. A recent prospective clinical trial showed that low versus intermediate positive end-expiratory pressure (PEEP) levels (8 vs 13 cm H2O) associated with low tidal ventilation had the same effect on ARDS patient survival. Nevertheless, both conventional and electrical impedance thoracic tomography studies indicate that stepwise PEEP recruitment maneuvers increase lung volume and the recruitment percentage of lung tissue, and higher levels of PEEP (18-26 cm H2O) are necessary to keep the ARDS lungs opened and assure a more homogeneous low tidal ventilation.

SUMMARY

Stepwise PEEP recruitment maneuvers can open collapsed ARDS lungs. Higher levels of PEEP are necessary to maintain the lungs open and assure homogenous ventilation in ARDS. In the near future, thoracic CT associated with high-performance monitoring of regional ventilation (electrical impedance tomography) may be used at the bedside to determine the optimal mechanical ventilation of ARDS patients.

Preliminary Experience with Airway Pressure Release Ventilation in a Trauma/Surgical Intensive Care Unit

BACKGROUND

Airway pressure-release ventilation (APRV) is a pressure-limited, time-cycled mode of mechanical ventilation. The purpose of this study was to evaluate our initial experience with the use of APRV in acutely injured, ventilated patients.

METHODS

Since March 2003, APRV has been used selectively in adult trauma patients with or at risk for acute lung injury/acute respiratory distress syndrome. Data were obtained before and during the 72 hours after switching to APRV. A retrospective analysis of these data was then performed.

RESULTS

Complete data were available on 46 of 60 patients (77%) for the first 72 hours of APRV. Before APRV, the average Pao2/Fio2 ratio was 243 and the average peak airway pressure was 28 cm H2O. Peak airway pressure decreased 19% (p = 0.001), Pao2/Fio2 improved by 23% (p = 0.017) and release tidal volumes improved by 13% (p = 0.020) over the course of the analysis.

CONCLUSION

APRV significantly improved oxygenation by alveolar recruitment and allowed for a reduction in peak airway pressures. This relatively new modality had favorable results and appears to be an effective alternative for lung recruitment in traumatically injured patients at risk for acute lung injury/acute respiratory distress syndrome.

Excitation patterns in three-dimensional electrical impedance tomography

Electrical impedance tomography (EIT) is a non-invasive technique that aims to reconstruct images of internal electrical properties of a domain, based on electrical measurements on the periphery. Improvements in instrumentation and numerical modeling have led to three-dimensional (3D) imaging. The availability of 3D modeling and imaging raises the question of identifying the best possible excitation patterns that will yield to data, which can be used to produce the best image reconstruction of internal properties. In this work, we describe our 3D finite element model of EIT. Through singular value decomposition as well as examples of reconstructed images, we show that for a homogenous female breast model with four layers of electrodes, a driving pattern where each excitation plane is a sinusoidal pattern out-of-phase with its neighboring plane produces better qualitative images. However, in terms of quantitative imaging an excitation pattern where all electrode layers are in phase produces better results.

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.

Alveolar recruitment in combination with sufficient positive end-expiratory pressure increases oxygenation and lung aeration in patients with severe chest trauma

OBJECTIVE

Investigation of oxygenation and lung aeration during mechanical ventilation according to the open lung concept in patients with acute lung injury or acute respiratory distress syndrome.

DESIGN

Retrospective analysis.

SETTING

Surgical intensive care unit of a university hospital.

PATIENTS

We retrospectively identified 17 patients with acute lung injury/acute respiratory distress syndrome due to pulmonary contusion who had thoracic helical computed tomography scans before and after ventilation with the open lung concept.

INTERVENTIONS

Baseline ventilation consisted of low tidal volumes (< or =6 mL/kg) and positive end-expiratory pressure (PEEP; 5-17 cm H2O). We briefly applied high inspiratory pressures for opening up collapsed alveoli. External PEEP and intrinsic PEEP were combined to keep recruited lung units open. We generated intrinsic PEEP by pressure-cycled high-frequency inverse ratio ventilation (80 min, inspiratory/expiratory ratio 2:1) and maintained our ventilatory strategy for 24 hrs. Then, after reducing total PEEP by decreasing respiratory rate, Pao2/Fio2 ratio was reevaluated. If it remained >300 mm Hg, weaning was started. If not, previous ventilator settings were resumed for another 24 hrs after recruiting the lungs once again.

MEASUREMENTS AND MAIN RESULTS

Physiologic variables and ventilator settings were obtained from routine charts. Data from computed tomography before and after the open lung concept were analyzed for volumetric quantification of lung aeration and collapse. All results are presented as median and range. During baseline ventilation, PEEP was 10 (range, 5-17) cm H2O and after recruitment 21 (range, 18-26) cm H2O. Opening pressures were 65 (range, 50-80) cm H2O. After recruitment, Pao2/Fio2 ratio was higher in all patients. Total lung volume increased from 2915 (range, 1952-4941) to 4247 (range, 2285-6355) mL and normally aerated volume from 1742 (range, 774-2941) to 2971 (range, 1270-5232) mL. Atelectasis decreased significantly from 604 (range, 147-1538) to 106 (range, 0-736) mL. Hyperinflation increased significantly from 5 (range, 0-188) to 62 (range, 1-424) mL, whereas poor aeration did not change substantially from 649 (range, 302-1292) to 757 (range, 350-1613) mL. No hemodynamic problems occurred.

CONCLUSIONS

Lung recruitment increased arterial oxygenation, normally aerated lung volume, and total lung volume while decreasing the amount of collapsed tissue. These results indicate that the open lung concept is a reasonable mode of ventilation for patients with severe chest trauma.