Underestimation Of Pressure Research Articles

Comparison of the Challenger Digital Applanation Tonometer and the Goldmann Applanation Tonometer

We compared the Challenger electronic tonometer to the Goldmann applanation tonometer in 70 eyes without corneal abnormalities. There was a good overall correlation between the two machines. Correlations between interobserver readings were excellent, demonstrating that obtaining reproducible measurements is possible with either machine. However, the scattergrams indicated that the Challenger tonometer gave consistently lower readings and showed more variability than the Goldmann tonometer. Additionally, the mean underestimation of intraocular pressure increased as the intraocular pressure increased above 20 mm Hg. Therefore, the Challenger electronic tonometer as presently calibrated is not accurate enough for clinical use in the detection and management of glaucoma.

Mean airway pressure and mean alveolar pressure during high-frequency jet ventilation in rabbits

Mean airway pressure underestimates mean alveolar pressure during high-frequency oscillatory ventilation. We hypothesized that high inspiratory flows characteristic of high-frequency jet ventilation may generate greater inspiratory than expiratory pressure losses in the airways, thereby causing mean airway pressure to overestimate, rather than underestimate, mean alveolar pressure. To test this hypothesis, we ventilated anesthetized paralyzed rabbits with a jet ventilator at frequencies of 5, 10, and 15 Hz, constant inspiratory-to-expiratory time ratio of 0.5 and mean airway pressures of 5 and 10 cmH2O. We measured mean total airway pressure in the trachea with a modified Pitot probe, and we estimated mean alveolar pressure as the mean pressure corresponding in the static pressure-volume relationship to the mean volume of the respiratory system measured with a jacket plethysmograph. We found that mean airway pressure was similar to mean alveolar pressure at frequencies of 5 and 10 Hz but overestimated it by 1.1 and 1.4 cmH2O at mean airway pressures of 5 and 10 cmH2O, respectively, when frequency was increased to 15 Hz. We attribute this finding primarily to the combined effect of nonlinear pressure frictional losses in the airways and higher inspiratory than expiratory flows. Despite the nonlinearity of the pressure-flow relationship, inspiratory and expiratory net pressure losses decreased with respect to mean inspiratory and expiratory flows at the higher rates, suggesting rate dependence of flow distribution. Redistribution of tidal volume to a shunt airway compliance is thought to occur at high frequencies.(ABSTRACT TRUNCATED AT 250 WORDS)

Evaluation of the Penàz servo-plethysmo-manometer for the continuous, non-invasive measurement of finger blood pressure.

Blood pressure in the finger was measured by a servo-plethysmomanometer constructed after the design of Penàz, which uses the principle of the unloaded arterial wall. The device contains a photoelectric plethysmograph mounted in an inflatable cuff and an electro-pneumatic transducer to control air pressure in the cuff via a servosystem. Comparison of simultaneous measurements of intra-arterial pressure in the brachial artery was performed on 33 patients suspected of having hypertension. In 12 patients evaluation of the technique could not be carried out due to technical failures or distorted blood pressure wave forms. Results of the remaining 21 patients show a mean underestimation of intra-arterial blood pressure by finger cuff blood pressure of 0.8 kPa (6 mm Hg), both for systolic and diastolic levels. The scatter range of the difference is from 1.9 to -3.5 kPa for systolic and 0.1 to -2.5 kPa for diastolic values. It appears that, although not all technical problems are solved, the Penàz servo-plethysmo-manometer is potentially an elegant method by which to arrive at the fully calibrated wave form of blood pressure in a finger in a non-invasive and continuous fashion.

Carbonic fluid inclusions in granulites from Tanzania — A comparison of geobarometric methods based on fluid density and mineral chemistry

High-density carbonic fluid inclusions with well-developed negative crystal shape occur in host-quartz, -plagioclase and -garnet in intermediate- to high-pressure granulites from the Furua Granulite Complex, southern Tanzania. They are most abundant in intercalated metapelitic units. Detailed microthermometric data on inclusions in combination with electron microprobe analyses of associated solid phases are presented for a kyanite-bearing garnet granulite and a garnet-two-pyroxene granulite, both selected from the same locality. The inclusions contain virtually pure CO 2 and are believed to be of primary syngenetic origin. This is indicated by their distribution in the host-minerals and by the homogenization temperatures which give well-defined single maxima between −30° and −50°C. The corresponding fluid pressures are only 1–3 kbar lower than the pressure range of 10–13 kbar that is inferred from the solid-phase geobarometers based on element partitioning in garnet-kyanite-plagioclase and orthopyroxene-clinopyroxene-garnet-plagioclase assemblages. Associated temperature estimates of 770–870°C are based on the compositions of garnet-biotite, garnet-clinopyroxene and orthopyroxene-clinopyroxene pairs. Although the CO 2 density data fit within the regional pressure regime of 7–12 kbar for the metamorphism in the Furua area, the detailed comparison with solid-phase geobarometers at the scale of a single outcrop reveals a small underestimate of pressure which may be explained by minor adjustment of the inclusions after peak metamorphism or by partial H 2O pressures in the order of 0.1 P tot to 0.2 P tot.

Effect of the cheeks and the compliance of alveolar gas on the measurement of respiratory variables

The distensibility of the upper airways was found to range in 8 subjects between 0.01 and 0.001 L/cm H 2O. This destensibility may cause a number of errors in standard respiratory measurements in normal subjects and in patients with airway obstruction due to delays in pressure and flow equilibrium between alveoli and mouth. The errors include overestimation of thoracic gas volume with the body plethysmograph; of airway inertance and of dynamic lung compliance; underestimation of respiratory resistance with the forced oscillation technique and with interruption pressure; underestimation of total pressure exerted by respiratory muscles with the P 0.1 technique during occlusions of the airway opening. The errors are often quite appreciable and may exceed 100%. Ways to reduce them or to compensate for them are discussed.

Problems in the plethysmographic assessment of changes in total lung capacity in asthma.

We studied the effect of abdominal gas compression on plethysmographically determined total lung capacity (TLC) in asthmatic patients before, during, and after treatment of induced bronchospasm. TLC was derived from panting maneuvers near residual volume, at functional residual capacity, and near TLC. Significant differences among these "derived TLC" values increased significantly during bronchospasm. Whether or not TLC appeared to increase, and by how much, depended on the level of the vital capacity from which it was derived. Individual increases in TLC during bronchospasm could not be explained by increases in abdominal gas volume or in the extent to which it was compressed and decompressed during panting. We postulate that during the Boyle's Law panting maneuver, pleural, and therefore alveolar, pressure swings may be nonhomogeneous and greater over lung regions subtended by closed airways than over regions in communication with the mouth. This would result in an underestimation of alveolar pressure swings as measured at the mouth and an overestimation of thoracic gas volume, and would account in large part for the observed increase in discrepancies between the "derived TLC" values in asthma as well as the dependence of apparent TLC changes on the level of the VC at which the panting maneuver is performed.

Closure of digital arteries in high vascular tone states as demonstrated by measurement of systolic blood pressure in the fingers

Finger systolic blood pressure (FSP) was measured indirectly in normal subjects and patients with primary Raynaud phenomenon by applying a thin-walled plastic cuff around the finger and a strain gauge more distally to detect volume changes. Inducing a high vascular tone in one or more fingers by direct cooling or intra-arterial noradrenaline infusion caused a marked drop in FSP in the exposed fingers, but not in the non-exposed fingers of the same hand. The fact that the non-exposed fingers retained the normal (arm systolic) pressure level is taken to indicate that palmar arch blood pressure also remained normal. In the high vascular tone state, a large transmural pressure difference must apparently be established before the digital arteries are forced open. The lowered opening pressure constitutes a manifestation of the closure phenomenon of the digital arteries described in patients with vasospastic arterial disease. It implies an underestimation of palmar arch systolic pressure measured indirectly on the fingers. FSP measured under these circumstances may be taken as an estimate of the vascular tone, and can be employed in diagnosis and quantification of vasospastic disorders.

Wind Pressures on Buildings-Probability Densities

By use of available statistics of building sites describing wind magnitude and direction, calculation of the recurrence interval for peak pressures may be performed if the probability distribution of pressure fluctuations is assumed, e.g., a Gaussian distribution. To perform the analysis adequately, the statistics of pressure fluctuations and their relationship to the governing physical parameters must be known. Fluctuating pressure data have been obtained on a scale model of an actual structure placed in a wind tunnel which simulated the characteristics of the atmospheric wind. The data were analyzed to obtain the probability distribution of the pressure fluctuations. The results show a remarkable similarity in probability distribution for large areas of the structure. In regions of positive pressure, the pressure distribution can be represented adequately by a Gaussian relationship. In regions of negative pressure, deviations from a Gaussian distribution are significant and can lead to a substantial underestimation of peak pressures if that distribution were assumed.

Matric Water Potential of Leaf Tissue—Measurement and Significance

A thermocouple psychrometer method, previously described for use in determining tissue and sap water potentials, has been adapted for determining matric potentials. Matric values ranging from approximately zero to −10 × 105 Pa were observed with wilting wheat leaves. Mean values of total tissue water potential and of its solute (osmotic), matric, and cell wall pressure components were −16.6, −18.3, −30 and +4.7 Pa × 105, respectively. Matric potentials are often ignored in investigations of plant water relations. This practice is shown to result in underestimation of wall pressures (in this case by a mean of 64 per cent) and sometimes in spuriously negative values.

Concentration Polarization in an Ultrafiltering Capillary

Concentration polarization, the accumulation of retained solute next to an ultrafiltering membrane, elevates osmotic pressure above that which would exist in the absence of polarization. For ultrafiltration in a cylindrical tube, use of the radially averaged solute concentration results in an underestimate of osmotic pressure, yielding an effective hydraulic permeability (k) less than the actual membrane hydraulic permeability (k(m)). The extent to which k and k(m) might differ in an ultrafiltering capillary has been examined theoretically by solution of the momentum and species transport equations for idealized capillaries with and without erythrocytes. For diameters, flow velocities, protein concentrations and diffusivities, and ultrafiltration pressures representative of the rat glomerular capillary network, results indicate that the effects of polarization are substantial without erythrocytes (k/k(m) = 0.7) and persist, but to a lesser extent, with erythrocytes (k/k(m) = 0.9), the reduction in polarization in the latter case being due to enhanced plasma mixing. In accord with recent experimental findings in rats, k is found to be relatively insensitive to changes in glomerular plasma flow rate.

Analysis of Power Plant Deaerator Under Transient Turbine Loads

In recent years, mechanical failure of trays and structural damage to steel members inside deaerating sections of power plant deaerators have been encountered during transient load operation of main turbines. These failures have been attributed to numerous causes, but underestimates of differential pressure across the tray stack and insufficient sizing of the pressure equalizers connecting the water storage tank and the deaerating section have been singled out as major factors. Design criteria of the pressure equalizers and tray stack enclosures should take into consideration rapid load reduction of the turbine. Under such mode of operation, pressure decay would produce flashing from the saturated water stored in the tank rushing upward through the pressure equalizers with possible flooding of the deaerating section if the equalizers were improperly sized. Concurrently, the differential pressure across the tray stack would increase by a factor of 10 to 20 times higher than normal maximum load. In such instances, the tray stack designed merely to meet a maximum stable load could not possibly withstand such severe dynamic loading. This paper presents a mathematical analysis of the transient operating behavior of a deaerator, with emphasis placed on sudden load reduction of the turbine. Items such as the quantity of flashing steam from the storage tank, physical properties of vapor and liquid in the deaerator, and the pressure losses through the pressure equalizers and tray stack are analyzed. From the analytical results, criteria of pressure equalizer sizing and tray stack enclosure design are established. A numerical example for design calculation is included for deaerator manufacturers and power plant process system engineers to use as reference for application in design.

Tonometric calibration for the rabbit eye.

Tonometric calibration for the rabbit eye reveals that the use of human calibration tables for the Schiotz tonometer results in a gross underestimate of intraocular pressure in the rabbit eye. The volume of corneal indentation is significantly lower in rabbit eyes than in human eyes for all scale readings. The variable ocular rigidities for different plunger weights and at different intraocular pressures preclude the creation of standard tables of pressure-volume relationships.