Should dynamic parameters for prediction of fluid responsiveness be indexed to the tidal volume?

Background: The respiratory variation in the pre‐ejection period (ΔPEP) has been used to predict fluid responsiveness in mechanically ventilated patients. Recently, we modified this parameter (PEPV) and showed that it was a reliable predictor for post‐cardiac surgery, mainly paced, patients when moderately low tidal volumes were used. One of the modifications involved tidal volume indexation, which had not been proposed before for dynamic parameters. The aim of the present animal study was to investigate whether indexation to tidal volume should be part of a new definition of dynamic parameters such as the case for our newly proposed PEPV. Methods: Eight prone, anesthetized piglets (23–27 kg) were subjected to a sequence of 25% hypovolemia, normovolemia, and 25% and 50% hypervolemia. At each volemic level, tidal volumes were varied in three steps: 6, 9, and 12 ml/kg. PEP variations (ms) and pulse‐pressure variation (PPV) were measured during the three tidal volume steps at each volemic level. Results: PEP variations increased significantly with increasing tidal volume at all volemic levels but 50% hypervolemia and were proportionally related to the tidal volume at normovolemia. PPV increased significantly with increasing tidal volume at all volemic levels and was roughly proportional to the tidal volume at all volemic levels but hypovolemia. Conclusion: Our study indicates that dynamic parameters are improved by indexing to tidal volume.