Pulse oximetry

Pulse oximetry is a non-invasive method which uses colorimetric techniques to monitor the oxygenation of a patient's blood. Pulse oximeters are small units that variously can be used in field medicine, at the bedside in a hospital or home care, and in the operating room. They complement more sophisticated blood gas analysis. Particularly in anesthesia or advanced field medicine where an artificial airway is inserted, direct carbon dioxide measurement with capnography is another complementary technique.

Principle

A sensor is placed on a relatively thin part of the patient's anatomy, usually a fingertip or earlobe. In the case of a neonate, the sensor can be placed around the heel of the foot. Then, light at two wavelengths is passed through the skin, and detected by a sensor on the other side of the skin. Changes in the spectrophotometric absorbance of each of the two wavelengths is measured, allowing determination of the absorbances due to the pulsing arterial blood alone. The measurement factors out venous blood, skin, bone, muscle, fat, and even (in most cases) fingernail polish. Based upon the ratio of changing absorbances of the red and infrared light caused by the difference in color between oxygen-carrying (bright red) and non-carrying (dark red or in severe cases blue) hemoglobin in the blood, a measure of oxygen saturation (the percent of hemoglobin molecules bound with oxygen molecules) can be made.

History

Pulse Oximetry was developed by Nellcor Incorporated in 1982, and introduced into the US operating room market in 1983. Prior to its introduction, a patient's oxygenation could only be determined by an arterial blood gas, a single point measure which typically took a minimum of 20-30 minutes processing by a laboratory. (In the absence of oxygenation, damage to the brain starts in 5 minutes with brain death in another 10-15 minutes). In the US alone, approximately $2 billion was spent annually on this measurement. With the introduction of pulse oximetry, a non-invasive, continuous measure of patient's oxygenation was possible, revolutionizing the practice of anesthesia and greatly improving patient safety. Prior to its introduction, studies in anesthesia journals estimated US patient mortality as a consequence of undetected hypoxemia at 2,000 to 10,000 deaths per year, with no known estimate of patient morbidity.

By 1987, the standard of care for the administration of a general anesthetic in the US included pulse oximetry. From the operating room, the use of pulse oximetry rapidly spread throughout the hospital, first in the recovery room, and then into the various intensive care units. Pulse oximetry was of particular value in the neonatal ICU, where the patients can suffer from inadequate oxygen levels, but also can be blinded by excessive concentrations of oxygen. Furthermore, obtaining an arterial blood gas from a neonatal patient is extremely difficult.

Indications

This is useful in any setting where a patient's oxygenation is unstable, including intensive care, surgery, postoperative recovery, emergency and hospital ward settings, pilots in unpressurized aircraft, and determining the effectiveness of or need for supplemental oxygen. Assessing a patient's need for oxygen is often referred to as the ultimate vital sign; human life is not feasible in the absence of oxygen. Although pulse oximetry is used to monitor oxygen delivery to peripheral tissues, it cannot determine the metabolism of oxygen, or the amount of oxygen actually being used by a patient. For this purpose, it is necessary to also measure carbon dioxide (CO₂) levels with arterial blood gas testing.

Limitations

This is a measure solely of oxygenation, not of ventilation, and is not a substitute for arterial blood gas analysis checked in a laboratory or by specialized sensors inserted through intravascular catheters. Oxygenation is generally not limited by ventilation. Hypoxia detectable by pulse oximetry is a relatively late finding in hypoventilation, the earlier finding being hypercarpia (carbon dioxide buildup). Ventilation, as measured by the minute volume, is actually more intimately intertwined with the carbon dioxide level in the blood, and this will build quickly in hypoventilation than hypoxemia. Pulse oximetry gives no indication of carbon dioxide levels, blood pH, or sodium bicarbonate levels. The metabolism of oxygen can be readily calculated by monitoring expired [[CO₂|carbon dioxide]].

Immediately after endotracheal intubation, a formerly hypoventilated patient may "blow off" too much carbon dioxide. The resultant hypocapnia can lead to metabolic alkalosis, and this is also not assessed by pulse oximetry. [{Capnography]], however, does measure the CO₂ Clearly, pulse oximetry is not a replacement for measurement of the partial pressure of oxygen and carbon dioxide afforded by an arterial blood gas test.

Falsely low readings may be caused by hypoperfusion of the extremity being used for monitoring (often due to the part being cold or from vasoconstriction secondary to the use of vasopressor agents); incorrect sensor application; highly callused skin; and movement (such as shivering), especially during hypoperfusion. To ensure accuracy, the sensor should return a steady pulse and/or pulse waveform before a measurement undertaken.

Falsely high or falsely low readings will occur when hemoglobin is bound to a molecule other than oxygen. In cases of carbon monoxide poisoning, the falsely high reading may delay the recognition of hypoxemia (low blood oxygen level). Cyanide poisoning can also give a falsely high reading.