***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 
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    ***EXAMPLES*** 
    Input: Closed-loop feedback computer-controlled vasopressor infusion has been previously described for maintaining blood pressure during spinal anaesthesia for caesarean section but there are limited data available comparing the relative performance of different vasopressors . The aim of this study was to compare the performance of norepinephrine versus phenylephrine in this system . Data from a r and omized , two-arm parallel group , double-blinded controlled trial were reanalyzed . 104 patients scheduled for elective caesarean section under spinal anaesthesia were r and omized to receive computer-controlled closed-loop infusion of either norepinephrine 5 µg ml−1 or phenylephrine 100 µg ml−1 . This was started immediately after induction of spinal anaesthesia and used an algorithm design ed to maintain systolic blood pressure near baseline until fetal delivery . Performance error calculations were used to compare the performance of the two vasopressors . The primary outcome was defined as the median absolute performance error . Median performance error , wobble and divergence were also compared . Median absolute performance error was smaller in the norepinephrine group ( median 3.79 [ interquartile range 2.82–5.17 ] % ) versus the phenylephrine group ( 4.70 [ 3.23–6.57 ] % , P = 0.028 ) . In addition , median performance error was smaller ( 0.75 [ −1.56–2.52 ] % ) versus 2.61 [ 0.83–4.57 ] % , P = 0.002 ) and wobble was smaller ( 2.85 [ 2.07–5.17 ] % ) versus 3.39 [ 2.62–4.90 ] % , P = 0.028 ) in the norepinephrine group versus the phenylephrine group . Divergence was similar between groups . The precision of the control of blood pressure was greater with norepinephrine compared with phenylephrine at the drug concentrations used BACKGROUND During spinal anesthesia for cesarean delivery  phenylephrine is the vasopressor of choice but can cause bradycardia . Norepinephrine has both β- and α-adrenergic activity suitable for maintaining blood pressure with less bradycardia . We hypothesized that norepinephrine would be superior to phenylephrine , requiring fewer rescue bolus interventions to maintain blood pressure . METHODS Eighty-five parturients having spinal anesthesia for elective cesarean delivery were r and omized to Group P ( phenylephrine 0.1μg/kg/min ) or Group N ( norepinephrine 0.05μg/kg/min ) fixed-rate  infusions  . Rescue bolus interventions of phenylephrine 100μg for hypotension , or ephedrine 5 mg for bradycardia with hypotension , were given as required to maintain systolic blood pressure . Maternal hemodynamic variables were measured non-invasively . RESULTS There was no difference between groups in the proportion of patients who required rescue vasopressor boluses ( Group P : 65.8 % [ n=25 ] vs. Group N : 48.8 % [ n=21 ] , P=0.12 ) . The proportion of patients who received ⩾1 bolus of phenylephrine was similar between groups ( Group P : 52.6 % [ n=20 ] vs. Group N : 46.5 % [ n=20 ] , P=0.58 ) . However , more patients received ⩾1 bolus of  ephedrine in the phenylephrine group ( Group P : 23.7 % [ n=9 ] vs. Group N : 2.3 % [ n=1 ] , P<0.01 ) . The incidence of emesis was greater in the  phenylephrine group ( Group P : 26.3 % vs. Group P : 16.3 % , P<0.001 ) . Hemodynamic parameters including heart rate , the incidence of bradycardia , blood pressure , cardiac output , cardiac index , stroke volume , and systemic vascular resistance and neonatal outcome were similar between groups ( all P<0.05 ) . CONCLUSION Norepinephrine fixed-rate infusion has efficacy for preventing hypotension and can be considered as an alternative to phenylephrine BACKGROUND : The use of phenylephrine as the first-line agent for prevention and treatment of  maternal hypotension during cesarean delivery ( CD ) may reduce cardiac output , posing a theoretical risk to mother and fetus . Norepinephrine has been suggested as a potential alternative , because its & bgr;-adrenergic effects might result in greater heart rate and cardiac output than phenylephrine . The use of norepinephrine to prevent and treat hypotension during CD is new , and its use as a bolus has not been fully determined in this context . The purpose of this study was to determine the effective norepinephrine dose , when given as intermittent intravenous ( IV ) boluses , to prevent postspinal hypotension in 90 % of women undergoing elective CD ( ED90 ) . METHODS : This was a prospect i ve , double-blind sequential allocation dose-finding study , using the biased coin up- and -down design . Forty-term pregnant women undergoing elective CD under spinal anesthesia received a set intermittent norepinephrine bolus of either 3 , 4 , 5 , 6 , 7 , or 8 µg every time their systolic blood pressure ( SBP ) fell to below 100 % of baseline . The primary outcome was the success of the norepinephrine regimen to maintain SBP at or above 80 % of baseline , from induction of spinal anesthesia to delivery of the fetus . Secondary outcomes included nausea , vomiting , hypertension ( SBP > 120 % of baseline ) , bradycardia ( < 50 bpm ) , upper sensory level of anesthesia to ice cold and umbilical artery and vein blood gases . The ED90 and 95 % confidence intervals ( CIs ) were estimated using both truncated Dixon and Mood and isotonic regression methods . RESULTS : The estimated ED90 of norepinephrine  was 5.49 µg ( 95 % CI , 5.15–5.83 ) using the truncated Dixon and Mood method and 5.80 µg ( 95 % CI , 5.01–6.59 ) using the isotonic regression method . CONCLUSIONS : The use of intermittent IV norepinephrine boluses to prevent spinal-induced hypotension in elective CD seems feasible and was not observed to be associated with adverse outcomes . Practically , we suggest an ED90 dose of 6 µg . Further work is warranted to eluci date the comparative effects of intermittent IV bolus doses of phenylephrine and norepinephrine , in terms of efficacy and safety Background The grading of recommendation , assessment , development and evaluation ( GRADE ) approach is widely implemented in health technology assessment and guideline development organisations throughout the world . GRADE provides a transparent approach to reaching judgements about the quality of evidence on the effects of a health care intervention , but is complex and therefore challenging to apply in a consistent manner . Methods We developed a checklist to guide the research er to extract the data required to make a GRADE assessment . We applied the checklist to 29 meta-analyses of r and omised controlled trials on the effectiveness of health care interventions . Two review ers used the checklist for each paper and used these data to rate the quality of evidence for a particular outcome . Results For most ( 70 % ) checklist items , there was good agreement between review ers . The main problems were for items relating to indirectness where considerable judgement is required . Conclusions There was consistent agreement between review ers on most items in the checklist . The use of this checklist may be an aid to improving the consistency and reproducibility of GRADE assessment s , particularly for inexperienced users or in rapid review s without the re sources to conduct assessment s by two research ers independently Background : During spinal anesthesia for cesarean delivery , phenylephrine can cause reflexive decreases in maternal heart rate and cardiac output . Norepinephrine has weak & bgr;-adrenergic receptor agonist activity in addition to potent & agr;-adrenergic receptor activity and therefore may be suitable for maintaining blood pressure with less negative effects on heart rate and cardiac output compared with phenylephrine . Methods : In a r and omized , double-blinded study , 104 healthy patients having cesarean delivery under spinal anesthesia were r and omized to have systolic blood pressure maintained with a computer-controlled infusion of norepinephrine 5 & mgr;g/ml or phenylephrine 100 & mgr;g/ml . The primary outcome compared was cardiac output . Blood pressure heart rate and neonatal outcome were also compared . Results : Normalized cardiac output 5 min after induction was greater in the norepinephrine group versus the phenylephrine group ( median 102.7 % [ interquartile range , 94.3 to 116.7 % ] versus 93.8 % [ 85.0 to 103.1 % ] , P = 0.004 , median difference 9.8 % , 95 % CI of difference between medians 2.8 to 16.1 % ) . From induction until uterine incision , for norepinephrine versus phenylephrine , systolic blood pressure and stroke volume were similar , heart rate and cardiac output were greater , systemic vascular resistance was lower , and the incidence of bradycardia was smaller . Neonatal outcome was similar between groups . Conclusions : When given by computer-controlled infusion during spinal anesthesia for cesarean delivery , norepinephrine was effective for maintaining blood pressure and was associated with greater heart rate and cardiac output compared with phenylephrine . Further work would be of interest to confirm the safety and efficacy of norepinephrine as a vasopressor in obstetric patients The aim of this paper is to evaluate the efficacy and safety of three different norepinephrine dosing regimens for preventing spinal hypotension in cesarean section . In this r and omized double-blinded controlled study , 120 parturients scheduled for elective section delivery under spinal anesthesia were assigned to 1 of 4 groups . In the control group , patients received saline infusion . In three norepinephrine groups , the infusion dosage regimens were 5 , 10 , and 15 μg/kg/h , respectively . Hypotension was treated with a rescue bolus of 10 μg norepinephrine . The study protocol was continued until the end of surgery . The primary outcome was the proportion of participants that underwent hypotension . The proportion of hypotension participants was significantly reduced in the norepinephrine groups ( 37.9 % , 20 % , and 25 % , respectively ) compared to that in the control group ( 86.7 % ) . However , the highest dose of norepinephrine ( 15 μg/kg/h ) result ed in more hypertension episodes . In addition , blood pressure was better maintained in the norepinephrine 5 μg/kg/h and 10 μg/kg/h groups than in the control group and 15 μg/kg/h group . No significant differences in other hemodynamic variables , adverse effects , maternal and neonatal blood gases , or Apgar scores were observed among the groups . In summary , for patients who undergo cesarean delivery under spinal anesthesia , infusion of 5–10 μg/kg/h norepinephrine was effective to reduce hypotension incidence without significant adverse effects on maternal and neonatal outcomes . Clinical Trial Registration Number is ChiCTR-INR-16009452 BACKGROUND Phenylephrine ( PE ) is currently the vasopressor of choice to prevent and treat spinal-induced hypotension at cesarean delivery ( CD ) . However , its use is often associated with reflex bradycardia . Norepinephrine ( NE ) has been put forward as an alternative vasopressor during CD due to its ability to treat hypotension while maintaining heart rate ( HR ) . Recent studies have focused on the role of NE used as an infusion with favorable results compared to PE . No studies have compared equipotent bolus doses of PE and NE at CD . We hypothesized that when used in equipotent doses as an intermittent bolus regimen to prevent and treat spinal-induced hypotension , NE would result in a reduction in the incidence of bradycardia compared to PE . METHODS This was a double-blind , r and omized clinical trial of women undergoing elective CD under spinal anesthesia . Women were r and omized to receive either PE 100 µg or NE 6 µg when the systolic blood pressure ( SBP ) was below baseline . In addition to the r and omized treatment , ephedrine was given intravenously to both groups if the SBP was below baseline and the HR < 60 bpm or if the SBP was < 80 % of baseline for 2 consecutive readings . The primary outcome was bradycardia ( HR < 50 bpm ) in the predelivery period . Secondary outcomes included hypotension ( SBP < 80 % of baseline ) , hypertension ( SBP > 120 % of baseline ) , tachycardia ( HR > 120 % of baseline ) , ≥2 episodes of bradycardia , nausea , vomiting , umbilical artery and vein blood gases , and Apgar scores . RESULTS One hundred twelve patients were r and omized . The incidence of bradycardia was lower in the NE group compared to the PE group ( 10.9 % vs 37.5 % ; P < .001 ; difference [ 95 % confidence interval { CI } ] , -26.8 % [ -41.8 % to - 
    Output:
    
However , evidence is insufficient to draw conclusions regarding the greater maternal CO and better BP control precision with the use of norepinephrine .
Conclusion : This systematic review and meta- analysis shows norepinephrine provides similar efficacy to manage maternal hypotension compared to phenylephrine ; additionally , showing advantage regarding certain side effects like bradycardia and IONV reduction .
Accordingly , norepinephrine is a promising alternative to phenylephrine .