Signal processing and calibration improve blood measurements

Article By : Marc Smith

Analog signal processing of heart pulses from reflective PPG sensor systems extracts data on heart rate, blood oxygen, volumetric behavior, and other parameters.

Pulse oximeters, a type of photoplethysmography (PPG) sensor, are electronic devices that measure a person’s blood-oxygen saturation (SpO₂) and heart-rate (HR) levels. Since their inception in the 1930s, these sensors have found their way into numerous applications such as surgery, post-anesthetic care units, neonatal care and neonatal ICU, emergency care, and respiratory patient monitoring. More recently, these sensors are used in wearable health-and-fitness applications.

A PPG sensor is an instrument that performs a volumetric measurement of an organ (which includes arteries). These sensors produce an optically obtained plethysmogram. Products on the market use specialized, high-performance analog front-ends (AFEs) that drive LEDs and detect reflected optical signals. The raw signals produced can be processed in multiple ways to yield various useful results such as:

• Pulse-oximetry (SpO₂) levels
• HR and HR variability (HRV) levels
• Systolic/diastolic blood volumetric behavior
• Peripheral perfusion index (PPI or PI)
• Pleth variability index (PVI)

This article describes the details of signal processing and calibration of the first two applications on this list: SpO₂ and HR.

SpO₂ fundamentals

A pulse oximeter, also referred to as a pulse ox sensor or monitor, noninvasively estimates arterial oxygen saturation (Sa O₂). Arterial oxygen saturation levels provide information on a patient’s respiratory function where a series of organs are responsible for taking in oxygen and expelling carbon dioxide. The lungs are the primary organs associated with this function, which implements gas exchange as we breathe. Other organs involved include veins and arteries, which are the oxygen and carbon dioxide transport pathways to and from cells, respectively [1,2].

SaO₂ measurements are traditionally done by using an in-vitro blood gas analyzer. Not only is this time-consuming, but it also requires a proper medical lab. Pulse ox, on the other hand, offers a technique for estimating SaO₂ with reasonable accuracy (i.e., 2% for medical-grade instruments) in a quick and non-invasive manner [1,2].

SpO₂ stands for peripheral capillary oxygen saturation. While a capillary is not technically considered an organ, the blood flow stems from arteries (an organ consisting of two or more tissue types). Via pulse ox, functional SpO₂ is measured as the ratio of oxygenated hemoglobin (hemoglobin with oxygen, HbO2) to total hemoglobin (oxygenated and non-oxygenated hemoglobin, HbO₂ + Hb). Non-oxygenated hemoglobin is also referred to as reduced hemoglobin. The formula for this is given by:

Oxygen saturation refers to the fact that hemoglobin molecules have a high affinity for oxygen molecules. So much so that all four iron sites in hemoglobin molecules will typically bond with oxygen molecules. This state, where the maximum number of oxygen molecule bonds exists, is referred to as oxygen saturation. Hemoglobin molecules characteristically have two states: zero oxygen molecules (non-oxygenated hemoglobin, Hb) or four oxygen molecules (oxygenated hemoglobin, HbO₂). Figure 1 below shows a diagram of a hemoglobin molecule and conveys how oxygen will typically bond at four binding sites.

Figure 1 In a hemoglobin molecule, oxygen will typically bond at four binding sites.

While SpO₂ is often referred to as the blood’s “oxygen saturation level,” the term can be somewhat misleading in its interpretation. SpO₂, or SpO₂, is actually a measure of lung function. That is, how well the hemoglobin molecules are binding with oxygen molecules while in the lungs.

SpO₂ levels are commonly presented as a percentage. Table 1 below shows typical pulse ox ranges. Please note that hypoxemia is a condition where arterial blood is oxygen deficient. Hypoxemia can cause hypoxia, a condition where body organs and tissue are oxygen deficient.

[Continue reading on EDN US: Reflective pulse oximetry]

Marc Smith is a former employee of Maxim Integrated. Questions or comments regarding this article may be directed to Steve Koh, steve.koh@maximintegrated.com.

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