Pulse Oximetry: An Introduction

Pulse Oximetry: An Introduction

Ashoka Reddy Komalla (Kakatiya Institute of Technology and Science, India)
DOI: 10.4018/978-1-5225-5152-2.ch007
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Pulse rate, body temperature, blood pressure, and respiratory rate are four vital signs indicating health status of a patient. Oxygen saturation of arterial blood (SaO2) is regarded as fifth vital sign of health status. Pulse oximeters are used in post-operative intensive care units for monitoring pulse rate and SaO2. They make non-invasive simultaneous estimation of pulse rate and SaO2 using photoplethysmogram (PPG) signals captured at red and IR wavelengths. This chapter describes the concept of oximetry, importance of non-invasive medical measurements, principle of pulse oximetry, and the block diagram approach for the design of pulse oximeters. It also presents an exhaustive review on various methods in-vogue for SaO2 estimation, identifies the problems associated with pulse oximeters. The critical limitation is that commercial pulse oximeters are as accurate as their calibration curves. Finally, it presents state-of-the-art research aimed at performance enhancement of pulse oximeters and directions for future work.
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Blood and Its Composition

About 55% of arterial blood is composed of a liquid called the plasma, 43% of the blood is made of red blood cells (RBCs, also known as erythrocytes), 1.5% white blood cells (leukocytes), and 0.5% platelets (thrombocytes). 90% of plasma is made of water with some proteins and other chemicals dissolved in it. On the other hand RBCs are mainly made of hemoglobin molecules. Hemoglobin is responsible for the transport of oxygen to various other cells of the body (Ganong, 1993). Typically each mm3 of blood contains nearly six million RBCs and each RBC is made of about 280 million hemoglobin molecules. The hemoglobin concentration in blood is between 134 and 173 g/l (Kasper, 2005).

Systemic and Pulmonary Circulation

Blood is circulated throughout the body by the systemic and pulmonary circulation system (Li, 2004). The flow of arterial blood, the replenishment of oxygen and exhaling of carbondioxide are controlled by the cardio-pulmonary system, comprising the heart, lungs and the blood vessels (arteries, veins and capillaries). Arteries (except the pulmonary artery, which carries oxygen depleted blood to the lungs for replenishment of oxygen) carry oxygenated blood from the heart to all parts of the body. Arteries terminate into capillaries and the blood in the capillaries provides oxygen and nutrients to the cells and picks up the waste including carbondioxide from the cells. The capillaries terminate to small veins and the small veins lead to bigger veins. Thus oxygen depleted blood containing additional waste is brought back to the right atrium of the heart through these veins. This blood then passes to right ventricle and gets pushed through the pulmonary artery to the lungs. In the lung capillaries, the exhale of carbondioxide and infusing of oxygen takes place and the oxygen-rich blood from the lungs returns back through the pulmonary veins to the heart, thus completing one cycle.

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