shock due to any reason carries a significant risk of mortality and morbidity , especially if the therapy is inappropriate .
aggressive fluid and/or vasoactive therapy aimed at achieving the desired end - points of early goal directed therapy ( egdt ) is the cornerstone of management of both adults and children with septic shock , and also following cardiac surgery .
specifically , achieving a central venous saturations ( scvo2 ) > 70% is one of the key therapeutic goals that has been shown to improve outcomes , as this reflects tissue oxygenation and oxygen delivery .
although the true value of mixed venous oxygen may be obtained by obtaining a sample from the pulmonary artery by swan ganz catherization , a scvo2 from an appropriately sited central venous catheter ( cvc ) is an acceptable surrogate .
the gold standard to measure scvo2 is analysis of the venous blood gas sample by a co - oximeter ( co - ox ) .
this is because the saturation is directly measured in a co - ox , whereas this value is calculated ( with many assumptions ) in a standard blood gas machine . in sick patients , binding of hemoglobin ( hb ) to oxygen
can not be assumed to be normal , and , furthermore , the position of the oxyhemoglobin curve may be shifted due to multiple reasons . however , due to the cost constraints , many intensive care units ( icu ) in india measure scvo2 by a standard abg ( stand abg ) machine , which is significantly less expensive .
this prospective , observational study from a multidisciplinary ( medical , cardiac , and surgical ) pediatric icu ( picu ) of a tertiary level hospital ( apollo hospitals , chennai ) was conducted to assess the validity of scvo2 levels obtained by stand abg in comparison with co - ox . in particular , with reference to reaching egdt endpoints in septic shock , we aimed to elucidate the false positive rates ( scvo2 > 70% by stand ab standard abg machine g but < 70% on co - ox ) that could potentially lead to inadequate resuscitation of this group , resulting in morbidity and mortality of these critically ill patients .
we also wanted to study the correlation of scvo2 obtained by the two methods under common conditions that shifted the oxygen dissociation curve ( odc ) to the right or left ( acidosis and alkalosis , temperature variations ) , as these conditions are frequent occurrences in critically sick icu patients .
sensitivity and specificity of standard abg machine against gold standard ( co - ox ) for scvo2 values in patients with shock was determined .
this was a prospective , observational study that included consecutive children over a period of 12 months from jan 2008 to 2009 who were admitted to a picu with central line in situ .
those with hemoglobinopathies and inappropriate central line placement ( as detected radiographically ) were excluded from the study .
paired central venous blood gas samples were taken with a maximum time interval of 1 min between sampling .
both blood samples were analyzed by stand abg ( bayer 248 blood gas analyzer ) and co - ox ( bayer diagnostics mfg ltd .
the machines were located in the icu premises and , hence , there was minimal time delay in processing the sample . the standard abg machine quality control of the stand abg and co - ox machines were maintained to reduce calibration errors .
both machines were calibrated by the biomedical department as per the hospital and concerned company policies .
all samples were collected in preconfigured heparinized syringes for blood gases ( bd india ) .
the cost per test in each modality was rs 450/- for standard abg test and rs 750/- for abg by co - ox . at the time of sampling ,
general patient demographics including diagnosis , clinical condition ( vitals , core temperature , need for inotropes , ventilation ) , and laboratory data ( hb level , lactate ) were recorded . after the sample was processed , the blood gas values were entered on a spreadsheet and cross - checked by two authors ( gs and vpa ) .
data obtained from paired samples was tested for correlation coefficient for ph , paco2 , pao2 ( measured values ) , and scvo2 ( measured in co - ox and calculated by stand abg ) .
a perfect agreement requires that all points obtained by plotting data pairs lie along the line of equality , while excellent correlation is obtained when the points lie along any straight line .
in addition , a difference in scale of values between two parameters does not affect correlation , but affects agreement .
altman assessment for agreement was used to compare the scvo2 measured by the two methods .
the groups included patients in shock , those with abnormalities in factors that shift the odc , viz .
ph , temperature , and hb levels and those without any abnormalities ( normal ) .
acidosis and alkalosis were defined as venous ph < 7.32 and ph > 7.42 , respectively .
temperature abnormality included patients with both hypothermia ( 97f ) and hyperthermia ( 99f ) .
patients were said to be in shock if they had at least three of the following clinical and laboratory evidence of shock : disproportionate tachycardia , low blood pressure for age , prolonged capillary refill time ( > 3 sec ) , poor peripheral pulses , core to peripheral temperature difference of > 2c , low urine output , or elevated lactate ( > 4 mmol / l ) . tests for correlation ( pearson 's coefficient ) and agreement ( bland  altman analysis ) were further performed on scvo2 values obtained in these subgroups .
sensitivity ( true positive ) and specificity ( true negatives ) for scvo2 values determined by stand abg versus the gold standard ( co - ox ) were calculated with particular reference to reaching egdt endpoints in septic shock .
the baseline characteristics of the study population are summarized in table 1 . from 82 patients ,
forty - four paired samples were collected in patients in shock , 54 in acidemic patients , 29 in alkalemic patients , 34 anemic patients , 32 in patients with either hypothermia or hyperthermia , and 31 in patients who did not fall in any of these categories ( normal ) .
baseline characteristics results obtained on blood gas analysis for the two groups are tabulated in table 2 . despite a statistically significant difference in the ph and scvo2 values measured by both the methods
, there was good linear correlation between these parameters [ table 2 , figures 1 and 2 ] . among the blood gas parameters , ph , paco2 , and pao2 showed positive good correlation when obtained by either method [ figures 1 and 2 ] . in the case of scvo2
, a positive correlation was obtained when the scvo2 was analyzed by either machine ( r = 0.69 , p = 0.001 ) [ figure 3 ] .
limits of agreement between scvo2 measured by standard abg machine and co - ox by bland  altman methodology gave 2.3% bias with 95% ci of  24.2% to 19.5% [ figure 4 ] .
comparison of blood gas parameters between standard blood gas machine ( stand abg ) and co - oximeter ( co - ox ) correlation between paco2 measured by stand abg and co - ox .
scatter diagram with regression estimate shows the positive good correlation between paco2 by stand abg and co - ox ( r = 0.84 , p = 0.001 ) ; paco2 abg = stand abg ; paco2 c = co - ox scatter diagram correlation between ph by stand abg and co - ox ( ph c ) .
scatter diagram with regression estimate shows the positive good correlation between ph by stand abg and co - ox ( r = 0.94 , p = 0.001 ) ; ph abg = ph analyzed by stand abg ; ph c = ph analyzed by co - oximeter ) correlation between scvo2 by stand abg and co - ox .
scatter diagram with regression estimate shows the positive good correlation between scvo2 by stand abg and co - ox ( r = 0.69 , p = 0.001 ) .
( scvo2 a = central venous saturation measured by standard blood gas machine ; scvo2 c = central venous saturation measured by co - oximeter ) agreement between scvo2 values obtained by the two machines .
scvo2 a = central venous saturation measured by standard blood gas machine , scvo2 c = central venous saturation measured by co - oximeter the sensitivity of the standard abg machine machine in detecting scvo2 in patients with shock was 84.21% , while the specificity was 93.18% . in 4 ( 2.84% ) samples ,
the standard abg machine gave false positive results , indicating that that shock endpoint had been reached ( scvo2 >
the scvo2 values obtained by the two methods in various subgroups are tabulated in table 3 .
all the subgroups , including patients with shock , had substantial correlation of the scvo2 values ( 0.6 - 0.8 , p = 0.001 ) whether it was analyzed by standard abg machine or co - ox . in those with alkalosis , the scvo2 values had good substantial correlation ( 0.90 , p = 0.001 ) .
however , presence of anemia showed only moderate correlation ( 0.49 , p = 0.001 ) .
aggressive fluid and/or vasoactive therapy aimed at achieving the desired end - points of egdt , including scvo2 > 70% , is one of the key therapeutic goals that has been shown to improve outcomes .
there have been no previous published studies comparing scvo2 values obtained by standard abg machine and co - ox . in our study , a positive good correlation ( r = 0.69 ) was obtained in the scvo2 values measured by both the machines in a heterogeneous group of critically sick children , i.e. , there is a strong , linear relationship unlikely to happen by chance ( p = 0.001 ) .
10 gm% , the correlation co - efficient showed only moderate positive correlation ( r = 0.49 , p = 0.001 ) .
the standard abg machine , which is widely used for blood gas analysis in many hospitals across india , measures the paco2 , pao2 , and ph of the sample .
all other values are calculated by the algorithm inbuilt in the machine . as against this
, most icus in the western world use the co - ox , which considered the gold standard .
the advantage of a co - ox compared to a standard blood gas machine is its capability to measure concentrations of oxygenated hb , reduced hb , carboxyhemoglobin , methemoglobin , and bicarbonate .
the co - ox is intuitively considered more reliable , especially in sick patients , where an algorithm - based calculation of scvo2 by a standard blood gas machine may be prone to fallacies .
knowledge regarding the limits of agreement and correlation of scvo2 values analyzed by stand abg versus co - ox across a wide range of typical icu conditions is important for critical care practitioners .
limits of agreement between scvo2 obtained by stand abg versus co - ox showed 2.3% bias with 95% ci of  24.2% to 19.5% by bland 
the same methodology was used in the subgroups with none , except the group with alkalosis showing agreement .
thus , although there was good correlation , the agreement levels were wide , suggesting that the wide difference is possibly machine related .
hemodynamic monitoring is a standard of care in the management of critically sick children to identify cardiovascular insufficiency , determine its cause , and evaluate the response to treatment .
global tissue oxygen delivery is commonly monitored using mixed venous oxygen saturation ( scvo2 ) level , which reflects the cardiac output , oxygen delivery ( do2 ) , peripheral extraction of oxygen , and consumption ( vo2 ) .
its value is related to four determinants : vo2 , cardiac output , hb , and oxygen saturation of hb .
scvo2 trends measured with the central venous catheter tip in right atrium is more easily measurable than the mixed venous saturations and is an acceptable surrogate .
inadequate do2 is presumed to be occurring if tissue oxygen extraction is markedly increased , as manifested by a decrease in scvo2 < 70 in any kind of shock .
hence , our particular focus of interest was to determine whether the stand abg performed adequately when assessing scvo2 in patients with shock . in samples where the sample was analyzed by stand abg and
the scvo2 was > 70% , the false positive rates were low and the degree of specificity and sensitivity was high , suggesting that the chance of missing a patient with uncorrected shock was low .
two of the 4 patients who had scvo2 < 70% on co - ox , but >
however , given the small sample size of false positives , we are underpowered at this stage to conclusively comment if fever tends to increase the rate of false positives .
overall , scvo2 obtained by the stand abg machine showed good overall correlation with the scvo2 obtained by co - ox , except in anemic patients who had hb <
our study was limited by the rather small number of paired samples in each subgroup .
larger number of samples would have eliminated the small margins of error and improved the strength of relationship ( determined by the p value ) occurring during the statistical evaluation .
furthermore , we did not process phosphate levels ( another variable that affects the odc ) .
it may be argued that even missing a single patient with scvo2 < 70% increases the probability of that patient being under - resuscitated with a higher chance of mortality .
however , in practice , the patient 's condition is considered in totality and is not based on a single variable ( in this case , scvo2 ) . therefore ,
if other parameters in egdt for septic shock are not achieved , an icu physician is unlikely to consider an isolated scvo2 value of > 70% as the end point of treatment .
intensive care is making a foray in a big way in the developing world with several institutes offering tertiary level care .
this machine is relatively  cheaper  ( approximately rs 400,000 ) as compared to the much more expensive co - ox ( approximately rs 10,00,000 - 14,00,000 ) and the cost per abg carried out by a stand abg is half of that performed by a co - ox . while it was encouraging that the less expensive equipment showed satisfactory correlation with the gold standard co - ox over a wide range of patient conditions , the wide range for limits of agreement was of concern .
tertiary care icus in developing nations may prefer the stand abg as the co - ox is much more expensive and there was a general perception that the accuracy of the readings from a stand abg are  in sync  with the co - ox .
our study indicates that the stand abg can be a satisfactory and cost - effective surrogate for the more expensive co - ox when assessing scvo2 for patients with shock . although the correlation was good over a range of conditions , the limits of agreement were wide and it performed particularly poorly in anemic patients
 aims : aggressive therapy aimed at desired end - points of early goal directed therapy ( egdt ) is the cornerstone of septic shock management . 
 a key endpoint that improves outcomes is achieving central venous saturation ( scvo2 ) > 70% . the gold standard to measure 
 scvo2 is by a co - oximeter ( co - ox).settings and design : this prospective , observational study from a multidisciplinary pediatric intensive care unit ( picu ) was conducted to assess the validity of scvo2 levels by standard abg ( stand abg ) machine in comparison with co - ox in conditions that shifted the oxygen dissociation curve ( odc ) to the right or left in sick children and controls.materials and methods : data from paired samples was tested for correlation coefficient for ph , paco2 , pao2 , and scvo2 . 
 tests for correlation ( pearson 's coefficient ) and agreement ( bland  altman analysis ) were performed on scvo2 values obtained in various subgroups . 
 sensitivity and specificity for scvo2 values determined by standard abg machine versus co - ox were calculated in reference to egdt endpoints.results:a total of 141 paired samples were collected from 82 children . despite a statistically significant difference in the ph and scvo2 , there was good linear correlation between these parameters . 
 limits of agreement between scvo2 measured by standard abg machine and co - ox by bland  
 altman gave 2.3% bias with 95% ci of -24.2% to 19.5% . 
 sensitivity and specificity of standard abg machine in detecting low scvo2 in shock was 84.21% and 93.18% respectively , while it was false positive in 4 samples.conclusions:the less expensive standard abg machine showed satisfactory correlation with gold standard co - ox over a range of patient conditions ; however , the wide range for agreement was of concern and it performed particularly poorly in anemic patients . 
Introduction
Materials and Methods
Results
Discussion