Andrew Lowe

A framework for intelligent medical diagnosis using the theory of evidence

In designing fuzzy logic systems for fault diagnosis, problems can be encountered in the choice of symptoms to use fuzzy operators and an inability to convey the reliability of the diagnosis using just one degree of membership for the conclusion. By turning to an evidential framework, these problems can be resolved whilst still preserving a fuzzy relational model structure. The theory of evidence allows for utilisation of all available information. Relationships between sources of evidence determine appropriate combination rules. By generating belief and plausibility measures it also communicates the reliability of the diagnosis, and completeness of information. In this contribution medical diagnosis is considered using the theory of evidence, in particular the diagnosis of inadequate analgesia is considered.

Non-invasive model-based estimation of aortic pulse pressure using suprasystolic brachial pressure waveforms

Elevated central arterial (aortic) blood pressure is related to increased risk of cardiovascular disease. Methods of non-invasively estimating this pressure would therefore be helpful in clinical practice. To achieve this goal, a physics-based model is derived to correlate the arterial pressure under a suprasystolic upper-arm cuff to the aortic pressure. The model assumptions are particularly applicable to the measurement method and result in a time-domain relation with two parameters, namely, the wave propagation transit time and the reflection coefficient at the cuff. Central pressures estimated by the model were derived from completely automatic, non-invasive measurement of brachial blood pressure and suprasystolic waveform and were compared to simultaneous invasive catheter measurements in 16 subjects. Systolic blood pressure agreement, mean (standard deviation) of difference was -1 (7)mmHg. Diastolic blood pressure agreement was 4 (4)mmHg. Correlation between estimated and actual central waveforms was greater than 90%. Individualization of model parameters did not significantly improve systolic and diastolic pressure agreement, but increased waveform correlation. Further research is necessary to confirm that more accurate brachial pressure measurement improves central pressure estimation.

Effect of tissue mechanical properties on cuff-based blood pressure measurements

This paper presents a 3D finite element upper arm model, validated by experiments as well as clinical data, used to study the error introduced in blood pressure measurements due to variability of arm tissue mechanical properties. The model consists of three separate cylindrical parts: soft tissue, bone and brachial artery. The artery volume changes under the cuff are used to represent the cuff pressure oscillations for analyzing blood pressure measurements. These oscillation trends are identical to observed clinical data. Also an upper arm simulator is designed and built for model validation. The model shows that the variation of soft tissue compressibility introduces an error up to 5% in blood pressure measurements. It is also revealed that the variation of the brachial artery and arm tissue stiffness has an insignificant effect on oscillometric blood pressure measurement method.

Arterial pulse wave velocity measurement: different techniques, similar results—implications for medical devices

Different characteristic points used for the evaluation of pulse wave velocity (PWV) give significantly different results. Hence, the accuracy of using these points is questionable. There is need for quantitative comparison of different techniques to determine PWV. Previous studies aimed at comparing different PWV measurement techniques have been noted, however, on a limited number of smaller animals (mice, dogs, etc.). This simulation-based study aims to compare different techniques for PWV measurement in a large representative human population. A computer model is developed for simulating the pressure wave propagation between the carotid and femoral arteries. Using relationships observed in clinical trials, the model input parameters for a statistically representative population are expressed in terms of a person’s age, gender and height. The model is used to calculate the carotid–femoral pressure ratio for different individuals, which is then parameterised into a number of features, and the equivalent propagation time is calculated using the phase-slope method. Using this time, the apparent phase velocity is determined and compared with PWV determined by the foot-to-foot technique. The two velocities compare well with a difference of 0.0059±0.0904 m/s. An averaging criterion for the calculation of apparent phase velocity has been tested and shown to give good estimates compared to the foot-to-foot technique. As it does not involve the identification of characteristic points on the measured pressure waves, the phase-slope method is more suitable for implementation in PWV monitors.

Biomechanical Basis of Oscillometric Blood Pressure Measuring Technique

Non-invasive blood pressure (BP) measurement has been used clinically for over a century to diagnose hypertension. Compared with the auscultatory technique, the oscillometric technique requires less professional training and is widely used in automatic BP measurement devices. Currently, most of these devices measure and record amplitude of cuff pressure oscillation, and then calculate diastolic and systolic pressure using characteristic ratios and designed algorithms. A finite element (FE) model is developed to study the biomechanical basis of this technique. The model identifies that errors were caused by mechanical factors of the soft tissue and the shape of the arm. By personalizing the parameters for each patient, the accuracy of the measurement will be improved for all age groups.Copyright

Mechanical Behaviour of Skin: A Review

Objective: The mechanical behaviour or the Young’s Modulus of the skin is measured as a ratio of the stress applied to the skin in vitro or in vivo over the skin deformation. The Young’s Modulus of skin is an important factor to estimate the characteristics of skin, to determine the course of a disease or to follow a cosmetic application. Methods: The mechanical behaviour of the skin is measured by changing the shape of skin by employing tensile, indentation, and suction and torsion tests. Results: Out of all the skin’s mechanical testing methods, suction tests are a common choice for skin testing, as they are easy to apply in vivo and consider both in-plane and normal loading conditions. Skin is found to be highly anisotropic and viscoelastic, with a range of Young’s Modulus between 5 kPa and 140 MPa. Conclusion: This paper reviews in vivo and in vitro reported values for Young’s Modulus of human skin for tensile, indentation, suction and torsion mechanical testing methods.

Arterial waveform parameters in a large, population-based sample of adults: relationships with ethnicity and lifestyle factors.

Little is known about how aortic waveform parameters vary with ethnicity and lifestyle factors. We investigated these issues in a large, population-based sample. We carried out a cross-sectional analysis of 4798 men and women, aged 50–84 years from Auckland, New Zealand. Participants were 3961 European, 321 Pacific, 266 Maori and 250 South Asian people. We assessed modifiable lifestyle factors via questionnaires, and measured body mass index (BMI) and brachial blood pressure (BP). Suprasystolic oscillometry was used to derive aortic pressure, from which several haemodynamic parameters were calculated. Heavy alcohol consumption and BMI were positively related to most waveform parameters. Current smokers had higher levels of aortic augmentation index than non-smokers (difference=3.7%, P<0.0001). Aortic waveform parameters, controlling for demographics, antihypertensives, diabetes and cardiovascular disease (CVD), were higher in non-Europeans than in Europeans. Further adjustment for brachial BP or lifestyle factors (particularly BMI) reduced many differences but several remained. Despite even further adjustment for mean arterial pressure, pulse rate, height and total:high-density lipoprotein cholesterol, compared with Europeans, South Asians had higher levels of all measured aortic waveform parameters (for example, for backward pressure amplitude: β=1.5 mm Hg; P<0.0001), whereas Pacific people had 9% higher loge (excess pressure integral) (P<0.0001). In conclusion, aortic waveform parameters varied with ethnicity in line with the greater prevalence of CVD among non-white populations. Generally, this was true even after accounting for brachial BP, suggesting that waveform parameters may have increased usefulness in capturing ethnic variations in cardiovascular risk. Heavy alcohol consumption, smoking and especially BMI may partially contribute to elevated levels of these parameters.

Different associations between beta-blockers and other antihypertensive medication combinations with brachial blood pressure and aortic waveform parameters

BACKGROUND Comparing the relationships of antihypertensive medications with brachial blood pressure (BP) and aortic waveform parameters may help clinicians to predict the effect on the latter in brachial BP-based antihypertensive therapy. We aimed to make such comparisons with new waveform measures and a wider range of antihypertensive regimens than examined previously. METHODS Cross-sectional analysis of 2933 adults (61% male; aged 50-84years): 1637 on antihypertensive treatment and 1296 untreated hypertensives. Sixteen medicine regimens of up to 4 combinations of drugs from 6 antihypertensive classes were analysed. Aortic systolic BP, augmentation index (AIx), excess pressure integral (EPI), backward pressure amplitude (Pb), reflection index (RI) and pulse wave velocity (PWV) were calculated from aortic pressure waveforms derived from suprasystolic brachial measurement. RESULTS Forest plots of single-drug class comparisons across regimens with the same number of drugs (for between 1- and 3-drug regimens) revealed that AIx, Pb, RI and/or loge(EPI) were higher (maximum difference=5.6%, 2.2mmHg, 0.0192 and 0.13 loge(mmHg⋅s), respectively) with the use of a beta-blocker compared with vasodilators and diuretics, despite no brachial systolic and diastolic BP differences. These differences were reduced (by 34-57%) or eliminated after adjustment for heart rate, and similar effects occurred when controlling for systolic ejection period or diastolic duration. CONCLUSIONS Beta-blocker effects on brachial BP may overestimate effects on aortic waveform parameters. Compared to other antihypertensives, beta-blockers have weaker associations with wave reflection measures and EPI; this is predominantly due to influences on heart rate.

Non-invasive blood pressure measurement algorithm using neural networks

The oscillometric method is the most commonly used automatic monitoring blood pressure measurement method nowadays.Height-based and Slope-based criteria are the two general means used to determine the systolic and diastolic pressures; howeverthey are disputed for their accuracy. Thus, the auscultatory method continues to be the gold-standard for these measurements.In this paper a newly developed cuff with piezofilm sensors and a pressure sensor to collect signals from the brachial artery isinvestigated. Using Neural Networks to classify the acquired pressure signals in various regions, an algorithm is developed andimplemented in signal processing and heart beat/heart rate detection software. The algorithm is tested on 258 measurementsfrom 86 subjects and shows good conformance to the standards set out by the Association for the Advancement of Medical Instrumentation and British Hypertension Society grade A criteria.

The Effect of Various Physical Phenomena on Wave Propagations in the Human Aorta

A physiologically-correct mathematical model of blood flow in the human aorta is developed from previously reported experimental data. The blood is assumed as a viscous fluid flowing through a compliant tube. This phenomenon is modeled by combining the Navier-Stokes’ equations and Laplace Law. The model is validated using experimental data collected at a leading specialist catheterisation laboratory. The mathematical model is then manipulated to derive a pressure transfer function between the aortic pressure and the pressure at the iliac bifurcation. The results of a comprehensive senstivity analysis carried out on this transfer function are discussed in this article. This study indicates that Coriolis and viscous effects insignificantly affect the wave propagation characteristics. The effect of arterial tapering on the transfer function is also recorded as insignificant. Changing the stiffness of the tube causes the pressure wave to travel faster through the system. The system natural frequency also increases when the tube wall is stiffened.Copyright