Does Iso-mechanical Power Lead to Iso-lung Damage?: An Experimental Study in a Porcine Model.

BACKGROUND: Excessive tidal volume, respiratory rate, and positive end-expiratory pressure (PEEP) are all potential causes of ventilator-induced lung injury, and all contribute to a single variable: the mechanical power. The authors aimed to determine whether high tidal volume or high respiratory rate or high PEEP at iso-mechanical power produce similar or different ventilator-induced lung injury. METHODS: Three ventilatory strategies-high tidal volume (twice baseline functional residual capacity), high respiratory rate (40 bpm), and high PEEP (25 cm H2O)-were each applied at two levels of mechanical power (15 and 30 J/min) for 48 h in six groups of seven healthy female piglets (weight: 24.2 ± 2.0 kg, mean ± SD). RESULTS: At iso-mechanical power, the high tidal volume groups immediately and sharply increased plateau, driving pressure, stress, and strain, which all further deteriorated with time. In high respiratory rate groups, they changed minimally at the beginning, but steadily increased during the 48 h. In contrast, after a sudden huge increase, they decreased with time in the high PEEP groups. End-experiment specific lung elastance was 6.5 ± 1.7 cm H2O in high tidal volume groups, 10.1 ± 3.9 cm H2O in high respiratory rate groups, and 4.5 ± 0.9 cm H2O in high PEEP groups. Functional residual capacity decreased and extravascular lung water increased similarly in these three categories. Lung weight, wet-to-dry ratio, and histologic scores were similar, regardless of ventilatory strategies and power levels. However, the alveolar edema score was higher in the low power groups. High PEEP had the greatest impact on hemodynamics, leading to increased need for fluids. Adverse events (early mortality and pneumothorax) also occurred more frequently in the high PEEP groups. CONCLUSIONS: Different ventilatory strategies, delivered at iso-power, led to similar anatomical lung injury. The different systemic consequences of high PEEP underline that ventilator-induced lung injury must be evaluated in the context of the whole body.

Extracorporeal CO2 Removal: The Minimally Invasive Approach, Theory, and Practice.

OBJECTIVES: Minimally invasive extracorporeal CO2 removal is an accepted supportive treatment in chronic obstructive pulmonary disease patients. Conversely, the potential of such technique in treating acute respiratory distress syndrome patients remains to be investigated. The aim of this study was: 1) to quantify membrane lung CO2 removal (VCO2ML) under different conditions and 2) to quantify the natural lung CO2 removal (VCO2NL) and to what extent mechanical ventilation can be reduced while maintaining total expired CO2 (VCO2tot = VCO2ML + VCO2NL) and arterial PCO2 constant. DESIGN: Experimental animal study. SETTING: Department of Experimental Animal Medicine, University of Göttingen, Germany. SUBJECTS: Eight healthy pigs (57.7 ± 5 kg). INTERVENTIONS: The animals were sedated, ventilated, and connected to the artificial lung system (surface 1.8 m, polymethylpentene membrane, filling volume 125 mL) through a 13F catheter. VCO2ML was measured under different combinations of inflow PCO2 (38.9 ± 3.3, 65 ± 5.7, and 89.9 ± 12.9 mm Hg), extracorporeal blood flow (100, 200, 300, and 400 mL/min), and gas flow (4, 6, and 12 L/min). At each setting, we measured VCO2ML, VCO2NL, lung mechanics, and blood gases. MEASUREMENTS AND MAIN RESULTS: VCO2ML increased linearly with extracorporeal blood flow and inflow PCO2 but was not affected by gas flow. The outflow PCO2 was similar regardless of inflow PCO2 and extracorporeal blood flow, suggesting that VCO2ML was maximally exploited in each experimental condition. Mechanical ventilation could be reduced by up to 80-90% while maintaining a constant PaCO2. CONCLUSIONS: Minimally invasive extracorporeal CO2 removal removes a relevant amount of CO2 thus allowing mechanical ventilation to be significantly reduced depending on extracorporeal blood flow and inflow PCO2. Extracorporeal CO2 removal may provide the physiologic prerequisites for controlling ventilator-induced lung injury.

Detecting Early Markers of Ventilator-Associated Pneumonia by Analysis of Exhaled Gas.

OBJECTIVES: The detection of microbial volatile organic compounds or host response markers in the exhaled gas could give an earlier diagnosis of ventilator-associated pneumonia. Gas chromatography-ion mobility spectrometry enables noninvasive, rapid, and sensitive analysis of exhaled gas. Using a rabbit model of ventilator-associated pneumonia we determined if gas chromatography-ion mobility spectrometry is able to detect 1) ventilator-associated pneumonia specific changes and 2) bacterial species-specific changes in the exhaled gas. DESIGN: Experimental in vivo study. SETTING: University research laboratory. SUBJECTS: Female New Zealand White rabbits. INTERVENTIONS: Animals were anesthetized and mechanically ventilated. To induce changes in the composition of exhaled gas we induced ventilator-associated pneumonia via endobronchial instillation of either Escherichia coli group (n = 11) or Pseudomonas aeruginosa group (n = 11) after 2 hours of mechanical ventilation. In a control group (n = 11) we instilled sterile lysogeny broth endobronchially. MEASUREMENTS AND MAIN RESULTS: Gas chromatography-ion mobility spectrometry gas analysis, CT scans of the lungs, and blood samples were obtained at four measurement points during the 10 hours of mechanical ventilation. The volatile organic compound patterns in the exhaled gas were compared and correlated with ventilator-associated pneumonia severity. Sixty-seven peak areas showed changes in signal intensity in the serial gas analyses. The signal intensity changes in 10 peak regions differed between the groups. Five peak areas (P_648_36, indole, P_714_278, P_700_549, and P_727_557) showed statistically significant changes of signal intensity. CONCLUSIONS: This is the first in vivo study that shows the potential of gas chromatography-ion mobility spectrometry for early detection of ventilator-associated pneumonia specific volatile organic compounds and species differentiation by noninvasive analyses of exhaled gas.

Positive End-expiratory Pressure and Mechanical Power.

BACKGROUND: Positive end-expiratory pressure is usually considered protective against ventilation-induced lung injury by reducing atelectrauma and improving lung homogeneity. However, positive end-expiratory pressure, together with tidal volume, gas flow, and respiratory rate, contributes to the mechanical power required to ventilate the lung. This study aimed at investigating the effects of increasing mechanical power by selectively modifying its positive end-expiratory pressure component. METHODS: Thirty-six healthy piglets (23.3 ± 2.3 kg) were ventilated prone for 50 h at 30 breaths/min and with a tidal volume equal to functional residual capacity. Positive end-expiratory pressure levels (0, 4, 7, 11, 14, and 18 cm H2O) were applied to six groups of six animals. Respiratory, gas exchange, and hemodynamic variables were recorded every 6 h. Lung weight and wet-to-dry ratio were measured, and histologic samples were collected. RESULTS: Lung mechanical power was similar at 0 (8.8 ± 3.8 J/min), 4 (8.9 ± 4.4 J/min), and 7 (9.6 ± 4.3 J/min) cm H2O positive end-expiratory pressure, and it linearly increased thereafter from 15.5 ± 3.6 J/min (positive end-expiratory pressure, 11 cm H2O) to 18.7 ± 6 J/min (positive end-expiratory pressure, 14 cm H2O) and 22 ± 6.1 J/min (positive end-expiratory pressure, 18 cm H2O). Lung elastances, vascular congestion, atelectasis, inflammation, and septal rupture decreased from zero end-expiratory pressure to 4 to 7 cm H2O (P < 0.0001) and increased progressively at higher positive end-expiratory pressure. At these higher positive end-expiratory pressure levels, striking hemodynamic impairment and death manifested (mortality 0% at positive end-expiratory pressure 0 to 11 cm H2O, 33% at 14 cm H2O, and 50% at 18 cm H2O positive end-expiratory pressure). From zero end-expiratory pressure to 18 cm H2O, mean pulmonary arterial pressure (from 19.7 ± 5.3 to 32.2 ± 9.2 mmHg), fluid administration (from 537 ± 403 to 2043 ± 930 ml), and noradrenaline infusion (0.04 ± 0.09 to 0.34 ± 0.31 µg · kg(-1) · min(-1)) progressively increased (P < 0.0001). Lung weight and lung wet-to-dry ratios were not significantly different across the groups. The lung mechanical power level that best discriminated between more versus less severe damage was 13 ± 1 J/min. CONCLUSIONS: Less than 7 cm H2O positive end-expiratory pressure reduced atelectrauma encountered at zero end-expiratory pressure. Above a defined power threshold, sustained positive end-expiratory pressure contributed to potentially lethal lung damage and hemodynamic impairment.

Targeted endomyocardial biopsy guided by real-time cardiovascular magnetic resonance.

BACKGROUND: Endomyocardial biopsies (EMB) are an important diagnostic tool for myocarditis and other infiltrative cardiac diseases. Routinely, biopsies are obtained under fluoroscopic guidance with a substantial radiation burden. Despite procedural success, there is a large sampling error caused by missing the affected myocardium. Therefore, multiple (>6) biopsies are taken in the clinical setting. In cardiovascular magnetic resonance (CMR), late gadolinium enhancement (LGE) depicts areas of affected myocardium in myocarditis or in other infiltrative cardiomyopathies. Thus, targeted biopsy under real-time CMR image guidance might reduce the problem of sampling error. METHODS: Seven minipigs of the Goettingen strain underwent radiofrequency ablation in the left ventricle. At least two focal lesions were induced on the lateral wall in five and the apex in two animals. Each ablation lesion was created by two consecutive 30 sec ablations (max. 30 W, temperature 60-64 °C). Biopsies were taken immediately after lesion induction using a commercially available 7 F conventional bioptome under fluoroscopic guidance at the ablation site. Afterwards the animals underwent CMR and lesion visualization by LGE at 3T. The lesions were then targeted and biopsied under CMR-guidance using a MR-conditional bioptome guided by a steerable catheter. Interactive real-time (RT) visualization of the intervention on an in-room monitor was based on radial FLASH with nonlinear inverse reconstruction (NLINV) at a temporal resolution of 42 ms. All samples underwent a standard histological evaluation. RESULTS: Radiofrequency ablation was successful in all animals. Fluoroscopy-guided biopsies were performed with a success rate of 6/6 minipigs - resulting in a nonlethal pericardial effusion in one animal. Visualization of radiofrequency lesions by CMR was successful in 7/7 minipig, i.e. at least one lesion was clearly visible. Localization and tracking of the catheters and the bioptome using interactive control of the imaging plane was achieved in 6/6 MP; however in the animal with a large pericardial effusion after EMB under fluoroscopy no further EMB was attempted for safety reasons. Biopsies under interactive RT-CMR guidance were successfully performed in 5/6 animals, in one animal the bioptome reached the lesion, however the forceps did not cut out a sample. Specimens obtained under CMR guidance contained part of the lesion in 6/15 (40%) myocardial specimens and in 4/5 (80%) animals in which samples were achieved. Conventional biopsies revealed ablation lesions in 4/17 (23.5%) specimens in 3/6 minipigs (50%). CONCLUSION: Focal lesions induced by radiofrequency ablation in a minipig model are a useful tool for CMR-guided biopsy studies. In contrast to fluoroscopy, CMR provides excellent visualization of lesions. Interactive real-time CMR allows excellent passive tracking of the instruments and EMB provides significantly superior sampling accuracy compared to fluoroscopy-guided biopsies. Nonetheless, further improvements of MR-compatible bioptomes and guiding catheters are essential before applying this method in a clinical setting.

Estimation of normal lung weight index in healthy female domestic pigs.

INTRODUCTION: Lung weight is an important study endpoint to assess lung edema in porcine experiments on acute respiratory distress syndrome and ventilatory induced lung injury. Evidence on the relationship between lung-body weight relationship is lacking in the literature. The aim of this work is to provide a reference equation between normal lung and body weight in female domestic piglets. MATERIALS AND METHODS: 177 healthy female domestic piglets from previous studies were included in the analysis. Lung weight was assessed either via a CT-scan before any experimental injury or with a scale after autopsy. The animals were randomly divided in a training (n = 141) and a validation population (n = 36). The relation between body weight and lung weight index (lung weight/body weight, g/kg) was described by an exponential function on the training population. The equation was tested on the validation population. A Bland-Altman analysis was performed to compare the lung weight index in the validation population and its theoretical value calculated with the reference equation. RESULTS: A good fit was found between the validation population and the exponential equation extracted from the training population (RMSE = 0.060). The equation to determine lung weight index from body weight was: [Formula: see text] At the Bland and Altman analyses, the mean bias between the real and the expected lung weight index was - 0.26 g/kg (95% CI - 0.96-0.43), upper LOA 3.80 g/kg [95% CI 2.59-5.01], lower LOA - 4.33 g/kg [95% CI = - 5.54-(- 3.12)]. CONCLUSIONS: This exponential function might be a valuable tool to assess lung edema in experiments involving 16-50 kg female domestic piglets. The error that can be made due to the 95% confidence intervals of the formula is smaller than the one made considering the lung to body weight as a linear relationship.

CD8+-T Cells With Specificity for a Model Antigen in Cardiomyocytes Can Become Activated After Transverse Aortic Constriction but Do Not Accelerate Progression to Heart Failure.

Heart failure due to pressure overload is frequently associated with inflammation. In addition to inflammatory responses of the innate immune system, autoimmune reactions of the adaptive immune system appear to be triggered in subgroups of patients with heart failure as demonstrated by the presence of autoantibodies against myocardial antigens. Moreover, T cell-deficient and T cell-depleted mice have been reported to be protected from heart failure induced by transverse aortic constriction (TAC) and we have shown recently that CD4+-helper T cells with specificity for an antigen in cardiomyocytes accelerate TAC-induced heart failure. In this study, we set out to investigate the potential contribution of CD8+-cytotoxic T cells with specificity to a model antigen (ovalbumin, OVA) in cardiomyocytes to pressure overload-induced heart failure. In 78% of cMy-mOVA mice with cardiomyocyte-specific OVA expression, a low-grade OVA-specific cellular cytotoxicity was detected after TAC. Adoptive transfer of OVA-specific CD8+-T cells from T cell receptor transgenic OT-I mice before TAC did not increase the risk of OVA-specific autoimmunity in cMy-mOVA mice. After TAC, again 78% of the mice displayed an OVA-specific cytotoxicity with on average only a three-fold higher killing of OVA-expressing target cells. More CD8+ cells were present after TAC in the myocardium of cMy-mOVA mice with OT-I T cells (on average 17.5/mm2) than in mice that did not receive OVA-specific CD8+-T cells (3.6/mm2). However, the extent of fibrosis was similar in both groups. Functionally, as determined by echocardiography, the adoptive transfer of OVA-specific CD8+-T cells did not significantly accelerate the progression from hypertrophy to heart failure in cMy-mOVA mice. These findings argue therefore against a major impact of cytotoxic T cells with specificity for autoantigens of cardiomyocytes in pressure overload-induced heart failure.

Total intravenous anaesthesia using propofol and sufentanil allows controlled long-term ventilation in rabbits without neuromuscular blocking agents.

The aim of this study was to evaluate a total intravenous anaesthesia (TIVA) protocol using propofol and sufentanil without neuromuscular blocking agents (NBAs) for a non-recovery lung pathology study in rabbits including 10 h of pressure-controlled ventilation. TIVA was started with 20 mg/kg/h propofol and 0.5 µg/kg/h sufentanil. The depth of anaesthesia was assessed by reflex testing and monitoring of spontaneous movements or respiratory efforts. Vital parameters were monitored to assess the effects of the TIVA protocol. The infusion rates were increased whenever reflex testing indicated inadequate depth of anaesthesia, and were reduced when vital parameters indicated unnecessarily deep levels. Median infusion rates of 35 mg/kg/h propofol and 2.0 µg/kg/h sufentanil were needed to ensure an adequate depth of anaesthesia. This protocol suppressed spontaneous movements, breathing and palpebral reflexes, but was unable to suppress corneal and pedal withdrawal reflexes. Since significant drops in arterial blood pressure (ABP) were observed and the animals were not exposed to painful procedures, positive corneal and pedal withdrawal reflexes were tolerated. In conclusion, propofol and sufentanil is a suitable combination for long-term anaesthesia in non-recovery lung pathology models in rabbits without painful procedures. ABP must be monitored carefully because of the circulatory side-effects, but it is an inappropriate surrogate marker for depth of anaesthesia. Due to the lack of neuromuscular blockade this TIVA protocol allows the adjustment of infusion rates based on reflex testing. The resulting decreased risk of unnoticed awareness is a decisive refinement in anaesthesia for similar studies including long-term mechanical ventilation in rabbits.