Effects of changes in blood pressure and airway pressures on parameters of fluid responsiveness

Systolic pressure variation (SPV) and stroke volume variation (SVV) are clinical indicators of fluid responsiveness. However, several factors may influence these parameters and thereby limit their usefulness. In this clinical study, we analysed SPV and SVV in comparison with static preload parameters during an increase in arterial blood pressure (BP) and airway pressure.

We studied 16 postoperative cardiac surgical and mechanically ventilated patients who, for clinical reasons, underwent haemodynamic monitoring by transpulmonary thermodilution (PiCCO, Pulsion Medical Systems, Munich, Germany) and continuously received norepinephrine. After baseline, the systolic BP was increased (from 105 +/- 10 to 135 +/- 14 mmHg) by briefly increasing the norepinephrine dosage and, after a return to baseline level, the tidal volume was increased from 6 to 10 ml kg(-1). Final measurements were again taken at baseline airway pressures. At each time point, three boluses of 15 ml of 0.9% saline solution (<8 degrees C) were injected into a central vein. Fluid status and sedation remained unchanged. Friedman ANOVA on ranks for repeated measurements was used for statistical analysis.

Heart rate (HR), cardiac index, intrathoracic blood volume and extravascular lung water remained constant throughout. However, the SPV (6 +/- 3 versus 5 +/- 2 mmHg) remained unchanged and the SVV decreased (12 +/- 5 versus 10 +/- 4%) with higher BP, whereas both parameters, in contrast to central venous and left atrial pressure, significantly increased with higher tidal volume.

In cardiac surgical patients with preserved cardiac index, SVV, but not SPV, decreased during an acute increase in BP, whereas both parameters, in contrast to cardiac filling pressures, significantly increased with higher tidal volume.