Dynamic parameters have some limitations that prevent their use in all circumstances. First, because in clinical practice the arterial pressure curve is obtained from fluid-filled catheters, several factors (air bubbles, kinks, clot formation, compliant tubing, excessive tubing length) may distort the signal. This problem can be ruled out by a “fast-flush test”16 but requires first careful examination of the BP curve by a caregiver before relying on PPV and SVV values. Second, in patients with cardiac arrhythmia, the beat-to-beat variation in stroke volume and hence in BP may no longer reflect the effects of mechanical ventilation. This is particularly true in patients with atrial fibrillation or frequent extrasystoles. In patients with few-and-far-between extrasystoles, the arterial pressure curve can still be analyzed if the cardiac rhythm is regular during at least one respiratory cycle. However, it definitely rules out the possibility of a continuous and automatic monitoring of this phenomenon, like done in the study by Hofer et al.1 Third, if pleural pressure changes are small over a single respiratory cycle, inspiration will not induce any significant change in left ventricular stroke volume, even in fluid-responsive patients. Small variations in pleural pressure may be observed in patients with spontaneous breathing activity, in patients receiving mechanical ventilation with small tidal volumes (eg, 6 mL/kg), or in patients with increased chest compliance (eg, open chest). In this context, caution should be exercised before concluding that a patient will not respond to a fluid challenge because PPV or SVV are low.