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.     

Nitrous oxide has different effects on the EEG and somatosensory evoked potentials during isoflurane anaesthesia in patients

Background: Electroencephalogram (EEG) and somatosensory evoked potentials (SEPs) are altered by inhalation anaesthesia. Nitrous oxide is commonly used in combination with volatile anaesthetics. We have studied the effects of nitrous oxide on both EEG and SEPs simultaneously during isoflurane burst-suppression anaesthesia.
Methods: Twelve ASA I-II patients undergoing abdominal or orthopaedic surgery were anaesthetized with isoflurane by mask. After intubation and relaxation the isoflurane concentration was increased to a level at which an EEG burst-suppression pattern occurred (mean isoflurane end-tidal concentration 1.9 (SD 0.2) %. With a stable isoflurane concentration, the patients received isoflurane-air-oxygen and isoflurane-nitrous oxide-oxygen (FiO₂ 0.4) in a randomized cross-over manner. EEG and SEPs were simultaneously recorded before, and after wash-out or wash-in periods for nitrous oxide. The proportion of EEG suppressions as well as SEP amplitudes for cortical N₂₀ were calculated.
Results: The proportion of EEG suppressions decreased from 53.5% to 34% ( P < 0.05) when air was replaced by nitrous oxide. At the same time, the cortical N₂₀ amplitude was reduced by 69% ( P < 0.01).
Conclusion: The results suggest that during isoflurane anaesthesia, nitrous oxide has a different effect on EEG and cortical SEP at the same time. The effects of nitrous oxide may be mediated by cortical and subcortical generators.     

Predictable nitrous oxide uptake enables simple oxygen uptake monitoring during low flow anaesthesia

The uptake rate of oxygen and nitrous oxide were studied during low flow anaesthesia with enflurane or isoflurane in nitrous oxide with either spontaneous or controlled ventilation. The excess gas flow and composition were analysed. The nitrous oxide uptake rate was in agreement with Severinghaus'formula _N20 1000.t^-0.5. The composition of excess gas was predictable and the following formula for oxygen uptake could be derived: O₂=fgO₂ -0.45 (fgN²O -(kg: 70.1000.t^-0.5)) where oxygen uptake rate (O₂, ml.min^-1) equals oxygen fresh gas flow (fgO₂) minus 0.45 times the difference between the fresh gas flow of nitrous oxide (fgN₂O), ml.min^-1 and estimated uptake of nitrous oxide. The equation assumes constant inspired gas concentrations of 30% oxygen and 65–70% nitrous oxide. The oxygen uptake rates calculated from this formula were in good agreement with measured uptake rates. Thus, continuous monitoring of oxygen uptake rates is possible by using only reliable flowmeters and analysis of inspried oxygen concentration.     

Sevoflurane low-flow anaesthesia: best strategy to reduce Compound A concentration

BACKGROUND: To define the best strategy to reduce Compound A production in Sevoflurane low-flow anaesthesia by experiments in vitro and in vivo of different absorbers and different anaesthesia machines. METHODS: In vitro Compound A has been measured at 45 degrees C in vitro following Sevoflurane interactions with potassium hydroxide, sodium hydroxide, soda lime, Dragersorb 800 Plus and Amsorb, a new absorber that does not contain sodium or potassium hydroxide. In vivo Compound A concentration in the anaesthesia circuit (inspiratory branch) has been measured using an indirect sampling method through absorber vials (SKC) with active coal granules, during low flows (500 ml/min) general anaesthesia using soda lime, Dragersorb 800 Plus or Amsorb as absorber. Compound A was also measured during low flows (500 ml/min) general anaesthesia using as carbon dioxide absorber soda lime with different anaesthesia machines. RESULTS: In vitro at 45 degrees C Compound A concentration with soda lime and Dragersorb 800 Plus was about 10 times higher than with Amsorb. In vivo the Compound A concentrations in the inspiratory branch of the circuit were lower in the group with Amsorb. CONCLUSION: The Compound A production is minimal with Amsorb as carbon dioxide absorber.     

Effect of low fresh gas flow rates on inspired gas composition in a circle absorber system

Study Objective: To determine the effects of fresh gas f ow on inspired gas composition during low flow anesthesia.

Design: Randomized trial with 2-hour observation periods in patients assigned to one of three groups.

Setting: Inpatient surgery clinic at a medical center.

Patients: Thirty-six patients undergoing abdominal surgery with low flow anesthesia. Interventions: Fresh gas flow was given at a starting rate of 5 L/min for 6 minutes. Thereafter, the fresh gas flow setting was nitrous oxide (N₂O) 1 L/min and oxygen (O₂) 0.6 L/min (Group 1), N₂O 0.5 L/min and O₂ 0.5 L/min (Group 2), and with a moderate surplus of N₂O and O₂ with respect to the patient's O₂ consumption (Group 3).

Measurements and Main Results: The inspired O₂ concentration (FIO₂) was measured using a paramagnetic technique, and N₂O levels were measured with infrared sensors; the inspired nitrogen concentration (FIN₂) was calculated by the following formula: (FIN₂) = 1 - FIO₂ - FIN₂O, where FIN₂O is the inspired N₂O concentration. After 1 hour of anesthesia, FIO₂ was significantly lower in Group 1 than in Groups 2 and 3 (p < 0.01), and FIN₂ was significantly higher in Groups 2 and 3 than in Group 1 (p < 0.01). After 2 hours of anesthesia, (FIN₂) returned to normal in Group 2 but continued to increase in Group 3. FIN₂O was close to 0.7% only in Group 1.

Conclusions: The same initial period of denitrogenation is not adequate to denitrogenate the circle system in all cases. The lower the fresh gas flow, the longer the initial period of denitrogenation should be. Various levels of fresh gas flow for low-flow anesthesia have been suggested, but none guarantees adequate control of inspired gas composition unless f owmeters are continuously adjusted.     

Respiratory effects of nitrous oxide during halothane or enflurane anaesthesia in children

The respiratory effects of nitrous oxide (N2O) were studied during halothane and enflurane anaesthesia in 12 children (mean age 46.4 +/- 29.3 months, mean weight 15.3 +/- 4.2 kg) during surgery under continuous extradural anaesthesia. Four equipotent anaesthetic states were studied in random order: 1) halothane 1 MAC in oxygen, 2) halothane 0.5 MAC + 50% N2O, 3) enflurane 1 MAC in oxygen, 4) enflurane 0.5 MAC +50% N2O. End-tidal fractions of CO2 (PetCO2) and halothane and enflurane were measured using infrared analysers. The respiratory variables (tidal volume VT, minute ventilation VE, respiratory frequency F, inspiratory time Ti, mean inspiratory flow VI, effective inspiratory time Ti/Ttot) were measured using a pneumotachograph. Significant changes were observed between the four states for VE, VI, F and PetCO2, whereas the values of VT, Ti and Ti/Tot did not differ significantly. The respiratory depressant effect of 1 MAC of either halothane alone or of the mixture of halothane and N2O was very similar. During enflurane anaesthesia, PetCO2 was less increased when N2O was substituted for enflurane, owing to a significant increase in respiratory frequency. A marked decrease in VE together with an increase in PetCO2 was observed during enflurane anaesthesia (states 3 and 4) when compared to the corresponding states during halothane anaesthesia (states 1 and 2). The respiratory depressant effect of enflurane is greater than that of halothane in unpremedicated children, even when substituting N2O for an equal MAC fraction of enflurane.2+ The effect of N2O on respiratory patterns seems to depend on the inhalational agent used and/or on the vesting respiratory frequency.     

Soda lime temperatures during low-flow sevoflurane anaesthesia and differences in dead-space

BACKGROUND: Sevoflurane degrades during low-flow anaesthesia to compound A, and high carbon dioxide absorbent temperatures cause increased degradation. The purpose of this investigation was to determine if larger tidal volumes, without increasing alveolar ventilation, decrease the temperature in the carbon dioxide absorber during low- and minimal-flow sevoflurane anaesthesia. METHODS: Prospective, randomized study, including 45 patients (ASA 1-2), scheduled for elective general or urology surgery. The patients were randomly assigned to one of three treatments. Patients in group 1 (NDS) received fresh gas flow of 1 litre/min without using additional dead-space volumes. In group 2 (DS + 1.0), the patients received fresh gas flow of 1 litre/min using additional dead-space volumes, placed between the Y-piece and the HME, and patients in group 3 (DS + 0.5) received the same technique with a fresh gas flow of 0.5 litre/min. The soda lime temperatures, dead-space volumes, end-tidal carbon dioxide, sevoflurane concentrations, ventilation volumes and pressures, absorbent weight and ear temperatures were measured. RESULTS: The maximum temperature of the soda lime was 44.1 +/- 1.1 degrees C in the NDS group, 37.8 +/- 0.8 degrees C in the DS + 1.0 group and 38.5 +/- 2.7 degrees C in the DS + 0.5 group (P<0.0001). The dead-space volume between the Y-piece the tracheal tube was 164 +/- 69 ml in the DS + 1.0 group and 196 +/- 15 ml in the DS + 0.5 group (P<0.05). The ventilator pressure were higher in the DS groups compared with the NDS group (P<0.001). Soda lime weight increased in all groups. End-tidal carbon dioxide, sevoflurane concentrations and ear temperatures were similar between the groups. CONCLUSION: Increasing dead-space volumes can reduce carbon dioxide absorber temperature during low- and minimal-flow sevoflurane anaesthesia.     

A double-blind comparison of lidocaine and mepivacaine during epidural anaesthesia

The effects of epidural anaesthesia with plain 2% lidocaine or mepivacaine were compared in 200 patients undergoing extracorporeal shock wave lithotripsy in a double-blind manner. The onset, spread, duration and quality of analgesia were similar in both groups. The numbers of patients who needed vasoconstrictor or atropine were almost equal in both the lidocaine and the mepivacaine groups. Mild but significant hypotension continued for a longer period in the mepivacaine group than in the lidocaine group. A transient decrease in arterial oxygen saturation was seen in two patients receiving lidocaine and in three patients receiving mepivacaine. Mild systemic toxicity was observed in eight patients in both groups, although serious complications such as convulsions did not occur. It is concluded that both drugs can be used equally safely for epidural anaesthesia, although the maximum recommended doses differ.     

Measurement of systemic oxygen uptake during low-flow anaesthesia with a standard technique vs. a novel method

We assessed agreement between measurement of systemic oxygen uptake using the Fick-derived method, and a novel method described by Biro, based on the difference in oxygen concentrations of the delivered fresh gas and the gas circulating in the circle system. Twenty-nine patients undergoing elective cardiac surgery were studied during stable haemodynamic and ventilatory conditions. Systemic oxygen uptake was measured using the two methods in each patient before and after cardiopulmonary bypass. Limits of agreement were found to be wide (-162 to 311 ml.min-1 before bypass, and -257 to 401 ml.min-1 after bypass), indicating poor agreement between the methods. No significant difference was found between the pre- and post cardiopulmonary bypass values for each method. We conclude that the Biro method, although attractive in terms of its simplicity, is an unreliable measure of systemic oxygen uptake under these conditions.     

Spinal anaesthesia with hyperbaric tetracaine: effect of age and body mass

To discover the influence of age and body mass on spinal analgesia with hyperbaric tetracaine, 60 patients (17-82 years) were administered a subarachnoid injection of 15 mg tetracaine in 3 ml of 5% glucose. The patients were in the lateral position during the injection and in the supine position thereafter. The maximal spread of analgesia (mean T4, range T1-T11) did not correlate with age or body mass. Any variable studied was not significantly different for the different age groups. In 29 patients (48%) the block required supplementation with fentanyl or general anaesthesia (five patients). Tourniquet-induced pain occurred in eight patients (28% of patients with tourniquet). Decreases in systolic arterial blood pressure, greater than 30% of the preanaesthetic value, correlated with the level of analgesia and were more frequent in patients over 50 years (P less than 0.01). Severe hypotension and bradycardia with transient serious changes in QRS-complexes occurred in one patient (analgesia level, T3). In conclusion, 15 mg of hyperbaric tetracaine provided a relatively high sensory spinal blockade without correlating with age or body mass. Supplementation of analgesia during surgery was needed in almost half of the patient material.     

Nitrous oxide uptake during spontaneous and controlled ventilation

The rate of uptake of nitrous oxide was studied in 40 orthopaedic patients anaesthetised with either enflurane or isoflurane in nitrous oxide and with either spontaneous or controlled ventilation. A variant of the Douglas bag method was used in combination with low fresh gas flows to a circle system. There were no significant differences in nitrous oxide uptake between the groups and the uptake rates followed'the square root of time concept', with an overall best fit curve of 1080.t^-0.505 ml.70 kg^-1.min^-1. During spontaneous ventilation, the nitrous oxide uptake rate was similar or even higher than the corresponding rate during controlled ventilation, in spite of lower minute volumes.     

Characteristics of muscle nerve sympathetic activity during general anaesthesia in humans

General anaesthesia influences the cardiovascular system at different levels. To increase our knowledge of how sympathetic outflow is affected by anaesthetic interventions, we used direct microelectrode recordings of muscle sympathetic activity (MSA) in the peroneal nerve in 18 patients scheduled for ENT-surgery. During induction of anaesthesia (propofol 2.1 mg/kg or methohexitone 1.4 mg/kg), all patients showed reductions in MSA (from 41 +/- 4 to 20 +/- 4 bursts/min). With endotracheal intubation there was a sudden increase of activity and in several patients the normal pulse-synchrony of MSA was lost temporarily. Maintenance of anaesthesia with nitrous oxide (n = 12) increased MSA and methohexitone (n = 2), propofol (n = 3) and isoflurane (n = 8) decreased MSA, while the effect of halothane (n = 3) varied. Baroreflex mechanisms were still operative but seemed to be depressed in relation to anaesthetic agent and depth. Laryngeal and surgical stimuli caused increases in MSA and blood pressure, lasting several minutes after the stimulation. It is concluded that the strength of MSA is profoundly influenced by the choice of anaesthetic agent. A suppression of activity is more common than an increase. Qualitatively, several sympathetic reflexes operate in a similar way during light anaesthesia as in awake subjects, but are depressed or absent during deep anaesthesia.     

Comparison of costs of different anaesthetic techniques

The costs of anaesthetic drugs, intravenous agents as well as gases, were studied for different anaesthetic techniques in a medium-sized operative procedure, cholecystectomy. Three anaesthetic breathing systems were used: a non-rebreathing system, a circle absorber system with medium fresh gas flows of 3-6 l/min, and a low-flow circle system. Anaesthesia without volatile inhalation agents used with a low-flow technique was the least expensive, and anaesthesia with isoflurane in a non-rebreathing system was the most expensive. The costs of anaesthesia without volatile inhalation agents in a non-rebreathing system, enflurane anaesthesia in a circle system with medium fresh gas flows, and isoflurane anaesthesia with low-flow technique were similar.     

Humidification during low-flow anesthesia in children

Purpose The aim of this study was to compare the effect of low-flow anesthesia with or without a heat and moisture exchanger with high-flow anesthesia on airway gas humidification in children. Methods One hundred twenty children were randomly assigned to one of three groups: low-flow anesthesia with 0.5l·min⁻¹ of total gas flow (LFA,n=40), low-flow anesthesia with 0.5l·min⁻¹ using a heat and moisture exchanger (HME,n=40), and high-flow anesthesia with 6l·min⁻¹ (HFA,n=40). The temperature and relative humidity of the inspired gas were measured throughout anesthesia. Results The relative humidity of the inspired gas in the HME group was increased compared with that of the LFA and HFA groups 20 min after induction (p<0.05). The airway humidification in the LFA group was higher than that in the HFA group 10 min after induction (p<0.05). The temperature of the inspired gas in the HME group was increased compared with that in the LFA and HFA groups after 70 min (P<0.05). Conclusion Low-flow anesthesia is less effective in providing adequate humidification of inspired gas than low-flow anesthesia with a heat and moisture exchanger, but significantly better than high-flow anesthesia in children.     

Influence of nitrous oxide on induction of anaesthesia with sevoflurane

Nitrous oxide is often used during inhalation induction of anaesthesiawith sevoflurane. Although the value of using nitrous oxideduring inhalation induction with other volatile anaestheticshas been studied, the popularity of sevoflurane induction andthe different characteristics of this agent make a study ofthe combination of nitrous oxide with this agent of interest.We compared induction times, oxygenation, and excitatory eventsduring inhalation induction of anaesthesia using sevoflurane,with and without nitrous oxide. We studied 64 female patients,randomly allocated to receive inhalation induction of anaesthesiausing sevoflurane with or without 50% nitrous oxide in the freshgas, using a co-axial breathing system (Mapleson D) and a freshgas flow rate of 3–6 litre min^–1. Mean time to inductionof anaesthesia (fall of an outstretched arm) was 102 s in bothgroups, but excitation (limb or head movement) was more frequentin those receiving nitrous oxide (10 patients) than in thosereceiving oxygen only (five patients) (P<0.05). Oxygenationwas similar in both groups. We conclude that nitrous oxide confersno advantage when anaesthesia is induced with sevoflurane inthis way. Br J Anaesth 2001; 87: 286–8     

Buprenorphine as premedication and as analgesic during and after light isoflurane-N2O-O2 anaesthesia. A comparison with oxycodone plus fentanyl

Sixty patients undergoing gynaecological laparotomies under isoflurane anaesthesia received 0.4 mg of buprenorphine sublingually or 0.12 mg/kg of oxycodone intramuscularly in random order for preanaesthetic medication. Patients premedicated with buprenorphine were given buprenorphine before, during and after anaesthesia and patients premedicated with oxycodone received fentanyl before and during anaesthesia and oxycodone after anaesthesia. Buprenorphine premedication produced less drowsiness and sedation and alleviated patients' apprehension significantly (P less than 0.05) less than oxycodone. Systolic and diastolic blood pressure and heart rate were significantly (P less than 0.05 to P less than 0.01) higher after intubation in the buprenorphine group when compared with the oxycodone plus fentanyl group. After anaesthesia, spontaneous respiration started rapidly; the return of consciousness and immediate recovery occurred at the same rate in both groups. In the recovery room moderate to severe pain was more common (P less than 0.05) in the oxycodone plus fentanyl group than in the buprenorphine group. The respiratory rate in the recovery room was lower among patients given buprenorphine, and two patients given buprenorphine developed severe respiratory depression. In the ward (2 to 24 h after operation) sublingual buprenorphine provided pain relief as good as intramuscularly administered oxycodone. No differences were noted in the incidence or severity of emetic symptoms between the groups. It is concluded that buprenorphine can provide good postoperative pain relief for gynaecological laparotomies performed under light isoflurane anaesthesia, but patients need to be monitored carefully after operation because of the possibility of respiratory depression.     

Nitrous oxide anaesthesia and vomiting

Eighty-seven patients undergoing routine laparoscopy were divided randomly into two groups to study the effect of nitrous oxide anaesthesia on the incidence of postoperative vomiting. Patients in group A received nitrous oxide as part of their anaesthetic, while in group B nitrous oxide was omitted. Significantly fewer patients in group B vomited when compared with group A (17 percent and 49 percent respectively; p less than 0.005). We suggest that an anaesthetic technique which avoids nitrous oxide may be especially indicated in patients undergoing laparoscopy.     

Speed of collapse of the non-ventilated lung during one-lung anaesthesia: the effects of the use of nitrous oxide in sheep

By enhancing gaseous uptake from the non-ventilated lung during procedures performed thoracoscopically, the rapid diffusion properties of nitrous oxide would be expected to speed lung collapse and so facilitate surgery. To assess the effect of nitrous oxide on the speed of absorptive lung collapse, a study was conducted using 11 anaesthetised sheep. Speed of collapse was assessed in an indirect manner by recording the time required in a closed-chest situation for the airway pressure distal to a single lung airway occlusion to decrease to - 1.0 kPa. The influence of nitrous oxide was assessed by comparing the time taken for this decrease in airway pressure when the animal was being mechanically ventilated with 50% nitrous oxide in oxygen with the time taken when using 100% oxygen. In all assessments, it was found that the decrease in airway pressure to - 1.0 kPa occurred in a shorter time when nitrous oxide was used. The findings lend support to the hypothesis that during thoracoscopic surgery, mechanical lung ventilation with an oxygen/nitrous oxide mixture will increase the rate of gaseous uptake from the non-ventilated lung and so hasten its absorptive collapse.     

Local anaesthesia and propofol-fentanyl sedation for carotid artery surgery

Background: In patients undergoing carotid artery surgery with local anaesthesia (LA), a sedative / analgesic pharmacological supplement is appropriate in most cases in order to provide comfort. This adjunct should not preclude continuous clinical neurological monitoring. The aim was to investigate if a combination of fentanyl and propofol to supplement LA would provide comfort for the patient, allow continuous clinical neurological monitoring and absence of difficulties for the anaesthetist, and good conditions for surgery, including insertion of a shunt if this should become necessary. Methods: During a 1-year period low doses of propofol and fentanyl were used to supplement LA in 36 cases of carotid artery surgery in 34 consecutive patients. A shunt was only used if neurological dysfunction occurred. Data on haemodynamics, pulmonary gas exchange, clinical neurological monitoring, and subjective opinions from patients, surgeons and anaesthetists were obtained. Morbidity within 30 days was documented. Results: Conversion to general anaesthesia was undertaken in one patient, previously operated on the same artery, who became unconscious due to a stroke during manipulation of the artery before arteriotomy. No other adverse outcome was found within 30 days. In the remaining 35 cases the procedures were carried out under LA. Stump pressures below 50 mmHg were found in 17/35 cases. Intraoperative neurological dysfunction was detected in 10/35 cases (stump pressures between 23 and 60 mmHg). Shunting was easily performed, and rapidly relieved the neurologic symptoms in all these patients. Intraoperative respiratory and haemodynamic control was satisfactory. Ease of performance, including clinical neurological monitoring, was acknowledged by both anaesthetists and surgeons, and all 33 patients (35 operations) who were accessible for a postoperative interview stated that they would prefer the same regimen in the case of further surgery. Conclusion: The number of cases in this open, uncontrolled study does not permit an evaluation of this anaesthetic and sedative technique in terms of neurological and cardiac outcome. Thus, we simply want to inform about our positive experiences regarding patient acceptance and ease of performance in all relevant respects when fentanyl and propofol are used to supplement LA for carotid artery surgery.     

Choice of cuff inflation medium during nitrous oxide anaesthesia

Air, saline and a mixture of nitrous oxide and oxygen were used to inflate the cuffs of tracheal tubes inserted into three groups of patients, during nitrous oxide anaesthesia. Large increases in intracuff pressure were seen in the air group, but no morbidity resulted. The intracuff pressure decreased in the gas mix group and leaks occurred in some patients. Initial adjustment of cuff pressure was more difficult in the saline group but the pressure remained stable thereafter.