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Biomedical subjects

J G Whitwam

Publications and source records attributed to J G Whitwam.

At least 37 records · Page 2Linked to original sources
PubMed1994

Clonidine has comparable effects on spontaneous sympathetic activity and afferent A delta and C-fiber-mediated somatosympathetic reflexes in dogs.

BACKGROUND: Clonidine, an alpha 2-adrenergic agonist, has been studied as an adjunct or alternative to spinal opioids in the management of moderate to severe pain. This study examined the relative effects of clonidine on efferent spontaneous sympathetic activity and afferent A delta and C fiber-mediated somatosympathetic responses. METHODS: Spontaneous and evoked sympathetic activity in renal sympathetic nerves, mediated by A delta and C fibers by means of supramaximal electrical stimulation of the radial and tibial nerves, were observed in anesthetized dogs. Incremental doses of clonidine were administered intrathecally or intravenously in each of five preparations followed by intravenous naloxone 2 mg and yohimbine 5 mg. RESULTS: Both spontaneous sympathetic outflow and afferent A delta- and C fiber-mediated somatosympathetic responses evoked by tibial nerve stimulation were depressed in a similar dose dependent manner by clonidine administered intrathecally or intravenously in a dose ratio of approximately 1:4. Intrathecal clonidine inhibited and eliminated both local spontaneous sympathetic outflow and tibial nerve evoked sympathetic responses but had no significant depressant effect on the radial nerve evoked sympathetic reflexes. When administered intravenously clonidine had a similar depressant effect on both radial and tibial nerve elicited reflexes and spontaneous sympathetic activity. CONCLUSIONS: Clonidine, administered intrathecally or intravenously, has a similar depressant effect on both spontaneous sympathetic outflow and afferent A delta- and C fiber-mediated somatosympathetic reflexes. When administered intrathecally it has little effect on reflexes evoked via the descending pathway by radial nerve stimulation.

Animals↗
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↗
PubMed1994

Anaphylactic/anaphylactoid reactions to anaesthetic and associated agents. Skin prick tests in aetiological diagnosis.

Fifty-one patients were referred in one year (1992) for investigation of immediate type anaphylactic/anaphylactoid reactions during anaesthesia. Skin prick tests were made with 23 anaesthetic and associated agents in the concentrations used clinically. Definite or probable causes were identified by immediate type wealing reactions, supported by the clinical history in 36 of the 46 in whom a diagnosis of anaphylaxis was made. These comprised mainly the neuromuscular relaxants, chiefly suxamethonium (18); atracurium (6); gallamine (2); one each alcuronium; pancuronium; vecuronium and tubocurarine, as well as alfentanil (1); Gelofusine (2); cefuroxime (1) and latex (2). The materials for performing the skin prick test are readily available and it can be very helpful in making important aetiological diagnoses.

Adjuvants, Anesthesia↗
PubMed1993

Effect of ICI197067, a kappa-opioid receptor agonist, spinally on A delta and C reflexes and intracerebrally on respiration.

Intrathecal (i.t.) injection of a kappa-opioid receptor agonist, ICI197067, caused a similar dose dependent depression of A delta and C fibre mediated nociceptive reflexes in renal sympathetic nerves due to supramaximal electrical stimulation of tibial nerves in anaesthetized dogs. A total dose of 8 mg i.t. abolished these reflexes. When administered into the 4th ventricle (i.c.v.) in a total dose range from 0.1-2.5 mg ICI197067 caused no respiratory depression; a total dose of 10 mg i.c.v. abolished both phrenic nerve activity and spontaneous respiration. The ED50 ratio of ICI197067 for depression of respiration (i.c.v.) and somatosympathetic reflexes (i.t.) is approximately 1.5:1 compared with 0.3:1 for fentanyl. ICI197067 i.c.v. caused a similar reduction in arterial pressure compare to fentanyl without comparable changes in heart rate. Thus in terms of cardiorespiratory depression and blockade of A delta and C fibre pathways kappa-opioid receptor agonists may be safer and more effective for producing spinal analgesia than mu-opioid receptor agonists.

Animals↗
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

Cardiorespiratory effects of conventional and high frequency ventilation in rabbits with bilateral pneumothoraces and surfactant depleted lungs.

We compared high frequency ventilation (HFV) to conventional mechanical ventilation (CMV) under normoxic and normocapnic condition in surfactant depleted rabbits with bilateral pneumothoraces. We hypothesized that lower airway pressures would be required with HFV under these conditions. We applied CMV and HFV in 8 anaesthetized rabbits with a prototype ventilator at frequencies of 30, 100, 200, and 300 cycles/min. A positive end-expiratory pressure (PEEP) just below the pressure sufficient to open the air leak from the pneumothoraces was applied at all frequencies. Airway pressures, gas exchange, heart rate, and mean arterial pressure were recorded. Peak airway pressure decreased significantly from 2.50 to 2.10 kPa when the frequency of ventilation was increased from 30 to 300 cycles/min. There were no significant changes in mean airway pressure, PaO2, arterial pH, heart rate, and mean arterial pressure when HFV was compared to CMV. In conclusion, during HFV peak airway pressures measured at the mouth were decreased. Our ability to maintain adequate gas exchange in the face of ongoing pulmonary air leaks may reflect lower alveolar pressures.

Animals↗
PubMed1993

Synergism between the antinociceptive effects of intrathecal midazolam and fentanyl on both A delta and C somatosympathetic reflexes.

The interaction between the antinociceptive effects of fentanyl and midazolam, administered intrathecally (i.t.), was examined in dogs. Midazolam 1 mg (i.t) depressed the A delta and C fibre mediated somatosympathetic reflexes to 68.3 and 85.2% of control values and then reduced the subsequent doses of fentanyl (i.t.) required to abolish these reflexes by 50%. After midazolam (1 mg, i.t.) the ED values for fentanyl were markedly less than the theoretical predicted additive values. This indicates synergism between the effects of fentanyl and midazolam.

Analgesics↗
PubMed1993

Differential effects of alfentanil, fentanyl, pethidine and lignocaine administered intrathecally on nociceptive responses evoked by low and high frequency stimulation of somatic nerves.

We have studied in anaesthetized dogs the effects of alfentanil, fentanyl, pethidine and lignocaine administered intrathecally on nociceptive responses evoked by low and high frequency supramaximal electrical stimulation of the tibial and radial nerves. Doses were selected to abolish both A delta and C fibre somatosympathetic reflexes to single stimuli. Pethidine and lignocaine eliminated reflex pressor and heart rate responses to repeated single stimuli and almost completely abolished responses to train stimulation. The pressor response to single stimuli was abolished by fentanyl and reduced by alfentanil, but these drugs did not reduce significantly this response to train stimulation, suggesting a stimulation rate-dependent effect. Of the opioids, only pethidine had an effect comparable to that of lignocaine. The absence of sympathetic block and antagonism by naloxone imply a lack of significant local anaesthetic effect. We suggest that the greater analgesic efficacy of pethidine is a result of endogenous synergism between a minor local anaesthetic and a major opioid effect.

Alfentanil↗
PubMed1993

Lack of a ceiling effect for intrathecal buprenorphine on C fibre mediated somatosympathetic reflexes.

We observed that buprenorphine 20 micrograms kg-1 i.v. in dogs caused an initial significant reduction in both A delta and C fibre mediated somatosympathetic reflexes evoked by tibial and radial nerve stimulation, to approximately 75% and 70% of control values. Larger doses (up to 100 micrograms kg-1 i.v.) had progressively less effect and the mean responses were depressed to only approximately 65% and 55% of control, indicating a ceiling effect. Buprenorphine 450 micrograms intrathecally (i.t.) completely abolished tibial C fibre reflexes, but 20% of A delta responses could not be eliminated with doses up to 1050 micrograms i.t. Fentanyl 100 micrograms kg-1 i.v. or 150 micrograms i.t. after buprenorphine i.v. or i.t., respectively, had little additional effect. This study confirms the limited ceiling effect of buprenorphine on nociceptive reflexes when administered systemically, and provides evidence that when administered i.t. in sufficient doses it abolishes the C responses (lack of ceiling effect for C responses), but the A delta responses show a plateau or ceiling effect.

Animals↗
PubMed1993

Specific enhancement by fentanyl of the effects of intrathecal bupivacaine on nociceptive afferent but not on sympathetic efferent pathways in dogs.

BACKGROUND: Bupivacaine alone, or in combination with opioids, has been shown to provide adequate pain relief without motor paralysis. This study examined the effects of bupivacaine administered intrathecally on sympathetic efferent and A delta- and C-fiber-mediated afferent pathways in dogs and the interactions with intrathecal fentanyl. METHODS: Spontaneous activity in renal sympathetic nerves was observed, as were reflex somatosympathetic responses mediated by A delta and C fibers evoked by supramaximal electrical stimulation of the tibial and radial nerve. Bupivacaine was administered intrathecally in doses of 0.5, 1, 2, and 3.5 mg, each in 0.5 ml, and 7 mg in 1 ml with or without pretreatment with 5.4 mg intrathecal fentanyl (ED25 for depression of C tibial reflexes) in each of five preparations. RESULTS: Bupivacaine caused a dose-dependent inhibition of both A delta- and C-fiber-mediated somatosympathetic responses evoked by tibial nerve stimulation. The depression of radial and tibial nerve reflexes and spontaneous renal sympathetic activity was similar. Pretreatment with fentanyl (5.4 micrograms, intrathecally) depressed tibial C-fiber reflexes by only 23.8% without any significant effect on either tibial A delta or radial A delta and C fiber responses. Fentanyl markedly enhanced the effect of subsequent doses of bupivacaine on tibial A delta and C reflexes without any additional effect on either spontaneous sympathetic activity or radial responses. CONCLUSIONS: Intrathecal bupivacaine has no selectivity for the afferent and efferent pathways, and intrathecal fentanyl acts synergistically to enhance the effect of bupivacaine on the afferent pathway without a measurable effect on sympathetic outflow.

Adrenergic Fibers↗
PubMed1993

Induction of microcurrents in critically ill patients in magnetic resonance systems.

OBJECTIVE: To determine whether electric current can be induced in intracardiac catheters, thermistor wires and pacing electrodes in patients during magnetic resonance imaging (MRI). DESIGN: Prospective laboratory study. SETTING: Postgraduate medical school hospital. SUBJECTS: A sheep heart model. INTERVENTIONS: None. MEASUREMENTS AND MAIN RESULTS: Voltage generated by saline 0.9% flowing through a magnetic field and distribution of current from a catheter tip within a sheep heart model were measured in a 0.15 Tesla MRI system. Resistance of loops formed by pacing wires, a pacing electrode, and a thermistor wire were measured in saline 0.9%. Effects of rapidly changing magnetic fields and the movement of the beating heart on epicardial pacing wires were calculated theoretically. A flow of 200 mL/min of saline 0.9% induced a current of 0.1 microampere (microA) (at 0.15 Tesla). From magnetic resonance images we derived a current density of approximately 0.004 microA/mm2 (at 0.15 Tesla). Internal resistance of pacing catheters and thermistor wires was > 1 megaohm (M omega). The maximum currents calculated (for a higher field strength of 1.5 Tesla) in a circuit formed by epicardial pacing wires were 80 microA, induced by the beating heart moving the wires through the magnetic field and 46 microA, induced by the rapidly changing magnetic fields. CONCLUSIONS: Current generated by flow of conducting fluid should be safe. Pacing catheters and thermistor wires should be safe if well insulated and disconnected from external electric connections. However, current induced in epicardial pacing wires may be a hazard, and precautions should be taken. External wire tips must be separated, insulated, and coiled to lie along the axis of the magnetic field. Electrocardiography is required, and defibrillation equipment should be available.

Animals↗
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

Relative effects of intrathecal administration of fentanyl and midazolam on A delta and C fibre reflexes.

The effects of fentanyl and midazolam, administered intrathecally, on somatosympathetic reflexes evoked by tibial nerve stimulation were investigated in 12 anaesthetized and paralysed dogs. Fentanyl depressed both the C and A delta fibre evoked reflexes in a dose ratio of approx 1:2. In contrast, midazolam had a greater effect on A delta compared with C fibre reflexes; while A delta reflexes were abolished by a total dose of 3 mg midazolam, C fibre reflexes were depressed by only 50%. The effect of fentanyl was reversed by naloxone (2 mg, i.v.) and that of midazolam by flumazenil (1 mg, i.v.). The results suggest that fentanyl and midazolam have different relative effects on A delta and C fibre pathways.

Anesthesia, Spinal↗