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Biomedical subjects

S L Snowdon

Publications and source records attributed to S L Snowdon.

At least 19 recordsLinked to original sources
PubMed1992

Effect of age, sex and anaesthetic technique on the pharmacokinetics of atracurium.

We have defined the pharmacokinetics of atracurium besylate 0.25 mg kg-1 in 41 patients anaesthetized with 0.9% isoflurane end-tidal, 0.5% halothane end-tidal or midazolam 3-10 mg as a supplement to 66% nitrous oxide in oxygen. The pharmacokinetic profile was affected by age, sex and anaesthetic technique. Advancing age was associated with a reduced clearance and a longer elimination half-time; clearance was greater and elimination half-time was shorter in males than in females. Clearance was also greater in patients anaesthetized with isoflurane than with the two other techniques. Age, sex and anaesthetic technique did not significantly affect the volume of distribution.

Adolescent↗
PubMed1991

Blood/gas solubility coefficient and blood concentration of enflurane during normothermic and hypothermic cardiopulmonary bypass.

The blood/gas solubility coefficient and blood concentration of enflurane were measured at intervals in 10 patients undergoing coronary artery revascularization with cardiopulmonary bypass (CPB) and moderate hypothermia. A constant end-tidal concentration of enflurane was maintained throughout the study. Blood/gas solubility coefficient was determined at 37 degrees C, which when combined with an initial single-step equilibration of the blood sample with air, permitted the accurate measurement of blood concentration. Blood/gas solubility coefficient and blood concentration both decreased significantly with the onset of CPB. During the period of hypothermia, blood/gas solubility as measured at 37 degrees C showed little change; however, there was a progressive, marked increase in blood concentration with a mean increase of 80% prior to rewarming. Therefore, the level of anesthesia provided by enflurane may lighten with the onset of CPB, and a deeper level will accompany any decrease in blood temperature. On rewarming, blood concentration levels rapidly returned to levels similar to those measured before cooling. The increased uptake and accumulation of volatile anesthetic agent that occurred as a result of the period of hypothermic CPB was rapidly cleared. The rapidity with which blood concentration responded to the changes occurring during CPB make it unlikely that there was any significant increase in myocardial depression in response to the raised blood concentration secondary to the hypothermia.

Anesthesia, Inhalation↗
PubMed1991

Nitrous oxide and internal tamponade during vitrectomy.

We analysed the nitrous oxide composition of the intraocular gas bubble following vitrectomy and fluid-air exchange in 12 patients. Samples were taken under standardised conditions at 20 minutes after completion of the fluid-air exchange. Analysis was conducted by a Medishield MS2 mass spectrometer. The percentage composition of nitrous oxide in the samples varied between 4 and 21% (mean 9%). This influx of nitrous oxide was associated with an increase of intraocular pressure. Pressure rises of greater than 20 mm Hg were not seen owing to venting of gas through the sclerostomies. An inverse relationship was noted between the extent of retinal detachment preoperatively and the amount of nitrous oxide entering the eye. A possible explanation for this relationship is proposed. The importance of nitrous oxide movement is stressed.

Adolescent↗
PubMed1990

Laboratory evaluation and use of the Engström metabolic computer in the clinical setting.

Laboratory evaluation and use of the Engström metabolic computer (EMC) in a clinical setting are described. The accuracy of the EMC was tested using an inert gas dilution technique. Mean errors in oxygen consumption (VO2) and CO2 production compared with predicted values were less than 2% of predicted, and generally less than 1%, but with an SD of less than or equal to 5.1%. At an FIO2 of greater than or equal to 0.7, the errors in VO2 were erratic and generally greater than 15%. The apparatus has the facility to assume an RQ in the calculation of VO2 and the errors in measured VO2 were less than 1% up to an FIO2 of 0.8.

Calibration↗
PubMed1988

Predicted and measured oxygen concentrations in the circle system using low fresh gas flows with oxygen supplied by an oxygen concentrator.

An oxygen concentrator based on a zeolitic molecular sieve has been used to supply piped oxygen in a District General Hospital; such oxygen contains 5% argon. If concentrator oxygen is used in the closed circuit, argon accumulates. The present investigation defines the extent and time course of argon accumulation in partially and fully closed breathing systems. A theoretical model is presented which predicts the extent and rate of inert gas accumulation. The predictions have been tested in a volunteer under laboratory conditions and in five anaesthetized patients. It is recommended that concentrator oxygen may be used for low flow anaesthesia of indefinite duration, provided the fresh oxygen flow to the circuit is at least twice the oxygen consumption. If the circuit is totally closed, periodic opening of the circuit is necessary.

Adult↗
PubMed1987

Potentiation of the neuromuscular blockade produced by alcuronium with halothane, enflurane and isoflurane.

The potentiation of alcuronium by halothane, enflurane and isoflurane was investigated using electromyography. In the first study, cumulative dose-response curves were constructed in four groups of 10 patients anaesthetized with one of the inhalation agents and nitrous oxide, or with fentanyl and droperidol (control). All three agents reduced the ED50 of alcuronium; the effect was marked with isoflurane (P less than 0.005) but less so with halothane (P less than 0.05) and enflurane (ns). In the second part of the investigation, designed primarily to test the duration of action of alcuronium with each agent, a single bolus dose of alcuronium 0.2 mg kg-1 was given to four similar groups (n = 5). The duration of action was significantly prolonged by enflurane (P less than 0.01) and isoflurane (P less than 0.05), but not by halothane. The possible reasons for this are discussed.

Action Potentials↗
PubMed1987

Pre-oxygenation and the parturient patient.

The effects of pre-oxygenation were studied by continuous expired oxygen analysis in twenty pregnant patients and ten nonpregnant controls. The Magill system and a demand valve breathing system were both studied with tidal and vital capacity breathing. Rapid pre-oxygenation was possible with the demand valve system but the technique was associated with air leaks around the mask. Vital capacity breathing with the Magill system was inefficient in practice because of rebreathing and air leaks, and the recently proposed four vital capacity breath technique provided suboptimal alveolar oxygenation in pregnant subjects. Oxygenation in pregnant subjects was significantly faster than in nonpregnant controls and was achieved after just 2 minutes of tidal breathing from a standard Magill system with a prefilled reservoir. Care was necessary to ensure a gas-tight seal around the facemask.

Adult↗
PubMed1986

Medical gas probes.

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Equipment Safety↗
PubMed1986

Experimental investigation of the cardiovascular and respiratory effects of increasing concentrations of isoflurane in the dog.

The cardiovascular and respiratory effects of the new inhalational anaesthetic agent isoflurane were investigated in dogs. Anaesthesia was induced with thiopentone after premedication with acepromazine. Isoflurane was administered with nitrous oxide and oxygen by spontaneous ventilation after base line values had been determined. Arterial blood pressure decreased as the concentration of administered isoflurane increased. Isoflurane produced a profound and dose related respiratory depression as measured by the increase in end tidal carbon dioxide levels. Isoflurane administration did not produce any visible muscle twitching.

Animals↗