Oximetry and Capnography

Chapter 9


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

Pulse oximetry is used to measure oxygen saturation (SpO2) by relying on the differential absorption spectra of deoxyhemoglobin (red light, 660 nm) and oxyhemoglobin (infrared light, 940 nm) (Figure 9-1).

The comparative ratio of light absorbance at these 2 wavelengths is calculated and calibrated against direct measurements of arterial oxygen saturation (SaO2) via blood gas measurements. This establishes the pulse oximeter’s measure of arterial saturation (SpO2).


Figure 9-1. Absorption spectra of human hemoglobins. Adapted from Miller RD. Miller’s Anesthesia. 8th ed. Philadelphia, PA: Elsevier; 2015. Copyright 2015, with permission from Elsevier.

Pulse oximeters typically consist of 2 light-emitting diodes, 1 emitting at the red spectrum and the other at the infrared spectrum. At the other end of the diodes, a detector is used to measure nonabsorbed energy. A microprocessor is used to subtract absorption by constant sources like bone and tissue and displays the final signal electronically as a waveform.

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.

Limitations of Pulse Oximetry

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

Abnormal hemoglobins, such as methemoglobin or carboxyhemoglobin (see Figure 9-1)

Nail polish (may need to be removed before testing), skin pigmentation

Darker skin pigmentation can lead to erroneously lower values of SpO2

Ambient (white) light interference

Poor perfusion states (decreased cardiac output, marked hypothermia)

Severe anemia

Intravenous dyes (like methylene blue)

Suboptimal probe positioning

Motion artifact

Pulse oximetry is inaccurate below an SpO2 level of 75%

It is not a good reflector of O2 status beyond 100% (see Figure 9-2)


Principles of Capnography

Capnography is the noninvasive monitoring of the concentration of carbon dioxide (CO2) in expired respiratory gases, presented in a continuous waveform display.

It may be worthwhile (and cost-effective) for offices, especially those that deal with a large volume of technology-dependent children, to have this modality available in the office to enhance monitoring.

Infrared spectrometry is the most commonly used method of capnography.

Infrared radiation passing through the sample chamber is absorbed by CO2.

The remaining unabsorbed radiation is detected by a semiconductor, which converts it into a continuously displayed electrical signal that is directly proportional to the concentration of CO2.

Since CO2 is produced in the tissues, transported in the blood, exchanged in the lungs, and expired through the airways, capnography is an integrated indicator of the functions of the respiratory, cardiovascular, and metabolic systems.

Though frequently used by anesthesiologists as a tool to monitor the adequacy of ventilation in the surgical setting, capnography continues to develop wider applications outside the perioperative setting.

The capnogram obtained during expiration is described in 3 phases (Figure 9-3).

Phase zero represents the gas from the anatomic dead space (trachea) that contains no CO2.


Figure 9-2. The oxygen dissociation curve demonstrates the relationship between partial pressure of oxygen (Po2) and the percentage of hemoglobin saturation with oxygen (Spo2). As can be seen from the curve, the relationship between Po2 and Spo2 is not linear but S-shaped. At higher Po2 levels, the curve flattens out, indicating that there is little incremental increase in Spo2. Fever, acidosis, and increased levels of diphosphoglycerate (DPG) reduce affinity for hemoglobin with oxygen, demonstrating a “shift to the right” for the curve, which leads to unloading of O2 to the tissues.

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.


Figure 9-3. Sample capnogram. PETCO2 = 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)

Resources for Families

Pulse Oximetry (American Thoracic Society). www.thoracic.org/patients/patient-resources/resources/pulse-oximetry.pdf

Using the Pulse Oximeter (World Health Organization). www.who.int/patientsafety/safesurgery/pulse_oximetry/who_ps_pulse_oxymetry_tutorial2_advanced_en.pdf

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Aug 8, 2019 | Posted by in PEDIATRICS | Comments Off on Oximetry and Capnography
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