EFFECTS OF THE RECTAL ADMINISTRATION OF DIAZEPAM: Diazepam Concentrations in Children Undergoing General Anaesthesia

Diazepam 1 mg/kg body weight was administered rectally to 14children (11–22.5 kg) before minor surgery under generalanaesthesia. Administration in solution (n = 7) resulted ina rapid increase in serum concentrations which were maintainedfor 8 h. Administration by suppository (n = 7) resulted in significantlylower serum concentrations at 10 and 20 min, but higher concentrationsat 2 h. At other intervals the concentrations did not differfrom those reached after administration of diazepam in rectalsolution. These results favour the use of a solution when rectaldiazepam is used for premedication in children.     

EXPOSURE TO TRACE AMOUNTS OF NITROUS OXIDE: Evaluation of urinary gas content monitoring in anaesthetic practice

The urinary content of nitrous oxide was monitored during routineanaesthetic practice. There was a linear correlation with simultaneousmeasurements of technical exposure measured with a personalpump-bag sampling system (r = 0.99). Monitoring of urinary nitrousoxide with headspace extraction and gas chromatographic analysisis a simple and accurate mode of biological monitoring of exposureto nitrous oxide in the atmosphere.     

METABOLIC GAS EXCHANGE DURING NITROUS OXIDE ANAESTHESIA WITH CONTROLLED VENTILATION

A system for metabolic gas exchange has been used during nitrousoxide-opioid anaesthesia incorporating a Servo Ventilator 900C and external analysers for oxygen and carbon dioxide. Oxygenconsumption and carbon dioxide excretion were calculated asdifferences in content between inspired and expired minute ventilation.Nitrous oxide uptake was calculated similarly, assuming it wasthe only other gas present in addition to oxygen and carbondioxide. The mean value for oxygen consumption was 3.25 ml kg^–1min^–1, declining by 8% during the 2 h of anaesthesia.The formula for the best fit curve of nitrous oxide uptake was18.3.t^–0.48 ml kg^–1 min^–1 when Fl_NO was 0.7.To simplify measurement procedures and avoid measurements ofexpiratory volume, we also calculated metabolic gas exchangewhen expiratory minute ventilation was expressed as a functionof inspiratory minute volume and nitrous oxide uptake. The lattervalue was obtained from the overall best fit curve for nitrousoxide uptake.     

SMALL ENDOTRACHEAL TUBES: Ventilator and intratracheal pressures during controlled ventilation

The use of small endotracheal tubes reduces the trauma of intubation.Ventilator and tracheal pressures were measured during controlledventilation with various tube dimensions and ventilation volumes.Ventilation with large volumes using small tracheal tubes resultsin high ventilator pressures. However, tracheal pressures areonly marginally greater than those obtained with larger tubes.Small endotracheal tubes and high ventilation volumes resultin a positive tracheal pressure at the end of expiration. Themeasured end-expiratory pressures are within the limits whichmight be used therapeutically (in PEEP). The force requiredto reshape endotracheal tubes of various dimensions to an "anatomical"shape was related to the tube dimensions; the beneficial effectsof preformed, "anatomically shaped" endotracheal tubes can beachieved by using small tubes of standard design. ^*Present address: Lidköpings Lasarett, 53100 Lidköping,Sweden.     

Low–flow anaesthesia does not increase the risk of microbial contamination through the circle absorber system

In the circle absorber system, a decrease in fresh gas flow means a higher degree of rebreathing, and, consequently, a higher temperature and humidity within the system. With our present hygienic routines, the circle system tubings are changed and decontaminated once daily. Thus, the same circle system is used for several patients each day. In order to evaluate whether the risk of bacterial contamination increased with the introduction of low-flow anaesthesia, 122 patients anaesthetized with either a low-flow technique (less than 1.5 l fresh gas flow/min) or with medium fresh gas flows (3-6 l/min) were studied. Bacterial samples were taken preoperatively from the oropharynx and postoperatively from five locations in the circle system. The patients were studied postoperatively for signs of respiratory tract infection. There were few positive bacteria cultures from the tubings in the circle system, regardless of fresh gas flow. No pathogens were found in the inspiratory tubings and no cases of postoperative respiratory tract infection could be related to bacterial spread from the anaesthesia machine. There were no indications that the present hygienic management was insufficient for the medium- or the low-flow circle system techniques.     

Nitrous Oxide Exposure during Routine Anaesthetic Work

Nitrous oxide exposure in a modern hospital during routine anaesthetic work was measured using a technical exposure measurement technique and compared to measurement of biologic exposure from urine samples. The study included different anaesthetic situations and also a study of the efficiency of close scavenging and general air-conditioning in reducing nitrous oxide exposure. Exposure to nitrous oxide varied greatly. The mean nitrous oxide exposure in the total material was 53 ppm corresponding to approximately half the Swedish control limit (100 ppm) for 8 h time-weighted average (TWA). The only anaesthetic situation regularly resulting in 8 h TWA exposure exceeding the control limit was paediatric anaesthesia (92 +/- 67 ppm, mean +/- s.d.). The use of close scavenging significantly reduced the 8 h TWA nitrous oxide exposure in paediatric anaesthesia. The reduction of exposure was not significant during other forms of anaesthesia where low levels were found when anaesthetic equipment with excess gas scavenging was used in theatres with non-recirculating air-conditioning. The correlation between conventional technical exposure measurement and urine headspace nitrous oxide measurement was good. Both theoretical arguments and practical experience indicate that this method can be used for assessing nitrous oxide exposure during routine anaesthetic work.     

VALIDATION OF A SYSTEM FOR MEASUREMENT OF METABOLIC GAS EXCHANGE DURING ANAESTHESIA WITH CONTROLLED VENTILATION IN AN OXYGEN CONSUMING LUNG MODEL

An oxygen consuming lung model was used for evaluation and validationof a technique for metabolic gas exchange measurements duringcontrolled ventilation. The technique comprised a Servo 900C ventilator (Siemens) and separate oxygen and carbon dioxideanalysers (Beck-man). Measurements of oxygen consumption andcarbon dioxide production were made either by measuring inspiredand expired ventilation and gas fractions in these volumes orby measuring gas fractions and calculating expired ventilationfrom inspired by transformation (Haldane). Irrespective of theFl_O2, measured values correlated well with lung model settings:measured values were within ±2% of simulated. When Haldanetransformation was used with an Fl_O2 of 0.5 there was a significantunderestimation of oxygen consumption. Carbon dioxide productionvalues correlated well irrespective of the Fl_O2 used or methodof measurement of ventilation volume. Metabolic gas exchangemeasurements by measuring both inspired and expired ventilationvolumes may be used when inert gases are not in equilibrium,for example during nitrous oxide anaesthesia.     

METHOHEXITONE ANAESTHESIA FOR MICROLARYNGOSCOPY: CIRCULATORY MODULATION WITH METROPROLOL AND DIHYDRALAZINE

Anaesthesia for microlaryngoscopy was induced and maintainedwith fentanyl 3 µg kg^– and methohexitone (initialbolus of 1–1.5 mg-kg^–1 plus subsequent infusionof 4 mg kg^–1 h^–1). Suxamethonium was used to induceneuro-muscular blockade. Twenty minutes before induction ofanaesthesia, previously normotensive patients (n = 35), andpatients with hypertension well controlled by beta-receptorantagonists (n = 16) were pretreated with metoprolol (M) 0.2mg kg^–1i.v. and dihydralazine (DH) 0.2 mg kg^–1i.v.,dihydralazine 0.2 mg kg^–1 i.v. alone, or saline. Arterialpressure (AP) and heart rate (HR) were monitored: any arrhythmiaand ST₆₀T changes were noted. After the methohexitone infusionwas stopped, the times for emergence and full recovery wereshort (median 2 min 15s and 5 min later, respectively). Pretreatmentwith M+DH abolished increases in AP and HR during endoscopy.Arrhythmias were observed in fewer pretreated patients thanin controls (P < 0.05). ST₆₀-Tchanges in the ECG indicatingmyocardial ischaemia were found in two of 19 M+DH and in sixof 21 saline-pretreated patients. One of these six patientsdeveloped a myocardial infarction. Pretreatment with dihydralazinealone attenuated the pressor response to microlaryngoscopy,but was associated with consistently high HR and an incidenceof arrhythmias as well as ST₆₀T changes similar to that foundafter saline.     

REBREATHING CHARACTERISTICS OF THE BAIN CIRCUIT An Experimental and Theoretical Study

The Bain circuit was studied in a model lung on the assumptionthat, in addition to the ratio of fresh gas flow to total ventilation(FG/E), different time fractions of the respiratory cycle might influencerebreathing. We found that the time fraction for active expiration(FE_t) governed rebreathing for each FG/E value. With FE_t, as an independentvariable, a theoretical formula was derived for rebreathing.Rearranging this formula made it possible to calculate the necessaryincrease in ventilation to keep end-tidal carbon dioxide constantfor each FG/E. Thus, at a fresh gas flow of 70 ml kg^-1 min^-1,I has to be increased 2.6 times. For spontaneously breathing patients inhalation anaestheticsthat do not depress carbon dioxide sensitivity seem to be bettersuited to use in the Bain circuit. The FE_CO2 can then kept constantthrough increased ventilation in spite of the concomitant increasein rebreathing