Pulse oximetry

How does pulse oximetry work?

For pulse oximetry measurement, a probe is attached to the finger, the toe or the earlobe. Finger clips and adhesive cuffs are available for selection. Passing light through the patient’s skin, the device measures light absorption. Thus, it determines the oxygen saturation of the blood pigment (haemoglobin) contained in the blood. The patient does not feel the measurement. There is no exposure to radiation involved, nor does the skin overheat. The examination takes only a few seconds or minutes and is absolutely painless. Besides measuring the oxygen saturation, pulse oximeters also display the heart rate in terms of numbers and curves. A reliable measurement of oxygen saturation cannot be made until the pulse curve is stable and the heart rate is completely in accord with the displayed heart rate.

Cyanotic heart defects

In certain heart defects, deoxygenated blood gets into the arterial systemic circulation, which leads to decreased oxygen saturation. This is the case for example in pulmonary atresia, transposition of the great arteries or hypoplastic left heart syndrome.

Not all heart defects are detected before birth. Even children with complex critical congenital heart defects can appear completely normal and inconspicuous in their first days of life. Some of these children still suckle strongly at their mother’s breast and have an almost normal skin colour, even though they have a cyanotic heart defect. In spite of the lack of oxygen, a bluish tint to the skin (cyanosis) does not occur immediately in many cases. Often, there is also no heart murmur, so that not all heart defects are detected by the paediatrician during the second preventive medical examination for infants, which usually takes place between the third and the tenth day of life. As a result, these children often leave the hospital undiagnosed and are only readmitted when their condition suddenly deteriorates due to their heart defect.

Pulse oximetry in newborns

Studies have shown that cases like those described above, which affect approximately a quarter of all children born with a heart defect, can benefit from pulse oximetry, as this method can detect a decrease in oxygen saturation even when there is no visible cyanosis. Several scientific investigations of the past decade have proven that routine pulse oximetry measurements in newborns during their first day of life, i.e. between the sixth and 24^th hour of life, can lead to early detection of these complex and often life-threatening heart defects. In the case of abnormal findings a paediatric cardiologist can establish the exact diagnosis on the basis of an ultrasound examination of the heart.

Earliest possible diagnosis is especially important in the case of such critical congenital heart defects so that the necessary treatments can be initiated in time. Studies have revealed that an early diagnosis can have a positive effect on the course of the disease.

Strengths and weaknesses of pulse oximetry

In both the routine care of heart patients as well as in intensive care and emergency care, pulse oximetry has proven to be an effective and inexpensive method to determine oxygen saturation. Concerning newborns, studies have verified that pulse oximetry has the potential to detect severe congenital heart defects that would otherwise go unnoticed. In this context the method also turned out to be a cost-effective and time-saving alternative to other techniques such as routine echocardiography.

However, it should be noted that pulse oximetry cannot detect all cases of severe congenital heart defects. Normal findings produced by pulse oximetry measurements in the first hours of life are therefore no absolute guarantee that a baby is healthy. Likewise, there is the possibility of false-positive results, which means that the measurements indicate an underlying congenital heart defect which is later not confirmed. Both cases can mean a great emotional burden for the parents. A correct measurement strategy (ideal time of measurement and placement of the probe, adequate threshold values) is therefore highly important. Experts recommend performing the examination on the first day of life and not before the sixth hour of life and placing the probe at the baby’s foot. Besides, they stress the importance of the baby being calm and at rest and showing a good pulse curve.

The following congenital heart defects can be detected by standardised pulse oximetry screening:

• Interrupted aortic arch
• Complex/critical coarctation of the aorta
• Hypoplastic left heart syndrome
• Duct-dependent systemic or pulmonary circulation
• Pulmonary atresia
• Total anomalous pulmonary venous drainage
• Transposition of the great arteries
• Various forms of functionally univentricular heart

Current state of the art and recommendations

In intensive care and emergency care, pulse oximetry is routinely used as a surveillance method. The same holds true for regular follow-up examinations of patients with congenital heart defects, since pulse oximetry provides an uncomplicated and quick means of measuring both heart rate and oxygen saturation.

Several studies have investigated whether pulse oximetry, used in connection with newborn examinations, promotes the detection of critical congenital heart defects. In particular, four large-scale prospective investigations from Sweden, Germany, the UK and Poland provided strong arguments for the introduction of routine pulse oximetry screening as a supplement to the usual newborn screenings.

All over the world, the use of this screening method within the scope of newborn examinations is currently on the rise and the number of hospitals performing routine pulse oximetry screenings in newborns is growing constantly.