Publications

We read with interest the article by Myatra et al (1) in a recent article in Critical Care Medicine, where the authors reported that in 20 patients with acute circulatory, two of whom had acute respiratory distress syndrome (ARDS), the absolute change (ΔPPV6–8) in pulse pressure variation (PPV) after increasing tidal volume (Vt) from 6 to 8 ml/lkg predicted body weight (PBW) (tidal volume challenge) reliably predicts fluid responsiveness (sensitivity 94% and specificity 100%) during low Vt (6 ml/lkg) ventilation. However, we are not convinced by their conclusion that this “tidal volume challenge test” may reliably identify the true fluid responders during low Vt. Will this approach work in patients with ARDS ventilated with a low Vt?

Indeed the main explanation for the poor predictive performance of PPV during a low Vt is that respiratory changes in pleural pressure (ΔPpl) are too low, such that PPV is low even in responders (false negatives) (2). More recently, Lansdorp et al (3) recruited 20 ventilated patients following coronary artery bypass grafting surgery, clearly showing that Vt is a relevant determinant of ΔPpl: the higher the Vt, the higher the ΔPpl. Hence, we agree with the authors that, in non-ARDS patients, raising the Vt (from 6 to 8 ml/kg PBW) increases the magnitude of ΔPpl (and then of PPV in responders), avoiding some of the false negatives observed during low Vt. Unfortunately, this is not necessarily the case in patients with ARDS, in whom the magnitude of ΔPpl (2) was mostly determined by chest wall elastance (Ecw) (R2 = 0.84) or the ratio of Ecw to respiratory system elastance (Ers, R2 = 0.69), but to a lesser extent by Vt (R2 = 0.12). Given that ΔPpl = Vt × ERS × (Ecw/ERS) (2), the inadequacy of Vt as a determinant of ΔPpl (2,4) (and of PPV (2)) can be accounted for by the great variability of Ecw/ERS (range, 0.08–0.80) (2,5) reported for ICU populations with ARDS and renders tidal volume challenge test questionable at best in such a population. In short, the clinician should be cautioned against using the tidal volume challenge test to identify responders masked by low Vt in patients with ARDS, in particular when the Ecw or Ecw/ERS is “too low” such that increases in Vt (i.e., 6–8 ml/Kg) will fail to produce sufficient increases in ΔPpl (2) to induce significant changes in PPV values (ΔPPV6–8) in responders. The question of how low is “too low” remains unsettled, however. Future studies are required to determine a cutoff value for Ecw or Ecw/ERS, below which the use of this test should be discouraged.