Pulse and systolic pressure variation assessment in partially assisted ventilatory support

Introduction

The use of pulse pressure variation (PPV) and systolic pressure variation (SPV) is possible during controlled mechanical ventilation (MV) [1]. Even in acute respiratory failure, controlled MV tends to be replaced by assisted ventilatory support, which may generate a tidal volume (Tv) inadequate to change the pulmonary venous flow and swing in pleural pressure [2]. This makes the use of dynamic indices unreliable. Our hypothesis was that during a pressure-support-assisted ventilatory (PSV) approach few imposed breaths (flow-triggered synchronized intermittent mechanical ventilation (SIMV)) could allow the monitoring of PPV and SPV. We therefore tested whether PPV and SPV during PSV + SIMV could be as accurate as in controlled MV.

Methods

A prospective case–control study. Thirty patients who met the criteria of weaning from controlled MV were included. PPV and SPV were measured, first, during 20 minutes in PSV with three per minute flow-triggered SIMV breaths (10 ml/kg, duration 5 s, inspiration to expiration ratio 1:3) (T1), and then during three consecutive breaths in controlled MV (respiratory rate 12/min, duration 5 s, inspiration to expiration ratio 1:3, Tv 10 ml/kg, positive end-expiratory pressure and FiO₂ as in PSV) (T2). Throughout 20 minutes of data collection, saline infusions were kept constant (3 ml/hour) without performing any fluid loading. Correlation and Bland–Altman analysis were used to compare respective values of PPV and SPV in the two modes of ventilation.

Results

Significant correlations were found between dynamic indices in SIMV during pressure support ventilation and those in controlled MV mode. The mean differences between two measurements were: PPV 0.6 ± 2.8% (limit of agreement: -5.0 and 6.2), SPV 0.5 ± 2.3 mmHg (limit of agreement: -4.0 and 5.1).

Conclusion

PPV and SPV measured during SIMV fitted with the findings in controlled MV. Dynamic indexes could be accurately monitored in patients breathing with assisted respiratory assistance adding an imposed large enough SIMV breath [3].