Pulse Pressure Variation Helps to Predict Fluid Responsiveness in Patients Ventilated With Low Tidal Volumes

Volume expansion is frequently used to treat critically ill patients with acute circulatory failure. The goal of volume expansion is to increase left ventricular stroke volume and consequently cardiac output. However, about 50% of patients with acute circulatory failure will respond to fluid challenge (preload- dependent patients). Therefore, the ability to predict fluid responsiveness in critically ill patients is crucial, particularly for ARDS patients because of increased alveolar-capillary membrane permeability, and avoiding unnecessary fluid loading has been shown to have a positive effect on patient outcome.Among the dynamic parameters used at the bedside to identify fluid responsiveness, pulse pressure variation (ΔRESPPP) is one of the most accurate in patients with acute circulatory failure receiving invasive mechanical ventilation. However, most studies evaluated patients ventilated with large tidal volumes (≥ 8 ml/kg). Therefore, the validity of ΔRESPPP to identify fluid responsiveness is still debated when lower tidal volumes are used.

The current literature about its performance during ventilation with low tidal volumes is unclear, and opposite conclusions have been drawn. The present study was designed to (1) determine the value of ΔRESPPP to predict fluid responsiveness in patients ventilated with low tidal volumes, and (2) to investigate whether a lower ΔRESPPP cut-off point should be used when patients are ventilated with low tidal volumes.

Methods: This cross-sectional, observational study included 37 critically ill patients with acute circulatory failure requiring fluid challenge. They were sedated and mechanically ventilated with Tidal Volume (VT) 6-7 ml/kg IBW (ideal body weight), monitored by pulmonary artery catheter and arterial line. Mechanical ventilation and hemodynamic parameters, including ΔRESPPP, were measured before and after fluid challenge with 1,000 ml crystalloids or 500 ml colloids. Fluid responsiveness was defined as an increase of at least 15% in cardiac index.

Patients were followed for 28 days or until discharge from the ICU.

Study Protocol Patients were sedated with midazolan and fentanyl (score of -4 to -5 in the Richmond Agitation Sedation Scale)and ventilated in controlled pressure or controlled volume mode (Servo I system v.12 or Servo 900 C, Siemens, Sweden) with VT < 8 ml/kg IBW (51 + 0.9[height in cm- 152.9] for men and 45.5 + 0.91[height in cm- 152.9] for women). Ventilatory and hemodynamic variables were measured before and after FC with the patients in a supine position. Zero pressure was measured at the midaxillary line. The correct position of the pulmonary artery catheter in West's zone 3 was checked as described in the literature.