A study of blood flow through bone marrow by a method of venous effluent collection

Effects of humeral intraosseous epinephrine in a pediatric hypovolemic cardiac arrest porcine model

Background

Aims of the study were to determine the effects of humerus intraosseous (HIO) versus intravenous (IV) administration of epinephrine in a hypovolemic, pediatric pig model. We compared concentration maximum (Cmax), time to maximum concentration (Tmax), mean concentration (MC) over time and return of spontaneous circulation (ROSC).

Methods

Pediatric pig were randomly assigned to each group (HIO (n=7); IV (n=7); cardiopulmonary resuscitation (CPR)+defibrillation (defib) (n=7) and CPR-only group (n=5)). The pig were anesthetized; 35% of the blood volume was exsanguinated. pigs were in arrest for 2 min, and then CPR was performed for 2 min. Epinephrine 0.01 mg/kg was administered 4 min postarrest by either route. Samples were collected over 5 min. After sample collection, epinephrine was administered every 4 min or until ROSC. The Cmax and MC were analyzed using high-performance liquid chromatography. Defibrillation began at 3 min postarrest and administered every 2 min or until ROSC or endpoint at 20 min after initiation of CPR.

Results

Analysis indicated that the Cmax was significantly higher in the IV versus HIO group (p=0.001). Tmax was shorter in the IV group but was not significantly different (p=0.789). The MC was significantly greater in the IV versus HIO groups at 90 and 120 s (p<0.05). The IV versus HIO had a significantly higher MC (p=0.001). χ² indicated the IV group (5 out of 7) had significantly higher rate of ROSC than the HIO group (1 out of 7) (p=0.031). One subject in the CPR+defib and no subjects in the CPR-only groups achieved ROSC.

Discussion

Based on the results of our study, the IV route is more effective than the HIO route.

Peripheral circulation

Blood flow (BF) increases with increasing exercise intensity in skeletal, respiratory, and cardiac muscle. In humans during maximal exercise intensities, 85% to 90% of total cardiac output is distributed to skeletal and cardiac muscle. During exercise BF increases modestly and heterogeneously to brain and decreases in gastrointestinal, reproductive, and renal tissues and shows little to no change in skin. If the duration of exercise is sufficient to increase body/core temperature, skin BF is also increased in humans. Because blood pressure changes little during exercise, changes in distribution of BF with incremental exercise result from changes in vascular conductance. These changes in distribution of BF throughout the body contribute to decreases in mixed venous oxygen content, serve to supply adequate oxygen to the active skeletal muscles, and support metabolism of other tissues while maintaining homeostasis. This review discusses the response of the peripheral circulation of humans to acute and chronic dynamic exercise and mechanisms responsible for these responses. This is accomplished in the context of leading the reader on a tour through the peripheral circulation during dynamic exercise. During this tour, we consider what is known about how each vascular bed controls BF during exercise and how these control mechanisms are modified by chronic physical activity/exercise training. The tour ends by comparing responses of the systemic circulation to those of the pulmonary circulation relative to the effects of exercise on the regional distribution of BF and mechanisms responsible for control of resistance/conductance in the systemic and pulmonary circulations.

Evidence of an autoregulatory mechanism of regional bone blood flow at hypotension

BACKGROUND

Blood flow in various organs is determined by an autoregulatory mechanism that guarantees constant organ perfusion over a wide range of arterial blood pressure changes. This physiological principle has been proven for the kidney, brain and intestinal tract, but so far not for bone. This study was carried out to determine whether there is an autoregulatory mechanism of bone or not.

METHODS

The fluorescent microsphere reference sample method was used to determine blood flow within the bone and kidneys. Eight anesthetized female New Zealand rabbits received left ventricular injections of fluorescent microspheres over a wide range of arterial pressure levels prior to removal of kidney, femur and tibia. Blood flow values were calculated by measurement of fluorescence intensity in kidney and bone and correlated to fluorescence intensity in the peripheral blood (reference sample).

RESULTS

Despite a reduction of mean arterial pressure from 100 to 80 mmHg bone blood flow remained constant. Further reduction of mean arterial pressure results in a linear decrease in bone blood flow.

CONCLUSION

The correlation between arterial pressure and organ perfusion in the bone is similar to blood flow within the kidney, indicating the presence of an autoregulated blood flow mechanism within the bone tissue.

Intraosseous vascular access: A review

Intraosseous cannulation is an increasingly common means of achieving vascular access for the administration of fluids and medications during the emergent resuscitation of both paediatric and adult patients. Improved tools and techniques for intraosseous vascular access have recently been developed, enabling the healthcare provider to choose from a wide range of devices and insertion sites. Despite its increasing popularity within the adult population, and decades of use in the paediatric population, questions remain regarding the safety and efficacy of intraosseous infusion. Although various potential complications of intraosseous cannulation have been theorized, few serious complications have been reported. This article aims to provide a review of the current literature on intraosseous vascular access, including discussion on the various intraosseous devices currently available in the market, the advantages and disadvantages of intraosseous access compared to conventional vascular access methods, complications of intraosseous cannulation and current recommendations on the use of this approach.

Microspheres accurately predict regional bone blood flow

Even though the microsphere method frequently is used to determinate bone blood flow, validation of this technique for bone blood flow measurement is incomplete. The method is based on the principle that injected microspheres are distributed with the arterial blood and trapped in the capillaries because of their diameter (15 microm). The number of spheres lodged in an organ is proportional to its blood flow. The number of radioactive or fluorescent microspheres in a specific organ is determined indirectly by measuring radioactivity or fluorescence intensity in the organ. In this study the reliability and precision of the microsphere method for determining bone blood flow was established using radioactive and fluorescent microspheres. Six female, anesthetized New Zealand rabbits received left ventricular injections of pairs of fluorescent and/or radioactive microspheres. The humerus, femur, and tibia were dissected in a standardized manner and blood flow was determined in each sample. Comparison of relative blood flow values showed an excellent correlation between radioactive and fluorescent microspheres. The percentage difference and variation between two simultaneously injected sets of microspheres was minimal for radioactive microspheres (0.8% +/- 9.6%) and for fluorescent microspheres (0.2% +/- 11.4%). Regional bone blood flow in different regions of the femur, tibia, or humerus ranged from 2.2-28.1 mL/minute/100 g, but there was no significant difference between right and left bone samples of the same region after repeated measurement. Radioactive and fluorescent microspheres allow precise determination of regional bone blood flow.

Validity of fluorescent microspheres method for bone blood flow measurement during intentional arterial hypotension

In this study, we compared bone blood flow values obtained by simultaneously injected fluorescent (FM) and radiolabeled microspheres (RM) at stepwise reduced arterial blood pressure. Ten anesthetized female New Zealand White rabbits received simultaneous left ventricular injections of FM and RM at 90, 70, and 50 mmHg mean arterial blood pressure (MAP). After the experiments, both kidneys and long bones of all four limbs were removed and dissected in a standardized manner. Radioactivity (corrected for decay, background, and spillover) and fluorescence were determined, and blood flow values were calculated. Relative blood flow values estimated for each bone sample by RM and FM were significantly correlated (r = 0.98, slope = 0.99, and intercept = 0.04 for 90 mmHg; r = 0.98, slope = 0.94, and intercept = 0.09 for 70 mmHg; r = 0.98, slope = 0.96, and intercept = 0.07 for 50 mmHg). Blood flow values (ml x min-1 x 100 g-1) of right and left bone samples determined at the different arterial blood pressures were identical. During moderate hypotension (70 mmHg MAP), blood flow in all bone samples remained unchanged compared with 90 mmHg MAP, whereas a significant decrease of bone blood flow was observed at severe hypotension (50 mmHg MAP). Our results demonstrate that the FM technique is valid for measuring bone blood flow. Differences in bone blood flow during altered hemodynamic conditions can be detected reliably. In addition, changes in bone blood flow during hypotension indicate that vasomotor control mechanisms, as well as cardiac output, play a role in setting bone blood flow.

Blood flow and blood volume in the femoral heads of healthy adults according to age: measurement with positron emission tomography (PET)

OBJECTIVE

To deepen understanding of hemodynamics in the femoral head, i.e., the essential factor in clarifying pathogenesis of hip disorders, this study examined blood flow and blood volume in the femoral heads of healthy adults, and their changes with age, by using positron emission tomography (PET).

METHODS

In 16 healthy adult males (age: 20-78 years old, mean age: 42 years), blood flow was measured by means of the H2(15)O dynamic study method, and blood volume was measured by means of the (15)O-labeled carbon monoxide bolus inhalation method.

RESULTS

Blood flow was 1.68-6.47 ml/min/100 g (mean +/- SD: 3.52 +/- 1.2), and blood volume was 1.67-6.03 ml/100 g (mean +/- SD: 3.00 +/- 1.27). Blood flow significantly decreased (p < 0.01) with age, and blood volume significantly increased (p < 0.05).

CONCLUSION

PET was useful in the measurement of blood flow and blood volume in the femoral heads. With age, physiological hemodynamic changes also increased in femoral heads.

A new in vivo method for the direct measurement of nutrient artery blood flow

A new method of directly measuring nutrient artery blood flow using ultrasonic probes is described. These probes have provided reproducible results in our experiments. Advantages of ultrasonic probes include the direct measurement of blood flow through small arteries, ease of use, accuracy of measurement, applicability to a wide range of vessel diameters, the capability of chronically monitoring blood flow over time using permanently implanted probes, and the ability to use the method in conjunction with previous methods of bone blood flow measurement. The method is limited to the extent that only the contribution of the nutrient artery can be measured and total bone blood flow cannot be assessed. Tibial nutrient arterial flow and cardiac output were measured in adult mongrel dogs. Two experiments were performed: 1) bilateral baseline tibial nutrient artery blood flow measurements over time and 2) tibial nutrient blood flow comparing inhaled anesthesia (halothane/nitrous oxide/oxygen) and intravenous anesthesia (pentobarbital [Nembutal]). In 15 mongrel dogs, tibial nutrient artery blood flow averaged 1.46 +/- 0.72 mL/min (0.09 +/- 0.05 percent of cardiac output and 2.75 +/- 1.95 mL/min/100 g of bone). No significant difference in tibial nutrient artery blood flow was observed between animals given intravenous and inhaled anesthesia (P > .05). As a basic research tool, transit-time ultrasonic blood flow technology may be useful. The method is relatively easy to use and may be applied to experimental models designed to investigate various physiologic and pathologic states frequently encountered in orthopedics (eg, shock, sepsis, fractures).

Assessment of the relationship between standard probe and implantable fiber measurements of cortical bone blood flow: a canine study

Laser Doppler flowmetry (LDF) has been used to assess cortical bone blood flow in various clinical situations, such as osteomyelitis and osteonecrosis. Standard metal-sheathed probes containing optical fibers, applied to cortical bone for perfusion measurements, require direct exposure of the bone surface for each measurement, making nonanesthetized assessments over time impractical. Implantable optical fibers offer a noninvasive method for evaluating cortical bone perfusion without repeated surgical exposure of the bone after initial surgical implantation of the fibers. In vitro studies have shown the reliability of laser Doppler (LD) fibers compared with those of the standard probe. This investigation studied the relationship between measurements of cortical bone perfusion obtained by implanted optical (LD) fibers and standard (LDF) probes in vivo. Midshaft tibial fractures were created in the right hindlimb of 11 adult, large (>25 kg) dogs and stabilized by low contact-dynamic compression plate fixation. Cortical bone blood flow was measured by LDF using standard probes and implantable fibers at five sites along the tibia prefracture, postfracture, immediately postplate application, and at 10 weeks postplating, immediately prior to euthanasia. The implantable fibers were secured onto the cortical bone via the plate and led through a percutaneous exit site. Histological examination of the inguinal and popliteal lymph nodes and soft tissue surrounding the fibers revealed mild inflammation. No significant correlation of blood flow assessed by the implantable fibers and standard probe occurred immediately postfracture (r < 0. 13, p > 0.62). However, a statistically significant correlation was seen postplate application at one of the measurement sites in the distal fracture fragment (r = 0.78, P < 0.003). The fibers remained intact and functional until an average of 3 weeks at which time they either fractured or were removed by the animals. This is the first in vivo study assessing the reliability of implantable fibers for the measurement of cortical bone blood flow. Further modification of the fibers will be necessary to improve their longevity and durability for assessment of cortical bone blood flow.

Measurement of blood flow to osseous tissue in dogs using the radiolabelled microsphere method

1. The purpose of this study was to examine bone blood flow in various intra- and extra-oral sites. 2. The radiolabelled microsphere method was utilized to assess osseous blood flow in the following regions of 10 dogs: rib, long bone, and anterior and posterior regions of the maxilla and mandible. 3. Samples of cancellous and cortical bone were also obtained from each of these regions with the exception of the maxilla and the anterior mandible. 4. Mean blood flow ranged from 3.71 +/- 0.81 (SE) ml min.-1 100 g-1 in the mandibular posterior cortical bone to 22.7 +/- 4.66 ml min-1 100 g-1 in the cancellous rib samples. 5. Blood flow to the cancellous tissue of the rib was significantly greater (P < 0.05) than the other tissues with the exception of maxillary posterior bone and cortical rib. 6. Results from this study indicate that blood flow to the maxillary posterior bone is relatively high, but blood flow in other intraoral osseous sites is significantly less than that of cancellous rib bone.

The effects of acidosis and alkalosis on long bone vascular resistance

This study used an ex vivo perfusion model to investigate the direct effects of acidosis and alkalosis on the vascular resistance of the canine tibia. Baseline vascular resistance (BVR) and the vascular smooth muscle response to bolus doses of norepinephrine (NE) (0.025-3.2 nmol) and periarterial sympathetic nerve stimulation (NS) (10-25 Hz: 9 V, 2 ms pulses, 10 s) were studied. In Group I, these parameters were measured at normal pH (duration 7.34-7.44) and then during acidosis (pH 7.2-7.33). In Group II, they were measured at normal pH and then during alkalosis (pH 7.47-7.58). In Group III (control), they were measured serially at a normal pH. Alkalosis increased BVR by 56% (p < 0.0001). Acidosis attenuated (18% reduction) and alkalosis enhanced (66% increase) the vasoconstrictor action of NE (p < 0.0001). Acidosis also attenuated (11% reduction) the effect of sympathetic NS (p = 0.012). It is concluded that perfusion pH influences the sensitivity of long bone resistance vessels to circulating NE and sympathetic NS. Thus, local concentration of hydrogen ions may provide bone with a mechanism to autoregulate blood flow.

The relationship between inert gas wash-out and radioactive tracer microspheres in measurement of bone blood flow: effect of decreased arterial supply and venous congestion on bone blood flow in an animal model

Several methods have been employed in the study of bone perfusion. We used a method of determining inert gas wash-out by mass spectrometry in the study of blood flow rates in pigs. The method was validated by comparison of the result obtained with inert gas wash-out to that with measurement by microspheres. Furthermore, the effect of decreased inlet flow and venous congestion on the bone perfusion data was tested. The undisturbed bone blood flow was not significantly different when measured with wash-out of inert gas (7 +/- 0.7 ml/min/100 g) or with microspheres (9 +/- 2.9 ml/min/100 g), and the methods were correlated. Perfusion was reduced significantly, to 20% of the original value, after arterial occlusion. The changes in wash-out curves and accumulation of radioactive tracer provided substantial evidence for impaired intraosseous circulation following venous obstruction also. In conclusion, the study showed that this method of determining inert gas wash-out is feasible for studies of local perfusion rates in bone. The flow rates obtained by wash-out correlated well with the results of microsphere studies. In this animal model, both methods detected a fivefold reduction in flow rate after clamping of the arterial inflow. Obstruction of the venous outflow also impaired blood flow and lowered the cellular supply.

Treatment of chronic traumatic bone wounds. Microvascular free tissue transfer: a 13-year experience in 96 patients

During a 13-year period at the Massachusetts General Hospital, Boston, Massachusetts, 97 microvascular free tissue transfers have been performed for soft-tissue reconstruction in 96 patients following bone debridement for chronic traumatic bone wounds. These 96 patients comprise a continuation study of 18 original patients reported in 1982. During a 13-year follow-up period (mean, 77.1 months), 95.8% of these 96 patients have enjoyed complete wound closure with a lack of drainage after the debridement and free tissue transfer. Most of the patients (89.6%) encountered in this study are ambulatory without assist and 5.2% of patients have undergone amputation. Twenty-three per cent of patients required subsequent segmental bone defect reconstruction in the lower extremity after infection eradication. The pathophysiology of chronic traumatic bony wounds is different from that of chronic hematogenous osteomyelitis and thus a high incidence of long-term successful management can be seen through complete wound debridement and adequate soft-tissue coverage.

The effect of anaesthesia on the bone blood flow of the rabbit

The effect of anaesthesia on the rate of bone blood flow in the rabbit was studied. Two estimates of bone blood flow were performed, using 15 microns radioactively labelled microspheres, in three groups of rabbits. The first group was anaesthetised and the interval between the two estimates was 5 min. The second group was also anaesthetised but the interval between estimates was 60 min. The third group of rabbits was conscious and the interval between the two estimates was 4 h. In the first group, small uniform falls in whole bone blood flow were observed (mean change = -5%). In the second group, larger and less predictable reductions were observed (mean change = -24%). In the third group (conscious), little change was observed in whole bone blood flow during the 4-h experiment (mean change = +7%). It is concluded that substantial falls can occur in bone blood flow in the rabbit during a 1-h anaesthetic. If bone blood flow is to be monitored under anaesthesia, then the effect of the anaesthetic on bone blood flow must be accurately defined. In conscious rabbits, estimates of whole bone blood flow performed at an interval of 4 h under control conditions will give reproducible results.

Assessment of bone marrow blood flow using positron emission tomography: no relationship with bone marrow cellularity

Bone marrow blood flow has been assessed using positron emission tomography and the 15O-labelled carbon dioxide steady-state technique. The measurements were performed at the site of the posterior iliac crest. The bone marrow blood flow was 10.0 ml/min/100 cm3 +/- 3.0 (SD) in normal volunteers. It was markedly increased in patients with polycythaemia vera (26.9 +/- 4.6), chronic granulocytic leukaemia (25.2 +/- 3.9) and myelofibrosis (35.1 +/- 7.3). However, bone marrow blood flow did not differ from normal in patients with aplastic anaemia, chronic haemolysis or chronic lymphocytic leukaemia. There was no relationship between bone marrow cellularity and bone marrow blood flow. The data show that bone marrow blood flow is markedly elevated in polycythaemia vera, myelofibrosis and chronic granulocytic leukaemia and suggest that bone marrow cellularity is not a major factor in regulating bone marrow blood flow.

Increased bone marrow blood flow in rabbits with acute hemolytic anemia

The bone marrow blood flow in the femur of the rabbit with acute hemolytic anemia was measured by the radioactive microsphere method. Acute hemolytic anemia was induced by a single intraperitoneal injection of phenylhydrazine. An increase in the bone marrow blood flow was detected three days after the induction of anemia and continued for about one week. The bone marrow blood flow of the anemic rabbit (66 ml/min/100 g marrow) was about three times that of the control (20 ml/min/100 g marrow). The increased blood flow to the bone marrow appeared to be correlated to the increased erythropoietic activity.

Measurement of absolute bone blood flow by positron emission tomography

A method of measuring bone blood flow has been developed using 18F sodium fluoride and positron emission tomography. The blood flow levels are in line with those obtained experimentally from microsphere embolisation. This investigative method could be applied to elucidate a number of clinical questions involving bone perfusion.

Blood flow rates in canine cortical and cancellous bone measured with 99Tcm-labelled human albumin microspheres

Regional differences in bone blood flow rates in the femur and the tibia of dogs were measured with 99Tcm-labelled microspheres. The measurements show an average flow rate of 3.7 ml (100 g)-1 x min-1 in cortical bone. A more rapid pace was found in red marrow containing areas, with an average flow rate in the femoral head of 19.7 ml blood (100g)-1 x min-1, and in the femoral neck of 50.3 ml blood (100 g)-1 x min-1. In the calcar femorale the average flow rate was 9.0 ml (100 g)-1 x min-1, and compared to cortical flow a positive correlation between strain and perfusion seems obvious. No difference between cortical bone in tibia and femur was found. The flow rates in the red marrow of the femoral neck are remarkably high, but the flow in the cortical bone is relatively low. It is concluded that handling of fragments of cortical bone and the associated soft tissue is presumably critical, and that the surgical technique has to be quite gentle to obtain optimal conditions for fracture healing.

Changes in blood-flow in the bones after osteotomy and osteosynthesis

The blood-flow can be measured with the method of "tracer microspheres". In 15 adult shepherd-dog bastards the blood-flow in the bones of the posterior extremity was determined. The blood-flow in callous bone is 6 times higher than in cortical bone. The values for blood-flow are decreasing from the proximal to the distal end, in cortical as well as in callous bone. In 10 of the 15 dogs the blood-flow was determined after manipulation of the bone. If the medullary cavity of the tibia was evacuated the blood-flow of the corticalis in the bone shaft diminished; it was reduced 3 times more than after removal of the periost. If an osteotomy was performed in the middle of the shaft, the blood-flow was more reduced distally than proximally. In the corticalis of the shaft the blood-flow of the proximal fragment drops only when an osteosynthesis is performed. In extended oblique fractures the upper ends of the fragments--proximally as well as distally--are endangered by a reduction of the blood-flow.

Observations on long bone medullary pressures in relation to arterial PO2, PCO2 and pH in the anaesthetized dog

To investigate the influence of variations in arterial oxygen tensions (PaO2), arterial carbon dioxide tensions (PaCO2), and arterial pH on long bone medullary pressures, seven anaesthetized dogs were investigated. Comparing the control medullary pressures, i.e. the mean medullary pressures obtained at the normal range of PaO2 (75--110 mmHg) with the mean medullary pressures corresponding to the range of PaO2 of less than 75 mmHg, statistically significant (P less than 0.05) decreases were seen in both epiphyseal, metaphyseal and diaphyseal medullary pressures, from 27.6 +/- 5.0 to 15.5 +/- 3.6 mmHg, from 23.5 +/- 2.9 to 13.9 +/- 2.3 mmHg and from 27.7 +/- 3.9 to 18.3 +/- 2.5 mmHg (all mean values +/- s.e. mean), respectively. Hyperoxia, hypocapnia, hypercapnia or metabolic acidosis had no effect on medullary pressures in any of the regions studied.

The effect of haemorrhage and bone fracture on bone marrow circulation

Blood flow was measured in the bone marrow and trabecular bone of the dog using 133Xe and hydrogen clearance technique. Estimated from hydrogen wash-out curves, flow in the marrow of the femoral diaphysis was 0.207 ml.min(-1).g(-1), in the marrow of the humerus 0.123ml.min(-1).g(-1) and in the trabecular bone of femoral metaphysis 0.173ml.min(-1).g(-1). Haemorrhagic hypotension and fracture reduced the medullary flow. After haemorrhage an exponential relationship was found between blood pressure and effective bone marrow and muscle blood flow. The correlation between systemic arterial and intramedullary pressures was found to be exponential and between intramedullary pressure and flow linear.

Studies on bone ion exchanges using multiple-tracer indicator-dilution techniques

Free diffusion appears to be the principal mechanism for movement of ions, known to concentrate in bone, across the capillaries of bone. The interstitial fluid space appears large enough to allow for determination of instantaneous fractional escape by the indicator-dilution method. The mechanism by which large molecules, such as the diphosphonate 99mTc-labeled EHDP or 99mTc-labeled pyrophosphate, pass through capillaries in bone is by passive diffusion. These molecules are larger than 85Sr and their lower extraction rate is presumably due to the effect of their sizes on passage through transcapillary clefts. A corollary of these studies might be that if transcapillary exchange is passive, then partition of anions and cations likely lies beyond the capillary and therefore is controlled by osteal cellular processes.

Tonometric measurement of dental pulpal and mandibular marrow blood pressures

Dental pulpal and mandibular marrow vascular pressures were studied with a tonometric method under conditions of autoperfusion and step flow controlled perfusion. The tonometric pulpal and marrow pressures averaged 17 +/- 5 mm Hg and 20 +/- 6 mm Hg, respectively. The venous end pressure was 16 +/- 8 mm Hg at a perfusion pressure of 156 +/- 28 mm Hg. The tonometric pulpal pressures were lower and were less variable than the comparable canulation pulpal pressures.

The effects of hypoxia on the caudal vertebrae of growing mice and rats

Images

Blood flow in canine tibial diaphysis estimated by iodoantipyrine- 125 I washout

Tibial bone blood flow was estimated in dogs by an indicator washout method using ¹²⁵I-labeled 4-iodoantipyrine (I-Ap). The tibial cortical bone/blood partition coefficient, λ, of I-Ap was (mean ± sd) 0.150 ± 0.027 for 17 dogs. Tibial cortical bone blood flow (mean ± sd) was 0.0152 ± 0.003 ml/ml bone per min by the residue function technique of Zierler applied to 32 washout curves. Exponential analysis by the successive subtraction method gave the same mean value and, if the flow were considered to traverse two regions in parallel, suggested that 22% (sd, ±7.4) of the indicator traversed a high-flow region (mean flow, 0.265 ml/ml bone per min; sd, ±0.194) and the remainder had a mean flow of 0.0118 ml/ml bone per min (sd, ± 0.003).

The control of the circulation in skeletal muscle during arterial hypoxia in the rabbit

1. The effects of arterial hypoxia on muscle blood flow were examined in normal unanaesthetized rabbits in relation to simultaneously determined changes in cardiac output, arterial pressure and heart rate. Muscle blood flow was estimated from the difference between total limb flow (local thermodilution) and the estimated skin flow (using a calibrated heat conductivity method). The role of the arterial chemoreceptors and baro-receptors in the control of muscle blood flow was examined and the nature of the sympathetic efferent discharge analysed.2. In mild hypoxia (P(O2) > 35 mm Hg) in the rabbit, muscle blood flow did not change, although cardiac output increased. During moderate hypoxia (P(O2) 30-35 mm Hg) there was initial vasoconstriction in muscle, followed by a return to control values paralleling the changes in cardiac output. In severe arterial hypoxia (P(O2) < 30 mm Hg) the initial vasoconstriction was less marked, and during the ;steady state' there was a large vasodilatation and increase in muscle blood flow, at a time when the cardiac output was not elevated.3. The early vasoconstriction in arterial hypoxia is mediated mainly through sympathetic vasoconstrictor nerves as a result of strong arterial chemoreceptor stimulation.4. Increased secretion of adrenaline is an important factor in restoring muscle blood flow to control values during moderate arterial hypoxia, and in elevating the muscle blood flow above these values in severe hypoxia. The peripheral dilator (beta-) effects of adrenaline oppose the peripheral constrictor (alpha-) effects resulting from increased activation of sympathetic constrictor nerves during arterial hypoxia.

Effects of arterial hypoxia on the cutaneous circulation of the rabbit

1. Changes in blood flow of the skin of the rabbit's ear and hind limb have been studied during arterial hypoxia by a calibrated heat conductivity method, together with changes in arterial pressure and aortic blood temperature.2. There is little change in the blood flow of the hind-limb skin during the early phase of arterial hypoxia, reflecting a balance between the local dilator effects of hypoxia and the increased constrictor activity mediated through the sympathetic nerves as a result of arterial chemoreceptor excitation. During more prolonged arterial hypoxia there is a small gradual dilatation of the vessels of the hind-limb skin as a result of some diminution in the initial intensity of vasoconstrictor activity.3. There is much more extensive vasodilatation in the ear than in the hind-limb skin during arterial hypoxia. Vasoconstrictor activity is slight in this region. Comparison of the ear responses to arterial and to primary tissue hypoxia suggests that in the former type of hypoxia stimulation of the arterial chemoreceptors inhibits thermoregulatory vasoconstriction to the ear, whilst in the latter type of hypoxia baroreceptor reflexes maintain or intensify it.