Dr Adrian Leung

Specialist

Introduction

When someone is deprived of oxygen, he cannot survive for more than a few minutes. If there is no blood flowing to one’s brain, he will become vegetative permanently, if not die, within a couple of minutes. This illustrate s the importance of both oxygen and blood.

Blood is pumped to lung by the heart. The reduced hemoglobin (Hb) will combine with the oxygen in the airsac and form Oxyhemoglobin (HbO2). Most of the oxygen content is carried in this way and only 2% of the oxygen is dissolved in the plasma. The HbO will be carried to the tissue organ in the bloodstream. Inside the capillary, the HbO will dissociate. The oxygen will be taken up for metabolism while the reduced hemoglobin will once again be pumped back to the lung and repeat the cycle.

Apple is red; banana is yellow. Anyone knows that. Fresh blood is red and a suffocated person turns purplish blue. This is a basic knowledge and the reason behind is in common. We take the apple as the example. Its skin absorbs most of the light in the light spectrum except red light, which is thus reflected. The reflected light will be picked up by our eyes and perceived as red. The same theory applies. Hemoglobin has different light absorption characteristics under different forms. HbO absorb most of the light except red, thus perceived as red. Reduced Hb will absorb red  light more, but then reflect light of dark purple spectrum. As such, it appears bluish purple. Their difference in light absorption is most significant in the red and infra-red spectrum (600nm~1000nm). Usage of this property will thus allow painless, non-invasive continuous assay of the Hb content. This is the background of  use of pulse oximeter.

History

Using the difference of their light absorption characteristics can be dated back to 1940. At that time, Millikan announced the theory in public. Since then, scholar and research scientist like Brinkman, Wood, Sekely, and Tait further refine the technology. But it was only up till 1964 the oximetry started to be clinically available. Shaw R invented HP47201A self-calibrating oximeter. It was bulky because it housed 8 light bulb with different wavelength. Although the oximeter is reliable and accurate, its bulkiness, costly, and fragility restrained it from extensive clinical usage. In the mid 1970s, Takatan utilised Light-Emitting-Diode (LED) to manufacture a reflective oximeter. It remains bulky and expensive. In 1980s, pulse oximeter was on market. It assumes the difference in light absorption is based on the pulse volume (because the saturation of hemoglobin will not fluctuate within mini-seconds). Based on this assumption, it provide oxygen saturation with good accuracy. With the advance in technology, the production of this oximeter became large scale. As cost of its installation is cheaper, it becomes indispensable in clinical use.

Background theory of current pulse oximeter

By clipping the sensor and LED to the finger tip, the system will work. The above component housed 2 parallel LED emitting light  of 660nm and 940nm wavelength. The lower component housed a sensor to pick up the transmitted light. By means of an analogue digital converter, the signal is quantified. Stronger signal means less light is absorped by soft tissue, bone and blood, and vice versa.

Transmitting type oximeter

Absorption of light depends on the arterial, capillary and venous blood, in addition to skin, bone, and soft tissue. Only the blood component will varies with pulse because the volume pumped in each pulse generated by the heart.

Assuming all variation in light absorption is due to change in blood volume generated by pulse, we can thus subtract the constant fraction of absorption due to skin, bone and soft tissue. The difference in absorption of the 2 wavelengths will allows saturation be calculated with the equation depicted below.

SaO2=k1R2+k2R+k3.

k1, k2 and k3 are constant variables generated by study on normal subjects. R is the difference of light absorption of the light of 2 wavelength within a finite time. R=$RED:$IR

Since the rate of light absorption difference is equal to the rate of pulse. the pulse oximeter can also provide the pulse rate.

Clinical application of pulse oximeter

Nowadays, pulse oximeter is widely used in the operation theatre, ICU and high dependency unit, resusciatation room, sleep study lab and cardiac unit. In the operation theatre and resuscitation room, the pulse oximeter can provide continuous assessment on the oxygen saturation, in particular to those with poor ventilation and critical patients. For patients requiring oxygen therapy with or without ventilation support, e.g. COAD and sleep apnea patient, the use of pulse oximeter can guide the adjustment of the oxygen dosage and ventilator setting. Since current pulse oximeter can provide swiftly assessment, the use becomes more extensive. It thus can be applicable to any condition when patients have cardiovascular impairment and respiratory disorder. The pulse oximeter can reflect the change in pulse rate and oxygen saturation in seconds.
As the cost of the equipment is lowered with the advance in technology and large scale production, it has extended its use in various nursing unit, including elderly home, and sports centers. Monitoring a patient during chest physiotherapy and sputum suction, and for people undergoing vigorous physical assessment and training provide a much higher safety margin.

Limitation of pulse oximeter

Although pulse oximeter has gained much support in both medical and recreational field, there remains a few scenario that it cannot provide accurate measurement. Since it uses dual wavelength light emission for the absorption profile study, it cannot eliminate the effect of dye or abnormal hemoglobin inside blood. Classical example include intravenous contrast and dye,  bilirubin, carboxyhemogloboin, methemoglobin, fetal hemoglobin, thalaessemia major and sickle cell anaemia patient.

The pulse oximeter also cannot accurately measure the oxygen saturation when the measuring limb is shaky, poor circulation etc. Even we use averaging technique, the slower response and decreased accuracy remain as problems.

As oxygen dissociate rapidly when oxygen saturation was between 45-70% (as shown in oxygen-dissociation curve), therefore, measurement made in the background of saturation less than 70% will have low accuracy and reliability. One should not depends on the reading made by the pulse oximeter for their clinical judgment when he expect the patient has seriously impaired oxygen saturation. However, by the time saturation is as low as 70%, it is almost not compatible with life.
Whenever there is a discrepancy of clinical presentation and the pulse oximeter reading, one should resort to formal blood gas analysis. 

Conclusion

As mentioned above, the technique of non-invasive analysis of blood oxygen saturation by light absorptiometry has became mature and clinically applicable. Due to its production cost being lowered and non-invasive nature, its application field has extended from clinical to recreational. Although it cannot provide very accurate reading in extreme of physiological conditions, it  can unquestionable be able to provide timely and accurate information on oxygen saturation and pulse rate in majority of clinical and recreational scenario. We have confidence that sooner or later it will be a piece of equipment that is indispensable in clinic, hospital, elderly home and sports center.

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