Effects of an incision on a subject under general anesthesia on the photoplethysmograph (PPG) and blood pressure (BP). This incision generated a sympathetic nervous system response which suppresses blood flow to the skin, which manifest itself in a reduced PPG amplitude. This provides a mechanism for gauging the depth of anesthesia.

The original figure was provided by Kirk Shelley, Yale Medical School and modified by Stephen Linder, Dartmouth College.

{{cite journal |last1=Shelley |first1=KH |title=Photoplethysmography: beyond the calculation of arterial oxygen saturation and heart rate. |journal=Anesthesia and analgesia |date=December 2007 |volume=105 |issue=6 Suppl |pages=S31-6, tables of contents |doi=10.1213/01.ane.0000269512.82836.c9 |pmid=18048895}}

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