E. O. Cruz1, Y. Sanaiha1, V. Dobaria1, P. Benharash1 1University of California Los Angeles,Cardiac Surgery,Los Angeles, CA, USA
Introduction:
Near infrared spectroscopy (NIRS) is a non-invasive commercially available method for measuring tissue oxygenation. However, cost and reliability have limited its widespread use. Currently, there is no cost-effective method to assess tissue oxygenation under non-pulsatile conditions such as cardiopulmonary bypass. Such a device might improve the reliable detection of limb ischemia and direct interventions to avoid permanent injury.
Methods:
The device used in this study consisted of a single photodetector and a pair of red (660 nm) and infrared (880 nm) light emitting diodes (LEDs) selected to discriminate between the absorption of oxymyoglobin and deoxymyoglobin at a depth of about 1cm. The LEDs and signals (20 point running average) were multiplexed and recorded at 100 Hz on a PC. Nine healthy volunteers were examined while a blood pressure cuff was used to first disrupt venous (30 mmHg x 90 seconds) and subsequently arterial flow (200 mmHg x 30 seconds). The red/infrared (R/IR) transmission ratio was calibrated and then recorded for each subject. A Wilcoxon-Mann-Whitney test was used to determine the significance of signals during each period of occlusion compared to the baseline noise, using alpha level P<0.05.
Results:
Amongst all volunteers, the amplitude of the average noise, or deviation from the baseline R/IR ratio, was 0.0091 +/- 0.004. At an average of 90 seconds after venous occlusion, the R/IR ratio decreased by 0.033 +/- 0.021 (P=0.01). Venous occlusion resulted in signal depression greater than the noise for that specific trial in 77.8% of participants, and the median signal to noise ratio was 16.49. After an average of 30 seconds of arterial occlusion, the R/IR ratio decreased by 0.035 +/- 0.008 (P<<0.01). All trials produced a signal depression greater than the noise during arterial occlusion, and the median signal to noise ratio was 10.92. In addition, a significant hyperemic rebound was detected following the restoration of arterial flow in 77.8% of trials.
Conclusion:
This simple device appears to successfully detect changes in tissue oxygenation under non-pulsatile settings. Further studies and adjustments to decrease system noise and add additional depths are warranted. The utilization of this simple NIRS system in patients on extracorporeal life support or cardiopulmonary bypass may assist in the early detection of limb ischemia.
