although surgeon has a preference for patient positioning , the sitting position has several advantages including the ease of setup , excellent intra - articular visualisation for all types of arthroscopic shoulder procedures , less intra - operative blood loss , a lower incidence of traction neuropathy , and ease of conversion to the open approach if needed .
surgeons who prefer the sitting position cite the ability to use general or interscalene brachial plexus block ( isbpb ) as an advantage .
isbpb is possible for patients in the sitting position ; however , it is poorly tolerated in patients in the lateral decubitus position .
interscalene brachial plexus block provides effective anaesthesia for most types of shoulder surgeries , including arthroplasty and fracture fixation .
when administered by an anaesthesiologist committed to and skilled in the technique , the block has an excellent rate of success and is associated with a relatively low complication rate .
interscalene brachial plexus block has several advantages for patients undergoing shoulder surgery : excellent anaesthesia , reduction in both intra - operative and post - operative doses of opiates , delay in the onset of post - operative pain , a shortened post - anaesthesia stay , and increased patient satisfaction .
one of the major disadvantages of the sitting position is cardiovascular instability during the shoulder procedure .
sudden , profound hypotension and bradycardia events ( hbes ) have been reported in more than 20% of patients undergoing shoulder arthroscopy in the sitting position .
these hbes may be a form of vasovagal syncope mediated by the bezold  jarisch reflex ( bjr ) , which happens when venous pooling and increased sympathetic tone cause a low volume , hyper contractile ventricle .
this leads to sudden activation of the parasympathetic nervous system and sympathetic withdrawal , causing bradycardia and hypotension .
animal studies reported that serotonin ( 5-hydroxy tryptamine 3 [ 5-ht3 ] ) , may be an important contributing factor to the occurrence of bjr in the settings of decreased blood volume , which can be blocked by antagonising the serotonin at the level of the receptors .
one previous study also reported that intravenous ( iv ) ondansetron attenuated the spinal induced hypotension that might be caused by bjr .
the present work is a prospective , randomised study , which attempts to assess if blocking of the serotonin receptors by ondansetron ( a selective 5-ht3 receptor antagonist ) can help in reducing the hbes associated with shoulder arthroscopy done in the sitting position under isbpb .
after obtaining institutional review board approval and informed , written consent , 100 american society of anesthesiologists ( asa ) physical status i or ii , age of 2050 years patients who were undergoing shoulder arthroscopy in the sitting position under isbpb were studied .
sample size calculation was done based on a previous study in which the incidence of hbes in control group and study group were 24% and 6% , respectively .
it was estimated that a minimum of 50 patients in each group would be required to have a 90% power of detecting a 50% difference in the hbes with 95% confidence interval .
patients with a history of coronary artery disease , cardiac conduction defects , uncontrolled diabetes and hypertension , asthma , and those receiving clonidine , beta - blocker or calcium channel - blocker therapy were excluded from the study .
patients underwent surgery for a variety of indications including rotator cuff repair , acromioplasty , and labral repair .
one hundred patients were randomly assigned to one of the two equal groups to receive either of the following : group c : 10 ml normal saline .
all patients received test drug iv over a period of 5 min before starting isbpb .
a total volume of 35 ml of local anaesthetic was selected to provide the highest rate of success of interscalene block administered using the nerve stimulation technique . after establishing iv access , non - invasive monitoring of blood pressure ( bp ) , oxygen saturation , and electrocardiogram ( ecg )
sedation was provided by iv administration of midazolam 1 mg and fentanyl 30 g before the block .
after the proper location of the nerve , the local anaesthetic solution was injected in incremental 5 ml boluses with intermittent aspiration .
all isbpb were performed by one anaesthesiologist , and all surgeries were performed by the same group of surgeons .
after the isbpb , non - invasive bp was measured every 5 min with more frequent measurements if needed , and continuous ecg monitoring .
beach chair position , which was achieved by elevating the back of the operating table to 6080 and flexing both the knees and hips to 90 with the patients feet resting properly on a footboard .
sensory blockade was evaluated every 5 min by pinprick test and the motor block was tested by asking the patient to raise his arm , to abduct / adduct the thumb , and to flex the forearm on the arm .
intra - operatively , midazolam boluses of 0.02 mg / kg and fentanyl 1 g / kg boluses were given if the patient complained of pain .
the amount of midazolam and fentanyl administered and the degree of sedation were recorded ( sedation score : 1 = awake , 2 = awake but sedated , 3 = asleep but responsive to verbal stimuli , and 4 = asleep but responsive to tactile stimuli ) .
a hbe was defined as ; heart rate decrease of more than 30 bpm in < 5 min or any decrease < 50 bpm , and/or a systolic bp decrease more than 30 mmhg in < 5 min or any decrease to < 90 mmhg .
a hbe was managed by iv injection of atropine ( 0.6 mg boluses ) or ephedrine ( 6 mg boluses ) .
each patient was observed for side effects such as horner 's syndrome ( hs ) , ipsilateral diaphragmatic paralysis , hoarseness of voice , difficulty in swallowing and complications such as pneumothorax , vascular injury ( external jugular vein puncture , haematoma formation , intra - arterial injection ) , epidural or spinal injection , neuropathy / nerve injury .
after the recovery , the pain was tested and recorded by using the verbal numerical rating scale ( vnrs : 0 = no pain , 10 = most severe pain imaginable ) ; starting immediately on arrival to the recovery room as 0 h and 1 , 3 , 6 , and 12 h later .
when the vnrs was more than 3 , iv pethidine 0.5 mg / kg was injected and recorded .
any complications as neurological disabilities , nausea , vomiting or respiratory difficulties were also recorded .
data were checked , entered , and analysed by using spss for windows version 19 ( armonk , ny : ibm corp ) .
data are expressed as mean  standard deviation for quantitative variables , number , and percentage for categorical variables .
t test and chi - square (  ) test used for comparison in between groups .
two patients were excluded from the study due to block failure ; one of them was randomised to group c and the second one was randomised to group t. the demographic data and surgical characteristics were comparable in both groups [ tables 1 and 2 ] .
the mean age in group c ( 33.88  9.39 years ) was very much comparable to the mean age in group t ( 34.28  8.8 years ) ( p > 0.05 ) .
side of surgery in both the groups was comparable and non - significant for statistical analysis .
to summate , all the demographic characteristics such as age , weight , asa grade , and side of the shoulder operated were comparable in both the groups and were found to be statistically non - significant ( p > 0.05 ) [ table 1 ] .
there was no difference in sedation score , nil per oral status and iv fluids in both the groups ; p > 0.05 , and statistically not significant [ table 2 ] .
characteristics of groups there were no signs and symptoms suggestive of spinal , epidural or stellate ganglion blockade or intra - vascular injection in two groups .
hypotension and bradycardia events occurred in three patients in group t ( 6.1% ) , and 11 patients in group c ( 22.44% ) which was significantly higher than in group c ; p - 0.030 [ table 3 ] .
the onset of these hbes was comparable in the two groups [ table 3 ] .
there was no significant difference in the block onset time in the two groups ; ( 21.82  3.6 min ) , and ( 22.1  2.5 min ) in groups c and t , respectively ; p - 0.186 [ table 4 ] .
the duration of analgesia was significantly longer in group c ( 8.1  3.3 h ) in comparison with group t ( 6.3  4.2 h ) ; p - 0.035 [ table 4 ] . details of hypotension bradycardia events nine patients in group c received ephedrine to treat hypotension , which was significantly higher compared to the other group ( three patients in group t ) [ table 5 ] .
one patient in group t and five patients in group c needed atropine to treat bradycardia [ table 5 ] .
there was no significant difference in the use of intra - operative midazolam or fentanyl [ table 5 ] .
the study was conducted with the aim to test the hypothesis that iv ondansetron would reduce the incidence of hbes in arthroscopic shoulder surgery in the sitting position after isbpb .
activation of the bjr is considered one of the proposed mechanisms for the occurrence of hbes .
the bjr is an inhibitory reflex that originates in the cardiac sensory receptors with vagal afferents , which are affected by chemical and mechanical stimuli .
this reaction may be related to venous pooling ( caused by the sitting position and epinephrine - induced beta 2-adrenergic effect ) and increased inotropy ( alfa - adrenergic effect of epinephrine ) .
increase in epinephrine levels may result from decreased venous return and carotid baroreceptor stimulation , as well as exogenously from epinephrine administered with the local anaesthetic or in the irrigating solution .
a low - volume hypercontractile ventricle causes stimulation of intra - myocardial mechanoreceptors ( c fibres ) , which is followed , in susceptible patients , by an abrupt withdrawal of sympathetic outflow and an increase in vagal tone with resultant bradycardia and hypotension were observed in this setting .
hypovolaemia not only causes stimulation of cardiac mechanoreceptors in the left ventricle that triggers the bjr and causes reflex bradycardia , vasodilatation and hypotension , but also results in the activation of thrombocytes to release serotonin which triggers chemoreceptors in the wall of the heart .
furthermore , stimulation of 5-ht3 receptors , which are g protein - coupled , ligand - gated , fast - ion channels , increases the activity of the vagal nerve .
one previous study in anaesthetised rats reported that bjr induced by serotonin participates in systemic hypotension and bradycardia .
this means that bjr can also be directly triggered by the activation of the serotonin 5-ht3 receptors .
based on these studies , we assumed that preventing the effect of serotonin by blocking 5-ht3 receptors by iv ondansetron can help in reducing the incidence of the hbes associated with shoulder arthroscopy done in the sitting position under isbpb .
our results demonstrated that iv injection of ondansetron 4 mg significantly reduced the incidence of hbes in our patients from 22.44% in the saline group ( group c ) to 6.1% after injection of 4 mg ondansetron ( group t ) .
this 22.44% incidence of hbes in the saline group is consistent with the previous reports of such events .
the reduction in hbes in our study after iv ondansetron is similar to a previous study done in rabbits , in which iv administration of granisetron 50 g / kg was efficacious in reducing bradycardia and hypotension associated with bjr . in one more study
, it was found that granisetron injection reduced the heart rate fluctuation and hypotension during head - up tilt table tests that are likely related to bjr . and
this is consistent with matrinek who concluded that injection of 4 mg ondansetron iv with atropine 0.6 mg could revert asystole during spinal anaesthesia and also with owczuk et al . who found that 8 mg ondansetron reduced the incidence of bradycardia and hypotension after spinal anaesthesia .
the combination of the sitting position , regional anaesthetic technique , an awake patient , and surgical procedure may precipitate vasovagal episodes , which if anticipated are of minor significance , but which in the extreme may lead to cardiac arrest .
iv uptake of local anaesthetic can be considered , but time from block placement to the onset of symptoms was 42.5  5.5 min and 44.1  2.7 min in group c and in group t , respectively .
this is significantly longer than that which would be expected for peak local anaesthetic uptake from an interscalene brachial ( isb ) which is approximately 30 min for bupivacaine and < 20 min for lidocaine with or without epinephrine .
roth et al . , using mepivacaine 10 mg / kg and tetzlaff et al . , using the same dose of alkalinised mepivacaine for isb reported no episodes of toxicity .
the central nervous system symptoms of lightheadedness and nausea were unlike those of local anaesthetic toxicity and promptly resolved after the administration of atropine , glycopyrrolate or ephedrine .
extensive spread of local anaesthetic to the epidural or subarachnoid space may cause cardiovascular and respiratory compromise but is rare .
previous studies reported interval between positioning and haemodynamic changes ranging between 12 and 24 min but may be delayed to 1 h. our patients had similar sequence of events to those undergoing tilt - table testing and the onset of symptoms was ( 13.7  2.1 min ) , ( 13.3  3.6 min ) , in groups c and t , respectively .
although the onset of analgesia was similar in all groups , the duration of analgesia was shorter in group t in comparison with group c. this can be explained by antagonising the effect of serotonin in controlling pain .
although potential mechanisms of this observation were not clear , but animal studies have clarified the anti - nociceptive mechanisms of the descending serotoninergic system at the level of the spinal cord .
it hyperpolarises the membrane of substantia gelatinosa neurons , inhibits the excitatory transmitter glutamate release of a - delta and c afferent fibres pre - synaptically and increases the inhibitory transmitters release including aminobutyric acid and glycine from the interneurons .
the shortened duration of analgesia observed in our study is consistent with fassoulaki et al . who reported that iv ondansetron leads to a faster regression of the sensory block after spinal anaesthesia with lidocaine .
furthermore , continuous iv administration of ondansetron decreased the analgesic potency of tramadol infusion for post - operative pain .
the combination of sitting position , isbpb , and surgical procedure may result in hbes , usually of minor significance if anticipated . in extreme cases
we conclude that iv ondansetron 4 mg can reduce the hbes during shoulder arthroscopy in the sitting position under isbpb .
 background and aims : sudden , profound hypotension and bradycardia events ( hbes ) have been reported in more than 20% of patients undergoing shoulder arthroscopy in the sitting position . 
 the present study was designed to know whether intravenous ( iv ) ondansetron ( selective 5-hydroxy tryptamine 3-antagonist ) can help in reducing the hbes associated with shoulder arthroscopy performed in sitting position under interscalene brachial plexus block ( isbpb).methods : a total of 100 patients ( age 2050 years ) undergoing shoulder arthroscopy performed in the sitting position under isbpb were assigned randomly to one of the two groups : group c received 10 ml of normal saline and group t received 4 mg of ondansetron diluted in 10 ml of normal saline ` iv . 
 all patients received isbpb using levobupivacaine 0.5% . 
 assessment of motor and sensory blockade , pulse rate , systolic blood pressure , respiration , and side effects were noted every 5 min for first 30 min and every 10 min till the end of surgery . 
 hbes were recorded in both groups.results:iv injection of ondansetron significantly reduces the incidence of hbes from 11 ( 22.44% in group c ) to 3 ( 6.1% in group t ) . 
 the duration of analgesia was significantly longer in group c ( 8.1  3.3 ) in comparison with group t ( 6.3  4.2 h).conclusion : we conclude that 4 ml of iv ondansetron can significantly reduce the hbes during shoulder arthroscopy done in the sitting position under isbpb . 
INTRODUCTION
METHODS
RESULTS
DISCUSSION
CONCLUSION