To understand what a drop in oxygen saturations mean, it is important to develop a look, listen and feel approach to assessing your patient. There are many reasons as to why oxygen saturation from a pulse oximeter may be low. To begin to understand oxygen saturations, we will begin with what oxygen saturation means, what haemoglobin is, and how oxygen is transported to the cells and carbon dioxide away. During this process, we will introduce you to the oxygen-haemoglobin dissociation curve.



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An article that discusses respiratory physiology and also respiratory assessment is:
Moore, T. 2007 Respiratory assessment in adults. Nursing Standard, 21, 49, 48-56.




You may also want to revise your knowledge of normal respiratory function by using any good anatomy and physiology text.


What does oxygen saturation mean?

Red blood cells contain a substance known as haemoglobin. Haemoglobin occurs inside red blood cells (erythrocyte) and consist of a globin molecule with four haem molecules attached to iron molecules, linked together by 4 protein chains – 2 α and 2 β. These are the components that bind with gases. For an illustration of haemoglobin, see figure 5.



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Figure 5. The structure of haemoglobin. Source: (Novartis Pharmaceuticals UK Ltd. ,2008) Source: http://164.109.71.105/Thalassaemia/General/Blood.html



This substance is responsible for the transport of oxygen from the alveoli to the cellular level and return carbon dioxide to the alveoli for gaseous exchange. Haemoglobin has an affinity or an attraction to oxygen. The strength of this affinity is determined by a number of factors, including alveolar pressure, pH, body temperature, level of 2,3-DPG and gas solubility in plasma (Marieb & Hoehn, 2010: 830-834). In normal function, haemoglobin has a strong affinity for oxygen at the alveolar level, and a reduced affinity at the cellular level allowing oxygen to be transported to the cells and exchanged for carbon dioxide at the cellular level. When oxygen is bound to haemoglobin, it forms the chemical oxyhaemoglobin. 97% of oxygen in plasma is taken up by haemoglobin forming oxyhaemoglobin. The remainder remains in the plasma (Moore, 2007: 49). The normal oxygen saturation SpO2 is 95-100% for an average adult (Booker, 2008: 31). In the case of patients with COPD, there is often a higher percentage of CO2 in the blood due to poor gas exchange. The body becomes adjusted to this higher level of CO2 and so from an oxygen saturation (SpO2) point of view the patient will often present with a chronically lower level (Lynes & Kelly, 2009: 51).



Watch the video below to see the structure of haemoglobin, and how oxygen is transported and released to the cells. This video will also briefly introduce you to the oxygen haemoglobin dissociation curve.




Video 1. Oxygen transportation. Source http://www.youtube.com/watch?v=WXOBJEXxNEo&feature=player_embedded





Pulse oximetery


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Figure 6. Pulse oximeter. Source http://www.tradekool.com




Pulse oximeters work by shining red light and infrared light through a thin or peripheral area of the body, for example and ear lobe or nail capillary bed. Red light is absorbed by haemoglobin and infrared light is absorbed by oxyhaemoglobin. The amount of both lights is measured on the other side of the oximeter giving a percentage of oxygen saturation (Booker, 2008: 39). The pulse oximeter however does not replace the need for an arterial blood gas analysis in an acutely unwell patient or in signs of a significant exacerbation of illness (Carter & Williams, 2008: 23). To examine the significance of pulse oximetry, it is important to understand how the readings are relevant to the oxygen haemoglobin dissociation curve.




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Figure 7. Oxygen haemoglobin dissociation curve. Source http://www.bronchialtimes.blogspot.com



The oxygen haemoglobin dissociation curve shows the relationship between oxygen saturation (SpO2) as the x axis, and arterial oxygen pressure (PO2) as the y axis. In this figure, the solid curve in the middle indicates that all variables affecting the relationship are within normal limits. The curve may shift to the left or right in the presence of the variables cited in the graph. As you can see saturation of oxygen (SpO2) is in proportion with PO2. For example, an SpO2 of 85% equals a PO2 of 60mmHg. From this it is easy to see that an oxygen saturation of 90% gives a PO2 of 70mmHg. You will also notice that at this point an increase in FiO2 will not affect PO2 in a major way compared to increasing FiO2 when a patient has an SpO2 of 85%. It is also important to note that once an oxygen saturation falls to between 80-85% it will rapidly fall away without the introduction of supplemental oxygen. It is for this reason that we should attempt to keep a patient’s oxygen saturations above 90% (Marieb & Hoehn, 2010: 831). This tool is useful if oxygen saturations are accurate. There are some limitations however in the use of pulse oximetry.



While a pulse oximeter is a valuable tool due to its quick and non invasive application, reliance on readings from a pulse oximeter should be considered in context with the overall clinical picture (Jones & Summers, 2010: 18). Pulse oximeters are not effective in the presence of carbon monoxide poisoning, as they sense carboxyhaemoglobin as oxyhaemoglobin (Booker, 2008: 40-41). Carter and Williams (2008: 20) cite movement artefact, peripheral circulation, and lighting conditions as a small part of the limitations or issues that may generate an inaccurate reading. Added to this may be the presence of cardiac arrhythmia or output, as a pulse oximeter requires an adequate and steady blood supply to the area in which it is applied (Booker, 2008: 31). In addition, Booker (2008: 41) argues that the presence of nail polish can affect the transmission of light via the oximeter, and nail polish or anything obstructing the nail bed should be removed. The pulse oximeter probe should be applied to a finger or toe with good capillary return and the probe should be shielded from any external light source that can interfere with or dilute the transmission of light through the area where the probe is applied (Carter &Williams, 2008: 21). Booker (2008: 31) states that the user of a pulse oximeter should be aware that a person with dark skin pigmentation, high bilirubin levels, or patients who have received an imaging contrast may provide an inaccurate SpO2 reading.



How does all this relate to my patient with COPD?


  • Due to poor gas exchange, patients with COPD will adapt to function with a higher level of circulating CO2, hence they may have a chronically lower SpO2. However, hypoxia should be treated, so supplemental oxygen should aim to maintain SpO2 at 90-92%.

  • Higher CO2 levels lead to decreased pH (acidosis) which can further reduce the affinity of haemoglobin for oxygen, as can an increase in temperature and reducing levels of 2,3-BPG. For this reason, it is important to control temperature in an acute exacerbation of COPD, as well as regularly monitor arterial blood gases (ABGs) to ascertain the blood pH and CO2 levels (see the oxygen haemoglobin dissociation curve).

  • All observations should be viewed in context with the patient. That is, if you found a reading on a pulse oximeter of less than 90% on room air, ask yourself the following – what is the respiratory rate and depth over 60 seconds? Is the patient cold in the extremities suggesting circulation problems? Is the patient using accessory muscles to breathe? Is the patient able to talk to you in full sentences without shortness of breath? If your patient has a normal respiratory rate and depth, and is able to talk to you in full sentences without becoming short of breath, then check the positioning of the pulse oximeter. If their extremities are cold, they may need to be warmed up to encourage blood flow to the periphery. If you have attended to this and are still getting a low reading, then consider applying oxygen. Are they wearing nail polish? When auscultating (listening to their chest via stethoscope), what sounds are you hearing. If you are not sure, seek advice from either a medical officer or consult with a senior registered nurse. There are many ways to learn respiratory assessment and it will add another dimension to your clinical skills to learn the techniques of respiratory assessment.

  • Pulse oximetry readings should not replace the need for arterial blood gas analysis or medical review for an unwell patient. There is another page on this site that explores oxygen further and its associated delivery devices.