🚤 Reduction of plethysmographic pulse wave amplitude (PPWA) has been proven to be a reliable method for detecting the IV injection of an exogenous vasopressor ( for e.g. The adrenaline in epidural test dose)
🚤 Currently, a numerical value has been added to new pulseoximeters indicating the PPWA, termed the perfusion index (PI), to augment its clinical applicability.
🚤i.e. PI is the numerical value of the amplitude of the plethysmographic pulse wave that is displayed on many pulse oximeters.
🚤 Using pulse oximetry, a variable amount of light is absorbed by pulsating arterial flow (AC) and a constant amount of light is absorbed by nonpulsating blood and tissue
(DC). The pulsating signal indexed against nonpulsating signal and expressed as ratio is commonly referred to as the perfusion index
🚤 It depends on the distensibility of the vascular wall and the intravascular pulse pressure. Usually the effect of autonomic impulses upon distensibility is so strong that it predominates the opposite effect of pulse pressure.
🚤 Decreases in PI resulting from pain and other stressful stimuli are due to vasoconstriction of the finger arterial bed rather than changes in the pulse pressure
Reference: The Efficacy of Perfusion Index as an Indicator for Intravascular Injection of Epinephrine-Containing Epidural Test Dose in Propofol-Anesthetized Adults, Anesth Analg 2009;108:549 –53)