Biomedical subjects

R K Millard

Publications and source records attributed to R K Millard.

8 recordsLinked to original sources

PubMed2001

Progress in non-invasive respiratory monitoring using uncalibrated breathing movement components.

The theory for optimal linear combination of uncalibrated breathing movements was developed and applied in non-invasive respiratory monitoring situations for assessment. 16 patients were monitored overnight for respiratory depression during postoperative pain treatment. Intranasal/extra-oral airway pressure monitoring and pulse oximetry signals were recorded at 50 Hz. Respiratory inductive plethysmography (RIP) provided guidance to nurses regarding sensitivity settings of the pressure device during slow, shallow breathing, and vital information about breathing movements to help distinguish central from obstructive apnoeas. Subsequent analysis showed that the principal components of the standardized RIP signals would be helpful in any automated identification of pressure indicator false alarms and could provide a simple means for supplementary breath classification. The sum and difference of the scaled RIP values tracked changes in tidal volume and indicated any breathing movement asynchrony or paradox associated with obstructions. A construction was developed for emulating RIP calibration predictions of relative changes in tidal volume to within about 1%, so that invasive or demanding monitoring preparations could be by-passed altogether. The necessary signal combination and linearcalibration model background is reviewed for this simple formulation, which arises from component analysis and least squares regression. The methods are illustrated for definitive non-invasive postoperative monitoring and calibration situations. Theoretical and physiological reasons for preferring the use of balanced ribcage and abdomen contributions to overall tidal volume are presented that also help clarify the greater limitations of traditional RIP monitoring practices.

Analgesics, Opioid↗

PubMed1999

Bootstrap resampling: a powerful method of assessing confidence intervals for doses from experimental data.

Bootstrap resampling provides a versatile and reliable statistical method for estimating the accuracy of quantities which are calculated from experimental data. It is an empirically based method, in which large numbers of simulated datasets are generated by computer from existing measurements, so that approximate confidence intervals of the derived quantities may be obtained by direct numerical evaluation. A simple introduction to the method is given via a detailed example of estimating 95% confidence intervals for cumulated activity in the thyroid following injection of 99mTc-sodium pertechnetate using activity-time data from 23 subjects. The application of the approach to estimating confidence limits for the self-dose to the kidney following injection of 99mTc-DTPA organ imaging agent based on uptake data from 19 subjects is also illustrated. Results are then given for estimates of doses to the foetus following administration of 99mTc-sodium pertechnetate for clinical reasons during pregnancy, averaged over 25 subjects. The bootstrap method is well suited for applications in radiation dosimetry including uncertainty, reliability and sensitivity analysis of dose coefficients in biokinetic models, but it can also be applied in a wide range of other biomedical situations.

Confidence Intervals↗

PubMed1999

Inductive plethysmography components analysis and improved non-invasive postoperative apnoea monitoring.

Twenty-nine patients were monitored overnight for breathing distress patterns during postoperative analgesia. Nasal flow apnoea monitoring and pulse oximetry data were recorded at 50 Hz. Respiratory inductive plethysmography (RIP) tracked tidal volume (TV) thoracoabdominal motion, and supplemented the flow monitoring by identifying detected apnoea type. TVs were computed from linear combinations of the RIP signals, but calibrations showed that multiple regression approaches with fitting errors <1% had highly variable coefficients and estimate precisions. Simple least squares theory showed that unstable parameter calculation and coefficient variation with signal conditions were inherent in RIP calibration models. Principal components (PC) methods were well suited to mitigating these problems because the RIP signals were highly correlated. The two PCs tracked the relative changes in TVs and indicated the degree of signal asynchrony, enabling improved uncalibrated monitoring. For accurately measuring RIP phase differences, the cross-correlation function was calculated. A simple version of PC analysis is developed, avoiding matrices, to help clarify how RIP calibration problems can be addressed. The methods are illustrated for calibration in normal breathing, and for postoperative monitoring during Cheyne-Stokes breathing. Sum and difference combinations of the RIP signals could discriminate central from obstructive apnoeas to help improve flow monitoring efficacy on-line.

Analgesia↗

PubMed1997

Indicator-dilution dispersion models and cardiac output computing methods.

The general theory of indicator-dilution methods provides a basis for computing improved cardiac output estimates. Interpretation is via indicator-dispersion modeling with Brownian motion of drifting particles. Detected curves indicate the distribution of passage times from the injection site: the local density random walk (LDRW) function of a Wiener process. Fitting the LDRW to 70 dye curves by nonlinear regression for examples, I show how all possible undistorted curves can, in principle, be simulated. I show via semilogarithmic plots that conventional exponential decay constructs systematically underestimate cardiac output by up to 8%. To help reconcile the predictions of LDRW-fitted dilution curves and contemporary practice, I show how curve-shape asymmetry (skewness) dramatically affects the enclosed areas. Mean transit times may overestimate blood volumes by 15-100% in very skewed thermodilution curves if the dispersion effects are overlooked. Triangle constructions, which accounted for hundreds of experimental findings, also have theoretical explanations. Curve-fitting methods reduce the extrapolation biases inherent in many computers and in any respiration-induced artifacts. Compatibility of cardiac output predictions from various dilution methods and modules becomes feasible.

Artifacts↗

PubMed1990

Self-tuning adaptive control of induced hypotension in humans: a comparison of isoflurane and sodium nitroprusside.

Induced hypotension is commonly used during surgery to decrease arterial pressure. Sodium nitroprusside and isoflurane are well-known hypotensive agents. The use of self-tuning adaptive control of induced hypotension was assessed with the use of sodium nitroprusside and isoflurane as hypotensive agents. Nineteen surgical patients were studied during closed-loop control of hypotension induced with sodium nitroprusside. This group of patients was compared with 10 similar patients in whom infusions of sodium nitroprusside were controlled manually by an anesthesiologist. Although the results of the two studies varied, no conclusion could be drawn regarding the superiority of either manual or closed-loop control. When manual versus automatic control of isoflurane-induced hypotension was assessed in a similar fashion, the two methods of induction were found to be comparable.

Algorithms↗

PubMed1989

Automatic arterial pressure regulation using isoflurane: comparison with manual control.

A self-tuning, closed-loop controller, based on the algorithm of Clarke and Gawthrop, was used to regulate the inspired concentration of isoflurane to reduce arterial pressure electively in 33 patients undergoing ENT surgery. The patients were allocated randomly to one of four groups and received differing doses of fentanyl and labetalol to vary the range of sensitivities to the hypotensive action of isoflurane. The performance of the controller was evaluated at two target arterial pressures (AP), by its response to simulated changes in AP and by a comparison with a further group of eight patients with manual control of AP. The controller's undershoot of AP (range 2.8 +/- 0.5-4.5 +/- 1.3 mm Hg) and % time spent within +/- 5 mm Hg of the target AP (range 83 +/- 3.4-89 +/- 2.2%) were acceptable and equalled the manual performance figures (range 3.3 +/- 0.8 mm Hg; 90 +/- 5%). The regulation of induced hypotension in all four groups was rapid, accurate, stable and reproducible.

Adolescent↗

PubMed1988

Clonidine premedication for isoflurane-induced hypotension. Sympathoadrenal responses and a computer-controlled assessment of the vapour requirement.

The effect of single-dose clonidine premedication on the vapour requirement for isoflurane-induced hypotension in patients undergoing middle ear or nasal surgery was evaluated in an open, controlled, randomized study. Inspired isoflurane concentration was regulated by a microcomputer-based, self-tuning control program when hypotension was required. Patients given clonidine 0.6 mg by mouth 2 h before operation required a mean inspired isoflurane concentration of 2.0% to induce hypotension (mean intra-arterial pressure 50 mm Hg) compared with 3.01% in the control group (P less than 0.05). Five out of 10 patients in the control group required a supplementary dose of labetalol 5 mg i.v. to achieve satisfactory hypotension, compared with one of 10 patients given clonidine premedication (Fisher's exact probability, 0.07). A mean concentration of 1.4% isoflurane was required to maintain hypotension in the clonidine group, compared with 2.3% in the control group (P less than 0.01). Plasma adrenaline and noradrenaline concentrations did not increase during induced hypotension in each group.

Adolescent↗

PubMed1987

On using a self-tuning controller for blood pressure regulation during surgery in man.

The generalized minimum-variance self-tuning controller of Clarke and Gawthrop has been used to adjust the flow rate of a modified Vickers Treonic IP4 syringe pump delivering phenylephrine to 20 patients undergoing lower abdominal surgery during epidural analgesia. This proved to be a very effective method of restoring and maintaining normal arterial pressure. The method has also been used to produce controlled hypotension in 18 patients undergoing plastic or neurosurgical procedures via sodium nitroprusside infusions. Valuable insight into patient responses to surgical stimuli, blood loss, fluid loads, opioids, relaxants and other agents was provided.

Anesthesia, Epidural↗