Accuracy of Non-invasive Assessment of Cardiac Output and Fluid Responsiveness in Patients Undergoing Bariatric Procedures.

Comparability of discontinuous non-invasive (sphygomanometric) and continuous semi-invasive and invasive (Masimo©; Nexfin© Monitoring ; FloTrac © Edwards Lifesciences) beat to beat measurement methods for the determination of cardiac output and fluid responsiveness in patients undergoing bariatric surgery.

Bariatric patients show limitations regarding the conventional monitoring options with ECG derivation, pulse oxymetric oxygen saturation and sphygmo-manometric blood pressure measurement. From a clinical point of view this results in an invasive arterial blood pressure monitoring. In addition to that, intraoperative extreme changes of the patients position on the table combined with the applied pneumoperitoneum during the minimally invasive laparoscopic surgery can significantly influence the cardiovascular parameters. An invasive arterial blood pressure monitoring is able to reproduce the blood pressure by stroke, but it does not give any information about the cardiac output. The sphygmomanometric blood pressure measurement also entails the risk of insufficient detection of a hypotonic phase in the measurement. Additional monitoring systems are currently available, which are capable of measuring completely non-invasive or semi-invasively different cardiovascular parameters such as cardiac output (CO) and volume responsiveness. In this study, the investigators will compare continuous and discontinuous cardiovascular monitoring procedures and their parameters. The measurements take place at specific times, under defined changes in the body position with the additional influence of the pneumoperitoneum.

These changes are recorded and compared at the same time during different measuring methods preoperatively, intraoperatively and postoperatively. The non-invasive Nexfin © Monitoring (Edwards Lifesciences) is to be evaluated and compared with further measurement methods (FloTrac © (Edwards Lifesciences)) as well as the sphygmomanometric upper arm blood pressure measurement. The investigators also compare the non-invasive and invasive continuous beat to beat blood pressure measurement with conventional, discontinuous sphygomanometric upper arm blood pressure measurement.

The planned study is the comparison of 3 different hemodynamic monitoring procedures. For the evaluation we use the method used by Bland and Altmann for calculating the mean deviation (bias) and the precision (mean value ± 2 standard deviations. In the case of multiple measurements , the modification of the Bland-Altman method is applied (repeated measurements). The number of cases was determined with n = 60 patients, followed by an intermediate evaluation. For a Bland-Altman analysis, the width w of the confidence interval for the Limits of Agreement is calculated as w = 6.79 • σ • 1 / √n, where n is number of cases and σ is the standard deviation. For a case count of n = 60, the result is w = 0.88 • σ and thus for this explorative study a sufficiently large number. In case of the dynamic variables, the percentage matching and the calculation of the kappa index are also used for the statistical evaluation.