1. Preoperative anxiety in adolescents undergoing surgery: a pilot study
Fortier MA, Martin SR, MacLaren Chorney J, Mayes LC, Kain ZN. Pediatric Anesthesia 2011, 21: 969-973.
This small pilot study addresses preoperative anxiety in adolescents aged 11 to 18 years. The study assessed anxiety at multiple points during the preoperative period including holding, separation from parents and at mask introduction. Anxiety was measured using a visual analog scale and by measuring heart rate, skin conductance and blood pressure. Potential risk factors such as baseline emotional state were measured using parental report scales. 80% of adolescents in this study had high levels and increasing levels of anxiety in the preoperative period both on self-reported and physiological measures. Predictors of anxiety included baseline anxiety, depression, somatization and a fearful temperament.
Take Home Message
This study is useful because it addresses a group not usually assessed for preoperative anxiety. It shows that a high number of adolescents are anxious in the preoperative period. It also highlights potential risk factors for preoperative anxiety. It would be interesting to know if there are any negative sequelae associated with preoperative anxiety in adolescents.
2. Anesthetic concerns for pediatric patients in an intraoperative MRI suite CD. McClain, MA. Rockoff and SG. Soriano. Current Opinion in Anesthesiology 2011; 24: 480-486.
Intraoperative MRI is used to improve resection during neurosurgical procedures. It allows more precise mapping during resection when structures shift as a result of resection itself. There are three types of iMRI setups: stationary patient and stationary magnet, mobile patient and stationary magnet, mobile magnet and stationary patient. The stationary patient and stationary magnet system allows frequent images to be taken. However, as the procedure is being carried out within the magnet, space is limited and all equipment needs to be MRI safe. MRI safe (poses no risk in an MRI environment) surgical instruments are of inferior quality to conventional instruments. Current Intraoperative MRI systems use a stationary patient and mobile magnet or a stationary magnet and mobile patient. The patient is placed within the magnet for scanning as required. The magnetic field moves relative to the patient, equipment and staff and this creates challenges unique to iMRI when compared to diagnostic MRI. The operating theatre is essentially a conventional one with conventional surgical instruments which are then counted and removed before the magnet is brought in. Anaesthetic equipment and monitoring is MRI safe or conditional (poses no hazard up to a specified limit). MRI Unsafe (hazardous in all MRI environments) anaesthetic equipment such as airway equipment, defibrillators, forced air warmers, temperature probes and fluid warmers must be removed and accounted for before scanning. The patient and the surgical field need to be draped to maintain sterility.
This paper outlines the different types of intraoperative MRI and discusses the advantages and disadvantages of each. This paper emphasizes the complexities of the iMRI environment and the necessity for special training of all staff involved. This paper also contains diagrams and photographs which better illustrate the challenges.
3. Preparation of Modern Anesthesia Workstations for Malignant Hyperthermia–susceptible Patients. A Review of Past and Present Practice. Kim TW, Nemergut ME. Anesthesiology 2011; 114:205–12
The maximum safe concentration for exposure to volatile agents in MH susceptible patients is not known. Guidelines have been published for the preparation of anaesthesia work stations to provide a trigger free anaesthetic for MH susceptible patients. The standard practice is to remove vaporizers, remove CO2 absorber, fresh gas flow hose and circuit. The machine needs to be flushed with a fresh gas flow of 10 l / min using the ventilator for at least 20 minutes. Studies showed that for older machines, which were basic in configuration, high flows resulted in a rapid purge of residual vapors.
Modern anaesthesia work stations incorporate more sophisticated ventilators. These machines contain more plastic and rubber parts which act as a reservoir for anaesthetic agent. The solubility of anaesthetic agents also affects the degree to which it is absorbed into plastic, rubber and silicone components. The more soluble agents such as sevoflurane and desflurane, required greater purge times. The paper also describes the technique of removing and autoclaving or replacing exchangeable internal components to speed up the process of purging some machines. This resulted in a significant reduction in the time required to purge a machine of anaesthetic vapors.
It has been found that for some workstations, low anaesthetic concentrations can be achieved and maintained at high flows but increase above acceptable levels when flows are decreased. This rebound effect is thought to occur for the following reasons:
1. Greater absorption of anaesthetic gas into plastic, rubber and silicone components which then act as a reservoir for agent.
2. The internal configuration of the anaesthetic machine creates non ventilated pockets which act as a reservoir for agent.
3. Fresh gas flow decoupling, introduced to allow more precise delivery of tidal volume, means that purging only occurs in a proportion of the ventilatory cycle where in older machines it would occur throughout the ventilatory cycle. For example, if purging is only occurring in 50% of the cycle then it would be expected to take twice as long to achieve a given concentration of agent for the same starting concentration and fresh gas flow rate. (See paper for detail)
The rebound phenomenon can be avoided by using high fresh gas flows for the duration of the anaesthetic and not just during the purge.
Take Home Message
This paper looks at purge recommendations for preparing the following machines for MH susceptible patients: Ohmeda Modulus I and II, Ohmeda Excel 210, Datex-Ohmeda AS/3, Narkomed GS, Drager Primus/Apollo, Drager Primus, Drager Fabius, Drager Fabius GS and Siemens KION. The authors report that they could not find any studies on purging the latest GE workstations. They recommend a comprehensive study of all anaesthesia machines currently in use and development of guidelines for their preparation.
Without work station specific guidelines the following should probably apply:
1. Allow sufficient time to purge. This varies from machine to machine as outlined in the paper. Of note is the fact that there are no studies for GE machines and as such application of shorter purge times, without guidelines from the manufacturer, is probably inadvisable.
2. Use high fresh gas flows of at least 10 l / in for the duration of the case to avoid the rebound phenomenon.
3. Remove and exchange any external breathing circuitry including CO2 absorber which act as significant reservoirs of agent. Flushing should occur through the ventilator as this apparatus is not easily replaced. The bag and attached circuits can easily be removed and replaced.
4. If practical, exchangeable internal components can be replaced to speed up purge.
5. The duration of the purge will be determined by the most soluble agent used on a particular machine.
4. Gastric emptying after overnight fasting and clear fluid intake: a prospective investigation using serial magnetic resonance imaging in healthy children. Schmitz a, Kellenberger CJ, Liamlahi R. et al. British Journal of Anaesthesia 107 (3): 425–9 (2011)
This study measures residual gastric fluid volume after fasting as well as gastric fluid volume up to 2 hours after ingestion of clear fluids. The authors report that there is little information on gastric fluid volumes in fasting children. In addition, they state that other studies looking at the volume of fluid in the stomach after clear fluids rely on nasogastric or orogastric aspiration which may underestimate the true volume and cannot measure the volume of air. They chose MRI as a non invasive method of measuring both gastric air and fluid volume. They also documented the time course of gastric emptying in individual children after 7 ml / kg of clear fluid. Measurements were taken at 30 minute intervals up to 120 minutes. The study included children between the ages of 6 - 14 years. It excluded children with gastrointestinal disease and functional disturbance. Children who were unable to tolerate an overnight fast or remain still in the MRI scanner for up to 3 minutes were excluded.
The main findings were that GFV decreased rapidly after clear fluid ingestion. Median half time for emptying was less than 30 minutes. There was significant inter individual variability with half time ranging between 18 minutes and 47.8 minutes. Gastric Air Volume showed even more inter individual variability and did not empty in a predictable manner. Residual GFV was found to be greater in this study than in previous studies which depended on aspiration techniques. There was no correlation between either residual GFV or half life of emptying and age, weight or BMI.
Take Home Message
Liberal 2 hour clear fluid fasting time for children regardless of age, weight or BMI is supported by this data. This is for healthy children undergoing elective surgery. Factors that may slow gastric emptying need to be considered.
See Journal Watch September 2009