Use of dynamic CT in acute respiratory distress syndrome (ARDS) with comparison of positive and negative pressure ventilation

A novel tree shrew model of lipopolysaccharide-induced acute respiratory distress syndrome

INTRODUCTION

Acute respiratory distress syndrome (ARDS) is a leading cause of respiratory failure, with substantial attributable morbidity and mortality. The small animal models that are currently used for ARDS do not fully manifest all of the pathological hallmarks of human patients, which hampers both the studies of disease mechanism and drug development.

OBJECTIVES

To examine whether the phenotypic changes of primate-like tree shrews in response to a one-hit lipopolysaccharides (LPS) injury resemble human ARDS features.

METHODS

LPS was administered to tree shrews through intratracheal instillation; then, the animals underwent CT or PET/CT imaging to examine the changes in the structure and function of the whole lung. The lung histology was analyzed by H&E staining and immunohistochemical staining of inflammatory cells.

RESULTS

Results demonstrated that tree shrews exhibited an average survival time of 3-5 days after LPS insult, as well as an obvious symptom of dyspnea before death. The ratios of PaO2 to FiO2 (P/F ratio) were close to those of moderate ARDS in humans. CT imaging showed that the scope of the lung injury in tree shrews after LPS treatment were extensive. PET/CT imaging with 18F-FDG displayed an obvious inflammatory infiltration. Histological analysis detected the formation of a hyaline membrane, which is usually present in human ARDS.

CONCLUSION

This study established a lung injury model with a primate-like small animal model and confirmed that they have similar features to human ARDS, which might provide a valuable tool for translational research.

A Comparative Study of Ex-Vivo Murine Pulmonary Mechanics Under Positive- and Negative-Pressure Ventilation

Increased ventilator use during the COVID-19 pandemic resurrected persistent questions regarding mechanical ventilation including the difference between physiological and artificial breathing induced by ventilators (i.e., positive- versus negative-pressure ventilation, PPV vs NPV). To address this controversy, we compare murine specimens subjected to PPV and NPV in ex vivo quasi-static loading and quantify pulmonary mechanics via measures of quasi-static and dynamic compliances, transpulmonary pressure, and energetics when varying inflation frequency and volume. Each investigated mechanical parameter yields instance(s) of significant variability between ventilation modes. Most notably, inflation compliance, percent relaxation, and peak pressure are found to be consistently dependent on the ventilation mode. Maximum inflation volume and frequency note varied dependencies contingent on the ventilation mode. Contradictory to limited previous clinical investigations of oxygenation and end-inspiratory measures, the mechanics-focused comprehensive findings presented here indicate lung properties are dependent on loading mode, and importantly, these dependencies differ between smaller versus larger mammalian species despite identical custom-designed PPV/NPV ventilator usage. Results indicate that past contradictory findings regarding ventilation mode comparisons in the field may be linked to the chosen animal model. Understanding the differing fundamental mechanics between PPV and NPV may provide insights for improving ventilation strategies and design to prevent associated lung injuries.

Positive- and Negative-Pressure Ventilation Characterized by Local and Global Pulmonary Mechanics

Rationale: There is continued debate regarding the equivalency of positive-pressure ventilation (PPV) and negative-pressure ventilation (NPV). Resolving this question is important because of the different practical ramifications of the two paradigms. Objectives: We sought to investigate the parallel between PPV and NPV and determine whether or not these two paradigms cause identical ventilation profiles by analyzing the local strain mechanics when the global tidal volume (Vt) and inflation pressure was matched. Methods: A custom-designed electromechanical apparatus was used to impose equal global loads and displacements on the same ex vivo healthy porcine lung using PPV and NPV. High-speed high-resolution cameras recorded local lung surface deformations and strains in real time, and differences between PPV and NPV global energetics, viscoelasticity, as well as local tissue distortion were assessed. Measurements and Main Results: During initial inflation, NPV exhibited significantly more bulk pressure-volume compliance than PPV, suggestive of earlier lung recruitment. NPV settings also showed reduced relaxation, hysteresis, and energy loss compared with PPV. Local strain trends were also decreased in NPV, with reduced tissue distortion trends compared with PPV, as revealed through analysis of tissue anisotropy. Conclusions: Apparently, contradictory previous studies are not mutually exclusive. Equivalent changes in transpulmonary pressures in PPV and NPV lead to the same changes in lung volume and pressures, yet local tissue strains differ between PPV and NPV. Although limited to healthy specimens and ex vivo experiments in the absence of a chest cavity, these results may explain previous reports of better oxygenation and less lung injury in NPV.

Continuous external negative pressure improves oxygenation and respiratory mechanics in Experimental Lung Injury in Pigs - A pilot proof-of-concept trial

BACKGROUND

Continuous external negative pressure (CENP) during positive pressure ventilation can recruit dependent lung regions. We hypothesised that CENP applied regionally to the thorax or the abdomen only, increases the caudal end-expiratory transpulmonary pressure depending on positive end-expiratory pressure (PEEP) in lung-injured pigs. Eight pigs were anesthetised and mechanically ventilated in the supine position. Pressure sensors were placed in the left pleural space, and a lung injury was induced by saline lung lavages. A CENP shell was placed at the abdomen and thorax (randomised order), and animals were ventilated with PEEP 15, 7 and zero cmH₂O (15 min each). On each PEEP level, CENP of - 40, - 30, - 20, - 10 and 0 cmH₂O was applied (3 min each). Respiratory and haemodynamic variables were recorded. Electrical impedance tomography allowed assessment of centre of ventilation.

RESULTS

Compared to positive pressure ventilation alone, the caudal transpulmonary pressure was significantly increased by CENP of ≤ 20 cmH₂O at all PEEP levels. CENP of - 20 cmH₂O reduced the mean airway pressure at zero PEEP (P = 0.025). The driving pressure decreased at CENP of ≤ 10 at PEEP of 0 and 7 cmH₂O (P < 0.001 each) but increased at CENP of - 30 cmH₂O during the highest PEEP (P = 0.001). CENP of - 30 cmH₂O reduced the mechanical power during zero PEEP (P < 0.001). Both elastance (P < 0.001) and resistance (P < 0.001) were decreased at CENP ≤ 30 at PEEP of 0 and 7 cmH₂O. Oxygenation increased at CENP of ≤ 20 at PEEP of 0 and 7 cmH₂O (P < 0.001 each). Applying external negative pressure significantly shifted the centre of aeration towards dorsal lung regions irrespectively of the PEEP level. Cardiac output decreased significantly at CENP -20 cmH₂O at all PEEP levels (P < 0.001). Effects on caudal transpulmonary pressure, elastance and cardiac output were more pronounced when CENP was applied to the abdomen compared with the thorax.

CONCLUSIONS

In this lung injury model in pigs, CENP increased the end-expiratory caudal transpulmonary pressure. This lead to a shift of lung aeration towards dependent zones as well as improved respiratory mechanics and oxygenation, especially when CENP was applied to the abdomen as compared to the thorax. CENP values ≤ 20 cmH₂O impaired the haemodynamics.

Dynamic single-slice CT estimates whole-lung dual-energy CT variables in pigs with and without experimental lung injury

Background

Dynamic single-slice CT (dCT) is increasingly used to examine the intra-tidal, physiological variation in aeration and lung density in experimental lung injury. The ability of dCT to predict whole-lung values is unclear, especially for dual-energy CT (DECT) variables. Additionally, the effect of inspiration-related lung movement on CT variables has not yet been quantified.

Methods

Eight domestic pigs were studied under general anaesthesia, including four following saline-lavage surfactant depletion (lung injury model). DECT, dCT and whole-lung images were collected at 12 ventilatory settings. Whole-lung single energy scans images were collected during expiratory and inspiratory apnoeas at positive end-expiratory pressures from 0 to 20 cmH₂O. Means and distributions of CT variables were calculated for both dCT and whole-lung images. The cranio-caudal displacement of the anatomical slice was measured from whole-lung images.

Results

Mean CT density and volume fractions of soft tissue, gas, iodinated blood, atelectasis, poor aeration, normal aeration and overdistension correlated between dCT and the whole lung (r² 0.75–0.94) with agreement between CT density distributions (r 0.89–0.97). Inspiration increased the matching between dCT and whole-lung values and was associated with a movement of 32% (SD 15%) of the imaged slice out of the scanner field-of-view. This effect introduced an artefactual increase in dCT mean CT density during inspiration, opposite to that caused by the underlying physiology.

Conclusions

Overall, dCT closely approximates whole-lung aeration and density. This approximation is improved by inspiration where a decrease in CT density and atelectasis can be interpreted as physiological rather than artefactual.

Effects of recruitment maneuvers with PEEP on lung volume distribution in canine models of direct and indirect lung injury

Lung recruitment maneuvers can help open collapsed lung units for sufficient oxygenation, and positive end expiratory pressure (PEEP) is used to keep the lung open after recruitment. However, the application of high PEEP levels may play a significant role in causing regional lung hyperinflation during mechanical ventilation. The authors sought to study the effects of PEEP targeting optimal oxygenation on regional lung volume distribution in a direct and an indirect acute respiratory distress syndrome (ARDS) model. ARDS was induced by either surfactant depletion or oleic acid injection in dogs. After lung recruitment, PEEP was decreased from 20 to 10 cmH2O in 2 cmH2O steps every 10 min to examine regional lung aeration by using computed tomography. Lung injury appeared to be localized in the model of surfactant depletion while it widely diffused after oleic acid infusion. At PEEP levels that achieved optimal oxygenation, nonaerated lung units decreased and normally aerated lung units enhanced, but hyperinflated areas increased significantly in both models (P < 0.05). Hyperinflated areas were greater in the surfactant depletion model than in the oleic acid model at PEEP levels applied (P < 0.05). Optimal oxygenation guided PEEP may cause hyperinflated in both focal lung injury and diffused lung injury post lung recruitment. Hyperinflation was more susceptible in focal lung injury than in diffused lung injury post lung recruitment.

Relative effects of negative versus positive pressure ventilation depend on applied conditions

PURPOSE

Comparisons of negative versus positive pressure ventilation have imperfectly matched the pressure-time profile or the lung volume history, or have incompletely applied in vivo negative pressure to include the complete thoracic wall and abdomen.

HYPOTHESIS

Negative pressure exerts the same pattern of lung distension as positive pressure when the pressure-time and volume history profiles are identical and the application of negative pressure is over the whole lung.

METHODS

(1) In isolated (ex vivo) and (2) intact (in vivo) mouse lungs (n = 4/group) (sealed chamber enclosing either the whole lung or whole mouse except for external airway opening), identical and inverse-tidal, square-wave pressure-time profiles were obtained with positive and negative pressure ventilation. (3) Following an identical volume history, surfactant-depleted rabbits (n = 7) were randomly assigned to sustained, static equivalent positive versus negative pressures. (4) Surfactant-depleted anesthetized rabbits (n = 10) with identical volume histories were randomized to positive versus negative ventilation with identical pressure-time characteristics.

RESULTS

Matched positive and negative pressure time profiles in ex vivo and in vivo mice resulted in identical tidal volumes. Identical (negative vs. positive) sustained static pressures resulted in similar PaO(2) and end expiratory lung volumes. Positive and negative ventilation with identical volume histories and pressure time characteristics showed no difference in oxygenation or lung volumes. Historical comparisons suggested better oxygenation with negative pressure when the volume history was not identical.

CONCLUSIONS

These data do not support major biological differences between negative and positive pressure ventilation when waveforms and lung volume history are matched.

Computed tomography findings from patients with ARDS due to Influenza A (H1N1) virus-associated pneumonia

PURPOSE

The purpose of this study was to retrospectively evaluate whether computed tomography (CT) findings have prognostic value for the prediction of mortality and severity of the clinical course in patients presenting with early stage of acute respiratory distress syndrome (ARDS) due to swine-origin influenza A (S-OIV).

MATERIALS AND METHODS

Chest CT (16-/64-row multidetector CT) of 23 patients (of whom 9 patients died) were retrospectively reviewed by three independent blinded observers. The CT findings were graded on a 3-point scale (1: normal attenuation, 2: ground-glass attenuation, 3: consolidation). The extent of each abnormality was determined by visually estimating the percentage (to the nearest 10%) of the affected lung parenchyma in each zone and multiplied by the CT-score described above.

RESULTS

All patients presented with a mixture of bilateral patchy consolidations and ground glass opacities. Spearman rank correlation in evaluation of the presence and extent of lung abnormalities by the three different observers was good (correlation coefficient, 0.876-0.922; p < 0.001). The overall CT-score in survivors (mean, 96.0 (± 26.2); range, 53-158) was significantly lower than that in non-survivors (mean, 116.2 (± 14.0); range, 101-139). ROC analysis revealed an area under curve of 0.79 (p = 0.021) for the CT score with an optimal cutoff value of a CT-score of 100 for prediction of survival, with a sensitivity of 100% and a specificity of 64% (accuracy, 78%). For this optimal cutoff, Kaplan-Meier estimator showed a significant difference for the survival ratio (p = 0.011).

CONCLUSION

In patients with severe ARDS due to S-OIV-infection, the CT-score has a prognostic value in the prediction of mortality.

Adrenal gland volume measurement in septic shock and control patients: a pilot study

OBJECTIVES

To compare adrenal gland volume in septic shock patients and control patients by using semi-automated volumetry.

METHODS

Adrenal gland volume and its inter-observer variability were measured with tomodensitometry using semi-automated software in 104 septic shock patients and in 40 control patients. The volumes of control and septic shock patients were compared and the relationship between volume and outcome in intensive care was studied.

RESULTS

The mean total volume of both adrenal glands was 7.2 ± 2.0 cm(3) in control subjects and 13.3 ± 4.7 cm(3) for total adrenal gland volume in septic shock patients (p < 0.0001). Measurement reproducibility was excellent with a concordance correlation coefficient value of 0.87. The increasing adrenal gland volume was associated with a higher rate of survival in intensive care.

CONCLUSION

The present study reports that with semi-automated software, adrenal gland volume can be measured easily and reproducibly. Adrenal gland volume was found to be nearly double in sepsis compared with control patients. The absence of increased volume during sepsis would appear to be associated with a higher rate of mortality and may represent a prognosis factor which may help the clinician to guide their strategy.

Regional lung aeration and ventilation during pressure support and biphasic positive airway pressure ventilation in experimental lung injury

INTRODUCTION

There is an increasing interest in biphasic positive airway pressure with spontaneous breathing (BIPAP+SBmean), which is a combination of time-cycled controlled breaths at two levels of continuous positive airway pressure (BIPAP+SBcontrolled) and non-assisted spontaneous breathing (BIPAP+SBspont), in the early phase of acute lung injury (ALI). However, pressure support ventilation (PSV) remains the most commonly used mode of assisted ventilation. To date, the effects of BIPAP+SBmean and PSV on regional lung aeration and ventilation during ALI are only poorly defined.

METHODS

In 10 anesthetized juvenile pigs, ALI was induced by surfactant depletion. BIPAP+SBmean and PSV were performed in a random sequence (1 h each) at comparable mean airway pressures and minute volumes. Gas exchange, hemodynamics, and inspiratory effort were determined and dynamic computed tomography scans obtained. Aeration and ventilation were calculated in four zones along the ventral-dorsal axis at lung apex, hilum and base.

RESULTS

Compared to PSV, BIPAP+SBmean resulted in: 1) lower mean tidal volume, comparable oxygenation and hemodynamics, and increased PaCO2 and inspiratory effort; 2) less nonaerated areas at end-expiration; 3) decreased tidal hyperaeration and re-aeration; 4) similar distributions of ventilation. During BIPAP+SBmean: i) BIPAP+SBspont had lower tidal volumes and higher rates than BIPAP+SBcontrolled; ii) BIPAP+SBspont and BIPAP+SBcontrolled had similar distributions of ventilation and aeration; iii) BIPAP+SBcontrolled resulted in increased tidal re-aeration and hyperareation, compared to PSV. BIPAP+SBspont showed an opposite pattern.

CONCLUSIONS

In this model of ALI, the reduction of tidal re-aeration and hyperaeration during BIPAP+SBmean compared to PSV is not due to decreased nonaerated areas at end-expiration or different distribution of ventilation, but to lower tidal volumes during BIPAP+SBspont. The ratio between spontaneous to controlled breaths seems to play a pivotal role in reducing tidal re-aeration and hyperaeration during BIPAP+SBmean.