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.     

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.     

Regional pulmonary transit times in humans

We measured the frequency distribution of erythrocyte (RBC) transit times in resected lobes of lungs in eight human subjects undergoing thoracotomy for peripheral lung tumors. RBC transit times were measured by the injection of radiolabeled blood flow and volume markers, which were counted in samples from the resected lung. In five of these subjects, the measurements from the resected lung were compared with preoperative measurements of the transit times of radiolabeled RBCs with a gamma camera-computer system. Time-activity curves from the cardiac chambers and the lung or its regions were obtained from which transit times were calculated by the centroid and deconvolution techniques. The reproducibility of transit times measured by this technique was assessed in another eight normal subjects, after sequential bolus injections of radiolabeled cells. The mean transit time of the upper lung region was longer (5.1 +/- 0.5 s) than that of the lower (4.1 +/- 0.6 s, P less than 0.05) in the preoperative study. Similarly, the mean transit time of the upper lung slice was longer (5.5 +/- 0.3 s) than that of the lower slice (3.8 +/- 0.3 s, P less than 0.05) in the resected lung specimens. We conclude that there was good agreement between these techniques and that there are long transit times in the upper regions of human lungs.     

Pulmonary capillaries are recruited during pulsatile flow.

Capillaries recruit when pulmonary arterial pressure rises. The duration of increased pressure imposed in such experiments is usually on the order of minutes, although recent work shows that the recruitment response can occur in <4 s. In the present study, we investigate whether the brief pressure rise during cardiac systole can also cause recruitment and whether the recruitment is maintained during diastole. To study these basic aspects of pulmonary capillary hemodynamics, isolated dog lungs were pump perfused alternately by steady flow and pulsatile flow with the mean arterial and left atrial pressures held constant. Several direct measurements of capillary recruitment were made with videomicroscopy. The total number and total length of perfused capillaries increased significantly during pulsatile flow by 94 and 105%, respectively. Of the newly recruited capillaries, 92% were perfused by red blood cells throughout the pulsatile cycle. These data provide the first direct account of how the pulmonary capillaries respond to pulsatile flow by showing that capillaries are recruited during the systolic pulse and that, once open, the capillaries remain open throughout the pulsatile cycle.     

Regional differences in neutrophil margination in dog lungs

We investigated the relationship between polymorphonuclear leukocyte (PMN) retention and erythrocyte (RBC) velocity in the lungs of mongrel dogs. Regional velocity was estimated by measuring regional RBC transit times and was correlated with the retention of PMN found in the same lung sample 10 min after the injection of a bolus of labeled cells. Data from the whole lung showed that the total number of cells marginated in the pulmonary vasculature was 2.4 times as great as the number present in the circulation and that this pool turned over at a rate of 1%/s. The regional data showed increased retention, indicating slower PMN turnover in the upper lung regions, which have longer transit times and therefore slower blood velocities than the RBC is attributed to a greater discrepancy between PMN and RBC is attributed to a greater discrepancy between PMN and capillary size and the fact that PMN are less deformable than RBC. The large number of capillary segments present in the lung allows neutrophils to move more slowly while RBC stream around them. We conclude that there are approximately 2.5 times as many PMNs marginated in the lung as there are in the total circulating blood volume of the dog and that the pulmonary marginated pool turns over at approximately 1%/s with slower turnover in the upper compared with the lower regions of the lung.     

Capillary recruitment and transit time in the rat lung

Presson, Robert G., Jr., Thomas M. Todoran, Bracken J. DeWitt, Ivan F. McMurtry, and Wiltz W. Wagner, Jr.Capillary recruitment and transit time in the rat lung.J. Appl. Physiol. 83(2): 543-549, 1997.Increasing pulmonary blood flow and the associated rise incapillary perfusion pressure cause capillary recruitment. The resultingincrease in capillary volume limits the decrease in capillary transittime. We hypothesize that small species with relatively high restingmetabolic rates are more likely to utilize a larger fraction ofgas-exchange reserve at rest. Without reserve, we anticipate thatcapillary transit time will decrease rapidly as pulmonary blood flowrises. To test this hypothesis, we measured capillary recruitment andtransit time in isolated rat lungs. As flow increased, transit timedecreased, and capillaries were recruited. The decrease in transit timewas limited by an increase in the homogeneity of the transit time distribution and an increased capillary volume due, in part, to recruitment. The recruitable capillaries, however, were nearly completely perfused at flow rates and pressures that were less thanbasal for the intact animal. This suggests that a limited reserve ofrecruitable capillaries in the lungs of species with high restingmetabolic rates may contribute to their inability to raiseO₂ consumption manyfold abovebasal values.

     

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.     

Oxygen delivery to skeletal muscle fibers: effects of microvascular unit structure and control mechanisms

The number of perfused capillaries in skeletal muscle varies with muscle activation. With increasing activation, muscle fibers are recruited as motor units consisting of widely dispersed fibers, whereas capillaries are recruited as groups called microvascular units (MVUs) that supply several adjacent fibers. In this study, a theoretical model was used to examine the consequences of this spatial mismatch between the functional units of muscle activation and capillary perfusion. Diffusive oxygen transport was simulated in cross sections of skeletal muscle, including several MVUs and fibers from several motor units. Four alternative hypothetical mechanisms controlling capillary perfusion were considered. First, all capillaries adjacent to active fibers are perfused. Second, all MVUs containing capillaries adjacent to active fibers are perfused. Third, each MVU is perfused whenever oxygen levels at its feed arteriole fall below a threshold value. Fourth, each MVU is perfused whenever the average oxygen level at its capillaries falls below a threshold value. For each mechanism, the dependence of the fraction of perfused capillaries on the level of muscle activation was predicted. Comparison of the results led to the following conclusions. Control of perfusion by MVUs increases the fraction of perfused capillaries relative to control by individual capillaries. Control by arteriolar oxygen sensing leads to poor control of tissue oxygenation at high levels of muscle activation. Control of MVU perfusion by capillary oxygen sensing permits adequate tissue oxygenation over the full range of activation without resulting in perfusion of all MVUs containing capillaries adjacent to active fibers.     

Pulmonary capillary recruitment during airway hypoxia in the dog.

To study the effect of hypoxia on the pulmonary capillaries, windows were inserted in the chest wall of 9 pentobarbital-anesthetized dogs. A microscope with an image-superimposing device was used to make drawings of the perfused capillaries. Summed lengths of individual perfused capillaries in the drawing were determined with a map-measuring tool. Total capillary length was constant between PaO2 of 160 and 70 Torr. As PaO2 fell below 70 Torr, recruitment of previously unperfused capillaries occurred in every case; at PaO2 of 40 Torr, the total length of perfused capillaries was about 4 times greater than during normoxia. There was no correlation between the recruitment of capillaries and alterations in left atrial pressure, only a weak correlation with cardiac output changes, but a very strong correlation with increased pulmonary artery pressure. This implies that recruitment was probably caused by vasoconstriction within the lung.     

Kinetic measurements of gas exchange in the intact pulmonary microcirculation.

The kinetics of gas exchange are monitored in an isolated perfused lung preparation contained within a plethysmograph. The lungs are perfused with buffer, and there is no gas exchange until a 2.0-ml bolus of reactant is injected into the perfusion system. Subsequent gas exchange produces a pressure transient that is related to the corresponding volume of exchanged gas. The observed rate of volume change is the result of two separate processes: 1) the rate of gas exchange during transit through the capillary bed and 2) the distribution of vascular transit times between the point of injection and the capillary bed. The latter is assessed by a control injection containing a dissolved inert gas that is liberated in the alveoli as the bolus enters the capillary bed. Analysis of the experimental curves permits the separation of these two processes. A model of exchange kinetics indicates that this method has the capability of measuring kinetic events occurring during gas exchange in the microcirculation under physiological conditions.     

Pulmonary arterial transit times

To begin to characterize the pulmonary arterial transport function we rapidly injected a bolus containing a radiopaque dye and a fluorescence dye into the right atrium of anesthetized dogs. The concentrations of the dye indicators were measured in the main pulmonary artery (fluoroscopically) and in a subpleural pulmonary arteriole (by fluorescence microscopy). The resulting concentration vs. time curves were subjected to numerical deconvolution and moment analysis to determine how the bolus was dispersed as it traveled through the arteriole stream tube from the main pulmonary artery to the arteriole. The mean transit time and standard deviation of the transport function from the main pulmonary artery to the arterioles studied averaged 1.94 and 1.23 s, respectively, and the relative dispersion (ratio of standard deviation to mean transit time) was approximately 64%. This relative dispersion is at least as large as those reported for the whole dog lung, indicating that relative to their respective mean transit times the dispersion upstream from the arterioles is comparable to that taking place in capillaries and/or veins. The standard deviations of the transport functions were proportional to their mean transit times. Thus the relative dispersion from the main pulmonary artery to the various arterioles studied was fairly consistent. However, there were variations in mean transit time even between closely adjacent arterioles, suggesting that variations in mean transit times between arteriole stream tubes also contribute to the dispersion in the pulmonary arterial tree.     

Selected contribution: measuring the response time of pulmonary capillary recruitment to sudden flow changes.

To determine how rapidly pulmonary capillaries recruit after sudden changes in blood flow, we used an isolated canine lung lobe perfused by two pumps running in parallel. When one pump was turned off, flow was rapidly halved; when it was turned on again, flow immediately doubled. We recorded pulmonary capillary recruitment in subpleural alveoli using videomicroscopy to measure how rapidly the capillaries reached a new steady state after these step changes in blood flow. When flow was doubled, capillary recruitment reached steady state in <4 s. When flow was halved, steady state was reached in approximately 8 s. We conclude that the pulmonary microcirculation responds rapidly to step changes in flow, even in the capillaries that are most distant from the hilum.     

Model analyses of capillary growth and tissue oxygenation during hypoxia

Theoretical analyses were used to determine whether capillary growth is an adaptive response to hypoxia. Parameter values were obtained from models of transverse sections of muscles in which individual fibers were distributed in square-ordered arrays and capillaries were added to the perimeters of individual fibers in the arrays. Increasing the number of capillaries up to 2.0 per fiber increased hypoxic tolerance by 157% above that expected for a Krogh cylinder. However, increasing the number of capillaries from 2.0 to 4.0 per fiber increased hypoxic tolerance by only 18% and, assuming the entire perimeter of each fiber was perfused with blood, increased hypoxic tolerance by only 11% over the value obtained when capillary-to-fiber ratio was 4.0. Capillary growth during normal maturation may result in capillary-to-fiber ratios around 2.0, near the upper limit for producing marked changes in hypoxic tolerance. Therefore, capillary growth may not be an adaptive response to ambient hypoxia because there is little or no gas transport benefit derived from the additional capillaries.     

Non-invasive Assessment of Microvascular and Endothelial Function

The authors have utilized capillaroscopy and forearm blood flow techniques to investigate the role of microvascular dysfunction in pathogenesis of cardiovascular disease. Capillaroscopy is a non-invasive, relatively inexpensive methodology for directly visualizing the microcirculation. Percent capillary recruitment is assessed by dividing the increase in capillary density induced by postocclusive reactive hyperemia (postocclusive reactive hyperemia capillary density minus baseline capillary density), by the maximal capillary density (observed during passive venous occlusion). Percent perfused capillaries represents the proportion of all capillaries present that are perfused (functionally active), and is calculated by dividing postocclusive reactive hyperemia capillary density by the maximal capillary density. Both percent capillary recruitment and percent perfused capillaries reflect the number of functional capillaries. The forearm blood flow (FBF) technique provides accepted non-invasive measures of endothelial function: The ratio FBF_max/FBF_base is computed as an estimate of vasodilation, by dividing the mean of the four FBF_max values by the mean of the four FBF_base values. Forearm vascular resistance at maximal vasodilation (FVR_max) is calculated as the mean arterial pressure (MAP) divided by FBF_max. Both the capillaroscopy and forearm techniques are readily acceptable to patients and can be learned quickly.The microvascular and endothelial function measures obtained using the methodologies described in this paper may have future utility in clinical patient cardiovascular risk-reduction strategies. As we have published reports demonstrating that microvascular and endothelial dysfunction are found in initial stages of hypertension including prehypertension, microvascular and endothelial function measures may eventually aid in early identification, risk-stratification and prevention of end-stage vascular pathology, with its potentially fatal consequences.     

Extension of the theory of the multiple-indicator dilution technique to metabolic systems with an arbitrary number of rate constants

The multiple indicator dilution technique consists in the instantaneous injection of a mixture of tracers into the arterial perfusate flow of a catheterized or isolated perfused organ, followed by the analysis of the effluent perfusate. The theory of this technique, which has hitherto been developed for cases where metabolism of a tracer is confined to sequestration described by a single rate constant, is extended in this paper to include an arbitrary number of metabolic rate constants. Partial differential equations with constant coefficients describing the events in a single capillary are derived by applying conventional compartmental analysis to infinitesimally small sections of the capillary. Methods for solving such systems in the time as well as in the frequency domain are developed. From the solutions, the impulse response of the whole organ is evaluated assuming variable capillary and uniform large-vessel transit times. In addition, an efficient method using much less computer time was developed, based on the approximation of the distribution of the capillary transit times by a sum of exponentials. Evaluation of moments (recoveries and mean transit times) is also treated. The results are applied to an example from hepatic lactate metabolism.     

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.     

Blood flow distribution with adrenergic and histaminergic antagonists

Superficial fibular nerve stimulation (SFNS) causes increased pre- and post-capillary resistances as well as increased capillary permeability in the dog hind paw. These responses indicate possible adrenergic and histaminergic interactions. The distribution of blood flow between capillaries and arteriovenous anastomoses (AVA) may depend on the relative effects of these neural inputs. Right hind paws of anesthetized heparinized dogs were vascularly and neurally isolated and perfused with controlled pressure. Blood flow distribution was calculated from the venous recovery of 85Sr-labeled microspheres (15 microns). The mean transit times of 131I-albumin and 85Sr-labeled microspheres were calculated. The effects of adrenergic and histaminergic antagonists with and without SFNS were determined. Phentolamine blocked the entire response to SFNS. Prazosin attenuated increases in total and AVA resistance. Yohimbine prevented increased total resistance, attenuated the AVA resistance increase, and revealed a decrease in capillary circuit resistance. Pyrilamine attenuated total resistance increase while SFNS increased capillary and AVA resistances. Metiamide had no effect on blood flow distribution with SFNS. The increase in AVA resistance with SFNS apparently resulted from a combination of alpha 1 and alpha 2 receptor stimulation but not histaminergic effects.     

Capillary perfusion patterns in single alveolar walls.

We studied capillary perfusion patterns in single alveolar walls through a transparent thoracic window implanted in pentobarbital-anesthetized dogs. The capillaries were maximally opened by brief inflation of a balloon in the left atrium to raise pressure. After the balloon was deflated and pulmonary hemodynamics returned to zone 2 baseline conditions, the capillaries that remained perfused in the observed field were videotaped with the use of in vivo microscopy. The cycle of elevated pressure and baseline observation was repeated three times. Perfusion of different capillaries during each of the observations would imply that the capillaries had characteristics that permitted flow to switch between segments. Perfusion of a specific set of pathways through the network each time would demonstrate that flowing blood sought a unique and repeatable combination of segments, presumably with the least total pathway resistance. We found that the same capillary segments were perfused 79% of the time, a strong indication that a reproducible combination of individual segmental resistances determined the predominant pattern of pulmonary capillary perfusion.     

The variance of the distribution of traversal times in a capillary bed

A random walk model of capillary tracer transit times is developed that treats simulataneously: plug flow in the capillary, radial and axial diffusion in the capillary cylinder and tissue annulus, and endothelial barriers to solute transport. The mean transit time is simply the volume of distribution divided by blood flow. Variance of transit times has additive terms for radial, axial, and barrier influences that are reduceable to variances of simpler models of capillary exchange. The dependence of variance on the solute diffusion coefficient is not monotonic, but has a minimum near 0·5 × 10?6 cm²/s for reasonable parameters and no barrier, Small molecules like inert gases are expected to have larger variances with higher diffusion coefficients, while larger molecules and barrier limited solutes will have the reverse dependence. Available literature data indicates that capillary heterogeneity will have a major influence on whole-body variance of transit times.     

Site of recruitment in the pulmonary microcirculation

Increasing the total surface area of the pulmonary blood-gas interface by capillary recruitment is an important factor in maintaining adequate oxygenation when metabolic demands increase. Capillaries are known to be recruited during conditions that raise pulmonary blood flow and pressure. To determine whether pulmonary arterioles and venules are part of the recruitment process, we made in vivo microscopic observations of the subpleural microcirculation (all vessels less than 100 microns) in the upper lung where blood flow is low (zone 2). To evoke recruitment, pulmonary arterial pressure was elevated either by an intravascular fluid load or by airway hypoxia. Of 209 arteriolar segments compared during low and high pulmonary arterial pressures, none recruited or derecruited. Elevated arterial pressure, however, did increase the number of perfused capillary segments by 96% with hypoxia and 165% with fluid load. Recruitment was essentially absent in venules (4 cases of recruitment in 289 segments as pressure was raised). These data support the concept that recruitment in the pulmonary circulation is exclusively a capillary event.