The amount of oxygen in 100 mL of blood is the total oxygen content.
Plasma carries a small fraction of oxygen called dissolved content, which is directly proportional to the partial pressure of oxygen in the blood (PaO2) measured by arterial blood gases analyzer. Dissolved content is negligible. Whether PaO2 is high or low does not significantly affect the amount of oxygen in the blood. Consequently, adjusting oxygen therapy based on PaO2 values is not supported by physiology. PaO2 (and its derivatives, such as the P/F ratio and the A-a gradient) may, however, provide important information about the lungs’ ability to transfer oxygen.
Hemoglobin carries the bulk of oxygen in the blood (combined content), which is directly proportional to the amount of hemoglobin in the unit of blood and the fraction of hemoglobin sites bound to the hemoglobin (oxygen saturation or SO2). Pulse oximetry can directly measure the oxygen saturation of the arterial blood (SpO2), which is the most practical surrogate for oxygen content and should be used to guide oxygen therapy.
Many do not realize that changes in hemoglobin and saturation affect oxygen content equally. Physiologically, a decrease in hemoglobin from 14 to 7 has the same effect on oxygen content as a decrease in saturation from 100 to 50 percent. The story does not end there. The amount of oxygen available on the tissue level (oxygen delivery) is proportional to oxygen content and cardiac output, which can triple to compensate for hypoxemia. Consider all this when you panic about somebody’s saturation falling below the comfortable level of 90%.
Lastly, one should exercise caution while interpreting saturation reported by the blood gases analyzer (SaO2), which many perceive is more accurate. This notion cannot be further from the truth. The blood gases analyzer does not measure SaO2 but calculates it from PaO2 based on many assumptions that are frequently inaccurate (oxyhemoglobin curve shift), making the value useless. Using SaO2 should only be considered when carbon monoxide poisoning is suspected or the pulse oximetry signal is unreliable.