Evaluation of methodologies for measurement of interstitial fluid pressure (Pi): physiological implications of recent Pi data

As one of the determinants of net transcapillary filtration, filling of initial lymphatics and interstitial compliance, interstitial fluid pressure (Pi) is of great physiological interest. Several methods have been developed to measure Pi, all measuring the fluid equilibration pressure, i.e., the pressure in a saline-filled tube brought into contact with the interstitium. The methods designed to establish such contact may be characterized as acute and chronic. With acute methods, i.e., needle without infusion, wick catheters, WIN, and micropipettes, measurements are made within minutes to a few hours after insertion of the device. The chronic methods are various capsules (perforated, porous, or Teflon rings) implanted into the tissue 4 to 6 weeks before measurements. A general finding in previous studies from different laboratories and in different species have been that the chronic methods give more strongly negative (subatmospheric) Pi than the acute methods, and that the chronic methods give much greater pressure responses to changes in IFV. By direct comparison of acute and chronic methods in the same tissue and site, all give similar Pi in steady state conditions, while an acute over- and dehydration results in far more pronounced pressure changes recorded with chronic than with acute devices. It is proposed that these transient pressure differences recorded by acute vs. chronic methods result from different physical properties of the capsule lining compared with that of the surrounding skin, in addition to a possible osmometer effect of the capsule lining. Recent data show that such methodological problems have resulted in great variation in the estimation of compliance of the interstitium and have overemphasized the importance of Pi as an edema-preventing mechanism.(ABSTRACT TRUNCATED AT 250 WORDS)     

“Artificial Lymphatic System”: A New Approach to Reduce Interstitial Hypertension and Increase Blood Flow, pH and pO2 in Solid Tumors

A mechanical drainage system, the artificial lymphatic system (ALS), consisting of a vacuum source and drain, is evaluated for its ability to aspirate the interstitial fluids responsible for the elevated interstitial fluid pressure (IFP) observed in solid tumors. IFP, pH, and pO₂ radial profiles were measured before and after aspiration using wick-in-needle (WIN) probes, needle pH and oxygen electrodes, respectively. Laser Doppler flowmetry measured temporal changes in blood flow rate (BFR) at the tumor surface during aspiration. The WIN probe and IFP profile data were analyzed using numerical simulation and distributed mathematical models, respectively. The model parameter, p _E reflecting central tumor IFP, was reduced from 15.3 to 5.7 mm Hg in neuroblastoma and from 13.3 to 12.1 mm Hg in Walker 256, respectively, following aspiration. The simulation demonstrated that spatial averaging inherent in WIN measurements reduced the calculated magnitude of the model parameter changes. IFP was significantly lower (p < 0.05), especially in regions surrounding the drain, and BFR was significantly higher (p < 0.05) following 25 and 45 min of aspiration, respectively; pH and pO₂ profiles increased following aspiration. The experimental and mathematical findings suggest that ALS aspiration may be a viable way of reducing IFP and increasing BFR, pO₂ and pH and should enhance solid tumor chemo and radiation therapy. © 2000 Biomedical Engineering Society. PAC00: 8719Tt, 8715Vv, 8719Uv, 8780-y, 8719Xx     

Interstitial compliance and transcapillary fluid balance in renal hypertensive rats

Local interstitial fluid volume (IFV) and pressure (IFP) were used to estimate interstitial compliance (= delta IFV/delta IFP) in skin and skeletal muscle of normotensive (NT) and renal hypertensive rats (HT). The IFV was measured as the extravascular 51Cr-EDTA space, and IFP with micropipettes (tip diameter 2-4 microns) connected to a servocontrolled counter-pressure system. After control measurements, overhydration was induced by infusion of saline, 10% of body weight i.v. with or without venous stasis. Alternatively, dehydration was induced by peritoneal dialysis with 20% glucose or by furosemide infusion with or without 24 h fluid deprivation. Control ECV averaged 24.94 and 24.73 ml per 100 g in NT and HT, respectively (P greater than 0.05). Control PV averaged 2.81 and 3.28 ml per 100 g in NT and HT, respectively (P = 0.061), and control IFP was more positive in HT than in NT: 0.4 mmHg in skin (P less than 0.05) and 0.2 mmHg in skeletal muscle (P greater than 0.05). Dehydration changed PV significantly more in HT than in NT (P less than 0.05). The interstitial volume-pressure curve was linear in dehydration and the initial part of overhydration but gradually levelled off, and the maximal rise in IFP was 1-1.5 mmHg in skin and muscle. Interstitial compliance was calculated from the dehydration part of the volume-pressure curve and was in NT 14% per mmHg both in skin and skeletal muscle. In HT, compliance during dehydration was 10.2 and 20.7% reduction in IFV per mmHg fall in IFP in skin and muscle, respectively, not significantly different from corresponding values in NT (P greater than 0.05). We conclude that HT had unaltered ECV and a tendency to higher PV, and that interstitial compliance was not significantly different in normotensive and hypertensive rats.     

Effect of pressure on transmural fluid flow in different de-endothelialised arteries

The effect of pressure on filtration across different de-endothelialised arteries has been studied experimentally and the existing theoretical model is validated. Segments of different arteries are excised, de-endotheliaslised and cannulated. Bovine serum albumin Krebs solution is used as perfusate. Transmural water flux is measured by following the movement of an air bubble in a calibrated capillary, which connects the artery to a pressure reservoir; the pressure of which is varied. The hydraulic conductivity L_p is calculated from the flux values. Using available experimental parameters in the case of the thoracic and abdominal aorta, a theoretical model is validated using the experimental results. As the elastic constant for the carotid artery is not available, the theoretical model is used to calculate the elastic constant at different transmural pressures. The values calculated are in the range −4·9×10⁻⁸ to −5·7×10⁻⁹ cm² dyne⁻¹ between 50 and 135 mm Hg. Both theoretical and experimental results show a decrease in L_p values with an increase in transmural pressure for the thoracic and abdominal aorta, whereas a different trend is observed in the case of the carotid artery. The L_p values increase at 90 mm Hg, as compared with 50 mm Hg, and with a further increase in transmural pressure the values decrease.     

Changes of phasic pleural pressure in awake dogs during exercise: Potential effects on cardiac output

Eighty experiments were performed with nine awake dogs to study the changes of phasic pleural pressure with exercise. The increased minute volume with exercise was obtained by more frequent pleural pressure swings and by a substantial extention of the pressure swings in both directions. The cyclic changes of stroke volume following the pressure swings support the hypothesis that alterations of pleural pressure affect the stroke volume and can act, if necessary, as a secondary pump for the circulation. Mean pleural pressure during exercise fell by 2.5 cm H₂O from the rest value of 12.1 cm H₂O. The absolute right atrial pressure during exercise (−2.69 mm Hg) was not different from that at rest (−2.39 mm Hg). However, the transmural right atrial pressure of 7.6 mm Hg during exercise was higher than the pressure of 6.2 mm Hg at rest because during exercise the right atrium was perfused by 38% higher blood flow than that at rest. The phasic pattern of right atrial pressure shows that there is good reason to assume that during inspiration the extrathoracic veins are collapsed at their entrance into the chest, but this collapse is removed during expiration. There is no reason to assume an effective, sustained collapse of extrathoracic veins. Rather we can visualize a rythmical change of flow in extrathoracic veins from transient limitation to transient acceleration.     

Extracardiac pressure changes do not alter contractile function of the isolated left ventricle

To determine whether or not extracardiac pressure has an effect on left ventricular contractile function, we analyzed pressure-volume relationships of six isolated, perfused canine hearts in an air-tight chamber. The chamber pressure was set at −60, −30, 0, 30 and 60 mm Hg and left ventricular pressure-volume relationships studied. The slope (E_es) and volume axis intercept (V₀) of the transmural pressure-volume relationship were used to compare the pump functions of an individual heart at the different extracardiac pressures applied. No significant difference in either E_es or V₀ was seen with different extracardiac pressures. During isovolumic ventricular contraction, developed left ventricular pressure did not change over the range of extracardiac pressures applied. The same was true during ejecting contraction; when the downstream pressure of the computer simulated afterload circuit and the venous filling pressure of the preload circuit were changed in parallel with the extracardiac pressure, pressur-volume loops remained identical throughout their course for all of the extracardiac pressures applied. We conclude that (1) transmural pressure is the overwhelmingly dominant loading factor governing LV contraction, and (2) myocardial contractile function is unaffected by the absolute value of extracardiac pressure.     

Compliance of the interstitial space in rats I. Studies on hindlimb skeletal muscle

Interstitial compliance, defined as the ratio between changes in interstitial fluid volume (ΔIFV) and interstitial fluid pressure (ΔIFP), was determined for rat skeletal muscle. IFV was measured as the extravascular distribution space for ⁵¹Cr-EDTA, while sharpened micropipettes connected to a servo-controlled counterpressure system were used to measure IFP. The experimental protocol was designed to bring about acute (2–4 h) and chronic (24–28h) tissue over- and dehydration. During dehydration, the average compliance was 0.056 ml/g dry weight · mmHg, corresponding to 1.40 ml/100 g wet tissue mmHg, and was not significantly different in acute and chronic experiments. In hydration (acute and chronic), compliance increased several-fold when IFV increased. Even at greatly increased IFV, IFP did not rise more than 1 to 1.5 mmHg above control level. Since control IFV amounts to 10 ml/100 g wet tissue, IFV will decrease by 14% when IFP falls by 1 mmHg from this control level. Provided unchanged interstitial protein mass the dehydration will cause interstitial fluid colloid osmotic pressure to increase by somewhat more than 1 mmHg—from a control level of 9 mmHg. Furthermore, since IFP was not increased by more than 1 to 1.5 mmHg during hydration, an increase in IFP plays a minor role in edema-prevention compared to dilution and/or washout of interstitial proteins.     

Non-invasive assessment of human large vein diameter, capacity, distensibility and ellipticity in situ: dependence on anatomical location, age, body position and pressure

The objective was to compare in-situ diameter, capacity and distensibility changes as well as ellipticity of large human veins of different locations, reveal alterations with age, body position and increased intraluminal pressure. Ultrasonographic assessment of mediolateral and anteroposterior diameters was performed of femoral, axillary and inner jugular veins, in erect and reclined positions as well as before and during controlled Valsalva test. Groups of young (24.0±0.4 years, n=11) and elderly (72.6±1.5 years, n=11) subjects were studied. Capacity of the femoral vein (reclined patients) gradually increased when a graded Valsalva test was applied. Its in situ distensibility was found to be 0.048±0.011 mm Hg⁻¹ between 0–15 mm Hg (1 mm Hg=0.133 kPa) pressure loads in reclined young subjects, which decreased to 0.009±0.005 mm Hg⁻¹ at 45–60 mm Hg. The femoral vein was considerably more rigid in the erect than in the reclined body position while an opposite correlation was found for the inner jugular vein. Axillary vein distensibility was very low and independent of body position. Ellipticity of femoral and axillary veins was minimal (the ratio of the two perpendicular diameters <1.25). Inner jugular veins were more elliptic in the erect position and without Valsalva (2.94±0.99 in young patients). Old femoral veins had higher capacity in the reclined position without Valsalva, while distensibility at low pressures was much more prominent in young vessels. The in situ femoral vein is more distensible in supine than in erect position, opposite to the inner jugular vein. The axillary vein is rather rigid in both positions. Only the inner jugular vein shows significant elipticity in situ. Aging decreases the distensibility of the femoral vein in reclined position.     

Rat brain interstitial fluid pressure measured with micropipettes

Interstitial fluid pressure (IFP) in the rat brain has been measured with micropipettes, 2-4 microns in diameter, after trepanation and incision of dura. IFP was compared with cerebrospinal fluid pressure (CSFP) measured by a 0.3-mm (OD) cannula in the lateral ventricle. Control IFP averaged 3.43 +/- 0.65 (SD) mmHg (n = 83) with a corresponding CSFP of 3.53 +/- 0.71 (n = 83). Pretreatment with indomethacin to reduce inflammation during preparation did not affect IFP and CSFP. Infusion of a 20% mannitol solution amounting to 5% of body weight resulted in an average IFP of -5.66 mmHg and CSFP of -4.56 mmHg 20 min after start of the infusion, after which a gradual rise in both pressures was observed. Similar results were obtained after infusion of hypertonic urea. Inflation of a cuff around the rat neck resulted in an immediate and similar rise in IFP and CSFP. Increasing the arterial CO2 partial pressure to 80 mmHg increased IFP to 12-14 mmHg and pial venular pressure to 13-15 mmHg, while CSFP rose to about 10 mmHg. The micropuncture technique for measurement of brain IFP is rapid and practically atraumatic and may be used in a variety of animals.     

Circulatory effects of internal jugular vein compression: A computer simulation study

The effects of compression of the internal jugular veins and the inferior vena cava are simulated using an equivalent electronic circuit, which included simulation of cardiocirculatory phenomena and special features of the cerebral circulation. Compression of the inferior vena cava resulted in a profound decrease in cardiac output (from 4.5 to 1.5l min⁻¹) and arterial pressure (from 140/85 to 50/35 mm Hg). Compression of the internal jugular veins resulted in a negligible slightly decreased. Cerebral capillary and internal jugular pressures were considerably increased, leading to obstruction of cerebral veins and increased pressure (from 9 to 22 mm Hg) and volume (from 120 to 145 ml) of the cerebrospinal fluid (CSF). Increased cerebral capsule compliance resulted in decreased CSF pressure (from 9 to 8.5 mm Hg), but CSF volume increased (from 120 to 190 ml). A small increase in brain volume (from 1000 ml to 1060 ml, 6% volume increase) was compensated for by an equal decrease in the volume of CSF. When brain volume was above 1080 ml, the absorption of CSF was reduced, and its pressure increased.     

Strategy for the treatment of noncompliant hypertensive hemodialysis patients

Hypertensive hemodialysis patients noncompliant for their medications do not benefit from pharmacologic advances in the treatment of high blood pressure, and increase their already high risk of cardiovascular complications. The medical staff often becomes frustrated by severe hypertension in those who refuse to take medicines at home, drink excessive fluids, miss multiple dialysis sessions and sign-off dialysis early. In addition to addressing the psychosocial, financial, educational and substance abuse problems which contribute to noncompliance, we have developed a medication strategy to serve as an at least interim means of lowering blood pressure. Antihypertensive agents which have long half-lives in renal failure (lisinopril) and/or are intrinsically long acting (transdermal clonidine and amlodipine) were administered on dialysis days by the unit personnel to those patients who did not or would not take that or any dose on their own. The lisinopril and amlodipine were assured to have been taken on at least the dialysis days (thrice weekly), and the clonidine patch replaced weekly. Sixteen patients were thus treated when they failed to reliably self-administer medications. They had a significant decline in the predialysis systolic pressure of 15 mm Hg (175 +/- 6 to 160 +/- 5 mm Hg), diastolic of 12 mm Hg (103 +/- 3 to 91 +/- 3 mm Hg), and mean pressure of 13 mm Hg (127 +/- 4 to 114 +/- 4 mm Hg). There was an improvement in post-dialysis bood pressures, with the mean pressure declining 13 mm Hg from 110 +/- 4 to 97 +/- 4 mm Hg. Many individuals had erratic blood pressure control, having intermittently missed dialysis and hence unit-administered medicine, as well as continued fluid or drug abuse. The patients had uniformly excellent acceptance of this regimen, even spontaneously requesting it, and had no appreciable adverse effects. In summary while noncompliance is being addressed by the entire medical team, dialysis unit administration of long-acting medicines helps many hypertensive dialysis patients who would otherwise be at increased risk for severe cardiovascular complications.     

Hydrocephalus shunts and waves of intracranial pressure

The majority of contemporary hydrocephalus valves are designed to introduce a low resistance to flow into the cerebrospinal fluid (CSF) drainage pathway, and an therefore intended to stabilise intracranial pressure (ICP) at a level close to the shunt's operating pressure. However, this goal cannot always be attained. Accelerated CSF drainage with vertical body posture in ventriculo-peritoneal shunts is one reason for the ICP decreasing below the shunt's operating pressure. Another possible factor has been studied: the impact of the pulsating pattern in the ICP on the operating pressure. Six popular constructions of medium-pressure valves were studied (Radionics Low-profile, Delta, Hakim Precision, Holter, Integra In-line and Hakim NMT). Valves were mounted in the testing rig in the UK. Shunt Evaluation Laboratory and perfused with de-ionised water at a rate of 0.3 ml min⁻¹, and proximal pulsating pressures of different amplitudes (from 2 to 30 mmHg peak-to-peak) and frequencies (70–10 cycles min⁻¹) were superimposed. Laboratory findings were compared with clinical material containing recordings of ICP made in patients to diagnose reasons for ventriculomegaly. The mean operating pressure decreased in all valves when the simulated amplitude of heart pulsations increased. The rate of this decrease was dependent on the type of valve (variable from 2.5 to 5 mm Hg per increase in peak-to-peak amplitude by 10 mm Hg). The decrease was not related to the frequency of the wave. The relationship between pulse amplitude and ICP in 35 patients with blocked shunts was strong (R=0.48; p<0.03; slope 0.14) and in 25 patients with properly functioning shunts was non-significant (R=0.057; p=0.765). Two examples of decrease in mean ICP in the presence of increased vasogenic ICP waves in shunted patients are presented. The shunt operating pressure, which ‘sets’ the ICP in shunted patients may be influenced by the dynamics of a patient's ICP waveform.     

Filtration pressure response to infusion of atrial natriuretic peptides

The present experiments were undertaken to assess the effect of an atrial extract (ANF) and of the synthetic atriopeptin II (APII) on filtration pressure of rat kidneys. Continuous recordings of stop flow pressure (SFP) were made to obtain an index of the change of glomerular capillary pressure produced by atrial peptides and its time course. Short-term infusion of ANF or APII increased SFP from 40.6±0.99 to 50.7±1.42 mm Hg (p<0.001) and from 44.0±1.28 to 52.7±1.75 mm Hg (p<0.001) respectively. The maximum response was achieved promptly. Return of SFP to control was slow: 20 minutes after termination of the infusion SFP was still elevated by 4.9±1.27 mm Hg (p<0.01). Tubule and stellate vessel pressures increased less than 2mm Hg, changes that were not significant. Arterial pressure fell 6 mm Hg (p<0.05). When arterial pressure was reduced by an aortic clamp to 85–90 mmHg prior to administration of APII the response of SFP was markedly blunted (from a mean increase of 9.0±1.07 mm Hg to 4.5±0.53 mm Hg). The increase of SFP probably reflects an increase of glomerular capillary pressure. The finding suggests that atrial peptides increase glomerular filtration rate at least in part by increasing filtration pressure.     

Right ventricular function in patients with chronic obstructive pulmonary disease

Summary Simultaneous right heart catheterization and radionuclide ventriculography were performed in 27 patients with a wide range of chronic obstructive pulmonary disease. Central hemodynamics and radionuclide studies were done at rest and during exercise. In the resting state the right ventricular ejection fraction (RVEF) was in the normal range (43.3±6%). During exercise a significant (p<0.001) decrease of RVEF to 38.8±6.7% occurred. The pumonary artery mean pressures were 19.9±3.8 at rest. During exercise a significant (p<0.001) increase to 41±9.8 mm Hg occurred. There was a linear relationship between pulmonary pressures and RVEF during exercise in patients with pulmonary artery pressures not exceeding 35 mm Hg. In patients with right ventricular end-diastolic wall thickness 6 mm a curvilinear relationship between these parameters could be observed with a flattening of the curve at higher pressures (>35 mm Hg) and lower ejection fractions (<35% RVEF). Radionuclide venticulography cannot substitute for right heart catheterization. Echocardiography is useful for interpretation of right ventricular ejection fractions in advanced chronic obstructive pulmonary disease.Abbreviations CI Cardiac index (l/min/m²) - CO Cardiac output (l/min) - COPD Chronic obstructive pulmonary disease - FEV₁ Forced expiratory volume in the first second (ml) - HR Heart rate (B/min) - PAd Pulmonary artery diastolic pressure (mm Hg) - PAP Pulmonary artery mean pressure (mm Hg) - PAs Pulmonary artery peak pressure (mm Hg) - PVR Pulmonary vascular resistance (dyn·s·cm^–5) - PwP Pulmonary capillary wedge pressure (mm Hg) - RAP Right arterial pressure (mm Hg) - Raw Airway resistance (cm H₂/l/s) - RNV Radionuclide ventriculogram - RV Residual volume (l) - RVEF Right ventricular ejection fraction (%) - RVEDVI Right ventricular enddiastolic volume index (ml/m²) - RVEDVI SVI RVEF (ml/m²) - RVESVI Right ventricular endsystolic index (m²/m²) - SVI Stroke volume index (ml/m²) - TLC Total lung capacity (l) - VC Vital capacity (l)     

Elevated ambulatory day-time blood pressure in women with polycystic ovary syndrome: a sign of a pre-hypertensive state?

The aim of this study was to evaluate office and 24 h bloodpressures in women with polycystic ovary syndrome (PCOS) andnormal controls in relation to insulin sensitivity. Office bloodpressure, insulin sensitivity (euglycaemic hyperinsulinaemicclamp) and body fat distribution (skin-folds, waistrhip ratio)were investigated in 36 women with PCOS and 55 controls [bodymass index (BMI) for PCOS patients, mean (confidence interval)263 (24.6–282); controls, 25.1 (24.0–26.9)], andambulatory 24 h blood pressure was recorded in representativesubgroups of 17 and 16 women respectively. Compared with thecontrols, the women with PCOS had higher day-time systolic (meanSD,12611 versus 11912 mm Hg, P < 0.05) and mean arterial bloodpressures (927 versus 867 mm Hg, P < 0.05) in ambulatoryrecordings, and greater increases in pulse rate (70%; P <0.05) from night to day-time recordings, whereas the groupsdid not differ significantly in day-time diastolic blood pressure(746 versus 706 mm Hg, P = 0.05) or in night-time recordings.The women with PCOS had a more pronounced truncal-abdominalfat distribution (P < 0.05) and a lower insulin-mediatedglucose disposal (P < 0.01) during the clamp. The increasedday-time blood pressures in women with PCOS persisted afteradjusting for BMI, body fat distribution and insulin resistance.It is concluded that women with PCOS have an increased prevalenceof labile blood pressure, which may indicate a pre-hypertensivestate, adding a further risk factor for cardiovascular diseasein these women.     

Regional,segmental, and temporal heterogeneity of cerebral vascular autoregulation

Autoregulation of cerebral blood flow is heterogeneous in several ways: regional, segmental, and temporal. We have found regional heterogeneity of the autoregulatory response during both acute reductions and increases in systemic arterial presure. Changes in blood flow are less in brain stem than in cerebrum during decreases and increases in cerebral perfusion pressure. Segmental heterogeneity of autoregulation has been demonstrated in two ways. Direct determination of segmental cerebral vascular resistance indicates that, while small cerebral vessels (<200 μm in diameter) make a major contribution to autoregulation during acute increases in pressure between 80 and 100 mm Hg, the role of large cerebral arteries (>200 μm) becomes increasingly important to the autoregulatory response at pressures above 100 mm Hg. Measurement of changes in diameter of pial vessels has shown that, during acute hypotension, autoregulation occurs predominantly in small resistance vessels (<100 μm). Finally, there is temporal heterogeneity of autoregulation. Sudden increases in arterial pressure produce transient increases in blood flow, which are not observed under steady-state conditions. In addition, the blood-brain barrier is more susceptible to hypertensive disruption after rapid, compared to step-wise, increases in arterial pressure. Thus, when investigating cerebral vascular autoregulation, regional, segmental, and temporal differences in the autoregulatory response must be taken into consideration.     

Quantitative assessment of ultrasound-induced resistance change in saline solution

The purpose of the study was the experimental assessment of the interaction coefficient characterising the influence of pressure on the conductivity of electrolyte solutions. Pressure pulses were applied to samples of 9 gl⁻¹ sodium chloride contained in cuboid measurement cells of identical cross-sectional dimensions but different thickness along the acoustic beam axis. The magnitude of the changes induced in cell resistance was recorded for three values of applied pressure increment (ΔP=0.94, 1.39 and 1.78MPa) and three values of cell thickness (e=0.58, 1.13 and 1.62mm). A thick, focused transducer generated short (0.1μs), unipolar pressure pulses. A model accounting for the characteristics of the pressure pulse and the geometry of the measurement cell was developed to predict the ultrasound-dependent changes in the measured electrical resistance. Despite some discrepancy between theoretical and experimental results, discussed in the paper, the results validated the order of magnitude of the interaction coefficient (10⁻⁹ Pa⁻¹). The predictions varied from about 50% (e=1.62 mm, ΔP=0.94MPa) to 77% (e=0.58mm, ΔP=1.78MPa) of the experimental values.     

Relationship between pulse-wave velocity and arterial elasticity

Pulse wave velocity (PWV) was measuredin situ in 11 isolated canine common carotid arteries. Seven arteries exhibited a linear PWV/pressure function at pressures ranging from 0 to 200 mm Hg. Four arteries yielded a linear relationship between PWV and pressure between 1 and 100 mm Hg; for these vessels the relationship was nonlinear at higher pressures. Seven arteries (five from the group which was linear up to 200 mm Hg and two from the group which was linear up to 100 mm Hg) were excised and presure/volume measurements were madein vivo. Using pressure/volume data, the Moens-Korteweg equation was evaluated as a predictor of the PWV/pressure relationship over the linear region. An expression was developed to anable prediction of the pressure/volume relationship using the coefficients at the linear PWV/pressure function; these predictions were evaluated. We found that, for this range, the Moens-Korteweg equation provides a very good basis for predicting the increase in PWV with increasing bias pressure. In addition, we found that the pressure/volume relationship of common carotid arteries is well represented by an exponential of the form V/V_o=Ke^αf(P), which was derived as the inverse solution to the Moens-Korteweg equation.     

Weibull distribution in physiological investigations

Conclusions 1. The systolic pressure and diastolic pressure in populations of practically healthy persons and patients is distributed according to the Weibull law (three-parameter distribution; P<0.05). 2. For a practically healthy population of persons the confidence limits of the DP are 50–95 mm Hg (P≤0.05), SP 93–153 mm Hg (P≤0.05). 3. For coronary disease patients the confidence limits for DP are 63–115 mm Hg and for SP 102–166 mm Hg (P≤0.05). In rheumatic heart disease patients these limits are equal to 55–106 mm Hg for DP and 95–160 mm Hg for SP (P≤0.05). Translated from Meditsinskaya Tekhnika, No. 3, pp. 8–12, May–June, 1986.     

Changes of peripheral venous tone and central transmural venous pressure during immersion in a thermo-neutral bath

Summary Peripheral venous tone, central venous and oesophageal pressures were recorded while the upright sitting subjects were immersed to the neck in a thermoneutral water bath. The central venous pressure rose from 3.4 to 15.2 mm Hg and the oesophageal pressure from –0.4 to +3.4 mm Hg. The transmural pressure, which determines the enddiastolic volume of the heart, increased by 8.0 mm Hg. Plethysmographic determinations of peripheral venous tone revealed a relaxation of the peripheral veins: after a quick initial decrease of the volume elasticity coefficient (E₁₅) from 16.6 to 13.5 mm Hg/ml/100 g tissue there is a continuous decline to 11.8 mm Hg/ml/100 g tissue after 3 hrs immersion. This relaxation persists for at least 1 hr after termination of immersion.This work was supported by Contract No. F44620-71-C-0117 of the Air Force Office of Scientific Research (OAR), European Office of Aerospace Research.with the technical assistance of H. Dannenberg