Hypovolaemia

The determination of intravascular volume in haemodynamically unstable patients is key to subsequent management. Administration of fluid remains the cornerstone of treatment but fluid loading in those patients not responsive to fluid is detrimental to outcome, as interstitial oedema impedes tissue oxygenation. Traditionally, static markers of cardiac preload such as central venous pressure have guided fluid therapy. Such markers have been shown to poorly predict fluid responsiveness. This has led to increased interest in dynamic variables such as the fluctuation of blood pressure and stroke volume in response to mechanical ventilation. These variables use the cyclical changes induced by positive pressure ventilation on right and left ventricular loading to measure changes in left ventricular stroke volume and arterial pressure. These dynamic indices, termed systolic pressure variation, pulse pressure variation and stroke volume variation, are superior in predicting fluid responsiveness in mechanically ventilated patients with a regular cardiac rhythm. In patients with spontaneous respiratory effort, the response of stroke volume to a passive leg raise is also an accurate indicator of fluid status.