Oximetry and Capnography

Sankaran Krishnan, MD, MPH

Pulse Oximetry

First described in the 1940s, pulse oximetry is now considered the “fifth vital sign” and has evolved to be the method of choice to monitor the oxygenation status of a patient.

Principles of Pulse Oximetry

•The waveform reflects the pulsatile nature of blood flow and is thus a marker of pulse or heart rate.

•SpO₂ is calculated by converting the absorption ratios with dedicated calibration algorithms stored in the microprocessor of the device. These algorithms were derived from blood gas measurements obtained in healthy volunteers who were breathing standard oxygen concentrations.

•These algorithms are not useful below an SpO₂ level of 75% because it is unethical to expose volunteers to oxygen concentrations that lead to lower SpO₂ levels.

•In most pulse oximeters, displayed SpO₂ represents the mean of the measurements obtained during the previous 3 to 6 seconds.

•Typical measuring sites include the finger, toe, pinna, and lobe of the ear.

•Masimo technology (Irvine, CA) uses a patented signal extraction technique to smooth out motion artifacts. Pulse oximeters equipped with this technology tend to more accurately reflect SpO₂ in young infants and children.

Normal Spo₂ Values

•Normal SpO₂ values in children are not well established.

•Readings vary with age and altitude.

—Typical SpO₂ values in healthy children at sea level range from 97% to 99%.

—In neonates and young infants, typical values are lower, ranging from 93% to 100%.

—Values are lower at higher altitudes.

—SpO₂ values demonstrate diurnal fluctuation, with lower readings in the early morning and peak readings in late afternoon.

Utility of Pulse Oximetry

•Monitoring in respiratory disorders associated with hypoxemia (eg, bronchiolitis, asthma, pneumonia)

•Monitoring during resuscitation

•Neonatal screening for congenital heart disease

•Prevention of hyperoxia, especially in neonates

•It is important to emphasize that pulse oximetry is not used to measure “oxygen level in the blood.” Rather, it is a measure of how well the hemoglobin is saturated with oxygen. There is no established “safe value” to discharge a child from the hospital. Clinical consideration is needed, and a child may be discharged from the hospital with a lower than normal SpO₂ value as long as he or she is clinically well otherwise.

•Erroneous readings may occur in the presence of any of the following:

—In phase 2, the curve increases sharply as CO₂-containing alveolar gas mixes with dead space. As expiration continues, more and more of the alveoli empty, and CO₂ concentration increases rapidly.

—In phase 3, a plateau is reached as end-expiration is reached.

—As the next inspiration begins, the CO₂ level decreases sharply to zero. The point at which the plateau ends, just before inspiration, is referred to as the end-tidal CO₂ concentration.

Clinical Applications of End-Tidal CO₂ Concentration Monitoring

•Confirming endotracheal intubation

•Assessing effectiveness of cardiopulmonary resuscitation

•Monitoring real-time alveolar ventilation in the intensive care unit setting

•Monitoring ventilation in the home ventilator setting (ventilator setting adjustments)

•Monitoring changes in dead space while the patient is receiving ventilator support (eg, mucus plugging, bronchospasm)

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