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Biomedical subjects

G G Lockwood

Publications and source records attributed to G G Lockwood.

36 records · Page 2Linked to original sources
PubMed1996

The relationship between anaesthetic uptake and cardiac output.

The hypothesis that anaesthetic uptake during maintenance of anaesthesia is related to cardiac output was tested on 21 patients undergoing cardiac surgery. Using a computer-controlled closed breathing system, enflurane was administered to maintain an end-expired concentration of 1%. Cardiac output was measured by thermodilution using a pulmonary artery catheter. A clear qualitative but not quantitative relationship was demonstrated. Changes in anaesthetic requirements at a constant end-expired concentration are a better guide to changes in cardiac output than changes in end-expired carbon dioxide with constant ventilation in patients undergoing cardiac surgery.

Anesthetics, Inhalation↗
PubMed1996

Uptake of desflurane during anaesthesia.

The amount of desflurane required to maintain an end-expired concentration of 8% was measured in 30 ASA 1 and 2 patients undergoing elective spinal surgery. The anaesthetic was administered using a computer-controlled closed circle system. After an initial period during which the expired concentration of desflurane was stabilised (4 min) the rate of uptake showed a bi-exponential decline. Mean cumulative usage of desflurane was 10.1 ml of liquid at 30 min, 14.8 ml at 60 min, 25.4 ml at 120 min, 35.8 at 180 min.

Anesthesia, Closed-Circuit↗
PubMed1996

The estimation of inspired desflurane concentration in a low-flow system.

We have examined the predictability of inspired desflurane concentration during low-flow anaesthesia using a to-and-fro breathing system. Twenty-two adult patients requiring mechanical ventilation of the lungs during surgery took part in the study. Using a fresh gas flow of 1 l.min-1, the ratio of inspired desflurane concentration to desflurane vaporizer setting was found to be approximately 0.75 after 9 min of anaesthesia and at 2 l.min-1 fresh gas flow the ratio was approximately 0.9 after 2 min of anaesthesia. These ratios were maintained throughout the procedure, except for a few minutes following each change in vaporizer setting.

Adolescent↗
PubMed1995

The uptake of enflurane during anaesthesia.

The uptake of enflurane at a constant end-expired concentration of 2% in oxygen was studied in 38 patients, ASA 1 or 2, undergoing elective orthopaedic procedures. The anaesthetic was administered using a computer-controlled closed circle system. Following an initial 4 min period during which the expired concentration of enflurane was established, the rate of uptake of enflurane showed a triexponential decline. The mean cumulative use of enflurane after 60 min was 10.7 ml, and after 120 min was 18.4 ml.

Adult↗
PubMed1995

The estimation of inspired isoflurane concentration in a low-flow system.

We have examined the predictability of inspired isoflurane concentration during low-flow anaesthesia using a to-and-fro breathing system. Twenty one adult patients requiring mechanical ventilation of the lungs during surgery took part in this study. Using a fresh gas flow of 2 l.min-1, the ratio of inspired isoflurane concentration to isoflurane vaporizer setting was found to be approximately 4/5th after 5 min of anaesthesia. The ratio was maintained throughout the procedure, except for a few minutes following each change in vaporizer setting.

Adult↗
PubMed1995

The effect of nitrous oxide on the performance of psychomotor tests. A dose-response study.

The effect of different nitrous oxide concentrations on the performance of psychomotor tests (inspection time, tapping frequency, critical flicker fusion, picture memory and time sense) was investigated in seven healthy volunteers. The results show a dose-related impairment of memory, a reduction in tapping frequency and an increase in inspection time. A paradoxical increase in critical flicker-fusion threshold was observed. There was no effect on critical fusion-flicker threshold or time sense. Recovery was incomplete for memory tests performed 20 min after withdrawal of nitrous oxide. Subjective effects persisted for several hours.

Adult↗
PubMed1995

The use of a portable refractometer to measure desflurane.

The measurement of volatile anaesthetic agents by infrared absorption is liable to interference by other gases which absorb infrared light. In particular, nitrous oxide may, in principle, interfere with the measurement of desflurane. On the other hand, the refractometer should be immune to such effects. We demonstrated that interactions may occur between desflurane and nitrous oxide with some gas analysers and that the refractometer remains a useful instrument.

Anesthetics↗
PubMed1994

The Ohmeda Rascal II. A new gas analyser for anaesthetic use.

The Ohmeda Rascal II is a multigas analyser and pulse oximeter for dedicated anaesthetic.use. It uses the Raman scattering of laser light to identify and quantify oxygen, nitrogen, carbon dioxide, nitrous oxide and three volatile anaesthetic agents. Its response times equal or better the published response times of infrared or photo-acoustic devices. It is linear within the clinical ranges of all gases and vapours, simple to use, requires no maintenance, holds its calibration well, and is a suitable monitor for clinical and research use.

Alcohols↗
PubMed1993

In vivo fluorine-19 magnetic resonance spectroscopy of cerebral halothane in postoperative patients: preliminary results.

This study reports the use of 19F MRS to study halothane in the brain of eight patients recovering from halothane anesthesia of short duration. Resonances attributable to halothane were observed up to 90 min after withdrawal of the anesthetic agent. The signal-to-noise ratio for an unlocalized spectrum acquired using a 6 cm surface coil was typically 20 with data collection times of 2 min. In seven patients a single resonance was seen with a mean (+/- SD) chemical shift of +43.3 (+/- 1.8) ppm, referenced to NaF at 0 ppm. This resonance exhibited a T1 value of between 0.5 and 1 s, and a T2* (estimated from the linewidth of the resonance) between 3.5 and 10 ms. In one patient two resonances were observed with chemical shifts of +38 and +41 ppm. Because we cannot exclude the possibility that this was due to field inhomogeneity, the significance of the last finding is uncertain. However, phantom studies show that the chemical shift of halothane in different environments (such as water, olive oil, methanol, and lecithin) can vary to an extent that accounts for the two resonances seen in our patient. These results demonstrate the feasibility of in vivo 19F MRS studies of fluorinated volatile agents in humans. The potential for clinical 19F MRS of fluorinated anesthetics is discussed.

Adult↗
PubMed1993

An evaluation of anaesthetic loss from a closed breathing system.

We present the results of a laboratory study of the loss of isoflurane from a to-and-fro system, ventilated with oxygen, using a standard ventilator connected to the system via a long corrugated tube (the trunk) in place of the reservoir bag. No conventional fresh gas supply is used and isoflurane is injected as a liquid directly into the soda lime canister. The loss of isoflurane from the system due to mixing in the trunk was generally less than that lost by absorption into rubber and plastic system components. Glass syringes used for injection of liquid isoflurane were found to be a potential source of much greater leaks. The results showed that a to-and-fro system ventilated with oxygen via an open trunk functions as a virtually completely closed anaesthetic breathing system.

Anesthesia, Closed-Circuit↗
PubMed1993

A computer-controlled closed anaesthetic breathing system.

We describe the design and working of a computer-controlled, closed anaesthetic breathing system which rapidly achieves and maintains a prescribed end-tidal concentration of isoflurane in oxygen. The system is simple to set up and not expensive; the only nonstandard component is a modified glass syringe. We have demonstrated that gas analysers may contribute as much as the patient to the accumulation of nitrogen within the breathing system. Details of our clinical experience with the system are presented in an accompanying article.

Algorithms↗
PubMed1993

True patient-controlled sedation.

A modified patient-controlled analgesia pump provided doses of propofol 3 mg or midazolam 0.1 mg in 0.3 ml, over 5.4 s, with no lockout, during transvaginal oocyte retrieval. Alfentanil 0.2 mg was administered at three points during the procedure, and on request. Patients were randomly assigned to receive either propofol (25 patients) or midazolam (22 patients). The mean age, weight, duration of procedure and dose of alfentanil were similar in both groups. Onset of sedation with propofol or midazolam took 70.6 (SD 22.4) and 106.3 (50.7) s respectively. Mean doses over the first 5 min were midazolam 2.7 (1.2) mg, and propofol 54 (18) mg. Thereafter requirements decreased: midazolam 0.065 (0.065) mg.min-1, propofol 2.1 (1.3) mg.min-1. All patients successfully completed the procedure; none required additional sedation. P-deletion, reaction time, and critical flicker fusion tests revealed similar depression in both groups immediately postoperatively. After 30 min the p-deletion and critical flicker fusion scores were still impaired in the midazolam, but not in the propofol, group.

Analgesia, Patient-Controlled↗
PubMed1993

The uptake of isoflurane during anaesthesia.

The uptake of isoflurane at a constant end-expired concentration of 1.5% in oxygen was studied in 15 women, ASA 1 or 2, undergoing elective total abdominal hysterectomy. The anaesthetic was administered by a simple computer-controlled to-and-fro closed system. After an initial period of wash-in to the system, the rate of uptake of isoflurane decreased bi-exponentially with a rapid reduction during the first 15 min. Perturbations from this bi-exponential decline reflect changes in cardiac output. The mean (SD) cumulative use of isoflurane was 4.5 (0.43) ml after 30 min and 7.3 (0.79) ml after 60 min.

Adult↗
PubMed1992

Clinical use of a small soda lime canister in a low-flow to-and-fro system.

It may be possible to overcome the classical disadvantages of a to-and-fro system by the use of a small soda lime canister. We report our experience of such a system in a series of 13 adult patients requiring mechanical ventilation of the lungs during surgery. The system was found to be convenient to use and the charge of soda lime had sufficient absorptive capacity to last throughout the great majority of surgical cases. Using a fresh gas flow of 1 l.min-1, the ratio of inspired isoflurane concentration to isoflurane vaporizer setting was found to be approximately one half after 10 min of anaesthesia. This ratio was maintained throughout the procedure, except for a few minutes following each change in vaporizer setting. A to-and-fro system using a small soda lime canister is a convenient alternative to the circle system for low-flow anaesthesia.

Adult↗
PubMed1991

Effect of ventilation and cardiac output on the uptake of anaesthetic agents from different breathing systems: a theoretical study.

We have investigated the influences of ventilation and cardiac output on uptake of anaesthetic with different breathing systems, by analysis of simple equations and by computer simulation. Increases in cardiac output and ventilation increased uptake from those systems which provided a constant inspired concentration, but not from completely closed systems with the vaporizer out of the circle (VOC), or when using the technique described by Lowe and Ernst. When the vaporizer was inside the circle, uptake increased with ventilation but not with cardiac output. With servo control of endtidal concentration, uptake increased with cardiac output but not with ventilation. When the fresh gas flow to VOC systems was increased from basal, independence of uptake from ventilation was well maintained until fresh gas flow approached alveolar ventilation, but the independence of uptake from cardiac output was lost much sooner.

Anesthesia, Closed-Circuit↗
PubMed1991

A low-flow to-and-fro system. Laboratory study of mixing of anaesthetic and driving gases during mechanical ventilation.

The insertion of a long deadspace tube between an anaesthetic breathing system and a ventilator produces only imperfect separation of the breathing system gas from the driving gas. This laboratory study has investigated different connecting tubes to establish the maximum tidal volume possible before ventilator gas contaminates the gas in a low-flow to-and-fro system (fresh gas flow 1 litre/minute). A larger volume tube enables the use of larger tidal volumes, and plastic corrugated tubes are slightly better than black rubber corrugated tubes in this respect. The maximum tidal volume possible without contamination decreases as ventilatory rate increases, but the maximum minute volume is increased. A 22 mm plastic corrugated tube of internal volume 1.5 litres should be adequate for clinical use with the to-and-fro system described in this study at a fresh gas flow of 1 litre/minute.

Anesthetics↗
PubMed1991

Evaluation of a small soda lime canister in a to-and-fro system.

The main disadvantages of the to-and-fro system (the bulky canister and the progressive increase in apparatus deadspace) may be overcome by the use of a smaller canister. In this laboratory study, we have evaluated a 160 g canister in a low-flow to-and-fro system (fresh gas flow 1 litre/minute). Two carbon dioxide productions of 150 and 200 ml/minute were simulated. The mean times to exhaustion, defined here as a 0.5 kPa rise in end-tidal PCO2, were 112 and 79 minutes in the 150 and 200 ml/minute carbon dioxide groups respectively. Ventilation to normacapnia or hypocapnia did not affect the times to exhaustion. The soda lime absorbed 16 litres of carbon dioxide before exhaustion, and this was not affected by minute volume or carbon dioxide production. A small soda lime canister is suitable for carbon dioxide absorption in a low-flow to-and-fro system for ventilated adults.

Absorption↗