Volume therapy in perioperative and critically ill patients remains a highly debatable subject. The clinician who is standing at the operating room or intensive care unit is frequently asking a question: Can we improve stroke volume, cardiac output, and hence hemodynamics by fluid administration? Part of the answer may stem from the fact that the end-point of fluid therapy is frequently unclear. It is therefore that perioperative fluid therapy is still a challenge as well as a source of frequent errors.

The traditional, widely used monitoring of filling pressures, e.g. central venous pressure (CVP), and pulmonary artery occlusion pressure, to predict fluid responsiveness for the optimization of cardiac preload is frequently inadequate and incorrect in critically ill patients.¹ Functional hemodynamic monitoring implies a therapeutic application, apart from diagnosis such as a therapeutic trial of fluid challenge to assess preload responsiveness. Newer ways for assessing preload responsiveness include monitoring changes in CVP during spontaneous breathing and variations in arterial pulse pressure, systolic pressure, and aortic flow variation in response to vena caval collapse during positive pressure ventilation or passive leg raising.² In this issue of the Acta Anaesthesiol Taiwan, an article by Hofer and Cannesson³ demonstrates various techniques for monitoring fluid responsiveness. They emphasize that the traditional monitoring parameters (CVP, pulmonary artery occlusion pressure) are not able to reflect preload and discriminate between responders and nonresponders to a fluid challenge. Therefore, reliable, sensitive, and specific variables are required. To date, many dynamic parameters including arterial systolic pressure variation⁴ and its expiratory component (Δup and Δdown) and the arterial pulse pressure variation (PPV),⁵ the Doppler aortic blood velocity variation,⁶ the pulse contour stroke volume variation (SVV),⁷ the pre-ejection period variation,⁸ and plethysmography systolic pressure variation and plethysmographic variability index⁹ have successively been shown to be useful tools to predict fluid responsiveness.

Both PiCCO plus system and LiDCO plus technique allow the simultaneous continuous assessment of SVV and PPV.³ Nevertheless, the Flotrac/Vigileo system can only display SVV on a continuous basis and even limits its accuracy of measurements in vasoplegic patients under hyperdynamic situations.¹⁰ Although PPV or systolic pressure variation can also provide hemodynamic monitoring, its lack of cardiac output assessment limits a direct control of the fluid loading effect. However, fluid responsiveness predicted by using SVV or PPV is not significantly different from each other until now.³ Nonetheless, plethysmography systolic pressure variation and plethysmographic variability index may tend to inaccuracy in low-perfusion conditions with increased vasomotor tone.¹¹ Besides, arrhythmias and increased intra-abdominal pressure may result in an erroneous assessment of fluid responsiveness.2, 12 Moreover, positive end-expiratory pressure may not affect functional hemodynamic assessment if the respirator settings are kept constant.3, 13 However, fluid responsiveness indicates a preload reserve can be reliably predicted by a simple passive leg-raising test using a cardiac output monitoring system.³

In brief, there are many new modalities for monitoring fluid responsiveness. Whether it is reliable or not needs to be interpreted with caution because we are often confronted with a variety of static parameters that do not provide a conclusive picture. Patient’s physical status such as respiratory conditions, inotropes administration, and different monitoring devices may all affect the measurements of hemodynamic parameters.