Reducing aortocaval compression: how much tilt is enough?
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Anesthesia, Obstetrical↗
Biomedical subjects
J G Whitwam
Publications and source records attributed to J G Whitwam.
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↗
Midazolam sedation reversed with flumazenil for cardioversion.
Midazolam was used for anaesthesia in 20 patients undergoing cardioversion; 10 received flumazenil, which caused immediate rapid reversal of anaesthesia, and these patients maintained SpO2 greater than 95%, breathing air, within 5-10 min. In contrast, patients in the placebo group were still partially sedated and required oxygen therapy for up to 2 h to maintain a normal SpO2. Arterial pressure, but not heart rate, also was greater in the flumazenil group in the recovery period.
Adult↗
Effect of low and high concentrations of alfentanil administered intrathecally on A delta and C fibre mediated somatosympathetic reflexes.
We have studied the effects of alfentanil administered intrathecally on somatosympathetic reflexes evoked by stimulation of radial and tibial nerves in 10 anaesthetized and paralysed dogs. In five animals, alfentanil was administered in doses of 100, 100, 200 and 400 micrograms in 0.8 ml and 800 micrograms in 1.6 ml prepared from the formulation of 500 micrograms ml-1. Five others received alfentanil (high concentration formulation, 5 mg ml-1) in doses of 500 and 2000 micrograms in 0.5 ml and 5000 micrograms in 1 ml. C fibre tibial nerve reflexes were depressed by 100 micrograms and abolished with doses of 200-400 micrograms, but the A delta response remained unaffected. A delta responses were depressed also and abolished, respectively, by doses of alfentanil 2000 micrograms and 5000 micrograms. At the larger, but not the smaller, concentration there was evidence of baroreflex sensitization probably caused by systemic absorption of the drug.
Alfentanil↗
Magnetic resonance for the anaesthetist. Part I: Physical principles, applications, safety aspects.
Anaesthetists are being increasingly involved in magnetic resonance (MR) procedures, both in patient care and as a research tool. This paper outlines the physical basis of nuclear magnetic resonance and describes its application in magnetic resonance imaging and spectroscopy. Principles of magnet design and safety relevant to anaesthetic practice in a magnetic resonance environment are discussed and guidelines for anaesthetic practice suggested. Some recent clinical magnetic resonance studies of anaesthetic interest are reviewed.
Anesthesiology↗
Magnetic resonance for the anaesthetist. Part II: Anaesthesia and monitoring in MR units.
Anaesthetists are increasingly involved in patient care during magnetic resonance imaging and spectroscopy. This paper describes a system which has been developed for the management of critically ill patients and the conduct of anaesthesia in a magnetic resonance unit with a 1.6 tesla whole body magnet. Difficulties which arise from working in a confined space in a high magnetic field are highlighted. Different approaches to anaesthesia, sedation and the modification of equipment for use in this environment are reviewed. The problems associated with patient monitoring within a magnetic field are discussed and some solutions are suggested. A transport system for critically ill patients is described and a protocol for management is outlined.
Anesthesia↗
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↗
A new optical transducer for arterial pressure measurement.
A new optical pressure transducer system (Viggo) has been assessed and compared with a standard P10 transducer (Spectramed) using a similar 20-gauge cannula, both in vitro in terms of linearity and frequency response and in vivo using an animal model. The linearity of the transducers was comparable; the resonant frequencies were 106 Hz and 75 Hz, respectively. However, the resonant frequency of the complete Spectramed system including 150-cm tubing was 11 Hz. The frequencies at which the output amplitude error exceeded 10% of the initial amplitude for the Viggo and for P10 with and without 150-cm tubing were 32 Hz, 24 Hz and 4 Hz, respectively. The principal advantage of the new transducer is that it is sufficiently compact to be mounted directly in the cannula at the wrist, so obviating the use of connecting tubing between the transducer and the cannula.
Animals↗
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↗
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↗
Prevention of pain during injection of propofol.
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Drug Therapy, Combination↗
Effects of circulating endogenous catecholamines on plasma glucose, potassium and magnesium.
1. To examine the metabolic effects of increases in circulating endogenous plasma catecholamines, we measured plasma glucose, potassium and magnesium in 14 patients undergoing elective coronary artery bypass grafting. The patients were randomized into two groups and received either sodium nitroprusside (a direct-acting vasodilator) or trimetaphan camsylate (a ganglion-blocking agent) for routine control of blood pressure during the operation. 2. There were significant differences between the two groups in the levels of all three metabolic variables studied. Plasma glucose levels rose in both groups, but were significantly higher in the sodium nitroprusside group [peak levels 9.14 (SEM 0.72)mmol/l compared with 6.71 (0.88) mmol/l, P less than 0.001, analysis of variance]. The cardioplegia solution caused a large increase in plasma magnesium in both groups but in the sodium nitroprusside group the level rose higher [to 1.59 (0.12)mmol/l compared with 1.34 (0.06)mmol/l] and fell faster (P less than 0.05, analysis of variance). In the group receiving sodium nitroprusside, plasma potassium fell, by a mean of 0.34mmol/l, as plasma catecholamine levels rose; no such fall was seen in the group receiving trimetaphan camsylate (P less than 0.05, analysis of variance). 3. It is concluded that the sympathoadrenal system is important in causing metabolic changes during cardiopulmonary bypass and may be relevant in other conditions such as acute myocardial infarction.
Adult↗
A new infant oscillatory ventilator.
A new, simple and inexpensive oscillatory ventilator is described in which a rotating jet mounted in the breathing duct generates cyclically positive and negative pressures in the airway with a sinusoidal flow waveform. Unlike conventional oscillatory ventilators it is free from restrictions to inspiratory or expiratory gas flows and open to atmosphere at all times, making it intrinsically a safe system for ventilation. A prototype rotating jet oscillatory ventilator designed for application in infants was evaluated in rabbits (mean weight 3.8 kg). The positive peak and mean airway pressures were significantly less during oscillatory ventilation at 300 and 420 b.p.m. compared with normal and high frequency positive pressure ventilation at 30 and 300 b.p.m., respectively, while maintaining blood-gas tensions within the normal range. An increase in the oscillatory frequency from 300 to 420 b.p.m. provided no further benefit in terms of airway pressure, tidal volume or blood-gas tensions.
Animals↗
Comparison of two ventilators used with the T-piece in paediatric anaesthesia.
The Nuffield 200 ventilator was compared with a new valveless ventilator (CW 200) in 20 children undergoing general anaesthesia for paediatric surgery. The new ventilator incorporates design features which overcome the main disadvantages of the Nuffield 200 and make it an inherently safer machine. At identical ventilator settings it produced a significantly greater tidal volume with a reduction in end-tidal carbon dioxide partial pressure. This may have advantages in avoiding pulmonary barotrauma in children.
Anesthesia, General↗
Minimum airway pressure weaning. Weaning from mechanical ventilation at 50 breaths/minute by reduction of inflation pressure using a pressure generator.
Ten patients, whose lungs were ventilated initially with intermittent positive pressure ventilation at conventional rates after myocardial revascularisation or cardiac valvular surgery, were weaned using a valveless high frequency jet ventilator at a constant rate of 50 breaths/minute. The withdrawal of ventilation was achieved, when clinical criteria permitted, by reduction of the tidal volume preset on the jet ventilator in successive stages; this was effected by stepwise decreases in the jet driving pressure. This new mode of weaning at a constant rate of 50 breaths/minute is associated with minimum peak airway pressures. Synchronisation of the patient's breathing with the valveless ventilator is not required and weaning is tolerated well by the patient. Arterial oxygen tension and saturation were maintained throughout weaning and did not decline after extubation of the trachea.
Adult↗
A low flow open circle system for anaesthesia. Part I: Laboratory evaluation at normal and high frequencies.
A circle breathing system was connected by deadspace tubing to an open system valveless ventilator. The minimum volume of this tubing, required to prevent dilution of anaesthetic gas in the breathing system by the driving gas of the ventilator, was determined at frequencies of 15, 30, 60, 100 and 150 breaths/minute, with tidal volumes that ranged from 100 to 1100 ml and a fresh gas supply to the circle system of 1, 2 and 4 litres/minute. At 15 breaths/minute, tidal volumes equal to or less than the deadspace volume could be used safely without any mixing with the ventilator driving gas, when a fresh gas flow of 2 litres/minute or above is supplied to the circle system. At 1 litre/minute of fresh gas flow, mixing occurred at tidal volumes less than the deadspace volume. Mixing of gas occurred in the system at frequencies greater than 30 breaths/minute even when the tidal volume was much less than the deadspace volume. However, at high frequencies of ventilation, since the tidal volume requirement decreases, deadspace tubing with a safe internal volume, that is greater than 600 ml, may be used up to 100 breaths/minute.
Anesthesia, Closed-Circuit↗
A low flow open circle system for anaesthesia. Part 2: Clinical evaluation.
A prototype valveless ventilator was attached by open deadspace tubing to a circle system and used to ventilate the lungs of 12 patients with low flows of anaesthetic gases for periods between 60 and 120 minutes during intra-abdominal surgery. Anaesthesia was induced with thiopentone and maintained with nitrous oxide 50% in oxygen and enflurane. This was reduced to 2 litres/minute after a 10-minute period of nitrogen wash out and stabilisation of anaesthetic gas concentration, with an initial anaesthetic gas flow of 6 litres/minute. The concentration of oxygen, carbon dioxide, nitrous oxide and enflurane were measured in the outflow from both the anaesthetic machine and the inspiratory limb of the circle system. The measured mean inspired oxygen and nitrous oxide concentrations showed no significant variation throughout the low flow period of the study. This new low flow open circle ventilation system appears to offer some advantages in terms of safety and versatility over other systems which are discussed.
Adult↗
Intermittent high frequency ventilation. Clinical evaluation of a new mode of ventilation.
In this first clinical study of a new mode of ventilation, called intermittent high frequency ventilation (IHFV), a comparison was made in ten patients after myocardial revascularization, between normal ventilation, high frequency ventilation (HFV) and IHFV at 15, 60, 100 and 160 breaths per min (b.p.m.). During IHFV the ventilation was interrupted for 3 s 7 times per min. The measured mean trapped gas volumes were 219 ml at 60 b.p.m. and 716 ml at 160 b.p.m. At 160 b.p.m. during IHFV, the mean, mean airway pressures, pulmonary artery (PAP) and capillary wedge pressures and the Paco2 were decreased, while the cardiac output increased. When the Paco2 was maintained constant, IHFV also allowed a reduction in the mean peak and mean airway pressures. This new mode of ventilation can prevent sustained increases in the lung volume and provide a reduction in intrapulmonary pressures. It also permits the measurement of the trapped gas volume and end-tidal CO2 gas concentrations several times a minute for the clinical management of patients during high frequency ventilation.
Evaluation Studies as Topic↗