Pulse Oximeter Readings – What Do They Mean?

The finger pulse oximeter detects and assesses changes in oxygen saturation in a record time. Pulse oximeter readings must be accompanied by the percentage of oxygen the person is breathing, and their respiratory rate, for the results to be meaningful.

Blood carries oxygen in two forms, the majority is bound to hemoglobin (oxyhemoglobin) and the rest is dissolved in the aqueous phase of blood (the plasma). The finger pulse oximeter measures the saturation of hemoglobin with oxygen. This is expressed as a percent saturation in which each gram of normal hemoglobin can hold 1.34 milliliters of oxygen.

The dissolved fraction is dependent upon the partial pressure of oxygen. Under normal conditions each 100-ml of blood contains about 20 ml of oxygen bound to hemoglobin and about 0.3 ml dissolved in plasma. The dissolved fraction is available to tissues first, and then the fraction bound to hemoglobin. So as tissues metabolize oxygen or if oxygen becomes difficult to pick up through the lungs the dissolved oxygen and the hemoglobin – bound oxygen will eventually become depleted. The finger pulse oximeter waits to sense the pulse of capillary blood from side of the capillaries, then using two different wavelengths of light calculates the percent of oxyhemoglobin from the total hemoglobin present. If oxygen transfer across the lungs or lung function is compromised and as tissues continue to metabolize oxygen, the percentage of oxyhemoglobin will decrease.

The pulse oximeter reading becomes the quantitative indicator of hypoxia. Pulse oximeter reading may be quantified as mild to moderate hypoxia if presented by a pulse of 90-95%. Moderate to severe hypoxia is represented by a pulse of 80-90%. Severe hypoxia is anything less then 80%, although the accuracy of pulse oximetry decreases generally below about 70%.

The primary utilities of the finger pulse oximeter are to determine the relative severity of cardiopulmonary or airway disease, monitor the effectiveness of therapy and monitor for deterioration in condition. Abnormal hemoglobin saturation is more useful to guide the physician in clinical decision making.