Preservation of humidity and heat of respiratory gases during anaesthesia--a laboratory investigation

Humidification and heating of anaesthetic gases are desirable to prevent respiratory tract damage and a fall in body temperature during operative procedures. Numerous studies on the humidity and temperature of inspiratory gases in different breathing systems for anaesthesia have been carried out, but comparisons are difficult since different methods have been used. In this laboratory set-up we studied a non-rebreathing system with and without humidifiers and a circle absorber system with low (0.5 l/min) or medium (5 l/min) fresh gas flows regarding their ability to heat and humidify anaesthetic gases. The humidity of inspired gases was acceptable in the non-rebreathing system using either a Bennett Cascade humidifier or disposable humidifiers and in the circle absorber system using a fresh gas flow of 5 l/min or less. The temperature of the inspired gases was highest with the Bennett Cascade humidifier, followed by the low-flow circle system. The circle absorber system used with low fresh gas flow gave higher inspiratory gas temperature and humidity than the non-rebreathing system with a good disposable humidifier.

Oxygen analyzers in anaesthesia: performance in a simulated clinical environment

Nine commercial oxygen analyzers were tested in a laboratory model stimulating clinical anaesthesia conditions. Fifteen test situations were used in order to study the effects of nitrous oxide, humidity, positive end-expiratory pressure, halothane, enflurane and isoflurane. Errors exceeding 8 vol% were not uncommon, the dominating source of error being humidity. Analyzers with efficient dehumidification of gases before analysis performed better than analyzers without this property.

Epidural fentanyl counteracts sympathetic gastric inhibition

Postoperative paralytic ileus is in part mediated by the sympathetic gastrointestino-gastrointestinal (GI-GI) reflex. The modulation of this reflex by epidural fentanyl (50 micrograms) was studied in chloralose-anesthetized, ventilated cats. The vagi were cut in the neck but could be efferently stimulated. Gastric volume, arterial pressure and heart rate were followed and the GI-GI reflex was elicited by intestinal distension, mesenteric afferent nerve stimulation or heating or capsaicin administered intra-arterially to an intestinal loop. Epidural fentanyl enhanced the gastric contraction response to efferent vagal stimulation and considerably counter-acted the GI-GI reflex inhibition of vagally induced tone. These effects were reversed by epidural naloxone (10 micrograms). In contrast, 50 micrograms of fentanyl i.v. only enhanced the effect of vagal stimulation but had no influence on the inhibitory GI-GI reflex responses. Apparently, epidural fentanyl may inhibit the GI-GI reflex by a spinal point of action.

Nitrous oxide exposure during routine anaesthetic work. Measurement of biologic exposure from urine samples and technical exposure by bag sampling

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.

Influence of high-frequency pulses on alveolar gas exchange

A high-frequency (HF) unit which can be used in combination with a volume-cycled ventilator was tested in an oxygen-consuming lung model. Alveolar O2 and CO2 concentrations were not related to superimposed HF pulsations. With constant alveolar ventilation each basal frequency resulted in identical alveolar fractions of O2 and CO2. Addition of HF pulsations did not change the volume of CO2 or O2 exchanged between alveolar and dead-space compartments. Superimposed HF pulsations may affect gas exchange, but the verification of this hypothesis requires further studies.

Material thrombogenicity in central venous catheterization: a comparison between soft, antebrachial catheters of silicone elastomer and polyurethane

In order to compare two types of long, soft central venous catheters with the same stiffness, 39 silicone elastomer (SE) and 36 polyurethane (PU) catheters were inserted in 75 patients via basilic or cephalic veins punctured at the cubital fossa. Mean duration of catheterization was 10.5 days. Scanning electron microscopy revealed that the SE catheters to have a more uniform, but somewhat rougher surface topography than the PU catheters. The platelet adhesion in vitro to the SE catheters was four times higher than to the PU catheters. The incidence of clinical thrombophlebitis in the arm veins was 36% with the SE catheters, and 5.5% with the PU catheters (p less than 0.01). No significant differences were found between the SE and PU catheters regarding the number and size of radiologic thrombi in the peripheral and central veins, catheter occlusion rate, and platelet adhesion to the inner side of the catheter tip at withdrawal. Platelet adhesion in vivo correlated with the duration of catheterization in both groups of catheters. Mechanical trauma to the vein endothelium seems to be of vital importance in thrombus formation, but not in the induction of clinical thrombophlebitis.

Material thrombogenicity in central venous catheterization III. A comparison between soft polyvinylchloride and soft polyurethane elastomer, long, antebrachial catheters

Soft venous catheters, 36 made of polyurethane (PU) and 37 made of polyvinylchloride (PVC), were inserted in 73 patients via basilic and cephalic veins punctured at the cubital fossa, and compared regarding their thrombogenicity. The mean duration of catheterization was 12.5 days. Two patients with PU- and six patients with PVC-catheters developed clinical thrombophlebitis 4-36 days after insertion. The radiological thrombi were rather small with both PU- and PVC-catheters in both peripheral and central veins with no significant difference between the catheters. Also, the incidence of thrombosis and the rate of catheter occlusions were similar. The surface topography of the PVC-catheters was relatively smoother, but less uniform than that of the PU-catheters. Platelet adhesion in vitro to the catheter surfaces, expressed as ATP-concentration in relation to catheter area after contact with human blood, was relatively low with both catheters. The results suggest that soft PU- and PVC-catheters have a similar thrombogenicity.

Influence of anaesthesia on blood flow to the calves during surgery

The blood flow to the calves during abdominal surgery under different kinds of general anaesthesia was measured by strain gauge plethysmography in 12 patients. The volume flow increased 100% during surgery in both light and deep anaesthesia, caused by a decrease in peripheral resistance. The increase in volume flow could partly be explained by the anaesthesia itself, possibly induced by the use of a nitrous oxide-oxygen gas mixture. The surgical procedure caused a further increase in volume flow. Possible underlying mechanisms are discussed.

Rebreathing characteristics of the Bain circuit. An experimental and theoretical study

The Bain circuit was studied in a model lung on the assumption that, in addition to the ratio of fresh gas flow to total ventilation (VFG/VE), different time fractions of the respiratory cycle might influence rebreathing. We found that the time fraction for active expiration (FEt) governed rebreathing for each VFG/VE value. With FEt as an independent variable, a theoretical formula was derived for rebreathing. Rearranging this formula made it possible to calculate the necessary increase in ventilation to keep end-tidal carbon dioxide constant for each VFG/VE. Thus, at a fresh gas flow of 70 ml kg-1 min-1, VI has to be increased 2.6 times. For spontaneously breathing patients inhalation anaesthetics that do not depress carbon dioxide sensitivity seem to be better suited to use in the Bain circuit. The FECO2 can then be kept constant through increased ventilation in spite of the concomitant increase in rebreathing.

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 routine anaesthetic practice. There was a linear correlation with simultaneous measurements of technical exposure measured with a personal pump-bag sampling system (r = 0.99). Monitoring of urinary nitrous oxide with headspace extraction and gas chromatographic analysis is a simple and accurate mode of biological monitoring of exposure to nitrous oxide in the atmosphere.

Microvascular changes due to repeated local pressure-induced ischemia: intravital microscopic study on hamster cheek pouch

Pressure ulcer and tracheal injury occurring after endotracheal intubation are clinical examples of pressure-induced tissue damage where magnitude and duration of the applied pressure are of major importance. The objective of this study was to investigate the efflux of macromolecules caused by repeated short-time ischemia induced by a pressure of 60mmHg, which causes circulatory standstill. Pressure was applied to the hamster cheek pouch and the efflux of the macromolecules of the microvasculature was evaluated by using the fluorescein isothiocyanate (FITC) dextran dye and intravital microscopy. The pressure was applied for either 1 or 5 min and repeated 8 times with a 10min restitution period in between. The repeated 1min pressure caused almost no change in microvascular efflux of FITC-dextran while 5min repeated pressure did. The gross mechanical impact on the tissue seems to be equal in the 1 and 5min exposure and thus cannot explain the difference in the developed permeability. Ischemia of such short time as 1 and 5min does not alter the energy state of the tissue significantly. The reactive hyperemia and the mediating vasoactive amines can differ markedly after 1 and 5min ischemia and can thus contribute to the permeability alteration. However, the role of vasoactive amines is unclear and further experiments are needed to determine their role.

Material thrombogenicity in central venous catheterization II. A comparison between plain silicone elastomer, and plain polyethylene, long, antebrachial catheters

Central venous catheters of two types were inserted through basilic or cephalic veins punctured at the fossa cubiti in 61 patients: 39 soft silicone elastomer (S.E.) and 22 stiffer polyethylene (P.E.) catheters were compared regarding their thrombogenicity. Mean duration of catheterization was 6.5 days. Radiological thrombi in the peripheral and central veins visualized by phlebography were significantly smaller, and the incidence of the mural thrombi in the central veins significantly lower with the S.E. than with the P.E.-catheters, but the use of S.E.-catheters did not reduce the incidence of clinical thrombophlebitis in arm veins: 14 cases in the group with S.E. and four in that with P.E.-catheters. In both groups, the maximum incidence of clinical thrombophlebitis occurred 4-8 days after catheterization. Later, there seems to be a low risk for the appearance of clinical thrombophlebitis with both catheters. Neither the surface topography of the catheter materials, nor the platelet adhesion on their surfaces in contact with human blood in vitro offered conclusive arguments for interpretation of their thrombogenicity in vivo. Catheter stiffness seems to play an important role in inducing mural thrombus formation in central venous cannulation in man.

Stiffness of central venous catheters

Catheter stiffness has been suggested to be a principal factor in the thrombogenesis encountered after central venous cannulation. However, no data have been published to date about the stiffness of central venous catheters. A method for measuring catheter stiffness has been developed. The force needed to deflect a catheter tip 1.2 mm from a fastening point was measured with the help of a cantilever beam (Grass Model DA-7). Six different sections of each catheter were measured, and the final results expressed as an average of these. Twenty-seven central venous catheters made of silicone elastomer, polyurethane, polyvinylchloride, polyethylene and teflon were tested. The bending stiffness, EI (E = elastic modulus of the material, I = moment of inertia of catheter (a geometrical property)) was below 16 X 10(-6) Nm2 for all catheters made of silicone elastomer, polyvinylchloride and polyurethane. Polyethylene catheters were stiffer, but could be made softer by reduction of their diameters. Teflon catheters were up to 10 times stiffer than the catheters in the soft group. Heparinization and radioopacity of catheters do not significantly alter their bending stiffness. In a concomitant study the results indicate that there is a significantly lower incidence of thrombus formation in catheters with a bending stiffness below 16 X 10(-6) Nm2.

The anaesthetic management of a patient with familial dysautonomia

Familial dysautonomia is a rare inherited disease of the nervous system, involving mainly peripheral sensory and sympathetic nerves. The autonomic instability is of greatest importance for the anaesthetist although the sensory defect is the most dramatic symptom of the disease. A case report of a 15-year-old girl undergoing two operations for severe scoliosis is presented with a discussion of the anaesthetic problems encountered in this disease.

An oxygen-consuming model lung for evaluation of anaesthetic circuits

We have constructed an oxygen-consuming model lung based on pure combustion of hydrogen. Besides consuming oxygen the model produces water vapour, and carbon dioxide is delivered to the lung, thus mimicking carbon dioxide production. Oxygen consumption and carbon dioxide production can be set at desired values. The model lung can be used for investigations on and evaluation of all types of anaesthesia circuits. Results in a circle system with carbon dioxide absorption show that FIO2 is related to the fresh gas flow. The end tidal CO2 level is correlated to the alveolar ventilation and cannot be used for predicting FIO2.

Postoperative sore throat related to tracheal tube cuff design

Recent reports of the incidence of postoperative sore throat following anaesthesia with tracheal intubation have claimed that low volume high pressure cuffs are preferable to those with high volume and low pressure. In this study similar methods were used for evaluating postoperative sore throat. Randomly selected tracheal tubes were used in 56 patients undergoing elective abdominal surgery, followed by direct questioning about sore throat on the first postoperative day. The incidence of postoperative sore throat was 41 per cent with high volume low pressure cuffed tubes and 55 per cent with low volume high pressure cuffed tubes. This difference is not statistically significant, but the tendency of the results is contradictory to those published earlier. The incidence of postoperative sore throat varies greatly if direct or indirect questioning is used and also varies between studies using the same method of questioning. Therefore the validity of this method for evaluating the influence of cuff design must be questioned. Postoperative sore throat is a symptom caused by many factors, such as the intubation procedure and the use of stylets or lubricants. The incidence of postoperative sore throat does not necessarily reflect damage caused by the tracheal tube cuff.

Oxygen and carbon dioxide concentrations in anaesthetic circuits ventilated with an AGA UV 705 in an oxygen consuming model lung

An oxygen consuming an carbon dioxide producing model lung was used for evaluation of anaesthetic circle systems. Two different circles, the AGA metal circle and the AGA Monosorb circle ventilated by an AGA UV-705 were tested with reference to inspired oxygen concentrations and end-tidal CO2 levels. Carbon dioxide is dependent on alveolar ventilation and CO2 production and thus was stable throughout the experiments. The oxygen concentration of inspired gas decreased progressively with decreasing fresh gas flow. When using the fresh gas inlet on the ventilator outlet block, the inspired oxygen concentration fell 8% as compared to using the fresh gas inlet in the circle. Fresh gas flows under 3 1/min should only be used with oxygen monitoring in the circle. The fresh gas inlet must be directly into the circle, and not in the ventilator outlet block, to avoid hypoxia.

Close scavenging experimental and preliminary clinical studies of a method of reducing anaesthetic gas contamination

Long-term exposure to low concentrations of anaesthetic gases is potentially hazardous. In spite of efforts to reduce the exposure of personnel by increased air conditioning, by scavenging of excess circuit gases and by improvements in anaesthetic equipment, high concentrations of gases have still been measured. Problems arise during mask induction, in paediatric anesthesia, and in certain situations with unavoidable leaks, especially in small rooms but also in recovery rooms. Experimental and preliminary clinical studies have shown that nitrous oxide pollution can be considerably reduced by using a separate close scavenging system in the area where high gas concentrations occur. The geometrical and flow dimensions of a close scavenging device used in paediatric anaesthesia are described.

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 controlled ventilation with various tube dimensions and ventilation volumes. Ventilation with large volumes using small tracheal tubes results in high ventilator pressures. However, tracheal pressures are only marginally greater than those obtained with larger tubes. Small endotracheal tubes and high ventilation volumes result in a positive tracheal pressure at the end of expiration. The measured end-expiratory pressures are within the limits which might be used therapeutically (in PEEP). The force required to reshape endotracheal tubes of varvious dimensions to an "anatomical" shape was related to the tube dimensions; the beneficial effects of performed, "anatomically shaped" endotracheal tubes can be achieved by using small tubes of standard design.

The tracheal mucosa microvasculature and microcirculation. Intravital microscopic observations in rabbits and a histologic study in man

A method for the intravital microscopic study of the tracheal mucosal microcirculation in the rabbit is described. A low tracheostomy is performed for ventilation, while a hole cut in the ventral portion of the trachea cranial to the tracheostomy is used for observations. The trachea is immobilized by a fixation device. Microscopic observations are made with a Leitz stereo microscope. The microvasculature of the rabbit tracheal mucosa is characterized by centripetally arranged arterioles which subdivide into a fairly sparse capillary network at the mucosal surface. The capillaries drain into venular networks at different levels of the mucosa and collect finally into circumferentially arranged veins which run mainly between the cartilages. For comparative purposes the microvasculature of the human tracheal mucosa has been analysed in clarified (Spalteholz) preparations. This investigation shows that the microvascular architecture is principally the same in the human and rabbit tracheal mucosa. There is, however, a much denser capillary network in the human tracheal mucosa.