While sophisticated haemodynamic monitoring as well as depth of neuromuscular block and sedation are established in daily clinical routine monitoring of analgesia remains poorly represented. The surgical pleth index (SPI) algorithm takes into account both central and peripheral sympathetic tone via the normalised heart beat interval and the plethysmographic pulse wave amplitude and has been described before. A more detailed investigation on the SPI and its behaviour under a posture maneuver shows the limitation of the device.