The protective effect of tight-fitting powered air-purifying respirators during chest compressions.

BACKGROUND: Airborne personal protective equipment is required for healthcare workers when performing aerosol-generating procedures on patients with infectious diseases. Chest compressions, one of the main components of cardiopulmonary resuscitation, require intense and dynamic movements of the upper body. We aimed to investigate the protective effect of tight-fitting powered air-purifying respirators (PAPRs) during chest compressions. METHODS: This single-center simulation study was performed from February 2021 to March 2021. The simulated workplace protection factor (SWPF) is the concentration ratio of ambient particles and particles inside the PAPR mask; this value indicates the level of protection provided by a respirator when subjected to a simulated work environment. Participants performed continuous chest compressions three times for 2 min each time, with a 4-min break between each session. We measured the SWPF of the tight-fitting PAPR during chest compression in real-time mode. The primary outcome was the ratio of any failure of protection (SWPF <500) during the chest compression sessions. RESULTS: Fifty-four participants completed the simulation. Overall, 78% (n = 42) of the participants failed (the measured SWPF value was less than 500) at least one of the three sessions of chest compressions. The median value and interquartile range of the SWPF was 4304 (685-16,191). There were no reports of slipping down of the respirator or mechanical failure during chest compressions. CONCLUSIONS: Although the median SWPF value was high during chest compressions, the tight-fitting PAPR did not provide adequate protection.

Association between loss of hypercoagulable phenotype, clinical features and complement pathway consumption in COVID-19.

Background: Coronavirus disease 2019 (COVID-19) features a hypercoagulable state, but therapeutic anticoagulation effectiveness varies with disease severity. We aimed to evaluate the dynamics of the coagulation profile and its association with COVID-19 severity, outcomes, and biomarker trajectories. Methods: This multicenter, prospective, observational study included patients with COVID-19 requiring respiratory support. Rotational thromboelastometry findings were evaluated for coagulation and fibrinolysis status. Hypercoagulable status was defined as supranormal range of maximum clot elasticity in an external pathway. Longitudinal laboratory parameters were collected to characterize the coagulation phenotype. Results: Of 166 patients, 90 (54%) were severely ill at inclusion (invasive mechanical ventilation, 84; extracorporeal membrane oxygenation, 6). Higher maximum elasticity (P=0.02) and lower maximum lysis in the external pathway (P=0.03) were observed in severely ill patients compared with the corresponding values in patients on non-invasive oxygen supplementation. Hypercoagulability components correlated with platelet and fibrinogen levels. Hypercoagulable phenotype was associated with favorable outcomes in severely ill patients, while normocoagulable phenotype was not (median time to recovery, 15 days vs. 27 days, P=0.002), but no significant association was observed in moderately ill patients. In patients with severe COVID-19, lower initial C3, minimum C3, CH50, and greater changes in CH50 were associated with the normocoagulable phenotype. Changes in complement components correlated with dynamics of coagulation markers, hematocrit, and alveolar injury markers. Conclusions: While hypercoagulable states become more evident with increasing severity of respiratory disease in patients with COVID-19, normocoagulable phenotype is associated with triggered by alternative pathway activation and poor outcomes.

Burnout among Japanese emergency medicine physicians: A multicentric questionnaire study.

Objective: Burnout negatively affects the wellness and performance of emergency physicians (EPs). This study aimed to clarify the actual prevalence of burnout and its associated factors among Japanese EPs. Methods: We conducted a cross-sectional questionnaire study of selected 27 Japanese emergency departments (EDs). We examined the Maslach Burnout Inventory-Human Services Survey score and its associations with ED-level- and EP-level factors in a multivariable analysis. Results: A total of 267 EPs (81.9%) completed survey. Of these, 43 EPs (16.1%) scored severe emotional exhaustion (EE), 53 (19.8%) scored severe depersonalization (DP), and 179 (67.0%) scored severe personal accomplishment (PA), and 24 (8.9%) scored severely in all three domains. In our multivariable analysis, emergency medical service centers were associated with severe PA scores (odds ratio [OR], 10.56; 95% confidence interval [CI], 1.78-62.66; p = 0.009). A 3 to 6 hour-sleep period was associated with severe EE scores (OR, 2.04; 95% CI, 1.04-3.98; p = 0.036), and EPs in their 20s were associated with severe DP scores (OR, 7.37; 95% CI, 1.41-38.38; p = 0.018). Conclusion: Our results suggest that 8.9% of Japanese EPs are in higher degrees of burnout. In particular, Japanese EPs scored more severely on PA. To avoid burnout in Japanese EPs, it is important to improve the working environment by ensuring more than 6 h of sleep, providing more support for young EPs, and taking effective action to combat low EP self-esteem.

Physical function and mental health trajectories in COVID-19 patients following invasive mechanical ventilation: a prospective observational study.

This prospective observational cohort study was performed to investigate the physical function and mental health trajectories of novel coronavirus disease 2019 (COVID-19) patients requiring invasive mechanical ventilation (IMV) after discharge from the intensive care unit (ICU). The study population consisted of 64 patients (median age, 60 years; 85.9% male; median IMV duration, 9 days). At ICU discharge, 28.1% of the patients had Medical Research Council (MRC) sum score < 48 points, and prolonged IMV was significantly associated with lower MRC sum score and handgrip strength. Symptoms were similar between groups at ICU discharge, and the symptoms most commonly reported as moderate-to-severe were impaired well-being (52%), anxiety (43%), tiredness (41%), and depression (35%). Although muscle strength and mobility status were significantly improved after ICU discharge, Edmonton Symptom Assessment System score did not improve significantly in the prolonged IMV group. EuroQol five-dimension five-level summary index was significantly lower in the prolonged than short IMV group at 6 months after ICU discharge. We found substantial negative physical function and mental health consequences in the majority of surviving COVID-19 patients requiring IMV, with prolonged period of IMV showing greater negative effects not only immediately but also at 6 months after discharge from the ICU.

Bedside cannulation for veno-venous extracorporeal membrane oxygenation using portable X-ray system in a coronavirus disease patient.

Transportation of patients with coronavirus disease (COVID)-19 outside isolation rooms should be avoided to prevent further spread of the disease. Here, we report a safe and accurate bedside cannulation method for veno-venous extracorporeal membrane oxygenation (VV-ECMO) in a COVID-19 patient in the intensive care unit. A 71-year-old man was admitted to our hospital and diagnosed as having COVID-19 pneumonia. We decided to initiate VV-ECMO therapy because maintaining blood oxygen saturation was difficult despite the mechanical ventilation. We placed two flat-panel detectors behind the patient's chest and the right inguinal area. We repeatedly imaged and monitored insertion of wires and cannulas using a portable X-ray system. Cannulas were successfully inserted in the appropriate position, and VV-ECMO was initiated without any complications. .

Relative platelet reductions provide better pathophysiologic signatures of coagulopathies in sepsis.

In sepsis-associated coagulopathies and disseminated intravascular coagulation, relative platelet reductions may reflect coagulopathy severity. However, limited evidence supports their clinical significance and most sepsis-associated coagulopathy criteria focus on the absolute platelet counts. To estimate the impact of relative platelet reductions and absolute platelet counts on sepsis outcomes. A multicenter retrospective observational study was performed using the eICU Collaborative Research Database, comprising 335 intensive care units (ICUs) in the United States. Patients with sepsis and an ICU stay > 2 days were included. Estimated effects of relative platelet reductions and absolute platelet counts on mortality and coagulopathy-related complications were evaluated. Overall, 26,176 patients were included. Multivariate mixed-effect logistic regression analysis revealed marked in-hospital mortality risk with larger platelet reductions between days one and two, independent from the resultant absolute platelet counts. The adjusted odds ratio (OR) [95% confidence intervals (CI)] for in-hospital mortality was 1.28[1.23-1.32], 1.86[1.75-1.97], 2.99[2.66-3.36], and 6.05[4.40-8.31] for 20-40%, 40-60%, 60-80%, and > 80% reductions, respectively, when compared with a < 20% decrease in platelets (P < 0.001 for each). In the multivariate logistic regression analysis, platelet reductions ≥ 11% and platelet counts ≤ 100,000/µL on day 2 were associated with high coagulopathy-related complications (OR [95%CI], 2.03 and 1.18; P < 0.001 and P < 0.001), while only platelet reduction was associated with thromboembolic complications (OR [95%CI], 1.43 [1.03-1.98], P < 0.001). The magnitude of platelet reductions represent mortality risk and provides a better signature of coagulopathies in sepsis; therefore, it is a plausible criterion for sepsis-associated coagulopathies.

Usefulness of Respiratory Mechanics and Laboratory Parameter Trends as Markers of Early Treatment Success in Mechanically Ventilated Severe Coronavirus Disease: A Single-Center Pilot Study.

Whether a patient with severe coronavirus disease (COVID-19) will be successfully liberated from mechanical ventilation (MV) early is important in the COVID-19 pandemic. This study aimed to characterize the time course of parameters and outcomes of severe COVID-19 in relation to the timing of liberation from MV. This retrospective, single-center, observational study was performed using data from mechanically ventilated COVID-19 patients admitted to the ICU between 1 March 2020 and 15 December 2020. Early liberation from ventilation (EL group) was defined as successful extubation within 10 days of MV. The trends of respiratory mechanics and laboratory data were visualized and compared between the EL and prolonged MV (PMV) groups using smoothing spline and linear mixed effect models. Of 52 admitted patients, 31 mechanically ventilated COVID-19 patients were included (EL group, 20 (69%); PMV group, 11 (31%)). The patients' median age was 71 years. While in-hospital mortality was low (6%), activities of daily living (ADL) at the time of hospital discharge were significantly impaired in the PMV group compared to the EL group (mean Barthel index (range): 30 (7.5-95) versus 2.5 (0-22.5), p = 0.048). The trends in respiratory compliance were different between patients in the EL and PMV groups. An increasing trend in the ventilatory ratio during MV until approximately 2 weeks was observed in both groups. The interaction between daily change and earlier liberation was significant in the trajectory of the thrombin-antithrombin complex, antithrombin 3, fibrinogen, C-reactive protein, lymphocyte, and positive end-expiratory pressure (PEEP) values. The indicator of physiological dead space increases during MV. The trajectory of markers of the hypercoagulation status, inflammation, and PEEP were significantly different depending on the timing of liberation from MV. These findings may provide insight into the pathophysiology of COVID-19 during treatment in the critical care setting.

Surgical strategy and optimal timing of tracheostomy in patients with COVID-19: Early experiences in Japan.

OBJECTIVE: Tracheostomy is an important surgical procedure for coronavirus disease-2019 (COVID-19) patients who underwent prolonged tracheal intubation. Surgical indication of tracheostomy is greatly affected by the general condition of the patient, comorbidity, prognosis, hospital resources, and staff experience. Thus, the optimal timing of tracheostomy remains controversial. METHODS: We reviewed our early experience with COVID-19 patients who underwent tracheostomy at one tertiary hospital in Japan from February to September 2020 and analyzed the timing of tracheostomy, operative results, and occupational infection in healthcare workers (HCWs). RESULTS: Of 16 patients received tracheal intubation with confirmed severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection, five patients (31%) received surgical tracheostomy in our hospital. The average consultation time for surgical tracheostomy was 7.4 days (range, 6 - 9 days) from the COVID-19 team to the otolaryngologist. The duration from tracheal intubation to tracheostomy ranged from 14 to 27 days (average, 20 days). The average time of tracheostomy was 27 min (range, 17 - 39 min), and post-wound bleeding occurred in only one patient. No significant differences in hemoglobin (Hb) levels were found between the pre- and postoperative periods (mean: 10.2 vs. 10.2 g/dl, p = 0.93). Similarly, no difference was found in white blood cell (WBC) count (mean: 12,200 vs. 9,900 cells /µl, p = 0.25). After the tracheostomy, there was no occupational infection among the HCWs who assisted the tracheostomy patients during the perioperative period. CONCLUSION: We proposed a modified weaning protocol and surgical indications of tracheostomy for COVID-19 patients and recommend that an optimal timing for tracheostomy in COVID-19 patients of 2 - 3 weeks after tracheal intubation, from our early experiences in Japan. An experienced multi-disciplinary tracheostomy team is essential to perform a safe tracheostomy in patients with COVID-19 and to minimize the risk of occupational infection in HCWs.

Efficacy of favipiravir for an end stage renal disease patient on maintenance hemodialysis infected with novel coronavirus disease 2019.

BACKGROUND: Novel coronavirus disease 2019 (COVID-19) refers to infection with the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pathogen, and has spread to pandemic levels since its inception in December 2019. While several risk factors for severe presentation have been identified, the clinical course for end-stage renal disease (ESRD) patients on maintenance hemodialysis with COVID-19 has been unclear. Previous studies have revealed that some antiviral agents may be effective against COVID-19 in the general population, but the pharmacokinetics and pharmacodynamics of these agents in ESRD patients remain under investigation. Favipiravir, an antiviral agent developed for treatment of influenza, is one candidate treatment for COVID-19, but suitable dosages for patients with renal insufficiency are unknown. Here we provide a first report on the efficacy of favipiravir in a patient with ESRD undergoing hemodialysis. CASE PRESENTATION: The case involved a 52-year-old woman with COVID-19 who had been undergoing maintenance hemodialysis three times a week for 3 years due to diabetic nephropathy. She had initially been treated with lopinavir/ritonavir and ciclesonide for 5 days, but developed severe pneumonia requiring invasive positive-pressure ventilation. Those antiviral agents were subsequently switched to favipiravir. She recovered gradually, and after 2 weeks was extubated once the viral load of SARS-CoV-2 fell below the limit of detection. Although concentrations of several biliary enzymes were elevated, no major adverse events were observed. CONCLUSION: Favipiravir may be an effective option for the treatment of COVID-19-infected patients with ESRD.

Clinical impact of visually assessed right ventricular dysfunction in patients with septic shock.

We retrospectively analyzed data from the Medical Information Mart for Intensive Care-III critical care database to determine whether visually-assessed right ventricular (RV) dysfunction was associated with clinical outcomes in septic shock patients. Associations between visually-assessed RV dysfunction by echocardiography and in-hospital mortality, lethal arrhythmia, and hemodynamic indicators to determine the prognostic value of RV dysfunction in patients with septic shock were analyzed. Propensity score analysis showed RV dysfunction was associated with increased risk of in-hospital death in patients with septic shock (adjusted odds ratio [OR] 2.15; 95% confidence interval [CI] 1.99-2.32; P < 0.001). In multivariate logistic regression analysis, RV dysfunction was associated with in-hospital death (OR 2.19; 95% CI 1.91-2.53; P < 0.001), lethal arrhythmia (OR 2.19; 95% CI 1.34-3.57; P < 0.001), and tendency for increased blood lactate levels (OR 1.31; 95% CI 1.14-1.50; P < 0.001) independent of left ventricular (LV) dysfunction. RV dysfunction was associated with lower cardiac output, pulmonary artery pressure index, and RV stroke work index. In patients with septic shock, visually-assessed RV dysfunction was associated with in-hospital mortality, lethal arrhythmia, and circulatory insufficiency independent of LV dysfunction. Visual assessment of RV dysfunction using echocardiography might help to identify the short-term prognosis of patients with septic shock by reflecting hemodynamic status.

A case of reexpansion pulmonary edema and acute pulmonary thromboembolism associated with diffuse large B-cell lymphoma treated with venovenous extracorporeal membrane oxygenation.

A 37-year-old man diagnosed with diffuse large B-cell lymphoma two weeks previously, visited our emergency department with sudden dyspnea. He had a severe respiratory failure with saturated percutaneous oxygen at 80% (room air). Chest radiography showed a large amount of left pleural effusion. After 1000 mL of the effusion was urgently drained, reexpansion pulmonary edema (RPE) occurred. Despite ventilator management, oxygenation did not improve and venovenous extracorporeal membrane oxygenation (VV-ECMO) was initiated in the intensive care unit. The next day, contrast-enhanced computed tomography showed a massive thrombus in the right pulmonary artery, at this point the presence of pulmonary thromboembolism (PTE) was revealed. Fortunately, the patient's condition gradually improved with anticoagulant therapy and VV-ECMO support. VV-ECMO was successfully discontinued on day 4, and chemotherapy was initiated on day 8. We speculated the following mechanism in this case: blood flow to the right lung significantly reduced due to acute massive PTE, and blood flow to the left lung correspondingly increased, which could have caused RPE in the left lung. Therefore, our observations suggest that drainage of pleural effusion when contralateral blood flow is impaired due to acute PTE may increase the risk of RPE. .

Reducing Aerosol Generation During Ventilator Weaning in a Coronavirus Disease 2019 Patient Using a Supraglottic Airway: A Case Report.

We report weaning from mechanical ventilation with no coughing in a patient with coronavirus disease 2019 (COVID-19). Substituting the endotracheal tube for a supraglottic airway (SGA), which is less stimulating to the trachea, can reduce coughing with weaning from mechanical ventilation and extubation. Personal protective equipment is in short supply worldwide. Reducing spread of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is beneficial in terms of occupational health of health care workers.

Factors associated with successful rescue intubation attempts in the emergency department: an analysis of multicenter prospective observational study in Japan.

AIM: It remains unclear whether physicians should change intubation approaches after the failed first attempt. We aimed to determine the rescue intervention approaches associated with a higher success rate at the second attempt in the emergency department (ED). METHODS: We analyzed the data from a prospective, multicenter, observational study - the second Japanese Emergency Airway Network Study. The current analysis included all patients who underwent emergency intubation from February 2012 through November 2017. We defined a rescue intubation attempt as a second intubation attempt with any change in intubation approaches (i.e., change in methods, devices, or intubators) from the failed first attempt. The outcome measure was second-attempt success. RESULTS: Of 2,710 patients with a failed first attempt, 43% underwent a second intubation attempt with changes in intubation approach (i.e., rescue intubation). Rescue intubation attempts were associated with a higher second-attempt success rate compared to non-rescue intubation attempts (adjusted odds ratio [OR], 1.78; 95% confidence interval [CI], 1.50-2.12). The rescue intubation approaches associated with a higher second-attempt success were changes from non-rapid sequence intubation (RSI) to RSI (adjusted OR, 2.04; 95% CI, 1.12-3.75), from non-emergency medicine (EM) residents to EM residents (adjusted OR, 2.02; 95% CI, 1.44-2.82), and from non-EM attending physicians to EM attending physicians (adjusted OR, 2.82; 95% CI, 2.14-3.71). CONCLUSIONS: In this large multicenter study, rescue interventions were associated with a higher second-attempt success rate. The data also support the use of RSI and backup by EM residents or EM attending physicians to improve the airway management performance after a failed attempt in the ED.

Relative bradycardia as a clinical feature in patients with coronavirus disease 2019 (COVID-19): A report of two cases.

We treated two patients with COVID-19 pneumonia requiring mechanical ventilation. Case 1 was a 73-year-old Japanese man. Computed tomography (CT) revealed ground-glass opacities in both lungs. He had severe respiratory failure with a partial pressure of oxygen in arterial blood/fraction of inspiratory oxygen ratio (P/F ratio) of 203. Electrocardiogram showed a heart rate (HR) of 56 beats/min, slight ST depression in leads II, III, and aVF, and mild saddle-back type ST elevation in leads V1 and V2. High-sensitivity cardiac troponin T (cTnT) level was slightly elevated. Despite a high fever and hypoxemia, his HR remained within 50-70 beats/min. Case 2 was a 52-year-old Japanese woman. CT revealed ground-glass opacities in the lower left lung. Electrocardiogram showed a HR of only 81 beats/min, despite a body temperature of 39.2 °C, slight ST depression in leads V4, V5, V6, and a prominent U wave in multiple leads. She had an elevated cTnT and a P/F ratio of 165. Despite a high fever and hypoxemia, her HR remained within 50-70 beats/min. Both patients had a poor compensatory increase in their HR, despite their critical status. Relative bradycardia could be a cardiovascular complication and is an important clinical finding in patients with COVID-19. .

Advancing emergency airway management practice and research.

Emergency airway management is one of the vital resuscitative procedures undertaken in the emergency department (ED). Despite its clinical and research importance in the care of critically ill and injured patients, earlier studies have documented suboptimal intubation performance and high adverse event rates with a wide variation across the EDs. The optimal emergency airway management strategies remain to be established and their dissemination to the entire nation is a challenging task. This article reviews the current published works on emergency airway management with a focus on the use of airway management algorithms as well as the importance of first-pass success and systematic use of rescue intubation strategies. Additionally, the review summarizes the current evidence for each of the important airway management processes, such as assessment of the difficult airway, preparation (e.g., positioning and oxygenation), intubation methods (e.g., rapid sequence intubation), medications (e.g., premedications, sedatives, and neuromuscular blockades), devices (e.g., direct and video laryngoscopy and supraglottic devises), and rescue intubation strategies (e.g., airway adjuncts and rescue intubators), as well as the airway management in distinct patient populations (i.e., trauma, cardiac arrest, and pediatric patients). Well-designed, rigorously conducted, multicenter studies that prospectively and comprehensively characterize emergency airway management should provide clinicians with important opportunities for improving the quality and safety of airway management practice. Such data will not only advance research into the determination of optimal airway management strategies but also facilitate the development of clinical guidelines, which will, in turn, improve the outcomes of critically ill and injured patients in the ED.

Techniques and outcomes of emergency airway management in Japan: An analysis of two multicentre prospective observational studies, 2010-2016.

OBJECTIVES: Continuous surveillance of emergency airway management practice is imperative in improving quality of care and patient safety. We aimed to investigate the changes in the practice of emergency airway management and the related outcomes in the emergency departments (EDs) in Japan. METHODS: We conducted an analysis of the data from two prospective, observational, multicentre registries of emergency airway management-the Japanese Emergency Airway Network (JEAN)-1 and -2 Registries from April 2010 through May 2016. RESULTS: We recorded 10,927 ED intubations (capture rate, 96%); 10,875 paediatric and adult patients were eligible for our analysis. The rate of rapid sequence intubation (RSI) use as the initial intubation method significantly increased from 28% in 2010 to 53% in 2016 (Ptrend=0.03). Likewise, the rate of video laryngoscope (VL) use as the first intubation device increased significantly from 2% in 2010 to 40% in 2016 (Ptrend<0.001), with a significant decrease in the rate of direct laryngoscope use from 97% in 2010 to 58% in 2016 (Ptrend<0.001). Concurrent with these changes, the overall first-attempt success rate also increased from 68% in 2010 to 74% in 2016 (Ptrend=0.02). By contrast, the rate of adverse events did not change significantly over time (Ptrend=0.06). CONCLUSION: By using data from two large, multicentre, prospective registries, we characterised the current emergency airway management practice, and identified their changes in Japan. The data demonstrated significant increases in the rate of RSI and VL use on the first attempt and the first-attempt success rate over the 6-year study period.

Emergency airway management by resident physicians in Japan: an analysis of multicentre prospective observational study.

AIM: To examine the success rates of emergency department airway management by resident physicians in Japan. METHODS: We conducted an analysis of a multicentre prospective registry (Japanese Emergency Airway Network Registry) of 13 academic and community emergency departments in Japan. We included all patients who underwent emergency intubation performed by postgraduate year 1 to 5 transitional or emergency medicine residents (resident physicians) between April 2010 and August 2012. Outcome measures were success rates by the first intubator, and by rescue intubator, according to the level of training. RESULTS: We recorded 4,094 intubations (capture rate, 96%); 2,800 attempts (2,800/4,094; 68%; 95% confidence interval (CI), 67%-70%) were initially performed by resident physicians. Overall success rate on the first attempt was 63% (1,767/2,789; 95%CI, 61%-64%); the rate improved over the first 3 years of training before reaching a plateau (P trend < 0.001). Success rate by the first intubator was 78% (2,185/2,800; 95%CI, 76%-79%); the rate steadily improved as level of training increased (P trend < 0.001). Of 597 failed intubation attempts by the first intubator, 41% (247/597; 95%CI, 37%-45%) of rescue attempts were performed by resident physicians. Success rate on the first rescue attempt was 76% (187/247; 95%CI, 70%-81%), and success rate by first rescue intubator was 89% (220/247; 95%CI, 85%-93%). These rates on rescue attempts steadily improved as level of training increased (both P trend < 0.001). Intubations were ultimately successful in 2,778 encounters (99.6%). CONCLUSION: In this multicentre study characterizing emergency airway management across Japan, we observed that emergency department intubations were primarily managed by resident physicians with acceptably high success rates overall.

X-ray structures of Aerococcus viridans lactate oxidase and its complex with D-lactate at pH 4.5 show an alpha-hydroxyacid oxidation mechanism.

L-Lactate oxidase (LOX) belongs to a family of flavin mononucleotide (FMN)-dependent alpha-hydroxy acid-oxidizing enzymes. Previously, the crystal structure of LOX (pH 8.0) from Aerococcus viridans was solved, revealing that the active site residues are located around the FMN. Here, we solved the crystal structures of the same enzyme at pH 4.5 and its complex with d-lactate at pH 4.5, in an attempt to analyze the intermediate steps. In the complex structure, the D-lactate resides in the substrate-binding site, but interestingly, an active site base, His265, flips far away from the D-lactate, as compared with its conformation in the unbound state at pH 8.0. This movement probably results from the protonation of His265 during the crystallization at pH 4.5, because the same flip is observed in the structure of the unbound state at pH 4.5. Thus, the present structure appears to mimic an intermediate after His265 abstracts a proton from the substrate. The flip of His265 triggers a large structural rearrangement, creating a new hydrogen bonding network between His265-Asp174-Lys221 and, furthermore, brings molecular oxygen in between D-lactate and His265. This mimic of the ternary complex intermediate enzyme-substrate-O(2) could explain the reductive half-reaction mechanism to release pyruvate through hydride transfer. In the mechanism of the subsequent oxidative half-reaction, His265 flips back, pushing molecular oxygen into the substrate-binding site as the second substrate, and the reverse reaction takes place to produce hydrogen peroxide. During the reaction, the flip-flop action of His265 has a dual role as an active base/acid to define the major chemical steps. Our proposed reaction mechanism appears to be a common mechanistic strategy for this family of enzymes.