A Review of the Risks and Benefits of Nitrous Oxide in Current Anaesthetic Practice

The safety of nitrous oxide: glass half-full or half-empty?

A systematic review of clinical trials confirms that including nitrous oxide in the gas mixture for general anaesthesia has minor short-term benefits and does not impact most patient safety outcomes. However, no risk-benefit analysis of nitrous oxide should ignore its known environmental effects. If continued nitrous oxide use is supported, strategies to minimise and monitor the contribution of medical nitrous oxide to global warming are vital.

Support-vector classification of low-dose nitrous oxide administration with multi-channel EEG power spectra

Support-vector machines (SVMs) can potentially improve patient monitoring during nitrous oxide anaesthesia. By elucidating the effects of low-dose nitrous oxide on the power spectra of multi-channel EEG recordings, we quantified the degree to which these effects generalise across participants. In this single-blind, cross-over study, 32-channel EEG was recorded from 12 healthy participants exposed to 0, 20, 30 and 40% end-tidal nitrous oxide. Features of the delta-, theta-, alpha- and beta-band power were used within a 12-fold, participant-wise cross-validation framework to train and test two SVMs: (1) binary SVM classifying EEG during 0 or 40% exposure (chance = 50%); (2) multi-class SVM classifying EEG during 0, 20, 30 or 40% exposure (chance = 25%). Both the binary (accuracy 92%) and the multi-class (accuracy 52%) SVMs classified EEG recordings at rates significantly better than chance (p < 0.001 and p = 0.01, respectively). To determine the relative importance of frequency band features for classification accuracy, we systematically removed features before re-training and re-testing the SVMs. This showed the relative importance of decreased delta power and the frontal region. SVM classification identified that the most important effects of nitrous oxide were found in the delta band in the frontal electrodes that was consistent between participants. Furthermore, support-vector classification of nitrous oxide dosage is a promising method that might be used to improve patient monitoring during nitrous oxide anaesthesia.

[Inhalation anesthesia in the cat: Development of cuff pressure when using nitrous oxide]

OBJECTIVE

During inhalation anesthesia with nitrous oxide in oxygen the pressure in the cuff of the endotracheal tube may increase due to diffusion of nitrous oxide into the cuff. The aim of the study was to investigate the development of cuff pressure during nitrous oxide anesthesia under clinical conditions in feline patients and to identify possible influencing factors such as tube size and gas flow rate.

MATERIAL AND METHODS

The prospective study included cats scheduled for inhalation anesthesia with nitrous oxide for a minimum duration of 60 minutes at the Department for Small Animals of the University of Leipzig. Cuff pressure was adjusted with a cuff manometer and its development was recorded.

RESULTS

In total, the cuff pressure values of 24 cats were recorded. Animals were allocated into groups by tube size (ID 4.0 mm and ID 4.5 mm) and by fresh gas flow rate: low flow rate (0.6 l/min) and high flow rate (3 l/min). During anesthesia, cuff pressure increased over time, with statistical significance occurring from 45 minutes onwards in comparison to the initial cuff pressure (p=0.005). After 60 minutes, there was a mean cuff pressure increase of 3 cmH2O. Despite this moderate mean increase, highly variable pressure values up to 48 cmH2O in individual animals were recorded. No cat reached the termination criterion of 60 cmH2O cuff pressure. Effects of tube size (p=0.63) and flow rate (p=0.334) on the cuff pressure were not evident.

CONCLUSION

After a period of 45 minutes of nitrous oxide administration, a significant increase in cuff pressure occurs in the cat. However, tube size and total gas flow rate do not seem to influence the cuff pressure development.

CLINICAL RELEVANCE

When using nitrous oxide during inhalation anesthesia, regular cuff pressure evaluation and correction are necessary and hence recommended in feline patients. As individual pressure changes may be highly variable, no fixed recommendations for optimal management are possible.

The surgical management and outcomes of kissing nevi of the eyelids

PURPOSE

To describe the clinical characteristics, surgical technique, and outcomes in patients with kissing nevi who underwent surgical management in two tertiary referral centers.

METHODS

Medical chart review was conducted for all patients who underwent surgical repair in Moorfields Eye Hospital and The Children's Hospital of Philadelphia. Demographics, medical history, lesion characteristics, surgical intervention, and outcomes were collected. Main outcome measures were surgical interventions as well as functional and cosmetic outcomes.

RESULTS

Thirteen patients were included. Mean age at presentation was 23.46 years (±19.35,4-61), and the mean number of surgeries per patients was 1.9 (±1.3,1-5). Initial procedure included incisional biopsy in three cases (23%) and complete excision and reconstruction in 10 cases (77%). Surgery involved the upper and lower anterior lamella in all cases, the upper posterior lamella in 4 patients (31%), and the lower posterior lamella in 2 patients (15%). Local flaps were utilized in 3 cases and grafts in 5 cases. Complications included: trichiasis (n = 2, 15%), lower eyelid ectropion (n = 2, 15%), mild ptosis (n = 1, 8%), and upper/lower punctal ectropion (n = 1, 8%). Twelve patients (92%) were satisfied with the final functional and cosmetic outcome. No recurrence or malignant transformation were observed in any patient.

CONCLUSION

The surgical management of kissing nevi can be challenging, and commonly includes the use of local flaps or grafts, often requiring multiple interventions. The approach should be based on lesion size and location, proximity and involvement of key anatomical landmarks, as well as individual facial characteristics. Surgical management has a favorable functional and cosmetic outcome in the majority of patients.

Perioperative strategies to reduce risk of myocardial injury after non-cardiac surgery (MINS): A narrative review

Myocardial injury is a frequent complication of surgical patients after having non-cardiac surgery that is strongly associated with perioperative mortality. While intraoperative anesthesia-related deaths are exceedingly rare, about 1% of patients undergoing non-cardiac surgery die within the first 30 postoperative days. Given the number of surgeries performed annually, death following surgery is the second leading cause of death in the United States. Myocardial injury after non-cardiac surgery (MINS) is defined as an elevation in troponin concentrations within 30 days postoperatively. Although typically asymptomatic, patients with MINS suffer myocardial damage and have a 10% risk of death within 30 days after surgery and excess risks of mortality that persist during the first postoperative year. Many factors for the development of MINS are non-modifiable, such as preexistent coronary artery disease. Preventive measures, systematic approaches to surveillance and treatment standards are still lacking, however many factors are modifiable and should be considered in clinical practice: the importance of hemodynamic control, adequate oxygen supply, metabolic homeostasis, the use of perioperative medications such as statins, anti-thrombotic agents, beta-blockers, or anti-inflammatory agents, as well as some evidence regarding the choice of sedative and analgesic for anesthesia are discussed. Also, as age and complexity in comorbidities of the surgical patient population increase, there is an urgent need to identify patients at risk for MINS and develop prevention and treatment strategies. In this review, we provide an overview of current screening standards and promising preventive options in the perioperative setting and address knowledge gaps requiring further investigation.

The effects of cardiovascular and orthopaedic surgery on vitamin concentrations: a narrative review of the literature and mechanisms of action

Given the rise in worldwide chronic diseases, supplemented by an aging population, the volume of global major surgeries, encompassing cardiac and orthopedic procedures is anticipated to surge significantly. Surgical trauma can be accompanied by numerous postoperative complications and metabolic changes. The present review summarized the results from studies assessing the effects of orthopedic and cardiovascular surgery on vitamin concentrations, in addition to exploring the possible mechanisms associated with changes in concentrations. Studies have revealed a potentially severe depletion in plasma/serum concentrations of numerous vitamins following these surgeries acutely. Vitamins C, D and B1 appear particularly vulnerable to significant depletions, with vitamin C and D depletions consistently transpiring into inadequate and deficient concentrations, respectively. The possible multifactorial mechanisms impacting postoperative vitamin concentrations include changes in hemodilution and vitamin utilization, redistribution, circulatory transport and absorption. For a majority of vitamins, there has been a lack of investigation into the effects of both, cardiac and orthopedic surgery. Additionally, studies were predominantly restricted to short-term postoperative investigations, primarily performed within the first postoperative week of surgery. Overall, results indicated that further examination is necessary to determine the severity and clinical significance of the possible depletions in vitamin concentrations that ensue cardiovascular and orthopedic surgery.

The international ENIGMA-II substudy on postoperative cognitive disorders (ISEP)

There is a large controversy as to whether nitrous oxide (N₂O) added to the anaesthetic gas mixture is harmful or harmless for postoperative cognitive function recovery. We performed a nested study in the ENIGMA-II trial and compared postoperative neurocognitive recovery of patients randomly receiving N₂O (70%) or Air (70%) in 30% O₂ during anesthesia. We included adults having non cardiac surgery. We compared recovery scores for episodic memory, decision making/processing speed and executive functions measured with the computerised Cambridge Neuropsychological Test Automated Battery (CANTAB). Assessments were performed at baseline, seven and ninety days. At first interim analysis, following recruitment of 140 participants, the trial was suspended. We found that the mean (95%CI) changes of scores for episodic memory were in the Pocock futility boundaries. Decision making/processing speed did not differ either between groups (P > 0.182). But for executive functions at seven days, the mean number (95% CI) of problems successfully solved and the number of correct box choices made was higher in the N2O group, P = 0.029. N₂O with the limitations of an interim analysis appears to have no harmful effect on cognitive functions (memory/processing speed). It may improve the early recovery process of executive functions. This preliminary finding warrants further investigations.

Hypnosis and progressive muscle relaxation for anxiolysis and pain control during extraction procedure in 8-12-year-old children: a randomized control trial

Introduction

Hypnosis (H) and Progressive Muscle Relaxation (PMR) have proven to be effective in a variety of medical settings; there is a paucity of their practical application in paediatric dentistry. The study aimed to comparatively evaluate the role of H and PMR on anxiety, heart rate (HR), oxygen saturation (SPO₂), blood pressure (BP), pain, and analgesic requirement during extraction in children.

Materials and methods

Sixty children aged 8–12 years undergoing primary molar extractions were randomly allocated to three groups—H, PMR, and control (C). The anxiety (proposed Visual Facial Anxiety scale), HR, and SPO₂ were measured pre/post-operatively with/without interventions (H, PMR, C) at 4 intervals. The BP and pain (Wong-Baker faces pain scale) were recorded pre- and post-operatively. Need for analgesic post-operatively was assessed.

Results

Statistically significant reduction in anxiety was noted post-extraction in H (0.30 ± 0.80), PMR (0.50 ± 0.69) (p < 0.001*). HR showed a statistically significant drop after H, PMR application. (p < 0.001*) No significant difference in SPO₂ was noted in the three groups (p > 0.05). Pain control was well achieved using H (85%), PMR (70%); BP was well-regulated in the H, PMR compared to C group (p < 0.001*). Need for analgesics was reduced in H (45%), PMR (50%) versus C (100%). Both techniques H, PMR were comparable in all measures.

Conclusion

Hypnosis and PMR are effective techniques for anxiolysis and pain control in paediatric dental patients.

An Electroencephalogram Metric of Temporal Complexity Tracks Psychometric Impairment Caused by Low-dose Nitrous Oxide

BACKGROUND

Nitrous oxide produces non-γ-aminobutyric acid sedation and psychometric impairment and can be used as scientific model for understanding mechanisms of progressive cognitive disturbances. Temporal complexity of the electroencephalogram may be a sensitive indicator of these effects. This study measured psychometric performance and the temporal complexity of the electroencephalogram in participants breathing low-dose nitrous oxide.

METHODS

In random order, 20, 30, and 40% end-tidal nitrous oxide was administered to 12 participants while recording 32-channel electroencephalogram and psychometric function. A novel metric quantifying the spatial distribution of temporal electroencephalogram complexity, comprised of (1) absolute cross-correlation calculated between consecutive 0.25-s time samples; 2) binarizing these cross-correlation matrices using the median of all channels as threshold; (3) using quantitative recurrence analysis, the complexity in temporal changes calculated by the Shannon entropy of the probability distribution of the diagonal line lengths; and (4) overall spatial extent and intensity of brain complexity, was quantified by calculating median temporal complexity of channels whose complexities were above 1 at baseline. This region approximately overlay the brain's default mode network, so this summary statistic was termed "default-mode-network complexity."

RESULTS

Nitrous oxide concentration correlated with psychometric impairment (r = 0.50, P < 0.001). Baseline regional electroencephalogram complexity at midline was greater than in lateral temporal channels (1.33 ± 0.14 bits vs. 0.81 ± 0.12 bits, P < 0.001). A dose of 40% N2O decreased midline (mean difference [95% CI], 0.20 bits [0.09 to 0.31], P = 0.002) and prefrontal electroencephalogram complexity (mean difference [95% CI], 0.17 bits [0.08 to 0.27], P = 0.002). The lateral temporal region did not change significantly (mean difference [95% CI], 0.14 bits [-0.03 to 0.30], P = 0.100). Default-mode-network complexity correlated with N2O concentration (r = -0.55, P < 0.001). A default-mode-network complexity mixed-effects model correlated with psychometric impairment (r2 = 0.67; receiver operating characteristic area [95% CI], 0.72 [0.59 to 0.85], P < 0.001).

CONCLUSIONS

Temporal complexity decreased most markedly in medial cortical regions during low-dose nitrous oxide exposures, and this change tracked psychometric impairment.

EDITOR’S PERSPECTIVE

Factors Influencing Clinical Correlates of Chronic Traumatic Encephalopathy (CTE): a Review

Chronic traumatic encephalopathy (CTE) is a neuropathologically defined disease reportedly linked to a history of repetitive brain trauma. As such, retired collision sport athletes are likely at heightened risk for developing CTE. Researchers have described distinct pathological features of CTE as well a wide range of clinical symptom presentations, recently termed traumatic encephalopathy syndrome (TES). These clinical symptoms are highly variable, non-specific to individuals described as having CTE pathology in case reports, and are often associated with many other factors. This review describes the cognitive, emotional, and behavioral changes associated with 1) developmental and demographic factors, 2) neurodevelopmental disorders, 3) normal aging, 4) adjusting to retirement, 5) drug and alcohol abuse, 6) surgeries and anesthesia, and 7) sleep difficulties, as well as the relationship between these factors and risk for developing dementia-related neurodegenerative disease. We discuss why some professional athletes may be particularly susceptible to many of these effects and the importance of choosing appropriate controls groups when designing research protocols. We conclude that these factors should be considered as modifiers predominantly of the clinical outcomes associated with repetitive brain trauma within a broader biopsychosocial framework when interpreting and attributing symptom development, though also note potential effects on neuropathological outcomes. Importantly, this could have significant treatment implications for improving quality of life.

Nitrous Oxide and Serious Long-term Morbidity and Mortality in the Evaluation of Nitrous Oxide in the Gas Mixture for Anaesthesia (ENIGMA)-II Trial

Background

The Evaluation of Nitrous Oxide in the Gas Mixture for Anaesthesia (ENIGMA)-II trial randomly assigned 7,112 noncardiac surgery patients at risk of perioperative cardiovascular events to 70% N2O or 70% N2 groups. The aim of this follow-up study was to determine the effect of nitrous oxide on a composite primary outcome of death and major cardiovascular events at 1 yr after surgery.

Methods

One-year follow-up was conducted via a medical record review and telephone interview. Disability was defined as a Katz index of independence in activities of daily living score less than 8. Adjusted odds ratios and hazard ratios were calculated as appropriate for primary and secondary outcomes.

Results

Among 5,844 patients evaluated at 1 yr, 435 (7.4%) had died, 206 (3.5%) had disability, 514 (8.8%) had a fatal or nonfatal myocardial infarction, and 111 (1.9%) had a fatal or nonfatal stroke during the 1-yr follow-up period. Exposure to nitrous oxide did not increase the risk of the primary outcome (odds ratio, 1.08; 95% CI, 0.94 to 1.25; P = 0.27), disability or death (odds ratio, 1.07; 95% CI, 0.90 to 1.27; P = 0.44), death (hazard ratio, 1.17; 95% CI, 0.97 to 1.43; P = 0.10), myocardial infarction (odds ratio, 0.97; 95% CI, 0.81 to 1.17; P = 0.78), or stroke (odds ratio, 1.08; 95% CI, 0.74 to 1.58; P = 0.70).

Conclusion

These results support the long-term safety of nitrous oxide administration in noncardiac surgical patients with known or suspected cardiovascular disease.

Nitrous oxide-related postoperative nausea and vomiting depends on duration of exposure

BACKGROUND

Inclusion of nitrous oxide in the gas mixture has been implicated in postoperative nausea and vomiting (PONV) in numerous studies. However, these studies have not examined whether duration of exposure was a significant covariate. This distinction might affect the future place of nitrous oxide in clinical practice.

METHODS

PubMed listed journals reporting trials in which patients randomized to a nitrous oxide or nitrous oxide-free anesthetic for surgery were included, where the incidence of PONV within the first 24 postoperative hours and mean duration of anesthesia was reported. Meta-regression of the log risk ratio for PONV with nitrous oxide (lnRR PONVN2O) versus duration was performed.

RESULTS

Twenty-nine studies in 27 articles met the inclusion criteria, randomizing 10,317 patients. There was a significant relationship between lnRR PONVN2O and duration (r = 0.51, P = 0.002). Risk ratio PONV increased 20% per hour of nitrous oxide after 45 min. The number needed to treat to prevent PONV by avoiding nitrous oxide was 128, 23, and 9 where duration was less than 1, 1 to 2, and over 2 h, respectively. The risk ratio for the overall effect of nitrous oxide on PONV was 1.21 (CIs, 1.04-1.40); P = 0.014.

CONCLUSIONS

This duration-related effect may be via disturbance of methionine and folate metabolism. No clinically significant effect of nitrous oxide on the risk of PONV exists under an hour of exposure. Nitrous oxide-related PONV should not be seen as an impediment to its use in minor or ambulatory surgery.

Does anaesthesia with nitrous oxide affect mortality or cardiovascular morbidity? A systematic review with meta-analysis and trial sequential analysis

BACKGROUND

The role of nitrous oxide in modern anaesthetic practice is contentious. One concern is that exposure to nitrous oxide may increase the risk of cardiovascular complications. ENIGMA II is a large randomized clinical trial currently underway which is investigating nitrous oxide and cardiovascular complications. Before the completion of this trial, we performed a systematic review and meta-analysis, using Cochrane methodology, on the outcomes that make up the composite primary outcome.

METHODS

We used conventional meta-analysis and trial sequential analysis (TSA). We reviewed 8282 abstracts and selected 138 that fulfilled our criteria for study type, population, and intervention. We attempted to contact the authors of all the selected publications to check for unpublished outcome data.

RESULTS

Thirteen trials had outcome data eligible for our outcomes. We assessed three of these trials as having a low risk of bias. Using conventional meta-analysis, the relative risk of short-term mortality in the nitrous oxide group was 1.38 [95% confidence interval (CI) 0.22-8.71] and the relative risk of long-term mortality in the nitrous oxide group was 0.94 (95% CI 0.80-1.10). In both cases, TSA demonstrated that the data were far too sparse to make any conclusions. There were insufficient data to perform meta-analysis for stroke, myocardial infarct, pulmonary embolus, or cardiac arrest.

CONCLUSION

This systematic review demonstrated that we currently do not have robust evidence for how nitrous oxide used as part of general anaesthesia affects mortality and cardiovascular complications.

Postsurgical inflammatory neuropathy: a report of five cases

BACKGROUND

Clinical and pathologic descriptions of postsurgical inflammatory neuropathy have been reported but this condition is still under recognized.

METHODS

We reviewed 5 cases of a biopsy-proven inflammatory neuropathy that occurred within 30 days after surgical procedures. These patients were seen at one center in a 2-year period.

RESULTS

All patients developed neuropathy symptoms with time delay between the surgery and the neuropathy onset. In all, the symptoms progressed up to the time of evaluation. Electrophysiological studies revealed mononeuropathy or asymmetrical polyneuropathy with active denervation. Nerve biopsies showed ischemic injury and perivascular inflammatory collections in all cases. All patients were treated with intravenous methylprednisolone and four of them showed clinical improvement. The non-responsive patient did not receive immunotherapy until 2 years after neuropathy onset.

CONCLUSIONS

The report illustrates that focal or asymmetrical neuropathy is typical of postsurgical inflammatory neuropathy and has a favorable outcome after intravenous corticosteroid treatment. The report underscores the importance for considering potentially treatable inflammatory neuropathies in the post-surgical setting.

Influence of nitrous oxide anesthesia, B-vitamins, and MTHFR gene polymorphisms on perioperative cardiac events: the vitamins in nitrous oxide (VINO) randomized trial

BACKGROUND

Nitrous oxide causes an acute increase in plasma homocysteine that is more pronounced in patients with the methylenetetrahydrofolate reductase (MTHFR) C677T or A1298C gene variant. In this randomized controlled trial, the authors sought to determine whether patients carrying the MTHFR C677T or A1298C variant had a higher risk for perioperative cardiac events after nitrous oxide anesthesia and whether this risk could be mitigated by B-vitamins.

METHODS

The authors randomized adult patients with cardiac risk factors undergoing noncardiac surgery, to receive nitrous oxide plus intravenous B-vitamins before and after surgery, or to nitrous oxide and placebo. Serial cardiac biomarkers and 12-lead electrocardiograms were obtained. The primary study endpoint was the incidence of myocardial injury, as defined by cardiac troponin I increase within the first 72 h after surgery.

RESULTS

A total of 500 patients completed the trial. Patients who were homozygous for either MTHFR C677T, or A1298C gene variant (n=98; 19.6%) had no increased rate of postoperative cardiac troponin I increase compared with wild-type and heterozygous patients (11.2 vs. 14.0%; relative risk 0.96; 95% CI, 0.85-1.07; P=0.48). B-vitamins blunted the rise in homocysteine, but had no effect on cardiac troponin I increase compared with patients receiving placebo (13.2 vs. 13.6%; relative risk 1.02; 95% CI 0.78 to 1.32; P=0.91).

CONCLUSIONS

Neither MTHFR C677T and A1298C gene variant, nor acute homocysteine increase are associated with perioperative cardiac troponin increase after nitrous oxide anesthesia. B-vitamins blunt nitrous oxide-induced homocysteine increase but have no effect on cardiac troponin I increase.

Leukocyte DNA Damage and Wound Infection after Nitrous Oxide Administration

Background:

Nitrous oxide inactivates methionine synthase and may lead to DNA damage and wound infection. By using single-cell gel electrophoresis (comet assay), the authors determined the effect of nitrous oxide on DNA damage in circulating leukocytes.

Methods:

In this double-blind, randomized controlled trial, 91 patients undergoing major colorectal surgery were randomized to receive 70% nitrous oxide (n = 31) or nitrous oxide-free anesthesia using 30 (n = 30) or 80% (n = 30) oxygen. Venous blood was collected before and 24 h after surgery. The primary outcome was extent of DNA damage, quantified as the percentage of DNA staining intensity in the comet tail using digital fluorescence microscopy. Incidence of postoperative wound infection was also recorded.

Results:

Nitrous oxide exposure was associated with a two-fold increase in the percentage of DNA intensity in tail (P = 0.0003), but not in the 30 (P = 0.181) or 80% oxygen groups (P = 0.419). There was a positive correlation between the duration of nitrous oxide exposure and extent of DNA damage, r = 0.33, P = 0.029. However, no correlation was observed in nitrous oxide-free patients. The proportions of postoperative wound infection, using the Centers for Disease Control and Prevention criteria, were 19.4% (6 of 31) in the 70% nitrous oxide group and 6.7% (2 of 30) in both the 30 and 80% oxygen groups, P = 0.21. An increase in DNA damage was associated with a higher risk of wound infection, adjusted odds ratio (95% CIs): 1.19 (1.07–1.34), P = 0.003.

Conclusions:

Nitrous oxide increased DNA damage compared with nitrous oxide-free anesthesia and was associated with postoperative wound infection.

Nitrous oxide anesthesia and plasma homocysteine in adolescents

BACKGROUND

Nitrous oxide inactivates vitamin B(12), inhibits methionine synthase, and consequently increases plasma total homocysteine (tHcy). Prolonged exposure to nitrous oxide can lead to neuropathy, spinal cord degeneration, and even death in children. We tested the hypothesis that nitrous oxide anesthesia causes a significant increase in plasma tHcy in children.

METHODS

Twenty-seven children (aged 10-18 years) undergoing elective major spine surgery were enrolled, and serial plasma samples from 0 to 96 hours after induction were obtained. The anesthetic regimen, including the use of nitrous oxide, was at the discretion of the anesthesiologist. Plasma tHcy was measured using standard enzymatic assays.

RESULTS

The median baseline plasma tHcy concentration was 5.1 μmol/L (3.9-8.0 μmol/L, interquartile range) and increased in all patients exposed to nitrous oxide (n = 26) by an average of +9.4 μmol/L (geometric mean; 95% confidence interval, 7.1-12.5 μmol/L) or +228% (mean; 95% confidence interval, 178%-279%). Plasma tHcy peaked between 6 and 8 hours after induction of anesthesia. One patient who did not receive nitrous oxide had no increase in plasma tHcy. Several patients experienced a severalfold increase in plasma tHcy (maximum +567%). The increase in plasma tHcy was strongly correlated with the duration and average concentration of nitrous oxide anesthesia (r = 0.80; P < 0.001).

CONCLUSIONS

Pediatric patients undergoing nitrous oxide anesthesia develop significantly increased plasma tHcy concentrations. The magnitude of this effect seems to be greater compared with adults; however, the clinical relevance is unknown.

Intraoperative Nitrous Oxide as a Preventive Analgesic

Preventive analgesia is defined as the persistence of the analgesic effects of a drug beyond the clinical activity of the drug. The N-methyl D-aspartate receptor plays a critical role in the sensitisation of pain pathways induced by injury. Nitrous oxide inhibits excitatory N-methyl D-aspartate sensitive glutamate receptors. The objective of our study was to test the efficacy of nitrous oxide as a preventive analgesic. We conducted a retrospective analysis of data from a subset of patients (n=100) randomly selected from a previous major multicentre randomised controlled trial on nitrous oxide (ENIGMA trial). Data analysed included postoperative analgesic requirements, pain scores and duration of patient-controlled analgesia during the first 72 postoperative hours. There was no significant difference in postoperative oral morphine equivalent usage (nitrous group 248 mg, no nitrous group 289 mg, mean difference -43 mg, 95% confidence interval 141 to 54 mg). However, patients who received nitrous oxide had a shorter duration of patient-controlled analgesia use (nitrous group 35 hours, no nitrous group 51 hours, mean difference -16 hours, 95% confidence interval -29 to -2 hours, P=0.022). There was no difference in pain scores between the groups. The shorter patient-controlled analgesia duration in the nitrous oxide group suggests that intraoperative nitrous oxide may have a preventive analgesic effect.

Systematic review: the use of nitrous oxide gas for lower gastrointestinal endoscopy

BACKGROUND

Nitrous oxide gas (N(2)O) has been proposed as an alternative to intravenous (i.v.) analgesia in patients undergoing lower gastrointestinal endoscopy.

AIM

To perform a systematic review of randomized studies where N(2)O was compared against control in patients undergoing either flexible sigmoidoscopy or colonoscopy.

METHODS

Electronic databases were searched; reference lists were checked and letters were sent to authors requesting data. Methodological quality was assessed. Data were tabulated on the duration and difficulty of the procedure, quality of sedation and speed of patient recovery.

RESULTS

A total of 11 studies were identified containing 623 patients. No differences were seen between groups for duration, difficulty of procedure or complications. Patient-reported pain was similar for N(2)O when undergoing flexible sigmoidoscopy vs. no sedation and when undergoing colonoscopy vs. i.v. sedation. Differences in delivery of N(2)O were identified. In all studies, N(2)O was associated with a more rapid recovery than i.v. sedation.

CONCLUSION

For patients undergoing colonoscopy, N(2)O provides comparable analgesia to i.v. sedation. The rapid psychomotor recovery with N(2)O enables quicker patient discharge and removes the need for a patient to be chaperoned. Benefit was not seen from N(2)O in patients undergoing flexible sigmoidoscopy possibly because it was delivered on demand rather than continuously.

Growth suppression of four cancer cells by hyperbaric nitrous oxide and methotrexate

BACKGROUND

Nitrous oxide concentration is easily controlled by respiratory ventilation. It suppresses bone marrow via the inhibition of thymidylate synthesis. The aim of this work was to determine the optimal pressure and exposure duration of nitrous oxide, as well as methotrexate concentration that maximizes the suppression of 4 cancer cells: CCRF-CEM, K562, A549 and MDA-MB-231.

METHODS

Each cancer cell was cultured in a hyperbaric chamber at 1, 2 and 3 atmosphere of 74% nitrous oxide for 24, 48, and 72 hours at 0, 0.3, 0.7, 1, 2, 5 and 10 microM methotrexate (MTX), respectively. The results were expressed in the ratio of the number of cancer cells cultured under specific conditions (S cells) to that under normal conditions (N cells).

RESULTS

The S/N ratio of CCRF-CEM cells was 87.4% in 24-hour culture, 95.0% in 48-hour culture and 115.9% in 72-hour culture (P < 0.05). The S/N ratio of K562 cells was 103.6% at 1 atm, 102.4% at 2 atm and 115.6% at 3 atm (P < 0.05). The S/N ratio of A549 cells was 94.3% at 1 atm, 94.1% at 2 atm, 99.3% at 3 atm, 96.2% in 24-hour culture, 99.2% in 48-hour culture and 99.3% in 72-hour culture (P > 0.05). However, the S/N ratio of MDA-MB 231 cells was 66.9% in 24-hour culture, 83.1% in 48 hour culture and 87.8% in 72-hour culture (P < 0.05).

CONCLUSIONS

Only the growth of the MDA-MB-231 cells was significantly reduced after a longer exposure time to nitrous oxide, but those of the other cells were not.

Nitrous oxide and perioperative cardiac morbidity (ENIGMA-II) Trial: rationale and design

BACKGROUND

Globally there are >200 million major surgical procedures undertaken annually, and about 20% of these involve patients who have coronary artery disease. Many receive nitrous oxide, which impairs methionine synthase, thus inhibiting folate synthesis and increasing postoperative homocysteine levels. Nitrous oxide anesthesia leads to postoperative endothelial dysfunction, and there is some evidence that it increases myocardial ischemia and, possibly, myocardial infarction. We have initiated the Nitrous oxide and perioperative cardiac morbidity (ENIGMA-II) Trial to test the hypothesis that in inpatients undergoing anesthesia for major noncardiac surgery, avoidance of nitrous oxide will reduce the incidence of death and major cardiovascular events.

METHODS

ENIGMA-II is a 7,000-patient, international randomized trial involving patients at risk of coronary artery disease undergoing noncardiac surgery. The patients, health care providers (except for the anesthesiologists), data collectors, and outcome adjudicators are blinded to whether patients receive nitrous oxide-containing or nitrous oxide-free anesthetic. The primary outcome is a composite of death and major nonfatal events (ie, myocardial infarction, cardiac arrest, pulmonary embolism, and stroke) at 30 days after surgery.

RESULTS

At present, ENIGMA-II has randomized >1,000 patients in 22 hospitals in 5 countries. To date, patients' mean age is 70 years, 66% are men, 38% have a history of coronary artery disease, 19% have a history of cerebrovascular disease, and 84% have a history of hypertension. Most patients have undergone intra-abdominal 28%, vascular 32%, and orthopedic 16% surgery.

CONCLUSIONS

The ENIGMA-II Trial will be the largest study yet conducted to ascertain the benefits and risks of removing nitrous oxide from the gas mixture in anesthesia. The results of this large international trial will guide the clinical care of the hundreds of millions of adults undergoing noncardiac surgery annually.

A comparison of waste gas concentrations during xenon or nitrous oxide anaesthesia

BACKGROUND AND OBJECTIVE

The aim of this study was to compare waste gas concentrations during xenon or nitrous oxide anaesthesia.

METHODS

A total of 64 patients were included in this study. Gas concentrations were measured with a mass spectrometer during anaesthesia. The probes were taken beside the patient's head and thorax and at a height of 180 cm above and at the floor level.

RESULTS

In both groups, waste gas concentrations peak after intubation and extubation. Waste gas levels during xenon anaesthesia are low compared with nitrous oxide.

CONCLUSIONS

The low waste gas levels of xenon seem to be beneficial compared to nitrous oxide.

Oxygen in neonatal and infant anesthesia--current practice in the UK

BACKGROUND

There is an increasing awareness of the role of oxygen free radicals in the pathogenesis of many conditions associated with prematurity, which has led to caution in the use of oxygen in neonatal resuscitation. We surveyed the practice of UK pediatric anesthetists with regard to oxygen use in neonatal and infant anesthesia.

METHODS

A postal questionnaire survey of 460 UK-based members of the Association of Pediatric Anesthetists of Great Britain and Ireland.

RESULTS

Responses were received from 247 pediatric anesthetists (54%). Seventy-five percent of anesthetists aim to avoid the use of 100% oxygen during routine infant anesthesia and 52% aim for an FiO(2) of < 0.4 in neonates. The factors most influencing choice of carrier gas are optimal oxygenation and the avoidance of pulmonary atelectasis. Sixteen percent stated that unavailability of medical air is a factor. Opinion was divided on concern about the effects of anesthetic agents on the developing brain. Moderate levels of concern were expressed about the potential toxic effects of oxygen on the eyes and lungs of premature newborns but this concern does not extend to term newborns. Only 20% of anesthetists had any recent knowledge of these issues.

CONCLUSIONS

This survey indicates that there is no consistency in attitudes and practices and demonstrates considerable variation in the use of oxygen during anesthesia in premature and newborn babies and infants. This may reflect the paucity of evidence in the anesthetic literature on the potential harmful effects of high concentrations of oxygen in vulnerable groups of infants.

When nitrous oxide is no laughing matter: nitrous oxide and pediatric anesthesia

Although often felt to be relatively innocuous, nitrous oxide can have significant metabolic effects in settings of abnormal vitamin B12 and B12-related metabolism in children. These conditions can be genetic or environmental. Symptoms may not appear until days to weeks after exposure to nitrous oxide. Although overt genetic diseases are relatively uncommon, the implications of nitrous oxide interactions with much more frequent but less symptomatically obvious single nucleotide polymorphisms are potentially more concerning. In addition, nitrous oxide can have direct and differing neurotoxic effects on both immature and aged brain, the clinical impact of which remains undetermined.

Preoperative folate and homocysteine status in patients undergoing major surgery

BACKGROUND & AIMS

Patients with folate deficiency and elevated homocysteine (Hcy) levels have increased risk of cerebrovascular and cardiovascular disease. They may also be at increased risk of complications after surgery because nitrous oxide interferes with folate metabolism. The aim of this study was to assess the incidence of folate deficiency and hyperhomocysteinemia in patients presenting for major surgery. We also tested the utility of a brief preoperative medical and dietary questionnaire to identify those with low folate or elevated Hcy levels.

METHODS

We enrolled 390 patients and obtained a preoperative fasting blood sample from each of the study participants (folate [n=386], homocysteine [n=387]).

RESULTS

Although only one patient had folate deficiency preoperatively (incidence +/-SD folate and Hcy concentrations were 23.7+/-5.2 nmol/l and 9.4+/-4.2 micromol/l, respectively. There was a negative correlation between folate and Hcy, r=-0.27, P<0.001. Multivariate analyses indicated that vegan status and folate supplementation prevented low folate status (P<0.05), while age of the patient, and history of heart failure and anaemia predicted elevated Hcy concentration.

CONCLUSIONS

Some factors identified by a brief medical and dietary questionnaire are associated with folate and homocysteine status. Hyperhomocysteinaemia occurs in about 7.5% of surgical patients; however, both low folate status and elevated Hcy concentration are less likely in those taking folate or vitamin B supplements. This has implications for patients undergoing nitrous oxide anaesthesia because of its inhibition of folate metabolism, and should prompt clinicians to consider folate and other nutritional supplementation before elective surgery.