SPI definition: The SPI (Surgical Pleth Index), called the Surgical Volume Stroke Index on the GE website, seems to be also referred to as the Surgical Stress Index or Surgical Injury Index, according to its listed references. The SPI is a number that can be used to monitor a patient’s hemodynamic response to surgical stimulation and analgesic medication during general anesthesia. The SPI reflects the patient’s increased sympathetic activity in response to painful (injury-sensitive) stimuli. SPI monitoring is obtained using a unique algorithm that processes off-the-shelf pulse waves. The measurement process of SPI: The measurement starts automatically once the pulse oxygen probe is connected to the patient. There is first a 3-minute learning process, followed by a base value, or the learning process can be initiated intraoperatively as needed. The values are gray in color and a “Learning” message is displayed. As the measurement continues, it changes to purple, forming a trend curve as time increases. The learning process is marked as a dashed vertical line in the trend. The value is also integrated in a sub-screen for anesthesia (also including entropy and neuromuscular transmission NMT modules) to visualize the impact of anesthetic and analgesic drugs on the management of general anesthesia. SPI Precautions: SPI is indicated for adult patients (>18 years of age) undergoing general anesthesia. SPI is contraindicated in patients with pacemakers and during the use of atropine. Factors affecting hemodynamic stability in anesthetized patients may also affect SPI. Clinical Use of SPI: SPI can be used to help assess acute injurious events during general anesthesia, as well as long-term status responses. In general, when the SPI rises, the patient is responding to the event. When the index decreases, the level of surgical reactivity decreases. Calculated range: 0-100. normal range: 30-80.