In vivo performance of a visible wavelength optical sensor for monitoring intestinal perfusion and oxygenation

Abstract

Traumatic injury resulting in hemorrhage is a prevalent cause of death worldwide. The current standard of care for trauma patients is to restore hemostasis by controlling bleeding and administering intravenous volume resuscitation. Adequate resuscitation to restore tissue blood flow and oxygenation is critical within the first hours following admission to assess severity and avoid complications. However, current clinical methods for guiding resuscitation are not sensitive or specific enough to adequately understand the patient condition. To better address the shortcomings of the current methods, an approach to monitor intestinal perfusion and oxygenation using a multiwavelength (470, 560, and 630 nm) optical sensor has been developed based on photoplethysmography and reflectance spectroscopy. Specifically, two sensors were developed using three wavelengths to measure relative changes in the small intestine. Using vessel occlusion, systemic changes in oxygenation input, and induction of hemorrhagic shock, the capabilities and sensitivity of the sensor were explored in vivo. Pulsatile and nonpulsatile components of the red, blue, and green wavelength signals were analyzed for all three protocols (occlusion, systemic oxygenation changes, and shock) and were shown to differentiate perfusion and oxygenation changes in the jejunum. The blue and green signals produced better correlation to perfusion changes during occlusion and shock, while the red and blue signals, using a new correlation algorithm, produced better data for assessing changes in oxygenation induced both systemically and locally during shock. The conventional modulation ratio method was found to be an ineffective measure of oxygenation in the intestine due to noise and an algorithm was developed based on the Pearson correlation coefficient. The method utilized the difference in phase between two different wavelength signals to assess oxygen content. A combination of measures from the three wavelengths provided verification of oxygenation and perfusion states, and showed promise for the development of a clinical monitor.

Keywords: hemorrhage; oxygenation; perfusion; shock; spectroscopy.

Fig. 1

(a) Systems diagram showing custom sensors sutured to the exterior of the small intestine at the jejunum, at separate locations. The corresponding signals were acquired using custom hardware and processed with software, as described in Sec. 2.4. Commercial sensors (transit-time flowmeter, arterial catheter, and SpO2 monitor) were placed on the cranial mesenteric artery and tongue (not pictured). Standard operating room (OR) equipment was used to monitor systemic vital signs. (b) The optical sensors used to probe the intestine are shown illuminating at operating intensity. The $470/630\mathrm{nm}$ sensor can be seen on the left and the $470/560\mathrm{nm}$ sensor can be seen on the right.

Fig. 2

Comparison of the signal changes in the (a) nonpulsatile component and (b) pulsatile component in response to the graded occlusions.

Fig. 3

Changes in nonpulsatile signal intensity measured in an optofluidic intestinal phantom in response to changes in flow rate for (a) absorption dominated blood phantom, as compared to (b) the same blood phantom with the addition of scattering particles. The general trend of increased flow leading to decreased signal is preserved in the absorbing case, but, upon addition of the scattering particles, the trend is reduced in the blue wavelength and opposite for the red wavelength.

Fig. 4

Comparison of the pulsatile waveforms for the different wavelengths in vivo. (a) The difference in timing of the peak, as well as the morphology of the signal, can be seen between the blue and red wavelengths ($\text{Pearson correlation coefficient}=-0.565$). (b) The similarity between both the timing and morphology of the signal can be seen between blue and green wavelengths ($\text{Pearson correlation coefficient}=0.999$).

Fig. 5

Optical signal changes in response to oxygenated blood phantom flowing through the intestinal phantom. For the shorter wavelengths, increased flow to the tissue results in reduced signal intensity returning to the detector. This trend is reversed for the longer wavelengths, where absorption does not dominate.

Fig. 6

Modulation ratios and systemic ${\mathrm{SpO}}_2$ versus time during the hypoxia episode for (a) rabbit 1 and (b) rabbit 2. Correlation coefficients between wavelengths on each sensor and the systemic ${\mathrm{SpO}}_2$ plotted over time during the hypoxia episode for (c) rabbit 1 and (d) rabbit 2. Vertical dashed lines indicate the beginning and end of the hypoxic period.

Fig. 7

Changes in response to hemorrhagic shock for (a–d) rabbit 1 and (e–h) rabbit 3 of the pulsatile components of the blue and red sensor (a,e), the pulsatile components of the blue and green sensor (b,f), the nonpulsatile components of blue and red sensor (c,g) and the nonpulsatile components of the blue and green sensor (d,h). The amplitude of the pulsatile components of the blue and green signals across both sensors can be seen to generally decrease during the shock segment (8 to 23 min) and return to near baseline during the recovery segment (25 to 60 min). There was a slight drop after initial recovery at 28 min as excess fluid was added between 28 and 33 min (especially noticeable in the blue pulsatile signal on both sensors and to a lesser extent on the green pulsatile signal). For the nonpulsatile signals, the blue and green signals increased slightly during shock and the red signal decreased slightly while all three nonpulsatile signals decreased upon recovery.

Fig. 8

Plots of the Pearson correlation coefficient between the red and blue signals for (a) rabbit 1 and (b) rabbit 3.