External pressure--blood flow relations during limb compression in man

The effect of increased tissue pressure on blood flow in subcutaneous and skeletal muscle tissue was studied in 8 healthy humans resting in horizontal position. Blood flow was measured by the local 133Xe washout technique in the mid-calf region. Tissue pressure in the depot area was increased by inflating a compression cuff, 50 cm wide (knee to ankle). Blood flow rates were obtained from compressed tissues with normal vasomotor tone, at papaverine-induced vasoparalysis and during distension of the compressed vessels. The latter was achieved by inflating a proximal (extra) compression cuff on the thigh 10 or 20 mmHg above the pressure level in the more distally placed compression cuff. Increased tissue pressure was a potent stimulus for arteriolar dilatation (autoregulation) in both tissues. The autoregulatory response was to some extent counteracted by an increase in local vascular resistance in the postcapillary section as evidenced by the results of simultaneous venous stasis. Blood flow ceased in vasoparalysed tissues as well as in tissues with normal vasomotor tone, when the compression cuff was inflated to the level of the local diastolic blood pressure. Maintaining external compression at the diastolic blood pressure level, blood flow reappeared in both tissues, when the compressed vessels were distended by adding the proximal (extra) compression. It is concluded, that blood flow cessation in compressed tissues is caused by a widespread arterial-arteriolar collapse in diastole, as the volume of blood injected during the systolic peak is too small to expand also the distal sections of the precapillary vessels.     

External pressure-blood flow relations during limb compression in man

The effect of increased tissue pressure on blood flow in subcutaneous and skeletal muscle tissue was studied in 8 healthy humans resting in horizontal position. Blood flow was measured by the local ¹³³Xe washout technique in the mid-calf region. Tissue pressure in the depot area was increased by inflating a compression cuff, 50 cm wide (knee to ankle). Blood flow rates were obtained from compressed tissues with normal vasomotor tone, at papaverine-induced vasoparalysis and during distension of the compressed vessels. The latter was achieved by inflating a proximal (extra) compression cuff on the thigh 10 or 20 mmHg above the pressure level in the more distally placed compression cuff. Increased tissue pressure was a potent stimulus for arteriolar dilatation (autoregulation) in both tissues. The autoregulatory response was to some extent counteracted by an increase in local vascular resistance in the postcapillary section as evidenced by the results of simultaneous venous stasis. Blood flow ceased in vasoparalysed tissues as well as in tissues with normal vasomotor tone, when the compression cuff was inflated to the level of the local diastolic blood pressure. Maintaining external compression at the diastolic blood pressure level, blood flow reappeared in both tissues, when the compressed vessels were distended by adding the proximal (extra) compression. It is concluded, that blood flow cessation in compressed tissues is caused by a widespread arterial-arteriolar collapse in diastole, as the volume of blood injected during the systolic peak is too small to expand also the distal sections of the precapillary vessels.     

Vascular Resistance in Peripheral Blood Vessels at Normotension and at Local Orthostatic Hypertension in Healthy Humans

The effect of changes in orthostatic pressure on vascular resistance in subcutaneous adipose tissue was examined in the forearm at normal tension and in the leg at local orthostatic hypertension in three normal subjects. Blood flow in subcutaneous tissue was measured by the local xenon-133 washout technique. Changes in orthostatic pressure were achieved by postural changes of the extremity. At levels above the heart where venous pressure is constant, there was autoregulation of blood flow over a wider arterial blood pressure range in leg than in forearm. When transmural pressure increased 25 mm Hg or more vascular resistance increased about 50 per cent in the dependent forearm and 300 per cent in the dependent leg. The effect of ischemia on vascular resistance was investigated in cutaneous tissue of hand and foot by means of the ¹³¹I-Antipyrine initial slope technique. Vascular resistance after maximum dilatation achieved after20, 30, and 35 min of ischemia was less in vessels of the hand than in vessels of the foot. Thus, adaptive structural changes of vessels subjected to increased blood pressure are present in cutaneous and subcutaneous adipose tissues in normal subjects.     

Effect of “Vein Pump” Activation upon Venous Pressure and Blood Flow in Human Subcutaneous Tissue

The effect of “vein pump” activation upon superficial venous pressure and blood flow in human subcutaneous adipose tissue was studied in 6 normals and 2 patients with venous insufficiency. Blood flow in subcutaneous tissue was measured at the lateral malleolus by the local ¹³³Xenon washout technique. with the subject placed in a supine position. During passive lowering of the leg blood flow decreased 50 per cent and total vascular resistance increased 136 per cent. Activation of the vein pump by continuously tipping the foot up and down caused a decrease in venous pressure of 5 mmHg in horizontal position. Venous pressure increased only by 8 mmHg when the leg was lowered during exercise. In this situation blood flow remained constant corresponding to an increase in vascular resistance of 42 per cent. However increasing venous pressure to 28 mmHg by venous stasis in the lowered leg during exercise caused an additional increase in vascular resistance of 82 per cent. In the patients with venous insufficiency exercise did not prevent the decrease in blood flow during lowering of the leg. Hence venous pressure elevation of 25 mmHg or more caused an additional increase in vascular resistance in subcutaneous tissue, “vasoconstrictor response”. It is concluded that this “vasoconstrictor response” depends on a vasoconstrictor impulse transmission from veins to arterioles, veno-arteriolar reflex.     

Subcutaneous and skeletal muscle vascular responses in human limbs to lower body negative pressure.

Cardiopulmonary baroreceptor unloading in humans comparably increases sympathetic discharge to skeletal muscle in the forearm and calf, but blood flow studies have disclosed differential rather than uniform vasomotor responses in the extremities. The aim of the present study was to address the issue of differential effects of orthostatic stress on forearm and calf vascular adjustment and to extend previous studies by determining changes in vascular responses separately in various vascular beds of the limbs. The local [133Xenon] washout method was used for recording blood flow rates in subcutaneous tissue and skeletal muscle. Simultaneous recordings from the forearm and calf were performed in 11 healthy young males during lower body negative pressure at -10 mmHg. Heart rate, arterial mean and pulse pressures did not change during lower body negative pressure. In the forearm blood flow rates decreased significantly, in subcutaneous tissue by 16 +/- 2% (mean +/- SEM) and in skeletal muscle by 16 +/- 1%. In the calf lower body negative pressure induced a significant decrease in blood flow rates of 17 +/- 3% in subcutaneous tissue and of 30 +/- 2% in skeletal muscle. This vasoconstriction in calf skeletal muscle was consistently disclosed in both legs and was about the same magnitude in each calf when studied with the one leg exposed to lower body negative pressure and the other outside the lower body negative pressure chamber. These findings suggest that during unloading of cardiopulmonary afferents, reflex sympathetic activation as an important autonomic adjustment to orthostatic stress is accompanied by uniform vasoconstriction in subcutaneous and skeletal muscle vascular beds of human limbs.     

Perceived Changes in the Intensity oi Arterial Pulsations as a Function oi Applied Cufi Pressure

A psychophysical scaling procedure was employed to investigate subjects' ability to discriminate pulsatile arterial sensations produced by applying an occluding cuff about the upper arm. Subjects (n = 8) were exposed to 5 presentations of 7 occluding cuff pressure levels ranging from above systolic to below diastolic arterial pressure. During each cuff inflation, subjects were instructed to attend to the pulsating sensations in the arm. When the cuff deflated, subjects adjusted the volume ofa tone to a level that matched their subjective estimate of pulsation intensity. Consistent with previous reports, subjects perceived maximum intensity pulsations when cuff pressure approximated calculated mean arterial pressure (MAP), 1/3 (systolic -diastolic) + diastolic. The perceived intensity ofthe sensations decreased monotonically as cuff pressure was varied in either the systolic or diastolic direction producing a highly symmetrical function on both sides of MAP. The gradient of arterial pulsations produced by the occluding cuff pressure, the accuracy of the judgments ofthe intensity of perceived pulsations, and the possibility that these pulsations may he more physiologically related to the hemodynamics of blood flow, suggest that these sensations may be employed as a more effective discriminative stimulus in a blood pressure biofeedback procedure.     

Effects of Low Tissue Temperature on Peripheral Vascular Control Mechanisms

Influence of low temperatures on vascular effects produced by adrenaline, noradrenaline and vasoconstrictor nerve stimulation was studied in the feet of ducks. Observations were made on isolated preparations as well as on intact ducks with one or both feet immersed in ice-water. Isolated preparations were perfused with physiological solution and information on changes in vascular resistance obtained by direct measurement of flow changes during perfusion at constant pressure or by measuring changes in perfusion pressure during perfusion at constant flow. In intact ducks changes in blood flow were recorded as changes in digital subcutaneous tissue temperature. The study revealed that in the duck foot a relatively large fraction of the resistance to blood flow is found in the arteries of the proximal part of the foot. During cooling of the foot the influence of vasoconstrictor nerve stimulation and low doses of adrenaline and noradrenaline on vascular resistance is gradually lost, and it is negligible at temperatures below 8°C. At these low temperatures the blood vessels stay dilated and blood flow through the web seems to be determined by the digital arterial blood pressure.     

Intrinsic Regulation of Blood Flow in Adipose Tissue

Previous studies on intact human subcutaneous tissue have shown, that blood flow remains constant during minor changes in perfusion pressure. This so-called auto regulatory response has not been demonstrable in isolated preparations of adipose tissue. In the present study on isolated, denervated subcutaneous tissue in female rabbits only 2 of 12 expts. revealed an auto regulatory response during reduction in arterial perfusion pressure. Effluent blood flow from the tissue in the control state was 15.5 ml/100 g·min (S.D. 6.4, n = 12) corresponding to slight vasodilatation of the exposed tissue. Following total ischemia all experiments showed a period with reactive hyperemia, and both duration of hyperemia and excess flow was related to the duration of the ischemia. This response therefore seems more resistant to the experimental procedure, while auto regulation of blood flow to lowered pressure is more susceptible to surgical exposure of the tissue. During elevation of arterial perfusion pressure blood flow in the isolated tissue showed a transient increase and then almost returned to the level during normotension, indicating an elevated vascular resistance. Raising of venous pressure elicited vasoconstriction with pronounced flow reduction. These two reactions may be important for local regulation of blood flow in subcutaneous tissue during orthostatic changes in arterial and venous pressure. It is concluded that the response in adipose tissue to changes in arterial pressure (auto regulation), venous pressure and total ischemia appear to be elicited by different mechanisms.     

Subcutaneous and skeletal muscle vascular responses in human limbs to lower body negative pressure

Jacobsen , T. N., Nielsen , H. V., Kassis , E. & Amtorp o S. 1992. Subcutaneous and skeletal muscle vascular responses in human limbs to lower body negative pressure. Acta Physiol Scand 144 , 247–252. Received 8 March 1991, accepted 7 Novcmber 1991. ISSN 0001–6772. Department of Cardiology, Gentofte Hospital, University of Copenhagen, Denmark Cardiopulmonary baroreceptor unloading in humans comparably increases sympathetic discharge to skeletal muscle in the forearm and calf, but blood flow studies have disclosed differential rather than uniform vasomotor responses in the extremities. The aim of the present study was to address the issue of differential effects of orthostatic stress on forearm and calf vascular adjustment and to extend previous studies by determining changes in vascular responses separately in various vascular beds of the limbs. The local [¹³³Xenon] washout method was used for recording blood flow rates in subcutaneous tissue and skeletal muscle. Simultaneous recordings from the forearm and calf were performed in 11 healthy young males during lower body negative pressure at —10 mmHg. Heart rate, arterial mean and pulse pressures did not change during lower body negative pressure. In the forearm blood flow rates decreased significantly, in subcutaneous tissue by 16 ± 2% (mean ± SEM) and in skeletal muscle by 16 ± l%. In the calf lower body negative pressure induced a significant decrease in blood flow rates of 17 ± 3% in subcutaneous tissue and of 30 ± 2% in skeletal muscle. This vasoconstriction in calf skeletal muscle was consistently disclosed in both legs and was about the same magnitude in each calf when studied with the one leg exposed to lower body negative pressure and the other outside the lower body negative pressure chamber. These findings suggest that during unloading of cardiopulmonary afferents, reflex sympathetic activation as an important autonomic adjustment to orthostatic stress is accompanied by uniform vasoconstriction in subcutaneous and skeletal muscle vascular beds of human limbs.     

Effect of spinal sympathetic blockade upon postural changes of blood flow in human peripheral tissues

The effect of head-up tilt upon subcutaneous and skeletal muscle blood flow in the crus was studied before and during epidural blockade in 10 subjects. Relative changes in blood flow were estimated by the local ¹³³Xe washout technique. In subcutaneous tissue head-up tilt induced a decrease in blood flow of about 40% and there was no difference in the vascular response to head-up tilt before and during epidural blockade. In skeletal muscle tissue essentially the same was found as head-up tilt decreased blood flow by about 26% the response being uninfluenced by epidural blockade. In 3 patients local nervous blockade was induced by Lidocaine in ¹³³Xe labelled subcutaneous tissue on one side. During epidural blockade and tilt blood flow increased by 12% whereas blood flow decreased by 30% on the control side. Thus epidural blockade had no influence on the vasoconstrictor response in subcutaneous tissue and skeletal muscle to head-up tilt whereas local blockade was able to prevent the response. Local mechanisms including the local veno-arteriolar reflex appear to play an important role for the observed maintenance of arterial blood pressure in the tilted position during central sympathetic blockade.     

Influence of circulating NE and Epi on adipose tissue vascular resistance and lipolysis in humans

The effects of circulating norepinephrine (NE) and epinephrine (Epi) on vascular resistance in subcutaneous adipose tissue and the calf as well as on plasma glycerol, an indicator of lipolysis, were studied in healthy volunteers. Adipose tissue blood flow was determined by the local clearance of 99mTcO-4 or 133Xe. The two isotopes gave similar results. Calf blood flow was determined by venous occlusion plethysmography. Intravenous infusion of NE caused increases in systolic and diastolic blood pressures, adipose tissue and calf vascular resistances, and plasma glycerol and a decrease in plasma insulin and heart rate, all of which were significant when arterial plasma NE was elevated from 1.17 +/- 0.14 to 8.38 +/- 0.30 nM (n = 16). Epi reduced diastolic and mean arterial pressures and adipose tissue and calf vascular resistances and increased plasma glycerol without affecting systolic blood pressure or plasma insulin. An increase of arterial plasma Epi from 0.20 +/- 0.03 to 1.15 +/- 0.05 nM (n = 6) was sufficient to induce vasodilatation in adipose tissue and lipolysis. Human adipose tissue differs from canine adipose tissue inasmuch as Epi causes vasodilatation in humans (present results) but vasoconstriction in the dog (previous results), presumably due to a predominance of vascular beta 2-adrenoceptors in human and beta 1-adrenoceptors in canine adipose tissue. Furthermore, Epi is a considerably more potent lipolytic hormone than NE in humans but not in the dog. Our results indicate that both NE and Epi may influence human adipose tissue blood flow and lipolysis as circulating hormones.     

Local and central sympathetic reflex control of blood flow in skeletal muscle and subcutaneous tissue in normal man

The effect of age and sex on relative changes in blood flow and vascular resistance in skeletal muscle and subcutaneous tissue during postural changes and during local increase in transmural pressure was studied in 33 healthy subjects. The intra-individual variation was studied in five subjects. Blood flow was measured by the local 133Xenon wash-out method. No relation to age or sex was seen in the centrally elicited sympathetic vasoconstrictor responses in subcutaneous tissue and skeletal muscle and in the locally elicited vasoconstriction in subcutaneous tissue. A small, but statistically significant, correlation to sex and age was found in the local sympathetic vasoconstrictor response in skeletal muscle. The age correlation was caused only by an attenuated response in the young subjects below 40 years of age and may be fortuitous. The intra-individual variation was acceptably small. Based on the present results, a reduction in blood flow in skeletal muscle and subcutaneous tissue during centrally or locally elicited sympathetic vasoconstriction of 10% or less should be considered abnormal. The local 133Xenon wash-out method is of value in examining patients suspected of dysfunction in the sympathetic part of the autonomic nervous system.     

Cutaneous and subcutaneous blood flow during general anaesthesia

Summary The vasodilator effect of anaesthetic agents on cutaneous vessels has often been investigated. In contrast, although subcutaneous tissue is concerned with metabolism and thermoregulation, the effects of anaesthesia on subcutaneous blood flow have not been well documented. The purpose of this study was to determine the magnitude of changes in cutaneous and subcutaneous blood flow during general anaesthesia in Man. Anaesthesia was induced with flunitrazepam in 15 patients before facial plastic surgery. Blood flow was estimated using heat thermal clearance (HC). Two HC sensors in different areas allowed the measurement of superficial and deep HC. Systolic (SABP), diastolic (DABP) and mean arterial blood pressure (MABP), heart rate (HR), and rectal and mean skin temperature were also recorded. After induction of anaesthesia, HR increased significantly (p<0.05) whereas SABP, DABP and MABP remained unchanged. The rectal-toe temperature gradient fell from 6.3±4.1° C to 3.4±1.1° C (p<0.01) suggesting a reduction in vasomotor tone. Superficial HC increased from 0.37±0.06 to 0.42±0.08 W · m^–1 · ° C^–1 (p<0.05) whereas deep HC decreased from 0.33±0.07 to 0.31±0.09 W · m^–1 · ° C^–1 (NS) and returned to the control value thereafter. Rectal temperature and mean skin temperature were unchanged. The changes in deep HC are similar to those previously observed in muscle during induction of anaesthesia. Our results show that anaesthesia mainly affects cutaneous blood flow, without any significant change in subcutaneous blood flow during the early phase of anaesthesia in human beings.     

Effect of Chronic Sympathetic Denervation upon Local Regulation of Blood Flow in Human Subcutaneous Tissue

The effect of chronic sympathetic denervation upon the vasoconstrictor response to an increase of vascular transmural pressure in human subcutaneous adipose tissue was investigated in 6 patients suffering from manual hyperhidrosis. Changes in transmural pressure were obtained either by postural changes of a forearm or by venous stasis of 30 mmHg. Blood flow was measured in the distal part of the forearm or crus by means of the local ¹³³Xenon washout technique. 2 patients were studied before and after sympathectomy. When the area under study was lowered about 40 cm below the jugular notch, blood flow decreased about 50 per cent preoperatively, about 30 per cent 24 h after the operation, but remained constant 4 days after or later. Similar results were obtained during venous stasis. Hence about 4 days after sympathectomy, the vasoconstrictor response to an increase in vascular transmural pressure was abolished. In 3 chronically sympathectomized patients blood flow remained constant in the denervated limb, but decreased significantly in the control limb. In another patient studied 580 days after surgery blood flow remained constant during lowering of the denervated forearm as well as during venous stasis. These findings might indicate that the vasoconstrictor response to an increase in vascular transmural pressure in human subcutaneous adipose tissue is due to a local nervous mechanism involving symphathetic adrenergic nerves, but a myogenic mechanism cannot be definitively excluded.     

Local regulation of subcutaneous blood flow and capillary filtration in limbs with occlusive arterial disease. Studies before and after arterial reconstruction

The aim of the study was to examine the local blood flow regulation and the capillary filtration rate in patients with occlusive arterial disease before and after arterial reconstructive surgery. Fourty-seven normal subjects and 99 patients were studied. Subcutaneous blood flow was measured on the forefoot by the local 133Xenon method. Forefoot arterial blood pressure was measured indirectly by cuff and strain-gauge technique. Capillary filtration rate was measured by strain-gauge plethysmography on the forefoot. The arterial and venous pressures of the forefoot were changed by elevating or lowering the foot in relation to heart level. In normal limbs autoregulation was demonstrated during elevation of the limb when blood flow remained almost constant despite the reduction in arterial and perfusion pressures. The local vasoconstrictor response to increased venous transmural pressure was demonstrated when the limb was lowered and blood flow decreased about 30% despite a constant perfusion pressure. In limbs with occlusive arterial disease both local blood flow regulation mechanisms became progressively more abnormal the severe the symptoms and the lower the distal blood pressure. Estimations of the changes in local vascular resistance suggested that the abnormalities in blood flow regulation in all but the severest cases are the result of changes in local perfusion pressure rather than the result of inability of the arteriolar smooth muscle to dilate and constrict in response to changes in arterial and venous pressures. After arterial reconstruction the two mechanisms generally normalized within about a week. However, disturbances occurred in some cases in the early postoperative period, possibly as the result of postoperative pain and stress. Postreconstructive hyperaemia developed in most limbs despite the early normalization of local blood flow regulation. Compared with normal limbs, the forefoot capillary filtration rate was reduced in limbs with occlusive arterial disease. In the early postoperative period the filtration rate remained reduced, but it increased to normal values within three months. Postreconstructive oedema developed independently of the normalization of blood flow regulation, and almost exclusively after femoro-distal by-pass surgery. The study supports the hypothesis that the postreconstructive oedema is a lymphoedema due to surgical trauma, rather than the result of microvascular derangement.     

‘Distensibility’ of the papaverine-relaxed vascular bed in human subcutaneous tissue

The effect of an increase in vascular transmural pressure upon the blood flow in two subcutaneous vascular beds, maximally dilated by papaverine was studied in 6 healthy humans. Blood flow was measured on the dorsum of the hand and at the lateral malleolus by the local ¹³³Xe washout technique. Increase in vascular transmural pressure was induced by lowering the labelled area various distances below heart level. Lowering the area caused an increase in blood flow. The increase was less pronounced in the legs than in the hand. As arterial perfusion pressure head remained constant during lowering, this indicates that the relative decrease in vascular resistance was smaller in the leg than in the hand. Experimental edema did not influence the relative decrease in vascular resistance. The results suggest that ‘distensibility’ of the resistance vessels is smaller in the leg than in the hand. This might be due to a structural adaptation of the vascular wall in vessels often subjected to increased hydrostatic pressure.     

Effect of spinal sympathetic blockade upon local regulation of blood flow in subcutaneous tissue.

The influence of spinal sympathetic blockade upon local regulation of blood flow in subcutaneous adipose tissue was investigated in six subjects. The effect of changes in orthostatic pressure on blood flow in subcutaneous tissue in the crus and distal forearm was measured before and after sympathetic blockade obtained by epidural anesthesia in 4 subjects and by a bilateral sympathectomy in 2 patients suffering from manual hyperhidrosis. Blood flow in subcutaneous tissue measured by 133Xe washout technique decreased by about 40 per cent when the limb was lowered, and remained constant during 30 cm elevation. This was found both before and after the blockade, though in one of the patients, the orthostatic decrease in blood flow was less pronounced 24 h after sympathectomy. Hence central sympathetic reflexes do not alter local orthostatic changes of blood flow in subcutaneous tissue. These changes therefore are most likely due to local mechanisms.     

Effect of Spinal Sympathetic Blockade upon Local Regulation of Blood Flow in Subcutaneous Tissue

The influence of spinal sympathetic blockade upon local regulation of blood flow in subcutaneous adipose tissue was investigated in six subjects. The effect of changes in orthostatic pressure on blood flow in subcutaneous tissue in the crus and distal forearm was measured before and after sympathetic blockade obtained by epidural anesthesia in 4 subjects and by a bilateral sympathectomy in 2 patients suffering from manual hyperhidrosis. Blood flow in subcutaneous tissue measured by ¹³³Xe washout technique decreased by about 40 per cent when the limb was lowered, and remained constant during 30 cm elevation. This was found both before and after the blockade, though in one of the patients, the orthostatic decrease in blood flow was less pronounced 24 h after sympathectomy. Hence central sympathetic reflexes do not alter local orthostatic changes of blood flow in subcutaneous tissue. These changes therefore are most likely due to local mechanisms.     

Local Reflex in Microcirculation in Human Subcutaneous Tissue

The effect of changes in transmural pressure in the vessels of the extremities on blood flow in subcutaneous adipose tissue was investigated in three healthy subjects. Changes in transmural pressure were obtained either by postural changes of a limb, by locally induced subatmospheric pressure, or by venous stasis. Blood flow in subcutaneous tissue was measured 10 cm distal to the fibular head and at the lateral malleolus in the leg, or in the distal part of the forearm by the local ¹³³Xenon washout technique. When transmural pressure increased 25 mmHg or more, blood flow decreased about 50 per cent due to an increase in vascular resistance, but remained constant in the contralateral side. This vasoconstrictor response to increase in transmural pressure could be blocked by infiltrating the area under study with phentolamine or lidocaine or by induced counterpressure. In these cases blood flow remained constant. The findings indicate that the vasoconstrictor response to increase in transmural pressure of 25 mmHg or more is due to a local nervous mechanism involving adrenergic nerves. The receptors are presumably stretch receptors placed in the small veins. The effector site is probably the arterioles. When the vasoconstrictor response is blocked, autoregulation of blood flow in subcutaneous tissue, i.e. maintenance of constant blood flow during changes in perfusion pressure head or during changes in transmural pressure, is revealed. The vasoconstrictor response to increase in transmural pressure will counteract an increase in transmural pressure in the exchange vessels and thereby act as an edema protecting factor.     

Regional blood flow during reactive hyperemia in canine myocardium as determined by local washout of Xenon-133

The mechanisms regulating vascular tone in the myocardium were studied in open-chest anesthetized dogs by occlusions of the left anterior descending coronary artery (LAD) for 3 to 600 s. Cumulative excess blood flow (flow in excess of control flow), and repayment of flow debt (cumulative excess blood flow divided by blood flow deficit) were calculated using local injections of Xenon-133 for blood flow measurements. Release of vascular occlusion following 3 s of ischemia was not associated with any measurable hyperemia. Cumulative excess blood flow increased with increasing duration of ischemia from 5 to 600 s, but the increment in excess flow per unit extention of the occlusion time showed a considerable decline. Blood flow in excess exceeded blood flow debt incurred during the occlusion of 10 s duration of 161%; with prolongation of ischemia to 600 s repayment of flow debt declined markedly to about 10%. Oxygen lack in the tissue elicited by perfusion of LAD—for 10 s with constant perfusion rate—with deoxygenated blood produced a fall in peripheral coronary resistance of about 40% which closely corresponds to the fall in resistance observed after a period of LAD occlusion of similar duration. The results lead to the conclusion that ‘vasodilator’ metabolites formed in the tissue during periods of arterial occlusion are of prime importance for the fall in the tone of the vascular smooth muscle cell occurring in the post-occlusion period. The findings argue against a myogenic component in this response.