Evaluation of indexing pulse pressure variation during low tidal ventilation : An experimental pilot study.

BACKGROUND

To reliably assess fluid responsiveness using pulse pressure variation (PPV), tidal volumes (VT) of at least 8 ml/kg of ideal bodyweight are recommended. This contrasts with the current recommendations for lung-protective mechanical ventilation, which advocate VTs between 6 and 8 ml/kg to minimize ventilator-induced lung injury.

OBJECTIVE

The aim of this study was to analyze whether indexing PPV to certain ventilatory parameters can be a possibility for VT-independent assessment of fluid responsiveness during mechanical ventilation with lower tidal volumes.

MATERIAL AND METHODS

Hemodynamic and ventilatory data were collected from eight anesthetized, paralyzed, intubated and mechanically ventilated pigs. Each animal was ventilated with four different VTs (4, 6, 8, and 12 ml/kg) during volume-controlled ventilation, across four intravascular fluid states: normovolemia; hypovolemia induced by bleeding and two stages of fluid resuscitation induced by retransfusion and additional fluid administration. The PPV values were indexed to various ventilatory parameters including VT, plateau pressure (Pplat) and driving pressure (∆P), as well as transpulmonary pressures and composite parameters, such as minute ventilation (MV), mechanical power and mechanical energy.

RESULTS

Indexing PPV to MV (PPV/MV) resulted in values with the smallest variation across different VTs, followed by PPV/VT, PPV/Pplat and PPV/∆P. These indexed parameters exhibited high ratios of explained variance (R) to regression slope (β), indicating reduced VT dependency. In each case, higher values reflected a greater calculated fluid deficit.

CONCLUSION

Indexing PPV to MV can be a feasible way to use dynamic parameters of fluid responsiveness across a wide spectrum of ventilator settings, such as during lung protective ventilation strategies involving lower tidal volumes. Future studies should evaluate the performance of the indexed parameters in guiding fluid therapy in the clinical setting and define thresholds.