Should pulse pressure variation be indexed to tidal volume?

Introduction

Pulse pressure variation (PPV) for prediction of fluid responsiveness depends on the tidal volume (VT), and using VTs of the recommended 6 ml/kg makes PPV unreliable [1]. So far, nobody has suggested how to handle the VT when interpreting PPV or other dynamic parameters, but we hypothesise that PPV is proportional to VT and thus that PPV should be indexed to VT. The aim was to investigate how three VT levels affected PPV at four different intravascular volumes.

Methods

The study was approved by the national animal ethical committee. Eight anesthetised and ventilated pigs (23 to 27 kg) were bled 25% of the blood volume (hypovolemia). PPV was measured at ventilation with VTs of 6, 9 and 12 ml/kg (VT6, VT9 and VT12, respectively). Thereafter, depleted blood was replaced with voluven (normovolemia) and PPV was again measured at the three VT levels and subsequently at the +25% and +50% hypervolemic levels also generated with voluven infusion.

PPV values were log-transformed and compared with a paired t test. Comparisons were made at each intravascular volume level between VT6 and VT9, VT9 and VT12, and VT6 and VT12. Because three comparisons were performed, P < 0.05/3 = 0.017 was considered significant.

Results

All comparisons for PPV at different VTs were significantly different (P < 0.001); see Table 1 for factorial increases in PPV. At -25% hypovolemia, PPV did not fully double with doubling of VT, whereas PPV slightly more than doubled at normovolemia and hypervolemia.

Table 1 Comparisons of PPV at different VTs

Conclusion

Our study in pigs showed that PPV strongly depends on the VT and that PPV is nearly proportional to the VT, indicating that the clinical value of PPV may be improved by TV indexation.