controlled hypotension has an established role in reducing intra - articular bleed and thereby improving visualisation during shoulder arthroscopy .
both these agents reduce mean arterial blood pressures ( mbp ) to an equivalent magnitude .
the haemodynamic parameter which is directly linked to pump pressures , intra - articular bleed and clarity of vision is the systolic blood pressure ( sbp ) .
an agent which achieves low sbp with concomitant preservation of autoregulation of vital organ blood flow would be the anaesthetic of choice . while hunt continues for the ideal anaesthetic maintenance agent for shoulder surgery , medline search ( ncbi database literature ; key words : volatile anaesthetic , isoflurane , sevoflurane ,
shoulder arthroscopy , controlled hypotension ) revealed no study on shoulder arthroscopic surgeries till date comparing isoflurane and sevoflurane inhalational techniques employed with concomitant interscalene brachial plexus block .
this study aims to compare the capability ( to achieve and maintain the desired haemodynamic status , target , tsbp = 90 mmhg , sbp ) and convenience ( of manipulations performed by the anaesthesiologist for maintaining such a status ) of these two inhalational agents in this subset of patients .
considering the comparability of isoflurane at 1.21.5 mac to sevoflurane at equal mac , we investigated the hypothesis that both agents are similar to each other in their capability to achieve the desired haemodynamic status .
we used the  process capability indices ( pcis ) as an additional statistical tool in analysing our data , which we believe lends to more accurate conclusions .
fifty - nine ( among 64 ) randomly assigned ( random number generator , random # generator , jess tucker , version 1.1.3 ,  2013 webberface , llc , ios application , category : utilities , info@webberface.com ) patients belonging to the american society of anesthesiologists ( asa ) status 1 and 2 , who underwent shoulder arthroscopic surgery over a 23-month period ( january 2014 to november 2015 ) were included in the study .
since an effective preliminary interscalene block formed an essential part of the anaesthetic procedure , patients , in whom the regional block was not performed ( n = 4 ; 1 in isoflurane and 3 in sevoflurane group ) or less than optimally effective before inhalational anaesthesia was instituted , were excluded from the study .
likewise , patients with severe pain at the operative site immediately after termination of inhalational anaesthesia indicating ineffectiveness of the block were also excluded from the study ( n = 1 , sevoflurane , [ consort chart 1 ] ) .
all patients were anaesthetised by the same anaesthesiologist , the first author and operated by the same surgeon .
after preoperative assessment and recording of baseline vitals , these hospital inpatients were premedicated with tablet ranitidine 150 mg , having fasted overnight before the surgical procedure .
no sedatives or opioids were used for premedication . the distribution of patients in both groups .
isb = interscalene block in the operating room , patients were administered intravenous ( i.v . )
ultrasound - guided ( sonosite , high frequency , linear probe , 13 - 6 mhz ) interscalene , in - plane block was performed in the supine position with a mixture of 6 ml lignocaine 2% and 25 ml levobupivacaine 0.25% .
the effectiveness of the block was confirmed by the abolition of sensations ( pinprick ) over c4c7 dermatomes and/or free and painless ( passive ) abduction in patients with painful shoulders .
injection vecuronium or rocuronium in 2 ed95 doses and ventilation was instituted to achieve normocarbia .
three - lead electrocardiogram , spo2 , non - invasive blood pressure ( nibp ) , end - tidal carbon dioxide and inhalational agent monitoring were done during the entire procedure .
nibp recording was done in 3 min intervals in the non - operative upper arm . with the patient in the lateral decubitus position
, datumvaporizer , meditec england , queenborough , kent meii , 5el ) or isoflurane ( abbott loan vaporizer , abbott australasia pty .
ltd , 32 - 34 lord street , botany nsw 2019 , australia ) at 1.21.5 mac dosages .
age - related iso - mac inhalational concentrations ( chronomac , application timeline , version 1.0 : ios application , 2011 ) were used to achieve the desired end - tidal concentrations ( 0.3% , 1.0% - minimum and 2% , 3.0% - maximum for isoflurane and sevoflurane , respectively ) for maintenance of anaesthesia .
muscle paralysis was achieved with bolus doses of injection vecuronium or rocuronium to obtain train - of - four counts 12 and controlled ventilation was carried out .
sbp , diastolic blood pressure ( dbp ) , mbp and heart rate were recorded every 3 min .
the study period was considered from the time of insertion of the arthroscope to its removal . before inserting the arthroscope and during the maintenance of anaesthesia
, efforts were made to attain the target ( t ) sbp of 90 mmhg using the following methods : method a- increasing the depth of anaesthesia by administering additional doses of fentanyl ( 1 g / kg ) with propofol ( 1 mg / kg ) and method b- pharmacological intervention using a /-blocker ( labetalol , minimum 20 mg and maximum 40 mg ) .
method b was followed only after failure of method a. any adverse events such as persistent hypotension ( 3 consecutive readings of sbps lower than target or mbp < 60 mmhg ) or severe bradycardia ( heart rate < 40 beats / min ) were treated ( saline bolus , i.v .
ephedrine and i.v . atropine ) . the arthroscopic pump ( continuous wave iii arthroscopy pump , arthrex , arthrex med .
gmbh , germany ) pressure was 3040 mmhg and irrigant flow at 40%50% were maintained throughout the procedure for adequate visualisation .
necessary efforts were made to avoid hypothermia , urinary bladder distension and position - related complications .
the data was checked for its distribution by anderson  darling ( ad ) test .
statistical analysis was performed using minitab 17.1.0 ,  2013 minitab inc . statistical software .
a minimum of 16 patients was required in each group to detect a mean blood pressure ( bp ) difference of 10 mmhg , and these differences and sample sizes were based on earlier similar studies on arthroscopy haemodynamics ( power 80% ,  = 0.05 ,  = 0.20 , with standard deviation [ sd ] of 10mmhg ) in each group
the study included a higher number of subjects than necessary owing to the methodology of evaluation employed ( pci analysis ) .
data was categorised and analysed by two methods [ flow chart 1 ] . describing the data evaluation methods , merits and demerits .
please see text first , as mean with sd and 95% confidence intervals for vital recordings were derived .
mean ( sd ) of mean of bps ( sbp , mbp , dbp ) and heart rate for both groups , mean ( sd ) of each patient 's maximum ( max ) and minimum ( min ) readings for both groups were noted separately .
each patient 's recordings were analysed ( stacked data ) for obtaining assembly fit data and separately for means ( with sds ) .
assembly fit data of sbp were re - derived for pcis after box - cox data transformation with lambda ,  = 0.5 as ad test showed its distribution as non - normal . setting the desired systolic pressure target ( t ) during the procedure ( tsbp = 90 mmhg , sbp ) for each patient
, we derived pcis for the desired range of maintained sbp i.e. , between 100 and 80 mmhg .
these formed the upper specification limit and lower specification limit ( usl and lsl ) of sbp(100,80 ) .
further , the capability of the inhalational agents was analysed using a wider range of sbps simply by extension of the upper and lower limits to 105 and 75 mmhg , respectively ( extended sls [ esls , sbp(105,75 ) ] ) .
similar to sbps , pcis for dbps ( tdbp = 60 mmhg ) and mbps ( tmbp = 70 mmhg ) too were derived for sls and for esls , separately ( sls for dbp(70,50 ) , mbp(80,60 ) and the esls for dbp(75,45 ) and mbp(85,55 ) ) . for these co - variables ,
hypothetical targets were considered for pci analysis . to determine the ability of the inhalational agent to maintain the blood pressure with respect to a predefined range ( sl or esl ) ,
cpm was useful in evaluating the convenience of the inhalational agent in achieving target blood pressure .
to analyse whether the fluctuations deviated to higher or lower side of expected limits , the pci , cpk was considered .
these three pcis were derived for assembly fit and for means data in each group . corresponding 
overall process indices , pp and ppk values too were derived for  group performance analysis .
the probability plots of blood pressures were analysed for 80 percentile values and distribution in each group .
the demographic characteristics , duration of surgery and the variety of surgical procedures are detailed in table 1 and consort chart 1 .
characteristics of patients , surgical procedures and haemodynamic parameter comparisons between the groups the primary targeted independent variable was the sbp , and co - variables were dbp and mbp . significantly lower mean of mean sbp and dbp was recorded for isoflurane ( independent sample t - test , p = 0.001 , 0.032 , respectively ) , but not for mbp ( p > 0.05 ) .
the mean of mean heart rates achieved was significantly higher in the isoflurane group ( p = 0.019 [ table 1 ] ) .
the comprehensive list of maximum and minimum blood pressure and heart rate averages is included in table 1 .
the derived pcis show higher values of cp , cpk and cpm for all blood pressures ( sbp , dbp and mbp ) at predefined usls and lsls for isoflurane patients compared to sevoflurane [ table 2 and figures 1 , 2 ] suggesting superior capability of maintaining blood pressures and in achieving target defined
negative values for cpk derived for all bp recordings [ tables 2 and 3 ] suggesting that the achieved bp parameters crossed the usls more frequently in this subset .
process capability report for systolic blood pressures of both groups , for overall and within processes and for two different specification limits ( specification limits and extended specification limits ) the process capability reports of systolic blood pressures for assembly fit data .
the graphs represent both for original ( non - transformed ) data and after box - cox transformation .
the right corner minor images represent original data before transformation the process capability report of systolic blood pressures for means data .
the lsl and usl lines represent sls process capability report for diastolic blood pressures and mean arterial blood pressures of both groups and for two different specification limits ( specification limit and extended specification limits ) probability plots distribution analysis revealed a higher percentage of patients outside the sls and esls for all blood pressure readings ( sbp , dbp and mbp ) for sevoflurane vis - a - vis isoflurane and higher 80 percentile values of blood pressures for sevoflurane [ tables 2 and 3 ] . a comparable number of patients in both groups required either anaesthetic intervention , pharmacological manipulation or both to achieve the desired blood pressures [ table 1 ] .
while comparison of the efficacy of isoflurane vis - a - vis sevoflurane in hypotensive anaesthesia for shoulder arthroscopy formed the primary objective of our study , a new analytical approach , process capability indices ( pcis ) , different from conventional statistical methodologies deriving significance from p values was additionally deployed to derive more practical and meaningful conclusions .
for various reasons ( refer flow chart ) , we felt that comparisons using conventional p values could highlight some but not all significant variations , and this could ultimately result in a divergence between 
for example , when the means ( and sds ) of the sbps in two groups ( isoflurane and sevoflurane ) were compared , lower mean sbp and minimum sbps were achieved with isoflurane indicating its enhanced hypotensive action .
surprisingly , the mean mbp , dbpmax , sbpmax and mbpmax , min were not significantly different between the groups . searching for a more inclusive analytical technique to overcome the possible inaccuracies of these comparisons , we found computing of pcis very useful and advantageous .
as an indicator of how closely a process is able to match the output to its overall specifications and to predefined targets , it is widely employed as a statistical tool to assess production , quality and process improvement efforts in engineering design .
pcis measure patient volumes against predetermined parameter limits rather than vice - versa in conventional statistical analysis .
uniquely , this methodology , therefore , takes into account a multitude of influencing factors , both known and unknown , to provide us a  capability measure . in short
a high value of cp ( close to 1 ) was observed in the isoflurane group considering sbp within 80 and 100 mmhg .
this translated into 87% of patients in the isoflurane group maintaining sbps between predefined limits vis - a - vis 65% of sevoflurane suggest superior 
the cp and cpk for blood pressures moved in consonance for isoflurane while following an off - process - centralisation with high variability for sevoflurane .
we observed isoflurane group patients had higher cpk values of 0.6 ( for the range of 80100 mmhg , sevoflurane 0.03 ) along with a greater percentage of negative cpk value in the sevoflurane group of all blood pressure data .
both these findings confirm that the process mean has fallen out of usls ( blood pressure readings over 100 mmhg systolic , 3% isoflurane patients vs. 32% sevoflurane ) thereby lending support to the superiority of isoflurane .
higher cpm for isoflurane suggests that irrespective of the predefined range considered , the ease of achieving target blood pressure ( 90 mmhg sbp ) was higher with this agent .
higher heart rates were recorded in the isoflurane group , but pcis were not computed for this non - targeted parameter . however , but for the additional pharmacological interventions necessitated by isoflurane , heart rates recorded would possibly have been higher .
cardiovascular effects specific to isoflurane have been detailed in several studies on animals , human volunteers , asa1 patients and in patients with coronary artery disease .
the circulatory effects of isoflurane are distinctly self - specific while sevoflurane resembles both isoflurane and halothane . a study of isoflurane and sevoflurane in healthy volunteers by philip malan and colleagues demonstrated that dose - dependent decrease of mbp could be blunted by concomitant use of n2o .
our study groups were comparable to these patients since n2o was used in all and the complete interscalene block created a pain - free status before inhalational agents were administered .
however , if not for n2o usage , the hypotensive effect of isoflurane demonstrated in our patients could have been more profound . in horses , isoflurane - induced decrease in mean , systolic and diastolic bps has been shown to be maximum at 80100 min of anaesthesia .
a dose - dependent hypotensive effect for both the inhalational agents at 12 mac has been established ; each , however , having a different attributed mechanism of action .
the variance in the magnitude of hypotension ( isoflurane versus sevoflurane ) is difficult to explain .
multiple animal as well as human volunteer studies have shown the myocardial depressant effects of regional anaesthetic drugs .
isoflurane acts synergistically with bupivacaine to decrease myocardial contractility in rats clearly demonstrating the additive myocardial suppressive activity induced by regional anaesthetic used concomitantly with general anaesthesia .
bupivacaine at a plasma concentration of 3  10 mol / l , a level normally achieved during regional anaesthesia shows negative ionotropic effects ( lower dp / dtmax ) when administered with isoflurane .
levobupivacaine has a higher negative effect on isotonic relaxation ( maxvr ) than bupivacaine at equipotent plasma concentrations , thus causing more diastolic dysfunction .
moreover , plasma concentrations are doubled under additional inhalational anaesthesia as compared to awake patients under local anaesthetic alone , and hence the effects too could be simply binary .
we have used a mixture of both levobupivacaine and lidocaine but have not measured plasma concentrations .
animal studies have shown that the cumulative dose of bupivacaine required for inducing cardiac depression and arrhythmia is higher ( though not statistically significant ) with concomitant 1mac sevoflurane vis
it has been suggested that enhanced myocardial depression could result from a  fast in - slow out mechanism of the local anaesthetic drug during repolarisation , in tandem with the heart rate .
our isoflurane patients did show higher heart rates than the sevoflurane group , but a simple extrapolation of our observations to explain the enhanced hypotensive effect may be an oversimplification .
the application of engineering indices for analysis of haemodynamic parameters in medicine may be questionable .
ninety - five percent confidence was used for power in contrast to true formulae of engineering indices where 99.99% confidence is routinely used .
the desired target blood pressure and sls for blood pressures would be more meaningful if set for each individual patient rather than for a group . individual variability of mac , autonomic tone , etc .
for pci analysis , the data should preferentially be normally distributed ; if not , a box - cox or johnson 's data transformation should be attempted .
further , a 3-parameter distribution analysis with lowest ad statistic value too would best analyse non - normal data , but all these data transformations have their limitations .
this study confirms the superiority of isoflurane over sevoflurane in achieving target sbps , a parameter that has been directly linked to intra - articular bleeding during arthroscopic procedures .
we used pcis as an additional practical tool that we believe , analyses better , the observed overall and within performances , with more valid outputs of  spec 's which are independent of capability index values .
isoflurane can provide better intraoperative haemodynamic status as compared to sevoflurane in patients undergoing shoulder arthroscopic surgery with preliminary interscalene blockade .
we also recommend that pcis be more frequently used for analysing haemodynamic and other similar data ( e.g. , pain scores ) in medical research to establish their true place among current statistical methodologies .
 background and aims : hypotensive anaesthesia reduces intra - articular bleed and promotes visualisation during arthroscopy . 
 the haemodynamic effects of inhalational agents isoflurane and sevoflurane were studied extensively , and both were found to reduce mean arterial pressures ( mbp ) to an equivalent magnitude . 
 we investigated the relative ability of isoflurane vis - a - vis sevoflurane to maintain the target systolic blood pressure ( sbp ) in patients undergoing shoulder arthroscopic procedures.methods:in a prospective randomised study , 59 patients in two groups of 30 and 29 patients each received concomitant general anaesthesia ( 1.21.5 mac of isoflurane and sevoflurane ) and interscalene brachial plexus block . 
 nitrous oxide was used in both groups . 
 intraoperatively , serial blood pressure recordings of sbp , diastolic blood pressure ( dbp ) , mbp and heart rates were done at every 3rd min intervals . 
 the manipulations needed to achieve target sbp ( t = 90 mmhg ) for optimal arthroscopic visualisation and treat unacceptable hypotensive episodes were noted . 
 conventional statistical tests and process capability index ( pci ) evaluation were both deployed for data analysis.results:lower mean sbp and dbps were recorded for isoflurane patients as compared to sevoflurane ( p < 0.05 , for mean , maximum and minimum recordings ) . 
 higher mean heart rates were recorded for isoflurane ( p < 0.05 ) . 
 pcis indicated that isoflurane was superior to sevoflurane in the ease of achieving target sbp of 90 mmhg as well as maintaining blood pressures in the range of 80100 mmhg.conclusion:isoflurane provides better intraoperative haemodynamic status vis - a - vis sevoflurane in patients undergoing shoulder arthroscopic surgery with preliminary interscalene blockade . 
 the pci can be a useful additional medical data analysis tool . 
INTRODUCTION
METHODS
RESULTS
DISCUSSION
CONCLUSION
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