Recurring fentanyl use is a significant factor contributing to opioid-related deaths, yet the physiological impact of repeat opioid use on breathing remains poorly understood. This study demonstrates that divergent ventilatory responses to opioids emerge following repeated fentanyl administration. These responses coincide with changes in oxygen consumption and inspiratory rhythmogenesis from the preBötzinger complex. These observations advance an understanding of the physiological basis for susceptibility and tolerance among individuals likely to succumb to opioid overdose.

Myocardial infarction (MI) compromises the cardiac microvascular endothelial barrier, increasing leakage and inflammation. HIF2α, predominantly expressed in cardiac endothelial cells during ischemia, has an unclear role in barrier function during MI. Here, we show that inducible, adult endothelial-specific deletion of Hif2α in mice leads to increased mortality, cardiac leakage, inflammation, reduced heart function, and adverse remodeling after MI. In parallel, human cardiac microvascular endothelial cells (HCMVECs) lacking HIF2α display impaired barrier integrity, reduced tight-junction proteins, increased cell death, and elevated IL-6 levels, effects that are alleviated by overexpressing ARNT, a key partner of HIF2α under hypoxic conditions. Interestingly, ARNT, but not HIF2α, directly binds the IL-6 promoter to suppress its expression. These findings suggest the HIF2α/ARNT axis as a protective mechanism in heart failure post-MI and identify potential therapeutic targets to support cardiac function. Endothelial-specific deletion of Hif2α results in increased leakage, inflammation, impaired heart function, and adverse remodelling post-myocardial infarction in mice, highlighting the protective role of the HIF2α‒ARNT axis in heart failure.

Introduction: Opioid-related morbidities have become a critical public health concern, with ultrapotent synthetic opioids (UPSO) such as fentanyl contributing to the rise in opioid-related fatalities. Respiratory depression is the cardinal trait of fentanyl overdose, leading to systemic hypoxia and subsequent cardiorespiratory collapse (CRC). There is growing recognition that fentanyl induced CRC (fiCRC) is unique from other forms of sudden CRC. Yet, the consequences of fiCRC and its reversal remain poorly understood. This is due in part to the limited preclinical studies investigating rescue from fiCRC. Objective: using a novel model of fiCRC and subsequent reversal, this ongoing study aims to define the impact of UPSO and naloxone rescue on in vivo survival outcomes. Methods and Results: Male and female C57BL/6J mice were used. UPSO fentanyl i.v. administration (30 to 500 mcg/kg) suppressed breathing and subsequently led to fiCRC. The return of spontaneous circulation (ROSC) was achieved by performing manual chest compressions in combination of epinephrine administration after 8 minutes of fiCRC. In subset of mice, naloxone was administered post ROSC. Outcomes after 72 hours were accessed. The latency of fiCRC (mean arterial pressure at 40 mmHg) exhibited a negative correlation with the doses of fentanyl administered. Resuscitation from fiCRC was unreliable across the dose spectrum, while naloxone consistently rescued the majority of fiCRC animals. Naloxone increased ROSC rate from 54% to 86% when the dose of fentanyl was 30 mcg/Kg (n=22). With naloxone rescue, we compared fiCRC with non-opioid-induced cardiac arrest followed by potassium chloride injection (0.8 mg/g). Both models showed severe lung edema as evidenced by Evans Blue staining. However, the lung wet/dry weight ratio was higher in fiCRC group than in the non-opioid treatment group (5.50±0.05 vs. 6.60±0.60, n=4, p<0.05). Survival in fiCRC group was significantly worse than in non-opioid treatment group (survival rate: 25% vs. 83%, n=17, p<0.05). Conclusions: Our study highlights two important findings. First, the reliable resuscitation of fiCRC necessitates the use of naloxone. Second, the likelihood of injury is significant following successful rescue from fiCRC by naloxone. Our continued work using this fiCRC model will lead to insights that can be leveraged for strategies to mitigate injury and improve survival following fentanyl overdose. R01 HL163965 & R01 DA057767. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Advancing care in Emergency Medicine (EM) requires the development of well-trained researchers, but our specialty has lower amounts of research funding compared to similar medical fields. Increasing the number of pathways available for research training supports the growth of new investigators. To address the need for more EM researchers, the Society of Academic Emergency Medicine and the American College of Emergency Physicians convened a Federal Research Funding Workgroup. Here, we report the workgroup recommendations regarding the creation of Research Training Fellowships using the T32 grant structure sponsored by the National Institutes of Health. After reviewing the history of NIH-grant supported research fellowships in EM, we outline the rationale and describe the core components of T32-supported research fellowships, including program design, fellow evaluation, and recruitment considerations.

Manual pulse checks for assessing return of spontaneous circulation (ROSC) during planned pauses of cardiopulmonary resuscitation (CPR) are central to all internationally endorsed cardiac arrest algorithms for advanced cardiac life support.1–3 Despite this, the rapid and accurate detection of central pulses in patients with spontaneous circulation by health care providers, even under laboratory conditions, is notoriously challenging.4–6 Manual pulse sensitivity can be further degraded under conditions of hypotension or large patient body habitus leading to a paradoxical scenario, termed pseudo-pulseless activity (pseudo-PEA), where manual pulses are not detected despite the presence of spontaneous coordinated cardiac activity on point-of-care ultrasound (POCUS).

Myocardial infarction (MI) significantly compromises the integrity of the cardiac microvascular endothelial barrier, leading to enhanced leakage and inflammation that contribute to the progression of heart failure. While HIF2α is highly expressed in cardiac endothelial cells (ECs) under hypoxic conditions, its role in regulating microvascular endothelial barrier function during MI is not well understood. In this study, we utilized mice with a cardiac-specific deletion of HIF2α, generated through an inducible Cre (Cdh5Cre-ERT2) recombinase system. These mice exhibited no apparent phenotype under normal conditions. However, following left anterior descending (LAD) artery ligation-induced MI, they showed increased mortality associated with enhanced cardiac vascular leakage, inflammation, worsened cardiac function, and exacerbated heart remodeling. These outcomes suggest a protective role for endothelial HIF2α in response to cardiac ischemia. Parallel investigations in human cardiac microvascular endothelial cells (CMVECs) revealed that loss of ecHif2α led to diminished endothelial barrier function, characterized by reduced tight-junction protein levels and increased cell death, along with elevated expression of IL6 and other inflammatory markers. These effects were substantially reversed by overexpressing ARNT, a critical dimerization partner for HIF2α during hypoxia. Additionally, ARNT deletion also led to increased CMVEC permeability. Interestingly, ARNT, rather than HIF2α itself, directly binds to the IL6 promoter to suppress IL6 expression. Our findings demonstrate the critical role of endothelial HIF2α in response to MI and identify the HIF2α/ARNT axis as a transcriptional repressor, offering novel insights for developing therapeutic strategies against heart failure following MI.

Dynamin-related protein 1 (Drp1) is a cytosolic GTPase protein that when activated translocates to the mitochondria, meditating mitochondrial fission and increasing reactive oxygen species (ROS) in cardiomyocytes. Drp1 has shown promise as a therapeutic target for reducing cardiac ischemia/reperfusion (IR) injury; however, the lack of specificity of some small molecule Drp1 inhibitors and the reliance on the use of Drp1 haploinsufficient hearts from older mice have left the role of Drp1 in IR in question. Here, we address these concerns using two approaches, using: (a) short-term (3 weeks), conditional, cardiomyocyte-specific, Drp1 knockout (KO) and (b) a novel, highly specific Drp1 GTPase inhibitor, Drpitor1a. Short-term Drp1 KO mice exhibited preserved exercise capacity and cardiac contractility, and their isolated cardiac mitochondria demonstrated increased mitochondrial complex 1 activity, respiratory coupling, and calcium retention capacity compared to controls. When exposed to IR injury in a Langendorff perfusion system, Drp1 KO hearts had preserved contractility, decreased reactive oxygen species (ROS), enhanced mitochondrial calcium capacity, and increased resistance to mitochondrial permeability transition pore (MPTP) opening. Pharmacological inhibition of Drp1 with Drpitor1a following ischemia, but before reperfusion, was as protective as Drp1 KO for cardiac function and mitochondrial calcium homeostasis. In contrast to the benefits of short-term Drp1 inhibition, prolonged Drp1 ablation (6 weeks) resulted in cardiomyopathy. Drp1 KO hearts were also associated with decreased ryanodine receptor 2 (RyR2) protein expression and pharmacological inhibition of the RyR2 receptor decreased ROS in post-IR hearts suggesting that changes in RyR2 may have a role in Drp1 KO mediated cardioprotection. We conclude that Drp1-mediated increases in myocardial ROS production and impairment of mitochondrial calcium handling are key mechanisms of IR injury. Short-term inhibition of Drp1 is a promising strategy to limit early myocardial IR injury which is relevant for the therapy of acute myocardial infarction, cardiac arrest, and heart transplantation. Dynamin-related protein 1 (Drp1) is a cytosolic protein that regulates mitochondrial fission. Heart ischemia/reperfusion (IR) injury results in Drp1 translocation to the mitochondrial outer mitochondrial membrane, mitochondrial swelling, and opening of the mitochondrial permeability transition pore (MPTP) associated with calcium overload, cytochrome C release, and reactive oxygen species generation (ROS). Inducible short-term cardiomyocyte-specific Drp1 ablation results in a reduction of Drp1 and RyR1 protein expression that results in cardioprotection as evidenced by inhibition of MPTP opening, cytochrome C release, ROS generation, and calcium overload. Pharmacological inhibition of Drp1 with the inhibitor Drpitor1a is similarly cardioprotective. In summary, Drp1 is necessary for impaired ROS generation and impaired calcium handling resulting in myocardial injury following IR. Short-term targeting of Drp1 is a promising strategy to limit early myocardial IR injury which is relevant for acute myocardial infarction, cardiac arrest, and heart transplantation.image

There are no conflicts of interest for any authors to report for this effort.

As outlined in their eloquent editorial “Community First Responders: When is the Juice Worth the Squeeze?” Drs. Delbridge and Chizmar outline the benefits of training citizens to provide basic life-saving techniques that not only hold great potential for saving lives as the “Stop the Bleed” campaign but also have real-world improved outcomes, as demonstrated by increased bystander cardiopulmonary resuscitation (CPR) rates, as shown by Cardiac Arrest Registry to Enhance Survival data.1 The authors discuss the challenges of implementing these strategies and those discussed by Botan et al,2 given the lack of homogeneity among EMS systems and communities.

information (RFI) from the National Institutes of Health (NIH) to solicit public commentary on priority directions in violence against women (VAW) research .

Sleep apnea, a clinical condition characterized by repetitive pauses in breathing that cause intermittent hypoxia (IH), affects a billion people worldwide, increases the risk for mild cognitive impairment, and is estimated to occur in up to 50% of individuals with Alzheimer’s Disease (AD). Previous work in wildtype mice has shown that IH impairs spatial memory that is associated with deficits in NMDAr-dependent synaptic physiology. However, a significant knowledge gap remains in understanding how IH may impact ultrastructural properties in the brain. Using large volume serial electron microscopy (EM), we studied mitochondrial abnormalities and synaptic density changes in the area CA1 of hippocampus of non-transgenic mice and CeAPP/PS1 mice, a mouse model of Aß amyloidosis, exposed to ten days of IH. Immediately after IH, mitochondria became elongated and appeared to undergo fusion (becoming 4x to 8x longer) in pyramidal CA1 neurons. This change in mitochondrial morphology reversed, e.g. mitochondrial length normalized relative to controls, when the IH-exposed mice were allowed to recover in room air for 10 days post-IH. We also found a 20-40% increase in synaptic density in proximal and distal dendrites of IH-exposed pyramidal CA1 neurons, yet the ratio of multi-synaptic boutons to single synaptic boutons in area CA1 did not appear to change. These observations indicate that changes in synaptic density occur without producing significant changes in axonal innervation patterns. Finally, using synchrotron source X-ray microscopy, and automated serial EM in IH-exposed brain tissue from CeAPP/PS1 transgenic mice, we documented an increased amyloid plaque number in both cortex (two-fold increase) and hippocampus (ten-fold increase). Large volume EM showed similarly elongated mitochondrial morphology changes in hippocampal neurons in CeAPP/PS1 mice when exposed to IH. These findings suggest that IH, such as that experienced with untreated sleep apnea, may drive ultrastructural changes that both contribute to mild cognitive impairment and promote the progression of AD and related dementias. NIH: R01NS107421; R01HL163965; R01DA057767. This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Rationale: Cardiac microvascular leakage and inflammation are triggered during myocardial infarction (MI) and contribute to heart failure. Hypoxia-inducible factor 2α ( Hif2α ) is highly expressed in endothelial cells (ECs) and rapidly activated by myocardial ischemia, but its impact in microvascular endothelial barrier function during MI is unclear. Objective: To test our hypothesis that the expression of Hif2α in ECs regulates cardiac microvascular permeability in infarcted hearts, which is through its binding partner aryl hydrocarbon nuclear translocator. (ARNT). Methods and Results: Experiments were conducted with mice carrying an inducible EC-specific Hif2α -knockout ( ecHif2α -/- ) mutation, with mouse cardiac microvascular endothelial cells (CMVECs) isolated from the hearts of ecHif2α -/- mice after the mutation was induced, and with human CMVECs and umbilical-vein endothelial cells transfected with ecHif2α siRNA. After MI induction, echocardiographic assessments of cardiac function were significantly lower, while measures of cardiac microvascular leakage (Evans blue assay), plasma IL6 levels, and cardiac neutrophil accumulation and fibrosis (histology) were significantly greater, in ecHif2α -/- mice than in control mice, and RNA-sequencing analysis of heart tissues from both groups indicated that the expression of genes involved in vascular permeability and collagen synthesis was enriched in ecHif2α -/- hearts. In cultured ECs, ec Hif2α deficiency was associated with declines in endothelial barrier function (electrical cell impedance assay) and the reduced abundance of tight-junction proteins, as well as an increase in the expression of inflammatory markers, all of which were largely reversed by the overexpression of ARNT. We also found that ARNT, but not Hif2α, binds directly to the IL6 promoter and suppresses IL6 expression. Conclusions: Endothelial HIF-2a protects from hypoxia-induced cardiac microvascular barrier dysfunction, promotes inflammation damage and represents a potential therapeutic target for cardioprotection, and prevention of fibrosis following acute ischemic injury.

Sepsis is a life-threatening organ dysfunction caused by a dysregulated host response to infection, with septic cardiomyopathy being a common and severe complication. Despite its significant clinical impact, the molecular mechanisms underlying sepsis-induced cardiomyopathy (SICM) remain incompletely understood. In this study, we performed a comparative analysis of whole transcriptome profiles using RNA sequencing in mouse hearts in two widely used mouse models of septic cardiomyopathy. CLP-induced sepsis was achieved by surgical cecal ligation and puncture, while LPS-induced sepsis was induced using a 5 mg/kg intraperitoneal (IP) injection of lipopolysaccharide (LPS). For consistency, we utilized sham-operated mice as the control for septic models. Our aim was to identify key genes and pathways involved in the development of septic cardiomyopathy and to evaluate the similarities and differences between the two models. Our findings demonstrated that both the CLP and lipopolysaccharide LPS methods could induce septic heart dysfunction within 24 h. We identified common transcriptional regulatory regions in the septic hearts of both models, such as Nfkb1, Sp1, and Jun. Moreover, differentially expressed genes (DEGs) in comparison to control were involved in shared pathways, including regulation of inflammatory response, regulation of reactive oxygen species metabolic process, and the JAK-STAT signaling pathway. However, each model presented distinctive whole transcriptome expression profiles and potentially diverse pathways contributing to sepsis-induced heart failure. This extensive comparison enhances our understanding of the molecular basis of septic cardiomyopathy, providing invaluable insights. Accordingly, our study also contributes to the pursuit of effective and personalized treatment strategies for SICM, highlighting the importance of considering the specific causative factors.

Introduction: Oxygen administration following cardiac arrest (CA) is associated with cardiac dysfunction, brain injury, and poor survival. These injuries are driven in part by mitochondrial injury, but the specific mechanisms underlying oxygen-induced mitochondrial injury are unclear. Here we hypothesized that oxygen-mediated injury following CA is influenced by the mitochondrial fission protein Dynamin Related Protein 1 (Drp1) and that targeting Drp1 could mitigate the effects of post-CA oxygen toxicity. Methods and Results: Mice underwent brief asystole CA (12 min) followed by cardiopulmonary resuscitation (CPR). One hour following successful CPR, mice were randomized to receive hyperoxia (33% O 2 ), normoxia (21% O 2 ), or hypoxia (10% O 2 ) in an environmental chamber for 6 hours. After exposure, the mice were returned to room air. Hyperoxia and normoxia groups exhibited lower 10-day survival rate compared to mice exposed to hypoxia (15% and 67% vs. 92%, n=13/group, P<0.05, respectively). The hyperoxia group also showed decreased myocardial fractional shortening (30.1±3.0% vs 45.0±1.4%) and neurological scores (5.8±0.8 vs 9.8±0.3) 48 hours post CA compared to the hypoxia group (n=13, P<0.05, respectively). Hyperoxia increased the expression of the activated form of Drp1 (Drp1-S616) to 134% in the heart (n=4, P<0.05) and decreased cardiomyocyte mitochondrial size to 75% (n=30, P<0.05) compared to hypoxia-treated CA mice. The hypoxia-dependent decrease in Drp1 activity was associated with a 46% increase in myocardial mitochondrial respiration (n=5, P<0.01) and a 32% reduction in ROS production vs hyperoxia-treated CA mice (n=6, P<0.05). Administration of the Drp1 GTPase inhibitor Drpitor1a to hyperoxia-exposed post-CA mice increased survival (40% vs 20%), myocardial function (31.5±4.4% vs 42.0±1.3%) and neurological scores (4.1±0.7 vs 9.0±0.0) compared to untreated hyperoxia-exposed post-CA mice (n=10, P<0.05, respectively). Conclusions: Oxygen dependent post-CA injury is partly dependent on the mitochondrial fission factor Drp1. Targeting Drp1 is a promising strategy to limit post-CA oxygen toxicity.

Introduction: Mitochondrial injury occurs following cellular ischemia in the setting of myocardial infarction, stroke, and cardiac arrest (CA). Effective strategies for reducing injury are limited. Hypoxia has shown benefit in reversing mitochondrial mediated neurodegenerative disease but is little studied in the setting of post-ischemic injury and is regarded as potentially harmful. In this study, we investigated the effects of mild hypoxia on global post-ischemic injury following CA. We hypothesized that brief mild hypoxia following CA would improve mitochondrial function resulting in improved cellular metabolism and physiological recovery. Methods and Results: Anesthetized C57BL6 mice underwent brief asystolic CA (12 min) followed bycardiopulmonary resuscitation(CPR). One hour following successful CPR, mice were randomized to receive a brief episode (6 hours) of normoxia (21% O2) or hypoxia (10% O2) in an environmental chamber. Post intervention, the mice were returned to room air (21% O2). Mild hypoxia improved cardiac mitochondrial complex 1 respiration in the heart by 28% and ATP-related oxygen consumption by 49% (n=3, P<0.05 vs normoxia, respectively) compared to normoxia treated mice. Hypoxia was further associated with improved glucose oxidation in heart as evidenced by decreases in pyruvate dehydrogenase kinase 4 (PDK4) gene and protein expression (n=4, P<0.05 vs normoxia, respectively) as well as decreases in pyruvate dehydrogenase phosphorylation (n=4, P<0.05, respectively) in heart. Hypoxia decreased lactate dehydrogenase (LDH) expression and lactate concentration in heart compared to normoxia treated controls (n=4, P<0.05 vs normoxia, respectively). ROS production in heart by 20% (n=6, P<0.05) was also decreased by mild hypoxia when compared to normoxia post CA. These changes were associated with improved recovery of myocardial function, neurological recovery, and 10-day survival following CA (n=13, P<0.05 vs normoxia, respectively). Conclusion: Brief mild hypoxia treatment following global ischemic injury paradoxically improved recovery of myocardial mitochondrial respiration and metabolism and was associated with improved physiological recovery and survival. Post-ischemia hypoxia is a potential therapeutic strategy for mitochondrial ischemic injury and requires further study. No financial disclosures. This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Myocardial infarction (MI) significantly compromises the integrity of the cardiac microvascular endothelial barrier, leading to enhanced leakage and inflammation that contribute to the progression of heart failure. While HIF2α is highly expressed in cardiac endothelial cells (ECs) under hypoxic conditions, its role in regulating microvascular endothelial barrier function during MI is not well understood. In this study, we utilized mice with a cardiac-specific deletion of HIF2α, generated through an inducible Cre (Cdh5Cre-ERT2) recombinase system. These mice exhibited no apparent phenotype under normal conditions. However, following left anterior descending (LAD) artery ligation-induced MI, they showed increased mortality associated with enhanced cardiac vascular leakage, inflammation, worsened cardiac function, and exacerbated heart remodeling. These outcomes suggest a protective role for endothelial HIF2α in response to cardiac ischemia. Parallel investigations in human cardiac microvascular endothelial cells (CMVECs) revealed that loss of ecHif2α led to diminished endothelial barrier function, characterized by reduced tight-junction protein levels and increased cell death, along with elevated expression of IL6 and other inflammatory markers. These effects were substantially reversed by over expressing ARNT, a critical dimerization partner for HIF2α during hypoxia. Additionally, ARNT deletion also led to increased CMVEC permeability. Interestingly, ARNT, rather than HIF2α itself, directly binds to the IL6 promoter to suppress IL6 expression. Our findings demonstrate the critical role of endothelial HIF2α in response to MI and identify the HIF2α/ARNT axis as a transcriptional repressor, offering novel insights for developing therapeutic strategies against heart failure following MI.

The Society for Academic Emergency Medicine (SAEM) and American College of Emergency Physicians (ACEP) are responding jointly to this request for information. SAEM and ACEP promote innovative and imaginative strategy development that will transform prehospital and emergent patient care. The National Institutes of Health (NIH) proposal to modify the current grant scoring system to prioritize these principles is a welcome change and we are supportive of these efforts. Focusing on research importance, feasibility, and scientific rigor will make it easier to interpret reviewer critiques and provide investigators with clearer information on score driving factors related to the science itself. While investigator and environment are important factors for successfully conducting research, reducing their overall importance may help with reducing bias and expanding the pool of meritorious applications. We also support combining significance and innovation into a single “importance” score, as the current format gives equal weight to innovation even though related sections tend to be brief. Furthermore, the existing significance section focuses largely on perceived need and scientific rationale with lesser consideration of the true magnitude of impact. Our societies are hopeful that by broadening the concept of overall importance, this new section can better weight funding decisions toward injuries and illnesses where improved outcomes would benefit the greatest number of people. We also agree that the current scoring system overly emphasizes an investigator's prior grant success and institutional reputation in determining an application's merit. Reliance on these factors detracts from the proposal's true “importance” and may perpetuate existing and ongoing challenges with the development and retention of junior investigators, including investigators from historically underrepresented groups. The proposed changes in this area will allow for increased emphasis on the development of highly novel or paradigm-shifting approaches to problems. Nonetheless, consideration of the capacity to execute the proposed work remains a critically important factor. The proposed method of scoring using a descriptive range of “fully capable” to “additional resources needed” deemphasizes quantitative scoring and we believe accomplishes the stated goals. However, we recommend specific clarification that the proposed scale reflects the entirety of the investigatory team (with due consideration of experience with prior collaboration). An important component of the current scoring system not addressed in the proposed changes is the handling of grant resubmissions. It is not clear why resubmissions have a written narrative that is score-driving and subject to bias. We recommend amending this process for each additional criteria to be marked yes or no as to “all major/moderate concerns appropriately addressed.” Reviewers could also have the option of describing “any new major/moderate concerns identified” in the resubmission process. Finally, while we recognize that these scoring revisions are important, we suggest that the NIH consider a more direct acknowledgment of our nation's need to fund a diverse pool of grant applicants. We believe the approach of the three-factor scoring system that emphasizes capability to perform the work is well aligned with this goal, but we would encourage the NIH to consider an explicit acknowledgment of ongoing disparities in our nation's health research programs addressed by these changes.

Introduction: Mitochondrial injury occurs following cardiac arrest (CA) and contributes to myocardial stunning and anoxic brain injury. Hypoxia has been proposed as a therapeutic strategy for neurodegenerative diseases associated with mitochondrial respiration defects, but its benefits in the post-CA setting remain unexplored. Here, we hypothesized mild hypoxia as therapy for post-CA mitochondrial injury. Methods and Results: Mice underwent brief asystolic CA (12 min) followed by cardiopulmonary resuscitation (CPR). One hour following successful CPR, mice were randomized to receive a brief episode (6 hours) of hyperoxia (33% O 2 ), normoxia (21% O 2 ), or hypoxia (10% O 2 ) in an environmental chamber. Post exposure, the mice were returned to room air (21% O 2 ). Hypoxia improved myocardial fractional shortening compared to normoxia and hyperoxia (Hypoxia: 45.6±2.3 %, Normoxia: 40.0±0.8 %, Hyperoxia: 29.0±4.0 %, n=5, P<0.05, respectively). Hypoxia also elevated neurological scores (Hypoxia: 8.4±1.3, Hyperoxia: 5.6±1.2, n=5, P<0.05) 48 hours post CA compared to hyperoxia. Ten-day survival was prolonged by hypoxia (Hypoxia: 87.5 %, Normoxia: 62.5 %, Hyperoxia: 12.5 %, n=10, P<0.05, respectively). Hypoxia also improved mitochondrial function (28% increase in basic oxygen consumption and 49% increase in ATP-related oxygen consumption, n=3, P<0.05, respectively) and a 20% decrease in ROS production (n=6, P<0.05) compared to normoxia. Conclusion: This study has two important findings: First , we demonstrate that oxygen availability in the early post-CA period is a critical determinant of long-term post-CA outcomes. Second , we show that mild hypoxic therapy in the early post-CA period improves early mitochondrial function and long-term post-CA outcomes. These findings reveal an important oxygen-dependent therapeutic window following resuscitation from CA that has implications for post-CA critical care.