High frequency pulsation (HFP) enabling tracheo-broncheal suctioning without interrupting mechanical ventilation

A new technique of tracheo-broncheal suctioning is presented which enables constant artificial ventilation with high frequency pulsation (HFP), a modification of high frequency ventilation. Beyond that the high frequency pulsation method permits ventilation of the patient with adjusted end-expiratory pressure. This technique prevents the decrease of arterial oxygen pressure which is usually seen in patients who are disconnected for some time from the respirator as well as during the suction procedure and during periods of decreased end-expiratory pressures.     

Functional closure of the femoral canal revealed by computed tomographic scanning

The lacuna vasorum contains, as well as the external iliac vessels, a space called the femoral canal which allows pulsation and other changes in the diameter of the two vessels. The canal is closed by a connective tissue septum which is too weak to prevent a herniation alongside the vessels. The functional closure of the canal was investigated by artificial augmentation of the intraluminal pressure of the vein in five specimens and by computed tomographic (CT) scanning during a Valsalva manoeuvre in ten normal persons. In the abdomen the intraluminal and extraluminal pressures are raised in parallel, with no consequence for the diameter of the vessels. In the area of the lacuna vasorum the intravenous pressure is conveyed down to the first competent valve which is located 0.5 cm distal to the inguinal ligament, while the extraluminal pressure is reduced to normal. At each rise of the intra-abdominal pressure the vein will therefore bulge and exactly fill the empty space of the femoral canal thus preventing herniations by an on-demand mechanism. By CT scanning it was found that the vein in the femoral canal increased its diameter 2.5-3.0 times during the Valsalva manoeuvre.     

Continuous venovenous renal replacement therapy using a pulsatile blood pump

The objective of this study was to evaluate the efficacy of a pulsatile pump for continuous renal replacement therapy in a pediatric-size animal model. A vacuum-driven, tubular, blood-pumping device was used in 13 pigs weighing 10.4+/-1.5 kg, connected to a neonatal hemofiltration circuit with an FH22 filter and a flow sensor. Three different flow rates [30 ml/min (8 cases), 15 ml/min (3 cases), and 5 ml/min (2 cases] were used over 2-h periods. Aspiration pressure, frequency of pulsation, blood flow rate, ultrafiltrate volume, pre- and post-filter pressures, heart rate, arterial blood pressure, temperature, pH, sodium, potassium, chloride, urea, creatinine, glucose, and hematocrit were measured at 30-min intervals. The mean ultrafiltrate flow was 0.54+/-0.33 ml/kg per min. The aspiration pressure and pulsation frequency needed to maintain blood flow remained stable throughout the experiment. There were no complications secondary to the use of this technique and no significant changes in heart rate, blood pressure, or analytical determinations. In conclusion, in this animal model, the pulsatile pump has been shown to be an effective method for continuous venovenous renal replacement therapy.     

Intramedullary pressure of the patella in chondromalacia

Summary Degenerative changes in load-bearing joints are often combined with altered intramedullary circulation. This may cause an alteration in intramedullary pressures. It has been said that articular symptoms are connected with a change in intramedullary pressures being high they are suspected to cause pain.In this investigation the intramedullary pressure has been evaluated in chondromalacia and osteoarthrosis of the patella. A comparison has been made with normal patellae.A biopsy needle was drilled into the intramedullary bone of the patella and connected with a registering unit.In a control group an average value of 19 mm Hg was registered. In the condromalacia group the mean intramedullary pressure was 44 mm Hg and in osteoarthrosis 37 mm Hg.     

In vitro measurement of pressure differences using manometry at various injection speeds during discography.

BACKGROUND CONTEXT: Lumbar discography has been widely used to evaluate discogenic low back pain. Anecdotal evidence suggests that pain reproduction during discography is more closely correlated with peak dynamic pressure than with static postinjection pressure. Although there can be a significant difference between dynamic and static pressures, to date most discographic evaluations use static pressure recorded postinjection (which is stable and easily measured). The use of readings taken after injection, rather than readings of maximum dynamic peak pressure recorded during injection, appear to increase false positives in lumbar discography. High-speed intradiscal injections also appear to have potentially confounding effects that may increase the rate of false-positive responses during lumbar discography. To date there has been no study for the evaluation of peak dynamic intradiscal pressures or for differentiating dynamic from static pressures in the nucleus pulposus (NP) in response to various speeds of intradiscal injection. PURPOSE: The goal of this study was to obtain additional data on potential confounding factors that could affect discographic results by assessment of pressures within the NP during discography at various injection speeds. The purpose of data collection was to more precisely evaluate pressure differences between dynamic and static pressure within the NP, evident during discography. STUDY DESIGN: In vitro laboratory study. SAMPLES: A total of 82 trials were performed on intervening discs of 82 porcine cadaver lumbar spines. METHODS: Dynamic and static intradiscal pressures were measured with manometry, using two pressure sensors simultaneously during intradiscal injection of contrast media at various speeds. The tip of a 25-gauge needle was placed in the center of the NP and connected with a pressure manometer, which recorded the pressure and therefore the speed of injection. A second pressure reading was obtained using a sensor tip connected to a transducer; the sensor tip was located separately in the same NP tissue. The needle and transducer locations were confirmed by fluoroscopy. OUTCOME MEASURES: At low controlled injection speeds (below 0.08 mL/s), the mean peak pressure difference in the NP was 4.06 (+/-1.52) psi. With high-speed injections of 0.08 mL/s or greater, the mean increased abruptly up to 14.52 (+/-4.11) psi (p<.05). The data indicate that injections applied slowly resulted in smaller differences in pressure within the NP, registered by both manometry and the needle sensor tip. Additional samples were taken using both devices to confirm this threshold level. RESULTS: With low injection speeds, especially those below 0.08 mL/s, differences between dynamic and static pressures on both pressure sensors were minor. These differences increased as injection speed became faster. However, at fast injection speeds of 0.08 mL/s and above, those differences were significantly higher. CONCLUSION: Dynamic and static intradiscal pressures are of similar value when measured by manometer and by needle sensor at slow injection speeds during discography. However, the pressure differences appeared to rapidly increase in response to incremental increases in injection speed. The data from these 82 samples suggest that uncontrolled high speeds of intradiscal injections are a potential confounding factor, which may increase false-positive responses during lumbar discography.     

Walking and running plantar pressure analysis before and after resection of tarsal coalition

BACKGROUND: Conservative treatment failure in symptomatic tarsal coalition usually mandates bar resection as a preferred operative alternative. The outcome of this procedure generally has been assessed by clinical measures. The purpose of our study was to evaluate whether plantar pressure distribution is normalized after bar resection. METHODS: This study compared three groups: nine candidates for resection of tarsal coalition, nine patients between 2 and 4 years after bar resection (not the same patients who were candidates for resection), and nine control subjects. The ankle and hindfoot were evaluated according to the American Orthopaedic Foot and Ankle Society Ankle-Hindfoot Scale. Mean plantar pressure and mean normalized impulses were calculated during walking and running. RESULTS: Significantly higher AOFAS scores were demonstrated in the postoperative group compared with the preoperative group (p<0.01). Medial midfoot pressures demonstrated the most consistent differences between groups. Preoperative feet had significantly higher medial midfoot pressures compared with the control group both during walking and running. Feet that had resection did not demonstrate significantly higher medial midfoot pressures during walking compared with the control group. However, during running, this segment's pressures were significantly higher both in preoperative (p=0.000) and in postoperative (p=0.023) feet compared with the control group. Heel segments revealed decreased pressures laterally both in preoperative feet and in postoperative feet compared with the control group. CONCLUSIONS: Running demonstrates that normal plantar pressures are not recreated after resection of tarsal coalition. However, close to normal walking pressure distribution is consistent with the favorable clinical outcome observed in most patients after bar resection. CLINICAL RELEVANCE: Regaining full recreational activity after resection of a tarsal coalition, i.e. running, may have implications on abnormal foot loading and torque, thus promoting degenerative changes in the subtalar and adjacent joints.     

New blood volume monitoring method for hemodialysis: A-V pressure gradient measurement by synchronized one-point reading

Abstract:  During hemodialysis, rapid ultrafiltration often causes symptomatic hypotension. To predict the occurrence of volume-dependent hypotension as early as possible, continuous hematocrit monitoring with the Crit-Line noninvasive monitor has been widely used to measure blood volume changes during hemodialysis. As another potential method of monitoring blood volume variations, we studied blood viscosity, which is theoretically associated with the pressure gradient across the dialyzer. Blood viscosity (calculated by the Hugen-Poiseuille formula) is a major determinant of the blood flow rate and is associated with the pressure difference between the postpump arterial (A) and venous (V) pressures. The A-V pressure gradient fluctuates due to pump pulsation, so we minimized this noise by always reading the pressure gradient at the same point out of 1400 partitions on the rotary pump. To test this synchronized one-point reading method, the A-V pressure gradient was measured using 3 different xanthan gum solutions and was found to be linearly proportional to the model blood flow rate. In an experimental dialysis system using a xanthan gum solution (300 mg/L), the A-V pressure gradient showed a gradual linear increase along with the ultrafiltration rate up to 1 L/h as the viscosity slowly increased in the dialyzer. The changes of blood volume shown by this method were significantly correlated with data obtained using the Crit-Line in 8 patients undergoing hemodialysis. This simple and inexpensive method may allow monitoring of blood volume changes and thus provide data that are beneficial for fluid management in hemodialysis patients suffering from clinical dialysis intolerance.     

Renal pelvic pressure responds with augmented increases to increments in intraabdominal pressure

BACKGROUND/PURPOSE: Flow of a fluid through a collapsible tube is under the influence of various factors including the external compressing pressure. Because the intraabdominal pressure may compress the ureter, an experimental study has been planned to determine and compare the normal intraabdominal and renal pelvic pressures and the alterations in renal pelvic pressure in response to the increments in intraabdominal pressure in the rabbits. METHODS: Eight adult rabbits were used for the experiment. Under general anesthesia, an urethral catheter, a nasogastric tube, and an intraperitoneal catheter were placed to measure intravesical (IVP), intragastric (IGP), and intraabdominal pressures (IAP), respectively. Intracranial pressure monitorization catheter was placed into the renal pelvis to monitor intrapelvic pressure (IPP). Basal pressure measurements have been recorded. The pressures have been recorded in every 5 minutes, and IAP has been increased gradually about 3 to 4 cm of water pressure in every step for 30-minute periods. RESULTS: Increases in the intrapelvic pressure values have been significantly higher than the increases in the IAP (P < .001). A significant correlation has been found between IPP and IAP (P = .000, r = 0.866). By using linear regression analysis the relationship has been found to be IPP = 7.303 + 1.985 (IAP). Intragastric pressure values have been higher compared with IAP values (P < .001), whereas intravesical pressures have not differed from IAP (P > .05). CONCLUSIONS: Elevations in IAP results in augmented increases in the IPP. Poiseuille and Laplace Laws suggest this augmented increase to resemble proximal ureteric obstruction. Increases in IAP may simulate proximal ureteric obstruction and may take part in the pathogenesis of hydronephrosis. J Pediatr Surg 36:901-904.     

Intracranial echo pulsation in brain death,brain tumor and intracranial hypertension

Summary The rise time of the midline echo pulsation curve was shorter than normal in both the group of space taking lesions in the cranium and the group of cerebral arteriosclerosis. The rise time in the group of space occupying cerebral lesions was observed shorter than that in the group of cerebral arteriosclerosis. The latency time was measured showing no significant difference among groups of space taking lesions in the cranium, cerebral arteriosclerosis and normal. There was no difference as well in the latency time between both sides of normal persons. It is to be noted that there was a tendency that the latency time was longer on the affected side than on the normal side in the group of intracranial space occupying lesions. Fifteen cases of the so-called brain death were observed, and they were tested by electroencephalography and pulsatile echo-encephalography. Carotid arteriograms were taken in 8 cases. The pulsation of intracranial echoes was still found to exist even when the electroencephalograms were flat, and immediately after the echo pulsation disappeared, the non-filling phenomena of carotid arteriograms were constantly observed, and the intracranial pressure was found higher up to the systemic blood pressure. This may mean that the cerebral circulation is interrupted, when the intracranial echo pulsation disappears. Therefore the method to analyse the intracranial echo pulsation may be suitable to determine the state of the so-called brain death when the cerebral circulation has ceased for a period of time beyond the tolerance of brains to survive.     

The relationship between change in blood pressure, blood pressure and time

Physiological homeostatic mechanisms and interventions by anaesthetists attempt to moderate excessive change in many biological variables during anaesthesia. These mechanisms may have fast or slow response times. This study describes how mean arterial blood pressure changes with time and how the change is dependent upon the pre-existing blood pressure. The results demonstrate the 'regression towards the mean' concept; low arterial blood pressures increase and high pressures decrease. The data are the result of all interactions and have been used to produce an 'envelope' into which 80% of all changes fall. Alarm systems using this envelope could warn of excessive changes that occur within short time intervals.     

Acute negative intracranial pressure effects on the auditory evoked response in rabbits

Recent clinical experience has shown that significant neurological symptoms and deficits occur in patients who have been shunted for hydrocephalus when "overshunting" produces unusual negative intracranial pressure (ICP). Therefore, the effect of acute negative ICP .on the early auditory evoked response (AER) was studied in the normal New Zealand rabbit. ICP was reduced to -50, -100, and -150 mm H2O below base line pressure. The AER after rarefaction auditory stimulation was obtained from ongoing electroencephalographic activity at the base line ICP and at each of the three negative ICP levels. Off-line statistical evaluation of the AER showed minimal changes in the absolute and interpeak latencies of N-0 to P-5 at some negative pressures. However, no statistically significant changes were observed for any of the measures for grouped data. At the negative ICP levels studied, the cerebrospinal fluid (CSF) pulse pressure was considerably augmented over base line measures. Such augmentation of the CSF pulse pressure may be the cause of the minimal effects observed on the AER, which may be due to a neuropraxic effect rather than ischemia from cerebral perfusion pressure changes.     

The experimental assessment of techniques of measuring biliary pressure.

A variety of different methods have been used for the measurement of pressures during operations on the biliary tract. However, there are few data on the experimental evaluation of the methods, and there has been no controlled comparative evaluation of the most commonly used techniques. In these experiments, we have compared the manometric technique of Caroli, the syringe barrel method of Daniel and White et alii, and the constant infusion apparatus of Cushieri. The experiments were performed in pigs because of the similarity of their biliary tract to that of man. Resting and opening pressures were recorded, and flow into the duodenum via the ampulla was measured simultaneously. The manometric and syringe barrel techniques gave highly reproducible measurements of resting and opening pressure, and valid measurements of opening pressure. The constant infusion apparatus gave reproducible measurements of resting pressure, but did not measure opening pressure. The manometric technique was shown to measure changes in the resistance of the choledochoduodenal junction in response to morphine and atropine.     

Blood pressure measurement: clinic, home, ambulatory, and beyond

Blood pressure traditionally has been measured in the clinic setting using the auscultatory method and a mercury sphygmomanometer. Technologic advances have led to improvements in measuring clinic blood pressure and allowed for measuring blood pressures outside the clinic. This review outlines various methods for evaluating blood pressure and the clinical utility of each type of measurement. Home blood pressures and 24-hour ambulatory blood pressures have improved our ability to evaluate the risk of target-organ damage and hypertension-related morbidity and mortality. Measuring home blood pressures may lead to more active participation in health care by patients and has the potential to improve blood pressure control. Ambulatory blood pressure monitoring enables measuring nighttime blood pressures and diurnal changes, which may be the most accurate predictors of risk associated with elevated blood pressure. Additionally, reducing nighttime blood pressure is feasible and may be an important component of effective antihypertensive therapy. Finally, estimating central aortic pressures and pulse wave velocity are 2 of the newer methods for assessing blood pressure and hypertension-related target-organ damage.     

The effect of walking speed on peak plantar pressure

BACKGROUND: Plantar pressure measurements often are used as a tool to evaluate pathologic gait. Previous studies, often done at self-selected walking speeds, have used peak plantar pressure to try to predict ulcer formation, compare surgical outcomes, and evaluate orthotic device efficacy. However, the relationship between walking speed and plantar pressures at specific plantar regions has not been clearly defined. METHODS: Twenty normal subjects walked on a treadmill at six speeds (0.75 to 2.00 m/s). In-shoe peak plantar pressure was measured at five plantar regions and compared across the range of speeds. RESULTS: Walking speed affected peak plantar pressure differently at the five examined plantar regions. The hallux and heel regions had the highest pressures, which increased linearly with faster speeds. The central and medial forefoot pressures initially increased but plateaued at the faster speeds, while the lateral forefoot had the lowest overall peak pressures, which decreased at the faster walking speeds. Therefore, significant quadratic effects were found at the forefoot. Best-fit regression equations defined distinct pressure-speed relationships at each plantar region (p < 0.0001). CONCLUSION: The effect of walking speed on peak plantar pressure varied with plantar region. To achieve more robust peak plantar pressure measurements, walking speed should be controlled. Determining the normal plantar function across a range of speeds can aid in the development of shoes and foot orthoses. The pressure-speed relationships presented in this study can be used as a comparative tool for evaluating the efficacy of clinical interventions for pressure reduction, especially when walking speed changes may confound the outcomes.     

Contact mechanics of normal tarsometatarsal joints

BACKGROUND: The current treatment of tarsometatarsal joint injuries is associated with suboptimal long-term results. The objective of the present study was to measure the contact mechanics of the tarsometatarsal joints in normal adult cadaveric feet in order to develop a foundation for more effective treatment. METHODS: Six fresh cadaveric lower legs and feet were subjected to four different axial compressive loads (0.5, 1.0, 1.5, and 2.0 times body weight) at each of five different positions. For each position, load, and tarsometatarsal joint, the contact pressures and areas were measured with use of pressure-sensitive film. Contact forces were calculated from the ratio of pressure to area. Contact pressure, area, and force were analyzed as a function of load, the specific tarsometatarsal joint, and foot position. RESULTS: The forces across these joints ranged from 2 to 541 N, but pressures ranged only from 0.5 to 5.7 MPa. In general, changes in load and foot position, in both the sagittal and the frontal plane, were associated with changes (p<0.05) in tarsometatarsal joint contact areas and forces. In contrast, the contact pressures across these joints varied minimally with changes in load and foot position. CONCLUSIONS: These data suggest that the tarsometatarsal joints are designed to regulate pressure in each joint by means of two mechanisms: (1) at small loads, an intrajoint mechanism regulates tarsometatarsal joint pressure by increasing contact area within the joint in response to increasing force, and (2) at larger loads, an interjoint mechanism engages to regulate tarsometatarsal joint pressure by redirecting force to other tarsometatarsal joints. Clinical Relevance: The data provide both absolute (normal contact forces, areas, and pressures) and relative (intrajoint and interjoint regulating mechanisms) performance (functional) criteria for the development of new treatments for diseased or traumatized tarsometatarsal joints.     

Fluid balance within the canine anterolateral compartment and its relationship to compartment syndromes

Fluid homeostasis within muscle compartments is maintained by four pressures: capillary blood pressure, capillary blood oncotic pressure, tissue-fluid pressure, and tissue fluid oncotic pressure. As determined in the canine anterolateral compartment, capillary blood pressure is 25 +/- 3 millimeters of mercury; capillary blood oncotic pressure, 26 +/- 3 millimeters of mercury, tissue-pbessure, -2 +/- 2 millimeters of mercury; and tissue-fluid oncotic pressure, 11 +/- 1 millimeters of mercury. The wick technique allows direct measurement of tissue-fluid pressure in skeletal muscle and, with minor modifications, is adapted to collect microsamples of interstitial fluid for determinations of tissue-fluid oncotic pressure. The wick technique detects very slight fluctuations in intracompartmental pressure such as light finger compression, injection of small volumes of fluid, and even pulsation due to adjacent arterial pressure. Adjacent muscle compartments may contain different tissue-fluid pressure due to impermeable osseofascial barriers. Our results obtained in canine muscle compartments pressurized by infusion of autologous plasma suggest that risks of muscle damage are significant at intracompartmental pressures greater than thirty millimeters of mercury.     

Pharmacological effect on pyeloureteric dynamics with a clinical perspective: a review of the literature

We searched to review experimental and clinical trials concerning the capabilities of impacting on the ureteric and pelvic activity by means of pharmacological stimulation. Ureteropyeloscopy may cause high renal pelvic pressure. The normal pressure is in the range of 5–15 mmHg whereas pressure of 410 mmHg has been measured during endoscopy. The threshold pressure for intrarenal reflux is about 35 mmHg. Studies in animals have revealed that high renal pelvic pressures may cause permanent damage to the renal parenchyma. Furthermore, it has been demonstrated that elevated pressures may entail an increased risk of several complications related to endourological procedures including bleeding, perforation and infection. In other words, means by which intrarenal pressure could be lowered during endourological procedures might be beneficial with respect to clinical outcomes. In vitro experiments support the existence of different receptors in the ureter and renal pelvis. The ureteric and pelvic responses to the corresponding neurotransmitters have been determined. It seems that α-adrenergic and cholinergic agents are stimulating whereas β-adrenergic agents inhibit ureteric activity. The effect may depend on the mode of administration. Drugs exerting advantageous effects in the pyeloureter may cause undesirable systemic side effects when administered intravenously. In animal studies, renal pelvic pressure can be significantly lowered by topical administration of β-adrenergic agonists without systemic side effects. In vivo human studies are necessary to clarify the exact dose–response relationship and the degree of urothelial absorption of a drug before clinical use may be adopted.     

Doppler assessment of pulmonary haemodynamics in chronic hypoxic lung disease.

Various methods of Doppler echocardiography are useful in the analysis of flow dynamics within the heart and the pulmonary circulation in patients with COPD. In addition, to distinguish patients with increased pulmonary artery pressures from those with normal pressures, Doppler techniques provide quantitative methods for estimating pulmonary artery pressures non-invasively. Doppler echocardiography can be performed repeatedly and can thus be used to assess serial changes in the clinical state of a patient or in the response to certain pharmaceutical interventions in the pulmonary vascular bed. The most useful and accurate method of estimating pulmonary artery pressures in patients with chronic hypoxic lung disease is the systolic trans-tricuspid gradient, calculated from tricuspid regurgitation detected by continuous wave Doppler echocardiography with estimation of the right ventricular pressure, followed by the acceleration time from pulmonary flow analysis using pulsed Doppler techniques. New contrast materials to enhance the continuous wave Doppler signal and transoesophageal echocardiography may provide even more satisfactory results in the future.     

Understanding and extending the Starling principle

The Starling Principle states that fluid movements between blood and tissues are determined by differences in hydrostatic and colloid osmotic (oncotic) pressures between plasma inside microvessels and fluid outside them. The Revised Starling Principle recognizes that, because microvessels are permeable to macromolecules, a balance of pressures cannot halt fluid exchange. In most tissues, steady oncotic pressure differences between plasma and interstitial fluid depend on low levels of steady filtration from plasma to tissues for which the Revised Principle provides the theory. Plasma volume is normally maintained by fluid losses from filtration being matched by fluid gains from lymph. Steady state fluid uptake into plasma only occurs in tissues such as intestinal mucosa and renal peri-tubular capillaries where a protein-free secretion of adjacent epithelia contributes significantly to interstitial fluid volume and keeps interstitial oncotic pressure low. Steady filtration rates in different tissues are disturbed locally by reflex changes in capillary pressure and perfusion. The rapid overall decline in capillary pressure after acute blood loss initiates rapid fluid uptake from tissue to plasma, that is, autotransfusion. Fluid uptake is transient, being rapid at first then attenuating but low levels may continue for more than an hour. The Revised Principle highlights the role of oncotic pressure of small volumes of interstitial fluid within a sub-compartment surrounding the microvessels rather than the tissue's mean interstitial fluid oncotic pressure. This maximizes oncotic pressure differences when capillary pressure are high and enhances initial absorption rates when pressures are low, accelerating short-term regulation of plasma volume.     

Flow Modulation of Pressure-sensitive Tone in Rat Pial Arterioles : Role of the Endothelium

Background: Cerebral arteriolar tone is modulated in response to changes in transmural pressure and luminal flow. The effect of flow on the relation between pressure and diameter has not been fully evaluated in these vessels. This study was conducted to investigate this interaction and to determine the role of the endothelium in mediating it.

Methods: Rat pial arterioles from the territory of the posterior cerebral artery were mounted in a perfusion myograph. In some arterioles, the endothelium was removed by air perfusion. Diameters were recorded at pressures from 20 to 200 mmHg in the presence and absence of flow (10 [mu]l/min). The response to flow (0-30 [mu]l/min) was recorded at 60 and 120 mmHg.

Results: In the absence of flow, endothelium-intact arterioles demonstrated tone at distending pressures between 40 and 140 mmHg. In the presence of flow, tone did not develop until pressure exceeded 100 mmHg, and the vessels remained active at pressures up to 200 mmHg. Endothelium-denuded arterioles developed tone at the same pressure when perfused as when unperfused, but perfused vessels were able to maintain active tone at higher pressures. At 60 mmHg, flow caused dilation if the endothelium was intact and constriction if it had been removed. At 120 mmHg, flow caused constriction. Endothelium-dependent flow-relaxation was inhibited by NG-nitro-l-arginine methyl ester (10-5 m) and abolished by indomethacin (10-5 m).