***TASK*** the task is to summarize an input biomedical literature in six sentences 
    ***INPUT*** the input is a biomedical literature 
    ***OUTPUT*** the output is the summary of an input biomedical literature in six sentences 
    ***DOCUMENTATION*** 
    ***EXAMPLES*** 
    Input: Background — Oxygen is commonly administered to patients with ST-elevation – myocardial infa rct ion despite previous studies suggesting a possible increase in myocardial injury as a result of coronary vasoconstriction and heightened oxidative stress . Methods and Results — We conducted a multicenter , prospect i ve , r and omized , controlled trial comparing oxygen ( 8 L/min ) with no supplemental oxygen in patients with ST-elevation – myocardial infa rct ion diagnosed on paramedic 12-lead ECG . Of 638 patients r and omized , 441 patients had confirmed ST-elevation – myocardial infa rct ion and underwent primary end-point analysis . The primary end point was myocardial infa rct size as assessed by cardiac enzymes , troponin I , and creatine kinase . Secondary end points included recurrent myocardial infa rct ion , cardiac arrhythmia , and myocardial infa rct size assessed by cardiac magnetic resonance imaging at 6 months . Mean peak troponin was similar in the oxygen and no oxygen groups ( 57.4 versus 48.0 & mgr;g/L ; ratio , 1.20 ; 95 % confidence interval , 0.92–1.56 ; P=0.18 ) . There was a significant increase in mean peak creatine kinase in the oxygen group compared with the no oxygen group ( 1948 versus 1543 U/L ; means ratio , 1.27 ; 95 % confidence interval , 1.04–1.52 ; P=0.01 ) . There was an increase in the rate of recurrent myocardial infa rct ion in the oxygen group compared with the no oxygen group ( 5.5 % versus 0.9 % ; P=0.006 ) and an increase in frequency of cardiac arrhythmia ( 40.4 % versus 31.4 % ; P=0.05 ) . At 6 months , the oxygen group had an increase in myocardial infa rct size on cardiac magnetic resonance ( n=139 ; 20.3 versus 13.1 g ; P=0.04 ) . Conclusion — Supplemental oxygen therapy in patients with ST-elevation – myocardial infa rct ion but without hypoxia may increase early myocardial injury and was associated with larger myocardial infa rct size assessed at 6 months . Clinical Trial Registration — URL : http://www . clinical trials.gov . Unique identifier : NCT01272713 Pathological formation of reactive oxygen species within the coronary circulation has been hypothesized to mediate some clinical manifestations of ischemic heart disease ( IHD ) by interfering with physiological regulation of coronary tone . To determine the degree to which coronary tone responds to acute changes in ambient levels of oxidants and antioxidants in vivo in a clinical setting , we measured the effect of an acute oxidative stress ( breathing 100 % oxygen ) on coronary capacitance artery diameter ( quantitative angiography ) and blood flow velocity through the coronary microcirculation ( intracoronary Doppler ultrasonography ) before and after treatment with the antioxidant vitamin C ( 3-g intravenous infusion ) in 12 IHD patients undergoing a clinical coronary interventional procedure . Relative to room air breathing , 100 % oxygen breathing promptly reduced coronary blood flow velocity by 20 % and increased coronary resistance by 23 % , without significantly changing the diameter of capacitance arteries .  Vitamin C administration promptly restored coronary flow velocity and resistance to a slightly suprabasal level , and it prevented the reinduction of coronary constriction with rechallenge with 100 % oxygen . This suggests that acute oxidative stress produces prompt and substantial changes in coronary resistance and blood flow in a clinical setting in patients with IHD , and it suggests that these changes are mediated by vitamin C-quenchable substances acting on the coronary microcirculation . This observation may have relevance for clinical practice BACKGROUND AND PURPOSE We sought to test the hypothesis that breathing 100 % oxygen for the first 24 hours after an acute stroke would not reduce mortality , impairment , or disability . METHODS Subjects admitted to the Central Hospital of Akershus , Norway , with stroke onset < 24 hours before admittance were allocated to 2 groups by a quasi-r and omized design using birth numbers . All patients with acute stroke admitted to hospital within 24 hours after a stroke were included and enrolled . Patients were allocated to a group that received supplemental oxygen treatment ( 100 % atmospheres , 3 L/min ) for 24 hours  ( n=292 ) or to the control group , which did not receive additional oxygen . Main outcome measures were 1-year survival , neurological impairment ( Sc and inavian Stroke Scale ) , and disability ( Barthel Index ) 7 months after stroke . RESULTS One-year survival was 69 % in the oxygen group and 73 % in the control group ( OR 0.82 ; 95 % CI 0.57 to 1.19 ; P=0.30 ) . Impairment scores and disability scores were comparable 7 months after stroke . Among patients with Sc and inavian Stroke Scale ( SSS ) scores of > /=40 , 82 % in the oxygen group and 91 % in the control group survived ( OR 0 . 45 ; 95 % CI 0.23 to 0.90 ; P=0.023 ) . For patients with SSS scores of < 40 , 53 % in the oxygen group and 48 % in the control group survived ( OR 1.26 ; 95 % CI 0.76 to 2.09 ; P=0.54 ) . CONCLUSIONS Supplemental oxygen should not routinely be given to nonhypoxic stroke victims with minor or moderate strokes . Further research is needed to give conclusive advice concerning oxygen supplementation for patients with severe strokes Importance Despite suggestions of potential harm from unnecessary oxygen therapy , critically ill patients spend substantial periods in a hyperoxemic state . A strategy of controlled arterial oxygenation is thus rational but has not been vali date d in clinical practice . Objective To assess whether a conservative protocol for oxygen supplementation could improve outcomes in patients admitted to intensive care units ( ICUs ) . Design , Setting , and Patients Oxygen-ICU was a single-center , open-label , r and omized clinical trial conducted from March 2010 to October 2012 that included all adults admitted with an expected length of stay of 72 hours or longer to the medical-surgical ICU of Modena University Hospital , Italy . The originally planned sample size was 660 patients , but the study was stopped early due to difficulties in enrollment after inclusion of 480 patients . Interventions Patients were r and omly assigned to receive oxygen therapy to maintain Pao2 between 70 and 100 mm Hg or arterial oxyhemoglobin saturation ( Spo2 ) between 94 % and 98 % ( conservative group ) or , according to st and ard ICU practice , to allow Pao2 values up to 150 mm Hg or Spo2 values between 97 % and 100 % ( conventional control group ) . Main Outcomes and Measures The primary outcome was ICU mortality . Secondary outcomes included occurrence of new organ failure and infection 48 hours or more after ICU admission . Results A total of 434 patients ( median age , 64 years ; 188 [ 43.3 % ] women ) received conventional ( n = 218 ) or conservative ( n = 216 ) oxygen therapy and were included in the modified intent-to-treat analysis . Daily time-weighted Pao2 averages during the ICU stay were significantly higher ( P < .001 ) in the conventional group ( median Pao2 , 102 mm Hg [ interquartile range , 88 - 116 ] ) vs the conservative group ( median Pao2 , 87 mm Hg [ interquartile range , 79 - 97 ] ) . Twenty-five patients in the conservative oxygen therapy group ( 11.6 % ) and 44 in the conventional oxygen therapy group ( 20.2 % ) died during their ICU stay ( absolute risk reduction [ ARR ] , 0.086 [ 95 % CI , 0.017 - 0.150 ] ; relative risk [ RR ] , 0.57 [ 95 % CI , 0.37 - 0.90 ] ; P = .01 ) . Occurrences were lower in the conservative oxygen therapy group for new shock episode ( ARR , 0.068 [ 95 % CI , 0.020 - 0.120 ] ; RR , 0.35 [ 95 % CI , 0.16 - 0.75 ] ; P = .006 ) or liver failure ( ARR , 0.046 [ 95 % CI , 0.008 - 0.088 ] ; RR , 0.29 [ 95 % CI , 0.10 - 0.82 ] ; P = .02 ) and new bloodstream infection ( ARR , 0.05 [ 95 % CI , 0.00 - 0.09 ] ; RR , 0.50 [ 95 % CI , 0.25 - 0.998 ; P = .049 ) . Conclusions and Relevance Among critically ill patients with an ICU length of stay of 72 hours or longer , a conservative protocol for oxygen therapy vs conventional therapy result ed in lower ICU mortality . These preliminary findings were based on unplanned early termination of the trial , and a larger multicenter trial is needed to evaluate the potential benefit of this approach . Trial Registration clinical trials.gov Identifier : NCT01319643 Hyperoxia causes hemodynamic alterations . We hypothesized that cardiovascular and autonomic control changes last beyond the end of hyperoxic period into normoxia . Ten healthy volunteers were r and omized to breathe either medical air or 100 % oxygen for 45 min in a double-blind study design . Measurements were performed before ( baseline ) and during gas exposure , and then 10 , 30 , 60 , and 90 min after gas exposure . Hemodynamic changes were studied by Doppler echocardiography . Changes in cardiac and vasomotor autonomic control were evaluated through changes in spectral power of heart rate variability and blood pressure variability . Cardiac baroreflex sensitivity was assessed by the sequence method . Hyperoxia significantly decreased heart rate and increased the high frequency power of heart rate variability , suggesting a chemoreflex increase in vagal activity since the slope of cardiac baroreflex was significantly decreased during hyperoxia . Hyperoxia increased significantly the systemic vascular resistances and decreased the low frequency power of blood pressure variability , suggesting that hyperoxic vasoconstriction was not supported by an increase in vascular sympathetic stimulation . These changes lasted for 10 min after hyperoxia ( p < 0.05 ) . After the end of hyperoxic exposure , the shift of the power spectral distribution of heart rate variability toward a pattern of increased cardiac sympathetic activity lasted for 30 min ( p < 0.05 ) , reflecting a resuming of baseline autonomic balance . Cardiac output and stroke volume were significantly decreased during hyperoxia and returned to baseline values ( 10 min ) later than heart rate . In conclusion , hyperoxia effects continue during return to normoxic breathing , but cardiac and vascular parameters followed different time courses of recovery The aim of this study was to assess whether arterial chemoreceptors are activated in chronic heart failure and to appreciate whether this activation is related to the degree of cardiac dysfunction . We examined whether administration of pure oxygen for 30 min to 35 patients with chronic heart failure was followed by hemodynamic modifications that would suggest a negative feedback ( inactivation ) of such chemoregulation . A decrease in heart rate and cardiac output was observed in all patients . The decrease in cardiac output and stroke volume was inversely correlated to the left ventricular ejection fraction . We observed that the lower the left ventricular ejection fraction , the smaller the decrease in heart rate and the greater the increase in systemic vascular resistance . The variation of stroke volume was related to the variation of systemic vascular resistance . In 12 patients , keeping heart rate constant during oxygen inhalation by adequate right atrial pacing prevented the decrease of cardiac output . These results suggest that chemoregulation remains generally effective in chronic heart failure : increase in Pao2 result ed in a reduction of cardiac output mainly related to the decrease of heart rate . However , when the left ventricular function is very poor , the central chemomediated regulation is impaired , and the direct peripheral effect of oxygen on systemic circulation appears to play a leading part Purpose : Oxygen-enhanced magnetic resonance (MR)-ventilation imaging of the lung is based on the inhalation of a high concentration of oxygen ( hyperoxia ) . However , the effect of hyperoxia on the pulmonary circulation is not yet fully understood . In this study the impact of hyperoxia on the pulmonary circulation was evaluated . Material s and Methods : Ten healthy volunteers were examined in a 1.5 T MRI system with contrast-enhanced perfusion MRI ( saturation recovery 2D turboFLASH ) of the lung and phase-contrast flow measurements in the pulmonary trunk . Both measurements were performed breathing room air ( RA ) and , subsequently , 100 % oxygen ( 15 L/min ) ( O2 ) . Results : The perfusion measurements showed a significant difference between RA and O2 for the pulmonary blood flow ( 181 vs. 257 mL/min/100 mL , P = 0.04 ) and blood volume ( 14 vs. 21 mL/100 mL , P = 0.008 ) . The mean 
    Output:
    
Oxygen delivery was not altered by hyperoxia .
Conclusions Hyperoxia may considerably decrease cardiac output and increase systemic vascular resistance , but effects differ between patient categories .
Heart failure patients were the most sensitive while no hemodynamic effects were seen in septic patients .
There is currently no evidence supporting the notion that oxygen supplementation increases oxygen delivery