Per Sejrsen

Inert Gas Diffusion Method for Measurement of Blood Flow: COMPARISON OF BOLUS INJECTION TO DIRECTLY MEASURED BLOOD FLOW IN THE ISOLATED GASTROCNEMIUS MUSCLE

The isolated cat gastrocnemius preparation was used to study the relation between the washout of 133 Xenon measured by external monitoring and blood flow measured directly by a dropcounter. The experimental possibility was thereby tested of recording the total tracer bolus and its subsequent washout by external monitoring, using the height H and the corresponding area A of the washout curve. The partition coefficient λ, and the amount of recirculation were evaluated in separate experiments. According to Zierler, the blood flow per gram of tissue should equal λ • H/A . This relation was verified experimentally. Exponential analysis was carried out but the results of it were not considered to support the hypothesis of a two compartment inparallel model in skeletal muscle. The shape of the washout curve did not suggest the existence of A-V shunts.

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

The isolated gastrocnemius muscle of the cat was used to study the relation between the desaturation of 133xenon measured by external monitoring, and blood flow measured directly by a dropcounter. The metered blood flow agreed with that calculated from the initial slope of the desaturation curve from tissue equilibrated by prolonged intra-arterial infusion and also by atraumatic gas labelling of a local area of the muscle (approximately 0.15% of the muscle weight). In this sense the concept of uniform distribution of blood flow in skeletal muscle has received substantial support. The identity of the results obtained by local gas labelling and by intra-arterial equilibration of the whole muscle with gas implies that there is, at least initially, diffusion equilibrium between tissue and blood in both. However, the clearance curves could in no instance be fitted by a single exponential function. This indicates that the diffusion equilibrium is not maintained after the initial phase. It is suggested that this is not due to the presence of anatomical arteriovenous shunts or other types of uneven perfusion, but to counter current exchange of inert gas through the walls of vessels, i.e. shunting by diffusion explains the shape of the desaturation curves.

Blood flow in the brood patch of Bantam hens: evidence of cold vasodilatation

Summary1.Blood flow was measured in the brood patch of Bantam hens by recording the washout of133Xe from the tissue in different experimental conditions.2.In hens incubating 5 eggs at normal temperature, cutaneous and subcutaneous blood flow in the brood patch averaged 0.31 ml·min−1·g−1 and 0.15 ml·min−1·g−1, respectively.3.Cooling the brood patch in restrained hens immediately increased cutaneous and subcutaneous blood flow by an average of 83% and 63%, respectively. The increase in blood flow was restricted to the site of cooling, while neighbouring skin areas showed little change in flow. This cold vasodilatation could also be elicited in hens incubating water circulated eggs and was unaffected by local anaesthetics.4.The observations suggest that the vasodilatation in response to cooling is due to a direct temperature influence on the smooth muscle cells of the brood patch vasculature.5.It is suggested that the cold vasodilatation may be important for providing increased heat transfer to the eggs at low ambient temperatures or when the parent bird returns to cool eggs after feeding excursions.

Monoexponential Extrapolation of Tracer Clearance Curves in Kinetic Analysis

Kinetic analysis of inert tracers shows that some of the most important parameters such as the average turnover rate and the total volume of distribution can he calculated only if the entire time course of the tracer clearance is known. This means that extrapolation beyond the observation period (to infinity) must necessarily be accomplished. This paper presents arguments to support the monoexponential extrapolating function which often is used without justification. The arguments show that one cannot in the general case assign any clearcut physical value to the intercept or exponential coefficient of the extrapolating function. Theoretically, a monoexponential tail of a tracer clearance curve obtained from a system in a steady state is reached when the slope of the curve is proportional to the curve. Under certain conditions this slope can be measured as an independent observation, and hence the monoexponentiality can be put to a fairly rigorous experimental test. This concept is illustrated by clearance studies of 51Cr-EDTA and 181I-thalamate from the isolated cat gastrocnemius muscle. Furthermore, it is demonstrated that monoexponential extrapolation as made before the appearance of the final exponential part of the outflow curve can cause considerable error in determination of the mean transit time and hence of the volume of distribution for the tracer. Even with an apparent recovery of the tracer of about 99.7%, the mean transit time was underestimated by 20%.

Diffusion Processes Invalidating the Intra-arterial Krypton-85 Beta-Particle Clearance Method for Measurement of Skin Blood Flow in Man

Krypton-85 dissolved in 0.9% NaCl was injected into the femoral artery of man; β particles were measured over the skin of the crus. Cutaneous mean blood flow was calculated using the integral equation of Zierler based on the mean transit time analysis of clearance curves recorded from the cutaneous tissue; these calculations gave estimates which were only about one-fifth those obtained by other methods. The low estimates were caused by an intercom-partmental exchange of 85krypton between the cutaneous and subcutaneous tissues, an exchange implying that both tissues influence the shape of the clearance curve recorded from the cutaneous tissue alone. This intercompart-mental exchange was effected not only by diffusion directly from the cutaneous to the subcutaneous tissue, but presumably also by transport from the blood streaming between the two tissues. The clearance rate of the tail of the curve was shown to correspond to that of the subcutaneous tissue. The errors distorting the clearance curve of β particles may invalidate the use of intra-arterial injection and measurement of clearance curves of β particles for measurement of blood flow in other tissues.

Diffusion bypass of xenon in brain circulation.

The diffusion bypass of 133Xe between arteries and veins was studied in the brains of anesthetized dogs. A mixture of 51Cr-labeled red cells and 133Xe was injected as a bolus into the internal carotid artery. Gas-tight venous samples were taken every 0.4–0.8 seconds from the superior sagittal sinus. An excess of 133Xe in the early venous blood samples over that expected from blood-tissue equilibration considerations was demonstrated in all the experiments. The excess was most marked in the first samples, and it was enhanced by lowering cerebral blood flow by decreasing the cerebral perfusion pressure to 20–30 mm Hg. We concluded that diffusion bypass of the tissue by 133Xe is the most likely explanation for these experimental findings. The fraction of the bolus which appeared to bypass the tissue amounted to about 2–3% at the normal perfusion pressure (i.e., normal blood flow) and about 10% at a reduced cerebral perfusion pressure of 20–30 mm Hg (i.e., about half of normal blood flow). We concluded that measurement of cerebral blood flow by a bolus injection of 133Xe in the internal carotid artery and external detection of the washout is not influenced significantly by the diffusion bypass of 133Xe in the normal flow range.

Blood flow rate during orthostatic pressure changes in the pulp skin of the first toe

OBJECTIVES Determination of the local regulation of cutaneous blood flow through nutritive capillaries and through arteriovenous anastomoses of the pulp of the first toe in response to passively induced orthostatic blood pressure changes in normal subjects and in patients with occlusive atherosclerotic disease. MATERIAL Six normal subjects, seven patients with unilateral, crural intermittent claudication and six patients with unilateral, chronic critical ischaemia. METHODS Blood flow rates were measured in supine subjects by the heat washout method (the sum of blood flow rate in arteriovenous anastomoses and blood flow rate in nutritive capillaries) and by the 133Xenon washout method (blood flow rate in nutritive capillaries) after local, atraumatic labelling. Measurements were made with (a) the toe passively elevated to 50 cm above heart level, (b) at heart level and (c) passively lowered to 50 cm below heart level. RESULTS Autoregulation of nutritive blood flow was present in normal subjects and in claudicants, but the local sympathetic veno-arteriolar axon reflex was absent in both groups. In patients with critical ischaemia blood flow rate was the same in the supine position and during lowering in arteriovenous anastomoses and in nutritive capillaries. The arteriovenous anastomoses had distinct and characteristic reaction patterns in response to lowering in each of the three examined groups and to elevation in normal subjects and in patients with intermittent claudication (not measured in patients with critical ischaemia). CONCLUSIONS The microvascular responses to changes of orthostatic blood pressure differed among the three groups (normal subjects, patients with intermittent claudication, patients with critical chronic leg ischaemia). The heat washout method may be used to detect the functional significance of occlusive atherosclerotic disease.

TRANSPORT OF BETA-BLOCKERS AND CALCIUM ANTAGONISTS BY DIFFUSION IN CAT MYOCARDIUM

Beta-blockers and calcium antagonists have been claimed to possess cardioprotective properties. This study addresses the question of whether a significant amount of these drugs will reach the cardiac myocytes during no-flow ischemia, where solute transport depends solely on diffusion. In anesthetized cats the hearts were excised. Apparent diffusion coefficients in cat myocardium at 37°C (D‘37) for 14C-verapamil (protein bound). 3H-metoprolol (lipophilic), 3H-atenolol (hydrophilic), and 3H-propranolol (lipophilic and protein hound) were determined by means of a “true transient diffusion” method and compared with the free diffusion coefficients in water (D37). D’37 of 14C-verapamil, 3H-metoprolol, 3H-atenolol, and 3H-propranolol (in cm2 s-1 105) were (mean ± SEM) 0.025 ± 0.002, 0.055 ± 0.003, 0.041 ± 0.007, and 0.025 ± 0.002, respectively. The mean diffusive progression of the concentration profile of 3H-metoprolol and 3H-atenolol into the tissue during 20 min was calculated to be 0.36 and 0.31 mm, respectively. The protein binding of 14C-verapamil and 3H-propranolol caused a significant fall in the progression to 0.24 mm for both drugs. These results indicate that, by diffusion, these drugs traverse the tissue too slowly to reach a significant amount of myocardium before myocyte necrosis occurs during conditions of no-flow. Cardioprotective drugs are probably most effective, provided sufficient amounts are present in the tissue prior to the ischemic episode or sufficient supply via collateral blood flow is achieved.

Neuropeptide Y reduces ovarian blood flow in the rabbit

Neuropeptide Y-containing nerve fibers have previously been demonstrated to innervate the mammalian ovary. These nerve fibers innervate primarily the vasculature. In this study we have developed a method for in vivo measurement of the ovary blood flow rate by means of the 133Xe method. Using this technique we measured the ovary blood flow rate and investigated the dose-response relationship between close intraarterial-injected NPY and the ovary blood flow rate. A monoexponential washout curve for 133Xe was found for the whole washout process, ensuring that the blood flow rate at any time could be calculated from the curve. We found a mean blood flow rate in the nonpregnant rabbit ovary at 43.6 +/- 4.4 ml.(100 g)-1.min-1 (mean +/- SEM). Injection of NPY (20, 200, 2000 pM) in the aorta close to a. ovarica resulted in a dose-dependent decrease in the ovarian blood flow rate with a maximum reduction to 40.7 +/- 6.3% (mean +/- SEM) of the control blood flow rate. These findings make it likely that receptors able to interact with NPY are present in the vasculature of the rabbit ovary.

Microvascular function in the peripheral vascular bed during ischaemia and oxygen-free perfusion

OBJECTIVES To determine the influence of acute ischaemia and absence of leukocytes on the microvascular function and capillary permeability in skeletal muscle. DESIGN Prospective, open study. SETTING University Department of Vascular Surgery and Institute of Medical Physiology. MATERIALS AND METHODS Ten isolated cat gastrocnemius muscles were perfused with oxygen-free Ringer-albumin solution through the femoral artery. At 5 microliters bolus with 14.8 MBq 51Cr-EDTA was injected through a side branch into the femoral artery, and the response function was detected over the muscle by a scintillation detector connected to a spectrometer and a computer. The perfusion coefficient was measured directly at the venous outlet. The response function was analysed in accordance with non-compartmental black box kinetic principles to give perfusion rate, capillary extraction fraction and capillary diffusional permeability-surface area product (PdS). In separate experiments the molecular size and the free diffusion coefficient of 51Cr-EDTA in water at 37 degrees C were determined by a modified true transient diffusion method. MAIN RESULTS During perfusion the PdS-product increased as a function of flow rate, f, in accordance with the linear regression line PdS = 1.78 + 0.15 f between 5 to 60 ml/100 g/min. This permeative conductance was identical to that found previously in a similar experimental set up with oxygenated whole blood perfusion. During oxygen free perfusion the perfusion rate was a linear function of arterial perfusion pressure, and autoregulation of blood flow did not occur in response to variations of arterial perfusion pressures. The free diffusion coefficient in water at 37 degrees C for 51Cr-EDTA was 7.4 x 10(-6) cm2/s (n = 36), which corresponds to a Stokes-Einstein molecular radius, rSE, of 0.439 nm. CONCLUSIONS In spite of complete anoxia and absence of normal microcirculatory flow regulating mechanisms there is no sign of changes in capillary diffusional permeability for smaller hydrophilic molecules and functional membrane damage is not elicited in the absence of oxygen under these conditions.