Background on Pulse Oximetry

Reference: Chapters 1 and 2 of Pulse Oximetry 2nd Edition by John TB Moyle

1. Historical Overview

Johann Heinrich Lambert first described the relationship between light absorption and the amount of substance that absorbs it in 1760, in Augsburg, Germany. August Beer later expanded on Lambert's ideas, publishing what is now known as the Beer-Lambert law in 1851.

The Hewlett-Packard Ear Oximeter

The earliest form of pulse oximetry was the Hewlett-Packard ear oximeter, which served as the standard for comparing initial pulse oximeters. This device used eight distinct wavelengths from an incandescent light source. These wavelengths were guided to the ear's pinna via a fiber optic cable. The same type of cable also guided transmitted light to the detector.

To measure arterial oxygen saturation, the device calculated the light absorption across these eight wavelengths. The device didn't distinguish between arterial and other types of blood, like venous or capillary. It approximated arterial saturation by heating the ear to increase blood flow. However, the device was bulky, required frequent calibration, and was intended mainly for respiratory studies. Despite its limitations, it was a significant advancement in continuous oxygen saturation monitoring.

The Prototype Pulse Oximeter

The prototype pulse oximeter used a halogen lamp as its light source. The light traveled to a fingertip probe through a bundle of glass fibers and then returned to the device via another fiber bundle. The incoming light was filtered at two specific wavelengths—650 nm and 805 nm—and then detected by semiconductor sensors to calculate oxygen saturation.

While pioneering, this prototype had several drawbacks. The probe was heavy, the fiber optic cables were hard to manage, and the device was not sensitive to low pulse pressure. Additionally, the analog electronics were susceptible to drift, and the device didn't strictly adhere to the Beer-Lambert law.

Modern Pulse Oximeters

Most contemporary pulse oximeters use two wavelengths—660 nm and 940 nm—produced by light-emitting diodes. A semiconductor photodetector in a compact probe measures the light, making it easy to attach to the ear or fingertip. A small, lightweight cable connects the probe to the main device. An exception exists for oximeters used near magnetic resonance scanners, where all electronic components are housed in the main unit, and light is transmitted via optical fibers.

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2. Optical Principles in Pulse Oximetry

Hemoglobin Structure

Human hemoglobin is a complex molecule with a molecular weight of 64,585 daltons. It consists of two pairs of polypeptide chains: the alpha-chains have 141 amino acid residues and the beta-chains have 146. Each of these chains is attached to one heme group containing a single iron atom in the ferrous state. This iron atom can reversibly bond with an oxygen molecule.

Though it's easy to say hemoglobin has four chains, this belies its intricate three-dimensional shape, which is crucial for its function. The heme groups attach to the chains through weak bonds, nesting in chain crevices. The 3D structure of these chains also affects optical absorption, a key element in pulse oximetry.

Spectrophotometry Basics

Spectrophotometry allows the measurement of a substance's concentration in a solution. A typical spectrophotometer uses radiant energy to illuminate a test sample in a cuvette. This energy often comes from a broad-spectrum incandescent lamp, and a monochromator filters it into a single-wavelength beam. The absorption of this energy is related to the substance's concentration by the Beer-Lambert law:

$$A = \log(I_0/I) = \epsilon c l$$

Here, $A$ is the absorbance, $c$ is concentration, $l$ is path length, $\epsilon$ is the molar extinction coefficient, $I_0$ is initial intensity, and $I$ is final intensity.

Spectrophotometric Oximetry

Oximetry uses the change in color or electromagnetic energy absorption of hemoglobin as it binds to oxygen. While the Beer-Lambert law is a cornerstone of this approach, its assumptions don't always hold for complex biological systems like blood. Therefore, pulse oximetry calibration is empirical and will be discussed later.

Conventional Pulse Oximeters

These devices typically use two wavelengths, usually 660 nm and 940 nm, to measure absorption through an extremity like a fingertip. They provide a value, $\text{SpO}_2$, roughly equal to arterial hemoglobin saturation ($\text{SaO}_2$). The range for in vivo spectrophotometry is limited to 600-1300 nm due to issues like skin pigment (melanin absorbs strongly below 600 nm) and water absorption (water absorbs strongly above 1300 nm).

The interaction between light and bodily tissues is complex, involving reflection, absorption, and scattering. Despite these complexities, pulse oximetry works reliably, in part due to unique behaviors of arterial blood during the cardiac cycle.

During diastole, the axis of erythrocytes align along the direction of blood flow, allowing light to pass through with minimal scattering. The way red blood cells (erythrocytes) are oriented changes during the heartbeat cycle. When the heart is relaxed (diastole), red blood cells align so their longer dimension runs parallel to the direction of blood flow. During contraction (systole), they rotate to be more perpendicular to the flow, which makes them absorb more light. These changes in orientation affect how much light is reflected back, which is key for devices like pulse oximeters that use reflected light to measure things like heart rate. This idea is also backed by how the electrical properties of moving blood change with the speed of blood flow.

Photoplethysmogram (PPG)

A photoplethysmogram (PPG) is a simple optical technique used to detect blood volume changes in the microvascular bed of tissues. It's commonly used in devices like pulse oximeters and some types of smartwatches to measure parameters like heart rate and oxygen saturation. Essentially, a light source illuminates the skin and a sensor measures the light that's either transmitted or reflected back. As blood pumps through the vessels, the amount of light absorbed changes, giving a waveform that's often similar to an arterial pulse.

Dicrotic notch appears in the PPG waveform and is generally related to the closure of the aortic valve. When the heart pumps, blood is ejected into the aorta and travels through the arteries. After the systolic phase (heart's contraction), the aortic valve closes to prevent backflow of blood into the left ventricle. This closure creates a small, brief increase in aortic pressure, which manifests as the dicrotic notch in the PPG waveform. The presence of this notch is often considered a sign of a normal, healthy cardiac system, but its characteristics can vary depending on several factors like heart rate, age, and arterial stiffness.

Special Cases and Future Directions

Various experimental approaches aim to improve the accuracy and reliability of pulse oximetry. For example, some have tried applying probes to central locations like the tracheal mucosa or pharynx, demonstrating potential for more accurate readings in specific conditions.