Original ContributionA mini-fluid challenge of 150 mL predicts fluid responsiveness using ModelflowR pulse contour cardiac output directly after cardiac surgery
Introduction
Fluid therapy is essential in the treatment of shock and hypoperfusion. A restrictive fluid policy has been shown to result in fewer complications compared to a more liberal fluid strategy [1], [2], [3]. After cardiac surgery, fluid overloading may contribute to pulmonary oedema and is associated with an increase in mortality and morbidity [4], [5]. Fluid boluses have been shown to notably contribute to a positive fluid balance after cardiac surgery and could be reduced without increasing renal complications using a protocolled algorithm for fluid administration [6], [7]. Predicting fluid responsiveness may minimize harmful fluid overloading in intensive care patients, especially in patients after cardiac surgery [8], [9], [10]. Additionally, a mini-fluid challenge may predict fluid responsiveness and concomitantly limit fluid loading in unresponsive patients [11], [12], [13], [14]. Mallat et al. has shown that only 100 mL of colloid infusion may be sufficient in a mainly septic patient population, using the change in stroke volume variation (SVV) and pulse pressure variation (PPV) to predict fluid responsiveness [15]. The use of these dynamic parameters to accurately predict a positive response to fluid administration, however, may be restricted to mechanically ventilated patients with tidal volumes > 8 mL/kg and having regular heart rates [16], [17], [18]. Furthermore, transthoracic echocardiography allows for the assessment of the aortic velocity time index (VTI) variations, and can be used to assess and predict fluid responses after infusion of only 50–100 mL of colloids [19], [20]. However, VTI measurements can be technically demanding and cannot provide continuous registration. Thus, several hemodynamic parameters and methods have been opted when using a mini-fluid challenge, with their own limitations as stated above. A minimal invasive parameter providing continuous registration during a mini-fluid challenge may be preferred.
Cardiac output (CO) by modified ModelflowR (COm) and by PulseCOR (COli) are minimal invasive and use arterial pressure pulse contour analysis to derive CO measurements. Modelflow CO measurement estimates CO using the three-element Windkessel model. It accounts for the aorta resistance to volume increase and pulsatile inflow and peripheral vascular resistance. PulseCO on the other hand uses an autocorrelation algorithm to calculate stroke volume using a pressure/volume relationship with calibration. COm and COli have been shown to compare reasonably well to thermodilution CO measurements, with COm showing the lowest bias and limits of agreement [21], [22], [23]. Previous research has shown that both pulse contour analyses are able to predict fluid responsiveness during passive leg raising [23].
The hypothesis of the current study was that the COm and COli can lower the amount of fluid that is needed to predict fluid responsiveness by mini-fluid challenges. We therefore studied the two pulse contour CO methods and analyzed the smallest amount of fluid needed to predict fluid responsiveness in mechanically ventilated patients after elective cardiac surgery.
Section snippets
Patients and methods
This study was conducted according to the principles of the Helsinki declaration. Ethical approval for this study was provided by the Medical Ethics Committee of the Leiden University Medical Center, Leiden, The Netherlands, ISRCTN37554354. In this pilot study, twenty-one consecutive patients undergoing elective coronary artery bypass grafting and/or valve replacement were included into the study after written informed consent was obtained prior to surgery. Exclusion criteria were: previous
Overall study population
Twenty-one patients (5 women) were included with baseline characteristics shown in Table 1. No significant bleeding (> 50 mL/h) occurred during the study period. Patient characteristics did not differ between fluid responders and non-responders. CVP, HR, and MAP or their change during fluid loading did not differ between groups (Table 2). These hemodynamic parameters were not able to predict a positive fluid response with any amount of mini-fluid challenge. Two patients had an increase in COm >
Discussion
Our study suggests that a mini-fluid challenge of 150 mL bolus of hydroxyethyl starch may be sufficient in predicting fluid responsiveness using ModelflowR pulse contour analysis of CO. A 150 mL fluid bolus may reduce the amount of the fluid challenge minimizing the risk of fluid overload.
The increase in CO induced by a fluid challenge is dependent on the patient's volume status, heart function and fluid type, among others [8], [10]. Fluid loading will shift the working point on the heart
Acknowledgements
None declared.
Disclosures
None declared.
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