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Biomedical subjects

W W Mapleson

Publications and source records attributed to W W Mapleson.

At least 37 records · Page 2Linked to original sources
PubMed1997

Quantitative study of Lowe's square-root-of-time method of closed-system anaesthesia.

Intermittent injection of liquid anaesthetic into a closed breathing system is particularly suitable in countries with limited resources. A method of calculating appropriate times and magnitudes of the injected doses was described by Lowe but the method has never been assessed rigorously. Such an assessment was the purpose of this study. The technique was used in a double-blind, randomized comparison of halothane, enflurane and isoflurane in oxygen-air, with 20 ASA I or II patients in each group, undergoing superficial or abdominal surgery. The prescribed times of injection were adhered to, but the doses, after the first two, were adjusted to maintain systolic arterial pressure within 20% of the reference preoperative value. Partial pressures of the anaesthetics were monitored but concealed from the investigator-anaesthetist. The mean doses found necessary for each anaesthetic were within 33% of those calculated to produce 1.3 MAC. However, end-tidal partial pressure (just before each dose) stabilized at a steady level of only 0.97, 0.42 and 0.77 MAC for halothane, enflurane and isoflurane, respectively. Recovery from enflurane was much more rapid than that from the other agents but no patient admitted to any dreams. We conclude that the rate of uptake of anaesthetic declines more slowly than predicted and that the patients receiving enflurane were less deeply anaesthetized because the greater hypotensive effect of enflurane led to the use of smaller doses.

Adult↗
PubMed1996

Effect of age on MAC in humans: a meta-analysis.

It is well known that MAC, the minimum alveolar concentration required to prevent movement in response to surgical incision in 50% of patients, decreases with age. Regression analysis showed that data for humans derived from a comprehensive literature survey were consistent, for age > 1 yr, with log10 MAC decreasing with increasing age at the same rate for all inhaled anaesthetics; approximately equivalent to 6% change per decade of age. With some slight reservation on differences between data from different institutions, the present data for humans are consistent (for age > 1 yr), with the equation MAC = a x 10bx where x = difference in age (in years) from 40, b = -0.00269 (95% confidence limits (CL) -0.0030, -0.0024) and a = MAC at age 40 yr, which, for anaesthetics currently in use clinically, is given by: halothane, 0.75%; isoflurane, 1.17%; enflurane, 1.63%; sevoflurane, 1.80%; desflurane 6.6%; nitrous oxide, 104%; with 95% CL of approximately +/- 7% (+/- 10% for desflurane, +/- 17% for enflurane).

Adolescent↗
PubMed1996

Comparison of arterialized venous with arterial blood propofol concentrations during sub-anaesthetic infusions in volunteers.

We have compared propofol concentrations in arterial and arterialized venous blood (from the radial artery at the wrist and a vein in the opposite forearm) in five volunteers during a sub-anaesthetic increasing infusion regimen designed to produce pseudo-steady-state arterial concentrations of 0.06, 0.17 and 0.43 micrograms ml-1. As in a previous study, we found no statistically significant difference between sampling sites (P = 0.66). However, changes in the method of arterialization yielded a reduction in the observed variability, with the arterialized venous propofol concentration lying within +/- 23% (95% reference interval) of the corresponding arterial concentration compared with 1 +/- 43%, as reported previously.

Analysis of Variance↗
PubMed1996

Interference of volatile anaesthetics with infrared analysis of carbon dioxide and nitrous oxide tested in the Dräger Cicero EM using sevoflurane.

In theory, setting an infrared multi-gas analyser to measure a volatile anaesthetic different from that in the sampled gas mixture may cause interference with carbon dioxide and nitrous oxide readings. The theory was investigated during evaluation of the Dräger Cicero EM anaesthetic workstation for the Medical Devices Agency. Interference occurred as predicted, and was most pronounced when the vapour analyser of the Cicero EM was deliberately and erroneously set to measure isoflurane, but with sevoflurane present in the gas mixture. With 6% sevoflurane in the gas mixture, the carbon dioxide reading decreased from 5% to 3.6%, and the nitrous oxide reading increased from 0% to 8% although, as the apparent isoflurane reading was 9%, the Cicero EM would alert the operator to the problem. However, operators are encouraged to ensure that, when using gas analysers such as that incorporated into the Cicero EM, the analyser is set to measure the correct volatile anaesthetic (the Cicero EM does this automatically when a Vapor vaporizer is attached) and the breathing system does not contain any other volatile anaesthetic agents.

Anesthesiology↗
PubMed1995

Measurement of propofol concentration in sheep blood and plasma: effect of storage at different temperatures.

In planning a study of the pharmacokinetics of propofol in sheep, contradictions were noted in the literature with regard to loss of propofol during storage of blood samples. This prompted a study of such loss from samples of sheep blood and plasma during storage at room temperature, +4 degrees C and -20 degrees C, for up to 17 days, over a range of concentrations from 1 to 20 micrograms/mL. Samples were drawn from 22 different sheep. Analysis was by the method of Adam et al. (1981). The best estimate of the overall mean loss rate was 0.7% per day with 95% confidence limits of 0.3% to 1.2% per day. The loss rate increased nonsignificantly with storage temperature. There were very small nonsignificant differences of loss rate between plasma and blood, between different concentrations, and between genders. There were significant differences of loss rate between sheep--up to about 2% per day in blood or plasma from any one sheep.

Animals↗
PubMed1995

Non-invasive estimation of venous admixture: validation of a new formula.

We have developed a computer program that estimates venous admixture (intra-pulmonary shunt) from four measurements: haemoglobin concentration, end-tidal carbon dioxide tension (PE'CO2), fractional inspired oxygen concentration (FIO2) and pulse oximetry (SpO2). The formula was tested on patients in an intensive therapy unit by using it to estimate shunt while it was measured simultaneously by a standard, invasive method. A total of 101 measurements were made in 29 patients. After correcting the systematic errors in the assumed differences between PE'CO2 and arterial PCO2, and between SpO2 and co-oximetrically measured SaO2, and correcting for a trend in the arteriovenous oxygen concentration difference (C(a-v))2) with shunt, the bias of the non-invasive minus invasive shunt differences was negligible, with no significant dependence on shunt. The limits of agreement were then +/- 16% shunt overall (+/- 13% within patients). When SaO2 was used instead of SpO2, the limits were +/- 11% (+/- 8% within patients).

Carbon Dioxide↗
PubMed1994

An assessment of prilocaine as a topical anaesthetic agent for fibreoptic bronchoscopy in comparison with lidocaine.

We have evaluated prilocaine as a topical anaesthetic agent for fibreoptic bronchoscopy in comparison with lidocaine in terms of efficacy and safety. Forty patients were included in a randomised double-blind parallel-group study. Efficacy was assessed using visual analogue scales, a patient ranking scale and the number of doses of local anaesthetic and intravenous sedative required. Measures of toxicity included peak plasma concentration of local anaesthetic, whether supplementary oxygen was needed and change in methaemoglobin concentration. For most of the outcome variables, the medians and quartiles were similar for the two local anaesthetics. However, the median peak plasma concentration of prilocaine (0.5 micrograms.ml-1) was less than one-third that of lidocaine (1.76 micrograms.ml-1). The merits and hazards of using multiple-regression modelling to improve the precision of the analysis of the results are considered. We conclude that prilocaine can be used successfully as a topical anaesthetic agent for fibreoptic bronchoscopy and is associated with a lower risk of toxicity.

Adult↗
PubMed1994

A novel method of measuring the concentration of anaesthetic vapours using a dew-point hygrometer.

The Antoine equation relates the saturated vapour pressure of a volatile substance, such as an anaesthetic agent, to the temperature. The measurement of the 'dew-point' of a dry gas mixture containing a volatile anaesthetic agent by a dew-point hygrometer permits the determination of the partial pressure of the anaesthetic agent. The accuracy of this technique is limited only by the accuracy of the Antoine coefficients and of the temperature measurement. Comparing measurements by the dew-point method with measurements by refractometry showed systematic discrepancies up to 0.2% and random discrepancies with SDS up to 0.07% concentration in the 1% to 5% range for three volatile anaesthetics. The systematic discrepancies may be due to errors in available data for the vapour pressures and/or the refractive indices of the anaesthetics.

Anesthesia, Inhalation↗
PubMed1993

A physiological model for the distribution of injected agents, with special reference to pethidine.

The model is based on Mapleson's Model P for inhaled anaesthetics, but has compartments for lungs, peripheral shunt, kidneys, portal bed, liver, other viscera, muscle, other lean, fat, brain, i.m. injection site and for various blood "pools". Intracellular and extracellular fluids are represented separately in each compartment and in blood. In each fluid, four forms of the agent are distinguished: unionized dissolved in water ("standard" form), ionized dissolved in water, unionized dissolved in lipid, and bound to protein. Equations define the equilibrium between these four forms in any one fluid and between blood and tissue. After changing two of the more uncertain numbers in the quantification for pethidine, good agreement was obtained between computed and published venous concentrations after single i.v. and i.m. injections, continuous i.v. infusions and repeated i.m. injections. The model can be used to make a wide variety of "what if" predictions.

Humans↗
PubMed1993

Efficiency of breathing systems A and D in the Carden Ventmasta ventilator.

We have compared the efficiency of the enclosed Magill attachment (System A) and the co-axial System D (Bain) in the Carden Ventmasta ventilator using both systems, each under five different ventilatory conditions, in each of five anaesthetized patients. Efficiency was assessed in terms of the effective alveolar ventilation as a fraction of the fresh gas flow. For System A, efficiency increased from a mean of 0.37 when the total ventilation was only 50% of the fresh gas flow, to a mean of 0.74 when ventilation was 2.3 times the fresh gas flow. The efficiency was substantially and significantly less with System D: 75% of that for System A at the smaller total ventilation (95% CI 65-85%) and 65% at the larger (95% CI 59-71%). A critical examination is made of conflicting definitions and terminology of the efficiency of breathing systems.

Adult↗
PubMed1992

Effects of posture, phonation and observer on Mallampati classification.

We have studied the effects of phonation and posture on the Mallampati classification of view of the pharyngeal structures. Differences between observers were allowed for by the experimental design and log-linear modelling. Sixty-four patients were assessed on the ward, sitting upright, with and without phonation, by each of two observers. Another 64 patients were assessed without phonation, but both upright and supine, again by both observers. Phonation (the patient saying "Ah") produced a marked, systematic improvement of view; moving to the supine posture produced a small, systematic, non-significant worsening of the view. Differences between observers were non-systematic but substantial. About 25% of patients phonated spontaneously. It is recommended that anaesthetists make their own assessments of Mallampati classification, with the patient in either of the postures but always either with or without phonation, and thereby gradually "calibrate" their assessments against the degree of difficulty encountered in intubation.

Humans↗