Effect of local limb temperature on pulse oximetry and the plethysmographic pulse wave.

We attempted to evaluate prospectively local thermoregulatory vasoconstriction and vasodilatation in 15 volunteers by means of pulse oximetry and plethysmography as measured by a finger tip-pulse oximeter. Recent studies [1] concluded that the plethysmo time interval (PTI) between the QRS complex on ECG and the upstroke of the plethysmographic wave of the pulse oximeter is useful in evaluating the peripheral circulatory status. Venous (both Vv cubitae) blood samples for determination of SvO2 were drawn, as well as pulse oximetry SpO2 oxygen saturation measurements and calculation of PTI were performed under conditions of normothermia, hypothermia and hyperthermia, before and after immersing the hands of the volunteers in a cold (15 degrees C) and warm (45 degrees C) waterbath. Two pulse oximeters simultaneously were used, one for each hand, in order to check to which extent SpO2, SvO2 and PTI might be influenced by gross limb temperature changes. Our results show that during local hyperthermia SpO2 significantly decreased and during local hypothermia SpO2 increased after immersing the one hand in a warm waterbath and the other hand in a cold waterbath at the same time. The peripheral SvO2 significantly increased in the warm hand and decreased in the cold hand. PTI remained unchanged after exposure to either a cold or a warm waterbath. The possibility of technical causes for the SpO2 changes were eliminated. Finger-tip pulse oximetry SpO2 readings change with limb temperature. The change in venous oxygen saturation can be explained by temperature dependent arteriovenous shunts in the periphery. The observed change in SpO2 probably reflects altered transmission of arterial pulsations to venous blood in the finger.