184 LASER DOPPLER MONITORING OF TISSUE PERFUSION

Abstract

Laser doppler (LD) velocimetry theoretically offers a noninvasive objective measurement of tissue perfusion, an important clinical parameter that is currently only subjectively assessed. The laser doppler (Med Pacific)uses a skin sensor to measure reflected light from a helium-neon laser that penetrates tissue about 15 mm over a 1 mm2 area. It utilizes the doppler principle to calculate the root-mean-square frequency of the reflected light. Because of the extremely small cross sectional area of the vessels in the microcirculation that this technique samples, this continuously monitored number represents changes in tissue blood flow. We used the LD to assess tissue perfusion on 10 critically ill neonates (480 to 1600 gm) in the NICU to evaluate the feasibility of its clinical application. During the study we recorded temperature, heart rate, blood pressure and hematocrit to determine what factors might influence perfusion. We monitored the infants during procedures that were stressful or would alter cardiac output such as ET tube suctioning, blood transfusion and intubation. The LD was very sensitive, in fact, too sensitive to changes in tissue blood flow. There was no correlation between any of our measured parameters and changes in perfusion as measured by the LD. Obvious stress with procedures such as suctioning and intubation caused immediate decreases in perfusion, but changes in the readings from minute to minute varied just as much. In fact, moving the skin sensor as little as 1 mm would cause as much as a 300% change in readings. The LD appears to be an extremely sensitive monitor of local tissue perfusion, being more indicative of effects of local factors (skin temperature changes or application pressure) rather than changes in cardiac output. Non-invasive measurement of perfusion as measured by laser doppler still appears to be problematic.