Distribution of perfusion

Assessing the pulmonary vascular responsiveness to oxygen with proton MRI

Ventilation-perfusion matching occurs passively and is also actively regulated through hypoxic pulmonary vasoconstriction (HPV). The extent of HPV activity in humans, particularly normal subjects, is uncertain. Current evaluation of HPV assesses changes in ventilation-perfusion relationships/pulmonary vascular resistance with hypoxia and is invasive, or unsuitable for patients because of safety concerns. We used a noninvasive imaging-based approach to quantify the pulmonary vascular response to oxygen as a metric of HPV by measuring perfusion changes between breathing 21% and 30%O2 using arterial spin labeling (ASL) MRI. We hypothesized that the differences between 21% and 30%O2 images reflecting HPV release would be 1) significantly greater than the differences without [Formula: see text] changes (e.g., 21-21% and 30-30%O2) and 2) negatively associated with ventilation-perfusion mismatch. Perfusion was quantified in the right lung in normoxia (baseline), after 15 min of 30% O2 breathing (hyperoxia) and 15 min normoxic recovery (recovery) in healthy subjects (7 M, 7 F; age = 41.4 ± 19.6 yr). Normalized, smoothed, and registered pairs of perfusion images were subtracted and the mean square difference (MSD) was calculated. Separately, regional alveolar ventilation and perfusion were quantified from specific ventilation, proton density, and ASL imaging; the spatial variance of ventilation-perfusion (σ2V̇a/Q̇) distributions was calculated. The O2-responsive MSD was reproducible (R2 = 0.94, P < 0.0001) and greater (0.16 ± 0.06, P < 0.0001) than that from subtracted images collected under the same [Formula: see text] (baseline = 0.09 ± 0.04, hyperoxia = 0.08 ± 0.04, recovery = 0.08 ± 0.03), which were not different from one another (P = 0.2). The O2-responsive MSD was correlated with σ2V̇a/Q̇ (R2 = 0.47, P = 0.007). These data suggest that active HPV optimizes ventilation-perfusion matching in normal subjects. This noninvasive approach could be applied to patients with different disease phenotypes to assess HPV and ventilation-perfusion mismatch.NEW & NOTEWORTHY We developed a new proton MRI method to noninvasively quantify the pulmonary vascular response to oxygen. Using a hyperoxic stimulus to release HPV, we quantified the resulting redistribution of perfusion. The differences between normoxic and hyperoxic images were greater than those between images without [Formula: see text] changes and negatively correlated with ventilation-perfusion mismatch. This suggests that active HPV optimizes ventilation-perfusion matching in normal subjects. This approach is suitable for assessing patients with different disease phenotypes.

The Pulmonary Vasculature

The pulmonary circulation is a low-pressure, low-resistance circuit whose primary function is to deliver deoxygenated blood to, and oxygenated blood from, the pulmonary capillary bed enabling gas exchange. The distribution of pulmonary blood flow is regulated by several factors including effects of vascular branching structure, large-scale forces related to gravity, and finer scale factors related to local control. Hypoxic pulmonary vasoconstriction is one such important regulatory mechanism. In the face of local hypoxia, vascular smooth muscle constriction of precapillary arterioles increases local resistance by up to 250%. This has the effect of diverting blood toward better oxygenated regions of the lung and optimizing ventilation-perfusion matching. However, in the face of global hypoxia, the net effect is an increase in pulmonary arterial pressure and vascular resistance. Pulmonary vascular resistance describes the flow-resistive properties of the pulmonary circulation and arises from both precapillary and postcapillary resistances. The pulmonary circulation is also distensible in response to an increase in transmural pressure and this distention, in addition to recruitment, moderates pulmonary arterial pressure and vascular resistance. This article reviews the physiology of the pulmonary vasculature and briefly discusses how this physiology is altered by common circumstances.

Early Endothelial Signaling Transduction in Developing Lung Edema

The lung promptly responds to edemagenic conditions through functional adaptations that contrast the increase in microvascular filtration. This review presents evidence for early signaling transduction by endothelial lung cells in two experimental animal models of edema, hypoxia exposure, and fluid overload (hydraulic edema). The potential role of specialized sites of the plasma membranes considered mobile signaling platforms, referred to as membrane rafts, that include caveolae and lipid rafts, is presented. The hypothesis is put forward that early changes in the lipid composition of the bilayer of the plasma membrane might trigger the signal transduction process when facing changes in the pericellular microenvironment caused by edema. Evidence is provided that for an increase in the extravascular lung water volume not exceeding 10%, changes in the composition of the plasma membrane of endothelial cells are evoked in response to mechanical stimuli from the interstitial compartment as well as chemical stimuli relating with changes in the concentration of the disassembled portions of structural macromolecules. In hypoxia, thinning of endothelial cells, a decrease in caveolae and AQP-1, and an increase in lipid rafts are observed. The interpretation of this response is that it favors oxygen diffusion and hinder trans-cellular water fluxes. In hydraulic edema, which generates greater capillary water leakages, an increase in cell volume and opposite changes in membrane rafts were observed; further, the remarkable increase in caveolae suggests a potential abluminal-luminal vesicular-dependent fluid reabsorption.

The impact of heterogeneity of the air-blood barrier on control of lung extravascular water and alveolar gas exchange

The architecture of the air-blood barrier is effective in optimizing the gas exchange as long as it retains its specific feature of extreme thinness reflecting, in turn, a strict control on the extravascular water to be kept at minimum. Edemagenic conditions may perturb this equilibrium by increasing microvascular filtration; this characteristically occurs when cardiac output increases to balance the oxygen uptake with the oxygen requirement such as in exercise and hypoxia (either due to low ambient pressure or reflecting a pathological condition). In general, the lung is well equipped to counteract an increase in microvascular filtration rate. The loss of control on fluid balance is the consequence of disruption of the integrity of the macromolecular structure of lung tissue. This review, merging data from experimental approaches and evidence in humans, will explore how the heterogeneity in morphology, mechanical features and perfusion of the terminal respiratory units might impact on lung fluid balance and its control. Evidence is also provided that heterogeneities may be inborn and they could actually get worse as a consequence of a developing pathological process. Further, data are presented how in humans inter-individual heterogeneities in morphology of the terminal respiratory hinder the control of fluid balance and, in turn, hamper the efficiency of the oxygen diffusion-transport function.

Integrative Computational Models of Lung Structure-Function Interactions

Anatomically based integrative models of the lung and their interaction with other key components of the respiratory system provide unique capabilities for investigating both normal and abnormal lung function. There is substantial regional variability in both structure and function within the normal lung, yet it remains capable of relatively efficient gas exchange by providing close matching of air delivery (ventilation) and blood delivery (perfusion) to regions of gas exchange tissue from the scale of the whole organ to the smallest continuous gas exchange units. This is despite remarkably different mechanisms of air and blood delivery, different fluid properties, and unique scale-dependent anatomical structures through which the blood and air are transported. This inherent heterogeneity can be exacerbated in the presence of disease or when the body is under stress. Current computational power and data availability allow for the construction of sophisticated data-driven integrative models that can mimic respiratory system structure, function, and response to intervention. Computational models do not have the same technical and ethical issues that can limit experimental studies and biomedical imaging, and if they are solidly grounded in physiology and physics they facilitate investigation of the underlying interaction between mechanisms that determine respiratory function and dysfunction, and to estimate otherwise difficult-to-access measures. © 2021 American Physiological Society. Compr Physiol 11:1501-1530, 2021.

Real-time effects of PEEP and tidal volume on regional ventilation and perfusion in experimental lung injury

BACKGROUND

Real-time bedside information on regional ventilation and perfusion during mechanical ventilation (MV) may help to elucidate the physiological and pathophysiological effects of MV settings in healthy and injured lungs. We aimed to study the effects of positive end-expiratory pressure (PEEP) and tidal volume (V_T) on the distributions of regional ventilation and perfusion by electrical impedance tomography (EIT) in healthy and injured lungs.

METHODS

One-hit acute lung injury model was established in 6 piglets by repeated lung lavages (injured group). Four ventilated piglets served as the control group. A randomized sequence of any possible combination of three V_T (7, 10, and 15 ml/kg) and four levels of PEEP (5, 8, 10, and 12 cmH₂O) was performed in all animals. Ventilation and perfusion distributions were computed by EIT within three regions-of-interest (ROIs): nondependent, middle, dependent. A mixed design with one between-subjects factor (group: intervention or control), and two within-subjects factors (PEEP and V_T) was used, with a three-way mixed analysis of variance (ANOVA).

RESULTS

Two-way interactions between PEEP and group, and V_T and group, were observed for the dependent ROI (p = 0.035 and 0.012, respectively), indicating that the increase in the dependent ROI ventilation was greater at higher PEEP and V_T in the injured group than in the control group. A two-way interaction between PEEP and V_T was observed for perfusion distribution in each ROI: nondependent (p = 0.030), middle (p = 0.006), and dependent (p = 0.001); no interaction was observed between injured and control groups.

CONCLUSIONS

Large PEEP and V_T levels were associated with greater pulmonary ventilation of the dependent lung region in experimental lung injury, whereas they affected pulmonary perfusion of all lung regions both in the control and in the experimental lung injury groups.

Physiology for the pulmonary functional imager

As pulmonary functional imaging moves beyond the realm of the radiologist and physicist, it is important that imagers have a common language and understanding of the relevant physiology of the lung. This review will focus on key physiological concepts and pitfalls relevant to functional lung imaging.

Severe hypoxemia during carinal resection in the lateral position under one-lung ventilation of a non-dependent lung: a case report

During one-lung ventilation (OLV) in the lateral position, the dependent, ventilated lung receives more blood flow than the non-dependent, non-ventilated lung owing to gravity, improving the match of ventilation and perfusion. Conversely, in the rare clinical situations when OLV is applied to the non-dependent lung, arterial oxygenation can get worse due to considerable shunt flow to the dependent non-ventilated lung. We report a case of severe hypoxemia during carinal resection under OLV of a non-dependent lung. In this case, OLV had to be applied to the non-dependent lung in the lateral position because the bronchus of the non-dependent lung was anastomosed with the trachea, whereas the bronchus of the dependent lung had already been resected for carinal resection. The subsequent hypoxemia resulting from the shunt flow to the dependent non-ventilated lung was treated successfully by ligating the pulmonary artery of the dependent lung.

Lung Circulation

The circulation of the lung is unique both in volume and function. For example, it is the only organ with two circulations: the pulmonary circulation, the main function of which is gas exchange, and the bronchial circulation, a systemic vascular supply that provides oxygenated blood to the walls of the conducting airways, pulmonary arteries and veins. The pulmonary circulation accommodates the entire cardiac output, maintaining high blood flow at low intravascular arterial pressure. As compared with the systemic circulation, pulmonary arteries have thinner walls with much less vascular smooth muscle and a relative lack of basal tone. Factors controlling pulmonary blood flow include vascular structure, gravity, mechanical effects of breathing, and the influence of neural and humoral factors. Pulmonary vascular tone is also altered by hypoxia, which causes pulmonary vasoconstriction. If the hypoxic stimulus persists for a prolonged period, contraction is accompanied by remodeling of the vasculature, resulting in pulmonary hypertension. In addition, genetic and environmental factors can also confer susceptibility to development of pulmonary hypertension. Under normal conditions, the endothelium forms a tight barrier, actively regulating interstitial fluid homeostasis. Infection and inflammation compromise normal barrier homeostasis, resulting in increased permeability and edema formation. This article focuses on reviewing the basics of the lung circulation (pulmonary and bronchial), normal development and transition at birth and vasoregulation. Mechanisms contributing to pathological conditions in the pulmonary circulation, in particular when barrier function is disrupted and during development of pulmonary hypertension, will also be discussed.