The Relationship between Calculated Effect-site Sevoflurane Levels and Awakening from Anaesthesia

We have previously described a system that displays real-time estimates of effect-site sevoflurane concentrations. Estimates of effect-site levels should be similar to minimum alveolar concentration (MAC) values, which are determined after allowing time for equilibrium. This study aimed to determine estimated effect-site sevoflurane concentrations at awakening from routine anaesthesia and to compare this with published estimates of MAC-awake. If these values were similar, this would validate our approach to the calculation of effect-site concentration. Sixty-five patients undergoing a variety of surgical procedures were observed. Prior to disconnection from the breathing circuit, forward estimates of effect-site sevoflurane were recorded. Patients were observed in the postanaesthesiacare unit and the time at which they responded to comm and wasrecorded. Age-adjusted effect-site sevoflurane at the time of awakeningwas determined. Correlation with patient, surgical and anaestheticfactors includingage, gender, ASA status and intraoperative opioid usage were explored. Mean age-adjusted calculated effect-site concentration at awakening was 0.59 (SD 0.27) vol%. This value is within the range of values determined for MAC-awake of sevoflurane. There was no correlation with any of the demographic or anaesthetic factors, but patients undergoing major surgery woke at a significantly lower mean sevoflurane level. These results support the use of effect-site sevoflurane concentration to guide administration of anaesthesia.

Direct measurement of nitrous oxide kinetics

BACKGROUND

Using conscious subjects, measurement of the effects of low concentrations of anaesthetic agents can allow the dynamics of onset and offset of the agent to be measured and kinetic values estimated. However, the tests have to be rapid and preferably assess cerebral function.

METHODS

We used a short version of the digit symbol substitution test (DSST) that allowed frequent measurement of the impairment caused by nitrous oxide. We compared 10 min of onset and offset of breathing 5% and 30% nitrous oxide in 30% oxygen, compared with 30% oxygen only. End-tidal nitrous oxide concentrations were used to predict the concentration in a central compartment, according to a range of T(1/2) values chosen to be consistent with possible cerebral blood flow values.

RESULTS

We studied 19 volunteers and estimated a mean response. Only 30% nitrous oxide decreased the DSST. When DSST scores were related to the values in the predicted central compartment, the best dose-effect relationship was found when the T(1/2) was 37 s, consistent with a regional blood flow of about 120 ml 100 g(-1) min(-1).

CONCLUSIONS

The onset of nitrous oxide effect on DSST is rapid, consistent with the perfusion of metabolically active cerebral cortical tissues. The rate of onset is greater than previous measures based on a motor test which involved the function of subcortical structures in the central nervous system.

The effect of a graphical interpretation of a statistic trend indicator (Trigg's Tracking Variable) on the detection of simulated changes

Anaesthesia involves processing large amounts of information over time. One task of the anaesthetist is to detect substantive changes in physiological variables promptly and reliably. It has been previously demonstrated that a graphical trend display of historical data leads to more rapid detection of such changes. We examined the effect of a graphical indication of the magnitude of Trigg's Tracking Variable, a simple statistically based trend detection algorithm, on the accuracy and latency of the detection of changes in a micro-simulation. Ten anaesthetists each viewed 20 simulations with four variables displayed as the current value with a simple graphical trend display. Values for these variables were generated by a computer model, and updated every second; after a period of stability a change occurred to a new random value at least 10 units from baseline. In 50% of the simulations an indication of the rate of change was given by a five level graphical representation of the value of Trigg's Tracking Variable. Participants were asked to indicate when they thought a change was occurring. Changes were detected 10.9% faster with the trend indicator present (mean 13.1 [SD 3.1] cycles vs 14.6 [SD 3.4] cycles, 95% confidence interval 0.4 to 2.5 cycles, P = 0.013. There was no difference in accuracy of detection (median with trend detection 97% [interquartile range 95 to 100%], without trend detection 100% [98 to 100%]), P = 0.8. We conclude that simple statistical trend detection may speed detection of changes during routine anaesthesia, even when a graphical trend display is present.

A New Zealand based cohort study of anaesthetic trainees' career outcomes compared with previously expressed intentions

Predicting workforce requirements is a difficult but necessary part of health resource planning. A 'snapshot' workforce survey undertaken in 2002 examined issues that New Zealand anaesthesia trainees expected would influence their choice of future workplace. We have restudied the same cohort to see if that workforce survey was a good predictor of outcome. Seventy (51%) of 138 surveys were completed in 2009 compared with 100 (80%) of 138 in the 2002 survey. Eighty percent of the 2002 respondents planned consultant positions in New Zealand. We found 64% of respondents were working in New Zealand (P < 0.01). We found that family ties were an important influence on the choice of country of residence for 80% of New Zealand based respondents but only 40% of those living outside New Zealand agreed or strongly agreed with this statement (P < 0.01). Remuneration influenced country of residence for 76% of those living outside New Zealand but was important for only 2% of those resident in New Zealand (P < 0.01). Salaries in New Zealand were predominantly between NZ$150,000 and $200,000 while those overseas received between NZ$300,000 and $400,000. Of those that are resident in New Zealand, 84% had studied in a New Zealand medical school compared with 52% of those currently working overseas (P < 0.01). Our study shows that stated career intentions in a group do not predict the actual group outcomes. We suggest that 'snapshot' studies examining workforce intentions are of little value for workforce planning. However we believe an ongoing program matching career aspirations against career outcomes would be a useful tool in workforce planning.

The Development of a System to Guide Volatile Anaesthetic Administration

We have developed and deployed within our operating rooms a system which provides real-time estimates of effect-site levels of inhalational anaesthetic agents along with forward predictions of end-tidal and effect-site concentrations. The initial aim of this project was to provide users of inhalational agents with tools similar to those available in target-controlled infusion systems. This paper describes the development and implementation of the system and outlines evaluation and uses of the system.

The prototype was developed by combining a locally developed data logging and trend display system with a model of uptake developed as a teaching tool in 1982. This uptake model performs as well as contemporary models of propofol uptake and distribution. Following initial evaluation, the system has been deployed in over half our operating rooms and uses data gathered from the Datex/GE Anaesthesia Delivery Unit anaesthetic machines. We have conducted a number of studies of the system itself, explored aspects of the underlying models, and used the system to investigate effect-site guided anesthesia and as a tool for data collection in other studies.

The system has been well accepted locally and has been shown to facilitate faster changes in inhalational levels. We have also seen a significant decrease in fresh gas flow rates over recent years and attribute this in part to the predictive system, which simplifies the task of determining the appropriate combination of gas flow and vapour dial setting. The system also provides a useful platform for a range of research projects.

The effect of predictive display on the control of step changes in effect site sevoflurane levels

Graphical displays of past and future levels of drugs may be a useful adjunct to manual dosing. We have previously found that a display of predicted future values speeds step changes in end-tidal sevoflurane. In this study anaesthetists made step changes of 0.3% in effect site sevoflurane, with and without the display and as increases and decreases. We analysed 91 changes. When the predictive display was present, users made larger vaporiser dial changes of 3.9% vs 3.1% (95% CI for the difference -1.3% to -0.01%, p = 0.046) reflected in larger end-tidal changes (95% CI for the difference -0.009 vol% to -0.34 vol%, p = 0.06). There was no difference in the speed of change (220 vs 227 s (95% CI for the difference -51 to 32 s)), or in the accuracy of the change. In this study the predictive display influenced the magnitude of the step changes made by anaesthetists but did not affect the speed or overall accuracy of the change.

Influence of general health and degree of surgical insult on long-term survival

BACKGROUND

This study investigated the relationship of American Society of Anesthesiologists Physical Status (ASA-PS) grade and degree of surgical insult to long-term postoperative survival.

METHODS

National death records to June 2007 were matched against records of patients undergoing elective surgery between January 1997 and December 2001. Stratified survival analysis was performed to allow baseline hazard functions to vary among four patient groups (15-64 years with no malignancy, at least 65 years with no malignancy, 15-64 years with malignancy and at least 65 years with malignancy).

RESULTS

Of 8134 patients, 6185 (76.0 per cent) were alive after a median follow-up of 7.1 (range 0-10.5) years. The overall mortality rate was 3.62 (95 per cent confidence interval (c.i.) 3.46 to 3.78) per 100 person-years. The 10-year probability of survival was significantly higher in ASA-PS I or II for minor or intermediate surgery (90.7 (89.1 to 92.1) per cent) than in ASA-PS I or II for major or complex major surgery (79.6 (77.5 to 81.6) per cent), ASA-PS III or IV for minor or intermediate surgery (41.2 (36.2 to 46.7) per cent) and ASA-PS III or IV for major or complex major surgery (44.6 (41.4 to 47.7) per cent) (P < 0.001). Priority of admission modified survival probabilities. Adjusted survival probabilities were lowest in the elderly with malignancy.

CONCLUSION

ASA-PS grade has a more significant and persistent effect on long-term survival than degree of surgical insult.

The effect of epidural analgesia on postoperative outcome after colorectal surgery

OBJECTIVE

The aim of this review was to determine the effects of epidural analgesia as it relates to outcome after colorectal surgery.

METHOD

We searched and reviewed studies that included colorectal surgery and epidural method of analgesia listed on the Pubmed, Medline, Embase and the Cochrane library database.

RESULTS

The majority of data demonstrate a superior effect of epidural analgesia on pain control after colorectal surgery. Well designed randomized controlled trials (RCT's) have also shown that epidural analgesia reduces the duration of ileus after colorectal surgery. Limited data suggest the additional benefit may be minimal after laparoscopic surgery or when epidural analgesia is used as part of a multimodal regime. Data does not convincingly show either a clear harmful or beneficial effect of epidural analgesia on rates of anastomotic leakage. Epidural analgesia may have beneficial effects on postoperative lung function, however due to low numbers, the effects on cardiovascular and thromboembolic complications are indeterminate. Length of hospital stay has not been shown to be shortened by sole use of an epidural and, although epidural analgesia may be apparently more costly, alternatives may incur higher indirect costs and decreased patient satisfaction.

CONCLUSION

Randomized controlled trials have shown a benefit for epidurals on postoperative pain relief, and ileus, and possibly respiratory complications. There is no proven benefit with regard to length of stay. There are a number of unresolved issues which further focussed RCT's may help clarify such as effects of epidural on complication rates after colorectal surgery.

Pharmacokinetics of sevoflurane uptake into the brain

Two recent studies have examined the pharmacokinetics of sevoflurane in adults. Lu et al.(Pharmacokinetics of sevoflurane uptake into the brain and body, Anaesthesia 2003; 58: 951-6) observed that jugular bulb sevoflurane concentration initially rose unexpectedly rapidly and then approached arterial concentrations unexpectedly slowly, suggesting that a blood-brain diffusion barrier exists. They also observed a large alveolar-arterial sevoflurane gradient, suggesting that an alveolar-arterial diffusion barrier exists. Nakamura et al. (Predicted sevoflurane partial pressure in the brain with an uptake and distribution model: Comparison with the measured value in internal jugular vein blood. Journal of Clinical Monitoring and Computing 1999; 15: 299-305) found no diffusion barriers. We used a computer model to analyse both data sets and show that the observations of Lu et al. can be explained by contamination of jugular samples with extracerebral blood. It is possible that the alveolar-arterial gradients observed by Lu et al. are due to discrepancies in conversions between blood concentrations and gas partial pressures. Our study suggests that there is no blood-brain diffusion barrier for sevoflurane and that the data of Lu et al. must be interpreted with caution.

The effect of cardiac output changes on end-expired volatile anaesthetic concentrations--a theoretical study

Cardiac output is one of the major determinants of the rate of uptake, and therefore the end-expired concentration (F(E')) of volatile anaesthetic agents. The purpose of this theoretical study was to explore the effect of cardiac output changes on F(E') for a range of volatile anaesthetics. A multicompartment model of anaesthetic uptake and distribution which produces constant values of F(E') was used. The minimum detectable change in cardiac output was determined for a variety of anaesthetic agents for four patterns of cardiac output change. The effect of a step change in cardiac output from 5 to 10 l.min(-1) was also recorded. The smallest cardiac output changes (average 33%) were detected with isoflurane. As blood solubility increased or decreased, larger cardiac output changes were needed before they could be detected. With a large step change in cardiac output and with increasing solubility, the final change in F(E') increased but the initial rate of change of F(E') is decreased. A significant cardiac output change will produce a change in volatile anaesthetic uptake. An unexpected change in F(E') should be considered as a possible signal of a sudden cardiac output change. The difference between agents may represent a balance between the amount of agent taken up and the size of the tissue 'sink' for that agent.

The effect of cardiac output changes on end-tidal volatile anaesthetic concentrations

Cardiac output is a major determinant of the rate of uptake of volatile anaesthetic agents. Theoretical work suggests a 30% change in cardiac output will produce a measurable change in end-tidal volatile agent. We present three patients in whom significant changes in haemodynamic parameters, consistent with a large fall in cardiac output, were accompanied by an increase in end-tidal volatile agent concentrations. The changes in end-tidal volatile agent concentrations were comparable in magnitude but in the opposite direction to the changes in end-tidal CO2. Clinically, an unexplained change in end-tidal concentration of a volatile agent may signify a large change in cardiac output.

A breathing circuit disconnection detected by anesthetic agent monitoring

PURPOSE

To describe a case involving a spontaneously breathing patient where a circuit disconnection was detected by a change in monitored anesthetic agent parameters.

CLINICAL FEATURES

A patient undergoing shoulder surgery was breathing spontaneously from a circle type anesthesia circuit via a laryngeal mask. A disconnection occurred between the heat and moisture exchanger (HME) and the circle system's Y-piece. As the gas sampling port was integrated into the HME a near normal pattern of CO2 continued to be displayed. The disconnection was noted because of a change in the graphical display of the volatile agent concentration.

CONCLUSIONS

Anesthetic circuit disconnection can be difficult to detect, especially in the spontaneously breathing patient. Capnometry may not detect a disconnection on the machine side of the gas sampling port. Changes in oxygen and volatile agent concentrations may provide an early indication of these types of disconnection.

Preoperative core temperatures in elective surgical patients show an unexpected skewed distribution

PURPOSE

To document the preoperative core temperature of adult elective surgical patients.

METHODS

A prospective audit obtained sublingual temperatures from 446 adult elective surgical patients on arrival in the preoperative holding area.

RESULTS

Temperatures ranged from 35.7 degrees C to 37.8 degrees C with a mean of 36.5 degrees C (0.4 SD). The median was 36.4 degrees C and the mode was 36.1 degrees C. There was a skewed distribution with a clustering of values at the lower end of the range. All recordings were within the accepted normothermic range.

CONCLUSION

The asymmetric distribution we observed differs from previously published normothermia data which shows a symmetrical distribution of temperatures. This skewed distribution has not previously been documented and we interpret it as being due to the effect of preoperative cooling factors.

Disconnect alarm failure in detection of common gas outlet disconnection

This laboratory study was prompted by two paediatric cases where low pressure alarms and capnography failed to detect common gas outlet disconnection when using a T-piece pump ventilator. A carbon dioxide producing model lung was ventilated using the Clare ventilator (a T-piece pump type ventilator) via an Ayres T-piece. The T-piece used has 3 mm diameter fresh gas tubing. Common gas outlet disconnects were carried out using varying fresh gas flows and ventilatory patterns. At fresh gas flows of less than 8 l/min, the low pressure alarm of the Clare ventilator failed to sound following common gas outlet disconnection. Oxygraphy showed a rapid rise in inspired oxygen. End-tidal CO2 rose, whilst inspired CO2 remained at zero. This experiment demonstrates that the Clare ventilator's low pressure alarm detects common gas outlet disconnection poorly when used with an Ayres T-piece with narrow fresh gas tubing. Graphical representation of airway oxygen content has merit as an alerting monitor for common gas outlet disconnection.

A modified Trigg's Tracking Variable as an 'advisory' alarm during anaesthesia

We wished to assess the accuracy of a modified form of Trigg's Tracking Variable (TTV) at detecting the onset of changes in systolic blood pressure. A computer model generated systolic blood pressures which changed to a new value after period of stability. A separate algorithm based on TTV indicated when a 'significant' change had been detected. This signal occurred when TTV had exceeded a set limit (0.8-0.99) a predetermined number of times (1-10). Five anaesthetists were shown 40 sets of data generated by same the computer model and asked to indicate the onset of changes. The greatest number of changes (94.1%) were correctly identified when TTV exceeded 0.92 on 4 consecutive determinations. The onset of the trend was detected after an average delay of 140 s. The anaesthetists correctly detected 85% of the changes after an average delay of 162 s. There was no statistically significant difference between the anaesthetists and the algorithm, although only one anaesthetist performed better than the modified TTV. The modified TTV detected changes in simulated invasive systolic blood pressures faster and more accurately than four of a group of five anaesthetists. Such simple trend detection systems may be useful as 'advisory' alarms.

Infusing liquid anaesthetics into a closed circuit

Closed circuit anaesthesia offers the user many advantages but practical problems impede its widespread use. When conventional vaporizers are employed adequate amounts of agent cannot be delivered into a totally closed circuit during the early stages of an anaesthetic. Direct injection, or infusion, of liquid anaesthetic into the circuit overcomes this problem. The standard method for injecting agents directly into circuits is that described by Lowe and Ernst. Their system can be approximated to a series of constant rate infusions, as is frequently done for propofol, and forms the basis of our technique. For anaesthesia without nitrous oxide, liquid isoflurane should initially be infused into the circuit at a rate in ml/hr of 14 + 0.4 x weight in kg. After five minutes the infusion rate is reduced to 20% of this value. These rates are altered in the light of measured concentrations and clinical responses.

Vaporization of isoflurane by liquid infusion

We investigated the vaporization of liquid isoflurane when infused directly into a circuit. Pooling of isoflurane occurred within the circuit tubing at infusion rates used during clinical practice when constant gas flows were used. Despite pooling, the concentration of isoflurane was linearly related to infusion rate. Cyclical gas flow, such as that seen in a circle system, increased vaporization so that pooling occurred only at the higher infusion rates used during the first five minutes of totally closed circuit anaesthesia. There were no major differences in pooling or the maximum concentration of isoflurane reached between 26 gauge needle and droplet administration of isoflurane: however the maximum concentration was reached more quickly by droplet administration. We conclude that direct infusion of liquid isoflurane into an anaesthetic circuit will result in complete vaporization during maintenance anaesthesia.

Analysis of uncertainty in theoretical methods of cardiac output measurement using the "Monte Carlo" technique

We have developed three models which describe the relationship between cardiac output and the uptake of volatile anaesthetic agents, based on the Fick equation, and determined if these models could provide useful methods of cardiac output measurement. Because many variables are involved in the calculation of cardiac output using these methods, a "Monte Carlo" simulation was performed to investigate the combined effect of uncertainties in several variables on the resultant cardiac output estimate. We found that the single-breath model was most accurate when the inspired concentration was large, while the rebreathing model was better with smaller inspired concentrations. The three-breath model was the least accurate under all conditions studied. Volatile anaesthetics were generally more accurate than nitrous oxide, with both enflurane and halothane more accurate than isoflurane. The "Monte Carlo" technique provides a valuable tool for analysis of errors in measurement methods.

Solubility characteristics of the ideal agent for measurement of cardiac output by soluble gas uptake methods

Measurements of the rate of uptake of soluble gases such as nitrous oxide or acetylene has been used as the basis of a method of cardiac output estimation since 1912. Using theoretical models of single, multiple and rebreathing techniques of cardiac output measurement, we determined the effect of solubility in blood and lung tissue on the changes in the final expired concentration of the gas. Decreasing lung tissue solubility increased the sensitivity of all three models to cardiac output changes. When the lung tissue/blood partition coefficient was 1, the optimum blood/gas partition coefficients were 2.6 for the single-breath model, 4.1 for the three breath-model and 3.5 for the rebreathing model. A selection of gases, including volatile anaesthetic agents was studied using the same models. Under most conditions, enflurane approximated most closely to the requirements for an ideal agent for use in this technique of cardiac output measurement.

Gas uptake from an unventilated area of lung: computer model of absorption atelectasis

A computer model of gas uptake from an area of nonventilated lung, such as a pulmonary lobe with an occluded bronchus or an alveolus with an occluded airway, is presented. Previous analyses have assumed that when an inert gas is present, equilibration of O2 and CO2 with mixed venous blood is sufficiently rapid to be treated as instantaneous. This is valid for insoluble gases such as N2 or He when the fractional concentration of inspired O2 (FIO2) is < or = 0.6 but is invalid for a relatively soluble gas such as N2O. When a mixture of O2 and an inert gas is breathed, the time for an area of unventilated lung to collapse depends on the solubility of the inert gas and FIO2. When the solubility is low (N2 or He), collapse takes longer than when 100% O2 is breathed, and the lower the FIO2 the longer the time to collapse. When the gas is more soluble (N2O) and FIO2 is > 0.3, collapse is more rapid than when 100% O2 is breathed.

Anaesthesia and the 'inert' gases with special reference to xenon

Xenon has many of the properties of the ideal anaesthetic agent and has been proposed as a suitable replacement for nitrous oxide in routine clinical anaesthesia. Xenon, krypton and argon are chemically inert under most circumstances, yet all have anaesthetic properties. Xenon is of particular interest because it is the only 'inert' gas which is an anaesthetic under normobaric conditions. Because of this property, xenon has an important place in the history of the development of theories of anaesthetic action and of concepts such as MAC. Cost is likely to be a major impediment to the regular use of xenon.

Effect of acute versus chronic Trichinella pseudospiralis infections on systemic cell-mediated immunity

Infection of the mouse with Trichinella pseudospiralis is accompanied by pronounced suppression of host inflammatory response. This study examines the effects of infection with this parasite on several key elements in cell-mediated immunity. Early down-regulation of host granulomatous response to subcutaneously implanted cotton string and delayed-type hypersensitivity (DTH) response to trinitrochlorobenzene (TNCB) was followed later during infection by normalization of these parameters compared to that seen in uninfected mice. Cytotoxic T lymphocyte responses to tumor-specific antigens expressed on the syngeneic P91 mastocytoma were depressed early following infection with T. pseudospiralis relative to that seen in uninfected mice but were similar in these two groups during the later stages of infection. Down-regulation of the components of cell-mediated reaction examined herein accompanied the presence of migratory larvae in the host.

Data recording and trend display during anaesthesia using 'MacLab'

A single screen display of variables monitored during anaesthesia may be ergonomically superior to the 'stack' of monitors seen in many anaesthetising locations. A system based on a MacLab (Analogue Digital Instruments) analogue-to-digital convertor used in conjunction with a Macintosh computer was evaluated. The system was configured to provide trend displays of up to eight variables on a single screen. It was found to be a useful adjunct to monitoring during anaesthesia. Advantages of this system are low cost, flexibility, and the quality of the software and support provided. Limitations of this and other similar systems are discussed.

Infectivity of Trichinella pseudospiralis isolated from carrion

The reproductive capacity index for Trichinella pseudospiralis infective larvae was similar for worms isolated from mouse carcasses on the day upon which mice were killed (day 0: 104.4 +/- 18.6 [mean +/- SD]) and on day 5 following mouse death (106.1 +/- 23.6), but was reduced for worms recovered from carcasses on day 10 postkill (PK: 22.7 +/- 5.7). Larvae isolated from mouse carcasses held at 24 C after day 10 PK were not infective. The percentage of viable worms (tightly coiled or moving) isolated from carrion was similar on days 0 and 5 PK but had declined to 40.4% by day 10 PK and showed a further reduction to 11.8% for worms isolated from carrion on day 15 PK. Viable worms were not recovered from carcasses after day 15 PK.

Alterations in the longevity and fecundity of adult Trichinella pseudospiralis related to method of isolation of infective larvae

The infectivity of Trichinella pseudospiralis infective larvae was reduced significantly following exposure to low pH or a combination of 1% pepsin at low pH compared to that for larvae isolated in phosphate-buffered saline (PBS) at pH 7.0. Reduction of host gastric pH by administration to mice of sodium bicarbonate solution in PBS was accompanied by an increase in the infectivity of larvae isolated in 1% pepsin/HCl (P/HCl) compared to that for worms inoculated into hosts given PBS alone. Fewer adult worms developing from larvae isolated in P/HCl became established in the host small bowel than was seen with larvae isolated in PBS; moreover, the fecundity in vitro of adult worms developing from P/HCl-isolated larvae was reduced below that for adults developing from larvae isolated from host muscle in PBS. More adult worms were recovered following infection of immune hosts with PBS-isolated larvae than were recovered from immune mice challenged with larvae isolated in P/HCl. Similar findings were observed in mice immunized by infection with Trichinella spiralis and challenged with T. pseudospiralis larvae isolated in either P/HCl or PBS. Immunization of mice with T. pseudospiralis larvae isolated by either method and challenged with larvae of T. spiralis resulted in recovery of similar percentages of the challenge inoculum.