Perfusion heterogeneity in rat lungs assessed from the distribution of 4-microm-diameter latex particles

Apnea causes microvascular perfusion maldistribution in isolated rat lungs

Obstructive sleep apnea is associated with significant cardiovascular disease, yet little is known about the effects of OSA on pulmonary microvascular perfusion. In a recent report, we showed that pulmonary microvascular perfusion was significantly mal-distributed in anesthetized, spontaneously breathing rats exposed to five episodes of obstructive apnea. We quantified microvascular perfusion by analyzing trapping patterns of 4 μm diameter fluorescent latex particles infused into the pulmonary circulation after the last episode. We could not determine if the perfusion maldistribution was due to the effects of large subatmospheric intrapleural pressures during apnea, or to precapillary OSA hypoxic vasoconstriction. To address this, we repeated these studies using isolated, buffer-perfused rat lungs (P_{pulm art} , 10 cm H₂ O) ventilated in a chamber (-5 to -15 cm H₂ O, 25 breaths/min; P_trachea  = 0). We simulated apnea by clamping the trachea and cycling the chamber pressures between -25 and -35 cm H₂ O for five breaths. After five apnea episodes, we infused 4 μm diam. fluorescent latex particles into the pulmonary artery. The number of particles recovered from the venous effluent was 74% greater in nonapneic isolated lungs compared to apneic lungs (P ≤ 0.05). Apneic lungs also had perfusion maldistributions that were 73% greater than those without apnea (P ≤ 0.05). We conclude that simulated apnea in isolated, perfused rat lungs produces significantly greater particle trapping and microvascular perfusion maldistribution than in nonapneic isolated lungs. We believe these effects are due to the large, negative intrapleural pressures produced during apnea, and are not due to hypoxia.

Obstructive apnea causes microvascular perfusion maldistribution in the lungs of rats

Obstructive sleep apnea (OSA) is associated with significant cardiovascular consequences, including pulmonary hypertension, yet little is known about its effects on pulmonary microvascular perfusion. To investigate effects of OSA on pulmonary microvascular perfusion, we clamped the tracheal cannulas of anesthetized, spontaneously breathing rats to simulate obstructive apnea. The clamp remained in place for 10 breaths before it was released to allow the animals to again breathe spontaneously. We repeated this protocol every 20 s until the rat experienced a total of five apneic episodes of 10 breaths each. We then infused into a femoral vein 10⁸ fluorescent latex particles (4 µm diameter), which became trapped within the pulmonary microcirculation. We removed the lungs, allowed them to air-dry, and quantified the particle distributions in sections of the lungs using dispersion index (DI) analysis, a method we developed previously. The log of the DI (logDI) is a measure of perfusion maldistribution. Greater log(DI) values correspond to greater maldistribution. Apneic lungs had average logDI values of 1.28 (SD 0.24). Rats not subjected to apnea had average logDI values of 0.85 (SD 0.08) ( P ≤ 0.05). Rats that received latex particles 10 min or 24 h after apnea had average logDI values of 0.97 (SD 0.31) and 0.84  (SD 0.38), respectively (not significant). Our results demonstrate, for the first time, that a few apneic events produced significant, but temporary, perfusion maldistribution within the pulmonary microcirculation. Repeated nightly episodes of apnea over months and years may explain why human patients with OSA suffer from significantly greater cardiovascular disease than those without OSA.

Positive pressure ventilation compresses pulmonary acinar microvessels but not their supply vessels

Pulmonary alveolar septal capillaries receive their perfusion from a web of larger surrounding acinar vessels. Using 4 μm diam. Latex particles, we showed that positive pressure ventilation narrowed the acinar vessels as evidenced by venous 4 μm particle concentrations and perfusate flows <50% of particle concentrations in negative pressure ventilated lungs. We aimed to understand the effects of positive and negative pressure ventilation on flows of larger particles through the lung. Isolated, ventilated rat lungs (air, transpulmonary pressures of 15/5 cm H2O, 25 breaths/min) were perfused with a hetastarch solution at Ppulm art/PLA pressures of 10/0 cm H2O. Red latex 7 μm (2.5 mg, 4.8 × 10⁶) or 15 μm (2.5 mg, 5.5 × 10⁵) particles were infused into each lung during positive or negative pressure ventilation. An equal number of green particles was infused 20 min later. Flows through lungs infused with 7 μm and 15 μm particles were not different from flows through lungs infused with 4 μm particles (p = 0.811). Venous particle concentrations of 7 μm particles relative to infused particles were lower in positive pressure lungs (0.03 ± 0.03%) compared to negative pressure lungs (0.17 ± 0.12%) (p = 0.041). Venous particle concentrations of 15 μm particles were not different between positive (2.3 ± 1.9%) and negative (3.3 ± 1.8%) pressure ventilation (p = 0.406). Together with our previous study, we conclude that 4 μm and 7 μm particles both enter acinar vessels but that the 7 μm particles are too large to flow through those vessels. In contrast, we conclude that 15 μm particles bypass the acinar vessels, flowing instead through larger intrapulmonary vessels to enter the venous outflow. These findings suggest that intrapulmonary vessels are organized as a web that allows bypass of the acinar vessels by large particles and, that these bypass vessels are not compressed by positive pressure ventilation.

Negative pressure ventilation enhances acinar perfusion in isolated rat lungs

We compared acinar perfusion in isolated rat lungs ventilated using positive or negative pressures. The lungs were ventilated with air at transpulmomary pressures of 15/5 cm H₂O, at 25 breaths/min, and perfused with a hetastarch solution at P_{pulm art}/P_LA pressures of 10/0 cm H₂O. We evaluated overall perfusability from perfusate flows, and from the venous concentrations of 4-µm diameter fluorescent latex particles infused into the pulmonary circulation during perfusion. We measured perfusion distribution from the trapping patterns of those particles within the lung. We infused approximately 9 million red fluorescent particles into each lung, followed 20 min later by an infusion of an equal number of green particles. In positive pressure lungs, 94.7 ± 2.4% of the infused particles remained trapped within the lungs, compared to 86.8 ± 5.6% in negative pressure lungs ( P ≤ 0.05). Perfusate flows averaged 2.5 ± 0.1 mL/min in lungs ventilated with positive pressures, compared to 5.6 ± 01 mL/min in lungs ventilated with negative pressures ( P ≤ 0.05). Particle infusions had little effect on perfusate flows. In confocal images of dried sections of each lung, red and green particles were co-localized in clusters in positive pressure lungs, suggesting that acinar vessels that lacked particles were collapsed by these pressures thereby preventing perfusion through them. Particles were more broadly and uniformly distributed in negative pressure lungs, suggesting that perfusion in these lungs was also more uniformly distributed. Our results suggest that the acinar circulation is organized as a web, and further suggest that portions of this web are collapsed by positive pressure ventilation.

Arterio-venous anastomoses in isolated, perfused rat lungs

Several studies have suggested that large-diameter (>25 μm) arterio-venous shunt pathways exist in the lungs of rats, dogs, and humans. We investigated the nature of these pathways by infusing specific-diameter fluorescent latex particles (4, 7, 15, 30, or 50 μm) into isolated, ventilated rat lungs perfused at constant pressure. All lungs received the same mass of latex (5 mg), which resulted in infused particle numbers that ranged from 1.7 × 10⁷ 4 μm particles to 7.5 × 10⁴ 50 μm particles. Particles were infused over 2 min. We used a flow cytometer to count particle appearances in venous effluent samples collected every 0.5 min for 12 min from the start of particle infusion. Cumulative percentages of infused particles that appeared in the samples averaged 3.17 ± 2.46% for 4 μm diameter particles, but ranged from 0.01% to 0.17% for larger particles. Appearances of 4 μm particles followed a rapid upslope beginning at 30 sec followed by a more gradual downslope that lasted for up to 12 min. All other particle diameters also began to appear at 30 sec, but followed highly irregular time courses. Infusion of 7 and 15 μm particles caused transient but significant perfusate flow reductions, while infusion of all other diameters caused insignificant reductions in flow. We conclude that small numbers of bypass vessels exist that can accommodate particle diameters of 7-to-50 μm. We further conclude that our 4 μm particle data are consistent with a well-developed network of serial and parallel perfusion pathways at the acinar level.

Inhaled thrombolytics reduce lung microclot and leukocyte infiltration after acute blood loss

We showed previously that a 30% blood loss in rats, without resuscitation, caused significant accumulation of microthrombi and leukocytes within the pulmonary circulation by 24 h. We hypothesized that the microthrombi formed spontaneously as a consequence of hemorrhage-induced stasis within the low-pressure pulmonary circuit and that the leukocytes were attracted to them. This suggested that elimination of the microthrombi, using an inhaled thrombolytic agent, could prevent the neutrophil sequestration after blood loss. To test this hypothesis, we removed 30% of the calculated blood volume from isoflurane-anesthetized, male Sprague-Dawley rats (350-500 g) over 5 min and allowed them to recover. Six hours later, we re-anesthetized the rats and nebulized tissue plasminogen activator (80 or 320 µg/kg), lactated Ringer's solution (LRS), or ipratropium bromide (i-bromide) into their lungs. We used i-bromide as a control after we discovered that nebulized LRS had thrombolytic properties. At 24 h, we removed and fixed the lungs and prepared sections for immunohistochemistry using antibodies against fibrinogen (microthrombi) and CD16 (leukocytes). Digital images of each section were obtained using a confocal microscope. Pixel counts of the images showed significantly less accumulation of microthrombi and leukocytes in lungs nebulized with tissue plasminogen activator or LRS than in non-nebulized lungs or in lungs nebulized with i-bromide (P ≤ 0.05). Lactated Ringer's solution becomes positively charged when nebulized (unlike i-bromide), suggesting that it eliminated microthrombi by fibrin depolymerization. We confirmed this using an in vitro assay. Our results demonstrate that lyses of microthrombi that accumulate in the lung after acute blood loss prevent subsequent leukocyte sequestration.

A method for quantifying blood flow distribution among the alveoli of the lung

This article describes a method for quantifying blood flow distribution among lung alveoli. Our method is based on analysis of trapping patterns of small diameter (4 μm) fluorescent latex particles infused into lung capillaries. Trapping patterns are imaged using confocal microscopy, and the images are analyzed statistically using SAS subroutines. The resulting plots provide a quantifiable way of assessing interalveolar perfusion distribution in a way that has not previously been possible. Methods for using this technique are described, and the SAS routines are included. This technique can be an important tool for learning how this critical vascular bed performs in health and disease.

Evidence for active control of perfusion within lung microvessels

Vasoconstrictors cause contraction of pulmonary microvascular endothelial cells in culture. We wondered if this meant that contraction of these cells in situ caused active control of microvascular perfusion. If true, it would mean that pulmonary microvessels were not simply passive tubes and that control of pulmonary microvascular perfusion was not mainly due to the contraction and dilation of arterioles. To test this idea, we vasoconstricted isolated perfused rat lungs with angiotensin II, bradykinin, serotonin, or U46619 (a thromboxane analog) at concentrations that produced equal flows. We also perfused matched-flow controls. We then infused a bolus of 3 μm diameter particles into each lung and measured the rate of appearance of the particles in the venous effluent. We also measured microscopic trapping patterns of particles retained within each lung. Thirty seconds after particle infusion, venous particle concentrations were significantly lower ( P ≤ 0.05) for lungs perfused with angiotensin II or bradykinin than for those perfused with U46619, but not significantly different from serotonin perfused lungs or matched flow controls. Microscopic clustering of particles retained within the lungs was significantly greater ( P ≤ 0.05) for lungs perfused with angiotensin II, bradykinin, or serotonin, than for lungs perfused with U46619 or for matched flow controls. Our results suggest that these agents did not produce vasoconstriction by a common mechanism and support the idea that pulmonary microvessels possess a level of active control and are not simply passive exchange vessels.

Microthrombus formation may trigger lung injury after acute blood loss

We showed previously that acute blood loss, without resuscitation, caused marked maldistribution of interalveolar perfusion. Because hemorrhage is a known risk factor for the development of lung injury, the goal of our present studies was to determine if there was a correlation between perfusion maldistribution and the subsequent development of lung injury after blood loss. Specifically, we wanted to know if the perfusion maldistribution might be due to microthrombus formation and/or leukocyte sequestration within the pulmonary microcirculation. We bled rats (30% blood loss) and harvested their lungs 45 min or 24 h later. Lungs were prepared for perfusion distribution analysis, Western blot analysis to measure whole-lung fibrinogen concentrations, and for immunohistochemistry to measure fibrin deposition and leukocyte deposition (CD16 fluorescence). Perfusion was significantly maldistributed at 45 min and 24 h (P < 0.05). At 45 min, whole-lung fibrinogen concentrations were less than half that in controls (P < 0.05), whereas numbers of fibrin microthrombi were 2.5-fold greater than control by 45 min (not statistically significant) and were 4.5-fold greater by 24 h (P = 0.01). Leukocyte deposition was two-fold greater than control by 45 min (not statistically significant) and was 4-fold greater by 24 h (P = 0.02). Fibrin-to-leukocyte nearest-neighbor distances remained unchanged (18.1 [SD, 1.1] μm) even as the numbers of both increased with time after blood loss. Our results suggest that soluble fibrinogen polymerized to insoluble fibrin within minutes after acute blood loss, which caused perfusion maldistribution and attracted leukocytes. The development of lung injury after blood loss may be a consequence of leukocyte chemoattraction to fibrin microthrombi that seem to form within minutes after blood loss.

Effects of posture on regional pulmonary blood flow in rats as measured by PET

Using small animal PET with (68)Ga-radiolabeled human albumin microspheres (Ga-68-microspheres), we investigated the effect of posture on regional pulmonary blood flow (PBF) in normal rats. This in vivo method is noninvasive and quantitative, and it allows for repeated longitudinal measurements. The purpose of the experiment was to quantify spatial differences in PBF in small animals in different postures. Two studies were performed in anesthetized, spontaneously breathing Wistar rats. Study 1 was designed to determine PBF in the prone and supine positions. Ga-68-microspheres were given to five prone and eight supine animals. We found that PBF increased in dorsal regions of supine animals (0.75) more than in prone animals (0.70; P = 0.037), according to a steeper vertical gradient of flow in supine than in prone animals. No differences in spatial heterogeneity were detected. Study 2 was designed to determine the effects of tissue distribution on PBF measurements. Because microspheres remained fixed in the lung, PET was performed on animals in the position in which they received Ga-68-microsphere injections and thereafter in the opposite posture. The distribution of PBF showed a preference for dorsal regions in both positions, but the distribution was dependent on the position during administration of the microspheres. We conclude that PET using Ga-68-microspheres can detect and quantify regional PBF in animals as small as the rat. PBF distributions differed between the prone and supine postures and were influenced by the distribution of lung tissue within the thorax.

HEMORRHAGE PROGRESSIVELY DISTURBS INTERALVEOLAR PERFUSION IN THE LUNGS OF RATS

Acute hemorrhage is often followed by devastating lung injury. However, why blood loss should lead to lung injury is not known. One possibility is that hemorrhage rapidly disturbs the distribution of microvascular perfusion at the alveolar level, which may be a triggering event for subsequent injury. We showed previously that a 30% blood loss in rats caused significant maldistribution of interalveolar perfusion within 45 min (J Trauma 60:158, 2006). In this report, we describe results of further exploration of this phenomenon. We wanted to know if perfusion distribution was disturbed at 15 min, when vascular pressures were significantly reduced by the blood loss, compared with those at 45 min, when the pressures had returned substantially toward normal. We hemorrhaged rats by removing 30% of their blood volume. We quantified interalveolar perfusion distribution by statistically analyzing the trapping patterns of 4-microm-diameter fluorescent latex particles infused into the pulmonary circulation 15 (red particles) and 45 min (green particles) after blood removal. We used confocal fluorescence microscopy to digitally image the trapping patterns in sections of the air-dried lungs and used pattern analysis to quantify the patterns in tissue image volumes that ranged from 1,300 alveoli to less than 1 alveolus. LogDI, a measure of perfusion maldistribution, increased from 1.00 +/- 0.15 at 15 min after blood loss to 1.62 +/- 0.24 at 45 min (P < 0.001). These values were 0.86 +/- 0.22 (15 min) and 1.12 +/- 0.24 (45 min) in control rats (P = 0.03). Hemorrhage caused the green (45 min)-to-red (15 min) particle distance to decrease from 35.9 +/- 6.5 to 28.0 +/- 5.1 microm (P = 0.024) and the red-to-green particle distance to remain unchanged (30.2 +/- 5.7 microm [red]; 31.5 +/- 10.0 microm [green] [n.s.]). We conclude that hemorrhage caused a progressive increase in interalveolar perfusion maldistribution over 45 min that did not correspond to reduced arterial pressures or altered blood gases. Our particle distance measurements led us to further conclude that this maldistribution occurred in areas that were perfused at 15 min rather than in previously unperfused areas .

Intrapulmonary shunt during normoxic and hypoxic exercise in healthy humans

This review presents evidence for the recruitment of intrapulmonary arteriovenous shunts (IPAVS) during exercise in normal healthy humans. Support for pre-capillary connections between the arterial and venous circulation in lungs of humans and animals have existed for over one-hundred years. Right-to-left physiological shunt has not been detected during exercise with gas exchange-dependent techniques. However, fundamental assumptions of these techniques may not allow for measurement of a small (1-3%) anatomical shunt, the magnitude of which would explain the entire A-aDO2 typically observed during normoxic exercise. Data from contrast echocardiograph studies are presented demonstrating the development of IPAVS with exercise in 90% of subjects tested. Technetium-99m labeled macroaggregated albumin studies also found exercise IPAVS and calculated shunt to be approximately 2% at max exercise. These exercise IPAVS appear strongly related to the alveolar to arterial PO2 difference, pulmonary blood flow and mean pulmonary artery pressure. Hypoxic exercise was found to induce IPAVS at lower workloads than during normoxic exercise in 50% of subjects, while all subjects continued to shunt during recovery from hypoxic exercise, but only three subjects demonstrated intrapulmonary shunt during recovery from normoxic exercise. We suggest that these previously under-appreciated intrapulmonary arteriovenous shunts develop during exercise, contributing to the impairment in gas exchange typically observed with exercise. Future work will better define the conditions for shunt recruitment as well as their physiologic consequence.

Direct demonstration of 25- and 50-microm arteriovenous pathways in healthy human and baboon lungs

Postmortem microsphere studies in adult human lungs have demonstrated the existence of intrapulmonary arteriovenous pathways using nonphysiological conditions. The aim of the current study was to determine whether large diameter (>25 and 50 microm) intrapulmonary arteriovenous pathways are functional in human and baboon lungs under physiological perfusion and ventilation pressures. We used fresh healthy human donor lungs obtained for transplantation and fresh lungs from baboons (Papio c. anubis). Lungs were ventilated with room air by using a peak inflation pressure of 15 cm H(2)O and a positive end-expiratory pressure of 5 cm H(2)O. Lungs were perfused between 10 and 20 cm H(2)O by using a phosphate-buffered saline solution with 5% albumin. We infused a mixture of 25- and 50-microm microspheres (0.5 and 1 million total for baboons and human studies, respectively) into the pulmonary artery and collected the entire pulmonary venous outflow. Under these conditions, evidence of intrapulmonary arteriovenous anastomoses was found in baboon (n = 3/4) and human (n = 4/6) lungs. In those lungs showing evidence of arteriovenous pathways, 50-microm microspheres were always able to traverse the pulmonary circulation, and the fraction of transpulmonary passage ranged from 0.0003 to 0.42%. These data show that intrapulmonary arteriovenous pathways >50 microm in diameter are functional under physiological ventilation and perfusion pressures in the isolated lung. These pathways provide an alternative conduit for pulmonary blood flow that likely bypasses the areas of gas exchange at the capillary-alveolar interface that could compromise both gas exchange and the ability of the lung to filter out microemboli.

Hemorrhage Causes Interalveolar Perfusion Maldistribution in the Lungs of Anesthetized Rats

BACKGROUND

Lung injury often occurs following hemorrhage and we hypothesized that this might be due to the effects of hemorrhage on perfusion distribution among alveoli. To test this, we measured interalveolar perfusion distribution in anesthetized, spontaneously breathing rats subjected to blood losses of 0%, 10%, 20%, or 30% of calculated blood volume.

METHODS

We measured interalveolar perfusion distribution by analyzing trapping patterns of 4-mum diameter fluorescent latex particles infused into the pulmonary circulation. The particles (2 x 10) were infused 1 hour after each animal had been bled, and the lungs were then removed and air-dried. Using a confocal fluorescence microscope, we collected images of the particles in eight sections of each lung. Each image encompassed 3,360 x 3,360 x 100 microm (approximately 5,000 alveoli), and included 3-4,000 particles. Particle distributions in the images were measured using the method of dispersion index (DI) analysis. A DI value of zero corresponds to a statistically random distribution; the more DI exceeds zero, the more the distribution is clustered or inhomogenous.

RESULTS

The largest DI values for the four groups were: 0%, 0.69 +/- 0.41; 10%, 0.57 +/- 0.58; 20%, 0.72 +/- 0.34; 30%, 1.38 +/- 0.41. The 30% blood loss group had a max DI value approximately twofold greater than those of the other three (p < 0.0001).

CONCLUSIONS

Our results suggest that interalveolar perfusion distribution becomes markedly maldistributed at blood losses of 30%. This contributes to ventilation-perfusion mismatching, and may be a precipitating event for lung injury following hemorrhage.

Functional CT in lung with a conventional scanner: simulations and sampling considerations

Due to rapid transit times, motion artefacts from breathing and the low signal intensity, functional computed tomography (f-CT) studies in lung tissue remain challenging with conventional CT scanners. The purpose of this study is to examine the accuracy of parameter estimates when performing deconvolution analysis with signals from lung tissue. The effects of partial volume averaging in lung tissue, differing transit times, variable vascular and capillary responses, expected noise levels, differing sampling rate and durations were simulated on a computer. Deconvolution using singular-value decomposition (SVD) analysis was performed for realistic lung signals using published and measured values of the arterial input and noise levels. The accuracy, bias and variance of the estimated residue functions and their associated parameter estimates were evaluated. We find that f-CT signals may be measured and analysed using SVD and other deconvolution approaches. Functional CT signals in the lung may be analysed provided that the rise and fall of the tissue and input curves are well sampled (regardless of sampling rate) and noise levels in the lung ROI tissue are approximately 20 HU or less, even for regions of interest that are mostly occupied by air. Estimates of the mean tissue transit time (MTT) are insensitive to air volume. Other decovolution methods such as fast Fourier transform methods provide more accurate estimates of PBF, whereas SVD approaches provide more accurate estimates of pulmonary blood volume and MTT. F-CT of the lung with a conventional scanner should be possible, when the extra dose is not a consideration.

Thromboxane receptor analog, U-46619, redistributes pulmonary microvascular perfusion in isolated rat lungs

Effects of vasoconstriction on the distribution of perfusion among alveoli are not well understood. To address this, we used a new method we developed to determine how microvascular perfusion distribution was affected by a potent vasoconstrictor, the thromboxane receptor analog U-46619. Our method was to infuse 4-microm-diameter fluorescent latex microspheres into the circulation of isolated rat lungs vasoconstricted with U-46619. We used a confocal microscope to image trapping patterns of the particles in dried sections of the lungs and then used dispersion index analysis to quantify the particle patterns in the images, which encompassed approximately 2,000 alveoli. Dispersion indexes revealed significantly more particle clustering (inhomogeneous distribution) in vasoconstricted lungs than in normal flow controls or in controls in which flow was reduced by either lowering pulmonary arterial pressure or raising left atrial pressure. These results suggest that vasoconstriction occurred in the microvessels themselves, which are much smaller vessels than those previously thought to be capable of vasoconstriction.