Reabsorption atelectasis in a porcine model of ARDS: regional and temporal effects of airway closure, oxygen, and distending pressure

Prone-position decreases airway closure in a patient with ARDS undergoing venovenous extracorporeal membrane oxygenation

PURPOSE

Airway closure is a interruption of communication between larger and smaller airways. The presence of airway closure during mechanical ventilation may lead to the overestimation of driving pressure (DP), introducing errors in the assessment of respiratory mechanics and in positive end-expiratory pressure (PEEP) setting on the ventilator. Patients with severe acute respiratory distress syndrome (ARDS) may exhibit the airway closure phenomenon, which can be easily diagnosed with a low-flow inflation. Prone positioning is a therapeutic manoeuver proven to reduce mortality in ARDS patients, and has been widely implemented also in patients requiring veno-venous extracorporeal membrane oxygenation (V-V ECMO). To date, the impact of prone positioning on changes in airway closure has not been described.

METHODS

We present an image analysis of the pressure waveform during volume-controlled ventilation and low-flow inflations before and after prone positioning in an ARDS patient on VV ECMO.

RESULTS

A high airway opening pressure level (23 cmH2O) was detected in the supine position during tidal ventilation. Airway closure was confirmed by using a low-flow inflation. Prone positioning significantly attenuated airway closure, with the airway opening pressure decreasing to 13 cmH2O. After re-supination, airway closure was lower as compared with supine position at baseline (17 cmH2O).

CONCLUSION

Prone positioning reduced airway closure in an ARDS patient on VV ECMO support.

Inter-lung asymmetrical airway closure cause insufflation delay between lungs in acute hypoxemic respiratory failure

BACKGROUND

Electrical Impedance Tomography (EIT) can quantify ventilation in the two lungs and be used to measure the airway opening pressure (AOP) of each lung. Asymmetrical AOPs can cause inter-lung insufflation delay.

OBJECTIVES

To assess the relation between AOP asymmetry and inter-lung insufflation delay at different PEEP levels.

METHODS

Patients with acute hypoxemic respiratory failure and airway closure were included. Low-flow pressure-volume curves and EIT signal were recorded during controlled ventilation and for some patients in pressure support ventilation.

RESULTS

23 patients were studied, 22 patients had ARDS, 9 patients had asymmetrical airway closure with an AOP of 10 [6-13] cmH20 in the sicker lung (AOPsicker) vs. 5 [3-9, ] cmH20 in the healthier lung. During a low flow inflation, the inter-lung inflation delay was 0 [0-112]ms vs. 1450 [375-2400]ms in patients without or with asymmetrical AOPs, p < 0.0001. This delay was correlated to the difference of AOP between the 2 lungs, Spearman R2 = 0.800, p < 0.0001. During tidal ventilation, median delay was 0 [0-62] ms vs. 150 [50-355] ms in patients without vs. with asymmetry, p = 0.019. Setting PEEP at the crossing point of a decremental EIT-based PEEP trial decreased the inter-lung insufflation delay. During pressure support insufflation delay could still be measured and was reduced by increasing PEEP from 5 to 10 cmH2O in patient with asymmetrical lung injury.

CONCLUSION

In asymmetrical airway closure, titrating PEEP can minimize inter-lung insufflation delay and can be monitored by EIT. Reducing the delay and reducing ventilation asymmetry is also feasible during pressure support ventilation when low flow inflation curves cannot be performed.

Impact of airway closure and lung collapse on inhaled nitric oxide effect in acute lung injury: an experimental study

BACKGROUND

Efficacy of inhaled therapy such as Nitric Oxide (iNO) during mechanical ventilation may depend on airway patency. We hypothesized that airway closure and lung collapse, countered by positive end-expiratory pressure (PEEP), influence iNO efficacy. This could support the role of an adequate PEEP titration for inhalation therapy. The main aim of this study was to assess the effect of iNO with PEEP set above or below the airway opening pressure (AOP) generated by airway closure, on hemodynamics and gas exchange in swine models of acute respiratory distress syndrome. Fourteen pigs randomly underwent either bilateral or asymmetrical two-hit model of lung injury. Airway closure and lung collapse were measured with electrical impedance tomography as well as ventilation/perfusion ratio (V/Q). After AOP detection, the effect of iNO (10ppm) was studied with PEEP set randomly above or below regional AOP. Respiratory mechanics, hemodynamics, and gas-exchange were recorded.

RESULTS

All pigs presented airway closure (AOP > 0.5cmH2O) after injury. In bilateral injury, iNO was associated with an improved mean pulmonary pressure from 49 ± 8 to 42 ± 7mmHg; (p = 0.003), and ventilation/perfusion matching, caused by a reduction in pixels with low V/Q and shunt from 16%[IQR:13-19] to 9%[IQR:4-12] (p = 0.03) only at PEEP set above AOP. iNO had no effect on hemodynamics or gas exchange for PEEP below AOP (low V/Q 25%[IQR:16-30] to 23%[IQR:14-27]; p = 0.68). In asymmetrical injury, iNO improved pulmonary hemodynamics and ventilation/perfusion matching independently from the PEEP set. iNO was associated with improved oxygenation in all cases.

CONCLUSIONS

In an animal model of bilateral lung injury, PEEP level relative to AOP markedly influences iNO efficacy on pulmonary hemodynamics and ventilation/perfusion match, independently of oxygenation.

Impact of Positive End-Expiratory Pressure and FiO2 on Lung Mechanics and Intrapulmonary Shunt in Mechanically Ventilated Patients with ARDS Due to COVID-19 Pneumonia

Purpose: This study aimed to investigate the effects of inspired oxygen fraction (FiO2) and positive end-expiratory pressure (PEEP) on gas exchange in mechanically ventilated patients with COVID-19. Methods: Two FiO2 (100%, 40%) were tested at 3 decreasing levels of PEEP (15, 10, and 5 cmH2O). At each FiO2 and PEEP, gas exchange, respiratory mechanics, hemodynamics, and the distribution of ventilation and perfusion were assessed with electrical impedance tomography. The impact of FiO2 on the intrapulmonary shunt (delta shunt) was analyzed as the difference between the calculated shunt at FiO2 100% (shunt) and venous admixture at FiO2 40% (venous admixture). Results: Fourteen patients were studied. Decreasing PEEP from 15 to 10 cmH2O did not change shunt (24 [21-28] vs 27 [24-29]%) or venous admixture (18 [15-26] vs 23 [18-34]%) while partial pressure of arterial oxygen (FiO2 100%) was higher at PEEP 15 (262 [198-338] vs 256 [147-315] mmHg; P < .05). Instead when PEEP was decreased from 10 to 5 cmH2O, shunt increased to 36 [30-39]% (P < .05) and venous admixture increased to 33 [30-43]% (P < .05) and partial pressure of arterial oxygen (100%) decreased to 109 [76-177] mmHg (P < .05). At PEEP 15, administration of 100% FiO2 resulted in a shunt greater than venous admixture at 40% FiO2, ((24 [21-28] vs 18 [15-26]%, P = .005), delta shunt 5.5% (2.3-8.8)). Compared to PEEP 10, PEEP of 5 and 15 cmH2O resulted in decreased global and pixel-level compliance. Cardiac output at FiO2 100% resulted higher at PEEP 5 (5.4 [4.4-6.5]) compared to PEEP 10 (4.8 [4.1-5.5], P < .05) and PEEP 15 cmH2O (4.7 [4.5-5.4], P < .05). Conclusion: In this study, PEEP of 15 cmH2O, despite resulting in the highest oxygenation, was associated with overdistension. PEEP of 5 cmH2O was associated with increased shunt and alveolar collapse. Administration of 100% FiO2 was associated with an increase in intrapulmonary shunt in the setting of high PEEP. Trial registration: NCT05132933.

Airway Closure in Patients With Cardiogenic Pulmonary Edema as a Cause of Driving Pressure Overestimation: The "Uncorking Effect"

Airway closure is an underestimated phenomenon reported in hypoxemic respiratory failure under mechanical ventilation, during cardiac arrest, and in patients who are obese. Because airway and alveolar pressure are not communicating, it leads to an overestimation of driving pressure and an underestimation of respiratory system compliance. Airway closure also favors denitrogenation atelectasis. To date, it has been described mainly in patients with ARDS and those with obesity. We describe three cases of airway closure in patients with hydrostatic pulmonary edema caused by cardiogenic shock, highlighting its resolution in a limited period of time (24 h) as pulmonary edema resolved. The waveforms show a biphasic reopening that we refer to as the "uncorking effect". The detection of airway closure may require setting positive end-expiratory pressure at or above the airway opening pressure to avoid the overestimation of driving pressure.

Effect of sigh in lateral position on postoperative atelectasis in adults assessed by lung ultrasound: a randomized, controlled trial

Background

Postoperative atelectasis occurs in 90% of patients receiving general anesthesia. Recruitment maneuvers (RMs) are not always effective and frequently associated with barotrauma and hemodynamic instability. It is reported that many natural physiological behaviors interrupted under general anesthesia could prevent atelectasis and restore lung aeration. This study aimed to find out whether a combined physiological recruitment maneuver (CPRM), sigh in lateral position, could reduce postoperative atelectasis using lung ultrasound (LUS).

Methods

We conducted a prospective, randomized, controlled trial in adults with open abdominal surgery under general anesthesia lasting for 2 h or longer. Subjects were randomly allocated to either control group (C-group) or CPRM-group and received volume-controlled ventilation with the same ventilator settings. Patients in CPRM group was ventilated in sequential lateral position, with the addition of periodic sighs to recruit the lung. LUS scores, dynamic compliance (Cdyn), the partial pressure of arterial oxygen (PaO₂) and fraction of inspired oxygen (FiO₂) ratio (PaO₂/FiO₂), and other explanatory variables were acquired from each patient before and after recruitment.

Results

Seventy patients were included in the analysis. Before recruitment, there was no significant difference in LUS scores, Cdyn and PaO₂/FiO₂ between CPRM-group and C-group. After recruitment, LUS scores in CPRM-group decreased significantly compared with C-group (6.00 [5.00, 7.00] vs. 8.00 [7.00, 9.00], p = 4.463e-11 < 0.05), while PaO₂/FiO₂ and Cdyn in CPRM-group increased significantly compared with C-group respectively (377.92 (93.73) vs. 309.19 (92.98), p = 0.008 < 0.05, and 52.00 [47.00, 60.00] vs. 47.70 [41.00, 59.50], p = 6.325e-07 < 0.05). No hemodynamic instability, detectable barotrauma or position-related complications were encountered.

Conclusions

Sigh in lateral position can effectively reduce postoperative atelectasis even without causing severe side effects. Further large-scale studies are necessary to evaluate it’s long-term effects on pulmonary complications and hospital length of stay.

Trial registration

ChiCTR1900024379. Registered 8 July 2019, 

Supplementary Information

The online version contains supplementary material available at 10.1186/s12871-022-01748-9.

Perioperative Pulmonary Atelectasis: Part I. Biology and Mechanisms

Pulmonary atelectasis is common in the perioperative period. Physiologically, it is produced when collapsing forces derived from positive pleural pressure and surface tension overcome expanding forces from alveolar pressure and parenchymal tethering. Atelectasis impairs blood oxygenation and reduces lung compliance. It is increasingly recognized that it can also induce local tissue biologic responses, such as inflammation, local immune dysfunction, and damage of the alveolar-capillary barrier, with potential loss of lung fluid clearance, increased lung protein permeability, and susceptibility to infection, factors that can initiate or exaggerate lung injury. Mechanical ventilation of a heterogeneously aerated lung (e.g., in the presence of atelectatic lung tissue) involves biomechanical processes that may precipitate further lung damage: concentration of mechanical forces, propagation of gas-liquid interfaces, and remote overdistension. Knowledge of such pathophysiologic mechanisms of atelectasis and their consequences in the healthy and diseased lung should guide optimal clinical management.

Bedside monitoring of lung volume available for gas exchange

BACKGROUND

Bedside measurement of lung volume may provide guidance in the personalised setting of respiratory support, especially in patients with the acute respiratory distress syndrome at risk of ventilator-induced lung injury. We propose here a novel operator-independent technique, enabled by a fibre optic oxygen sensor, to quantify the lung volume available for gas exchange. We hypothesised that the continuous measurement of arterial partial pressure of oxygen (PaO2) decline during a breath-holding manoeuvre could be used to estimate lung volume in a single-compartment physiological model of the respiratory system.

METHODS

Thirteen pigs with a saline lavage lung injury model and six control pigs were studied under general anaesthesia during mechanical ventilation. Lung volumes were measured by simultaneous PaO2 rate of decline (VPaO2) and whole-lung computed tomography scan (VCT) during apnoea at different positive end-expiratory and end-inspiratory pressures.

RESULTS

A total of 146 volume measurements was completed (range 134 to 1869 mL). A linear correlation between VCT and VPaO2 was found both in control (slope = 0.9, R2 = 0.88) and in saline-lavaged pigs (slope = 0.64, R2 = 0.70). The bias from Bland-Altman analysis for the agreement between the VCT and VPaO2 was - 84 mL (limits of agreement ± 301 mL) in control and + 2 mL (LoA ± 406 mL) in saline-lavaged pigs. The concordance for changes in lung volume, quantified with polar plot analysis, was - 4º (LoA ± 19°) in control and - 9° (LoA ± 33°) in saline-lavaged pigs.

CONCLUSION

Bedside measurement of PaO2 rate of decline during apnoea is a potential approach for estimation of lung volume changes associated with different levels of airway pressure.

A physiological approach to understand the role of respiratory effort in the progression of lung injury in SARS-CoV-2 infection

Deterioration of lung function during the first week of COVID-19 has been observed when patients remain with insufficient respiratory support. Patient self-inflicted lung injury (P-SILI) is theorized as the responsible, but there is not robust experimental and clinical data to support it. Given the limited understanding of P-SILI, we describe the physiological basis of P-SILI and we show experimental data to comprehend the role of regional strain and heterogeneity in lung injury due to increased work of breathing.

In addition, we discuss the current approach to respiratory support for COVID-19 under this point of view.

Changes in shunt, ventilation/perfusion mismatch, and lung aeration with PEEP in patients with ARDS: a prospective single-arm interventional study

Background

Several studies have found only a weak to moderate correlation between oxygenation and lung aeration in response to changes in PEEP. This study aimed to investigate the association between changes in shunt, low and high ventilation/perfusion (V/Q) mismatch, and computed tomography-measured lung aeration following an increase in PEEP in patients with ARDS.

Methods

In this preliminary study, 12 ARDS patients were subjected to recruitment maneuvers followed by setting PEEP at 5 and then either 15 or 20 cmH₂O. Lung aeration was measured by computed tomography. Values of pulmonary shunt and low and high V/Q mismatch were calculated by a model-based method from measurements of oxygenation, ventilation, and metabolism taken at different inspired oxygen levels and an arterial blood gas sample.

Results

Increasing PEEP resulted in reduced values of pulmonary shunt and the percentage of non-aerated tissue, and an increased percentage of normally aerated tissue (p < 0.05). Changes in shunt and normally aerated tissue were significantly correlated (r = − 0.665, p = 0.018). Three distinct responses to increase in PEEP were observed in values of shunt and V/Q mismatch: a beneficial response in seven patients, where shunt decreased without increasing high V/Q; a detrimental response in four patients where both shunt and high V/Q increased; and a detrimental response in a patient with reduced shunt but increased high V/Q mismatch. Non-aerated tissue decreased with increased PEEP in all patients, and hyperinflated tissue increased only in patients with a detrimental response in shunt and V/Q mismatch.

Conclusions

The results show that improved lung aeration following an increase in PEEP is not always consistent with reduced shunt and V/Q mismatch. Poorly matched redistribution of ventilation and perfusion, between dependent and non-dependent regions of the lung, may explain why patients showed detrimental changes in shunt and V/Q mismatch on increase in PEEP, despite improved aeration.

Trial registration

ClinicalTrails.gov, NCT04067154. Retrospectively registered on August 26, 2019.

Microphysiological systems modeling acute respiratory distress syndrome that capture mechanical force-induced injury-inflammation-repair

Complex in vitro models of the tissue microenvironment, termed microphysiological systems, have enormous potential to transform the process of discovering drugs and disease mechanisms. Such a paradigm shift is urgently needed in acute respiratory distress syndrome (ARDS), an acute lung condition with no successful therapies and a 40% mortality rate. Here, we consider how microphysiological systems could improve understanding of biological mechanisms driving ARDS and ultimately improve the success of therapies in clinical trials. We first discuss how microphysiological systems could explain the biological mechanisms underlying the segregation of ARDS patients into two clinically distinct phenotypes. Then, we contend that ARDS-mimetic microphysiological systems should recapitulate three critical aspects of the distal airway microenvironment, namely, mechanical force, inflammation, and fibrosis, and we review models that incorporate each of these aspects. Finally, we recognize the substantial challenges associated with combining inflammation, fibrosis, and/or mechanical force in microphysiological systems. Nevertheless, complex in vitro models are a novel paradigm for studying ARDS, and they could ultimately improve patient care.

EIT monitors valid and robust regional ventilation distribution in pathologic ventilation states in porcine study using differential DualEnergy-CT (ΔDECT)

It is crucial to precisely monitor ventilation and correctly diagnose ventilation-related pathological states for averting lung collapse and lung failure in Intensive Care Unit (ICU) patients. Although Electrical Impedance Tomography (EIT) may deliver this information continuously and non-invasively at bedside, to date there are no studies that systematically compare EIT and Dual Energy CT (DECT) during inspiration and expiration (ΔDECT) regarding varying physiological and ICU-typical pathological conditions such as atelectasis. This study aims to prove the accuracy of EIT through quantitative identification and monitoring of pathological ventilation conditions on a four-quadrant basis using ΔDECT. In a cohort of 13 pigs, this study investigated systematic changes in tidal volume (TV) and positive end-expiratory pressure (PEEP) under physiological ventilation conditions. Pathological ventilation conditions were established experimentally by single-lung ventilation and pulmonary saline lavage. Spirometric data were compared to voxel-based entire lung ΔDECT, and EIT intensities were compared to ΔDECT of a 12-cm slab of the lung around the EIT belt, the so called ΔDECT_Belt. To validate ΔDECT data with spirometry, a Pearson’s correlation coefficient of 0.92 was found for 234 ventilation conditions. Comparing EIT intensity with ΔDECT_(Belt), the correlation r = 0.84 was found. Normalized cross-correlation function (NCCF) between scaled global impedance (EIT) waveforms and global volume ventilator curves was r = 0.99 ± 0.003. The EIT technique correctly identified the ventilated lung in all cases of single-lung ventilation. In the four-quadrant based evaluation, which assesses the difference between end-expiratory lung volume (ΔEELV) and the corresponding parameter in EIT, i.e. the end-expiratory lung impedance (ΔEELI), the Pearson’s correlation coefficient of 0.94 was found. The respective Pearson’s correlation coefficients implies good to excellent concurrence between global and regional EIT ventilation data validated by ventilator spirometry and DECT imaging. By providing real-time images of the lung, EIT is a promising, EIT is a promising, clinically robust tool for bedside assessment of regional ventilation distribution and changes of end-expiratory lung volume.

A novel concept for evaluation of pulmonary function utilizing PaO2/FiO2 difference at the distinctive FiO2 in cellular ex vivo lung perfusion-an experimental study

For more accurate lung evaluation in ex vivo lung perfusion (EVLP), we have devised a new parameter, PaO₂ /FiO₂ ratio difference (PFD); PFD_1-0.4  = P/F ratio at FiO₂ 1.0 - P/F ratio at FiO₂ 0.4. The aim of this study is to compare PFD and transplant suitability, and physiological parameters utilized in cellular EVLP. Thirty-nine human donor lungs were perfused. At 2 h of EVLP, PFD_1-0.4 was compared with transplant suitability and physiological parameters. In a second study, 10 pig lungs were perfused in same fashion. PFD_1-0.4 was calculated by blood from upper and lower lobe pulmonary veins and compared with lobe wet/dry ratio and pathological findings. In human model, receiver operating characteristic curve analysis showed PFD_1-0.4 had the highest area under curve, 0.90, sensitivity, 0.96, to detect nonsuitable lungs, and significant negative correlation with lung weight ratio (R²  = 0.26, P < 0.001). In pig model, PFD_1-0.4 on lower and upper lobe pulmonary veins were significantly associated with corresponding lobe wet/dry ratios (R²  = 0.51, P = 0.019; R²  = 0.37, P = 0.060), respectively. PFD_1-0.4 in EVLP demonstrated a significant correlation with lung weight ratio and allowed more precise assessment of individual lobes in detecting lung edema. Moreover, it might support decision-making in evaluation with current EVLP criteria.

Acute Lobar Atelectasis

Lobar atelectasis (or collapse) is an exceedingly common, rather predictable, and potentially pathogenic companion to many forms of acute illness, postoperative care, and chronic debility. Readily diagnosed by using routine chest imaging and bedside ultrasound, the consequences from lobar collapse may be minor or serious, depending on extent, mechanism, patient vulnerability, abruptness of onset, effectiveness of hypoxic vasoconstriction, and compensatory reserves. Measures taken to reduce secretion burden, assure adequate secretion clearance, maintain upright positioning, reverse lung compression, and sustain lung expansion accord with a logical physiologic rationale. Both classification and logical approaches to prophylaxis and treatment of lobar atelectasis derive from a sound mechanistic knowledge of its causation.

Quantitative CT assessment of lung injury after successful cardiopulmonary resuscitation in a porcine cardiac arrest model of different downtimes

Background

Utilize quantitative computed tomography (QCT) to detect and evaluate the severity of lung injury after successful cardiopulmonary resuscitation (CPR) in a porcine cardiac arrest (CA) model with different downtimes.

Methods

Twenty-one male domestic pigs weighing 38±3 kg were randomized into 3 groups: the sham group (n=5), the ventricular fibrillation (VF) 5 min (VF5) group (n=8), and the VF 10 min (VF10) group (n=8). VF was induced and untreated for 5 (VF5 group) or 10 (VF10 group) min before the commencement of manual CPR. Eight animals (8/8, 100%) in VF5 and 6 (6/8, 75%) in VF10 were successfully resuscitated. Chest QCT scans and arterial blood gas tests were performed at baseline and 6 h post-resuscitation. The QCT score, volume, and weight of ground-glass opacification (GGO), which was defined as poorly aerated regions with a CT value ranging from -500 Hounsfield units (HU) to -100 HU, and intense parenchymal opacification (IPO), which was defined as a non-aerated area with a CT value greater than -100 HU, were quantitatively measured.

Results

Significantly shorter durations of CPR and fewer defibrillations were observed in the VF5 group compared with the VF10 group [duration of CPR: VF5 (6±0 minutes) versus VF10 (8.3±1.5 minutes), P<0.05; numbers of defibrillation: VF5 (1±0) versus VF10 (2.2±0.8), P<0.05]. Compared with the baseline or sham animals, declining gas exchanges (end-tidal CO₂, PO₂, oxygen index) were observed in both VF groups; however, there were no significant differences in gas exchanges between the VF groups. Compared with the VF5 group, the GGO QCT score, volume, and weight were significantly greater in the VF10 group (P=0.002, 0.001, and 0.002 respectively), while no significant differences were found in the IPO QCT score, volume, or weight between two the VF groups (P=0.354, 0.447, and 0.512 respectively).

Conclusions

QCT analysis enables unique non-invasive assessments of different lung injuries (IPO and GGO lesions) that can clearly distinguish heterogeneous lesions and allow for early detection and quantitative monitoring of the severity of lung injury following CPR. QCT could provide a basis for clinical early ventilation strategy management after CPR.

Looking closer at acute respiratory distress syndrome: the role of advanced imaging techniques

PURPOSE OF REVIEW

Advanced imaging techniques have provided invaluable insights in understanding of acute respiratory distress syndrome (ARDS) and the effect of therapeutic strategies, thanks to the possibility of gaining regional information and moving from simple 'anatomical' information to in-vivo functional imaging.

RECENT FINDINGS

Computed tomography (CT) led to the understanding of several ARDS mechanisms and interaction with mechanical ventilation. It is nowadays frequently part of routine diagnostic workup, often leading to treatment changes. Moreover, CT is a reference for novel techniques both in clinical and preclinical studies. Bedside transthoracic lung ultrasound allows semiquantitative regional analysis of lung aeration, identifies ARDS lung morphology and response to therapeutic maneuvers. Electrical impedance tomography is a radiation-free, functional, bedside, imaging modality which allows a real-time monitoring of regional ventilation. Finally, positron emission tomography (PET) is a functional imaging technique that allows to trace physiologic processes, by administration of a radioactive molecule. PET with FDG has been applied to patients with ARDS, thanks to its ability to track the inflammatory cells activity.

SUMMARY

Progresses in lung imaging are key to individualize therapy, diagnosis, and pathophysiological mechanism at play in any patient at any specified time, helping to move toward personalized medicine for ARDS.

High-frequency oscillatory ventilation guided by transpulmonary pressure in acute respiratory syndrome: an experimental study in pigs

BACKGROUND

Recent clinical studies have not shown an overall benefit of high-frequency oscillatory ventilation (HFOV), possibly due to injurious or non-individualized HFOV settings. We compared conventional HFOV (HFOV_con) settings with HFOV settings based on mean transpulmonary pressures (P_Lmean) in an animal model of experimental acute respiratory distress syndrome (ARDS).

METHODS

ARDS was induced in eight pigs by intrabronchial installation of hydrochloric acid (0.1 N, pH 1.1; 2.5 ml/kg body weight). The animals were initially ventilated in volume-controlled mode with low tidal volumes (6 ml kg^- 1) at three positive end-expiratory pressure (PEEP) levels (5, 10, 20 cmH₂O) followed by HFOV_con and then HFOV P_Lmean each at PEEP 10 and 20. The continuous distending pressure (CDP) during HFOV_con was set at mean airway pressure plus 5 cmH₂O. For HFOV P_Lmean it was set at mean P_L plus 5 cmH₂O. Baseline measurements were obtained before and after induction of ARDS under volume controlled ventilation with PEEP 5. The same measurements and computer tomography of the thorax were then performed under all ventilatory regimens at PEEP 10 and 20.

RESULTS

Cardiac output, stroke volume, mean arterial pressure and intrathoracic blood volume index were significantly higher during HFOV P_Lmean than during HFOV_con at PEEP 20. Lung density, total lung volume, and normally and poorly aerated lung areas were significantly greater during HFOV_con, while there was less over-aerated lung tissue in HFOV P_Lmean. The groups did not differ in oxygenation or extravascular lung water index.

CONCLUSION

HFOV P_Lmean is associated with less hemodynamic compromise and less pulmonary overdistension than HFOV_con. Despite the increase in non-ventilated lung areas, oxygenation improved with both regimens. An individualized approach with HFOV settings based on transpulmonary pressure could be a useful ventilatory strategy in patients with ARDS. Providing alveolar stabilization with HFOV while avoiding harmful distending pressures and pulmonary overdistension might be a key in the context of ventilator-induced lung injury.

Expiratory Flow Limitation During Mechanical Ventilation

Expiratory flow limitation (EFL) is present when the flow cannot rise despite an increase in the expiratory driving pressure. The mechanisms of EFL are debated but are believed to be related to the collapsibility of small airways. In patients who are mechanically ventilated, EFL can exist during tidal ventilation, representing an extreme situation in which lung volume cannot decrease, regardless of the expiratory driving forces. It is a key factor for the generation of auto- or intrinsic positive end-expiratory pressure (PEEP) and requires specific management such as positioning and adjustment of external PEEP. EFL can be responsible for causing dyspnea and patient-ventilator dyssynchrony, and it is influenced by the fluid status of the patient. EFL frequently affects patients with COPD, obesity, and heart failure, as well as patients with ARDS, especially at low PEEP. EFL is, however, most often unrecognized in the clinical setting despite being associated with complications of mechanical ventilation and poor outcomes such as postoperative pulmonary complications, extubation failure, and possibly airway injury in ARDS. Therefore, prompt recognition might help the management of patients being mechanically ventilated who have EFL and could potentially influence outcome. EFL can be suspected by using different means, and this review summarizes the methods to specifically detect EFL during mechanical ventilation.

Effect of Bronchoconstriction-induced Ventilation-Perfusion Mismatch on Uptake and Elimination of Isoflurane and Desflurane

BACKGROUND

Increasing numbers of patients with obstructive lung diseases need anesthesia for surgery. These conditions are associated with pulmonary ventilation/perfusion (VA/Q) mismatch affecting kinetics of volatile anesthetics. Pure shunt might delay uptake of less soluble anesthetic agents but other forms of VA/Q scatter have not yet been examined. Volatile anesthetics with higher blood solubility would be less affected by VA/Q mismatch. We therefore compared uptake and elimination of higher soluble isoflurane and less soluble desflurane in a piglet model.

METHODS

Juvenile piglets (26.7 ± 1.5 kg) received either isoflurane (n = 7) or desflurane (n = 7). Arterial and mixed venous blood samples were obtained during wash-in and wash-out of volatile anesthetics before and during bronchoconstriction by methacholine inhalation (100 μg/ml). Total uptake and elimination were calculated based on partial pressure measurements by micropore membrane inlet mass spectrometry and literature-derived partition coefficients and assumed end-expired to arterial gradients to be negligible. VA/Q distribution was assessed by the multiple inert gas elimination technique.

RESULTS

Before methacholine inhalation, isoflurane arterial partial pressures reached 90% of final plateau within 16 min and decreased to 10% after 28 min. By methacholine nebulization, arterial uptake and elimination delayed to 35 and 44 min. Desflurane needed 4 min during wash-in and 6 min during wash-out, but with bronchoconstriction 90% of both uptake and elimination was reached within 15 min.

CONCLUSIONS

Inhaled methacholine induced bronchoconstriction and inhomogeneous VA/Q distribution. Solubility of inhalational anesthetics significantly influenced pharmacokinetics: higher soluble isoflurane is less affected than fairly insoluble desflurane, indicating different uptake and elimination during bronchoconstriction.

Bedside selection of positive end-expiratory pressure by electrical impedance tomography in hypoxemic patients: a feasibility study

Background

Positive end-expiratory pressure (PEEP) is a key element of mechanical ventilation. It should optimize recruitment, without causing excessive overdistension, but controversy exists on the best method to set it. The purpose of the study was to test the feasibility of setting PEEP with electrical impedance tomography in order to prevent lung de-recruitment following a recruitment maneuver. We enrolled 16 patients undergoing mechanical ventilation with PaO₂/FiO₂ <300 mmHg. In all patients, under constant tidal volume (6–8 ml/kg) PEEP was set based on the PEEP/FiO₂ table proposed by the ARDS network (PEEP_ARDSnet). We performed a recruitment maneuver and monitored the end-expiratory lung impedance (EELI) over 10 min. If the EELI signal decreased during this period, the recruitment maneuver was repeated and PEEP increased by 2 cmH₂O. This procedure was repeated until the EELI maintained a stability over time (PEEP_EIT).

Results

The procedure was feasible in 87% patients. PEEP_EIT was higher than PEEP_ARDSnet (13 ± 3 vs. 9 ± 2 cmH₂O, p < 0.001). PaO₂/FiO₂ improved during PEEP_EIT and driving pressure decreased. Recruited volume correlated with the decrease in driving pressure but not with oxygenation improvement. Finally, regional alveolar hyperdistention and collapse was reduced in dependent lung layers and increased in non-dependent lung layers.

Conclusions

In hypoxemic patients, a PEEP selection strategy aimed at stabilizing alveolar recruitment guided by EIT at the bedside was feasible and safe. This strategy led, in comparison with the ARDSnet table, to higher PEEP, improved oxygenation and reduced driving pressure, allowing to estimate the relative weight of overdistension and recruitment.

Electronic supplementary material

The online version of this article (doi:10.1186/s13613-017-0299-9) contains supplementary material, which is available to authorized users.

Quadratic function between arterial partial oxygen pressure and mortality risk in sepsis patients: an interaction with simplified acute physiology score

Oxygen therapy is widely used in emergency and critical care settings, while there is little evidence on its real therapeutic effect. The study aimed to explore the impact of arterial oxygen partial pressure (PaO₂) on clinical outcomes in patients with sepsis. A large clinical database was employed for the study. Subjects meeting the diagnostic criteria of sepsis were eligible for the study. All measurements of PaO₂ were extracted. The primary endpoint was death from any causes during hospital stay. Survey data analysis was performed by using individual ICU admission as the primary sampling unit. Quadratic function was assumed for PaO₂ and its interaction with other covariates were explored. A total of 199,125 PaO₂ samples were identified for 11,002 ICU admissions. Each ICU stay comprised 18 PaO₂ samples in average. The fitted multivariable model supported our hypothesis that the effect of PaO₂ on mortality risk was in quadratic form. There was significant interaction between PaO₂ and SAPS-I (p = 0.007). Furthermore, the main effect of PaO₂ on SOFA score was nonlinear. The study shows that the effect of PaO₂ on mortality risk is in quadratic function form, and there is significant interaction between PaO₂ and severity of illness.

Zero expiratory pressure and low oxygen concentration promote heterogeneity of regional ventilation and lung densities

Background

It is not well known what is the main mechanism causing lung heterogeneity in healthy lungs under mechanical ventilation. We aimed to investigate the mechanisms causing heterogeneity of regional ventilation and parenchymal densities in healthy lungs under anesthesia and mechanical ventilation.

Methods

In a small animal model, synchrotron imaging was used to measure lung aeration and regional‐specific ventilation (sV̇). Heterogeneity of ventilation was calculated as the coefficient of variation in sV̇ (CV_sV̇). The coefficient of variation in lung densities (CV_D) was calculated for all lung tissue, and within hyperinflated, normally and poorly aerated areas. Three conditions were studied: zero end‐expiratory pressure (ZEEP) and F_IO₂ 0.21; ZEEP and F_IO₂ 1.0; PEEP 12 cmH₂O and F_IO₂1.0 (Open Lung‐PEEP = OLP).

Results

The mean tissue density at OLP was lower than ZEEP‐1.0 and ZEEP‐0.21. There were larger subregions with low sV̇ and poor aeration at ZEEP‐0.21 than at OLP: 12.9 ± 9.0 vs. 0.6 ± 0.4% in the non‐dependent level, and 17.5 ± 8.2 vs. 0.4 ± 0.1% in the dependent one (P = 0.041). The CV_sV̇ of the total imaged lung at PEEP 12 cmH₂O was significantly lower than on ZEEP, regardless of F_IO₂, indicating more heterogeneity of ventilation during ZEEP (0.23 ± 0.03 vs. 0.54 ± 0.37, P = 0.049). CV_D changed over the different mechanical ventilation settings (P = 0.011); predominantly, CV_D increased during ZEEP. The spatial distribution of the CV_D calculated for the poorly aerated density category changed with the mechanical ventilation settings, increasing in the dependent level during ZEEP.

Conclusion

ZEEP together with low F_IO₂ promoted heterogeneity of ventilation and lung tissue densities, fostering a greater amount of airway closure and ventilation inhomogeneities in poorly aerated regions.

There is no cephalocaudal gradient of computed tomography densities or lung behavior in supine patients with acute respiratory distress syndrome

BACKGROUND

There is debate whether pressure transmission within the lungs and alveolar collapse follow a hydrostatic pattern or the compression exerted by the weight of the heart and the diaphragm causes collapse localized in the areas adjacent to these structures. The second hypothesis proposes the existence of a cephalocaudal gradient in alveolar collapse. We aimed to define whether or not lung density and collapse follow a 'liquid-like' pattern with homogeneous isogravitational layers along the cephalocaudal axis in acute respiratory distress syndrome lungs.

METHODS

Acute respiratory distress syndrome patients were submitted to full lung computed tomography scans at positive end-expiratory pressure (PEEP) zero (before) and 25 cmH2 O after a maximum-recruitment maneuver. PEEP was then decreased by 2 cmH2 O every 4 min, and a semi-complete scan performed at the end of each PEEP step.

RESULTS

Lung densities were homogeneous within each lung layer. Lung density increased along the ventrodorsal axis toward the dorsal region (β = 0.49, P < 0.001), while there was no increase, but rather a slight decrease, toward the diaphragm along the cephalocaudal axis and toward the heart. Higher PEEP attenuated density gradients. At PEEP 18 cmH2 O, dependent lung regions started to collapse massively, while best compliance was only reached at a lower PEEP.

CONCLUSIONS

We could not detect cephalocaudal gradients in lung densities or in alveolar collapse. Likely, external pressures applied on the lung by the chest wall, organs, and effusions are transmitted throughout the lung in a hydrostatic pattern with homogeneous consequences at each isogravitational layer. A single cross-sectional image of the lung could fully represent the heterogeneous mechanical properties of dependent and non-dependent lung regions.

Effects of superimposed tissue weight on regional compliance of injured lungs

Computed tomography (CT), together with image analysis technologies, enable the construction of regional volume (VREG) and local transpulmonary pressure (PTP,REG) maps of the lung. Purpose of this study is to assess the distribution of VREG vs PTP,REG along the gravitational axis in healthy (HL) and experimental acute lung injury conditions (eALI) at various positive end-expiratory pressures (PEEPs) and inflation volumes. Mechanically ventilated pigs underwent inspiratory hold maneuvers at increasing volumes simultaneously with lung CT scans. eALI was induced via the iv administration of oleic acid. We computed voxel-level VREG vs PTP,REG curves into eleven isogravitational planes by applying polynomial regressions. Via F-test, we determined that VREG vs PTP,REG curves derived from different anatomical planes (p-values<1.4E-3), exposed to different PEEPs (p-values<1.5E-5) or subtending different lung status (p-values<3E-3) were statistically different (except for two cases of adjacent planes). Lung parenchyma exhibits different elastic behaviors based on its position and the density of superimposed tissue which can increase during lung injury.

Mild loss of lung aeration augments stretch in healthy lung regions

Inspiratory stretch by mechanical ventilation worsens lung injury. However, it is not clear whether and how the ventilator damages lungs in the absence of preexisting injury. We hypothesized that subtle loss of lung aeration during general anesthesia regionally augments ventilation and distension of ventilated air spaces. In eight supine anesthetized and intubated rats, hyperpolarized gas MRI was performed after a recruitment maneuver following 1 h of volume-controlled ventilation with zero positive end-expiratory pressure (ZEEP), FiO2 0.5, and tidal volume 10 ml/kg, and after a second recruitment maneuver. Regional fractional ventilation (FV), apparent diffusion coefficient (ADC) of (3)He (a measurement of ventilated peripheral air space dimensions), and gas volume were measured in lung quadrants of ventral and dorsal regions of the lungs. In six additional rats, computed tomography (CT) images were obtained at each time point. Ventilation with ZEEP decreased total lung gas volume and increased both FV and ADC in all studied regions. Increases in FV were more evident in the dorsal slices. In each lung quadrant, higher ADC was predicted by lower gas volume and by increased mean values (and heterogeneity) of FV distribution. CT scans documented 10% loss of whole-lung aeration and increased density in the dorsal lung, but no macroscopic atelectasis. Loss of pulmonary gas at ZEEP increased fractional ventilation and inspiratory dimensions of ventilated peripheral air spaces. Such regional changes could help explain a propensity for mechanical ventilation to contribute to lung injury in previously uninjured lungs.

Intra-breath arterial oxygen oscillations detected by a fast oxygen sensor in an animal model of acute respiratory distress syndrome

Background

There is considerable interest in oxygen partial pressure (Po₂) monitoring in physiology, and in tracking Po₂ changes dynamically when it varies rapidly. For example, arterial Po₂ (PaO2) can vary within the respiratory cycle in cyclical atelectasis (CA), where PaO2 is thought to increase and decrease during inspiration and expiration, respectively. A sensor that detects these PaO2 oscillations could become a useful diagnostic tool of CA during acute respiratory distress syndrome (ARDS).

Methods

We developed a fibreoptic Po₂ sensor (<200 µm diameter), suitable for human use, that has a fast response time, and can measure Po₂ continuously in blood. By altering the inspired fraction of oxygen (FIO2) from 21 to 100% in four healthy animal models, we determined the linearity of the sensor's signal over a wide range of PaO2 values in vivo. We also hypothesized that the sensor could measure rapid intra-breath PaO2 oscillations in a large animal model of ARDS.

Results

In the healthy animal models, PaO2 responses to changes in FIO2 were in agreement with conventional intermittent blood-gas analysis (n=39) for a wide range of PaO2 values, from 10 to 73 kPa. In the animal lavage model of CA, the sensor detected PaO2 oscillations, also at clinically relevant PaO2 levels close to 9 kPa.

Conclusions

We conclude that these fibreoptic PaO2 sensors have the potential to become a diagnostic tool for CA in ARDS.

First-time imaging of effects of inspired oxygen concentration on regional lung volumes and breathing pattern during hypergravity

PURPOSE

Aeroatelectasis can develop in aircrew flying the latest generation high-performance aircraft. Causes alleged are relative hyperoxia, increased gravity in the head-to-foot direction (+Gz), and compression of legs and stomach by anti-G trousers (AGT). We aimed to assess, in real time, the effects of hyperoxia, +Gz accelerations and AGT inflation on changes in regional lung volumes and breathing pattern evaluated in an axial plane by electrical impedance tomography (EIT).

METHODS

The protocol mimicked a routine peacetime flight in combat aircraft. Eight subjects wearing AGT were studied in a human centrifuge during 1 h 15 min exposure of +1 to +3.5Gz. They performed this sequence three times, breathing AIR, 44.5 % O2 or 100 % O2. Continuous recording of functional EIT enabled uninterrupted assessment of regional lung volumes at the 5th intercostal level. Breathing pattern was also monitored.

RESULTS

EIT data showed that +3.5Gz, compared with any moment without hypergravity, caused an abrupt decrease in regional tidal volume (VT) and regional end-expiratory lung volume (EELV) measured in the EIT slice, independently of inspired oxygen concentration. Breathing AIR or 44.5 % O2, sub-regional EELV measured in the EIT slice decreased similarly in dorsal and ventral regions, but sub-regional VT measured in the EIT slice decreased significantly more dorsally than ventrally. Breathing 100 % O2, EELV and VT decreased similarly in both regions. Inspired tidal volume increased in hyperoxia, whereas breathing frequency increased in hypergravity and hyperoxia.

CONCLUSIONS

Our findings suggest that hypergravity and AGT inflation cause airway closure and air trapping in gravity-dependent lung regions, facilitating absorption atelectasis formation, in particular during hyperoxia.