Nasal nitric oxide and regulation of human pulmonary blood flow in the upright position

Gravity is an important but secondary determinantof regional pulmonary blood flow in upright primates

ROBB W. GLENNY,1,2 SUSAN BERNARD,1 H. THOMAS ROBERTSON,1AND MICHAEL P. HLASTALA1,2

Departments of 1Medicine and of 2Physiology and Biophysics,University of Washington, Seattle, Washington 98195

Glenny, Robb W., Susan Bernard, H. Thomas Robert-son, and Michael P. Hlastala. Gravity is an important butsecondary determinant of regional pulmonary blood flow inupright primates. J. Appl. Physiol. 86(2): 623–632, 1999.—Original studies leading to the gravitational model of pulmo-nary blood flow and contemporary studies showing gravity-independent perfusion differ in the recent use of laboratoryanimals instead of humans. We explored the distribution ofpulmonary blood flow in baboons because their anatomy,serial distribution of vascular resistances, and hemodynamicresponses to hypoxia are similar to those of humans. Fourbaboons were anesthetized with ketamine, intubated, andmechanically ventilated. Different colors of fluorescent micro-spheres were given intravenously while the animals were inthe supine, prone, upright (repeated), and head-down (re-peated) postures. The animals were killed, and their lungswere excised, dried, and diced into ,2-cm3 pieces with thespatial coordinates recorded for each piece. Regional bloodflow was determined for each posture from the fluorescentsignals of each piece. Perfusion heterogeneity was greatest inthe upright posture and least when prone. Using multiple-stepwise regression, we estimate that 7, 5, and 25% ofperfusion heterogeneity is due to gravity in the supine, prone,and upright postures, respectively. Although important, grav-ity is not the predominant determinant of pulmonary perfu-sion heterogeneity in upright primates. Because of anatomicsimilarities, the same may be true for humans.

regional perfusion; spatial heterogeneity; fluorescent micro-spheres

THE PULMONARY CIRCULATION is generally thought to be alargely passive circuit in which blood flow distributionis predominantly determined by the hydrostatic gradi-ent due to gravity. This perspective has dominated boththe interpretation and direction of studies related topulmonary perfusion for the past three decades. How-ever, recent studies that used high-resolution methods(12, 20, 28) and experiments performed in microgravity(33) have now shown that pulmonary perfusion is muchmore heterogeneous than can be explained by gravityalone.

Initial observations on regional pulmonary blood flowdocumented increasing blood flow down the lung (37),and a gravitational mechanism was postulated (39).The association between gravity and regional bloodflow was confirmed by changing the direction or magni-tude of gravity relative to the vertical axis of the lung

(29, 38). Methods used in these studies had relativelylow-spatial resolution that could not measure variabil-ity of isogravitational perfusion and, hence, could notquantify the relative contribution of gravity to bloodflow heterogeneity.

A second fundamental difference between contempo-rary studies and original studies that led to the gravita-tional model of pulmonary blood flow distribution is theuse of laboratory animals rather than humans. Al-though many studies (5, 10, 20, 28, 30, 31, 35) haveconfirmed gravity-independent perfusion heterogene-ity in different species, these observations may notapply to humans. Hughes (22) proposed that, in quadru-peds, gravity may not be as important a determinant ofpulmonary blood flow distribution because the postureof quadrupeds produces relatively smaller lung vol-umes compared with that of humans. Most laboratoryanimals also have a more muscularized vascular sys-tem with a smaller fraction of resistance in the micro-vascular segments (25).

Laboratory animals have been used in recent studiesbecause high-resolution studies are precluded in hu-mans by the necessary postmortem methods. Althoughhumans are the only true mammalian biped, baboonsspend most of their time in the upright posture andhave pulmonary structures and physiology remarkablysimilar to those of humans. The baboon pulmonaryvascular tree parallels the human system from its grossanatomy to the degree of muscularization at the arterio-lar and venular level (25). Gas exchange and hemody-namic responses to hypoxia in the baboon are alsosimilar to humans (16). In the baboon, changes in gasexchange with postural changes are identical to thosein humans (19), and they tolerate the upright posturewell.

We, therefore, explored the spatial distribution ofpulmonary blood flow in baboons in upright, head-down, supine, and prone postures to answer the ques-tion, How important is gravity in determining regionalpulmonary blood flow in an animal model similar tohumans?

METHODS

Experimental protocol. The study was approved by theUniversity of Washington Animal Care Committee. Fourmale baboons (Papio anuba), weighing 23.5–33.0 kg, werechemically restrained with intramuscular ketamine injec-tions, intubated, and mechanically ventilated with air. Tidalvolumes (8–10 ml/kg) and rates were adjusted to produceinitial arterial PCO2 of 35–40 Torr. Once set, tidal volumesand ventilatory rates were not altered. Anesthesia was main-tained with intravenous and intramuscular ketamine. A right

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internal jugular cordis and a carotid catheter were placedwith the use of local anesthesia. A flow-directed pulmonaryarterial catheter was introduced through the right internaljugular cordis. Two forearm peripheral veins were cannu-lated.

Prone and supine postures were obtained by laying theanimals on a horizontal table with their backs parallel to theground. Animals were placed in the head-down posture bydangling their torso over the edge of a table and using theirthighs to support their weight. Their arms and head wereallowed to hang freely. A custom-made chair kept the anesthe-tized animals in a seated upright posture with their backsparallel to the gravitational vector. The animals stabilized ineach posture for 20 min before physiological data wereacquired and microspheres injected.

Fluorescent 15-µm-diameter microspheres (FluoSpheres,Molecular Probes, Eugene, OR) of seven different colors(blue-green, green, yellow-green, orange, red, crimson, andscarlet) were injected intravenously through a forearm veinover 30 s in 5 ml of saline followed by a 10-ml saline flush. Themicrospheres were sonicated and vortexed before injection.Microspheres of one color were injected while the animalswere in one of four postures: upright, head down, supine, orprone. The upright and head-down postures were repeatedfor a total of six postures. The order of postures and color usedin each posture were varied across animals to negate anyeffect of order. Repeated postures were not performed consecu-tively. We injected 2 3 106 microspheres of the first five colorsand 4 3 106 crimson and scarlet microspheres. The seventhcolor was injected at the end of the postural study toinvestigate the effects of prostacyclin on perfusion distribu-tions; data from this injection are not included here. Beforeeach microsphere injection, two sets of stacked breaths wereadministered; arterial blood gases obtained; cardiac outputsdetermined by thermal dilution; and systemic, pulmonary,and airway pressures recorded.

After the final microsphere injection, each animal wasdeeply anesthetized, given heparin, exsanguinated, and killedby intravenous pentobarbital sodium. A sternotomy was

performed, large-bore catheters were placed in the pulmo-nary artery and left atrium, and the thoracic aorta was tiedoff. The lungs were perfused with 2% dextran (mol wt 74,000)in normal saline until they were clear of blood, removed fromthe chest, and allowed to dry inflated at an airway pressure of25 cmH2O.

When dry, the lungs were coated with Kwik Foam (DAP,Dayton, OH), suspended vertically in a plastic-lined squaredbox, and embedded in rapidly setting urethan foam (2 lb.polyol and isocyanate, International Sales, Seattle, WA) tocreate a rigid form to which a three-dimensional coordinatesystem was applied. The foam block was sliced and cut intouniformly sized 1.9-cm3 cubes. Foam adhering to lung pieceswas removed, and each lung piece was weighed and assigneda three-dimensional coordinate and lobe designation.

The fluorescent signals for each color were determined byextracting the fluorescent dyes from each piece with anorganic solvent and then measuring the concentration offluorescence in each sample (9). Spillover from adjacent colorswas corrected by using a matrix inversion method. Relativeblood flow to each lung piece was calculated by dividing themeasured fluorescence of each piece by the mean fluorescenceof all pieces for that color. The data set for each baboonconsisted of an x, y, and z coordinate (Fig. 1) and weight andrelative flow for each lung piece in each posture. The relativeflow to each lung piece in each posture was determined bydividing the fluorescent signal by the weight of each lungpiece and normalizing it to the mean.

To minimize observed flow heterogeneity caused by artifactor measurement noise, pieces weighing ,50 mg were notincluded, thus eliminating uncertainty in flow and in weight.Also, 16–22 pieces containing .25% airway by visual inspec-tion were excluded before analyses in each of the fouranimals.

Statistical analysis. Relative weight-normalized flows areused for all analyses and are hereafter referred to as flow orperfusion. Values are means 6 SD or 95% confidence inter-vals (CI). Pearson’s correlation coefficient (r) calculated be-tween perfusions to lung pieces within a baboon is used to

Fig. 1. Orthogonal coordinate system used to designatespatial coordinates of lung pieces and directional perfu-sion gradients. [From Glenny (8).]

624 REGIONAL PULMONARY BLOOD FLOW IN BABOONS

quantify the relationship between regional perfusions indifferent postures. The coefficient of variation (CV 5 100·SD/mean) is used to characterize perfusion heterogeneity withineach animal over space. ANOVA is used to determine thedifferences between hemodynamic and gas exchange vari-ables among postures. Paired t-tests are used to compareperfusion heterogeneity among postures.

Least squares linear regression is used to characterizedirectional gradients of blood flow. Vertical gradients arecharacterized by the slope of regional flow as a function ofdistance up the lung from the most dependent surface.Because the lung is not uniformly distributed in space (basallung regions are also dorsal), the spatial distributions of lungparenchyma along the three orthogonal axes are not indepen-dent of one another. Ventral-to-dorsal gradients of perfusionin the upright posture are, therefore, explored only withinisogravitational planes by simple linear regression of bloodflow as a function of distance along the ventral-dorsal direc-tion. Similarly, cephalad-to-caudad gradients of perfusion inthe supine and prone postures are explored within isogravita-tional planes by simple linear regression of blood flow as afunction of distance along the cephalad-caudad direction.Radial distances to each lung piece are determined from thespatial coordinates of each piece and the ipsilateral hilum ofeach lung as identified in the transverse lung slices. Radialgradients of perfusion are determined as a function of radialdistance for each piece before and after the subtraction of anyperfusion gradients in the cephalad-caudad direction. Statis-tical significance of any gradient is determined by a two-tailed t-test compared with zero.

The relative contribution of the hydrostatic gradient tototal perfusion heterogeneity can be estimated by usingmultiple-stepwise linear regression. We use a simple model inwhich blood flow distribution, Q̇, to each piece, i, in a givenposture, Q̇posture i, is determined by a structural component offlow, Q̇ i, a hydrostatic pressure, heighti (height up the lung),and a residual component, ei, for each piece

Q̇posture i 5 a · Q̇ i 1 b · heighti 1 ei (1)

This equation is applied to the upright (repeated), supine,and prone postures. ‘‘Structure’’ is defined as the averagevalue of flow for a given piece (designated by i ranging from 1to n lung pieces in a given animal) across all postures andreplications. Because repeat measurements were obtained inthe upright and head-down postures, Q̇ i is calculated fromthe average within each of these postures (producing a singleobservation for each posture). In this context, structure isdefined as those influences that remain constant regardless ofposture and hydrostatic gradients. The hydrostatic effect canbe thought of as adding or subtracting flow from the structurecomponent. The r2 values from multiple-stepwise regressionquantifies how much of the variability in Q̇posturei is determinedby variability in Q̇ i and heighti.

Because flow appears to decrease in the most dependentlung regions, a linear relationship will not account for thiscurvature. A quadratic function of flow as a function of heighti

2

is, therefore, also used to quantify the variability in flow

Q̇posture i 5 a · Q̇ i 1 b · heighti 1 d · heighti2 1 ei (2)

RESULTS

Physiological data. Basic physiological data for eachanimal are shown in Fig. 2. Gas exchange and hemody-namics remained relatively stable in baboons across allpostures. Peak airway pressure increased slightly dur-ing head-down posture, although not statistically signifi-cantly by ANOVA. Alveolar-arterial O2 differences arecalculated by using respiratory quotients of 1.0 becauseof the high-metabolic rates induced by ketamine (6).

Spatial heterogeneity. The number of lung piecesfrom each animal and the mean spatial CV in thedifferent postures are presented in Table 1. Pulmonaryperfusion heterogeneity was greatest in the uprightposture, averaging 65.3%. Compared with the uprightposture, blood flow variability was significantly less in

Fig. 2. Hemodynamic (A) and gas-exchange (B) variables for all animals in all postures. No significant change isobserved in any of the variables other than an increase in peak airway pressure (A, middle) during head-downpostures. Order of postures varied for each animal and does not correspond to order on horizontal axes. PaO2

,Arterial PO2; PaCO2

, arterial PCO2; (A-a)PO2, alveolar-arterial PO2 difference.

625REGIONAL PULMONARY BLOOD FLOW IN BABOONS

head-down, supine, and prone postures (Table 1). Perfu-sion heterogeneity was also significantly less (P 5 0.03)in the prone compared with supine posture.

Vertical gradients. Figure 3 presents reconstructedplanar images of blood flow distribution in one animalduring upright posture. A transverse section demon-strates the heterogeneity of perfusion within an iso-gravitational plane, whereas the sagittal section showsthe gradients of perfusion from cephalad to caudad andventral to dorsal. Figure 4 shows the vertical distribu-tion of blood flow in upright and head-down postures.Vertical gradients in the upright posture average 20.088relative flow units/cm up the lung (Table 2); i.e., ifregional blood flow at one vertical level in the lung isequal to the mean flow (relative flow 5 1.0), regionalblood flow will be 1.88 times the mean flow at a level 10cm lower in the lung. Slopes are negative because flowdecreases toward greater lung heights. In the uprightposture, flow decreases in the basal-to-apical direction,whereas, in the head-down posture, flow decreasesalong the apical-to-basal axis. The 95% CI on the slopeis 20.101 to 20.075 relative flow units/cm. In theupright posture, a strong relationship exists betweenheight up the lung and relative blood flow (mean r2 5

0.474; 95% CI, 0.368–0.579). When the animal isturned head down, vertical gradients are less (Table 2),averaging 20.048 relative flow units/cm (95% CI,20.067 to 20.028). The relationship between verticalheight and blood flow is also weaker in the head-downposture (mean r2 5 0.242; 95% CI, 0.121–0.364).

The difference in the vertical gradients betweenupright and head-down postures suggests that a mech-anism in addition to gravity is responsible for a verticalgradient of perfusion in the upright posture. Thisadditional influence can be explored by averaging theblood flow to each piece between opposing postures,thus nullifying the gravitational influence by cancelingits effect. Figure 5 shows that a vertical gradientremains in the upright lung after the gravitationalinfluence is removed. On average, the vertical gradientin the upright posture after the effect of gravity isnullified is 20.020 relative flow units/cm (95% CI,20.050–0.010). Hence, there appears to be an anatomicbias for blood flow to be greater in the caudad lungregions independent of gravity. This anatomic bias isconfirmed by cephalad-caudad gradients of perfusionwhen animals are supine. Figure 5 shows the increasein blood flow from cephalad to caudad in a supine

Table 1. Perfusion heterogeneity

Animal No. Lung Pieces

CVspatial, %

Upright 1 Upright 2 Head down 1 Head down 2 Supine Prone

1 1,302 62.2 64.1 48.4 41.3 52.3 45.42 1,208 53.1 43.1 42.5 48.4 39.0 34.23 1,265 77.0 76.2 60.4 65.0 58.1 51.74 1,629 72.0 74.4 60.4 58.8 58.7 44.0

Mean6SD 66.1610.6 64.5615.2 52.969.0* 53.4610.6* 52.069.2* 43.867.2*†95% CI 49.1–83.0 40.3–88.6 38.6–67.1 36.6–70.2 37.5–66.6 32.3–55.3

CVspatial, spatial coefficient of variance; CI, confidence interval. *Statistically different from upright posture, P , 0.05; †statisticallydifferent from supine posture, P , 0.05.

Fig. 3. Reconstructed images of a trans-verse and sagittal plane from 1 animalduring upright posture. Each squarerepresents location and relative bloodflow to a lung piece in the given plane.Note heterogeneity of perfusion in iso-gravitational plane. Note also that flowis not randomly distributed, but ratherneighboring pieces tend to have similarmagnitudes of flow. Cephalad-caudad(vertical) gradient is apparent in sagit-tal section.


animal. The gradients are small with an average of20.016 relative flow units/cm (95% CI, 20.036–0.000).

Figure 6 presents the vertical distribution of bloodflow for one animal in supine and prone postures. Onaverage, the vertical gradient during the supine pos-ture was 20.055 relative flow units/cm (95% CI, 20.073to 20.037), and, while the animal was prone, thisgradient did not differ from zero (95% CI, 20.014–0.01).

Vertical gradients in the different postures are sum-marized in Fig. 7. Vertical gradients of perfusion varyamong different postures, suggesting that factors otherthan hydrostatic pressure are important determina-tions of blood flow distribution in the vertical direction.

Partitioning flow heterogeneity. When multiple-stepwise regression is used to quantify the determi-

nants of perfusion heterogeneity in three postures, astructural component accounts for 59, 79, and 80% ofthe total variability in the upright, supine, and pronepostures, respectively (Table 3). Stated another way,when the animals are upright, about two-thirds of thevariability in regional pulmonary perfusion are due tosome factor that remains consistent across all of thepostures. After we account for this source of perfusionheterogeneity in the upright posture, height up thelung accounts for 25% of the total variability. Similarly,height up the lung accounts for only 7 and 5% ofperfusion heterogeneity when the animals are supineand prone, respectively.

When flow is modeled as a quadratic function ofheight up the lung, the relative contributions of struc-ture and height do not change significantly. This sug-

Fig. 4. Vertical distributions of blood flow in 1 animal in upright (A) and head-down postures (B). Independent anddependent axes have been reversed so that the graphs are similar to those familiarized by West (36). Linearregressions are performed with height up the lung as the independent variable. This animal had the weakestrelationship between blood flow and vertical height up the lung. n, No. of lung pieces.

Table 2. Vertical gradients of blood flow in different postures

AnimalNo.

Posture

Upright Head down Supine Prone

Slope r2 Slope r2 Slope r2 Slope r2

1 20.090 0.528 20.018 0.034 20.035 0.057 20.002 0.00020.104 0.670 0.004 0.002

2 20.086 0.605 20.048 0.297 20.062 0.308 0.005 0.00320.054 0.354 20.074 0.536

3 20.078 0.264 20.049 0.167 20.077 0.226 20.016 0.01220.081 0.286 20.062 0.232

4 20.105 0.534 20.069 0.327 20.048 0.113 0.028 0.06920.111 0.553 20.069 0.345

Mean6SD 20.08860.018 0.47460.152 20.04860.027 0.024260.175 20.05560.023 0.17660.113 0.00460.018 0.02160.03295% CI 20.101–20.075 0.368–0.579 20.067–20.028 0.121–0.364 20.073–20.037 0.066–0.286 20.014–0.021 20.010–0.053

Slopes are derived from least squares linear fits of relative blood flow per lung piece as a function of height up the lung (cm). Goodness of fit isrepresented by r2 for each posture. Repeat measures of perfusion distribution are shown for each animal in upright and head-down postures.


gests that the proposed zone 4 of decreasing flow in thedependent lung regions is not significant in these fouranimals.

Ventral-to-dorsal gradients. In the upright posture,regional perfusion increased significantly toward dor-sal regions in only one animal. The ventral-to-dorsal

gradient did not differ from zero in the other threeanimals.

Cephalad-to-caudad gradients. Whereas some ani-mals have significant gradients, a general patterncannot be discerned. In the supine posture, regionalperfusion increased significantly toward caudad re-

Fig. 5. Distributions of blood flow in 1 animal in upright (A) and supine postures (B). A: vertical perfusiondistribution after nullifying effect of gravity by averaging blood flow to each piece between opposing postures. Notesmall gradient of perfusion toward caudad regions. Independent and dependent axes have been reversed so thatgraphs are similar to those familiarized by West (36). B: horizontal distribution of blood flow along the lung insupine posture. Note small gradient of perfusion toward caudad regions. Linear regressions are performed withheight up the lung as the independent variable. n, No. of lung pieces.

Fig. 6. Vertical distributions of blood flow in 1 animal in supine (A) and prone (B) postures. Independent anddependent axes have been reversed so that graphs are similar to those familiarized by West (36). Linear regressionsare performed with height up the lung as the independent variable. n, No. of lung pieces.


gions in two animals and toward the cephalad regionsin one animal. In the prone posture, regional perfusionincreased significantly toward caudad regions in onlyone animal and was not different from zero in the otherthree.

Radial gradients. Figure 8 shows blood flow per pieceas a function of radial distance from the ipsilateral lungto each piece. Although all four animals had smalldecreases (20.15 to 20.01 relative flow units/cm) inperfusion from hila to periphery, the magnitude andvariability in the gradients preclude statistical signifi-cance (95% CI, 20.16–0.03). On average, radial dis-tance from the hilum explains ,7% of the variability inregional blood flow.

DISCUSSION

The data obtained from this study are unique in thatthey describe regional pulmonary blood flow in a pri-mate model with lung structure and vascular anatomysimilar to humans. The methods used allow regionalblood flow measurements to be compared among differ-ent gravitational vectors (changing postures). The dataare of high-fidelity and spatial resolution without recon-struction artifacts of imaging methods. The importantfindings of this study are as follows: 1) pulmonary bloodflow is heterogeneously distributed in the uprightprimate model, 2) the relative contribution of gravity topulmonary perfusion heterogeneity is similar in supineand prone primates compared with other laboratoryanimals, 3) gravity plays a greater role in perfusiondistribution when animals are upright, and 4) althoughimportant, gravity remains a secondary determinant ofregional pulmonary blood flow distribution in the up-right primate. These observations confirm recent stud-ies of pulmonary perfusion heterogeneity in micrograv-ity and provide a new perspective from which to exploredeterminants of pulmonary blood flow distribution.

We estimate that 25% of the observed perfusionheterogeneity in the upright primate can be attribut-able to the gravity-induced hydrostatic gradient downthe lung. The residual perfusion heterogeneity ob-served in isogravitational planes cannot be explainedby hydrostatic gradients. This is an overestimate be-cause our experimental design attributes changes inperfusion observed with postural changes to an effect ofhydrostatic pressure differences. Mediastinal struc-tures, abdominal contents, and the diaphragm likelyshift position in the head-down compared with theupright posture. Any alterations in regional perfusioninduced by these structural changes are attributed to agravitational effect. Any change in regional ventilationthat alters local alveolar O2 pressures may invoke localchanges in perfusion through hypoxic pulmonary vaso-constriction. Hence, we may be overestimating the

Fig. 7. Means and 95% confidence intervals on vertical gradients ofblood flow as a function of height up the lung in different postures.See text for details of slope estimations. Vertical gradients of perfu-sion vary among different postures, suggesting that factors otherthan hydrostatic pressure are important.

Table 3. Relative contribution of structure and heightup the lung to perfusion heterogeneity in the upright,supine, and prone posture

AnimalNo.

r2

Structurer2 Structure

1 Heightr2 Addedby Height

Upright

1 0.64 0.82 0.182 0.52 0.81 0.293 0.65 0.88 0.234 0.55 0.85 0.30Mean 0.59 0.84 0.25

Supine

1 0.77 0.79 0.022 0.71 0.86 0.253 0.91 0.95 0.044 0.78 0.86 0.18Mean 0.79 0.86 0.07

Prone

1 0.77 0.78 0.012 0.72 0.77 0.053 0.93 0.96 0.034 0.79 0.89 0.10Mean 0.80 0.85 0.05

Repeated measures in upright posture were run separately andthen averaged to obtain a single value in each animal.

Fig. 8. Radial distribution of blood flow in 1 animal during uprightposture. Radial distance is measured to each lung piece from theipsilateral hilum. n, No. of lung pieces.


effect of gravity on perfusion heterogeneity in a singleposture.

Our estimate that gravity is responsible for 25% ofperfusion heterogeneity in the upright posture is consid-erably greater than what we predicted from a similarstudy in supine and prone dogs (11). One explanationfor the lesser gravitational effect in these dogs is asmaller vertical gradient in supine and prone dogs (14cm) compared with upright and head-down baboons (25cm). This hypothesis is supported by our observationthat primates and dogs have similar gravitationalcontributions in the supine and prone postures. Asecond explanation for this lesser gravitational effect indogs may be inherent differences between quadrupedsand primates, as proposed by Hughes (22). Hughes etal. (23) believe that relative lung volumes may besmaller in quadrupeds because of their horizontalposture and that gravity plays a less important role atsmaller lung volumes. Most laboratory animals alsohave a more muscularized pulmonary vascular systemwith a smaller fraction of resistance in the microvascu-lar segments (25). If a smaller fraction of vascularresistance resides in microvascular segments of pri-mates and these segments have relatively little tone,hydrostatic pressures may have a greater influence onblood flow distribution (22).

Vertical gradients of perfusion may result from hydro-static pressure differences between the top and bottomof the lung and regional structural differences in thepulmonary vasculature. If the hydrostatic pressuredifferences were the sole determinant of regional perfu-sion, vertical gradients of perfusion should be similar inpostures with equivalent vertical heights. The verticalgradient of perfusion is much greater in the uprightthan in the head-down posture, and it is also greater inthe supine compared with the prone posture, suggest-ing that anatomic factors may be important in determin-ing the vertical distributions of blood flow. These slopesare similar to those seen in other laboratory animals(10, 20, 31, 35).

Radial blood flow gradients and dorsal perfusionbiases are small in upright primate lungs. Prior studieshave documented a large hilar-to-peripheral gradientin dogs (17) and humans (18) by using single-photon-emission computed tomography. Radial gradients havebeen variably documented (35) or dismissed (30) byothers using nonimaging methods. A dorsal blood flowbias has been suggested to explain the more homoge-neous flow distribution in the prone vs. supine posture(4). Although blood flow is more uniform in prone thanin supine baboons, a dorsal predisposition for perfusionis not apparent.

Supporting evidence. In 1970, Reed and Wood (34)first suggested that pulmonary blood flow may not be ashomogeneous as predicted by the gravitational model.Using methods with improved spatial resolution, theyfound that perfusion is not uniform within isogravita-tional planes and concluded that ‘‘the interrelation-ships of determinants of regional pulmonary blood flowin the intact animal are sufficiently complex so thatachievement of an adequate description by a relatively

simple model is not possible at this time.’’ Greenleafand associates (15) confirmed the observation of hetero-geneous blood flow within isogravitational planes usingmethods with even greater spatial resolution. A num-ber of recent studies have confirmed the large heteroge-neity of isogravitational pulmonary blood flow in avariety of laboratory animals (10, 20, 28, 31, 35). Inthose studies exploring isogravitational perfusion het-erogeneity, the CV within horizontal planes was almostas large as the perfusion heterogeneity across allplanes (12, 20). In standing, awake horses with hydro-static differences of 50 cmH2O between the top andbottom of the lung, blood flow increased up the lungrather than down the lung as predicted by the gravita-tional model (20). Exhaled gas-concentration profilesfrom humans in microgravity on the space shuttlequantify the role played by gravity in determiningperfusion, ventilation, and ventilation-perfusion hetero-geneity (32, 33). In these studies, oscillations in ex-haled CO2 were analyzed, and gravity was estimated tobe responsible for 38% (95% CI, 22–54%) of perfusionheterogeneity in standing humans (33).

Methodological considerations. Embolization of micro-spheres has been used extensively to measure regionalorgan blood flow (13, 20). Because of the particulatenature of the microspheres, there has been concernthat microspheres may not faithfully represent bloodflow distribution at the capillary level (26). Using lungpieces similar in size to those in the present study,Melsom and co-workers (28) demonstrated that 15-µm-diameter microsphere estimates of regional blood flowcorrelated well (r 5 0.99) with estimates from a nonpar-ticulate (molecular) marker. Studies by Beck and Re-hder (4) have confirmed that regional pulmonary bloodflow determined by 15-µm microspheres correlated well(r 5 0.91) with regional perfusion measured by indica-tor dilution techniques that used 99mTc-labeled redblood cells. Deposition of microspheres is a generallyaccepted and well-validated method for measuringblood flow to regions with volumes similar to those inthis study (,2 cm3).

Drying lungs at total lung capacity (TLC) alters theirconformation from the in vivo state. Because micro-spheres were administered over a number of breaths,their distribution represents blood flow over the entirerange of tidal breathing. Studies by Liu et al. (27)suggest that displacement for any given piece from itsin vivo position to its TLC position is probably quitesmall and insignificant compared with the magnitudeof the gradients and heterogeneity observed. When theflow per piece is normalized by weight, the observationsin this study are comparable with earlier regionalperfusion studies that used xenon, in which flow wasmeasured per alveolar volume. Because all alveoli arenearly uniform in size at TLC, the weight of anindividual lung piece should be roughly proportional tothe number of alveoli per piece. Because airway compo-sition may artifactually add to the observed perfusionheterogeneity, Melsom and associates (28) comparedperfusion heterogeneity between central lung regionscontaining airways and peripheral pieces void of con-


ducting airways. They found that perfusion heterogene-ity was similar between the two regions and concludedthat the observed isogravitational perfusion heteroge-neity is real and not artifactually increased by lungparenchymal heterogeneity.

Lung volume and shape are difficult to assess in thehead-down posture; they are probably smaller becauseabdominal contents push the diaphragm in a cephaladdirection. The effect may be local along the diaphrag-matic surface or impact the entire lung. The onlyevidence we have that this effect is small is that airwaypressures increased minimally in head-down posture.

Reconciling discrepancies. Spatial distribution of pul-monary blood flow was first studied directly in elegantphysiological experiments during the early 1960s. Ra-dioactive gases were used to measure regional distribu-tion of blood flow by one of two methods (1, 2, 7, 24):either an insoluble gas was given intravenously andradioactive counts recorded over the chest wall, or asoluble gas was inhaled and gas clearance from thealveolar space measured in a similar fashion. In all ofthese studies, blood flow appeared to increase withdistance down the lung.

Because the scintillation counters in these initialstudies were unable to measure perfusion variabilitywithin isogravitational planes, only the vertical hetero-geneity of perfusion was observed. Spatial resolution ofrecent animal experiments can be reduced to the levelof earlier experiments using chest wall scintillationcounters by averaging data within isogravitationalplanes. When high-resolution data sets are averaged toyield a few isogravitational planes, vertical height upthe lung becomes the key determinant of blood flowdistribution. Thus recent findings do not conflict witholder data; higher resolution methods simply revealother mechanisms that could not be observed previ-ously. Even higher resolution observations representa-tive of blood flow at the alveolar capillary level willshow a greater degree of heterogeneity and a lessercontribution of gravity to perfusion variability.

Isogravitational heterogeneity and zone model. Ban-nister and Torrance (3) and Howell et al. (21) were thefirst to publish on the notion of alveolar pressuresinfluencing pulmonary blood flow. Hughes, West, andothers (23, 24, 37) synthesized the observations exist-ing at the time into a model of pulmonary blood flowdistribution (39) describing horizontal ‘‘zones’’ of flow.Within these vertically stacked zones, regional bloodflow is determined by the relationship between thethree pressures (arterial, venous, and alveolar).

In a branching vascular tree, structural heterogene-ities produce variabilities in local driving pressures andresistances. Traditionally, the gravitational model ispresented with single arterial and venous hydrostaticpressures at a given vertical height (39). Whereas thehydrostatic pressure is determined by a stagnant col-umn of blood, pressure drops and driving pressures atthe microvascular level are determined by vascularbranching angles and serial resistances to flowingblood. Driving pressures within isogravitational planesshould, therefore, be heterogeneous (26). When re-

gional conditions are considered, the driving pressuresof interest are not arterial and venous pressures butrather the local microvascular and alveolar pressures.

Because of this heterogeneity in driving pressures,multiple zonal conditions must exist within a horizon-tal plane (14). In a departure from the classic gravita-tional model, each isogravitational plane can, there-fore, have a distribution of pressure relationships.Gravity exerts its effects on blood flow distribution byincreasing the hydrostatic pressure toward dependentlung regions, altering the statistical distribution ofthese regions. The frequency distribution likely shiftstoward zone 2 and 3 conditions down the lung, eventu-ally resulting in only zone 3 conditions in the mostdependent regions.

Conclusions and implications for gas exchange. Thegravitational hypothesis that attributes distribution ofboth blood flow and ventilation in the lung has been acornerstone of pulmonary physiology. We wish to ad-vance the concept that gravity alone cannot adequatelyaccount for recent experimental observations of pulmo-nary perfusion; additional factors must be considered.These proposals are based on new concepts of fractalvascular trees and perfusion heterogeneity (12, 13);they offer a new perspective from which to exploredeterminants of regional blood flow.

The reorientation induced by this new perspectiveprovides important insights into the primary functionof gas exchange in the lung. Despite heterogeneousdistribution of perfusion at the alveolar-capillary level,gases are exchanged efficiently. The traditional modelof gas exchange proposes a small unit of ventilation andits companion capillaries. The common effect of gravityis invoked to explain matching of ventilation andperfusion (22). However, given the degree of isogravita-tional perfusion heterogeneity, matching of ventilationand perfusion cannot be governed by gravity alone.This prediction was verified recently in studies by Priskand colleagues (32) in Spacelab Life Sciences (SLS)-1and SLS-2; under conditions of microgravity, ventilation-perfusion inequalities persist. The authors of this studyconclude that the principle determinants of ventilation-perfusion inequalities are not gravitational in origin.

We thank Heather McCown and John Wehrich, University ofWashington Primate Center, for technical assistance with the animalexperiments and Dowon An for assistance with lung sample process-ing.

Address for reprint requests: R. W. Glenny, Univ. of Washington,Division of Pulmonary and Critical Care Medicine, Box 356522,Seattle, WA 98195 (E-mail: [email protected]).

Received 1 June 1998; accepted in final form 27 October 1998.

REFERENCES

1. Anthonisen, N. R., and J. Milic-Emili. Distribution of pulmo-nary perfusion in erect man. J. Appl. Physiol. 21: 760–766, 1966.

2. Ball, W. C., P. B. Stewart, L. G. S. Newsham, and D. V. Bates.Regional pulmonary function studied with xenon. J. Clin. Invest.41: 519–531, 1962.

3. Banister, J., and R. W. Torrance. The effects of trachealpressure upon flow-pressure relations in the vascular bed ofisolated lungs. Q. J. Exp. Physiol. 45: 352–367, 1960.

4. Beck, K. C., and K. Rehder. Differences in regional vascularconductances in isolated dog lungs. J. Appl. Physiol. 61: 530–538, 1986.


5. Caruthers, S. D., and T. R. Harris. Effects of pulmonary bloodflow on the fractal heterogeneity in sheep lungs. J. Appl. Physiol.77: 1474–1479, 1994.

6. Dejong, C. H., N. E. Deutz, and P. B. Soeters. A simple newmethod for repeated in vivo cerebral cortex flux measurement inrats. Lab. Anim. Sci. 42: 280–285, 1992.

7. Dollery, C. T., and P. M. S. Gillam. The distribution of bloodand gas within the lungs measured by scanning after administra-tion of 133Xe. Thorax 18: 316–325, 1963.

8. Glenny, R. W. Spatial correlation of regional pulmonary perfu-sion. J. Appl. Physiol. 72: 2378–2386, 1992.

9. Glenny, R. W., S. Bernard, and M. Brinkley. Validation offluorescent-labeled microspheres for measurement of regionalorgan perfusion. J. Appl. Physiol. 74: 2585–2597, 1993.

10. Glenny, R. W., W. J. Lamm, R. K. Albert, and H. T. Robert-son. Gravity is a minor determinant of pulmonary blood flowdistribution. J. Appl. Physiol. 71: 620–629, 1991.

11. Glenny, R. W., L. Polissar, and H. T. Robertson. Relativecontribution of gravity to pulmonary perfusion heterogeneity. J.Appl. Physiol. 71: 2449–2452, 1991.

12. Glenny, R. W., and H. T. Robertson. Fractal properties ofpulmonary blood flow: characterization of spatial heterogeneity.J. Appl. Physiol. 69: 532–545, 1990.

13. Glenny, R. W., and H. T. Robertson. Fractal modeling ofpulmonary blood flow heterogeneity. J. Appl. Physiol. 70: 1024–1030, 1991.

14. Glenny, R. W., and H. T. Robertson. Regional differences in thelung: a changing perspective on blood flow distribution. In:Complexity in Structure and Function of the Lung, edited by M. P.Hlastala and H. T. Robertson. New York: Dekker, 1998, p.461–481.

15. Greenleaf, J. F., E. L. Ritman, D. J. Sass, and E. H. Wood.Spatial distribution of pulmonary blood flow in dogs in leftdecubitus position. Am. J. Physiol. 227: 230–244, 1974.

16. Guenter, C. A., D. R. McCaffree, L. J. Davis, and V. Smith.Hemodynamic characteristics and blood gas exchange in thenormal baboon. J. Appl. Physiol. 25: 507–510, 1968.

17. Hakim, T. S., G. W. Dean, and R. Lisbona. Effect of bodyposture on spatial distribution of pulmonary blood flow. J. Appl.Physiol. 64: 1160–1170, 1988.

18. Hakim, T. S., R. Lisbona, and G. W. Dean. Gravity-indepen-dent inequality in pulmonary blood flow in humans. J. Appl.Physiol. 63: 1114–1121, 1987.

19. Herman, C. M., L. D. Homer, and D. L. Horwitz. Effects ofsedation and posture on pulmonary gas exchange in the baboon.J. Appl. Physiol. 30: 498–501, 1971.

20. Hlastala, M. P., S. L. Bernard, H. H. Erickson, M. R. Fedde,E. M. Gaughan, R. McMurphy, M. J. Emery, N. Polissar, andR. W. Glenny. Pulmonary blood flow distribution in standinghorses is not dominated by gravity. J. Appl. Physiol. 81: 1051–1061, 1996.

21. Howell, J. B. L., S. Permutt, D. F. Proctor, and R. L. Riley.Effect of inflation of the lung on different parts of pulmonaryvascular bed. J. Appl. Physiol. 16: 71–76, 1961.

22. Hughes, J. M. B. Invited Editorial on pulmonary blood flowdistribution in exercising and in resting horses. J. Appl. Physiol.81: 1049–1050, 1996.

23. Hughes, J. M. B., J. B. Glazier, J. E. Maloney, and J. B. West.Effect of lung volume on the distribution of pulmonary blood flowin man. Respir. Physiol. 4: 58–72, 1968.

24. Kaneko, K., J. Milic-Emili, M. B. Dolovich, A. Dawson, andD. V. Bates. Regional distribution of ventilation and perfusion asa function of body position. J. Appl. Physiol. 21: 767–777, 1966.

25. Kay, M. J. Comparative morphologic features of the pulmonaryvasculature in mammals. Am. Rev. Respir. Dis. 128: S52–S57,1983.

26. Kuhnle, G. E., A. R. Pries, and A. E. Goetz. Distribution ofmicrovascular pressure in arteriolar vessel trees of ventilatedrabbit lungs. Am. J. Physiol. 265 (Heart Circ. Physiol. 34):H1510–H1515, 1993.

27. Liu, S., S. S. Margulies, and T. A. Wilson. Deformation of thedog lung in the chest wall. J. Appl. Physiol. 68: 1979–1987, 1990.

28. Melsom, M. N., T. Flatebo, J. Kramer-Johansen, A. Aulie,O. V. Sjaastad, P. O. Iversen, and G. Nicolaysen. Both gravityand non-gravity dependent factor determine regional blood flowwithin the goat lung. Acta Physiol. Scand. 153: 343–353, 1995.

29. Michels, D. B., and J. B. West. Distribution of pulmonaryventilation and perfusion during short periods of weightlessness.J. Appl. Physiol. 45: 987–998, 1978.

30. Nicolaysen, G., J. Shepard, M. Onizuka, T. Tanita, R. S.Hattner, and N. C. Staub. No gravity-independent gradient ofblood flow distribution in dog lung. J. Appl. Physiol. 63: 540–545,1987.

31. Parker, J. C., J. L. Ardell, C. R. Hamm, S. A. Barman, andP. J. Coker. Regional pulmonary blood flow during rest, tilt, andexercise in unanesthetized dogs. J. Appl. Physiol. 78: 838–846,1995.

32. Prisk, G. K., A. R. Elliot, H. J. B. Guy, J. M. Kosonen, andJ. B. West. Pulmonary gas exchange and its determinantsduring sustained microgravity on Spacelabs SLS-1 and SLS-2. J.Appl. Physiol. 79: 1290–1298, 1995.

33. Prisk, G. K., H. J. B. Guy, A. R. Elliot, and J. B. West.Inhomogeneity of pulmonary perfusion during sustained micro-gravity on SLS-1. J. Appl. Physiol. 76: 1730–1738, 1994.

34. Reed, J. H., and E. H. Wood. Effect of body position on verticaldistribution of pulmonary blood flow. J. Appl. Physiol. 28:303–311, 1970.

35. Walther, S., K. Domino, N. Polissar, R. W. Glenny, and M. P.Hlastala. Pulmonary blood flow distribution has a hilar-to-peripheral gradient in awake, prone sheep. J. Appl. Physiol. 82:678–685, 1997.

36. West, J. B. Regional differences in the lung. Chest 74: 426–437,1978.

37. West, J. B., and C. T. Dollery. Distribution of blood flow andventilation-perfusion in the lung, measured with radioactiveCO2. J. Appl. Physiol. 15: 405–410, 1960.

38. West, J. B., C. T. Dollery, M. E. Matthews, and P. Zardini.Distribution of blood flow and ventilation in saline-filled lung. J.Appl. Physiol. 20: 1107–1117, 1965.

39. West, J. B., C. T. Dollery, and A. Naimark. Distribution ofblood flow in isolated lung: relation to vascular and alveolarpressures. J. Appl. Physiol. 55: 1341–1348, 1964.


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