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Introduction

Pulse oximetry is a non-invasive technology that quickly tells doctors your blood oxygen saturation and pulse rate using a simple clip-on device. Originally developed by engineer Takuo Aoyagi in 1972, it leverages the different light absorption properties of oxygenated and deoxygenated hemoglobin. This guide walks through the science and steps behind how a pulse oximeter works, from the basic components to the real-time calculation of SpO2. Whether you're a curious patient or a student of medical technology, following these steps will give you a clear understanding of the process.

What You Need

• A standard pulse oximeter (finger clip style)
• Your own finger or a test subject’s finger
• Two light-emitting diodes (red ~660 nm and infrared ~940 nm)
• A photodiode light detector
• A microprocessor to run the algorithm
• Basic knowledge of light absorption and hemoglobin

Step-by-Step Process of Pulse Oximetry

1. Step 1: Understand the Challenge – Measuring Arterial vs. Venous Blood

   Blood oxygen levels are primarily concerned with arterial blood, which carries oxygen from the lungs to tissues. However, finger tissue contains skin, bone, and both arterial and venous blood. Early oximeters could only measure total light absorption through the tissue, which gave inaccurate results because they included venous blood and other structures. The key insight is to isolate the arterial component by using the pulsatile nature of blood flow.

2. Step 2: Learn the Light Absorption Patterns of Hemoglobin

   Hemoglobin absorbs light differently depending on whether it carries oxygen. Oxyhemoglobin (HbO2) absorbs less red light and more infrared light, whereas deoxyhemoglobin (Hb) absorbs more red and less infrared. This difference is the foundation for measuring oxygen saturation. Pulse oximeters use two specific wavelengths: red (around 660 nm) and infrared (around 940 nm). The ratio of absorption at these two wavelengths correlates directly with the percentage of saturated hemoglobin.

3. Step 3: Place the Device on a Finger

   The pulse oximeter clips onto a thin, translucent body part like a fingertip, toe, or earlobe. On one side of the clip is a pair of LEDs (one red, one infrared) and on the opposite side is a photodiode detector. The finger should be at heart level and free from nail polish or excessive movement for accurate reading.

4. Step 4: Emit Light Through the Tissue

   The LEDs rapidly alternate between red and infrared light, shining through the finger. The light passes through skin, bone, blood, and other tissue. Most of the light is absorbed or scattered; only a fraction reaches the photodiode. The detector measures the intensity of transmitted light for each wavelength separately.

5. Step 5: Detect the Pulsatile Component (The Pulse)

   Every time your heart beats, the volume of arterial blood in the finger increases slightly, causing a temporary increase in light absorption. This pulsatile signal appears as a periodic variation in the transmitted light intensity. The oximeter isolates this AC component (pulsatile) from the constant DC component (tissue, venous blood, bone). This is the breakthrough Aoyagi realized: the pulse itself can be used to filter out the non-arterial contributions.

6. Step 6: Calculate the Ratio of Absorption

   For each heartbeat, the oximeter computes the ratio of the changes in light absorption at red vs. infrared wavelengths. This ratio (R) is defined as:

   

   R = (AC_red / DC_red) / (AC_IR / DC_IR)

   This ratio eliminates the effects of skin color, nail thickness, and baseline absorption. It directly reflects the relative proportions of oxyhemoglobin and deoxyhemoglobin.

7. Step 7: Convert the Ratio to Oxygen Saturation (SpO2)

   The oximeter contains a built-in lookup table or calibration curve derived from empirical studies on healthy volunteers. The R value is mapped to a percentage of oxygen saturation, typically ranging from 70% to 100%. For lower saturations, the relationship becomes nonlinear, so devices are less accurate below 80%. The result is displayed as SpO2 on the screen, along with pulse rate.

8. Step 8: Verify and Monitor Continuously

   Modern pulse oximeters update the reading every few seconds, averaging over several pulses to improve accuracy. They also produce a plethysmograph waveform that shows the pulse amplitude, helping clinicians assess perfusion quality. If the signal is weak (e.g., due to cold hands or low blood pressure), the device may show an error or unstable reading.

Tips for Understanding Pulse Oximetry

• Know the limitations: Pulse oximeters measure functional saturation (oxyhemoglobin vs. total hemoglobin capable of carrying oxygen). They cannot distinguish carboxyhemoglobin or methemoglobin without special multi-wavelength sensors.
• Placement matters: Use a clean, well-perfused finger. Avoid nail polish, artificial nails, or excessive motion. For best results, keep the hand at heart level.
• The history helps: Takuo Aoyagi’s observation of the pulse artifact led to the modern technique. Remember that the pulse signal is not noise—it’s the key to isolating arterial blood.
• Compare readings: In clinical settings, a consistent SpO2 above 95% is usually normal, but values may drop temporarily during exercise or at high altitude. If a reading seems off, check the pleth waveform and try a different finger.
• Not a replacement for arterial blood gas: Pulse oximetry gives spot check or continuous monitoring, but it is not as accurate as a direct blood draw for determining oxygen content and acid-base status.