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Results for “Deterministic Measurement”

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PubMed1994

Radiation exposure in endovascular surgery of the head and neck.

PURPOSE: To evaluate the radiation risk to the operator and the patient during endovascular surgery of the head and neck. METHODS: The dose was measured using thermoluminescence dosimeters attached at the body surface of the operator and the patient during 15 endovascular surgeries (3 for arteriovenous malformation, 8 for dural arteriovenous fistulas, and 4 for other disorders of the head and neck). The dose was measured at seven sites on the operator and at five sites on the patient. RESULTS: The mean number of digital subtraction angiography studies and fluoroscopy time were 21 +/- 10 and 73 +/- 24 minutes, respectively. The equivalent dose range at each site in the operator was 0.12 to 0.88 mSv (glabella), 0.06 to 1.1 and 0 to 0.09 mSv (neck, outside and inside the protector, respectively), 0 to 0.20 mSv (left should, inside the protector), 0.09 to 1.99 mSv (left arm), 0.05 to 3.55 mSv (left hand), and 0 to 0.49 mSv (pubis, inside the protector). Those in the patients were 3.1 to 136 mSv (glabella), 13 to 5441 mSv (right temporal area), 4 to 186 mSv (left temporal area), 0.1 to 51 mSv (neck), and 0 to 0.62 mSv (pubis). CONCLUSIONS: The total doses at the operator's eyes and left hand during the course of a year may exceed the dose limits recommended by the International Commission on Radiological Protection. Operators should wear not only body protectors, but also thyroid protectors and lead glass spectacles. The equivalent dose at the right temporal area of the patient may exceed the deterministic dose for transient erythema or alopecia of the skin even in one endovascular procedure.

Adolescent↗

PubMed2000

Parasite-mediated and direct competition in a two-host shared macroparasite system.

This paper investigates the local dynamical behaviour of a deterministic model describing two host species experiencing three forms of competition: direct competition, apparent competition mediated by macroparasites, and intra-specific (density-dependent) competition. The problem of algebraic intractability is sidestepped by adopting a geometric approach, in which an array of maps is constructed in parameter space, each structured by bifurcation surfaces which mark qualitative changes in system behaviour. The maps provide both a succinct and a comprehensive overview of the stability and feasibility structure of the system equilibria, from which can be deduced the possible modes of local dynamical behaviour. A detailed examination of these maps shows that (i) the system is highly sensitive to the effect of infection on fecundity with synchronous sustained cycles readily generated by Hopf bifurcations; (ii) for a broad range of parameter values, pertinent to actual biological systems, apparent competition mediated by macroparasites is sufficient, on its own, to explain host exclusion; (iii) direct competition reinforces parasite-mediated competition to expand the host exclusion region; and (iv) the condition for host exclusion can be expressed simply in a form which holds for both micro- and macroparasite models and which involves just two key indices, measuring tolerance to the infection and the strength of direct competition. The techniques used in this paper are not restricted to the analysis of host-parasite systems but can be applied to a wide range of nonlinear population models. They are therefore as relevant to the analysis of such general issues as exploitative competition and trophic interactions as they are to specific epidemiological problems.

Animals↗

PubMed2005

Fractal and noisy CBV dynamics in humans: influence of age and gender.

The complexity of spontaneous cerebral blood volume (CBV) fluctuations can emerge from random, fractal, or chaotic processes. Our aims were to define the contribution of these patterns to the observed complexity and to evaluate the effect of age and gender on it. The total hemoglobin content as the measure of CBV was monitored by near-infrared spectroscopy on volunteers (men n = 19, age = 20 to 78 years; women n = 23, age = 21 to 79 years). Random and fractal patterns were distinguished by the spectral index (beta). Chaos was identified by surrogate analysis of the correlation dimension (a static chaotic parameter, the dimension of the correlation integral) and the largest Lyapunov exponent (a dynamic chaotic parameter, the rate of exponential divergence of the system states from a perturbed initial condition over the chaotic attractor). In spontaneous CBV fluctuations, both fast random and slow fractal dynamics are present separately in their spectra by a cutoff frequency, f'. Below f' the pattern is fractal, in that power rises inversely with frequency as 1/f(beta). f' decreases with age in men and women alike (F1: up to 0.12+/-0.06 Hz versus F2: up to 0.05+/-0.04 Hz at P = 0.015, and M1: up to 0.16+/-0.05 Hz versus M2: up to 0.11+/-0.04 Hz at P = 0.044). Neither pre- nor postmenopausal age groups (1 and 2, respectively) showed a (low)beta gender difference. Surrogate analysis showed that CBV dynamics cannot be characterized on the grounds of deterministic chaos. Cerebral blood volume fluctuates in a complex, bimodal manner in humans, in that the fast dynamics has no structure, while the slow dynamics exhibits a self-similar, that is, fractal temporal structure. The range of fluctuation amplitudes produced by fractal dynamics is always larger than that of random fluctuations, and it shrinks with an altered structuring in aging women only.

Adult↗

PubMed2002

X-ray dose and associated risks from radiofrequency catheter ablation procedures.

The objectives of this study were to quantify the ionizing radiation exposure to patient and operator during radiofrequency (RF) catheter ablation and to estimate the risks associated with this exposure. The study consisted of 50 RF ablation procedures, all performed in the same electrophysiology laboratory. Occupational dose to two cardiologists who performed the procedures was measured using film badges and extremity thermoluminescent dosemeters (TLDs). Absorbed dose to the patients' skin was measured using TLDs. Dose-area product (DAP) was also measured. The effective dose to the cardiologists was less than 0.15 mSv per month. The mean equivalent dose to the cardiologists' left hand and forehead was 0.24 mSv and 0.05 mSv, respectively, per RF ablation procedure, which was more than twice the mean dose for the other cardiology procedures carried out in the centre. Yearly occupational dose to the cardiologists was much lower than the relevant statutory dose limits. The mean skin dose, fluoroscopy time and DAP to patients were 0.81 Gy, 67 min and 123 Gycm(2), respectively, with a maximum of 3.2 Gy, 164 minutes and 430 Gycm(2), respectively. Mean effective dose to patients was 17 mSv, from which the excess risk of developing fatal cancer is 0.1%. Six of the patients (12%) received a skin dose above the threshold dose for radiation skin injury (2 Gy), but no skin injuries were reported. Patient skin dose and DAP were closely correlated and this allows DAP to be used to monitor patient skin dose in real-time. DAP levels were locally adopted as diagnostic reference levels (DRLs) that provide an indication during a procedure that a patient is at risk of suffering deterministic skin injury.

Adolescent↗

PubMed1979

A statistical description of the human tracheobronchial tree geometry.

Most physiological studies which made use of lung geometry have utilized average deterministic models of the tracheobronchial tree geometry, such as Weibel's Model A (1963). However, as shown by morphometric studies, it is well known that there are significant inter-subject and intra-subject variabilities in the structural components of the human lung. Hence, inherent inaccuracies exist when deterministic dimensions for lung geometry are used. In this paper, a statistical description of the lung geometry is presented. Using Weibel's Model A as the underlying average model, probability distributions for the lengths and the diameters of airways and for the number and volume of alveoli are proposed based on morphometric data. As a check for consistency, the probability distribution of the functional residual capacity is derived from those associated with airways and alveoli and it is compared with reported data. Results of this comparison are favorable, suggesting that the statistical description presented herein represents a self-consistent model for lung geometry which can be used for studies of problems related to pulmonary physiology.

Bronchi↗

PubMed1996

Chaos-related deterministic regulation of heart rate variability in time- and frequency domains: effects of autonomic blockade and exercise.

OBJECTIVES: To study non-linear complexity or chaotic behaviour of heart rate in short time series and its dependence on autonomic tone. METHODS: Ten healthy individuals (5 men, mean age 44 years) were investigated at rest, after intravenous injections of propranolol (0.15 mg/kg), followed by atropine (0.03 mg/kg). On another occasion, investigation was made during exercise on a bicycle ergometer at 40% and at 70% of maximal working capacity. Heart rate variability was assessed by: local sensitive dependence on initial conditions as quantitated by the dominant Lyapunov exponent, coefficient of variation of heart rate, power spectral analysis of high- and low-frequency bands and the 1/f-slope of the very-low-frequency band and time domain analysis. RESULTS: The approximate dominant Lyapunov exponent was positive at rest and remained positive during autonomic blockade and during exercise. The exponent decreased significantly with propranolol+atropine and even more so during exercise but did not attain zero. At baseline approximate predictability was lost after about 30 s whereas after autonomic blockade or exercise it was lost after about 60 s. The 1/f-slope remained unaltered around -1. As expected, power in high- and low-frequency bands as well as time domain index decreased significantly with autonomic blockade. The low-frequency band and time domain index were affected by exercise. CONCLUSIONS: Heart rate variability of sinus rhythm in healthy individuals has characteristics suggestive of low-dimensional chaos-like determinism which is modulated but not eliminated by inhibition of autonomic tone or by exercise. The dominant Lyapunov exponent characterises heart rate variability independent or the other investigated measures.

Adult↗

PubMed2003

Memory effects in microscopic traffic models and wide scattering in flow-density data.

By means of microscopic simulations we show that noninstantaneous adaptation of the driving behavior to the traffic situation together with the conventional method to measure flow-density data provides a possible explanation for the observed inverse-lambda shape and the wide scattering of flow-density data in "synchronized" congested traffic. We model a memory effect in the response of drivers to the traffic situation for a wide class of car-following models by introducing an additional dynamical variable (the "subjective level of service") describing the adaptation of drivers to the surrounding traffic situation during the past few minutes and couple this internal state to parameters of the underlying model that are related to the driving style. For illustration, we use the intelligent-driver model (IDM) as the underlying model, characterize the level of service solely by the velocity, and couple the internal variable to the IDM parameter "time gap" to model an increase of the time gap in congested traffic ("frustration effect"), which is supported by single-vehicle data. We simulate open systems with a bottleneck and obtain flow-density data by implementing "virtual detectors." The shape, relative size, and apparent "stochasticity" of the region of the scattered data points agree nearly quantitatively with empirical data. Wide scattering is even observed for identical vehicles, although the proposed model is a time-continuous, deterministic, single-lane car-following model with a unique fundamental diagram.

Journal Article↗

PubMed1992

Potential improvements in rate of genetic gain from marker-assisted selection in dairy cattle breeding schemes.

The value of marker-assisted selection in dairy cattle breeding schemes is predicted by a deterministic model. In these schemes, associations between markers and milk production were assessed from production records of daughters of a grandsire by a multiple regression model with random marker effects. By tracing markers from the grandsire to grandoffspring, deviations of grandoffspring from their full-sib family mean were predicted. Predictions of the within-family variance of the grandoffspring accounted for by markers amounted to up to 13.3%. This figure decreased when the number of daughters of the grandsire analyzed decreased and, less markedly, when the distance between flanking markers increased. Prediction of within-family deviations hardly improved rates of genetic gain in conventional progeny testing schemes; equal numbers of young bulls were born annually. Genetic gain and improvement of genetic gain because of prediction of within-family deviations were much higher in nucleus schemes. In these shemes, with short optimized generation intervals, conventional selection was mainly for pedigree information and did not use the within-family variance. Analysis of highly polymorphic markers in daughters of both grandsires accounted for 4.1 to 13.3% of the within-family variance, which increased rates of gain by 9.5 to 25.8% and 7.7 to 22.4% in open and closed nucleus schemes, respectively. Risk of breeding schemes, measured by the variance of the selection response, was not increased by the use of markers.

Animals↗

PubMed1991

A deterministic computer simulation model of life-cycle lamb and wool production.

A deterministic mathematical computer model was developed to simulate effects on life-cycle efficiency of lamb and wool production from genetic improvement of performance traits under alternative management systems. Genetic input parameters can be varied for age at puberty, length of anestrus, fertility, precocity of fertility, number born, milk yield, mortality, growth rate, body fat, and wool growth. Management options include mating systems, lambing intervals, feeding levels, creep feeding, weaning age, marketing age or weight, and culling policy. Simulated growth of animals is linear from birth to inflection point, then slows asymptotically to specified mature empty BW and fat content when nutrition is not limiting. The ME intake requirement to maintain normal condition is calculated daily or weekly for maintenance, protein and fat deposition, wool growth, gestation, and lactation. Simulated feed intake is the minimum of availability, DM physical limit, or ME physiological limit. Tissue catabolism occurs when intake is below the requirement for essential functions. Mortality increases when BW is depressed. Equations developed for calculations of biological functions were validated with published and unpublished experimental data. Lifetime totals are accumulated for TDN, DM, and protein intake and for market lamb equivalent output values of empty body or carcass lean and wool from both lambs and ewes. These measures of efficiency for combinations of genetic, management, and marketing variables can provide the relative economic weighting of traits needed to derive optimal criteria for genetic selection among and within breeds under defined industry production systems.

Animals↗

PubMed2003

Dynamic nature of electrocardiographic waveform predicts rescue shock outcome in porcine ventricular fibrillation.

STUDY OBJECTIVE: Survival decreases with duration of ventricular fibrillation, and it is possible that failed rescue shocks increase myocardial damage. Structure in the ECG signal during ventricular fibrillation can be quantified by using the scaling exponent, a dimensionless measure that correlates with ventricular fibrillation duration. This study examined whether the scaling exponent could predict rescue shock success and whether unsuccessful rescue shocks altered the structure of the ventricular fibrillation waveform and the responsiveness to subsequent rescue shocks. METHODS: Ventricular fibrillation was electrically induced in 44 anesthetized swine, which were randomly assigned to receive 70-J biphasic rescue shocks at 2, 4, 6, 8, or 10 minutes. If rescue shocks failed, up to 2 subsequent rescue shocks were performed at 2-minute intervals. The scaling exponent was calculated at 1-second intervals from ECG to quantify the organization of the ventricular fibrillation waveform. RESULTS: A total of 92 rescue shocks were delivered, of which 23 successfully converted ventricular fibrillation to an organized rhythm (immediate success). After these 23 rescue shocks, 14 swine sustained organized rhythms for more than 30 seconds (sustained success). Lower scaling exponent values were associated with increased probability of successful rescue shocks. Receiver operating characteristic curves had an area under the curve of 0.86 for immediate rescue shock success and 0.93 for sustained rescue shock success. Failed rescue shocks increased the rate of scaling exponent increase over time but did not appear to affect subsequent rescue shock success when the scaling exponent was taken into account. CONCLUSION: Highly deterministic ventricular fibrillation, reflected by a low scaling exponent, predicted rescue shock success regardless of antecedent failed rescue shocks. In addition, unsuccessful rescue shocks might decrease post-rescue shock ventricular fibrillation waveform organization.

Animals↗

PubMed2002

[A mathematical model of the annual risk of tuberculosis infection in Cali, Colombia].

OBJECTIVE: The annual risk of infection (ARI) for tuberculosis is the probability that an individual without previous contact with the tubercle bacillus has of being infected during the course of a year. The ARI is the most appropriate indicator for estimating the degree of tuberculosis infection in a population. The objective of this paper was to estimate the ARI and its trends in the city of Cali, Colombia, using data provided by the Municipal Secretariat of Health. METHODS: We used a deterministic model of the dynamics of pulmonary tuberculosis. The flows among the population subgroups were based on the natural history of the disease, taking vaccination into account. Using the data from the Municipal Secretariat of Health, we estimated the initial conditions and the values of the parameters. RESULTS: The mean ARI values were 1.24% in the 1970s, 0.93% in the 1980s, and 0.85% in the 1990s. In order to assess trends, we attempted to predict the annual risk, utilizing a nonlinear least-squares adjustment of the data on the overall percentage for each year. With that approach, we projected that the ARI in 2003 would be 1.3%, indicating a return to the patterns found in the 1970s. CONCLUSIONS: The estimated risk of tuberculosis infection in Cali during the decades of the 1970s, the 1980s, and the 1990s was very high in comparison with the risk in countries such as the Netherlands, which in 1985 had an ARI of 0.012%. However, the ARI in Cali is not so high in comparison to indices for other countries of South America, which range from 0.5% to 1.5%. This model and the simulation it produced showed a rising trend in the ARI for Cali, as well as demonstrated that the ARI will tend to continue to rise if control measures are not improved.

Colombia↗

PubMed2004

Visual discrimination learning in dwarf goats and associated changes in heart rate and heart rate variability.

We studied visual discrimination learning in a group of Nigerian dwarf goats using a computer-based learning device which was integrated in the animals' home pen. We conducted three consecutive learning tasks (T1, T2 and T3), each of which lasted for 13 days. In each task, a different set of four visual stimuli was presented on a computer screen in a four-choice design. Predefined sequences of stimulus combinations were presented in a pseudorandom order. Animals were rewarded with drinking water when they chose the positive stimulus by pressing a button next to it. Noninvasive measurements of goats' heartbeat intervals were carried out on the first and the last 2 days of each learning task. We analysed heart rate (HR) and heart rate variability (HRV) of resting animals to study sustained physiological effects related to general learning challenge rather than acute excitement during an actual learning session. The number of trials to reach the learning criterion was 1000 in T1, when visual stimuli were presented to the goats for the first time, but decreased to 210 in T2 and 240 in T3, respectively. A stable plateau of correct choices between 70% and 80% was reached on Day 10 in T1, on Day 8 in T2 and on Day 6 in T3. We found a significant influence of the task and of the interaction between task and day on learning success. Whereas HR increased throughout T1, this relationship was inverted in T2 and T3, indicating different effects on the HR depending on how familiar goats were with the learning task. We found a significant influence of the task and the interaction between task and time within the task on HRV parameters, indicating changes of vagal activity at the heart. The results suggest that changes in HR related to learning were predominantly caused by a withdrawal of vagal activity at the heart. With regard to nonlinear processes in heartbeat regulation, increased deterministic shares of HRV indicated that the animals did not really relax until the end of T3. Comparing changes of HR and HRV in T3 and in a subsequent postexperiment (PE), we assume a positive effect of such cognitive challenges once the task had been learned by the animals.

Animals↗

PubMed2003

Incorporation of a laser range scanner into image-guided liver surgery: surface acquisition, registration, and tracking.

As image guided surgical procedures become increasingly diverse, there will be more scenarios where point-based fiducials cannot be accurately localized for registration and rigid body assumptions no longer hold. As a result, procedures will rely more frequently on anatomical surfaces for the basis of image alignment and will require intraoperative geometric data to measure and compensate for tissue deformation in the organ. In this paper we outline methods for which a laser range scanner may be used to accomplish these tasks intraoperatively. A laser range scanner based on the optical principle of triangulation acquires a dense set of three-dimensional point data in a very rapid, noncontact fashion. Phantom studies were performed to test the ability to link range scan data with traditional modes of image-guided surgery data through localization, registration, and tracking in physical space. The experiments demonstrate that the scanner is capable of localizing point-based fiducials to within 0.2 mm and capable of achieving point and surface based registrations with target registration error of less than 2.0 mm. Tracking points in physical space with the range scanning system yields an error of 1.4 +/- 0.8 mm. Surface deformation studies were performed with the range scanner in order to determine if this device was capable of acquiring enough information for compensation algorithms. In the surface deformation studies, the range scanner was able to detect changes in surface shape due to deformation comparable to those detected by tomographic image studies. Use of the range scanner has been approved for clinical trials, and an initial intraoperative range scan experiment is presented. In all of these studies, the primary source of error in range scan data is deterministically related to the position and orientation of the surface within the scanner's field of view. However, this systematic error can be corrected, allowing the range scanner to provide a rapid, robust method of acquiring anatomical surfaces intraoperatively.

Aged↗

PubMed1997

Circadian rhythmic fractal scaling of heart rate variability in health and coronary artery disease.

BACKGROUND: In clinical cardiology, heart rate variability is a putative index of autonomic cardiovascular function. Signs of reduced vagal activity are not only associated with an enhanced risk of sudden cardiac death, but such impaired heart rate variability became a new predictor of sudden cardiac death and other mortality in patients with a variety of diseased states. HYPOTHESIS: It is postulated (1) that the time structure (chronome) of heart rate variability in clinical health includes a circadian rhythm and deterministic chaos, the latter gauged by the correlation dimensions of RR intervals; and (2) that this chronome is altered in patients with coronary artery disease (CAD). METHODS: From 24-h Holter records of 11 healthy controls and 10 patients with CAD, 500-s sections around 02:00, 06:00, 10:00, 14:00, 18:00 and 22:00 hours were analyzed for smoothed RR intervals sampled at 4 Hz. Correlation integrals were estimated for embedding dimensions from 1 to 20 with a 1.0-s time lag, using an algorithm modified from Grassberger and Procaccia. The Wilcoxon signed-rank test compares circadian end points assessed by cosinor between the CAD patients and age-matched controls. RESULTS: A circadian rhythm characterizes the correlation dimension of healthy subjects peaking during the night (p < 0.005). Patients with CAD have a lowered correlation dimension (p < 0.05) and an altered circadian variation which requires the consideration of an approximately 12-h (circasemidian) component. CONCLUSION: The results demonstrate the sensitivity of circadian rhythms for the detection of disease. A partial 24- to 12-h (circadian-to-circasemidian) frequency multiplication (or partial variance transposition) in CAD of the correlation dimension, apart from being a potential clue to the etiology of the disease, adds a new feature to a chronocardiology combining, with the fractal scaling, an assessment of circadian and circasemidian components as measures of predictable variability to be tested for use in diagnosis, prognosis, and as putative guides to treatment timing.

Adult↗

PubMed2003

Radiation doses in interventional radiology procedures: the RAD-IR study: part I: overall measures of dose.

PURPOSE: To determine patient radiation doses for interventional radiology and neuroradiology procedures, to identify procedures associated with higher radiation doses, and to determine the effects of various parameters on patient doses. MATERIALS AND METHODS: A prospective observational study was performed at seven academic medical centers. Each site contributed demographic and radiation dose data for subjects undergoing specific procedures in fluoroscopic suites equipped with built-in cumulative dose (CD) and dose-area-product (DAP) measurement capability compliant with International Electrotechnical Commission standard 60601-2-43. The accuracy of the dosimetry was confirmed by comprehensive measurements and by frequent consistency checks performed over the course of the study. RESULTS: Data were collected on 2,142 instances of interventional radiology procedures, 48 comprehensive physics evaluations, and 581 periodic consistency checks from the 12 fluoroscopic units in the study. There were wide variations in dose and statistically significant differences in fluoroscopy time, number of images, DAP, and CD for different instances of the same procedure, depending on the nature of the lesion, its anatomic location, and the complexity of the procedure. For the 2,142 instances, observed CD and DAP correlate well overall (r = 0.83, P <.000001), but correlation in individual instances is poor. The same is true for the correlation between fluoroscopy time and CD (r = 0.79, P <.000001). The correlation between fluoroscopy time and DAP (r = 0.60, P <.000001) is not as good. In 6% of instances (128 of 2,142), which were principally embolization procedures, transjugular intrahepatic portosystemic shunt (TIPS) procedures, and renal/visceral artery stent placements, CD was greater than 5 Gy. CONCLUSIONS: Most procedures studied can result in clinically significant radiation dose to the patient, even when performed by trained operators with use of dose-reducing technology and modern fluoroscopic equipment. Embolization procedures, TIPS creation, and renal/visceral artery stent placement are associated with a substantial likelihood of clinically significant patient dose. At minimum, patient dose data should be recorded in the medical record for these three types of procedures. These data should include indicators of the risk of deterministic effects as well as the risk of stochastic effects.

Adolescent↗

PubMed1994

Alpha particles are extremely damaging to developing hemopoiesis compared to gamma irradiation.

Estimates of risk of stochastic effects from contamination with alpha-particle-emitting radionuclides are based on equivalent doses which take into account the RBE of the high-LET radiation. ICRP has recommended a dose-weighting factor, wR, of 20 for alpha-particle radiation. It is assumed that the RBEs for deterministic effects are considerably less than those for stochastic effects. However, the offspring of mice injected with 30 Bq g-1 239Pu at 13 days gestation develop a persistent deficit in hemopoietic stem cells which is primarily the result of damage to their regulatory microenvironment. Their spatial distribution in the marrow is also perturbed, and recent observations on those mice suggested a considerably higher factor than 20. To define a more realistic RBE for hemopoiesis, the effects of external gamma irradiation during the fetal development period have been compared directly with those of 239Pu incorporated via placental transfer on the development of hemopoietic tissue. Pregnant mice were irradiated with 60Co gamma rays (a) continuously from day 13 of gestation to birth at 0.15 or 0.6 Gy/day; (b) six repeated acute doses (0.6 Gy/min) at 0.1 or 0.3 Gy from day 13 of gestation; (c) one acute dose of 0.6 or 1.8 Gy on day 15 of gestation. The spatial distribution of hemopoietic stem cells in 8-week-old offspring was then determined and compared to that resulting from alpha-particle irradiation. In each case, the higher dose was required to match the results for alpha particles, suggesting an RBE for developing hemopoiesis of 250-360 compared to a continuous gamma-ray dose and a rather lower value of 130-180 compared to a single acute dose of gamma rays. This contrasts greatly to values for direct irradiation of the stem cells but argues that the effective RBE, measured for long-term effects in vivo, is the more realistic. It is concluded that an all-embracing factor can be grossly misleading in the specification of protection guidelines and can greatly underestimate the risks of exposure to alpha particles.

Alpha Particles↗

PubMed2004

The effectiveness and cost-effectiveness of microwave and thermal balloon endometrial ablation for heavy menstrual bleeding: a systematic review and economic modelling.

OBJECTIVES: To estimate the clinical effectiveness and cost-effectiveness of microwave endometrial ablation (MEA) and thermal balloon endometrial ablation (TBEA) for heavy menstrual bleeding (HMB), compared with the existing (first-generation) endometrial ablation (EA) techniques of transcervical resection (TCRE) and rollerball (RB) ablation, and hysterectomy. DATA SOURCES: Electronic databases, bibliographies of articles, and also experts in the field and relevant industry bodies were asked to provide information. REVIEW METHODS: A detailed search strategy was carried out to identify systematic reviews and controlled trials of MEA and TBEA versus first-generation techniques for EA. In addition to electronic database searching, reference lists were hand-searched and information sought from manufacturers of EA devices and by experts in the field. A deterministic Markov model was developed to assess cost-effectiveness. Data for the model were taken from a range of sources. RESULTS: The systematic review of first-generation EA techniques versus hysterectomy found that EA offered an alternative to hysterectomy for HMB, with fewer complications and a shorter recovery period. Satisfaction and effectiveness were high for both MEA and TBEA. Costs were lower with EA although the difference narrows over time. Second-generation EA techniques are an alternative treatment to first-generation techniques for HMB, and first-generation techniques are known to offer an alternative to hysterectomy. Although no trials of second-generation techniques and hysterectomy have been undertaken, it seems reasonable to assume that second-generation techniques also offer an alternative surgical treatment. Using the model to assess cost-effectiveness, costs were very slightly higher for MEA when compared to TBEA, and differences in quality-adjusted life-years (QALYs) were negligible. For MEA compared with transcervical resection of the endometrium (TCRE) and RB ablation, costs were slightly lower with MEA and MEA accrued very slightly more QALYs. Compared with hysterectomy, MEA costs less and accrues slightly fewer QALYs. For TBEA compared with TCRE and RB ablation, costs were lower with TBEA and TBEA accrued slightly more QALYs. Compared with hysterectomy, TBEA costs moderately less and accrues moderately fewer QALYs. CONCLUSIONS: Overall, there were few significant differences between the outcomes of first- and second-generation techniques including bleeding, satisfaction and QoL measures and repeat surgery rates. Second-generation techniques had significantly shorter operating and theatre times and there appear to be fewer serious perioperative adverse effects with second-generation techniques and postoperative effects are similar. Compared with hysterectomy, TCRE and RB are quicker to perform and result in shorter hospitalisation and faster return to work. Hysterectomy results in more adverse effects and is more expensive, although the need for retreatment leads this difference to decrease over time. Satisfaction with hysterectomy is initially higher, but there is no significant difference after 2 years. The economic model suggests that second-generation techniques are more cost-effective than first-generation techniques of EA for HMB. Both TBEA and MEA appear to be less costly than hysterectomy, although the latter results in more QALYs. Further research is suggested to make direct comparisons of the cost-effectiveness of second-generation EA techniques, to carry out longer term follow-up for all methods of EA in RCTs, and to develop more sophisticated modelling studies. Further research is also recommended into HMB to establish health-state utility values, its surgical treatment, convalescence, complications of treatment, symptoms and patient satisfaction.

Adult↗

PubMed2006

Modelling the impact of antiretroviral use in resource-poor settings.

BACKGROUND: The anticipated scale-up of antiretroviral therapy (ART) in high-prevalence, resource-constrained settings requires operational research to guide policy on the design of treatment programmes. Mathematical models can explore the potential impacts of various treatment strategies, including timing of treatment initiation and provision of laboratory monitoring facilities, to complement evidence from pilot programmes. METHODS AND FINDINGS: A deterministic model of HIV transmission incorporating ART and stratifying infection progression into stages was constructed. The impact of ART was evaluated for various scenarios and treatment strategies, with different levels of coverage, patient eligibility, and other parameter values. These strategies included the provision of laboratory facilities that perform CD4 counts and viral load testing, and the timing of the stage of infection at which treatment is initiated. In our analysis, unlimited ART provision initiated at late-stage infection (AIDS) increased prevalence of HIV infection. The effect of additionally treating pre-AIDS patients depended on the behaviour change of treated patients. Different coverage levels for ART do not affect benefits such as life-years gained per person-year of treatment and have minimal effect on infections averted when treating AIDS patients only. Scaling up treatment of pre-AIDS patients resulted in more infections being averted per person-year of treatment, but the absolute number of infections averted remained small. As coverage increased in the models, the emergence and risk of spread of drug resistance increased. Withdrawal of failing treatment (clinical resurgence of symptoms), immunologic (CD4 count decline), or virologic failure (viral rebound) increased the number of infected individuals who could benefit from ART, but effectiveness per person is compromised. Only withdrawal at a very early stage of treatment failure, soon after viral rebound, would have a substantial impact on emergence of drug resistance. CONCLUSIONS: Our analysis found that ART cannot be seen as a direct transmission prevention measure, regardless of the degree of coverage. Counselling of patients to promote safe sexual practices is essential and must aim to effect long-term change. The chief aims of an ART programme, such as maximised number of patients treated or optimised treatment per patient, will determine which treatment strategy is most effective.

Anti-Retroviral Agents↗