Use of Peripheral Perfusion Index Derived From the Pulse Oximetry Signal as a Noninvasive Indicator of Perfusion

Alexandre Pinto Lima, MD, Peter Beelen, RN, Jan Bakker, MD, PhD

Disclosures

Crit Care Med. 2002;30(6) 

In This Article

Discussion

We studied whether a perfusion index calculated from the pulse oximetry signal, and available on-line in some monitoring systems, can reflect clinical signs of decreased peripheral perfusion (capillary refill time and central-to-toe temperature difference) in critically ill patients. Because no data were available on normal values for this perfusion index, we also studied the variation of this variable in healthy individuals. We show that a PFI of 1.4 can be used to detect abnormal peripheral perfusion in critically ill patients, corresponding with the median value found in the healthy volunteers. In addition, changes in this perfusion index adequately reflect changes in clinical signs of peripheral perfusion and thus can be used to assess effect of therapeutic interventions on peripheral perfusion.

During circulatory failure associated with hypovolemia and low cardiac output, redistribution of blood flow caused by increased vasoconstriction results in decreased perfusion of the skin.[1] Therefore, in critically ill patients, skin perfusion frequently is used to assess adequacy of global blood flow. Clinical signs of poor skin perfusion consist of a cold, pale, clammy, and mottled skin. Recently, techniques have become available to measure perfusion of the skin. Laser Doppler flow measurements and capillary microscopy[6] can adequately quantify changes in capillary blood flow but are not readily available in the emergency department or intensive care unit.

When blood supply to the skin decreases, the temperature of the skin also decreases. Therefore, measurements of skin temperature have been used to indicate decreases in skin blood flow as a marker of vasoconstriction and poor oxygen delivery.[3,2] Also, peripheral skin temperature has been advocated as a marker of the severity of shock.[4] In addition, because vasoconstriction of the skin reduces body heat loss, the difference between the core temperature and skin temperature may increase. The central-to-toe temperature difference therefore has been used to diagnose and treat patients with global blood flow abnormalities.[3,5] To have this parameter of peripheral perfusion available online, at least two temperature probes are necessary, and the skin temperature probe should be carefully affixed. These requirements may limit the use of these variables in emergency situations and clinically unstable patients.

Pulse oximetry is a monitoring technique used in almost every trauma and critically ill patient. Monitoring of pulse oximetry during surgery is mandatory in many countries. The principle of the pulse oximetry is the difference in absorbance of light with different wavelengths (660 and 940 nm) by oxygenated hemoglobin. Other tissues, such as connective tissue, bone, and venous blood, also absorb light and thus affect the resulting signal. However, whereas the arterial component of the signal is pulsatile, the absorption of light by other tissues is fairly constant. So, to have a proper estimate of the arterial oxygen saturation of the hemoglobin, the pulse oximetry has to distinguish the pulsatile component from the nonpulsatile component, where the pulsatile component is used subsequently to calculate the arterial oxygen saturation.[10,11] When the signal is weak, for example, during vasoconstriction, the pulse oximetry signal requires amplification up to ×109.[10] Although analysis of the pulse oximeter waveform has been used to assess the volume status of patients during major surgery,[7] the amplification necessary during a low signal (vasoconstriction, hypovolemia) could limit its clinical application in critically ill patients. The perfusion index, used in this study, is calculated as the ratio between the pulsatile and the nonpulsatile component of the light reaching the detector of the pulse oximeter. When peripheral hypoperfusion exists, the pulsatile component decreases, and because the nonpulsatile component does not change, the ratio decreases. Because the amplification necessary during the low signal affects both the pulsatile and nonpulsatile component, the ratio between these components is not affected. Although this variable has been incorporated in some monitoring systems as a parameter of peripheral perfusion, no data are available on the variation in the normal population. Also, no studies have been published on the relationship between the index and clinically used variables of peripheral perfusion in critically ill patients.

In the current study, we found a skewed and wide range of PFI values in healthy volunteers. We found no significant differences in the distribution of PFI values before or after a meal in this group of volunteers. Also, no differences were found between volunteers with or without chronic disease associated with vascular (or microvascular) abnormalities (e.g., hypertension, diabetes) or between smokers and nonsmokers. The variation in PFI was not related to differences in capillary refill times because these were all normal in the volunteers. Unfortunately, it was impossible to measure other indexes of peripheral perfusion, for example, the central-to-toe temperature difference in these volunteers.

By constructing a receiver operating characteristic, we found the median value of the healthy volunteers to be the best discriminating cutoff value to detect an abnormal core-to-toe temperature difference. This cutoff value also resulted in adequate predictability to detect an abnormal capillary refill time. Although this cutoff value suggests that 50% of the healthy volunteers had an abnormal peripheral perfusion, the two groups probably do not compare. Most of the critically ill patients were treated with vasoactive agents and were likely to have a disturbed regulation of peripheral circulation. Probably the cutoff value to detect abnormal peripheral circulation in the healthy volunteers is closer to the lower limit of normal reported by the manufacturer (0.3) representing the 5th percentile in our study. In addition, changes in clinical indicators of peripheral perfusion were met by concordant changes of the PFI in all patients.

Although poor peripheral perfusion often accompanies circulatory failure, the practical application of these indexes and the relationship with central hemodynamics or tissue oxygenation are not well studied. Assessment of capillary refill time has been found difficult in emergency situations, whereas the application of toe temperature measurements is often very limited in emergency medicine.[12] In adult cardiac surgery patients and patients with cardiogenic shock, a crude correlation between the central-to-toe temperature difference and cardiac output has been reported.[13,2] In pediatric patients, both capillary refill time and the central-to-toe temperature difference was not related to global hemodynamics or blood lactate concentrations.[14] However, in general pediatric patients, most of whom had septic shock, these indexes of peripheral perfusion correlated significantly with global hemodynamics and blood lactate concentrations.[14] In contrast with this study, the central-to-toe temperature difference has been found of limited value in adult patients with septic shock.[2] Also in our study, changes in cardiac output did not correlate with changes in clinical signs of poor peripheral perfusion or the PFI. These different findings could be related to the heterogeneity in skin blood flow regulation during changes in global blood flow and associated sympathetic nerve activity.[15] Nevertheless, improvements in peripheral perfusion after treatment are associated with improved outcome in patients with circulatory shock.[16] The PFI represents an easily obtainable measure of peripheral perfusion and thus could be used to monitor the effect of therapy on peripheral perfusion in critically ill patients.

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