Stroke Volume Variation and Pleth Variability Index- Are They Two Faces of the Same Coin?

Abstract

Introduction: Fluid requirement in patients undergoing major bowel surgery has been a matter of debate and no consensus has been reached. Stroke Volume Variation (SVV) and Pleth Variability Index (PVI) are dynamic indicators of preload. One is invasive and well established where as the other, is noninvasive and relatively new.Aim: This study was designed to compare SVV and PVI when used simultaneously and continuously in patients undergoing major open abdominal surgery. Materials and Methods: This prospective observational study was conducted on patients undergoing major open abdominal surgery. They were monitored continuously with FloTrac® to measure SVV and Masimo pulse oximeter to measure PVI, along with standard monitoring. Readings of both SVV and PVI were noted at baseline and every 10 minutes thereafter till the end of surgery. SVV was used for clinical decision-making and fluid infusion. The displayed PVI at each of these time points was observed for concurrence between the pair of measurements.Statistical analysis was performed using the SPSS 15.0 software. SVV and PVI measurements were compared by correlation coefficients and Bland-Altman analysis. Interclass correlation coefficients with respect to individual patient data are expressed using R software.Results: A total of 516 pairs of measurements were obtained from 15 patients. The pattern of variation of both SVV and PVI was similar in all patients. Mean PVI reading was ≈2-3% higher than SVV. When individual readings of PVI and SVV were plotted against each other, there was very weak positive correlation (r=0.3742). Bland-Altman plot showed the scatter to be wide, reiterating lack of agreement. The median (IQR) difference between SVV and PVI was -2% (-4 to -1%) but the range was very wide (-18% to 9%). Perfusion Index (PI) was > 0.5 in 95% of all individual readings (490/516).Conclusion: PVI overestimates and is unreliable as an indicator of fluid requirement as compared to SVV. PVI cannot be substituted for SVV in patients undergoing major open abdominal surgery.