Arnoldo Santos

Cardiovascular imaging: what have we learned from animal models?

Cardiovascular imaging has become an indispensable tool for patient diagnosis and follow up. Probably the wide clinical applications of imaging are due to the possibility of a detailed and high quality description and quantification of cardiovascular system structure and function. Also phenomena that involve complex physiological mechanisms and biochemical pathways, such as inflammation and ischemia, can be visualized in a non-destructive way. The widespread use and evolution of imaging would not have been possible without animal studies. Animal models have allowed for instance, (i) the technical development of different imaging tools, (ii) to test hypothesis generated from human studies and finally, (iii) to evaluate the translational relevance assessment of in vitro and ex-vivo results. In this review, we will critically describe the contribution of animal models to the use of biomedical imaging in cardiovascular medicine. We will discuss the characteristics of the most frequent models used in/for imaging studies. We will cover the major findings of animal studies focused in the cardiovascular use of the repeatedly used imaging techniques in clinical practice and experimental studies. We will also describe the physiological findings and/or learning processes for imaging applications coming from models of the most common cardiovascular diseases. In these diseases, imaging research using animals has allowed the study of aspects such as: ventricular size, shape, global function, and wall thickening, local myocardial function, myocardial perfusion, metabolism and energetic assessment, infarct quantification, vascular lesion characterization, myocardial fiber structure, and myocardial calcium uptake. Finally we will discuss the limitations and future of imaging research with animal models.

Experimental methods for flow and aerosol measurements in human airways and their replicas

Abstract Recent developments in the prediction of local aerosol deposition in human lungs are driven by the fast development of computational simulations. Although such simulations provide results in unbeatable resolution, significant differences among distinct methods of calculation emphasize the need for highly precise experimental data in order to specify boundary conditions and for validation purposes. This paper reviews and critically evaluates available methods for the measurement of single and disperse two‐phase flows for the study of respiratory airflow and deposition of inhaled particles, performed both in vivo and in replicas of airways. Limitations and possibilities associated with the experimental methods are discussed and aspects of the computational calculations that can be validated are indicated. The review classifies the methods into following categories: 1) point‐wise and planar methods for velocimetry in the airways, 2) classic methods for the measurement of the regional distribution of inhaled particles, 3) standard medical imaging methods applicable to the measurement of the regional aerosol distribution and 4) emerging and nonconventional methods. All methods are described, applications in human airways studies are illustrated, and recommendations for the most useful applications of each method are given. Graphical abstract Figure. No caption available.

Deterioration of Regional Lung Strain and Inflammation during Early Lung Injury

Rationale: The contribution of aeration heterogeneity to lung injury during early mechanical ventilation of uninjured lungs is unknown. Objectives: To test the hypotheses that a strategy consistent with clinical practice does not protect from worsening in lung strains during the first 24 hours of ventilation of initially normal lungs exposed to mild systemic endotoxemia in supine versus prone position, and that local neutrophilic inflammation is associated with local strain and blood volume at global strains below a proposed injurious threshold. Methods: Voxel‐level aeration and tidal strain were assessed by computed tomography in sheep ventilated with low Vt and positive end‐expiratory pressure while receiving intravenous endotoxin. Regional inflammation and blood volume were estimated from 2‐deoxy‐2‐[(18)F]fluoro‐d‐glucose (18F‐FDG) positron emission tomography. Measurements and Main Results: Spatial heterogeneity of aeration and strain increased only in supine lungs (P < 0.001), with higher strains and atelectasis than prone at 24 hours. Absolute strains were lower than those considered globally injurious. Strains redistributed to higher aeration areas as lung injury progressed in supine lungs. At 24 hours, tissue‐normalized 18F‐FDG uptake increased more in atelectatic and moderately high‐aeration regions (>70%) than in normally aerated regions (P < 0.01), with differential mechanistically relevant regional gene expression. 18F‐FDG phosphorylation rate was associated with strain and blood volume. Imaging findings were confirmed in ventilated patients with sepsis. Conclusions: Mechanical ventilation consistent with clinical practice did not generate excessive regional strain in heterogeneously aerated supine lungs. However, it allowed worsening of spatial strain distribution in these lungs, associated with increased inflammation. Our results support the implementation of early aeration homogenization in normal lungs.

Metabolic Reprogramming in the Heart and Lung in a Murine Model of Pulmonary Arterial Hypertension

A significant glycolytic shift in the cells of the pulmonary vasculature and right ventricle during pulmonary arterial hypertension (PAH) has been recently described. Due to the late complications and devastating course of any variant of this disease, there is a great need for animal models that reproduce potential metabolic reprograming of PAH. Our objective is to study, in situ, the metabolic reprogramming in the lung and the right ventricle of a mouse model of PAH by metabolomic profiling and molecular imaging. PAH was induced by chronic hypoxia exposure plus treatment with SU5416, a vascular endothelial growth factor receptor inhibitor. Lung and right ventricle samples were analyzed by magnetic resonance spectroscopy. In vivo energy metabolism was studied by positron emission tomography. Our results show that metabolomic profiling of lung samples clearly identifies significant alterations in glycolytic pathways. We also confirmed an upregulation of glutamine metabolism and alterations in lipid metabolism. Furthermore, we identified alterations in glycine and choline metabolism in lung tissues. Metabolic reprograming was also confirmed in right ventricle samples. Lactate and alanine, endpoints of glycolytic oxidation, were found to have increased concentrations in mice with PAH. Glutamine and taurine concentrations were correlated to specific ventricle hypertrophy features. We demonstrated that most of the metabolic features that characterize human PAH were detected in a hypoxia plus SU5416 mouse model and it may become a valuable tool to test new targeting treatments of this severe disease.

Heart-lung interactions in acute respiratory distress syndrome: pathophysiology, detection and management strategies

Acute respiratory distress syndrome (ARDS) is the most severe form of acute respiratory failure characterized by diffuse alveolar and endothelial damage. The severe pathophysiological changes in lung parenchyma and pulmonary circulation together with the effects of positive pressure ventilation profoundly affect heart lung interactions in ARDS. The term pulmonary vascular dysfunction (PVD) refers to the specific involvement of the vascular compartment in ARDS and is expressed clinically by an increase in pulmonary arterial (PA) pressure and pulmonary vascular resistance both affecting right ventricular (RV) afterload. When severe, PVD can lead to RV failure which is associated to an increased mortality. The effect of PVD on RV function is not only a consequence of increased pulmonary vascular resistance as afterload is a much more complex phenomenon that includes all factors that oppose efficient ventricular ejection. Impaired pulmonary vascular mechanics including increased arterial elastance and augmented wave-reflection phenomena are commonly seen in ARDS and can additionally affect RV afterload. The use of selective pulmonary vasodilators and lung protective mechanical ventilation strategies are therapeutic interventions that can ameliorate PVD. Prone positioning and the open lung approach (OLA) are especially attractive strategies to improve PVD due to their effects on increasing functional lung volume. In this review we will describe some pathophysiological aspects of heart-lung interactions during the ventilatory support of ARDS, its clinical assessment and discuss therapeutic interventions to prevent the occurrence and progression of PVD and RV failure.

Assessment of regional pulmonary blood flow using 68Ga-DOTA PET

BackgroundIn vivo determination of regional pulmonary blood flow (PBF) is a valuable tool for the evaluation of many lung diseases. In this study, the use of 68Ga-DOTA PET for the in vivo quantitative determination of regional PBF is proposed. This methodology was implemented and tested in healthy pigs and validated using fluorescent microspheres. The study was performed on young large white pigs (n = 4). To assess the reproducibility and consistency of the method, three PET scans were obtained for each animal. Each radiotracer injection was performed simultaneously to the injection of fluorescent microspheres. PBF images were generated applying a two-compartment exchange model over the dynamic PET images. PET and microspheres values were compared by regression analysis and Bland–Altman plot.ResultsThe capability of the proposed technique to produce 3D regional PBF images was demonstrated. The correlation evaluation between 68Ga-DOTA PET and microspheres showed a good and significant correlation (r = 0.74, P < 0.001).ConclusionsAssessment of PBF with the proposed technique allows combining the high quantitative accuracy of PET imaging with the use of 68Ga/68Ge generators. Thus, 68Ga-DOTA PET emerges as a potential inexpensive method for measuring PBF in clinical settings with an extended use.

Effects on Pulmonary Vascular Mechanics of Two Different Lung-Protective Ventilation Strategies in an Experimental Model of Acute Respiratory Distress Syndrome

Objectives: To compare the effects of two lung-protective ventilation strategies on pulmonary vascular mechanics in early acute respiratory distress syndrome. Design: Experimental study. Setting: University animal research laboratory. Subjects: Twelve pigs (30.8 ± 2.5 kg). Interventions: Acute respiratory distress syndrome was induced by repeated lung lavages and injurious mechanical ventilation. Thereafter, animals were randomized to 4 hours ventilation according to the Acute Respiratory Distress Syndrome Network protocol or to an open lung approach strategy. Pressure and flow sensors placed at the pulmonary artery trunk allowed continuous assessment of pulmonary artery resistance, effective elastance, compliance, and reflected pressure waves. Respiratory mechanics and gas exchange data were collected. Measurements and Main Results: Acute respiratory distress syndrome led to pulmonary vascular mechanics deterioration. Four hours after randomization, pulmonary vascular mechanics was similar in Acute Respiratory Distress Syndrome Network and open lung approach: resistance (578 ± 252 vs 626 ± 153 dyn.s/cm5; p = 0.714), effective elastance, (0.63 ± 0.22 vs 0.58 ± 0.17 mm Hg/mL; p = 0.710), compliance (1.19 ± 0.8 vs 1.50 ± 0.27 mL/mm Hg; p = 0.437), and reflection index (0.36 ± 0.04 vs 0.34 ± 0.09; p = 0.680). Open lung approach as compared to Acute Respiratory Distress Syndrome Network was associated with improved dynamic respiratory compliance (17.3 ± 2.6 vs 10.5 ± 1.3 mL/cm H2O; p < 0.001), driving pressure (9.6 ± 1.3 vs 19.3 ± 2.7 cm H2O; p < 0.001), and venous admixture (0.05 ± 0.01 vs 0.22 ± 0.03, p < 0.001) and lower mean pulmonary artery pressure (26 ± 3 vs 34 ± 7 mm Hg; p = 0.045) despite of using a higher positive end-expiratory pressure (17.4 ± 0.7 vs 9.5 ± 2.4 cm H2O; p < 0.001). Cardiac index, however, was lower in open lung approach (1.42 ± 0.16 vs 2.27 ± 0.48 L/min; p = 0.005). Conclusions: In this experimental model, Acute Respiratory Distress Syndrome Network and open lung approach affected pulmonary vascular mechanics similarly. The use of higher positive end-expiratory pressures in the open lung approach strategy did not worsen pulmonary vascular mechanics, improved lung mechanics, and gas exchange but at the expense of a lower cardiac index.

Position-dependent distribution of lung ventilation — A feasability study

The aim of this feasibility study was to determine whether the measurement setup and study protocol were able to show the effect that lung disease, body position and different levels of positive end expiratory pressure (PEEP) have on lung function. By means of a motorized rotation table and gravity sensors six pigs were rotated in steps of 30° from left to right lateral position. Regional ventilation distributions, measured by electrical impedance tomography (EIT), oxygenation and compliance measurements were performed at each position. Both, experimental and measurement setup as well as the parameters chosen to characterize lung function appear suitable for analyzing the effects of PEEP and rotation in healthy and injured lungs. The initial results show that the distribution of regional ventilation was highly gravity-dependent especially in sick lungs. Furthermore lateral rotation showed significant recruitment effects on previously collapsed lung tissue as witnessed by the increases in oxygenation at all PEEPs.