Biomedical subjects

A M Zbinden

Publications and source records attributed to A M Zbinden.

At least 37 records · Page 2Linked to original sources

PubMed1997

Epidural epinephrine and clonidine: segmental analgesia and effects on different pain modalities.

BACKGROUND: It is not known whether epidural epinephrine has an analgesic effect per se. The segmental distribution of clonidine epidural analgesia and its effects on temporal summation and different types of noxious stimuli are unknown. The aim of this study was to clarify these issues. METHODS: Fifteen healthy volunteers received epidurally (L2-L3 or L3-L4) 20 ml of either epinephrine, 100 microg, in saline; clonidine, 8 microg/kg, in saline; or saline, 0.9%, alone, on three different days in a randomized, double-blind, cross-over fashion. Pain rating after electrical stimulation, pinprick, and cold perception were recorded on the dermatomes S1, L4, L1, T9, T6, T1, and forehead. Pressure pain tolerance threshold was recorded at S1, T6, and ear. Pain thresholds to single and repeated (temporal summation) electrical stimulation of the sural nerve were determined. RESULTS: Epinephrine significantly reduced sensitivity to pinprick at L1-L4-S1. Clonidine significantly decreased pain rating after electrical stimulation at L1-L4 and sensitivity to pinprick and cold at L1-L4-S1, increased pressure pain tolerance threshold at S1, and increased thresholds after single and repeated stimulation of the sural nerve. CONCLUSIONS: Epidural epinephrine and clonidine produce segmental hypoalgesia. Clonidine bolus should be administered at a spinal level corresponding to the painful area. Clonidine inhibits temporal summation elicited by repeated electrical stimulation and may therefore attenuate spinal cord hyperexcitability.

Adrenergic alpha-Agonists↗

PubMed1996

Analgesic effect in humans of subanaesthetic isoflurane concentrations evaluated by evoked potentials.

The aim of this study was to see if an analgesic effect of subanaesthetic concentrations of isoflurane could be detected with evoked potentials elicited by nociceptive stimuli. We studied 10 healthy volunteers breathing three steady-state subanaesthetic concentrations of isoflurane (0.08, 0.16 and 0.24 vol% end-tidal). Reaction time, subjective pain intensities and evoked vertex potentials to laser (LEP) and electrical (SEP) stimuli were recorded and compared with auditory evoked potentials (AEP). Compared with baseline, the subanaesthetic concentrations of isoflurane did not change the latencies of the evoked potentials, but caused a significant reduction in the amplitudes of the LEP and SEP at 0.16 and 0.24 vol% and of the AEP at all three concentrations. There were no changes in perceived pain intensity, and isoflurane produced similar reductions in evoked potentials elicited by both nociceptive and non-nociceptive stimuli. The reaction time was increased significantly at 0.24 vol% isoflurane. The results demonstrated that subanaesthetic isoflurane concentrations caused similar changes in evoked potentials with both painful and non-painful stimuli, with no effect on perceived pain intensity.

Acoustic Stimulation↗

PubMed1996

Fuzzy logic control of inspired isoflurane and oxygen concentrations using minimal flow anaesthesia.

In order to evaluate the performance of feedback fuzzy logic control of inspired oxygen and isoflurane concentrations, we studied 30 patients undergoing discectomy for lumbar (n = 26) or cervical (n = 4) disc herniation. Patients were allocated random to one of two groups: a standard group (n = 15) with low flow anaesthesia (1.2-1.3 litre min-1) and manual control of gas concentrations; and a fuzzy group (n = 15) with minimal flow (0.5 litre min-1) and fuzzy logic feedback control of gas concentrations. Fuzzy logic control achieved and maintained very accurately the desired isoflurane concentration. Oxygen concentration was controlled more precisely than in the standard group. Delivery and costs of oxygen and nitrous oxide were significantly lower in the fuzzy group (P < 0.01). Accumulation of foreign gases was observed in one patient during low flow and in 11 patients during minimal flow anaesthesia. In conclusion, fuzzy logic control of inspired oxygen and isoflurane concentration during minimal flow anaesthesia was reliable and reduced anaesthetic gas delivery and costs.

Adult↗

PubMed1996

Psychophysical and electrophysiological responses to experimental pain may be influenced by sedation: comparison of the effects of a hypnotic (propofol) and an analgesic (alfentanil).

Sedation may influence the responses of some experimental pain models used to test analgesic efficacy. In this study we compared the effects of a sedative (propofol) and analgesic (alfentanil) on: nociceptive reflex to single and repeated electrical stimulations; mechanical pressure pain; and evoked potentials elicited by nociceptive (electrical and laser) and non-nociceptive (acoustical) stimulation. We studied 12 healthy volunteers with two subanaesthetic concentrations of propofol and two analgesic concentrations of alfentanil. Both propofol and alfentanil increased the threshold for nociceptive reflex to single electrical stimulations, but only alfentanil increased the threshold for nociceptive reflex to repeated electrical stimulations. The pressure pain tolerance thresholds were increased significantly by alfentanil, whereas propofol significantly decreased the thresholds (hyperalgesia). Propofol and alfentanil induced similar reductions in the amplitudes of the evoked potentials elicited by nociceptive (electrical and laser) and non-nociceptive (acoustical) stimulation, whereas only alfentanil reduced the perceived pain to nociceptive stimulations. We have shown that sedation can influence both the psychophysical and electrophysiological responses of some experimental pain tests used to measure analgesic efficacy, and that propofol in subhypnotic doses, has no analgesic effect on painful electrical and heat stimulations, but has a hyperalgesic effect on mechanical pressure pain.

Adult↗

PubMed1996

Effect of racemic mixture and the (S+)-isomer of ketamine on temporal and spatial summation of pain.

We have compared the analgesic efficacy of the racemic mixture and the stereoisomer (S+) of the NMDA antagonist ketamine. In a double-blind, three-way crossover, placebo-controlled study, we assessed the following: pain evoked by small/large area pressure stimuli, pain detection threshold and pain ratings to small/large area of heat stimuli, pain detection threshold and pain rating to heat stimuli of brief/long duration, summation pain threshold and pain ratings to repeated heat/electrical stimuli, side effects and reaction time. Plasma concentrations of 350 ng ml-1 for ketamine (racemic) and 180 ng ml-1 for ketamine (S+) were tried. We found that ketamine (racemic) prolonged the reaction time more than ketamine (S+). Both drugs affected pain caused by repeated stimuli or stimuli of long duration equally or more than a single stimulus of short duration. They also affected pain evoked from large areas equally or more than pain evoked from small areas. The (S+)-isomer was approximately twice as potent as the racemic mixture of ketamine in inhibiting central summation.

Adult↗

PubMed1996

Fuzzy logic control of mechanical ventilation during anaesthesia.

We have examined a new approach, using fuzzy logic, to the closed-loop feedback control of mechanical ventilation during general anaesthesia. This control system automatically adjusts ventilatory frequency (f) and tidal volume (VT) in order to achieve and maintain the end-tidal carbon dioxide fraction (FE'CO2) at a desired level (set-point). The controller attempts to minimize the deviation of both f and VT per kg body weight from 10 bpm and 10 ml kg-1, respectively, and to maintain the plateau airway pressure within suitable limits. In 30 patients, undergoing various surgical procedures, the fuzzy control mode was compared with human ventilation control. For a set-point of FE'CO2 = 4.5 vol% and during measurement periods of 20 min, accuracy, stability and breathing pattern did not differ significantly between fuzzy logic and manual ventilation control. After step-changes in the set-point of FE'CO2 from 4.5 to 5.5 vol% and vice versa, overshoot and rise time did not differ significantly between the two control modes. We conclude that to achieve and maintain a desired FE'CO2 during routine anaesthesia, fuzzy logic feedback control of mechanical ventilation is a reliable and safe mode of control.

Adolescent↗

PubMed1996

Factors associated with hypotension and bradycardia after epidural blockade.

In order to identify patient-, anesthesia-, and surgery-related factors influencing the probability of hypotension and bradycardia after epidural blockade, an observational study was conducted on 1050 nonpregnant patients. Backward stepwise logistic regression was performed on the variables hypotension (systolic blood pressure < 90 mm Hg) and bradycardia (heart rate < or = 45 bpm). Hypotension and bradycardia occurred in 158 and 24 patients, respectively. The probability of hypotension increased when epidural fentanyl was administered (odds ratio [OR] = 2.18; 95% confidence interval [CI] = 1.16-4.11), with body weight and spread of epidural analgesia, and decreased when a tourniquet was used (OR = 0.01, CI = 0.01-0.02) and bupivacaine instead of carbonated lidocaine was administered (OR = 0.28, CI = 0.14-0.60). Sensitivity and specificity of the model were 89% and 88%, respectively. The probability of bradycardia was less in women (OR = 0.05, CI = 0.01-0.41) and when a tourniquet was used (OR = 0.04, CI = 0.02-0.09). Sensitivity and specificity were 50% and 97%, respectively. In conclusion, our analysis can contribute to identification of patients at high risk to develop hypotension and bradycardia after epidural blockade. If bupivacaine instead of carbonated lidocaine is used and epidural fentanyl is not administered a decrease in the incidence of hypotension may be anticipated.

Adjuvants, Anesthesia↗

PubMed1996

Five oxygen-nitrous oxide proportioning systems compared.

The majority of contemporary gas delivery modules, on anaesthetic workstations, are equipped with oxygen-nitrous oxide proportioning systems which should prevent the delivery of hypoxic gas mixtures. We investigated five modules, of two different types, using fresh gas flows ranging from 50 mL min-1 to 20 L min-1 with minimally acceptable proportions of oxygen and maximally acceptable proportions of nitrous oxide set at the flow control valves. The oxygen concentrations of the resulting gas mixtures were measured at the fresh gas outlet of the gas delivery modules. All the modules prevented the delivery of hypoxic fresh gas flows both under normal and abnormal working conditions (unequal supply of gas pressures). All systems showed increased O2 concentrations at fresh gas flows below 1 L min-1. The systems can be used with low flow anaesthesia techniques with one exception (Dameca) also with minimal flow techniques.

Anesthesiology↗

PubMed1995

The accuracy of the flowrate in flush-devices of disposable pressure transducers.

BACKGROUND: Arterial and venous pressure is commonly measured using fluid filled catheters. To avoid obstruction they are continuously spilled by a flush-device. The accuracy of the flowrate has not been investigated previously. METHODS: The accuracy of 5 different flush-devices available in Switzerland was checked for flowrate when factory new, after a single sterilization with ethylen-oxide, in a long-term test over 96 hours, after repeated handling of the integrated bypass and under application of a pulsing counter-pressure. RESULTS: Flow is linearly related to differential-pressure and is constant over time. The flow of each flush-device at 200 mmHg differential-pressure was below the indicated 3 ml per hour (1.69 - 2.49 ml/h). Sterilization in two types produced a significant but not relevant difference in flowrate. Longtime-use, bypass actuation and pulsing pressure did not alter the flowrate significantly (p <0.05). In two factory-new flush-devices and 4 re-used ones a plugged capillary induced cessation of flow. CONCLUSIONS: Flowrate in flush-devices is accurate under sterilization, longtime use, bypass-actuation and pulsing counter pressure. A plugged capillary occurred in a few new and reused flush-devices, which can be the explanation for clotted catheters in clinical use.

Blood Pressure Determination↗

PubMed1995

Carbon dioxide analysers: accuracy, alarm limits and effects of interfering gases.

Six mainstream and twelve sidestream infrared carbon dioxide (CO2) analysers were tested for accuracy of the CO2 display value, alarm activation and the effects of nitrous oxide (N2O), oxygen (O2) and water vapour according to the ISO Draft International Standard (DIS)#9918. Mainstream analysers (M-type): Novametrix Capnogard 1265; Hewlett Packard HP M1166A (CO2-module HP M1016A); Datascope Passport; Marquette Tramscope 12; Nellcor Ultra Cap N-6000; Hellige Vicom-sm SMU 611/612 ETC. Sidestream analysers: Brüel & Kjaer Type 1304; Datex Capnomac II; Marquette MGA-AS; Datascope Multinex; Ohmeda 4700 OxiCap (all type S1: respiratory cycles not demanded); Biochem BCI 9000; Bruker BCI 9100; Dräger Capnodig and PM 8020; Criticare Poet II; Hellige Vicom-sm SMU 611/612 A-GAS (all type S2: respiratory cycles demanded). The investigations were performed with premixed test gases (2.5, 5, 10 vol%, error < or = 1% rel.). Humidification (37 degrees C) of gases were generated by a Dräger Aquapor. Respiratory cycles were simulated by manually activated valves. All monitors complied with the tolerated accuracy bias in CO2 reading (< or = 12% or 4 mmHg of actual test gas value) for wet and dry test gases at all concentrations, except that the Marquette MGA-AS exceeded this accuracy limit with wet gases at 5 and 10 vol% CO2. Water condensed in the metal airway adapter of the HP M1166A at 37 degrees C gas temperature but not at 30 degrees C. The Servomex 2500 (nonclinical reference monitor), Passport (M-type), Multinex (S1-type) and Poet II (S2-type) showed the least bias for dry and wet gases. Nitrous oxide and O2 had practically no effect on the Capnodig and the errors in the others were max. 3.4 mmHg, still within the tolerated bias in the DIS (same as above). The difference between the display reading at alarm activation and the set point was in all monitors (except in the Capnodig: bias 1.75 mmHg at 5 vol% CO2) below the tolerated limit of the DIS (difference < or = 0.2 vol%). The authors conclude that the tested monitors are safe for clinical used (except those failing the DIS limits). The accuracy of the CO2-reading (average of mean absolute bias) is better in the M-type than in the S1- or S2-type analysers although no statistical (nor clinical) significant differences could be detected. Most manufacturers work with stricter limits than those proposed by the DIS.

Blood Gas Analysis↗

PubMed1995

Arterial pressure control with isoflurane using fuzzy logic.

Arterial pressure is still one of the most important measures in estimating the required dose of inhaled anaesthetics. It is measured easily and reacts rapidly which makes it suitable as a variable for feedback control of depth of anaesthesia. Fuzzy logic, a novel approach to feedback control, was used to control arterial pressure in 10 patients during intraabdominal surgery by automatic adjustment of the concentration of isoflurane in fresh gas. During anaesthesia, fuzzy control periods of 45-min duration were alternated randomly with human control periods of equal duration. During the skin incision period (-3 to + 12 min) 48.2% of all fuzzy control pressure values were within +/- 10% of the desired mean arterial pressure compared with 40.4% of the human control values (P < 0.05). The corresponding values for the remainder of the operation were 78.3% and 83.2%, respectively. Thus fuzzy out-performed human control at skin incision, but was slightly inferior during the rest of the operation. We conclude that fuzzy logic is a promising new technique for control of isoflurane delivery during routine anaesthesia.

Adult↗

PubMed1995

Analgesic effect in humans of subanaesthetic isoflurane concentrations evaluated by experimentally induced pain.

The analgesic effect of subanaesthetic concentrations of ether, trichloroethylene, methoxyflurane and halothane has been investigated previously using either clinical assessment or pain threshold measurements, but with conflicting results. The purpose of the present study was to evaluate the analgesic effect of isoflurane using experimental pain models. We studied 12 healthy volunteers at three randomly chosen subanaesthetic isoflurane concentrations: low (0.10-0.14 vol%), middle (0.16-0.20 vol%) and high (0.22-0.26 vol%). We used thermal pain detection and pain tolerance thresholds to argon laser stimulation, pressure pain detection and pain tolerance thresholds, immersion of the hand in ice water, and the nociceptive reflex to single and repeated (temporal summation) electrical stimulations, as experimental models to assess analgesia. There were no significant changes in the response to heat, cold or mechanical pressure at any of the subanaesthetic concentrations of isoflurane used. The nociceptive reflex thresholds to single stimulations, but not the thresholds for repeated stimulations, were significantly increased in all three isoflurane groups compared with baseline values. The difference between the different isoflurane concentrations was not statistically significant. In experimental pain models, subanaesthetic isoflurane concentrations have little or no analgesic potency.

Adult↗

PubMed1995

A multifactorial analysis to explain inadequate surgical analgesia after extradural block.

A multivariate analysis of inadequate extradural analgesia was carried out prospectively on 1051 patients undergoing lumbar extradural anaesthesia for surgery performed on structures innervated by T10-S5. Ninety-six patients (9%) experienced pain during surgery. Age, extradural fentanyl, diazepam sedation and duration of surgery had no significant influence. We found some weak evidence that the type of surgery affects the risk of feeling pain. The probability of pain increased with increasing weight, except in overweight women, and was significantly greater for both shorter and taller patients, relative to patients of average height. The probability of pain decreased with increasing dose of local anaesthetic, increasing spread of extradural analgesia, addition of adrenaline, and fentanyl or thiopentone sedation. In conclusion, patient-, surgery- and anaesthesia-related factors influence the risk of inadequate extradural analgesia. If such factors are taken into account, an increase in the success rate may be anticipated.

Adolescent↗

PubMed1995

Temporal summation during extradural anaesthesia.

We have investigated in 10 patients the effect of extradural anaesthesia on temporal summation by comparing pain thresholds to single and repeated (five impulses at 2 Hz) electrical stimuli and compared these tests with pinprick and cold stimulation. Bupivacaine 0.5% (20 ml) was injected at L2-3. After extradural anaesthesia the threshold to repeated stimuli was significantly lower than the threshold to single stimuli (P = 0.0007). Nine patients lost cold sensation and 10 patients pinprick sensation. Pain to single electrical stimulation disappeared in six patients and pain to repeated electrical stimulation in one. Pain may be evoked by temporal summation of repeated electrical stimuli even when pinprick sensation, cold sensation and pain to single electrical stimuli are inhibited. Thus temporal summation should be taken into consideration when extradural analgesia is assessed.

Adult↗

PubMed1995

The effect of N-methyl-D-aspartate antagonist (ketamine) on single and repeated nociceptive stimuli: a placebo-controlled experimental human study.

Ketamine is a noncompetitive N-methyl-D-aspartate (NMDA) receptor channel blocker known to inhibit "wind-up" and hence central hyperexcitability of dorsal horn neurons. We sought to assess the effect of ketamine on single and repeated nociceptive stimuli. A placebo-controlled, human (12 volunteers) experimental study was conducted in which several psychophysical (pain detection and tolerance thresholds, magnitude ratings) and electrophysiologic (withdrawal reflex) techniques were used 1) to investigate whether a ketamine (0.5 mg/kg) bolus followed by a 20-min infusion (9 micrograms.kg-1.min-1) inhibits central temporal summation to repeated nociceptive electrical stimuli, and 2) to assess quantitatively the hypoalgesic potency using several experimental nociceptive stimuli (argon laser, pressure, electrical). Facilitation of the withdrawal reflex to and pain rating of repeated electrical stimuli (five pulses at 2 Hz) were inhibited by ketamine. Reflex and pain rating to a single stimulus did not change. The pressure pain detection and tolerance thresholds were increased significantly by ketamine, whereas the laser heat pain and tolerance thresholds remained stable compared with placebo. The stimulus response function showed that ketamine reduced the responses to the highest electrical stimulus intensities (1.4, 1.6, and 1.8 times the reflex threshold). We conclude that ketamine inhibits central temporal summation in humans and has a marked hypoalgesic effect on high intensity nociceptive stimuli.

Analgesics↗

PubMed1995

Resistance, reverse flow and opening pressure of unidirectional valves.

Based on a new preliminary standard of the 'Comité Européen de Normalisation', the following unidirectional valves were tested with regard to resistance, opening pressure, reverse flow and dislocation: the Dräger inspiratory and expiratory valves, the Engström inspiratory valve, the Ohmeda valve, the Siemens Ventilator 710 inspiratory and expiratory valves, the Siemens Ventilator 900C unidirectional valve of the absorber and the Megamed 700 inspiratory and expiratory valves of the circle system 219 (Megamed 700 CS 219). The following valves fulfilled all Comité Européen de Normalisation requirements: Dräger inspiratory and expiratory valves, Siemens 900 absorber valve and Megamed 700 CS 219 inspiratory valve. The Siemens 710 valve and the Megamed 700 CS 219 expiratory valve did not meet the requirements for flow resistance. The Ohmeda and Siemens 710 valves and the Engström inspiratory valve did not fulfil the Comité Européen de Normalisation requirements for reverse flow. In addition, the Engström inspiratory valve did not comply with the dislocation test. The requirements for the opening pressure were met by all the valves tested. Valves with the disc in a horizontal position achieved better results than those with the disc in a vertical position. These measurements, showing the differences in the performance of various types of valves confirm the feasibility of the standards proposal.

Anesthesiology↗

PubMed1995

Effect of partial pressure on solubility of isoflurane in blood.

The effect of partial pressure of isoflurane on its solubility in blood of different haematocrit was determined at different isoflurane partial pressures, using a syringe--flask method and gas chromatography with blood from one donor. The solubility of isoflurane decreased with increasing blood haematocrit (39.5%, 51.9% and 61.9%) and isoflurane partial pressure from 1 MAC to 4 MAC. The solubility in 50% and 60% haematocrit blood at 2, 3 and 4 MAC was significantly different from the solubility in normal blood at 1 MAC (P < 0.05). We conclude that the solution of isoflurane in blood (especially in high haematocrit blood) does not obey Henry's law.

Adult↗

PubMed1994

Automated determination of midazolam in human plasma by high-performance liquid chromatography using column switching.

An automated gradient high-performance liquid chromatographic method using a column-switching technique was developed in order to determine and quantify midazolam (separated from the metabolite alpha-hydroximidazolam) in human plasma. After dilution with an internal standard (flurazepam) solution, containing 20% acetonitrile, 400 microliters of the plasma samples were injected onto a precolumn (17 x 4.6 mm I.D., C18 Corasil 37-53 microns) and retained. Proteins and polar plasma components were washed out using a 0.1 M sodium hydroxide solution, followed by an equilibration with a phosphate buffer of pH 8.0. After column-switching midazolam and flurazepam were eluted and transferred to the analytical column (RP-select B) in the backflush mode, separated by gradient elution and detected at 230 nm by ultraviolet detection. Precision of replicate analyses on the same day was 1.5% for midazolam and 0.7% for flurazepam. Recovery of midazolam was in the range 80-89% and the detection limit was 2 ng/ml plasma.

Autoanalysis↗