Principles of Pulse Oximetry

To solve this, pulse oximeters use photoplethysmography. This technique relies on the fact that arterial blood is pulsatile. As the heart beats, a bolus of arterial blood enters the finger, causing a transient increase in volume and, consequently, a transient increase in light absorption.

The device separates the light signal into two components:

1. DC Component (Constant): This represents the light absorbed by non-pulsatile elements, such as bone, tissue, venous blood, and steady-state capillary blood.
2. AC Component (Variable): This represents the light absorbed by the pulsating arterial blood.

By focusing only on the AC component (the change in absorption during a pulse), the oximeter can isolate the arterial blood signal from the background "noise" of the surrounding tissue.

Calculating Oxygen Saturation ($SpO_2$)

The value displayed on the monitor is known as $SpO_2$ (peripheral capillary oxygen saturation). This is an estimate of the arterial oxygen saturation ($SaO_2$) measured via an Arterial Blood Gas (ABG) analysis.

The mathematical relationship used by the device involves the ratio of the pulsatile (AC) to the constant (DC) components for both wavelengths. Let $R$ be the ratio:

$$R = \frac{\left( \frac{AC}{DC} \right)_{red}}{\left( \frac{AC}{DC} \right)_{ir}}$$

The device then uses a calibration curve (an empirical algorithm) to convert this ratio $R$ into a saturation percentage.

💡 Exam Tip: $SpO_2$ vs. $SaO_2$

Always remember that $SpO_2$ is a non-invasive estimate provided by the pulse oximeter, whereas $SaO_2$ is the gold standard measurement obtained directly from arterial blood via an ABG. While they usually correlate closely, $SpO_2$ can be inaccurate in certain physiological conditions.

Clinical Limitations and Interference

Because pulse oximetry relies on light absorption and pulsatile blood flow, several factors can lead to inaccurate readings:

1. Poor Perfusion

If a patient is in shock, hypothermic, or has vasoconstriction (due to medications or cold), the "pulse" (the AC component) becomes very weak. If the device cannot detect a clear pulse, it cannot distinguish arterial blood from tissue, leading to an error or an inability to provide a reading.

2. Dyshemoglobins (Carbon Monoxide)

One of the most dangerous limitations is the presence of Carboxyhemoglobin (COHb). Carbon monoxide binds to hemoglobin much more strongly than oxygen does. Crucially, COHb absorbs light at a wavelength very similar to oxygenated hemoglobin ($HbO_2$). Consequently, a pulse oximeter may "see" COHb as $HbO_2$ and report a falsely high, normal oxygen saturation even though the patient is suffering from severe carbon monoxide poisoning.

3. Motion Artifact

Physical movement of the sensor or the patient's limb can create "pseudo-pulses" that the device mistakes for arterial blood flow, leading to erratic or false readings.

4. External Interference

• Nail Polish: Darker colors (especially blue, black, or green) can block the light transmission, leading to inaccurate readings.
• Ambient Light: Extremely bright surgical lights or direct sunlight can interfere with the light sensors on the device.

Summary Table: Factors Affecting Accuracy