Accuracy of auscultatory blood pressure measurement with a long cuff.

Intra-arterial blood pressure was compared with simultaneous auscultatory measurements in 37 subjects with a wide range of blood pressures and arm circumferences; six cuffs of various lengths and widths were used. Nineteen subjects had an arm circumference of 34 cm or more (mean 40 cm) and the other 18 were considered to be non-obese and had a mean arm circumference of 30 cm. With each larger cuff, in terms of bladder surface area, auscultatory blood pressure decreased a few mm relative to intra-arterial pressure both for systolic and for diastolic measurements. Apart from diastolic pressure measured with the two 12 cm wide cuffs (12 X 23 cm, 12 X 30 cm) in the obese group all other auscultatory measurements differed less than 5% from intra-arterial pressure, albeit with considerable variability among the subjects. The differences in error among measurements with the four largest cuffs in the obese group (13 X 30 cm, 14 X 30 cm, 14 X 38 cm, and a conical cuff) were clinically irrelevant, and there was even less to choose among all six cuffs in the non-obese subjects. These results suggest that auscultatory blood pressure may be measured with acceptable accuracy with a single long bladdered cuff both in subjects with large arms and in subjects with normal sized arms.     

Design and evaluation of a device for measurement of interface pressure

An electropneumatic device has been designed to measure the interface pressure profile under compression bandages. The device uses commercially available pneumatic sensors (Talley Group Ltd, SJ235/3) and measures interface pressure at up to eight sites simultaneously along the lower limb, with an accuracy of ± 3 mmHg. Measurements can be made in one of two modes: continuously at a rate of up to three samples per second with the results output to a suitable display device, or single measurements of interface pressure can be made and displayed on a digital display incorporated in the device. This enables the monitoring and recording of interface pressure under compression bandages during either ambulation or passive recumbency. The electropneumatic system is described together with its following characteristics: the hysteresis of the Talley pneumatic sensors, the accuracy of pressure measurement and the maximum achievable sample rate. Dynamic measurements in a single normal volunteer are shown.     

Accuracy and repeatability of a pressure measurement system in the patellofemoral joint

The objective of this study was to assess how accurately and repeatably the Iscan system measures force and pressure in the natural patellofemoral joint. These measurements must be made to test widely held assumptions about the relationships between mechanics, pain and cartilage degeneration. We assessed the system's accuracy by using test rigs in a materials testing machine to apply known forces and force distributions across the sensor. The root mean squared error in measuring resultant force (for five trials at each of seven load levels) was 6.5±4.4% (mean±standard deviation over all trials at all load levels), while the absolute error was −5.5±5.6%. For force distribution, the root mean squared error (for five trials at each of five force distributions) was 0.86±0.58%, while the absolute error was −0.22±1.03%. We assessed the repeatability of the system's measurements of patellofemoral contact force, pressure and force distribution in four cadaver specimens loaded in continuous and static flexion. Variability in measurement (standard deviation expressed as a percentage of the mean) was 9.1% for resultant force measurements and 3.0% for force distribution measurements for static loads, and 7.3% for resultant force and 2.2% for force distribution measurements for continuous flexion. Cementing the sensor to the cartilage lowered readings of resultant force by 31±32% (mean±standard deviation), area by 24±13% and mean pressure by 9±34% (relative to the uncemented sensor). Maximum pressure measurement, however, was 24±43% higher in the cemented sensor than in the uncemented sensor. The results suggest that the sensor measures force distribution more accurately and repeatably than absolute force. A limitation of our work, however, is that the sensor must be cemented to the patellar articular surface to make the force distribution measurements, and our results suggest that this process reduces the accuracy of force, pressure and area measurements. Our results suggest that the Iscan system's pressure measurement accuracy and repeatability are comparable to that of Fuji Prescale film, but its advantages are that it is thinner than most Fuji Prescale film, it measures contact area more accurately and that it makes continuous measurements of force, pressure and area.     

Repeatability of subject/bed interface pressure measurements

The repeatability of a technique for measuring interface pressures has been assessed. Pressure was measured using a Talley SA500 Pressure Evaluator under six anatomical sites (occiput, scapula, elbow, sacrum, buttock, and heel) of six healthy subjects lying supine on a Clinifloat mattress (trademark, SSI). For each site of each subject, four repeat readings were taken per day on four separate days. Mean pressures varied significantly between subjects (p < 0.02), though differences in mean pressures between sites were greater. Pressure was not significantly related to subject mass. The overall repeatability of the technique was ± 0.77 kPa (± 5.8 mm Hg) which was much smaller than the range of pressures found under different sites (2.72 kPa or 20.4 mm Hg at the sacrum to 9.00 kPa or 67.5 mm Hg at the heel). Repeatability varied from site to site, from ± 0.47 kPa (± 3.5 mm Hg) at the buttocks to ± 1.20 kPa (± 9.0 mm Hg) at the heel. Measurements were found to vary significantly more between days than between repeats on the same day (p < 0.02).     

Accuracy of alternative representations for integrated biochemical systems

The Michaelis-Menten formalism often provides appropriate representations of individual enzyme-catalyzed reactions in vitro but is not well suited for the mathematical analysis of complex biochemical networks. Mathematically tractable alternatives are the linear formalism and the power-law formalism. Within the power-law formalism there are alternative ways to represent biochemical processes, depending upon the degree to which fluxes and concentrations are aggregated. Two of the most relevant variants for dealing with biochemical pathways are treated in this paper. In one variant, aggregation leads to a rate law for each enzyme-catalyzed reaction, which is then represented by a power-law function. In the other, aggregation produces a composite rate law for either net rate of increase or net rate of decrease of each system constituent; the composite rate laws are then represented by a power-law function. The first variant is the mathematical basis for a method of biochemical analysis called metabolic control, the latter for biochemical systems theory. We compare the accuracy of the linear and of the two power-law representations for networks of biochemical reactions governed by Michaelis-Menten and Hill kinetics. Michaelis-Menten kinetics are always represented more accurately by power-law than by linear functions. Hill kinetics are in most cases best modeled by power-law functions, but in some cases linear functions are best. Aggregation into composite rate laws for net increase or net decrease of each system constituent almost always improves the accuracy of the power-law representation. The improvement in accuracy is one of several factors that contribute to the wide range of validity of this power-law representation. Other contributing factors that are discussed include the nonlinear character of the power-law formalism, homeostatic regulatory mechanisms in living systems, and simplification of rate laws by regulatory mechanisms in vivo.     

Numerical simulation of noninvasive blood pressure measurement

In this paper, a simulation model based on the partially pressurized collapsible tube model for reproducing noninvasive blood pressure measurement is presented. The model consists of a collapsible tube, which models the pressurized part of the artery, rigid pipes connected to the collapsible tube, which model proximal and distal region far from the pressurized part, and the Windkessel model, which represents the capacitance and the resistance of the distal part of the circulation. The blood flow is simplified to a one-dimensional system. Collapse and expansion of the tube is represented by the change in the cross-sectional area of the tube considering the force balance acting on the tube membrane in the direction normal to the tube axis. They are solved using the Runge-Kutta method. This simple model can easily reproduce the oscillation of inner fluid and corresponding tube collapse typical for the Korotkoff sounds generated by the cuff pressure. The numerical result is compared with the experiment and shows good agreement.     

The essentials of direct xylem pressure measurement

This paper discusses the essentials of the oil‐filled pressure probe technique in the measurement of negative xylem pressures, focusing in particular on the technique and physics underlying our recent, successful experiment which has rekindled the debate on the validity of the Cohesion–Tension theory. We illustrate a number of general problems associated with the cell pressure probe and xylem pressure probe techniques, and propose appropriate criteria for micropipette construction. We enumerate factors dealing with the cavitation problem and suggest methods for eliminating air seeds in the system. We introduce reliable criteria for the successful measurement of xylem pressure, and emphasize the importance of the probe pressure relaxation test. Several problems regarding the controversy over the Cohesion–Tension theory are also discussed. We discuss the correlation between xylem pressure and the transpiration rate, the existence of absolute negative xylem pressure in intact plants, the most negative values of xylem pressure measured by the pressure probe, the agreement between the pressure probe and pressure bomb techniques, and the vulnerability to cavitation (tensile strength) of pressure probes.     

Accuracy of circular contact area measurements with thin-film pressure sensors

Contact area is often used to characterize the biomechanical properties of joints, especially in testing of injury and joint replacement. Several methods have been developed to measure contact area, including piezo-resistive thin-film arrays. The purpose of this study was to determine the accuracy with which one of these systems (Tekscan, Inc., South Boston, MA) could measure the contact area of flat-ended circular indenters of varying known sizes. Static loads ranging from 1000 to 7000 N were applied to four flat, circular indenters (1140, 2027, 3167, and 4560 mm(2)) and the contact areas were recorded with Tekscan 5076 sensor. Similar testing was carried out on a 4000 sensor. I-scan software (Tekscan Inc., South Boston, MA) was used to analyze the Tekscan-recorded area measurements. The Tekscan data were also post-processed to filter out sensel signal intensity values that were at least two standard deviations from the average sensel signal intensity values of the sensor matrix. Unprocessed Tekscan measurements with the 5076 sensor had area percent errors ranging from 5% to 27%. The filtering algorithm reduced most errors to less than 1%. Similar trends of improved accuracy with post-filtering were found with the 4000 sensor. While this method of thresholding out the sensels with the lowest signal intensity values may not work for all surfaces and indenter shapes, it provides a new approach to improve the accuracy of contact area measurements collected with the Tekscan system.