Distribution of pulmonary capillary transit times in recruited networks

When pulmonary blood flow is elevated, hypoxemia can occur in the fastest-moving erythrocytes if their transit times through the capillaries fall below the minimum time for complete oxygenation. This desaturation is more likely to occur if the distribution of capillary transit times about the mean is large. Increasing cardiac output is known to decrease mean pulmonary capillary transit time, but the effect on the distribution of transit times has not been reported. We measured the mean and variance of transit times in single pulmonary capillary networks in the dependent lung of anesthetized dogs by in vivo videofluorescence microscopy of a fluorescein dye bolus passing from an arteriole to a venule. When cardiac output increased from 2.9 to 9.9 l/min, mean capillary transit time decreased from 2.0 to 0.8 s. Because transit time variance decreased proportionately (relative dispersion remained constant), increasing cardiac output did not alter the heterogeneity of local capillary transit times in the lower lung where the capillary bed was nearly fully recruited.     

Transit time dispersion in the pulmonary arterial tree

Knowledge of the contributions of arterialand venous transit time dispersion to the pulmonary vascular transittime distribution is important for understanding lung function and forinterpreting various kinds of data containing information aboutpulmonary function. Thus, to determine the dispersion of blood transittimes occurring within the pulmonary arterial and venous trees, imagesof a bolus of contrast medium passing through the vasculature ofpump-perfused dog lung lobes were acquired by using an X-ray microfocalangiography system. Time-absorbance curves from the lobar artery andvein and from selected locations within the intrapulmonary arterial tree were measured from the images. Overall dispersion within the lunglobe was determined from the difference in the first and second moments(mean transit time and variance, respectively) of the inlet arterialand outlet venous time-absorbance curves. Moments at selected locationswithin the arterial tree were also calculated and compared with thoseof the lobar artery curve. Transit times for the arterial pathwaysupstream from the smallest measured arteries (200-µm diameter) wereless than ~20% of the total lung lobe mean transit time. Transittime variance among these arterial pathways (interpathway dispersion)was less than ~5% of the total variance imparted on the bolus as itpassed through the lung lobe. On average, the dispersion that occurredalong a given pathway (intrapathway dispersion) was negligible. Similar results were obtained for the venous tree. Taken together, the resultssuggest that most of the variation in transit time in theintrapulmonary vasculature occurs within the pulmonary capillary bedrather than in conducting arteries or veins.

     

Effects of lung inflation and blood flow on capillary transit time in isolated rabbit lungs.

In a previous study, direct measurements of pulmonary capillary transit time by fluorescence video microscopy in anesthetized rabbits showed that chest inflation increased capillary transit time and decreased cardiac output. In isolated perfused rabbit lungs we measured the effect of lung volume, left atrial pressure (Pla), and blood flow on capillary transit time. At constant blood flow and constant transpulmonary pressure, a bolus of fluorescent dye was injected into the pulmonary artery and the passage of the dye through the subpleural microcirculation was recorded via the video microscope on videotape. During playback of the video signals, the light emitted from an arteriole and adjacent venule was measured using a video photoanalyzer. Capillary transit time was the difference between the mean time values of the arteriolar and venular dye dilution curves. We measured capillary transit time in three groups of lungs. In group 1, with airway pressure (Paw) at 5 cmH2O, transit time was measured at blood flow of approximately 80, approximately 40, and approximately 20 ml.min-1.kg-1. At each blood flow level, Pla was varied from 0 (Pla less than Paw, zone 2) to 11 cmH2O (Pla greater than Paw, zone 3). In group 2, at constant Paw of 15 cmH2O, Pla was varied from 0 (zone 2) to 22 cmH2O (zone 3) at the same three blood flow levels. In group 3, at each of the three blood flow levels, Paw was varied from 5 to 15 cmH2O while Pla was maintained at 0 cmH2O (zone 2).(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of positive airway pressure on capillary transit time in rabbit lung

We used fluorescence videomicroscopy to measure the passage of fluorescent dye through the subpleural microcirculation of the lung. With the rabbit in the left lateral decubitus position, the subpleural microcirculation was viewed either through a transparent parietal pleural window located in the superior part of the chest or directly with the chest open. There was no physical contact with the chest or lung. The rabbit was anesthetized, paralyzed, and mechanically ventilated with 100% O2. The dye was injected into the right ventricle during a 2-min apneic period to eliminate lung movement due to ventilation. The video signal of the passage of the dye was analyzed frame by frame by use of digital image processing to compensate for cardiogenic oscillations of the lung surface. Gray scale levels of an arteriole and adjacent venule were measured every 1/30 s. Capillary transit time was determined from the difference between the concentration-weighted mean time values of the arteriolar and venular dye dilution curves. We studied the effect of airway pressure (0-20 cmH2O) on transit time. Cardiac output was measured at different airway pressures by the thermal dilution technique. Capillary transit time averaged 0.60 s at functional residual capacity. Right ventricular-to-arteriolar transit time was four times as large as the capillary transit time. An increase in airway pressure from 0-5 to 20 cmH2O resulted in a fourfold increase in both capillary and arterial transit times and a threefold decrease in cardiac output.     

结缔组织生长因子在肺纤维化初期肺动脉中的表达

本研究观察了博莱霉素(bleomycin,BLM)诱导肺纤维化初期肺动脉压、肺动脉壁Ⅰ、Ⅲ型胶原的含量以及肺动脉壁结缔组织生长因子(connective tissue growth factor,CTGF)免疫阳性表达和分布.用气管内一次性滴注BLM(5 mg/kg体重)的方法复制肺纤维化动物模型大鼠;用右心漂浮导管技术检测肺动脉压;用天狼星红胶原纤维特异染色和偏振光观察肺动脉Ⅰ、Ⅲ型胶原;用免疫组织化学法检测肺动脉壁CTGF表达.结果显示:滴注BLM后第14天,大鼠肺动脉压高于对照组大鼠(P<0.05);肺动脉主干和肺内动脉壁Ⅰ、Ⅲ型胶原的染色面积大于对照组大鼠(P<0.05,P<0.01),肺动脉主干血管壁Ⅰ、Ⅲ型胶原染色面积的比值高于对照组大鼠(P<0.05);肺动脉主干和肺内动脉壁CTGF免疫染色面积均大于对照组大鼠,平均光密度也高于对照组大鼠(均P<0.05);增多的CTGF免疫阳性细胞主要分布在肺动脉的平滑肌层和内皮层.以上结果表明,在BLM致肺纤维化形成初期肺动脉高压和肺血管壁结构重塑过程中,肺动脉壁平滑肌层和内皮层CTGF表达增多,这可能是肺动脉高压维持和发展的机制之一.     

Comparison of direct and indirect measurements of pulmonary capillary transit times

Using in vivo microscopy, we made direct measurements of pulmonary capillary transit time by determining the time required for fluorescent dye to pass from an arteriole to a venule on the dependent surface of the dog lung. Concurrently, in the same animals, pulmonary capillary transit time was measured indirectly in the entire lung using the diffusing capacity method (capillary blood volume divided by cardiac output). Transit times by each method were the same in a group of five dogs [direct: 1.75 +/- 0.27 (SE) s; indirect: 1.85 +/- 0.33 s; P = 0.7]. The similarity of these transit times is important, because the widely used indirect determinations based on diffusing capacity are now shown to coincide with direct measurements and also because it demonstrates that measurements of capillary transit times on the surface of the dependent lung bear a useful relationship to measurements on the capillaries in the rest of the lung.     

Transit time dispersion in pulmonary and systemic circulation: effects of cardiac output and solute diffusivity

We present an in vivo method for analyzing the distribution kinetics of physiological markers into their respective distribution volumes utilizing information provided by the relative dispersion of transit times. Arterial concentration-time curves of markers of the vascular space [indocyanine green (ICG)], extracellular fluid (inulin), and total body water (antipyrine) measured in awake dogs under control conditions and during phenylephrine or isoproterenol infusion were analyzed by a recirculatory model to estimate the relative dispersions of transit times across the systemic and pulmonary circulation. The transit time dispersion in the systemic circulation was used to calculate the whole body distribution clearance, and an interpretation is given in terms of a lumped organ model of blood-tissue exchange. As predicted by theory, this relative dispersion increased linearly with cardiac output, with a slope that was inversely related to solute diffusivity. The relative dispersion of the flow-limited indicator antipyrine exceeded that of ICG (as a measure of intravascular mixing) only slightly and was consistent with a diffusional equilibration time in the extravascular space of approximately 10 min, except during phenylephrine infusion, which led to an anomalously high relative dispersion. A change in cardiac output did not alter the heterogeneity of capillary transit times of ICG. The results support the view that the relative dispersions of transit times in the systemic and pulmonary circulation estimated from solute disposition data in vivo are useful measures of whole body distribution kinetics of indicators and endogenous substances. This is the first model that explains the effect of flow and capillary permeability on whole body distribution of solutes without assuming well-mixed compartments.     

Microvascular pressure profile in intact in situ lung.

We measured the microvascular pressure profile in lungs physiologically expanded in the pleural space at functional residual capacity. In 29 anesthetized rabbits a caudal intercostal space was cleared of its external and internal muscles. A small area of endothoracic fascia was surgically thinned, exposing the parietal pleura through which pulmonary vessels were clearly detectable under stereomicroscopic view. Pulmonary microvascular pressure was measured with glass micropipettes connected to a servo-null system. During the pressure measurements the animal was kept apneic and 50% humidified oxygen was delivered in the trachea. Pulmonary arterial and left atrial pressures were 22.3 +/- 1.5 and 1.6 +/- 1.5 (SD) cmH2O, respectively. The segmental pulmonary vascular pressure drop expressed as a percentage of the pulmonary arterial to left atrial pressure was approximately 33% from pulmonary artery to approximately 130-microns-diam arterioles, 4.5% from approximately 130- to approximately 60-microns-diam arterioles, approximately 46% from approximately 60-microns-diam arterioles to approximately 30-microns-diam venules, approximately 9.5% from 30- to 150-microns-diam venules, and approximately 7% for the remaining venous segment. Pulmonary capillary pressure was estimated at approximately 9 cmH2O.     

Physiological neutrophil sequestration in the lung: visual evidence for localization in capillaries

Although the lung is known to be a major site of neutrophil margination, the anatomic location of these sequestered cells within the lung is controversial. To determine the site of margination and the kinetics of neutrophil transit through the pulmonary microvasculature, we infused fluorescein isothiocyanate-labeled canine neutrophils into the pulmonary arteries of 10 anesthetized normal dogs and made fluorescence videomicroscopic observations of the subpleural pulmonary microcirculation through a window inserted into the chest wall. The site of fluorescent neutrophil sequestration was exclusively in the pulmonary capillaries with a total of 951 labeled cells impeded in the capillary bed for a minimum of 2 s. No cells were delayed in the arterioles or venules. Transit times of individual neutrophils varied over a wide range from less than 2 s to greater than 20 min with an exponential distribution skewed toward rapid transit times. These observations indicate that neutrophil margination occurs in the pulmonary capillaries with neutrophils impeded for variable periods of time on each pass through the lung. The resulting wide distribution of transit times may determine the dynamic equilibrium between circulating and marginated neutrophils.     

Characteristics of the pulmonary transport functions for heat and dye in pulmonary edema and orthostasis

The aim of this study was to investigate whether changes in the distribution of pulmonary blood flow and disturbances of the pulmonary microcirculation can be detected by use of inflow-outflow indicator-dilution measurements. In 18 anesthetized (N2O-piritramide) mongrel dogs 221 thermal-indocyanine green dye indicator dilution kinetics were recorded in the pulmonary artery and aorta after central venous indicator injection. The lagged normal density function was used as a model for the pulmonary transport functions for heat and dye. The parameters of the lagged normal density function were computed by a non-linear least squares procedure by iterative convolution. After baseline measurements, in nine dogs, pulmonary edema was induced by central venous application of oleic acid. In nine other dogs, measurements were performed before and after postural changes. Our data show that both the microvascular injury caused by oleic acid edema and the perfusion heterogeneity caused by orthostasis can be detected by the indicator dilution technique since the both relative dispersion and skewness of the transport functions for heat and dye were significantly increased after these interventions.     

Computational modeling of RBC and neutrophil transit through the pulmonary capillaries.

A computational model of the pulmonary microcirculation is developed and used to examine blood flow from arteriole to venule through a realistically complex alveolar capillary bed. Distributions of flow, hematocrit, and pressure are presented, showing the existence of preferential pathways through the system and of large segment-to-segment differences in all parameters, confirming and extending previous work. Red blood cell (RBC) and neutrophil transit are also analyzed, the latter drawing from previous studies of leukocyte aspiration into micropipettes. Transit time distributions are in good agreement with in vivo experiments, in particular showing that neutrophils are dramatically slowed relative to the flow of RBCs because of the need to contract and elongate to fit through narrower capillaries. Predicted neutrophil transit times depend on how the effective capillary diameter is defined. Transient blockage by a neutrophil can increase the local pressure drop across a segment by 100--300%, leading to temporal variations in flow and pressure as seen by videomicroscopy. All of these effects are modulated by changes in transpulmonary pressure and arteriolar pressure, although RBCs, neutrophils, and rigid microspheres all behave differently.     

The reactivity of the skeletal muscle arterioles of rats to noradrenaline after whole-body gamma irradiation at a dose of 1 Gy]

Male Wistar rats were exposed by total gamma-irradiation at 1 Gy, Reactions of skeletal muscle arterioles and mean arterial pressure on intravenous doses of noradrenaline (0.1, 0.3, 1.0 and 3.0 micrograms/kg) was studied by intravital microscopy in acute experiments 1, 3 and 5-6 days after irradiation. The exposure causes arterial hypotension on day 1 after that as well as marked reduction of spontaneous arteriole vasomotions and decrease of arteriole constrictions at any doses and dates under study. There are no differences of arterial pressure reaction amplitudes in per cent between control and exposed animals at any doses and dates under investigation.     

Extravascular lung water in the exercising horse.

Seven Standardbred horses were exercised on a treadmill at speeds (approximately 12 m/s) producing maximal heart rate, hypoxemia, and a mean pulmonary arterial pressure of approximately 75 mmHg. Extravascular lung water was measured by using transients in temperature and electrical impedance of the blood caused by a bolus injection of cold saline solution. Lung water was approximately 3 ml/kg body wt when standing but did not increase significantly with exertion. We conclude that any increase in fluid extravasation from the pulmonary hypertension accumulates in the lung at a level that is less than that detectable by this method. At maximal exertion, the volume of blood measured between the jugular vein and the carotid artery increased by approximately 8 ml/kg, and the actively circulating component of the systemic blood volume increased by approximately 17 ml/kg with respect to corresponding values obtained when walking before exertion. These volume increases, reflecting recruitment and dilatation of capillaries, increase the area for respiratory gas exchange and offset the reduced transit times that would otherwise be imposed by the approximately eightfold increase in cardiac output at maximal exertion.     

A mathematical model of indicator extraction by the pulmonary endothelium via saturation kinetics

When a bolus containing a nonpermeating indicator and an indicator which permeates the endothelial cell membrane by a saturable process is injected into the blood flowing into the lung, the instantaneous extraction ratio curves measured in the pulmonary venous outflow are asymmetric with respect to the nonpermeating indicator curve. If the bolus contains a sufficient quantity of the permeating indicator that the capillary concentration begins to saturate the transfort mechanism, the extraction ratio curves are concave upward as well. The purpose of this study was to determine whether a mathematical model which represents endothelial extraction by Michaelis-Menten kinetics could explain the time variation in the instantaneous extraction ratio curves. The venous concentration curves were assumed to be the result of the endothelial transfort and distributed capillary input and transit times. In addition, we evaluated a method for estimating the kinetic parameters (K_m and V_max) of the saturable transfort process in such an organ. The results of simulations indicate that the important features of the data can be reproduced by the model, and that useful estimates of the kinetic parameters will be obtained from linear multiple regression analysis of the venous concentration curves if the standard deviation of the capillary input time distribution is not less than that of the capillary transit time distribution.     

Steep head-down tilt has persisting effects on the distribution of pulmonary blood flow.

Head-down tilt has been shown to increase lung water content in animals and alter the distribution of ventilation in humans; however, its effects on the distribution of pulmonary blood flow in humans are unknown. We hypothesized that head-down tilt would increase the heterogeneity of pulmonary blood flow in humans, an effect analogous to the changes seen in the distribution of ventilation, by increasing capillary hydrostatic pressure and fluid efflux in the lung. To test this, we evaluated changes in the distribution of pulmonary blood flow in seven normal subjects before and after 1 h of 30 degrees head-down tilt using the magnetic resonance imaging technique of arterial spin labeling. Data were acquired in triplicate before tilt and at 10-min intervals for 1 h after tilt. Pulmonary blood flow heterogeneity was quantified by the relative dispersion (standard deviation/mean) of signal intensity for all voxels within the right lung. Relative dispersion was significantly increased by 29% after tilt and remained elevated during the 1 h of measurements after tilt (0.84 +/- 0.06 pretilt, 1.09 +/- 0.09 calculated for all time points posttilt, P < 0.05). We speculate that the mechanism most likely responsible for our findings is that increased pulmonary capillary pressures and fluid efflux in the lung resulting from head-down tilt alters regional blood flow distribution.     

Effect of hypoxia on pulmonary blood flow-segmental vascular resistance relationship in perfused cat lungs

To investigate the effect of alveolar hypoxia onthe pulmonary blood flow-segmental vascular resistance relationship, wedetermined the longitudinal distribution of vascular resistance whileincreasing blood flow during hyperoxia or hypoxia in perfused catlungs. We measured microvascular pressures by the micropipetteservo-null method, partitioned the pulmonary vessels into threesegments [i.e., arterial (from main pulmonary artery to 30- to50-µm arterioles), venous (from 30- to 50-µm venules to leftatrium), and microvascular (between arterioles and venules)segments] and calculated segmental vascular resistance. Duringhyperoxia, total resistance decreased with increased blood flow becauseof a reduction of microvascular resistance. In contrast, duringhypoxia, not only microvascular resistance but also arterial resistancedecreased with increase of blood flow while venous resistance remainedunchanged. The reduction of arterial resistance was presumably causedby arterial distension induced by an elevated arterial pressure duringhypoxia. We conclude that, during hypoxia, both microvessels andarteries >50 µm in diameter play a role in preventing furtherincreases in total pulmonary vascular resistance with increased bloodflow.

     

Cellular disposition of transported polyamines in hypoxic rat lung and pulmonary arteries

The polyamines putrescine, spermidine (SPD), and spermine are a family of low-molecular-weight organic cations essential for cell growth and differentiation and other aspects of signal transduction. Hypoxic pulmonary vascular remodeling is accompanied by depressed lung polyamine synthesis and markedly augmented polyamine uptake. Cell types in which hypoxia induces polyamine transport in intact lung have not been delineated. Accordingly, rat lung and rat main pulmonary arterial explants were incubated with [(14)C]SPD in either normoxic (21% O(2)) or hypoxic (2% O(2)) environments for 24 h. Autoradiographic evaluation confirmed previous studies showing that, in normoxia, alveolar epithelial cells are dominant sites of polyamine uptake. In contrast, hypoxia was accompanied by prominent localization of [(14)C]SPD in conduit, muscularized, and partially muscularized pulmonary arteries, which was not evident in normoxic lung tissue. Hypoxic main pulmonary arterial explants also exhibited substantial increases in [(14)C]SPD uptake relative to control explants, and autoradiography revealed that enhanced uptake was most evident in the medial layer. Main pulmonary arterial explants denuded of endothelium failed to increase polyamine transport in hypoxia. Conversely, medium conditioned by endothelial cells cultured in hypoxic, but not in normoxic, environments enabled hypoxic transport induction in denuded arterial explants. These findings in arterial explants were recapitulated in rat cultured main pulmonary artery cells, including the enhancing effect of a soluble endothelium-derived factor(s) on hypoxic induction of [(14)C]SPD uptake in smooth muscle cells. Viewed collectively, these results show in intact lung tissue that hypoxia enhances polyamine transport in pulmonary artery smooth muscle by a mechanism requiring elaboration of an unknown factor(s) from endothelial cells.     

How well mixed is inert gas in tissues?

The washout of inert gas from tissues typically follows multiexponential curves rather than monoexponential curves as would be expected from homogeneous, well-mixed compartment. This implies that the ratio for the square root of the variance of the distribution of transit times to the mean (relative dispersion) must be greater than 1. Among the possible explanations offered for multiexponential curves are heterogeneous capillary flow, uneven capillary spacing, and countercurrent exchange in small veins and arteries. By means of computer simulations of the random walk of gas molecules across capillary beds with parameters of skeletal muscle, we find that heterogeneity involving adjacent capillaries does not suffice to give a relative dispersion greater than one. Neither heterogeneous flow, nor variations in spacing, nor countercurrent exchange between capillaries can account for the multiexponential character of experimental tissue washout curves or the large relative dispersions that have been measured. Simple diffusion calculations are used to show that many gas molecules can wander up to several millimeters away from their entry point during an average transit through a tissue bed. Analytical calculations indicate that an inert gas molecule in an arterial vessel will usually make its first vascular exit from a vessel larger than 20 micron and will wander in and out of tissue and microvessels many times before finally returning to the central circulation. The final exit from tissue will nearly always be into a vessel larger than 20 micron. We propose the hypothesis that the multiexponential character of skeletal muscle tissue inert gas washout curves must be almost entirely due to heterogeneity between tissue regions separated by 3 mm or more, or to countercurrent exchanges in vessels larger than 20 micron diam.     

Instant of vascular occlusion defined with laser-Doppler flowmetry.

Derivation of capillary pressure from tracings postarterial (AO) or -venous (VO) occlusion requires back extrapolation to an instant near the time of occlusion. This instant is difficult to identify because of pressure artifacts created by the occlusion maneuver. Theoretically, when the flow in the main artery (or veins) is stopped instantaneously, the flow in the arterioles (or venule) will stop after a short time delay (perhaps less than 100 ms). When flow had stopped in the main artery and in the arteriole, the pressure in the main artery at that instant would equal the pressure in the arterioles. We sought to identify the instant when flow stops in the arterioles and venules after AO and VO, respectively. In an isolated perfused dog left lower lobe preparation flow in the main vessels were monitored with inline flow probes, whereas flow in the microcirculation was monitored with laser-Doppler flow (LDF) probe placed on the lung surface. A sudden decline in arterial flow was detected by the LDF probe after 54 ms, while a sudden decline in venous flow was detected in the venules after 35 ms. These time delays were used as wave transmission time across the arterial and venous trees. Consequently, it was concluded that after AO, flow in the arterioles would stop 54 ms after it had become zero in the main artery, while after VO flow in the venules would stop 35 ms after it had become zero in the main vein. The pressure post-AO and post-VO was read at these instants (54 and 35 ms after flow in the main vessel reached zero).(ABSTRACT TRUNCATED AT 250 WORDS)     

Oxygen transport in the rat brain cortex at normobaric hyperoxia

The distribution of oxygen tension (PO(2)) in microvessels and in the tissues of the rat brain cortex on inhaling air (normoxia) and pure oxygen at atmospheric pressure (normobaric hyperoxia) was studied with the aid of oxygen microelectrodes (diameter = 3-6 microm), under visual control using a contact optic system. At normoxia, the PO(2) of arterial blood was shown to decrease from [mean (SE)] 84.1 (1.3) mmHg in the aorta to about 60.9 (3.3) mmHg in the smallest arterioles, due to the permeability of the arteriole walls to oxygen. At normobaric hyperoxia, the PO(2) of the arterial blood decreased from 345 (6) mmHg in the aorta to 154 (11) mmHg in the smallest arterioles. In the blood of the smallest venules at normoxia and at normobaric hyperoxia, the differences between PO(2) values were smoothed out. Considerable differences between PO(2) values at normoxia and at normobaric hyperoxia were found in tissues at a distance of 10-50 microm from the arteriole walls (diameter = 10-30 microm). At hyperbaric hyperoxia these values were greater than at normoxia, by 100-150 mmHg. In the long-run, thorough measurements of PO(2) in the blood of the brain microvessels and in the tissues near to the microvessels allowed the elucidation of quantitative changes in the process of oxygen transport from the blood to the tissues after changing over from the inhalation of air to inhaling oxygen. The physiological, and possibly pathological significance of these changes requires further analysis.