Correlation between ventilation and perfusion determines VA/Q heterogeneity in endotoxemia

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.

Ventilation/Perfusion Relationships and Gas Exchange: Measurement Approaches

Ventilation-perfusion ( V ˙ A / Q ˙ ) matching, the regional matching of the flow of fresh gas to flow of deoxygenated capillary blood, is the most important mechanism affecting the efficiency of pulmonary gas exchange. This article discusses the measurement of V ˙ A / Q ˙ matching with three broad classes of techniques: (i) those based in gas exchange, such as the multiple inert gas elimination technique (MIGET); (ii) those derived from imaging techniques such as single-photon emission computed tomography (SPECT), positron emission tomography (PET), magnetic resonance imaging (MRI), computed tomography (CT), and electrical impedance tomography (EIT); and (iii) fluorescent and radiolabeled microspheres. The focus is on the physiological basis of these techniques that provide quantitative information for research purposes rather than qualitative measurements that are used clinically. The fundamental equations of pulmonary gas exchange are first reviewed to lay the foundation for the gas exchange techniques and some of the imaging applications. The physiological considerations for each of the techniques along with advantages and disadvantages are briefly discussed. © 2020 American Physiological Society. Compr Physiol 10:1155-1205, 2020.

Pulmonary blood flow increases in damaged regions directly after acid aspiration in rats

BACKGROUND

After gastric aspiration events, patients are at risk of pulmonary dysfunction and the development of severe acute lung injury and acute respiratory distress syndrome, which may contribute to the development of an inflammatory reaction. The authors' aim in the current study was to investigate the role of the spatial distribution of pulmonary blood flow in the pathogenesis of pulmonary dysfunction during the early stages after acid aspiration.

METHODS

The authors analyzed the pulmonary distribution of radiolabeled microspheres in normal (n = 6) and injured (n = 12) anesthetized rat lungs using positron emission tomography, computed tomography, and histological examination.

RESULTS

Injured regions demonstrate increased pulmonary blood flow in association with reduced arterial pressure and the deterioration of arterial oxygenation. After acid aspiration, computed tomography scans revealed that lung density had increased in the injured regions and that these regions colocalized with areas of increased blood flow. The acid was instilled into the middle and basal regions of the lungs. The blood flow was significantly increased to these regions compared with the blood flow to uninjured lungs in the control animals (middle region: 1.23 [1.1; 1.4] (median [25%; 75%]) vs. 1.04 [1.0; 1.1] and basal region: 1.25 [1.2; 1.3] vs. 1.02 [1.0; 1.05], respectively). The increase in blood flow did not seem to be due to vascular leakage into these injured areas.

CONCLUSIONS

The data suggest that 10 min after acid aspiration, damaged areas are characterized by increased pulmonary blood flow. The results may impact further treatment strategies, such as drug targeting.

Nitric oxide transport in blood: a third gas in the respiratory cycle

The trapping, processing, and delivery of nitric oxide (NO) bioactivity by red blood cells (RBCs) have emerged as a conserved mechanism through which regional blood flow is linked to biochemical cues of perfusion sufficiency. We present here an expanded paradigm for the human respiratory cycle based on the coordinated transport of three gases: NO, O₂, and CO₂. By linking O₂ and NO flux, RBCs couple vessel caliber (and thus blood flow) to O₂ availability in the lung and to O₂ need in the periphery. The elements required for regulated O₂-based signal transduction via controlled NO processing within RBCs are presented herein, including S-nitrosothiol (SNO) synthesis by hemoglobin and O₂-regulated delivery of NO bioactivity (capture, activation, and delivery of NO groups at sites remote from NO synthesis by NO synthase). The role of NO transport in the respiratory cycle at molecular, microcirculatory, and system levels is reviewed. We elucidate the mechanism through which regulated NO transport in blood supports O₂ homeostasis, not only through adaptive regulation of regional systemic blood flow but also by optimizing ventilation-perfusion matching in the lung. Furthermore, we discuss the role of NO transport in the central control of breathing and in baroreceptor control of blood pressure, which subserve O₂ supply to tissue. Additionally, malfunctions of this transport and signaling system that are implicated in a wide array of human pathophysiologies are described. Understanding the (dys)function of NO processing in blood is a prerequisite for the development of novel therapies that target the vasoactive capacities of RBCs.

Electrical impedance tomography and heterogeneity of pulmonary perfusion and ventilation in porcine acute lung injury

BACKGROUND

The heterogeneity of pulmonary ventilation (V), perfusion (Q) and V/Q matching impairs gas exchange in an acute lung injury (ALI). This study investigated the feasibility of electrical impedance tomography (EIT) to assess the V/Q distribution and matching during an endotoxinaemic ALI in pigs.

METHODS

Mechanically ventilated, anaesthetised pigs (n=11, weight 30-36 kg) were studied during an infusion of endotoxin for 150 min. Impedance changes related to ventilation (Z(V)) and perfusion (Z(Q)) were monitored globally and bilaterally in four regions of interest (ROIs) of the EIT image. The distribution and ratio of Z(V) and Z(Q) were assessed. The alveolar-arterial oxygen difference, venous admixture, fractional alveolar dead space and functional residual capacity (FRC) were recorded, together with global and regional lung compliances and haemodynamic parameters. Values are mean+/-standard deviation (SD) and regression coefficients.

RESULTS

Endotoxinaemia increased the heterogeneity of Z(Q) but not Z(V). Lung compliance progressively decreased with a ventral redistribution of Z(V). A concomitant dorsal redistribution of Z(Q) resulted in mismatch of global (from Z(V)/Z(Q) 1.1+/-0.1 to 0.83+/-0.3) and notably dorsal (from Z(V)/Z(Q) 0.86+/-0.4 to 0.51+/-0.3) V and Q. Changes in global Z(V)/Z(Q) correlated with changes in the alveolar-arterial oxygen difference (r(2)=0.65, P<0.05), venous admixture (r(2)=0.66, P<0.05) and fractional alveolar dead space (r(2)=0.61, P<0.05). Decreased end-expiratory Z(V) correlated with decreased FRC (r(2)=0.74, P<0.05).

CONCLUSIONS

EIT can be used to assess the heterogeneity of regional pulmonary ventilation and perfusion and V/Q matching during endotoxinaemic ALI, identifying pivotal pathophysiological changes.

Hemodynamics in experimental gastric juice induced aspiration pneumonitis

OBJECTIVE

To characterize hemodynamic changes during experimental aspiration pneumonitis, paying special attention to echocardiographic assessment.

DESIGN AND SETTING

Animal study in a university-based research laboratory.

SUBJECTS

Fourteen mechanically ventilated New Zealand white rabbits

INTERVENTIONS

We instilled 1 ml/kg human gastric juice (mean pH: 4.1+0.2) intratracheally. Hemodynamic and respiratory parameters were measured every hour for 4 h, associated with a transthoracic echocardiography.

MEASUREMENTS AND RESULTS

Lung injury occurred within 1 hour with a marked decrease in PaO(2)/FIO(2) and an increase in plateau pressure; after this initial drop the ratio remained stable throughout the experiment. Seven rabbits experienced only a mild to moderate alteration in lung oxygenation function as defined by a PaO(2)/FIO(2) ratio above 200 (group A), while the other seven developed a severe alteration with a ratio below 200 (group B). At the end of the experiment pH and cardiac output were lower in group B than in group A. Using a PaO(2)/FIO(2) threshold value of 150, pH, mean arterial pressure, and cardiac output were lower in the animals with the more severe hypoxemia. Neither left nor right ventricular dysfunction occurred during the experiment, and no animal experienced circulatory failure

CONCLUSION

Experimental aspiration pneumonitis after intratracheal infusion of human gastric juice is characterized by a stable fall in PaO(2)/FIO(2). Animals with the most severe lung injury experienced a lower systemic arterial pressure, cardiac output, and metabolic acidosis without circulatory failure or cardiac dysfunction.

Microsphere maps of regional blood flow and regional ventilation

Systematically mapped samples cut from lungs previously labeled with intravascular and aerosol microspheres can be used to create high-resolution maps of regional perfusion and regional ventilation. With multiple radioactive or fluorescent microsphere labels available, this methodology can compare regional flow responses to different interventions without partial volume effects or registration errors that complicate interpretation of in vivo imaging measurements. Microsphere blood flow maps examined at different levels of spatial resolution have revealed that regional flow heterogeneity increases progressively down to an acinar level of scale. This pattern of scale-dependent heterogeneity is characteristic of a fractal distribution network, and it suggests that the anatomic configuration of the pulmonary vascular tree is the primary determinant of high-resolution regional flow heterogeneity. At approximately 2-cm(3) resolution, the large-scale gravitational gradients of blood flow per unit weight of alveolar tissue account for <5% of the overall flow heterogeneity. Furthermore, regional blood flow per gram of alveolar tissue remains relatively constant with different body positions, gravitational stresses, and exercise. Regional alveolar ventilation is accurately represented by the deposition of inhaled 1.0-microm fluorescent microsphere aerosols, at least down to the approximately 2-cm(3) level of scale. Analysis of these ventilation maps has revealed the same scale-dependent property of regional alveolar ventilation heterogeneity, with a strong correlation between ventilation and blood flow maintained at all levels of scale. The ventilation-perfusion (VA/Q) distributions obtained from microsphere flow maps of normal animals agree with simultaneously acquired multiple inert-gas elimination technique VA/Q distributions, but they underestimate gas-exchange impairment in diffuse lung injury.

Impact of lung volume on MR signal intensity changes of the lung parenchyma

PURPOSE

To test the hypothesis that, in magnetic resonance (MR) imaging of healthy individuals, equal relative changes in lung volume cause equal relative changes in MR signal intensity of the lung parenchyma.

MATERIALS AND METHODS

In two experimental runs, 10 volunteers underwent spirometrically monitored MR imaging of the lungs, with MR images acquired at 10 incremental lung volumes ranging from total lung capacity to 10% above residual volume. Average signal intensity, signal variability, and signal intensity integrals were calculated for each volunteer and for each lung volume. The effect of lung volume on signal intensity was quantified using linear regression analysis complemented by the runs test. Slopes and intercepts of regression lines were compared with an analysis of covariance. Slopes of the lines of best fit for lung volumes and signal intensities from the two runs were compared to the slope of the line of identity. Comparisons between the two runs were visualized using Bland and Altman plots.

RESULTS

The slopes of the 10 individual regression lines yielded no significant differences (F = 1.703, P = 0.101; F = 1.321, P = 0.239). The common slopes were -0.556 +/- 0.027 (P = 0.0001) for the first and -0.597 +/- 0.0031 (P = 0.0001) for the second experimental run. Both slopes displayed no significant nonlinearity (P = 0.419 and P = 0.067). There was a strong association between changes in lung volumes (rs = 0.991, P = 0.0001) and changes in signal intensity (rs = 0.889, P = 0.0001) in the two experimental runs. Lines of best fit for lung volume and signal intensities were not significantly different from the slope of the line of identity (P = 0.321 and P = 0.212, respectively).

CONCLUSION

Equal changes in lung volume cause equal changes in MR signal intensity of the lung parenchyma. This linear and reproducible phenomenon could be helpful in comparing pulmonary MR signal intensity between individuals.

Effect of posture on regional gas exchange in pigs

Although recent high-resolution studies demonstrate the importance of nongravitational determinants for both pulmonary blood flow and ventilation distributions, posture has a clear impact on whole lung gas exchange. Deterioration in arterial oxygenation with repositioning from prone to supine posture is caused by increased heterogeneity in the distribution of ventilation-to-perfusion ratios. This can result from increased heterogeneity in regional blood flow distribution, increased heterogeneity in regional ventilation distribution, decreased correlation between regional blood flow and ventilation, or some combination of the above (Wilson TA and Beck KC, J Appl Physiol 72: 2298-2304, 1992). We hypothesize that, although repositioning from prone to supine has relatively small effects on overall blood flow and ventilation distributions, regional changes are poorly correlated, resulting in regional ventilation-perfusion mismatch and reduction in alveolar oxygen tension. We report ventilation and perfusion distributions in seven anesthetized, mechanically ventilated pigs measured with aerosolized and injected microspheres. Total contributions of pulmonary structure and posture on ventilation and perfusion heterogeneities were quantified by using analysis of variance. Regional gradients of posture-mediated change in ventilation, perfusion, and calculated alveolar oxygen tension were examined in the caudocranial and ventrodorsal directions. We found that pulmonary structure was responsible for 74.0 +/- 4.7% of total ventilation heterogeneity and 63.3 +/- 4.2% of total blood flow heterogeneity. Posture-mediated redistribution was primarily oriented along the caudocranial axis for ventilation and along the ventrodorsal axis for blood flow. These mismatched changes reduced alveolar oxygen tension primarily in the dorsocaudal lung region.

PKQuest: a general physiologically based pharmacokinetic model. Introduction and application to propranolol

BACKGROUND

A "physiologically based pharmacokinetic" (PBPK) approach uses a realistic model of the animal to describe the pharmacokinetics. Previous PBPKs have been designed for specific solutes, required specification of a large number of parameters and have not been designed for general use.

METHODS

This new PBPK program (PKQuest) includes a "Standardhuman" and "Standardrat" data set so that the user input is minimized. It has a simple user interface, graphical output and many new features: 1) An option that uses the measured plasma concentrations to solve for the time course of the gastrointestinal, intramuscular, intraperotineal or skin absorption and systemic availability of a drug - for a general non-linear system. 2) Capillary permeability limitation defined in terms of the permeability-surface area products. 4) Saturable plasma and tissue protein binding. 5) A lung model that includes perfusion-ventilation mismatch. 6) A general optimization routine using either a global (simulated annealing) or local (Powell) minimization applicable to all model parameters.

RESULTS

PKQuest was applied to measurements of human propranolol pharmacokinetics and intestinal absorption. A meal has two effects: 1) increases portal blood flow by 50%; and 2) decreases liver metabolism by 20%. There is a significant delay in the oval propranolol absorption in fasting subjects that is absent in fed subjects. The oral absorption of the long acting form of propranolol continues for a period of more than 24 hours.

CONCLUSIONS

PKQuest provides a new general purpose, easy to use, freely distributed www.pkquest.com and physiologically rigorous PBPK software routine.

PKQuest: volatile solutes - application to enflurane, nitrous oxide, halothane, methoxyflurane and toluene pharmacokinetics

BACKGROUND: The application of physiologically based pharmacokinetic models (PBPK) to human studies has been limited by the lack of the detailed organ information that is required for this analysis. PKQuest is a new generic PBPK that is designed to avoid this problem by using a set of "standard human" default parameters that are applicable to most solutes. RESULTS: PKQuest is used to model the human pharmacokinetics of the volatile solutes. A "standard human" value for the lipid content of the blood and each organ (klip) was chosen. This set of klip and the oil/water partition coefficient then specifies the organ/blood partition for each organ. Using this approach, the pharmacokinetics of inert volatile solute is completely specified by just 2 parameters: the water/air and oil/water partition coefficients. The model predictions of PKQuest were in good agreement with the experimental data for the inert solutes enflurane and nitrous oxide and the metabolized solutes halothane and toluene. METHODS: The experimental data that was modeled was taken from previous publications. CONCLUSIONS: This approach greatly increases the predictive power of the PBPK. For inert volatile solutes the pharmacokinetics are determined just from the water/air and oil/water partition coefficient. Methoxyflurane cannot be modeled by this PBPK because the arterial and end tidal partial pressures are not equal (as assumed in the PBPK). This inequality results from the "washin-washout" artifact in the large airways that is established for solutes with large water/air partition coefficients.PKQuest and the worked examples are available on the web www.pkquest.com.

Spatial distribution of ventilation and perfusion in anesthetized dogs in lateral postures

We aimed to assess the influence of lateral decubitus postures and positive end-expiratory pressure (PEEP) on the regional distribution of ventilation and perfusion. We measured regional ventilation (VA) and regional blood flow (Q) in six anesthetized, mechanically ventilated dogs in the left (LLD) and right lateral decubitus (RLD) postures with and without 10 cmH(2)O PEEP. Q was measured by use of intravenously injected 15-microm fluorescent microspheres, and VA was measured by aerosolized 1-microm fluorescent microspheres. Fluorescence was analyzed in lung pieces approximately 1.7 cm(3) in volume. Multiple linear regression analysis was used to evaluate three-dimensional spatial gradients of Q, VA, the ratio VA/Q, and regional PO(2) (Pr(O(2))) in both lungs. In the LLD posture, a gravity-dependent vertical gradient in Q was observed in both lungs in conjunction with a reduced blood flow and Pr(O(2)) to the dependent left lung. Change from the LLD to the RLD or 10 cmH(2)O PEEP increased local VA/Q and Pr(O(2)) in the left lung and minimized any role of hypoxia. The greatest reduction in individual lung volume occurred to the left lung in the LLD posture. We conclude that lung distortion caused by the weight of the heart and abdomen is greater in the LLD posture and influences both Q and VA, and ultimately gas exchange. In this respect, the smaller left lung was the most susceptible to impaired gas exchange in the LLD posture.

Lung albumin accumulation is spatially heterogeneous but not correlated with regional pulmonary perfusion

The contribution of pulmonary perfusion heterogeneity to the development of regional differences in lung injury and edema is unknown. To test whether regional differences in pulmonary perfusion are associated with regional differences in microvascular function during lung injury, pigs were mechanically ventilated in the prone position and infused with endotoxin (Escherichia coli 055:B5, 0.15 microg. kg(-1). h(-1); n = 8) or saline (n = 4) for 4 h. Extravascular albumin accumulation and perfusion were measured in multiple approximately 0.7-ml lung regions by injecting pigs with radiolabeled albumin and radioactive microspheres, respectively. Extravascular albumin accumulation was spatially heterogeneous but not correlated with regional perfusion. Extravascular albumin accumulation was greater in dorsal than ventral regions, and regions with similar albumin accumulation were spatially clustered. This spatial organization was less evident in endotoxemic than control pigs. We conclude that there are regional differences in lung albumin accumulation that are spatially organized but not mediated by regional differences in pulmonary perfusion. We speculate that regional differences in microvascular pressure or endothelial function may account for the observed distribution of extravascular albumin accumulation.

Endotoxemia increases relative perfusion to dorsal-caudal lung regions

Changes in the spatial distribution of perfusion during acute lung injury and their impact on gas exchange are poorly understood. We tested whether endotoxemia caused topographical differences in perfusion and whether these differences caused meaningful changes in regional ventilation-to-perfusion ratios and gas exchange. Regional ventilation and perfusion were measured in anesthetized, mechanically ventilated pigs in the prone position before and during endotoxemia with the use of aerosolized and intravenous fluorescent microspheres. On average, relative perfusion halved in ventral and cranial lung regions, doubled in caudal lung regions, and increased 1.5-fold in dorsal lung regions during endotoxemia. In contrast, there were no topographical differences in perfusion before endotoxemia and no topographical differences in ventilation at any time point. Consequently, endotoxemia increased regional ventilation-to-perfusion ratios in the caudal-to-cranial and dorsal-to-ventral directions, resulting in end-capillary PO2 values that were significantly lower in dorsal-caudal than ventral-cranial regions. We conclude that there are topographical differences in the pulmonary vascular response to endotoxin that may have important consequences for gas exchange in acute lung injury.