Description of a technique for anaesthetizing pregnant ewes for fetal surgery.

General anaesthesia in 12 pregnant ewes undergoing surgery for fetal physiological research was supplemented with an intravenous infusion of remifentanil. This allowed us to employ a lighter plane of surgical anaesthesia and to use intermittent positive pressure ventilation. Our aim was to improve fetomaternal outcome. We monitored maternal pulse, blood pressure, transcutaneous oxygen saturation and end-tidal carbon dioxide levels. Remifentanil doses of 0.75-2.0 microg/kg/min were needed and typically this allowed halothane concentrations of 1-1.5% to be used for maintenance of anaesthesia. Surgery lasted up to 2.5 h. All 12 ewes and their singleton fetuses survived the peri- and postoperative period in good condition.

Effect of videotape feedback on anaesthetists' performance while managing simulated anaesthetic crises: a multicentre study.

The aim of this study was to examine the performance of anaesthetists while managing simulated anaesthetic crises and to see whether their performance was improved by reviewing their own performances recorded on videotape. Thirty-two subjects from four hospitals were allocated randomly to one of two groups, with each subject completing five simulations in a single session. Individuals in the first group completed five simulations with only a short discussion between each simulation. Those in the second group were allowed to review their own performance on videotape between each of the simulations. Performance was measured by both 'time to solve the problem' and mental workload, using anaesthetic chart error as a secondary task. Those trainees exposed to videotape feedback had a shorter median 'time to solve' and a smaller decrease in chart error when compared to those not exposed to video feedback. However, the differences were not statistically significant, confirming the difficulties encountered by other groups in designing valid tests of the performance of anaesthetists.

Investigating hypoxemia during apnea: validation of a set of physiological models.

UNLABELLED: The aim of our study was to validate the Nottingham Physiology Simulator (NPS) for examining pulmonary denitrogenation and apnea by reproducing the methodology and results of previous clinical studies. Only four studies provided sufficient detail in their description of their methodology to allow accurate reproduction by using the NPS or provided a sufficiently detailed description of their subjects to allow accurate modelling. The results of the NPS recreation of the studies were within 13% of the values found clinically in all cases and were within 2% in the majority of cases. The four studies included healthy and morbidly obese patients, conscious and anesthetized patients, and included examination of the effect of denitrogenation and apnea on plasma pH and on lung and arterial oxygen and carbon dioxide tensions at various lung volumes. IMPLICATIONS: We used mathematical, physiological models to recreate the methods and subjects of four clinical studies investigating oxygenation and low oxygen levels during cessation of breathing. Our aim was to validate the models, allowing theoretical investigations into this area. The blindly recreated results closely matched the clinical studies, validating the models.

Factors determining the onset and course of hypoxemia during apnea: an investigation using physiological modelling.

UNLABELLED: We used the Nottingham Physiology Simulator to examine the onset and course of hypoxemia during apnea after pulmonary denitrogenation. The following factors, as possible determinants of the hypoxemia profile, were varied to examine their effect: functional residual capacity, oxygen consumption, respiratory quotient, hemoglobin concentration, ventilatory minute volume, duration of denitrogenation, pulmonary venous admixture, and state of the airway (closed versus open). Airway obstruction significantly reduced the time to 50% oxyhemoglobin saturation (8 vs 11 min). Provision of 100% oxygen rather than air to the open, apneic patient model greatly prolonged time to 50% oxyhemoglobin saturation (66 vs 11 min). Hemoglobin concentration, venous admixture, and respiratory quotient had small, insignificant effects on the time to desaturation. Reduced functional residual capacity, short duration of denitrogenation, hypoventilation, and increased oxygen consumption significantly shortened the time to 50% oxyhemoglobin saturation during apnea. IMPLICATIONS: Reduction in oxygen levels during cessation of breathing is dangerous and common in anesthetic practice. We used validated, mathematical, physiological models to reveal the impact of physiological factors on the deterioration of oxygen levels. This study could not be performed on patients and reveals important information.

Estimation of alveolar deadspace fraction using arterial and end-tidal CO2: a factor analysis using a physiological simulation.

The alveolar deadspace as a fraction of alveolar ventilation (VDalv/VTalv), while technically difficult to measure, is an objective monitor of pulmonary disease progression and a predictor of successful weaning from mechanical ventilation. The aim of the study was to examine the relationship between the arterial to end-tidal PCO2 gradient (Pa-E'CO2) and VDalv/VTalv and between (Pa-E'CO2)/PaCO2 and VDalv/VTalv using the Nottingham Physiology Simulator, an original, validated physiology simulation. The relationships were observed while pulmonary shunt, anatomical deadspace, ventilatory minute volume and metabolic rate were varied. The relationship between Pa-E'CO2 and VDalv/VTalv was non-linear and was affected significantly by all the factors except anatomical deadspace. The relationship between (Pa-E'CO2)/PaCO2 and VDalv/VTalv (best fit: VDalv VTalv = 1.135 x (Pa-E'CO2)/PaCO2-0.005) during normal physiological conditions was approximately linear and less influenced by physiological variation. Shunt and anatomical deadspace caused some inaccuracy, although they are unlikely to prevent the clinical usefulness of this formula.

A physiology simulator: validation of its respiratory components and its ability to predict the patient's response to changes in mechanical ventilation.

We aimed to validate the mathematical validity and accuracy of the respiratory components of the Nottingham Physiology Simulator (NPS), a computer simulation of physiological models. Subsequently, we aimed to assess the accuracy of the NPS in predicting the effects of a change in mechanical ventilation on patient arterial blood-gas tensions. The NPS was supplied with the following measured or calculated values from patients receiving intensive therapy: pulmonary shunt and physiological deadspace fractions, oxygen consumption, respiratory quotient, cardiac output, inspired oxygen fraction, expired minute volume, haemoglobin concentration, temperature and arterial base excess. Values calculated by the NPS for arterial oxygen tension and saturation (PaO2 and SaO2), mixed venous oxygen tension and saturation (PvO2 and SvO2), arterial and mixed venous carbon dioxide tension (PaCO2 and PvCO2) and arterial pH were accurate compared with measured values. Subsequently, arterial gas responses to changes in minute volume of FiO2 were measured in 31 patients and were compared with the NPS prediction for each response. The 95% limits of agreement in predicting the magnitude of change were: arterial oxygen tension -2.07 to 2.47 kPa; PaCO2 -0.33 to 0.67 kPa; and pH -0.023 to 0.033. This investigation has validated respiratory components of the NPS. We recommend the NPS as a clinical tool for predicting the effects of alterations in mechanical ventilation in stable patients in the intensive care unit.

Role of vasopressin in the haemodynamic response to laparoscopic cholecystectomy.

We studied 10 patients undergoing laparoscopic cholecystectomy (group 1) and five control patients (group 2). We measured heart rate, arterial pressure, right atrial pressure (RAP), cardiac index (CI), systemic vascular resistance index (SVRI), intrathoracic pressure (ITP), plasma osmolality, adrenaline, noradrenaline and arginine vasopressin (aVP) concentrations, and serum renin activity (SRA), and calculated the atrial transmural pressure gradient (ATPG). We recorded significant decreases in mean arterial pressure (MAP), SVRI and CI in both groups (P < 0.05) after induction of anaesthesia. MAP and SVRI increased (P < 0.01) while CI decreased further in group 1 patients during the pneumoperitoneum. In group 1 plasma aVP concentration increased after insufflation of the pneumoperitoneum to a level sufficient to cause the recorded haemodynamic changes. ATPG decreased in group 1 patients during the pneumoperitoneum and this is a recognized trigger for aVP release.

Low-flow anaesthesia.

Although many anaesthesia machines are equipped with circle rebreathing systems, inhalational anaesthesia remains frequently performed using relatively high fresh-gas flows. The major advantages of rebreathing techniques can be achieved only if the fresh-gas flow is reduced to 1 l.min-1 or less. Although there are potential risks associated with low-flow anaesthesia, modern anaesthesia machines meet all the technical requirements for the safe use of low-flow techniques if they are used in conjunction with equipment for monitoring inhaled and exhaled gas concentrations; these monitors are already increasingly available and, in the near future, are likely to become an obligatory safety standard in many countries. For both economic and ecological reasons, the use of new inhalational anaesthetics, with low tissue solubility and low anaesthetic potency, can be justified only if the efficiency of administration is optimised by using low-flow anaesthetic techniques.

A comparison of droperidol and cyclizine in the prevention of postoperative nausea and vomiting associated with patient-controlled analgesia.

Fifty ASA 1 or 2 patients scheduled to undergo major gynaecological surgery were allocated randomly to one of two groups. All patients received a standard anaesthetic regimen. Patients in group 1 received droperidol 1.25 mg given intravenously 20 min prior to the end of surgery and a patient-controlled analgesia infusion containing morphine 1 mg.ml-1 and droperidol 0.05 mg.ml-1. Patients in group 2 received cyclizine 50 mg by slow intravenous injection 20 min prior to the end of surgery and a patient-controlled analgesia infusion containing morphine 1 mg.ml-1 and cyclizine 2 mg.ml-1. Fifteen of 25 patients (60%) in group 1 and 18 (72%) of 25 in group 2 suffered no nausea or vomiting postoperatively. Two patients (8%) in group 1 and three (12%) in group 2 suffered severe postoperative nausea or vomiting. We conclude that cyclizine is as effective as droperidol in the prevention of postoperative nausea and vomiting when included in a patient-controlled analgesia infusion using morphine.

Relationship of peak flow rate and peak velocity time during voluntary coughing.

The differences in phonation between men and women are thought to occur from anatomical differences in the larynx. However, it is not known if there are any differences in cough dynamics between the sexes. We investigated this by asking 100 healthy, non-smoking adults (50 male) to perform a voluntary cough into a tussometer. Each volunteer coughed at four different lung volumes, ranging from total lung capacity to functional residual capacity. There was a positive correlation between peak velocity time and cough peak flow rate in both males (r = 0.73, P < 0.001) and females (r = 0.78, P < 0.001). Multiple regression analysis showed that height (P < 0.05) and sex (P < 0.001) were significant determinants of the relationship between peak flow rate and peak velocity time. In a height-matched subgroup, sex differences remained significant (P < 0.05). This may be related to anatomical differences in laryngeal structure and may have implications when using tussometry to assess laryngeal function.

A prospective study to assess the demand for a high dependency unit.

We performed a prospective study in our teaching hospital to determine the demand for a High Dependency Unit where none had existed before. An admission criteria protocol was designed for medical and surgical patients and this was used in an intense surveillance of 22 acute wards over 2 weeks. It was predicted that a high dependency unit of eight beds would operate to capacity 50% of the time. An analysis of this type over a short period of time using similar admission criteria could be used to assist in the prediction of the necessary size of a High Dependency Unit facility in any hospital.

Local anaesthesia for cannulation. Has practice changed?

A questionnaire study was undertaken to assess the influence of recently published, simple and conclusive research on the practice of anaesthetists in four centres. The research had clearly demonstrated the benefit of subcutaneous infiltration of local anaesthetic in reducing the pain of intravenous cannulation. Of the 81% who responded, 71% were aware of the research; 43% of these anaesthetists had altered their clinical practice as a result of the research and 73% used local anaesthesia for cannulae of 18 gauge or less, compared with only 46% of those who were unaware of the research. Practice comparisons were made between centres and grades of anaesthetist. Senior house officers were significantly less likely to be aware of the research than other grades. Anomalies were identified between the apparent awareness of the research and routine practice agreeing with the study findings. The value of research and the incorporation of clinical findings into everyday practice is discussed.

Effect of topical anaesthesia on the motor performance of vocal cords as assessed by tussometry.

Tussometry involves a continuous measurement of airflow produced by a cough manoeuvre displayed as an airflow-time wave. There is a rapid rise to its peak (cough peak flow rate) and the time taken to reach the peak (peak velocity time) is determined by the laryngeal opening at the onset of cough. Cough peak flow and peak velocity time were measured in 10 healthy volunteers before and after topical anaesthesia of the larynx with lignocaine 100 mg sprayed under indirect laryngoscopy. Adequacy of anaesthesia was established by touching the cords deliberately with a fibreoptic laryngoscope. All subjects had excellent anaesthesia of the larynx. However, cough peak flow rate and peak velocity time remained unchanged following topical anaesthesia. We conclude that topical anaesthesia of the larynx does not impair the motor performance of the vocal cords during a voluntary cough.