Changes in pleth variability index and detection of hypotension during spinal anaesthesia for caesarean section.

Background: Hypotension occurs commonly during caesarean section under spinal anaesthesia. As the onset may be rapid, it may fail to be detected by non-invasive blood pressure (BP) measurement in a timely fashion. Pulse oximeters can continuously and non-invasively measure tissue perfusion (perfusion index (PI)) and its variation with the respiratory cycle (pleth variability index (PVI)). As with stroke volume variation, PVI predicts fluid responsiveness in mechanically ventilated patients with a high degree of sensitivity and specificity. Whilst there have been very few studies in spontaneously breathing patients, a volunteer study revealed that PVI decreased with straight leg raising. As stroke volume variation is known to increase during spinal anaesthesia, we hypothesized that this would also be the case for PVI and that this rise could be used to identify the onset of hypotension.