Proof of concept non-invasive estimation of peripheral venous oxygen saturation, 2017, Khan et al.

[SNT Gatchaman]

Proof of concept non-invasive estimation of peripheral venous oxygen saturation
Khan, Musabbir; Pretty, Chris G.; Amies, Alexander C.; Balmer, Joel; Banna, Houda E.; Shaw, Geoffrey M.; Geoffrey Chase, J.

Pulse oximeters continuously monitor arterial oxygen saturation. Continuous monitoring of venous oxygen saturation (SvO2) would enable real-time assessment of tissue oxygen extraction (O2E) and perfusion changes leading to improved diagnosis of clinical conditions, such as sepsis.

This study presents the proof of concept of a novel pulse oximeter method that utilises the compliance difference between arteries and veins to induce artificial respiration-like modulations to the peripheral vasculature. These modulations make the venous blood pulsatile, which are then detected by a pulse oximeter sensor. The resulting photoplethysmograph (PPG) signals from the pulse oximeter are processed and analysed to develop a calibration model to estimate regional venous oxygen saturation (SpvO2), in parallel to arterial oxygen saturation estimation (SpaO2). A clinical study with healthy adult volunteers (n = 8) was conducted to assess peripheral SvO2 using this pulse oximeter method. A range of physiologically realistic SvO2 values were induced using arm lift and vascular occlusion tests. Gold standard, arterial and venous blood gas measurements were used as reference measurements. Modulation ratios related to arterial and venous systems were determined using a frequency domain analysis of the PPG signals.

A strong, linear correlation (r2 = 0.95) was found between estimated venous modulation ratio (RVen) and measured SvO2, providing a calibration curve relating measured RVen to venous oxygen saturation. There is a significant difference in gradient between the SpvO2 estimation model (SpvO2 = 111 − 40.6*R) and the empirical SpaO2 estimation model (SpaO2 = 110 − 25*R), which yields the expected arterial-venous differences. Median venous and arterial oxygen saturation accuracies of paired measurements between pulse oximeter estimated and gold standard measurements were 0.29 and 0.65%, respectively, showing good accuracy of the pulse oximeter system.

The main outcome of this study is the proof of concept validation of a novel pulse oximeter sensor and calibration model to assess peripheral SvO2, and thus O2E, using the method used in this study. Further validation, improvement, and application of this model can aid in clinical diagnosis of microcirculation failures due to alterations in oxygen extraction.

Link | PDF (BioMedical Engineering OnLine)

 

[SNT Gatchaman]

Part of a PhD thesis (section 8): Non-invasive peripheral perfusion monitoring using photoplethysmography (2016, University of Canterbury, NZ).

 

[SNT Gatchaman]

Conventional pulse oximetry relies on the pulsatile nature of arterial blood and differential absorption of oxyhaemoglobin and de-oxyhaemoglobin at red (RD) and infrared (IR) wavelengths to estimate SpaO2.

Venous blood in the periphery is typically non-pulsatile in nature. Being dependent on pulsatile blood volume changes to make measurements, conventional pulse oximeter sensors can only determine SpaO2. Thus, SvO2 estimation cannot be provided by conventional pulse oximeters (SpvO2). Currently, no available commercial equipment provides continuous, non-invasive measurement or estimate of SvO2.

Reliable SvO2 monitoring is an important element of perfusion monitoring and a necessary measure to determine tissue oxygen extraction capability, which can be used as a clinical marker of microcirculatory failures. In addition, continuous monitoring of the difference between SaO2 and SvO2 would enable the tracking of alterations in tissue perfusion, in real-time. These alterations are very common in sepsis

An artificial pulse generation (APG) system was developed to artificially modulate the peripheral venous blood using a digit pressure cuff. These modulations are detected by a pulse oximeter sensor and the resultant PPG signals are extracted for analysis. The measured transmission from the PPG signal related to venous blood and the SvO2 from gold standard blood gas measurement were used to develop a calibration curve. The main outcome of this study is a proof of the concept and a calibration curve to estimate SpvO2 and accuracy of the sensor.

 

[SNT Gatchaman]

Non-invasive measurement of SvO2 would be helpful in sepsis / cardiogenic shock etc. It might be helpful in LC/ME.

They referenced their device against arterial and venous direct O2 measurements in healthy young adults. The device they were assessing was on the fingertip like the usual pulse-ox meter we're familiar with. Just proximal is a small pressure cuff surrounding the finger that could induce a sub-systolic (50%) pressure, pulsed to affect the venous side.

One of the tests (#3) had another pressure cuff over the upper arm at supra-systolic values to cease all blood flow to the hand for 4-6 minutes and make the muscles ischaemic.

From baseline SvO2 values (measured in blood) of 76-94%, they dropped after 4-6 minutes ischaemic time to 42-78%.

In Test 3, a venous sample was taken immediately before the arm pressure cuff was completely deflated to get a measure of the true ischemic, oxygen desaturated blood.

The recovery of oxygen desaturated blood to baseline oxygen saturation level after ischemia is very rapid during reactive hyperemia in healthy adults. For Subjects 5 and 6, a venous sample was taken approximately 30s after blood flow was restored to the arm. Blood gas analysis of those samples revealed SvO2 had already returned to baseline level.