Role of positive airway pressure on pulmonary acinar perfusion heterogeneity.

Perfusion of the pulmonary acinus has been shown to be generally homogeneous, but there is a significant component that is heterogeneous. To investigate the contribution of the alveolar septal capillary network to acinar perfusion heterogeneity, the passage of fluorescent dye boluses through the subpleural microcirculation of isolated dog lung lobes was videotaped using fluorescence microscopy. As the videotapes were replayed, dye-dilution curves were recorded from each of the tributary branches of Y-shaped venules that drained single acini. For each Y-shaped venule, the mean appearance time difference between the pair of tributary branches was calculated from the dye curves. When the complex septal capillary networks were derecruited by high positive airway pressure, venular perfusion became proportionally more homogeneous. This result shows that septal capillary resistance and pathlength differences are important contributors to intra-acinar perfusion heterogeneity.

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.

Pulmonary capillary perfusion: intra-alveolar fractal patterns and interalveolar independence.

Pulmonary capillary perfusion was analyzed from videomicroscopic recordings to determine flow switching characteristics among capillary segments in isolated, blood-perfused canine lungs. Within each alveolus, the rapid switching pattern was repetitive and was, therefore, nonrandom (fractal dimensions near 1.0). This self-similarity over time was unexpected in a network widely considered to be passive. Among adjacent alveoli, the relationship among the switching patterns was even more surprising, for there was virtually no relationship between the perfusion patterns (coefficients of determination approaching zero). These findings demonstrated that the perfusion patterns in individual alveolar walls were independent of their next-door neighbors. The lack of dependence among neighboring networks suggests an interesting characteristic: the failure of one alveolar-capillary bed would leave its neighbors relatively unaffected, a feature of a robust design.

Perfusion heterogeneity in the pulmonary acinus.

There is little information on the distribution of acinar perfusion because it is difficult to resolve blood flow within such small regions. We hypothesized that the known heterogeneity of arteriolar blood flow and capillary blood flow would result in heterogeneous acinar perfusion. To test this hypothesis, the passage of fluorescent dye boluses through the subpleural microcirculation of isolated dog lobes was videotaped by using fluorescence microscopy. As the videotapes were replayed, dye-dilution curves were recorded from each of the tributary branches of Y-shaped venules that drained an acinus. From the dye curves, we calculated the mean appearance time of each curve. The difference in mean appearance times between venular tributary branches was small in most cases. In 43% of the observed venular branch pairs, the dye curves were essentially superimposable (the mean appearance-time difference was <5%); and in another 42%, the mean appearance-time difference between curves was 5-10%. From these results, we conclude that acinar perfusion is unexpectedly homogeneous.

Anatomic distribution of pulmonary vascular compliance.

Previously, the pressure changes after arterial and venous occlusion have been used to characterize the longitudinal distribution of pulmonary vascular resistance with respect to vascular compliance using compartmental models. However, the compartments have not been defined anatomically. Using video microscopy of the subpleural microcirculation, we have measured the flow changes in approximately 40-micron arterioles and venules after venous, arterial, and double occlusion maneuvers. The quasi-steady flows through these vessels after venous occlusion permitted an estimation of the compliance in three anatomic segments: arteries > 40 microns, veins > 40 microns, and vessels < 40 microns in diameter. We found that approximately 65% of the total pulmonary vascular compliance was in vessels < 40 microns, presumably mostly capillaries. The transient portions of the pressure and flow data after venous, arterial, and double occlusion were consistent with most of the arterial compliance being upstream from most of the arterial resistance and most of the venous compliance being downstream from most of the venous resistance.

Capillary recruitment and transit time in the rat lung.

Increasing pulmonary blood flow and the associated rise in capillary perfusion pressure cause capillary recruitment. The resulting increase in capillary volume limits the decrease in capillary transit time. We hypothesize that small species with relatively high resting metabolic rates are more likely to utilize a larger fraction of gas-exchange reserve at rest. Without reserve, we anticipate that capillary transit time will decrease rapidly as pulmonary blood flow rises. To test this hypothesis, we measured capillary recruitment and transit time in isolated rat lungs. As flow increased, transit time decreased, and capillaries were recruited. The decrease in transit time was 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 than basal for the intact animal. This suggests that a limited reserve of recruitable capillaries in the lungs of species with high resting metabolic rates may contribute to their inability to raise O2 consumption manyfold above basal values.

Computer determination of perfusion patterns in pulmonary capillary networks.

Individual pulmonary capillaries are not steadily perfused. By using in vivo microscopy, it can readily be demonstrated that perfusion continually switches between capillary segments and between portions of the network within a single alveolar wall. These changes in capillary perfusion occur even when upstream pressure and flow are constant. Flow switching between capillary segments in the absence of hemodynamic changes in large upstream vessels suggests that capillary perfusion patterns could be random. To calculate the probability that perfusion patterns could occur by chance, it is necessary to know the total number of possible perfusion patterns in a given capillary network. We developed a computer program that can determine every possible perfusion pattern for any given capillary network, and from that information we can calculate whether perfusion of individual segments in the network is random. With the results of the computer program, we have obtained statistical evidence that some capillary segments in a network are nonrandomly perfused.

Pulmonary capillary diameters and recruitment characteristics in subpleural and interior networks.

In vivo microscopic observations of pulmonary capillaries are limited to subpleural networks that are less dense than interior networks. In addition to the density difference, subpleural and interior capillary diameters may differ, although there are conflicting data on this point. We measured the diameters of subpleural and interior capillaries in rats and dogs. Subpleural diameters were 30% larger in rats and 20% larger in dogs. Because diameter and density differences might cause differences in recruitment between subpleural and interior networks, we measured subpleural and interior recruitment by counting the number of red blood cells per 10 microns of alveolar wall in histological cross sections of rapidly frozen rat lungs. Lung inflation pressures of 4, 12, and 25 cmH2O created a wide range of capillary recruitment in different groups of animals. Red blood cell counts for interior and subpleural capillaries moved in parallel and progressively increased as inflation pressures were reduced. These data demonstrate that recruitment in subpleural capillaries accurately reflect recruitment in interior capillaries and validate the use of in vivo microscopic observations of subpleural capillaries to investigate pulmonary capillary recruitment in general.

Effect of capillary pressure and lung distension on capillary recruitment.

To investigate the effect of capillary pressure and alveolar distension on capillary recruitment, we used video-microscopy to quantify capillary recruitment in individual subpleural alveolar walls. Canine lobes were perfused with autologous blood either while inflated by positive airway pressure or while inflated by negative intrapleural pressure in the intact thorax with airway pressure remaining atmospheric. Low flow rates minimized the arteriovenous pressure gradient (< 5 mmHg), permitting capillary pressure estimation by averaging these pressures. Capillary pressure was varied stepwise from airway pressure to 30 mmHg above airway pressure. Capillary recruitment always began as capillary pressure exceeded airway pressure. At low positive airway pressures, the capillaries of the excised lobes opened suddenly over a narrow pressure range. AT higher airway pressures and in the intact thorax, recruitment occurred over a wide range of capillary pressures. We conclude that capillary perfusion begins when intracapillary pressure just exceeds alveolar pressure but that further increases in capillary pressure recruit capillaries depending on tension in the alveolar wall, whether imposed by positive airway pressure or by gravity when the lung is suspended in an intact thorax.

Distribution of pulmonary capillary red blood cell transit times.

In theory, red blood cells can pass through the pulmonary capillaries too rapidly to be completely saturated with oxygen during exercise. This idea has not been directly tested because the transit times of the fastest red blood cells are unknown. We report the first measurements of the entire transit time distribution for red blood cells crossing single subpleural capillary networks of canine lung using in vivo fluorescence videomicroscopy and compare those times with the distribution of plasma transit times in the same capillary networks. On average, plasma took 1.4 times longer than red blood cells to pass through the capillary bed. Decreased transit times with increased cardiac output were mitigated by both capillary recruitment and a narrowing of the transit time distribution. This design feature of the pulmonary capillary bed kept the shortest times from falling below the theoretical minimum time for complete oxygenation.

Measuring pulmonary microvessel diameters using video image analysis.

To directly determine the pressure-diameter relationship of individual pulmonary microvessels, it is necessary to measure the width of the column of blood in the vessel because microvascular walls are invisible when using intravital microscopy. To identify the margins of the blood column accurately, we developed a method for computer enhancement and measurement of vessel images. After recording microvessels on videotape, consecutive frames from the videotape were digitized by a computer. Pixels that changed from frame to frame (moving erythrocytes) were turned white, and unchanging pixels were turned black. In this way an image of the erythrocyte column with distinct edges was produced. The width of this column was measured with a heuristic technique involving interactions between the computer and the user. The measurements were reproducible and accurate. This technique has been used to measure microvascular diameters over a range of well-defined microvascular pressures and construct precise pressure-diameter curves.

Stability of alveolar capillary opening pressures.

Little is known about the stability of the process by which pulmonary capillaries open. To investigate this process, pulmonary capillary perfusion patterns in isolated pump-perfused canine lobes were studied using video microscopy. After pump flow was set to perfuse one-half of the capillaries, the pump was turned off and all of the capillaries emptied. Turning the pump back on reopened the capillaries. The on-off cycle was repeated six times. If the same capillaries were perfused during each observation, it would demonstrate that there were stable and significant differences between individual capillary opening pressures, causing consistent recruitment of those capillaries with the lowest opening pressures. Alternatively, variable perfusion patterns would result if capillary opening pressures changed between observations, if the differences in opening pressures between capillary segments were negligible, or if experimental conditions changed between cycles. The perfusion pattern was more reproducible than expected by chance alone, which indicated the existence of stable differences among alveolar capillary opening pressures.

Effect of increasing flow on distribution of pulmonary capillary transit times.

The complex morphology of the pulmonary capillary network causes capillary transit times to be dispersed about a mean. It is known that flow-induced decreases in mean capillary transit time are partially offset by capillary recruitment and distension, but the effect of these factors on the rest of the distribution of transit times is unknown. We have studied the relationship between blood flow, capillary recruitment, and the distribution of transit times in isolated canine lungs with videomicroscopy. Doubling baseline lobar blood flow recruited capillaries. All transit times in the distribution decreased, as did relative dispersion. Doubling flow again caused a further decrease in transit times, but neither capillary recruitment nor relative dispersion changed significantly. We conclude that capillary transit times become more homogeneous as lobar flow increases from low to intermediate levels. Further increases in flow across a fully recruited network are associated with decreases in transit times but not with more homogeneous capillary perfusion.

Temporal capillary perfusion patterns in single alveolar walls of intact dogs.

Pulmonary gas exchange reserve in the form of recruitable capillaries was first described in the 1930s, when in vivo microscopy was used to demonstrate that not all capillaries were perfused during basal conditions and that perfusion of individual capillaries varied over time. These important observations have never been directly confirmed, nor have the hemodynamic causes of the variation been investigated. We used videomicroscopy to record nine consecutive pulmonary capillary perfusion patterns during a 40-min period. Confirming the original work, we found considerable perfusion variation in about one-half of the capillaries. These variations did not correlate with changes in pulmonary arterial pressures or cardiac outputs, suggesting that factors more subtle than large-vessel hemodynamics affected capillary perfusion consistency. In contrast to this variable group, one-half of the capillary segments were consistently perfused during at least eight of the nine observations and were interconnected to form preferential pathways across the alveolar wall.

Direct measurement of pulmonary microvascular distensibility.

Pulmonary vascular distensibility has an important influence on pulmonary hemodynamics. Although many measurements of distensibility have been made on large pulmonary vessels, there is less information on microvascular distensibility. We have measured the distensibility of the smallest (< 70-microns-diam) precapillary arterioles and postcapillary venules. Isolated dog lobes, at 2.5 cmH2O transpulmonary pressure, were perfused at low flows, which caused the arteriovenous pressure gradient to be very small and thereby permitted accurate estimation of microvascular pressure. As microvascular pressure was systematically varied between 0 and 30 mmHg, subpleural microvascular diameters were determined from computer-enhanced images obtained by videomicroscopy. Arteriolar and venular distensibilities were not different from each other. The microvascular pressure-diameter relationship was alinear with distensibility coefficients of 1-3% mmHg-1, values that are of the same order of magnitude as previously measured distensibilities of 100- to 1,000-microns-diam canine pulmonary vessels.

Evaluation of a new fluid warmer effective at low to moderate flow rates.

BACKGROUND: The tendency of intravenous fluid exiting the heat exchanger of a fluid warmer to cool to room temperature increases as the rate of infusion slows and the length of tubing between the heat exchanger and the patient increases. Thus, slow to moderate flow rates result in the delivery of fluid near room temperature despite the use of a fluid warmer. The volumes infused even at low flow rates may be large relative to the size of infants and children and may result in a significant decrease in patient temperature. METHODS: A new warmer (Hotline, Level 1 Technologies) that actively heats the fluid in the delivery tubing was evaluated and compared to two different conventional dry-wall warmers: the model DW1000A (Baxter Health Care) and the FloTem IIe (DataChem). Cold blood (4-10 degrees C) and room temperature saline (22 degrees C) were pumped through the warmers and the delivered temperature was measured as the flow rate was varied from 50 to 12,000 ml/h. RESULTS: The Hotline was more effective than the Baxter or the FloTem IIe at flow rates between 50 and 6,000 ml/h for saline and at flow rates between 50 and 3,000 ml/h for blood. Insulating the tubing beyond the heat exchangers of the conventional warmers improved their performance, but the delivered temperatures were still less than those of the Hotline at low flow rates. CONCLUSIONS: The Hotline is more effective than conventional warmers at slow flow rates, and may be useful for preventing hypothermia when large volumes of fluid relative to patient size are infused at slow rates.

A review of the detection and treatment of venous air embolism.

Venous air embolism is a common potential complication of several surgical procedures and should be understood by anesthesiologists. The first part of this article, which appeared in a previous issue (Sept/Oct 1991;18(5):29-37), reviewed the pathophysiologic aspects. This concluding segment discusses the detection of venous air embolism and its prevention and treatment.

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.

Perioperative fluid and transfusion management.

Optimal perioperative fluid management in pediatric patients entails a knowledge of the effects of preoperative fasting, perioperative third space losses, and hemorrhage on the patient's fluid compartments. We explain which of the various available intravenous fluids should be used to correct various fluid and electrolyte losses that may occur. The authors also review techniques for limiting homologous transfusion requirements and discuss certain complications associated with blood transfusion.