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Biomedical subjects

J G Lerou

Publications and source records attributed to J G Lerou.

11 recordsLinked to original sources
PubMed1995

A system model for halothane closed-circuit anesthesia. Structure considerations and performance evaluation.

BACKGROUND: Previously, the authors described a physiologic model for closed-circuit inhalational anesthesia. The basic version of this system model was clinically validated for isoflurane. An extended version adopted nonpulmonary elimination causing a constant fraction of anesthetic to be irreversibly lost. This version improved the accuracy of the model for enflurane. The model's performance for other inhalational anesthetics that are not biochemically inert, such as halothane, remained to be evaluated. METHODS: The current study quantified the predictive performance of four versions of the model by comparison of the predicted and measured alveolar halothane concentration-time profiles in 53 patients. Version A did not incorporate nonpulmonary elimination, whereas version D adopted a nonlinear hepatic nonpulmonary elimination following Michaelis-Menten kinetics. A and D used fixed partition coefficients. Their counterparts, A' and D', were formulated to examine the impact of age-adjusted partition coefficients on the accuracy of our model. Each concentration measured by mass spectrometry was compared to four predicted concentrations calculated by four computer simulations (one per version). For each patient, the authors calculated the root mean squared error (rmse; typical error size), bias (systematic component), and scatter of the prediction errors. RESULTS: Fifty-three patients were anesthetized with 330 ml of liquid halothane via 426 bolus injections during more than 61 h; 21,890 alveolar concentrations (average 0.6 vol%) were measured. Version D' showed the best overall performance with an rmse of 19.6 +/- 7.2%, a bias of 0.5 +/- 15.9%, and a scatter of 13.2 +/- 3.5% (mean +/- SD). CONCLUSIONS: The model incorporating nonpulmonary elimination and age-adjusted partition coefficients (D') is sufficiently reliable and accurate to represent halothane closed-circuit anesthesia. This system model, with its various versions, is a valuable tool to predict the dynamics of isoflurane, enflurane, and halothane for clinical, educational, and research purposes.

Adult↗
PubMed1993

General anesthesia for surgical repair of intracranial aneurysm in pregnancy: effects on fetal heart rate.

A 30-year-old nulliparous woman underwent surgery for a ruptured aneurysm of the left vertebral artery in gestational week 27. The fetal heart rate (FHR) was monitored continuously with an abdominal Doppler transducer. Anesthesia was induced with midazolam, fentanyl, and thiopental and maintained with fentanyl, isoflurane, and nitrous oxide 67% in oxygen. Surgery was performed under moderate hypotension (mean arterial pressure +/- 70 mmHg) and moderate hyperventilation (arterial carbon dioxide pressure +/- 33 mmHg). There was a complete disappearance of FHR variability without decelerations or bradycardia. In the night following surgery, the patient was sedated with large parenteral doses of midazolam and fentanyl. Despite this sedation, some FHR variability reappeared within 40 minutes after discontinuation of the inhalation anesthetics. After discontinuation of parenteral midazolam and fentanyl, normal FHR variability returned within 60 minutes. In week 41 of pregnancy, a healthy girl of 4015 gm was born.

Adult↗
PubMed1993

The predictive performance of a system model for enflurane closed-circuit inhalational anesthesia.

BACKGROUND: Previously, the authors described a system model for closed-circuit inhalational anesthesia, and demonstrated close agreement between end-tidal isoflurane concentrations measured in their clinical study and those predicted by the model. The predictive performance of their model has not, however, been tested for anesthetics featuring nonpulmonary elimination (NPE). METHODS: The authors quantified the predictive performance of two versions (A and C) of the model in 50 patients by comparing the predicted and the measured alveolar concentration-time profiles after bolus injections of liquid enflurane into the expiratory limb of the closed system. Version A did not incorporate NPE, but version C emulated NPE by adopting the irreversible loss of a fraction of the enflurane present in the arterial hepatic blood flow (0.131, derived from a mass balance study performed by others). For each concentration measured by mass spectrometry, the authors used computer simulations of version A and C to calculate a predicted concentration for both versions. For each patient, the authors calculated the bias (indicating systematic over- or underprediction) and the scatter of the prediction errors (indicating typical error size). RESULTS: The authors administered a total of 379 ml of liquid enflurane via 466 injections. A total of 18,432 alveolar concentrations (one per 10-s period; average concentration = 0.96 vol%) were measured. The bias and the scatter, both given as mean (and SD), were 10.0 (13.1)% and 11.8 (3.9)% for version A and -0.8 (11.4)% and 11.4 (2.8)% for C. The bias for version C was closer to zero; the scatters were similar. CONCLUSIONS: Version C incorporating NPE performs better than version A. The accuracy that was obtained should encourage the use of version C for clinical, teaching, research, economic, and ecologic purposes.

Adult↗
PubMed1992

Factors affecting magnitude and time course of neuromuscular block produced by suxamethonium.

This study was designed to identify factors that significantly alter the magnitude and duration of suxamethonium-induced neuromuscular block in patients with an apparently normal genotype for pseudocholinesterase. One hundred and fifty-six adults (ages 18-65 yr) were allocated to 13 subgroups. Patients in each subgroup received suxamethonium 50-2000 micrograms kg-1. The mechanographic response of the adductor pollicis brevis muscle to ulnar nerve stimulation was recorded. The ED50 was found to be 167 micrograms kg-1, ED90 was 316 micrograms kg-1 and ED95 was 392 micrograms kg-1. The duration of action (delta t) was in agreement with earlier published results. The magnitude of block was dose-related and decreased with increasing onset time (ton) and pseudocholinesterase activity (PChA). Neither age nor gender affected the degree of suxamethonium-induced block. Delta t was dose-related, decreased with increasing PChA, and was shorter for women. Age and ton had no effect on delta t.

Adolescent↗
PubMed1991

A system model for closed-circuit inhalation anesthesia. II. Clinical validation.

Recently, we described a basic model and its more elaborate variants to predict the uptake and distribution of inhalational anesthetics during closed-circuit anesthesia. As an initial clinical validation of the linear, continuous, 14-compartment basic model, the current study examined its predictive performance in 50 patients by comparing quantitatively the predicted and the measured alveolar concentration-time profiles after bolus injections of liquid isoflurane into the closed system during mechanical ventilation. The two versions of the model studied differed in the size of their peripheral shunt, as 0% (version A) and 16% (version B) of the cardiac output. A total of 15,744 alveolar concentrations of isoflurane (one per 10s period) were measured by mass spectrometry. For each measured concentration we used computer simulations of version A and version B to calculate a predicted concentration for both versions. For each patient we calculated the bias (indicating over- or underprediction) and the scatter of the prediction errors (indicating the typical error size). The bias and the scatter of the prediction errors, both given as mean (and standard deviation), were 2.25 (13.59) and 12.51 (5.84)% for version A and 12.00 (14.97) and 14.12 (6.54)% for B. Version A performed better than B: both the bias (P = 0.008) and the scatter (P less than 0.0001) were closer to zero for A. Logistic regression analysis showed for version A that scatter, but not bias, increased with age (P = 0.002). Gender, body mass index (weight x height-2), and number of injections per hour did not influence scatter or bias.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1991

A system model for closed-circuit inhalation anesthesia. I. Computer study.

Developing a custom computer program to simulate the uptake, distribution, and elimination of inhalational anesthetics allows the anesthesiologist to address specific problems, but extensive skills are required to translate the involved processes first into a set of mathematical equations and then into a satisfactory computer program. The first step is often facilitated by solutions offered in the literature. The second step demands computer proficiency that is often not available, but this problem can be obviated by means of a special-purpose simulation language (SPSL). We therefore constructed a model for closed-circuit inhalation anesthesia with the aid of the block-structured SPSL TUTSIM. Noticeable differences with previous models are that the linear, 14-compartment basic model does not assume a constant alveolar concentration and mimics circulation times through the use of blood pools. Advanced features of the SPSL were used to develop variants of the basic model to simulate feedback-controlled isoflurane administration, nitrous oxide uptake, and the impact of a nonlinearity by incorporating the effect of enflurane on cardiac output. Two variants were concatenated to form a multiple model showing the concentration and second-gas effects. The model was capable of reproducing the anesthetic uptake from previous experimental studies for nitrous oxide. After its validation for other anesthetic agents, the model can be used for clinical, teaching, and research purposes. The SPSL freed the authors from the problems associated with computer programming and allowed them to concentrate on the structure of the model.

Anesthesia, Closed-Circuit↗
PubMed1990

Evaluation of long sampling tubes for remote monitoring by mass spectrometry.

The long sampling tubes required for remote mass spectrometry alter the sampling system's performance characterized by sample flow, residence time, and 10 to 90% response time. We searched for an easy-to-handle tube with (1) a length of 30 m, (2) sample flow less than 50 ml.min-1, and (3) residence and response times approaching those predicted by our mathematical model. We tested tubes of various geometries and various commercially available materials by using them as inlet catheters for a quadrupole mass spectrometer (Centronic 200 MGA, Centronic Ltd, Craydon, UK). We measured their responses at 0 to 10% (on transients) and 10 to 0% (off transients) step changes in gas concentration for nitrogen, argon, nitrous oxide, oxygen, and carbon dioxide and 0 to 3% and 3 to 0% for halothane, enflurane, and isoflurane. With 5 polyethylene tubes, halothane response times were up to 38 times longer than predicted. One 30-m polyethylene tube combined a 158-ms response time for nitrogen and argon with a 2,205-ms response time for halothane. Teflon, polyvinyl chloride, and stainless steel also proved to be unsuitable because of unacceptable signal distortion: the carbon dioxide response time for a 30-m Teflon tube was 2,600 ms. A glass tube showed the least signal distortion but was hard to handle. Our requirements were fulfilled by a 29.77-m tube made from nylon with a 1.00-mm inside diameter to which a 0.23-m length of nylon with a 0.25-mm inside diameter was added at the patient end. It offers (1) sample flow equals 46 ml.min-1, (2) residence time equals 11.1 seconds, and (3) response times approaching our theoretical predictions, that is, 159, 164, 180, 159, 188, 302, 298, and 300 ms (means of on and off transients) for nitrogen, argon, nitrous oxide, oxygen, carbon dioxide, halothane, enflurane, and isoflurane, respectively. This tube allows the accurate monitoring of breathing frequencies up to 25 and 50 breaths/min for volatile agents and gases, respectively.

Anesthetics↗
PubMed1988

Automated charting of physiological variables in anesthesia: a quantitative comparison of automated versus handwritten anesthesia records.

Eight physiological variables--tidal volume, breathing rate, end-tidal carbon dioxide fraction, oxygen fraction in the anesthetic circuit, oxygen saturation by pulse oximetry, systolic and diastolic blood pressure, and heart rate--recorded on-line by a commercially available automated system were compared with the same variables recorded on handwritten anesthesia records. We quantified the differences between the automated and handwritten records generated from the same 30 patients (2,412 minutes of general anesthesia for elective eye surgical procedures). Considering the design of the study, we claim that the differences between both records were caused by the incompleteness or inaccuracy of the handwritten records, except in two instances. The amounts of missing or erroneous data for these eight physiological variables were expressed as fraction ("error fractions") of the time being recorded, designated EFm and EFe, respectively. For the first five variables the EFm on the handwritten records ranged between 0.23 and 0.31, and the EFe ranged between 0.01 and 0.06. For the last three variables the EFm range was 0.08 to 0.13, and the EFe range was 0.05 to 0.11. Most of these missing or erroneous data occurred during the period of induction (first 15 minutes) and at the end of the case (last 10 minutes). The EFm and EFe during induction had increased to 0.62 and 0.26, respectively, and to 0.76 and 0.06, respectively, at the end of the case. Erroneous data were observed on the automated records for the tidal volume during induction (EFe = 0.0044) and for the oxygen fraction during maintenance (EFe = 0.0024). The effect of averaging by the recordkeeper is discussed. The results of this study indicate the clinical relevance of automated record keeping.

Anesthesia↗
PubMed1987

The clinical use of the Ohmeda Automated Anesthesia Record Keeper integrated in the Modulus II Anesthesia System. A preliminary report.

While performing his complex array of tasks, the anesthesiologist is also responsible for maintaining an anesthetic record. Up to now, this has been done by hand. The clinical use of automated anesthesia record keeping is presently evaluated. The anesthesia records generated by the Ohmeda Automated Anesthesia Record Keeper integrated in the Modulus II Anesthesia System is compared to hand written records. The differences between the two records of identical patients are quantified as erroneous or missing data. With the criteria adapted, we found significant and clinically relevant differences which stress the importance of automated record keeping.

Anesthesiology↗
PubMed1986

The influence of tube geometry on the performance of long sampling tubes in respiratory mass spectrometry.

Long sampling tubes allow remote patient monitoring by mass spectrometry. The choice of adequate tube geometry and material, which may minimise signal distortion and signal transmission delay, needs quantification of the relations between tube dimensions and factors determining tube performance (sample flow, residence time, response time). A mathematical model is presented which considers ideal tubes (no gas-tube interaction) with various geometries. It is shown for tubes with length L and uniform radius r that the response time for nitrogen estimated by the expression 0.0046 L will differ less than 10% from the exact value if L/r3 less than 1.5 X 10(12). The limited number of commercially available radii prevents free choice of sample flow for uniform tubes of a given length. However, tubes consisting of two parts with different diameters can provide any desired sample flow. The model indicates that residence and response times are effectively reduced if optimum radii are used for these two parts, and that a tube with an exponentially rising radius as a function of distance may halve residence and response times, compared with a uniform tube providing identical sample flow. Model predictions can be used to judge experimental results obtained with real tubes made of various materials. Experimental data in corroboration of the model are presented.

Anesthesiology↗