Inert gas diffusion method for measurement of blood flow using saturation techniques. Comparison with directly measured blood flow in isolated gastrocnemius muscle of the cat

Altering blood flow does not reveal differences between nitrogen and helium kinetics in brain or in skeletal miracle in sheep

In underwater diving, decompression schedules are based on compartmental models of nitrogen and helium tissue kinetics. However, these models are not based on direct measurements of nitrogen and helium kinetics. In isoflurane-anesthetized sheep, nitrogen and helium kinetics in the hind limb (n = 5) and brain (n = 5) were determined during helium-oxygen breathing and after return to nitrogen-oxygen breathing. Nitrogen and helium concentrations in arterial, femoral vein, and sagittal sinus blood samples were determined using headspace gas chromatography, and venous blood flows were monitored continuously using ultrasonic Doppler. The experiment was repeated at different states of hind limb blood flow and cerebral blood flow. Using arterial blood gas concentrations and blood flows as input, parameters and model selection criteria of various compartmental models of hind limb and brain were estimated by fitting to the observed venous gas concentrations. In both the hind limb and brain, nitrogen and helium kinetics were best fit by models with multiexponential kinetics. In the brain, there were no differences in nitrogen and helium kinetics. Hind limb models fit separately to the two gases indicated that nitrogen kinetics were slightly faster than helium, but models with the same kinetics for both gases fit the data well. In the hind limb and brain, the blood:tissue exchange of nitrogen is similar to that of helium. On the basis of these results, it is inappropriate to assign substantially different time constants for nitrogen and helium in all compartments in decompression algorithms.

Hyperbaric conditions

Exposure to elevated ambient pressure (hyperbaric conditions) occurs most commonly in underwater diving, during which respired gas density and partial pressures, work of breathing, and physiological dead space are all increased. There is a tendency toward hypercapnia during diving, with several potential causes. Most importantly, there may be reduced responsiveness of the respiratory controller to rising arterial CO₂, leading to hypoventilation and CO₂ retention. Contributory factors may include elevated arterial PO₂, inert gas narcosis and an innate (but variable) tendency of the respiratory controller to sacrifice tight control of arterial CO₂ when work of breathing increases. Oxygen is usually breathed at elevated partial pressure under hyperbaric conditions. Oxygen breathing at modest hyperbaric pressure is used therapeutically in hyperbaric chambers to increase arterial carriage of oxygen and diffusion into tissues. However, to avoid cerebral and pulmonary oxygen toxicity during underwater diving, both the magnitude and duration of oxygen exposure must be managed. Therefore, most underwater diving is conducted breathing mixtures of oxygen and inert gases such as nitrogen or helium, often simply air. At hyperbaric pressure, tissues equilibrate over time with high inspired inert gas partial pressure. Subsequent decompression may reduce ambient pressure below the sum of tissue gas partial pressures (supersaturation) which can result in tissue gas bubble formation and potential injury (decompression sickness). Risk of decompression sickness is minimized by scheduling time at depth and decompression rate to limit tissue supersaturation or size and profusion of bubbles in accord with models of tissue gas kinetics and bubble formation and growth.

Inert gas clearance from tissue by co-currently and counter-currently arranged microvessels

To elucidate the clearance of dissolved inert gas from tissues, we have developed numerical models of gas transport in a cylindrical block of tissue supplied by one or two capillaries. With two capillaries, attention is given to the effects of co-current and counter-current flow on tissue gas clearance. Clearance by counter-current flow is compared with clearance by a single capillary or by two co-currently arranged capillaries. Effects of the blood velocity, solubility, and diffusivity of the gas in the tissue are investigated using parameters with physiological values. It is found that under the conditions investigated, almost identical clearances are achieved by a single capillary as by a co-current pair when the total flow per tissue volume in each unit is the same (i.e., flow velocity in the single capillary is twice that in each co-current vessel). For both co-current and counter-current arrangements, approximate linear relations exist between the tissue gas clearance rate and tissue blood perfusion rate. However, the counter-current arrangement of capillaries results in less-efficient clearance of the inert gas from tissues. Furthermore, this difference in efficiency increases at higher blood flow rates. At a given blood flow, the simple conduction-capacitance model, which has been used to estimate tissue blood perfusion rate from inert gas clearance, underestimates gas clearance rates predicted by the numerical models for single vessel or for two vessels with co-current flow. This difference is accounted for in discussion, which also considers the choice of parameters and possible effects of microvascular architecture on the interpretation of tissue inert gas clearance.

Prolonged exercise-induced stimulation of skeletal muscle glucose uptake is due to sustained increases in tissue perfusion and fractional glucose extraction

CONTEXT

The mechanisms behind the positive effects of physical activity on glucose metabolism in skeletal muscle and the time course of the effects need to be more elucidated.

OBJECTIVE

The aim was to examine the prolonged effects of an acute bout of one-legged exercise on local skeletal muscle glucose utilization and tissue perfusion.

DESIGN AND SETTING

Interstitial glucose concentration, local tissue perfusion, glucose uptake, and effects of insulin infusion were studied 12 h after an acute bout of exercise and without prior exercise.

PARTICIPANTS

Ten healthy subjects, five women and five men, participated in the study.

INTERVENTION

Microdialysis measurements, (133)Xe clearance, and a 2-h hyperinsulinemic euglycemic clamp were performed on two occasions.

MAIN OUTCOME MEASURES

We measured interstitial glucose concentration and tissue perfusion in the quadriceps femoris muscle of both legs.

RESULTS

Tissue perfusion (3.3 ± 0.6 ml × 100 g(-1) × min(-1) vs. 1.4 ± 0.2 ml × 100 g(-1) × min(-1); P = 0.007) and basal glucose uptake (2.3 ± 0.5 μmol × 100 g(-1) × min(-1) vs. 0.9 ± 0.2 μmol × 100 g(-1) × min(-1); P = 0.006) were increased in the leg that had exercised compared to the resting leg; the findings in the resting leg were comparable to those in the control experiment without prior exercise. The relative effect of insulin on fractional skeletal muscle glucose uptake was the same in all experimental settings, and insulin did not affect tissue perfusion.

CONCLUSIONS

The prolonged stimulatory effect of physical exercise on skeletal muscle glucose uptake was mediated via vascular effects combined with an increase in basal glucose transport independent of enhancement of insulin responses.

Catecholamine regulation of local lactate production in vivo in skeletal muscle and adipose tissue: role of -adrenoreceptor subtypes

CONTEXT

The regulation of lactate production in skeletal muscle (SM) and adipose tissue (AT) is not fully elucidated.

OBJECTIVE

Our objective was to investigate the catecholamine-mediated regulation of lactate production and blood flow in SM and AT in healthy, normal-weight subjects by using microdialysis.

METHODS

First, lactate levels in SM and AT were measured during an iv norepinephrine infusion (n = 11). Local blood flow was determined with the 133Xe-clearance technique. Second, muscle lactate was measured during hypoglycemia and endogenous epinephrine stimulation (n = 12). Third, SM was perfused with selective beta(1-3)-adrenoreceptor agonists in situ (n = 8). Local blood flow was measured with the ethanol perfusion technique.

RESULTS

In response to iv norepinephrine, the fractional release of lactate (difference between tissue and arterial lactate) increased by 40% in SM (P = 0.001), whereas remaining unchanged in AT. Blood flow decreased by 40% in SM (P < 0.005) and increased by 50% in AT (P < 0.05). In response to hypoglycemia, epinephrine increased 10-fold, and the fractional release of lactate in SM doubled (P < 0.0001). The blood flow remained unchanged. The beta2-agonist, terbutaline, caused a marked concentration-dependent increase of muscle lactate and blood flow (P < 0.0001). The beta(1)-agonist, dobutamine, induced a discrete increase of muscle lactate (P < 0.0001), and the blood flow remained unchanged. The beta3-agonist, CPG 12177, did not affect muscle lactate or blood flow.

CONCLUSIONS

Catecholamines stimulate lactate production in SM, but not in AT. In SM, the beta2-adrenoreceptor is the most important beta-adrenergic receptor subtype in the regulation of lactate production.

Countercurrent compartmental models describe hind limb skeletal muscle helium kinetics at resting and low blood flows in sheep

AIMS

This study evaluated the relative importance of perfusion and diffusion mechanisms in compartmental models of blood : tissue helium exchange in a predominantly skeletal muscle tissue bed in the sheep hind limb. Helium has different physiochemical properties from previously studied gases and is a common diluent gas in underwater diving where decompression schedules are based on theoretical models of inert gas kinetics.

METHODS

Helium kinetics across skeletal muscle were determined during and after 20 min of helium inhalation, at separate resting and low steady-states of femoral vein blood flow in six sheep under isoflurane anaesthesia. Helium concentrations in arterial and femoral vein blood were determined using gas chromatographic analysis and femoral vein blood flow was monitored continuously. Parameters and model selection criteria of various perfusion-limited or perfusion-diffusion compartmental models of skeletal muscle were estimated by simultaneous fitting of the models to the femoral vein helium concentrations for both blood flow states.

RESULTS

A model comprising two parallel perfusion-limited compartment models fitted the data well but required a 51-fold difference in relative compartment perfusion that did not seem physiologically plausible. Models that allowed a countercurrent diffusion exchange of helium between arterial and venous vessels outside of the tissue compartments provided better overall fit of the data and credible parameter estimates.

CONCLUSIONS

These results suggest a role of arterial-venous diffusion in blood : tissue helium equilibration in skeletal muscle.

Marked heterogeneity of human skeletal muscle lipolysis at rest

In this study, variations in lipolysis among different muscle groups were examined by measuring local net glycerol release in vivo in healthy, normal-weight subjects (n = 11) during rested, postabsorptive conditions. Microdialysis of the gastrocnemius, deltoid, and vastus lateralis muscle regions revealed that extracellular glycerol concentrations of these three muscle regions were 84.7 +/- 6.7, 59.7 + 7.3, and 56.4 +/- 7.5 micro mol/l, respectively, and the arterial plasma glycerol concentration was 44.8 +/- 2.3 micro mol/l (P = 0.0003-0.006, gastrocnemius vs. others). Local tissue blood flow, as measured by Xe clearance, did not differ among the regions. Net glycerol release was significantly higher in gastrocnemius muscle than in the two other regions. There were no regional differences in glycerol uptake when studied during glycerol infusion. Gastrocnemius muscle showed a dominance of type 1 fibers (70%), whereas the vastus lateralis muscle had equal distribution of fiber types (P = 0.02). No differences in intramuscular triaclyceride content, perimuscular fat, or the adipocyte-specific protein perilipin were observed among the muscle regions. Triglyceride turnover in the gastrocnemius muscle was 3.3 + 1.4% over 24 h, which is about 10 times more rapid than the turnover rate in subcutaneous adipose tissue (P < 0.01). Thus there were marked differences in lipolytic activity among skeletal muscle groups at rest, possibly reflecting variations in fiber type.

Evidence for a major role of skeletal muscle lipolysis in the regulation of lipid oxidation during caloric restriction in vivo

A lipolytic process in skeletal muscle has recently been demonstrated. However, the physiological importance of this process is unknown. We investigated the role of skeletal muscle lipolysis for lipid utilization during caloric restriction in eight obese women before and after 11 days of very low-calorie diet (VLCD) (2.2 MJ per day). Subjects were studied with indirect calorimetry and microdialysis of skeletal muscle and adipose tissue in order to analyze substrate utilization and glycerol (lipolysis index) in connection with a two-step euglycemic-hyperinsulinemic (12 and 80 mU/m(2). min) clamp. Local blood flow rates in the two tissues were determined with (133)Xe-clearance. Circulating free fatty acids and glycerol decreased to a similar extent during insulin infusion before and during VLCD, and there was a less marked insulin-induced reduction in lipid oxidation during VLCD. Adipose tissue glycerol release was hampered by insulin infusion to the same extent ( approximately 40%) before and during VLCD. Skeletal muscle glycerol release was not influenced by insulin before VLCD. However, during VLCD insulin caused a marked (fivefold) (P < 0.01) increase in skeletal muscle glycerol release. The effect was accompanied by a fourfold stimulation of skeletal muscle blood flow (P < 0.01). We propose that, during short-term caloric restriction, the reduced ability of insulin to inhibit lipids, despite a preserved antilipolytic effect of the hormone in adipose tissue, is caused by an augmented mobilization of fat from skeletal muscle, and that a physiological role of muscle lipolysis provides a local source of fatty acids.

Diffusion-limited tissue equilibration and arteriovenous diffusion shunt describe skeletal muscle nitrous oxide kinetics at high and low blood flows in sheep

This study evaluated the relative importance of perfusion and diffusion mechanisms in compartmental models of blood : tissue inert gas exchange in skeletal muscle. Nitrous oxide kinetics in a hind limb skeletal muscle bed were determined during and after 20 min of nitrous oxide inhalation, at separate low and high steady states of hind limb blood flow in five sheep under halothane anaesthesia. Nitrous oxide concentrations in arterial and femoral vein blood were determined using gas chromatographic analysis and femoral vein blood flow was monitored continuously. Parameters and model selection criteria of various perfusion- or diffusion-limited structural models of skeletal muscle were estimated by simultaneous fitting of the models to the mean observed femoral vein nitrous oxide concentration for both blood flow states. Purely perfusion-limited models fit the data poorly. Models that allowed a diffusion-limited exchange of nitrous oxide between a perfusion-limited tissue compartment and an unperfused deep compartment provided better overall fit of the data and credible parameter estimates. The data was best described by allowing, in addition to diffusion-limited tissue equilibration, counter current diffusion of nitrous oxide between arterial and venous blood. The level of tissue blood flow modifies the magnitudes of both these diffusion effects. These results suggest a dual role of diffusion in blood : tissue inert gas equilibration in skeletal muscle.

The physiological kinetics of nitrogen and the prevention of decompression sickness

Decompression sickness (DCS) is a potentially crippling disease caused by intracorporeal bubble formation during or after decompression from a compressed gas underwater dive. Bubbles most commonly evolve from dissolved inert gas accumulated during the exposure to increased ambient pressure. Most diving is performed breathing air, and the inert gas of interest is nitrogen. Divers use algorithms based on nitrogen kinetic models to plan the duration and degree of exposure to increased ambient pressure and to control their ascent rate. However, even correct execution of dives planned using such algorithms often results in bubble formation and may result in DCS. This reflects the importance of idiosyncratic host factors that are difficult to model, and deficiencies in current nitrogen kinetic models. Models describing the exchange of nitrogen between tissues and blood may be based on distributed capillary units or lumped compartments, either of which may be perfusion- or diffusion-limited. However, such simplistic models are usually poor predictors of experimental nitrogen kinetics at the organ or tissue level, probably because they fail to account for factors such as heterogeneity in both tissue composition and blood perfusion and non-capillary exchange mechanisms. The modelling of safe decompression procedures is further complicated by incomplete understanding of the processes that determine bubble formation. Moreover, any formation of bubbles during decompression alters subsequent nitrogen kinetics. Although these factors mandate complex resolutions to account for the interaction between dissolved nitrogen kinetics and bubble formation and growth, most decompression schedules are based on relatively simple perfusion-limited lumped compartment models of blood: tissue nitrogen exchange. Not surprisingly, all models inevitably require empirical adjustment based on outcomes in the field. Improvements in the predictive power of decompression calculations are being achieved using probabilistic bubble models, but divers will always be subject to the possibility of developing DCS despite adherence to prescribed limits.

Nutritive and non-nutritive blood flow: rest and exercise

There is growing evidence to support the notion of two vascular routes within, or closely associated with skeletal muscle. One route is in intimate contact with muscle cells (hence is known as 'nutritive') and the other functions as a vascular shunt (and has had the interesting misnomer of 'non-nutritive'). Recent findings suggest that the 'non-nutritive' route may, in part, be those vessels in closely associated (interlacing?) connective tissue that nourishes attached fat cells, and may form the basis of 'marbling' of muscle in obesity. In addition, embolism studies using various size microspheres indicate that the 'non-nutritive' vessels are likely to be capillaries fed by terminal arterioles that branch from the same transverse arterioles as those supplying terminal arterioles of the muscle capillaries (i.e. two vascular systems operating in parallel). The proportion of flow distributed between the two routes is tightly regulated and controls muscle metabolism and contraction by regulating hormone and substrate delivery as well as product removal. Because a high proportion of nutritive flow may elevate the set point for basal metabolism, a low proportion of nutritive flow in muscle at rest confers an evolutionary advantage, particularly when food is scarce. In addition, the proportion of flow that is carried by the non-nutritive routes at rest affords a flow reserve that can be switched to the nutritive route to amplify nutrient supply during exercise. Alternatively the non-nutritive route may allow flow to escape when active muscle contraction compresses its nutritive capillaries. Thus rhythmic oscillation of blood flow between the non-nutritive and nutritive networks may aid the muscle pump.

Mucosal/submucosal blood flow in the small intestine in pigs determined by local washout of 133Xe and microsphere techniques

In 11 anaesthetized pigs a laparotomy was performed and the mucosal and submucosal blood flow rate in the small intestine of the pig was determined by a local application of 133Xe and by 6.5-microns radioactive microspheres. The 133Xe washout plotted in a semilogarithmic diagram showed a multiexponential configuration. As localization studies of 133Xe in the intestinal mucosa showed a constant high concentration of 133Xe in the luminal part of the mucosa due to shunting by diffusion, the initial slope of the 133Xe washout was used for blood flow determination in the mucosa/submucosa. There was a good relationship between blood flow determined by the two techniques. The correlation coefficient, R, between the two techniques was 0.89.

Inert gas wash-out from tissue: model analysis

Model simulations on variously arranged two-compartment models are performed to provide a basis for interpreting the observed non-monoexponential (or non-linear logarithmic) wash-out time courses of inert gases from tissue. The variables considered are: blood flow, tissue volume, solubility of gas in tissue and blood, and diffusive conductances (diffusing capacities) for tissue/blood gas transfer and for gas transfer between tissue compartments. The wash-out is studied in terms of both mean tissue partial pressure and effluent venous blood partial pressure. Diffusion limitation within a tissue-blood capillary unit is shown to produce logarithmic wash-out rates which increase or decrease during wash-out, depending on the functional structure of the unit. On the other hand, in a system consisting of dissimilar tissue-blood capillary units arranged in parallel, the logarithmic wash-out rate decreases with wash-out time. It is shown that the conventional analysis of nonlinear logarithmic wash-out may overestimate or underestimate tissue perfusion or the extent of its inhomogeneity.

Temporal muscle blood flow in common migraine

Temporal muscle blood flow (TMBF) was measured by the local 133-Xenon washout technique in 61 patients suffering from common migraine. Nineteen were re-examined in the course of spontaneous attacks. Muscle tension was quantified by surface EMG. During the attacks, median TMBF increased insignificantly, 33% on the ipsilateral and 24% on the contralateral side. During 15 unilateral attacks, the ipsilateral-contralateral ratio of TMBF was 1.02. Isometric and dynamic work tests showed intact metabolic regulation of TMBF. These results speak against a general vasomotor disturbance of the extracranial tissues during attacks of common migraine. There was also no indication that ischemia of the temporal muscle might explain the pain.

Dual role of diffusion in tissue gas exchange: blood-tissue equilibration and diffusion shunt

The role of diffusion in tissue gas exchange is investigated using a simple mathematical model which incorporates both tissue-blood equilibration and gas transfer between arterial and venous vessels with counter-current flow, leading to 'diffusion shunt'. Both increasing the diffusion coefficient of the gas considered or decreasing the blood flow results in two antagonistic effects: (i) improvement of blood/tissue equilibration, (ii) increase in extent of diffusion shunt. The diffusion shunt retards inert gas wash-out (local tissue clearance) and leads thus, if not taken into account, to an underestimation of capillary blood flow calculated from the wash-out rate constant. For O2 (and CO2) the diffusion shunt reduces the efficacy of blood/tissue transfer, but its extent is expected to be only moderate because of the chemical combination of these gases in blood.

Elements for modeling inert gas washout from heterogeneous tissues

Inert gas washout from tissue may be analyzed using Krogh's cylinder or a simplified model thereof, represented by a series arrangement of compartments. In particular with limited axial diffusion in tissue, washout from this model is multi-exponential, the rate of washout accelerating with time. To explain the experimentally observed multicomponent washout, in which washout retards with time, heterogeneity of flow and/or diffusing capacity to volume is required, either in series or in parallel compartments. While there appears to be a structural basis for both types of heterogeneity, parallel and series, a distinction between both on the basis of washout curves alone is impossible.

Diffusion-perfusion relationships in skeletal muscle: models and experimental evidence from inert gas washout

In order to study the dependence of blood-tissue gas exchange upon diffusion, the simultaneous washout of two inert gases of differing diffusivity was investigated in isolated-perfused dog gastrocnemius preparations. The muscles were equilibrated with CH4 and SF6 via arterial blood. The washout kinetics were determined from venous blood samples analyzed by gas chromatography. The results revealed the following features: The washout of the test gases was pronouncedly multi-exponential, and could be described by three exponential components when analyzed to 5% of the initial value. The non-exponential washout was attributed to unequal distribution of capillary blood flow to tissue volume. The mean ratio of washout rate constants CH4/SF6 was within 1.10-1.25 and was even smaller than the ratio expected for pure perfusion limitation (1.46). Therefore, no evidence for effective tissue-blood diffusion limitation was obtained. The observed washout rate constant ratio could be explained by a model with veno-arterial back diffusion which more strongly retards washout kinetics of the better diffusible gas (CH4) as compared to the less diffusible gas (SF6).

A method for determination of blood flow with 133Xe in human nasal mucosa

The 133Xe wash-out technique was used as a method for calculation of the blood flow in human nasal mucosa. The disappearance of 133Xe from the nasal mucosa was followed using scintillation detectors. In laryngectomies it was shown that the disappearance of 133Xe from the mucosa depended mainly on the blood flow. Leakage of 133Xe from the mucosa to the nasal air was unimportant. The disappearance rate of 133Xe was decreased by oxymetazoline and increased by histamine, mostly likely due to the effects of these vasoactive drugs on the mucosa blood flow. The 133Xe wash-out technique would seem to be a simple method to study nasal mucosal blood-flow changes in humans.

Measurement of myometrial blood flow in rabbits by washout of xenon-133 after atraumatic local labelling

In vivo experiments were performed on the uterine tissue of non-pregnant estradiol treated rabbit anesthetized with sodium pentobarbitone. The average blood flow calculated from the washout of Xenon-133 after atraumatic labelling from the uterine surface was 25.4 ml . min-1.(100 g)-1 (range 7.4-66.6), and after local injection of a tracer in isotonic saline directly into the myometrium 24.4 ml.min-1.(100 g)-1(range 8.7-45.3). During the experiments a monoexponential washout curve was found for the whole washout process both after atraumatic labelling and when the injected volume was 5 microliter. No trauma of injection was observed using this injection volume. The results support the applicability of a monocompartmental model for the washout of inert gas from the myometrium. Thus myometrial blood flow can be calculated from the Xenon-133 washout rate at any time interval during the washout process.

Regional blood flow in canine myocardium as determined by local washout of a freely diffusable radioactive indicator

The aim of this study has been to examine the utility of the washout of a freely diffusable radioactive indicator as a measure of regional myocardial blood flow in open-chest anesthetized dogs. The method employed was direct intramyocardial injection of Xenon-133 followed by measurement of its gamma-radiation. The experimental washout curves show, after a short acceleration period, monoexponential washout of the indicator over two decades. We found indications of insignificant veno-arterial shunting by diffusion of the blood flow level measured, insignificant arterial recirculation of the indicator, and minimal radioactive contribution to precordial residue versus time curves arising from right heart or non-myocardial tissue. We suggest that diffusion equilibrium between the tissue in the counting field and the blood leaving it is maintained during the linear down slope of two decades of the clearance curve, and that local blood flow can be calculated from washout rate constant obtained from this part of the curve. The method employing intramyocardial injection of Xenon-133 was found to give the same results as atraumatic epicardial labelling indicating negligible effect of the injection trauma and supporting the validity of the local injection method using small volumes (2-10 microliter).

Capillary permeability measured by bolus injection, residue and venous detection

Kinetic analysis of residue and outflow curves of gamma-emitting indicators such as chromium-51-EDTA and iodide-131-thalamate from skeletal muscle gives the possibility to determine the extraction fraction and the plasma flow, and from these two values the capillary diffusion capacity can be calculated (Sejrsen 1970, preliminary report). This is possible both for the transport from blood to tissue and from tissue to blood. THis alternative method has been compared in the autoperfused cat gastrocnemius preparation with the indicator diffusion method based on venous registration of a diffusible test indicator and an intravascular reference indicator (Chinard et al. 1955, Crone 1963). The results of the five independent measurements show good agreement. Calculation of the permeability Pd based on a capillary surface area of 7 000 cm2/100 g of tissue gives values of 1.05 . 10(-5), 1.10 . 10(-5), and 1.16 . 10(-5) cm/s, which is in agreement with results obtained by other investigators. The permeability was equal in both directions, and thus the capillary membrane seems to function as a symmetrical membrane. Using an area of 5 000 cm2/100 g which presumably is more realistic at the plasma flow range used gives Pd values around 1.5 . 10(-5) cm/s. The effective pore area is calculated to constitute 1/50 000 of the capillary surface area. Calculation of volumes of distribution in the muscle tissue gave intravascular plasma volumes of 1.5 to 2.0 ml/100 g, an extravascular volume of 12.4 and 15.2 ml/100 g and a final monoexponential component constituting a compartment of 5.4 and 7.0 ml/100 g from residue and venous curves, respectively. The last mentioned compartment constitutes nearly 50 per cent of the extravascular space, and it is suggested, that it is located inside the sarcoplasmic reticulum, which anatomically constitutes about 50 per cent of the interstitial space. The total area of contact between the longitudinal and the transversal tubules in this subsystem, which is the membrane of the lateral saccus, is estimated to about 6 times the capillary surface area at a plasma flow of 15 ml/100 g . min which gives a permeability about 60 times lower for this membrane compared to the capillary membrane.

Evaluation of oral submucosal blood flow at dental injection sites by radioactive Xenon clearance in beagle dogs

Oral submucosal blood flow was measured at dental injection sites in anesthetized dogs and compared with subcutaneous blood flow. Blood flow was calculated from the measured half-time for clearance of radioactive xenon dissolved in saline. By this method, oral submucosal blood flow was much greater than subcutaneous blood flow and was comparable in magnitude to values reported for cerebral blood flow. Injection of a solution containing 1:100,000 epinephrine at the mandibular canal produce a delayed clearance of isotope which was three times as long as the clearance time for solutions without epinephrine.

Measurement of diaphragmatic blood flow and oxygen consumption in the dog by the Kety-Schmidt technique

To assess energy expenditure of the diaphragm directly, a method was devised for percutaneous catheterization of the left inferior phrenic vein in dogs. Necropsy studies, including retrograde injection of india ink and measurement of radioactivity in diaphragmatic muscle strips, suggested that the territory drained by the inferior phrenic vein was uniformly perfused, and that there were no major anastomoses between this bed and adjacent ones. Diaphragmatic blood flow ( Q di) was calculated from the integrated diaphragmatic arteriovenous difference of (85)Kr by the Kety-Schmidt technique. Diaphragmatic oxygen consumption ( Vo(2) di) was determined as the product of Q di and the diaphragmatic arteriovenous oxygen content difference [(A-V)O(2) di]. When lightly anesthetized dogs breathed quietly, Q di was 22+/-SD 6 ml/min/100 g, (A-V)O(2) di was 6.1+/-SD 2.5 ml/100 ml, and VO(2) di averaged 1.2+/-SD 0.3 ml/min/100 g. This represented 1.0+/-SD 0.2% of total body oxygen consumption. VO(2) di remained relatively constant during quiet breathing, whereas Q di varied directly with cardiac output and reciprocally with (A-V)O(2) di. The oxygen consumption of the noncontracting diaphragm was 60+/-SD 20% of the level measured during quiet breathing. The energy expended by the diaphragm to support simple hyperventilation was small. A 100% increase in minute ventilation, induced by inhalation of 5% CO(2) in 21% or 14% O(2), increased Q di 13%, (A-V)O(2) di 19%, and VO(2) di 40%. The diaphragm consumed 0.13+/-SD 0.09 ml O(2) for each additional liter of ventilation. In four dogs, pneumonia appeared to increase VO(2) both by increasing minute ventilation and by increasing the energy cost per liter of ventilation.

A physiological approach to the assessment of disease activity in rheumatoid arthritis

A method is described for assessing the in vivo oxygen consumption and lactate production rates of human knee joints. It is based on the rate of fall of P(o2) and the rate of rise in lactate concentration in an intra-articular saline pool after interruption of the circulation to the joint with an arterial tourniquet. Studies in 5 control patients with degenerative joint disease and 29 patients with rheumatoid arthritis showed a 2- to 3-fold higher mean oxygen uptake rate and a 10- to 12-fold higher mean lactate appearance rate in the saline in the rheumatoid joints with severe disease compared to the control joints. These metabolic variables correlated with tissue metabolic demand as estimated in synovial biopsies. (133)Xe washout from the intra-articular space, which reflects joint circulatory flow, showed a 3-fold greater mean washout rate from the rheumatoid joints (48 studies) than control joints (7 studies) with extensive overlap between the two groups. (133)Xe washout rate correlated with knee joint inflammation estimated both clinically and histologically. After synovectomy in four patients, the operated knee showed a greater fall in lactate production than the opposite knee in three of these patients. Neither knee joint oxygen uptake nor (133)Xe washout rate changed significantly. Intra-articular corticosteroid injection (eight patients) resulted in decreased lactate production and a decreased (133)Xe washout rate in the injected knee and variable results in the untreated knee. Oxygen uptake again was unchanged after therapy.

The penetration of inulin, sucrose, mannitol and tritiated water from the interstitial space in muscle into the vascular system

1. An isolated gastrocnemius muscle from cat was perfused with a modified Ringer fluid containing the extracellular tracers mannitol, sucrose and inulin and in some cases tritiated water (HTO). When equilibration of the tracers between the perfusate and the tissue was obtained, perfusion with tracer-free fluid was started.2. The fractional rate of escape, Phi (t), of the tracers was followed. For all tracers it declined steadily with time. The rate of decline was faster for the smaller extracellular molecules when any combination of two extracellular tracers was studied.3. Arrest of flow for 1-2 min lead to an increase in Phi (t) for the extracellular tracers when flow was resumed. There was little or no effect upon Phi (t) for HTO.4. Doubling the rate of flow had no effect upon Phi (t) for the extracellular tracers, while that of HTO increased.5. We conclude that the cause of the decline in Phi (t) for the extracellular tracers is concentration gradients in the interstitial space. The findings raise problems in methods for studying capillary permeability by means of the fractional rate of escape of tracers from the interstitial space into the vascular system.

Blood flow in cutaneous tissue in man studied by washout of radioactive xenon

Washout of 133 xenon from a local depot in cutaneous tissue has been investigated in man to develop a method for blood flow measurement in that tissue. The tissue was labeled both by injecting 133 xenon in saline solution intracutaneously and by a nontraumatic epicutaneous application technique. The initial selectivity of the labeling and the subsequent accumulation of 133 xenon in subcutaneous tissue were examined by radioautography and by a special tissue separation technique. From the observations a two-compartment clearance model is suggested by which the cutaneous blood flow can be obtained separately from that of the subcutaneous tissue. Blood flow in the cutaneous tissue in the lateral region of the leg was 5.7 (SD 1.3) ml/100 g · min at an ambient temperature of 19° to 22°C. There were variations between different skin sites.

Experimental myocardial infarction in the dog. Comparison of myocardial blood flow within, near, and distant from the infarct

Control measurements of myocardial blood flow were made in intact dogs by a method of 133 Xe clearance, using a catheter positioned radiographically in the circumflex artery. Acute myocardial infarction was then caused by tying the anterior descending artery. A small catheter was inserted distally beyond the obstructed artery into the infarct, and another proximally toward the anterior ventricular artery supplying adjacent left ventricular muscle. Their ends were brought out as the chest was closed. 133 Xe in solution was injected serially over 4 hours to determine myocardial blood flow within the infarct, in adjacent muscle, and in more distant muscle supplied by the circumflex artery. Myocardial blood flow in the circumflex artery usually fell as a result of infarction. A constant finding was that adjacent flow exceeded the more distant flow by 6% up to 43% (mean 20%) regardless of either spontaneous or induced changes in blood pressure. The difference was not related to the collateral flow rate and was abolished by dipyridamole. Perfusion of muscle adjacent to the infarct was less homogenous than that of more distant muscle.

In the normal dog rates of myocardial blood flow in the distribution of the circumflex and anterior descending branches of the left coronary artery were the same, contrasting with lower rates for the right.