2.3 Pulse Oximetry (SpO2) and Blood Gas Basics

Pulse Oximetry (SpO2) and Blood Gas Basics

1. Principles of Pulse Oximetry (SpO2)

Pulse oximetry is a non-invasive, continuous method for monitoring the functional oxygen saturation of arterial hemoglobin ($SpO_2$). It combines two basic physical principles: spectrophotometry (which uses light absorption to identify substances) and photoplethysmography (which tracks blood volume changes to detect arterial pulses).

Spectrophotometric Principles

Hemoglobin exists in two primary states in healthy arterial blood: oxygenated hemoglobin ($O_2Hb$, carrying oxygen) and deoxygenated or reduced hemoglobin ($HHb$, without oxygen). These two forms of hemoglobin have distinct light absorption characteristics:

• Red Light (660 nm): Deoxygenated hemoglobin ($HHb$) absorbs approximately ten times more red light than oxygenated hemoglobin ($O_2Hb$).
• Infrared Light (940 nm): Oxygenated hemoglobin ($O_2Hb$) absorbs more infrared light than deoxygenated hemoglobin ($HHb$).

Photoplethysmography (PPG) and the Pulse Signal

A pulse oximeter probe contains two light-emitting diodes (LEDs) — one red (660 nm) and one infrared (940 nm) — and a photodetector. The LEDs are placed on one side of a tissue bed (like a finger), and the photodetector is placed on the opposite side. The monitor rapidly cycles the LEDs on and off (Red on, then IR on, then both off) hundreds of times per second. The "both off" phase allows the monitor to measure and subtract ambient light.

The light passing through the tissue is measured by the photodetector. The received signal is divided into two distinct components:

1. DC Component (Constant): Represents the constant absorption of light by non-pulsatile tissue, including bone, muscle, connective tissue, skin, capillary beds, and venous blood.
2. AC Component (Pulsatile): Represents the variable absorption of light caused by the surge of arterial blood into the tissue bed during cardiac systole (pulse). The expansion of the arterial vessel increases the path length and the volume of blood, absorbing more light.

By isolating the AC component, the pulse oximeter filters out the constant absorption of tissue and venous blood, ensuring that it only analyzes the oxygen saturation of the moving arterial blood.

The Ratio-of-Ratios (R) and Calibration

The monitor calculates the Ratio-of-Ratios (R value), which represents the normalized red absorption divided by the normalized infrared absorption: $R = \frac{(AC / DC)_{660}}{(AC / DC)_{940}}$

Because light absorption is highly complex and depends on tissue scattering, the R value cannot be converted to $SpO_2$ using a simple theoretical formula. Instead, manufacturers conduct clinical trials with healthy human volunteers breathing varying mixtures of oxygen to generate an empirical calibration curve.

• An R value of 0.4 typically corresponds to 100% oxygen saturation.
• An R value of 1.0 corresponds to approximately 85% oxygen saturation.
• An R value of 3.4 corresponds to 0% oxygen saturation.

The monitor's microprocessor looks up the calculated R value on this calibration curve to display the patient's $SpO_2$ percentage.

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2. Sensor Types and Placement Considerations

Selecting the proper sensor type and site is essential for signal quality and patient safety.

• Transmissive Sensors: These are the most common sensors, where light passes directly through the tissue bed (finger, toe, earlobe, or infant foot) to a detector on the opposite side.
• Reflective (Backscatter) Sensors: These sensors place the LEDs and the photodetector side-by-side. They measure light that is reflected or backscattered off the bone and deep tissue, commonly applied to the forehead or temple.

[!WARNING] Placement Precautions: Never place a pulse oximeter sensor on the same limb as a blood pressure cuff. When the cuff inflates, it occludes arterial blood flow, causing a loss of the pulsatile (AC) signal, resulting in false alarms or "no signal" errors. Additionally, avoid limbs with arterial lines, venous access lines, or sites with severe local edema.

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3. Clinical Limitations and Sources of Error

While pulse oximeters are highly reliable, several physiological and environmental factors can compromise their accuracy, representing critical troubleshooting scenarios for BMETs.

1. Motion Artifact

Physical movement (such as shivering, tapping, or patient transport) causes rapid changes in the optical path length and tissue thickness. These changes create artificial pulsations that mimic arterial flow. Standard monitors may interpret this motion as a pulse, resulting in a false heart rate (often locking onto the frequency of the movement) and a falsely low $SpO_2$ reading (typically defaulting to approximately 85%, which is the calibration crossover point). Advanced signal extraction technologies (like Masimo SET) use adaptive filters to isolate and discard motion noise.

2. Poor Perfusion (Low Signal-to-Noise Ratio)

In patients experiencing shock, hypothermia, severe hypotension, or vasoconstriction due to vasoactive medications, the blood flow to the extremities is severely reduced. The AC component becomes extremely small, dropping below the sensor's detection threshold. The monitor will display a "Low Perfusion" message or fail to show a reading. BMETs can resolve this by moving the sensor to a central site with higher perfusion, such as the forehead or earlobe, or by warming the patient's hand.

3. Dyshemoglobins (Abnormal Hemoglobins)

Standard dual-wavelength pulse oximeters assume that only two forms of hemoglobin are present ($O_2Hb$ and $HHb$). They cannot distinguish other hemoglobin variants:

• Carboxyhemoglobin (COHb): In carbon monoxide poisoning, CO binds to hemoglobin. COHb absorbs red light (660 nm) in a manner almost identical to oxygenated hemoglobin. The pulse oximeter interprets COHb as $O_2Hb$, resulting in a falsely high $SpO_2$ reading (e.g., 99% when the patient is actually hypoxic). Diagnosis requires a laboratory co-oximeter.
• Methemoglobin (MetHb): Occurs when the iron in hemoglobin is oxidized to the ferric state ($Fe^{3+}$), preventing oxygen binding. MetHb absorbs light heavily at both 660 nm and 940 nm. This drives the R-ratio to 1.0, forcing the monitor to display an $SpO_2$ reading of approximately 85%, regardless of actual oxygenation levels.

4. Ambient Light Interference

Bright external light (surgical lamps, fluorescent ceiling lights, infrared warming lamps, or direct sunlight) can flood the photodetector, saturating the sensor and drowning out the minute changes in the LED light signals. This can be resolved by covering the sensor with a dark, opaque shield.

5. Nail Polish and Skin Pigmentation

Dark nail polish (especially black, blue, or green colors) absorbs red and infrared light, reducing the light reaching the photodetector and causing falsely low $SpO_2$ readings. BMETs should advise clinicians to rotate the sensor (e.g., placing it sideways on the finger) or remove the nail polish.

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4. Blood Gas Basics: SpO2 vs. SaO2

To understand the clinical significance of pulse oximetry, BMETs must understand its relation to Arterial Blood Gas (ABG) analysis.

• Arterial Blood Gas (ABG): An invasive test where a blood sample is drawn directly from an artery (typically the radial or femoral artery) and analyzed in a clinical laboratory. It is the diagnostic gold standard for oxygenation, ventilation, and acid-base balance.
• SaO2 (Arterial Oxygen Saturation): The actual percentage of hemoglobin bound to oxygen in arterial blood, measured directly in the lab using a co-oximeter. Unlike a bedside pulse oximeter, a co-oximeter uses 4 or more wavelengths of light to directly measure and distinguish $O_2Hb$, $HHb$, $COHb$, and $MetHb$.
• PaO2 (Partial Pressure of Oxygen): The pressure exerted by the oxygen dissolved in the physical plasma of arterial blood, measured in mmHg. Normal range is 80 to 100 mmHg.
• PaCO2 (Partial Pressure of Carbon Dioxide): The pressure of carbon dioxide dissolved in blood plasma (normal: 35 to 45 mmHg), indicating the adequacy of lung ventilation.
• pH: Measures the acidity or alkalinity of the blood (normal: 7.35 to 7.45).
• HCO3- (Bicarbonate): Represents the metabolic (renal) component of acid-base balance (normal: 22 to 26 mEq/L).