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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
PubMed1994

Variation in the arterial to end-tidal PCO2 difference during one-lung thoracic anaesthesia.

We have measured the arterial to end-tidal PCO2 difference (PaCO2-PE'CO2) in 22 patients undergoing pulmonary resection in the lateral thoracotomy position during two-lung ventilation (TLV) and after transition to one-lung ventilation (OLV). With OLV for each patient, the practice of correcting the estimate by an initial measurement of (PaCO2-PE'CO2) was evaluated by subtracting the initial (PaCO2-PE'CO2) from subsequent values measured at 10-min intervals. Net (uncorrected) and corrected differences during OLV were analysed using ANOVA. (PaCO2-PE'CO2) values during TLV and OLV were similar: mean (SD) 1.3 (0.6) kPa and 1.2 (0.7) kPa, respectively (ns). Mean (PaCO2-PE'CO2) varied in the range 0.2-2.5 kPa, while maximum (PaCO2-PE'CO2) was 0.3-2.8 kPa. The mean (SD) of 133 pairs of measurements with OLV was 1.1 (0.7) kPa. Even after correction, mean (PaCO2-PE'CO2) varied in the range -0.7 to 0.8 kPa; individual extreme values were from -1.3 to 1.7 kPa. Variation between patients was found to be greater than variation within patients for both net and corrected differences (F ratio = 37.0 and 10.9, respectively), although calculating a corrected difference did reduce variation between patients from a mean square value of 2.44 to 0.61. The wide variation in (PaCO2-PE'CO2) suggests that the accuracy of estimation of PaCO2 by monitoring PE'CO2, although improved by the use of a corrected difference, remains questionable during OLV.

Adult↗
PubMed1994

Failure of pain relief after surgery. Attitudes of ward staff and patients to postoperative analgesia.

Three surveys of postoperative patients and surgical ward staff were conducted in an 840-bed university hospital to ascertain the severity of pain after surgery and the reasons why patients in pain do not receive more of the analgesia prescribed for them. In the first survey, 206 inpatients were questioned within 24 h following operation, and 25.2% of patients experienced moderate pain whilst 9.2% experienced severe pain but received only 36% of their prescribed analgesics. In the second survey, 176 staff reported that the commonest reasons why patients in pain did not receive analgesics were that they did not request analgesia, were too sleepy or refused the dose. In the final survey, of 200 inpatients questioned, 88% had experienced pain since operation, 97% had been offered analgesics, 14% refused a dose. Twenty-nine percent were moderately or very concerned about injections. The beliefs of both ward staff and patients could have contributed to this failure of pain relief.

Adolescent↗
PubMed1993

Effect of age, gender and anaesthetic technique on the pharmacodynamics of atracurium.

We have measured in 38 patients the plasma concentration profile of atracurium and its effect on the electromyographic first response of the train-of-four. One of three techniques was used to supplement anaesthesia with 66% nitrous oxide in oxygen, 0.9% isoflurane (end-tidal), 0.5% halothane (end-tidal) or midazolam 3-10 mg. A four-parameter threshold pharmacodynamic model was fitted to the data in each patient. Compared with a group of patients anaesthetized with an i.v. technique, the steady-state plasma concentration producing 50% block (Cpss50) was reduced by halothane, and to a greater extent by isoflurane. The rate constant for exit from the effect compartment (k(eo)) correlated negatively with age and was greater in female patients, but unaffected by anaesthetic technique. The values of gamma, the slope of the concentration-response curve, and of the threshold (Cpss theta) were not affected significantly by age, sex or anaesthetic technique.

Adolescent↗
PubMed1993

Flow reversal through a Mark III halothane vaporizer.

A fault in the assembly of a Matrix Large Animal Circle anaesthetic machine resulted in reversal of fresh gas flow through the vaporizer. The fault was discovered only after the sudden development of excessive depth of anaesthesia in two equine patients. Laboratory investigations were conducted to determine the effect of flow reversal on vaporizer output. Results indicated that output concentration was approximately doubled under these conditions.

Anesthesia, Closed-Circuit↗
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↗