Dual-ratio approach to pulse oximetry and the effect of skin tone

Abstract

Significance: Pulsatile blood oxygen saturation ( ${\mathrm{SpO}}_2$ ) via pulse oximetry is a valuable clinical metric for assessing oxygen delivery. Individual anatomical features, including skin tone, may affect current optical pulse oximetry methods.

Aim: We developed an optical pulse oximetry method based on dual-ratio (DR) measurements to suppress individual anatomical confounds on ${\mathrm{SpO}}_2$ .

Approach: We designed a DR-based finger pulse oximeter, hypothesizing that DR would suppress confounds from optical coupling and superficial tissue absorption. This method is tested using Monte Carlo simulations and in vivo experiments.

Results: Different melanosome volume fractions in the epidermis, a surrogate for skin tone, cause changes in the recovered ${\mathrm{SpO}}_2$ on the order of 1% in simulation and in vivo. Different heterogeneous pulsatile hemodynamics cause greater changes on the order of 10% in simulations. ${\mathrm{SpO}}_2$ recovered with DR measurements showed less variability than the traditional single-distance (SD) transmission method.

Conclusions: For the models and methods considered here, ${\mathrm{SpO}}_2$ measurements are strongly impacted by heterogeneous pulsatile hemodynamics. This variability may be larger than the skin tone bias, which is a known confound in ${\mathrm{SpO}}_2$ measurements. The partial suppression of variability in the ${\mathrm{SpO}}_2$ recovered by DR suggests the promise of DR for pulse oximetry.

Keywords: blood oxygen saturation; dual ratio; hemodynamics; melanin; near-infrared spectroscopy; optical pulse oximetry.

Fig. 1

(a) Schematic of the measurement geometry. Two sources (Src.; 1 and 2) and two detectors (Det.; A and B) were utilized in a transmission geometry through the finger to achieve a DR set. Data were collected from all four possible SD source-detector pairs (i.e., 1A, 1B, 2A, 2B). Src. 1/Det. A and Src. 2/Det. B were spaced 20-mm apart. (b) Photo of the probe which utilized four optical fibers (i.e., one for each optode) and a standard pulse-oximeter finger clip. The finger was oriented, so the sources were on the nail/knuckle side, and Src. 1 was placed behind the nail such that light did not enter the finger through the nail.

Fig. 2

MC finger model. The model consists of a cylinder 80-mm long and 15 mm in diameter with a 15-mm diameter hemisphere representing the fingertip. Source (Src.) 1 is placed at $(15\widehat{x})\mathrm{mm}$, Src. 2 at $(35\widehat{x})\mathrm{mm}$, detector (Det.) A at $(15\widehat{x}+15\widehat{z})\mathrm{mm}$, and Det. B at $(35\widehat{x}+15\widehat{z})\mathrm{mm}$. (a) $xz$ slice at $y=0\mathrm{mm}$ of the finger model. (b)–(d) $yz$ slice at $x=40\mathrm{mm}$. (b) Map of the absorption coefficient (${\mu }_a$) for the lowest volume fraction of melanosomes ([M]) case ($[\mathrm{M}]=0.013$) and the 800-nm wavelength. (c) Map of the reduced scattering coefficient (${\mu }_s^{\prime }$) for the 800-nm wavelength. (d) Slice of the finger model with the same color scale as panel (a).

Fig. 3

Color swatches of each subject’s skin tone obtained from a photo of the back of the subject’s hand. For each subject, the Monk scale value, gray-scale value (using Rec.ITU-R BT.601-7), and normalized red-green-blue (RGB) values are also reported.

Fig. 4

MC-derived average total optical path lengths ($⟨L⟩\mathrm{s}$) and the average difference in total optical path lengths ($\overline{\mathrm{\Delta }⟨L⟩}\mathrm{s}$) for different optical wavelengths ($\lambda \mathrm{s}$) as a function of volume fraction of melanosomes ([M]). Also, see Table 4. (a) $⟨L⟩$ for the SD pair formed by source 1 and detector A (Fig. 2). (b) $\overline{\mathrm{\Delta }⟨L⟩}$ for the DR set [Fig. 2; Eq. (7)]. (c) Ratio of $⟨L⟩$ at 690 nm over $⟨L⟩$ at 830 nm. (d) Ratio of $\overline{\mathrm{\Delta }⟨L⟩}$ at 690 nm over $⟨L⟩$ at 830 nm. Note: Subplots (c) and (d) are on the same scale but not on the same range.

Fig. 5

The sensitivity to local absorption change ($\mathcal{S}$) at 800 nm for the five different modeled tissues (Fig. 2) as a function of the volume fraction of melanosomes ([M]). Solid lines show the $\mathcal{S}$ for the SD measurement type and dashed lines for the DR measurement type.

Fig. 6

Spatial maps of the sensitivity to local absorption change ($\mathcal{S}$) at 800 nm for the SD) (a)–f) or DR (g)–(l) measurement types and volume fraction of melanosomes ([M]) of 0.013 (a), (b), (e), (g), (h), and (k) or 0.430 (c), (d), (f), (i), (j), and (l). Panels (e), (f), (k), and (l) show a zoomed view of the region indicated by the magenta box in panels (a), (c), (g), and (i), respectively.

Fig. 7

Example hemodynamics measured by SD and DR for subject H. (a) Folding average for two periods of the band-passed temporal traces of change in oxy-hemoglobin concentration ($\mathrm{\Delta }[{\mathrm{HbO}}_2]$) and change in deoxy-hemoglobin concentration ($\mathrm{\Delta }[\mathrm{Hb}]$) measured by SD 1A (Fig. 1). (b) Phasors for (a) which have the values: $\widetilde{[{\mathrm{HbO}}_2]}=(7.0\angle -9.1\mathrm{deg})\mathrm{nM}$ and $\widetilde{[\mathrm{Hb}]}=(1.7\angle -20.1\mathrm{deg})\mathrm{nM}$. (c) Same as panel (a) but measured by DR instead. (d) Phasors for (c) which have the values: $\widetilde{[{\mathrm{HbO}}_2]}=(48\angle 1.7\mathrm{deg})\mathrm{nM}$ and $\widetilde{[\mathrm{Hb}]}=(1.7\angle -48.6\mathrm{deg})\mathrm{nM}$. Note: The assumed volume fraction of melanosomes ([M]) for this example is 0.013, and the phase reference is $\mathrm{\Delta }[{\mathrm{HbO}}_2]+\mathrm{\Delta }[\mathrm{Hb}]$ measured by DR; see Sec. 2.4.1 for further details on analysis. Note: See footnote $k$ on page 30 regarding recovered pulsatile blood oxygen saturation (${\mathrm{SpO}}_2$).

Fig. 8

Recovered pulsatile blood oxygen saturation (${\mathrm{SpO}}_2$) (we consider these ${\mathrm{SpO}}_2$ measurements uncalibrated because they do not represent ${\mathrm{SaO}}_2$ due to different partial volume effects, and typical ${\mathrm{SpO}}_2$ techniques would effectively apply a calibration factor to these values to recover an ${\mathrm{SaO}}_2$ surrogate) in vivo from either SD or DR measurements using an assumed volume fraction of melanosomes ([M]) of 0.013 or 0.430. Subjects are ordered from light to dark skin tones according to Sec. 2.4.2 and Fig. 3.

Fig. 9

Simulation of tissue oxy-hemoglobin concentration ($[{\mathrm{HbO}}_2]$) and deoxy-hemoglobin concentration ([Hb]) phasors leading to recovered phasors and pulsatile blood oxygen saturation (${\mathrm{SpO}}_2$) measured by SD or DR. Hemodynamics are only in the dermis (BV and BF) and muscle (only BV). BV oscillations have a saturation of 95%. Volume fraction of melanosomes ([M]) is modeled as 0.013. (a) Dermis phasors: $(2.26\angle 18.3\mathrm{deg})\mu \mathrm{M}$ for $[{\mathrm{HbO}}_2]$ and $(1.11\angle -39.5\mathrm{deg})\mu \mathrm{M}$ for [Hb]. (b) SD recovered phasors: $(1.13\angle 4.4\mathrm{deg})\mu \mathrm{M}$ for $[{\mathrm{HbO}}_2]$ and $(0.17\angle -30.3\mathrm{deg})\mu \mathrm{M}$ for [Hb]. (c) Muscle phasors: $(1.90\angle 0\mathrm{deg})\mu \mathrm{M}$ for $[{\mathrm{HbO}}_2]$ and $(0.10\angle 0\mathrm{deg})\mu \mathrm{M}$ for [Hb]. (d) DR recovered phasors: $(1.20\angle 1.9\mathrm{deg})\mu \mathrm{M}$ for $[{\mathrm{HbO}}_2]$ and $(0.10\angle -22.1\mathrm{deg})\mu \mathrm{M}$ for [Hb].

Fig. 10

Simulated ratios of the changes in optical data ($\mathcal{Y}$) at 830 nm and 690 nm as a function of modeled [M]. Changes in $\mathcal{Y}$ are the numerators of Eq. (1) or Eq. (3) which are also written in the legend for either SD or DR. (a) Simulation with homogeneous BV oscillations in the whole tissue with phasor values of $(0.95\angle 0\mathrm{deg})\mu \mathrm{M}$ for oxy-hemoglobin concentration ($[{\mathrm{HbO}}_2]$) and $(0.05\angle 0\mathrm{deg})\mu \mathrm{M}$ for deoxy-hemoglobin concentration ([Hb]). (b) Simulation with BV oscillations only in the dermis and muscle again with phasor values of $(0.95\angle 0\mathrm{deg})\mu \mathrm{M}$ for $[{\mathrm{HbO}}_2]$ and $(0.05\angle 0\mathrm{deg})\mu \mathrm{M}$ for [Hb]. (c) Simulation with BF and BV oscillations in the dermis and only BV in the muscle. This is the same simulation as Fig. 9, and the simulated phasor values may be found there.

Fig. 11

Simulated recovered pulsatile blood oxygen saturation (${\mathrm{SpO}}_2$) for SD or DR. Solid lines are the case where the true volume fraction of melanosomes ([M]) is known and used to recover ${\mathrm{SpO}}_2$, whereas dashed lines assume a value for [M] of 0.013. For all simulations, the true arterial blood oxygen saturation (${\mathrm{SaO}}_2$) from BV) oscillations is 95%. (a) Simulation with homogeneous BV oscillations in the whole tissue with phasor values of $(0.95\angle 0\mathrm{deg})\mu \mathrm{M}$ for oxy-hemoglobin concentration ($[{\mathrm{HbO}}_2]$) and $(0.05\angle 0\mathrm{deg})\mu \mathrm{M}$ for deoxy-hemoglobin concentration ([Hb]). (b) Simulation with BV oscillations only in the dermis and muscle again with phasor values of $(0.95\angle 0\mathrm{deg})\mu \mathrm{M}$ for $[{\mathrm{HbO}}_2]$ and $(0.05\angle 0\mathrm{deg})\mu \mathrm{M}$ for [Hb]. (c) Simulation with BF and BV oscillations in the dermis and only BV in the muscle. This is the same simulation as Fig. 9, and the simulated phasor values may be found there.