Fluid administration is the first line of treatment in patients with acute circulatory failure. Dedicated algorithms and protocols of anaesthetic care regarding fluid therapy are key factors to prevent perioperative hypovolaemia or hypervolaemia, which are both known to increase morbidity and length of hospital stay.  Fluid responsiveness [i.e. the increase in stroke volume (SV) after a fluid challenge] is limited to about 50% of critically ill or surgical patients. For this reason, fluid challenge administration should be based on predictors of fluid responsiveness. Static indexes, such as central venous pressure and pulmonary wedge pressure, are unsuited for this purpose, but the dynamic indexes, such as pulse pressure variation (PPV) and stroke volume variation (SVV), reliably predict the effect of fluid challenge administration during controlled mechanical ventilation when a tidal volume (VT) of at least 8 ml/kg is used.

Use of an intra-operative lung-protective ventilation strategy (VT of < 8 ml/kg predicted body weight, PBW) is associated with a better outcome and is now suggested as standard practice in the operating room. Such small VTs limit the assessment of fluid responsiveness in surgical patients by means of dynamic indexes. To overcome this VT-related limitation of PPV and SVV, the prediction of fluid responsiveness can be achieved by applying functional haemodynamic tests aimed at increasing venous return and enhancing right ventricle preload dependence. Among these, the interruption of positive pressure ventilation (the so-called end-expiratory occlusion test, EEOT), initially proposed in critically ill patients, has been tested in two studies of elective surgical patients, with conflicting results. In patients ventilated with a mean VT of 8.2 ml/kg, the EEOT was unable to reliably predict fluid responsiveness, while the opposite was demonstrated in patients ventilated with a mean VT of 6.8 ml/kg. 

Myatra et al. recently proposed a new fluid responsiveness test called “the tidal volume challenge”. They demonstrated that an increase in the absolute value of PPV ≥3.5% induced by a transient increase in tidal volume from 6 to 8 mL/kg for 1 minute could reliably predict the increase in cardiac output in response to a fluid bolus performed at a tidal volume of 6 mL/kg whereas the PPV value obtained at 6 mL/kg tidal volume was unreliable for this purpose. Similar results were found for stroke volume variation (SVV) obtained from a contour analysis cardiac output monitor (threshold value: 2.5%). Thus, using a tidal volume challenge might overcome the limitations of PPV as a predictive index of fluid responsiveness during low tidal volume ventilation.

Recently Messina A et al. also demonstrate that the changes in PPV and SVV obtained after Tidal Volume Challenge are reliable and comparable to the changes in CI and SVI obtained after EEOT performed at 8 ml kg PBW in predicting fluid responsiveness in neurosurgical patients.

We hypothesised that the baseline reliability of the dynamic indices in elective surgical patients undergoing protective lung ventilation would be enhanced by the use of functional haemodynamic tests. Therefore, we designed this study to assess the sensitivity and specificity of PPV and SVV changes after tidal volume challenge in predicting fluid responsiveness in a renal transplant recipient under general anaesthesia with lung protective ventilation