🔵 SSI is an index which measures the surgical stress response in patients under anesthesia
🔵 It assess the balance between the intensity of surgical stimulation and the level of antinociception (e.g. Opioid analgesia , neuraxial or nerve blockade)
🔵 SSI uses two continuous cardiovascular variables, both obtained from Photo Plethysmography (PPG) waveforms of SpO2
(1) The interval between successive hearts beats (HBI)
(2) PPG amplitude (PPGA)
🔵Photoplethysmography (PPG), i.e. pulse oximetry, is primarily used to produce an estimation of the relative concentration of oxyhemoglobin in blood.
🔵 PPG is related to volume changes and contains information about the peripheral blood circulation, including skin vasomotion. Skin vasomotion is controlled by the sympathetic nervous system, which is activated during surgical stress.
🔵 Changes in PPG amplitude (PPGA) reflect changes in the peripheral vascular bed, controlled by the sympathetic nervous system . Increased PPGA response has been associated with nociception during general anesthesia.
🔵SSI values near 100 correspond to a high stress level, and values near zero to a low stress level.
🔵 In trials, SSI correlated positively with the intensity of painful stimuli and negatively with the analgesic concentration
🔵 SSI has been shown to be capable of differentiating decreases in HR achieved with opioid from those accomplished with a beta blocker (Ahonen et al. 2007).
🔵 An optimal range for SSI during anesthesia has not yet been recommended.
Reference: Measurements of adequacy of anesthesia and level of consciousness during surgery and intensive care, Johanna Wennervirta, Department of Anesthesiology and Intensive Care Medicine, Helsinki University Hospital
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