Oxygen delivery to the tissues each minute is the product of arterial oxygen content and cardiac output. Hence oxygen delivery can be compromised as much by a low haemoglobin concentration or low cardiac output as by a fall in the SaOdos. Following circulation through the tissues, the average oxygen saturation in the venous blood returning to the right side of the heart (mixed venous blood) is typically about 75% in healthy individuals at rest, a figure which implies a considerable “reserve” in the oxygen delivery system. The level of oxygenation of peripheral venous blood, however, will vary depending on local metabolism and oxygen consumption. The reserve in the system is called upon, for example, during exercise when the contracting muscles extract more oxygen such that the saturation of venous blood falls. Relatively greater extraction of oxygen by vital organs also occurs if cardiac output is impaired resulting again in reduction in joingy giriÅŸ mixed venous saturation. The complex regulatory mechanisms involved are reviewed in detail in the physiology section of the British Thoracic Society emergency oxygen guideline .
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O2) and partial pressure (PO2) is described graphically by the oxygen–haemoglobin dissociation curve (ODC) (figure 1). As defined above, SO2 represents the overall percentage of haemoglobin binding sites which are occupied by oxygen. Each haemoglobin molecule can bind reversibly up to four oxygen molecules; in addition, haemoglobin has the property that the binding of one oxygen molecule facilitates the binding of subsequent oxygen molecules. Consequently, the affinity of each haemoglobin molecule for oxygen increases until all four of its binding sites are occupied. This binding of oxygen to the haemoglobin molecule accounts for the increasing slope of the ODC at low levels of oxygenation. At higher oxygenation, the curve flattens off as all the haemoglobin molecules approach full saturation, resulting in the characteristic sigmoid (s-shaped) appearance (figure 1).
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Provided that haemoglobin and circulatory function are normal, a patient’s arterial oxygen saturation (measured either directly on arterial blood, (SaO2), or estimated by pulse oximetry, (SpO2)) gives information about the amount of oxygen that is available to the metabolising tissues.
The partial pressure of oxygen (also known as the oxygen tension) is a concept which often causes confusion. In a mixture of gases, the total pressure is the sum of the contributions of each constituent, with the partial pressure of each individual gas representing the pressure which that gas would exert if it alone occupied the volume. In a liquid (such as blood), the partial pressure of a gas is equivalent to the partial pressure which would prevail in a gas phase in equilibrium with the liquid at the same temperature. With a mixture of gases in either the gas or liquid phase, the rate of diffusion of an individual gas is determined by the relevant gradient of its partial pressure, rather than by its concentration. This allows blood to carry an enormously greater concentration (content) of oxygen than, for example, water (or blood plasma). Measurement of PO2, therefore, does not give direct information about the amount of oxygen carried by blood. The use of arterial PO2 (PaO2) as a valid index of arterial oxygenation is justified because measurements are interpreted with implicit assumptions about the ODC. In addition, PaO2 is important because most oxygen-dependent physiological systems, such as oxygen sensing, respond to changes in PO2 in their microenvironment.