Traditionally, cyanosis( blue/gray coloration of skin) has been relied on for detecting hypoxia ( reduction of oxygen in tissues), for example, during post-operative recovery or from severe respiratory disease. But severe hypoxia usually precedes cyanosis. Severe respiratory failure generally occurs when arterial saturation of hemoglobin falls to 85-90%, whereas cyanosis does not usually appear until saturation falls to 75% - the normal saturation of venous blood. Without invasive monitoring, detecting hypoxia in the critical 75-90% range was largely guesswork, leaving medical and nursing staff without clear evidence to guide their practice. Pulse oximetry replaced guesswork with evidence.
Both arterial blood gas (ABG) and pulse oximetry reveal patients’ arterial blood oxygen saturation – the percent to which hemoglobin is filled with oxygen. However, ABG analysis measures arterial blood oxygen saturation at only one moment in time, pulse oximetry measures it continuously. That means caregivers can detect changes in saturation practically as they happen and help spot trends before the patient becomes symptomatic from hypoxemia.
What the words “Pulse Oximeter” literally mean:Pulse = the changes in arterial blood with every heart beat
Oxi = oxygen
Meter = measurement
Pulse oximetry measures both pulse and saturation of hemoglobin. Radial (wrist) pulse is normally easily measured, and wel are all familiar with using radial measurement. Although measurement by pulse oximetry can be useful, it is rarely the reason for using a pulse oximeter. Oximetry is almost always commenced to measure hemoglobin saturation.
Before we get into the specifics of using a pulse oximeter and it’s applications, we need to review the mechanics of how oxygen is transported in the blood. We also need to review what the numbers mean. Basically, a saturation of 97% of the total amount of hemoglobin in the body is filled with oxygen molecules.
A range of 96% to 100% is generally considered normal. Anything below 90% could quickly lead to life-threatening complications.
Hemoglobin is the active oxygen-carrying part of the erythrocyte (red blood cell). Hemoglobin is a compound of iron (hem) and four polypeptide (globin) chains. Each globin chain is linked to one atom of iron, each of which can carry four molecules of oxygen. As each molecule of oxygen contains two atoms of oxygen (O2), each hemoglobin molecule can carry eight atoms of oxygen. This makes hemoglobin a very efficient means of oxygen transport: each gram of hemoglobin can carry 1.34 ml of oxygen.
What is saturation?Human blood carries oxygen in two ways: Dissolved in plasma and attached to hemoglobin. The amount of oxygen in plasma at normal atmospheric pressure is only about 3% of total oxygen carriage. This is measured in ‘blood gases’. Since most oxygen is carried by hemoglobin, there are three factors that will influence the amount of oxygen delivery to body cells:
- Tissue perfusion.
- The amount of hemoglobin.
- The saturation of hemoglobin by oxygen.
If all hem molecules bind with an oxygen molecule (O2), then total body hemoglobin is fully saturated (100% saturated). When breathing air (21% oxygen), it is rare for hemoglobin to be fully saturated. But the high affinity of hem for oxygen causes near-total saturation of arterial blood in health: usually about 97%.
As hemoglobin unloads oxygen to tissues (at capillary levels), intracapillary hemoglobin saturation progressively falls. Normal venous saturation is about 75%. The high affinity of hem for oxygen inhibits uploading of oxygen when saturation is low, so the margin between ‘healthy’ saturation levels (95-98%) and respiratory failure (usually 85-90%) is narrow. If oxyhemoglobin is low (below 90%) inadequate amounts of oxygen will reach body cells!
How Oximeters Work
Saturation (S) is therefore measured in peripheral (p) capillaries; hence saturation of peripheral oxygen (SpO2). In health, this should accurately reflect the arterial saturation of oxygen (SaO2) so oximetry saturations are often called SaO2, although SpO2 remains the more accurate term.
To calculate the difference between full and empty capillaries, oximetry measures light absorption over a number of pulses, usually five. This causes the short delay before readings are obtained. General Oxygen Protocol requires titrating oxygen to maintain an SPO2 of 94% for most patients, and 92% for those who are Carbondioxide ( CO2) retainers.
Pulse oximetry can be measured at any place where a pulse is accessible. In practice, oximetry is usually measured on fingers, the earlobe or, with infants, the bridge of the nose.
Pulse oximeters are used to monitor patients who have actual or potential respiratory problems. Although 100% saturation is not normal when breathing air, it can be achieved when supplementary oxygen is given. Oxygen, like any drug, can have toxic effects. So if oximetry consistently shows 100% saturation, patients may be receiving unnecessarily high levels of oxygen. However, 100% saturation may compensate for other problems of oxygen carriage, for example anemia, and nurses should consult medical staff to establish whether any change in oxygen therapy is appropriate.
AlarmsAs a rule of thumb, respiratory failure usually occurs when saturation (SpO2) falls to 90%, although some patients with chronic respiratory disease may tolerate lower saturations. Caregivers should consider the patient’s normal respiratory function and clarify the point at which medical staff needs to be informed of any changes. Alarm limits should be set at a level that identifies any significant change in saturation. Setting lower alarm limits of 90% may be appropriate when saturation is 95%, but inappropriate if saturation is fluctuating at 90-91%. If setting alarm limits below 90%, caregivers should be cautious about the very narrow margin remaining before respiratory failure. Setting a lower alarm limit of 85% or less should always be avoided! Oxygen delivery to tissues, including vital organs, is likely to be inadequate at this level, and such low saturations usually require urgent medical intervention (intubation and artificial ventilation).
Oximetry may be used for ‘spot checks’ or a continuous measurement. Measurements should always be considered in the context of the whole person. A ‘spot check’ or single measurement of hemoglobin saturation might suggest respiratory problems. Example: a patient with no history of chronic respiratory disease who has a saturation of 90% may have an acute problem, such as a chest infection. But the value of isolated measurements is limited and trends are more important than absolute figures. Changes in saturation identify deterioration or improvement, caused either by changes in pathology, response to treatment, or both.Pulse Oximetry Protocol generally states that continuous pulse oximetry is recommended for patients who are requiring FIO2 of 40% or greater.
Limitations & Increasing Reliability of Readings:Again, any ‘spot check’ or any single measurement or observation means little on its own and should be placed in the context of the whole person. Like any aspect of technology, oximetry is an aid to observation and total patient care, not a substitute.
Peripheral vasoconstriction – Oximetry relies on detecting a stable pulse. In order for pulsatile flow to be detected, there must be sufficient perfusion (blood flow) in the monitored areas. If peripheral(surface) pulses are weak or absent, readings can be difficult to obtain. This can give false low measurements compared with central saturation, which perfuses the brain and other vital organs. Patients most at risk for low perfusion states are those with hypotension, hypovolemia (low blood volume), and hypothermia, and of course those in cardiac arrest. Patients who are cold but not hypothermic may have vasoconstriction (narrowing of blood vessels) in their fingers and toes that can also compromise arterial blood flow.
If vasoconstriction is a problem, try moving the sensor to the ear lobe or warming the extremity to enhance perfusion (blood flow).
Dysrhythmias (irregular muscle contraction of the heart) such as atrial fibrillation may cause inadequate and irregular perfusion and unreliably low saturation measurements.
Because pulse measurements are calculated from a very few pulses, heart rate should be measured manually with a blood pressure monitor. With irregular rhythm, apex-radial deficits should be checked, and where there are significant differences, both should be regularly monitored.
Never apply the pulse oximeter sensor on a finger of an arm that is using an automatic blood pressure cuff. Blood flow to the finger will be cut off whenever the cuff inflates.
Other at-risk patients include those on mechanical ventilation, and those with cardiac or respiratory disease that could affect oxygenation. A patient with cardiomyopathy (heart disease), for example, may be at risk for acute heart failure if pulmonary edema develops. Pulse oximetry can identify changes in the patient’s pulmonary status before fulminating symptoms occur, which means early, lifesaving interventions can be implemented.
Motion Artifact – The most common cause of inaccurate SpO2 readings is movement. Movement affects the ability of the light to travel from the light-emitting diode (LED) to the photodetector. Rhythmic movement, such as Parkinsonian tremors and seizure activity, as well as shivering, exercise, and vibrations caused by ground or air transport, can cause problems with detecting saturation and will measure false high pulse readings.
To overcome these problems, move the sensor to the ear as it is usually least affected by motion.
Ambient Light – Because pulse oximeters measure the amount of light transmitted through arterial blood, bright light that shines directly on the sensor –whether from the sun or an overhead exam light can skew the readings.
To fix this problem, simply move the sensor or cover it with something opaque, like a washcloth.
Anything that absorbs light may cause false-low readings.
Possible sources of error include:
Dried blood which should always be removed for infection control as well as for aesthetic reasons.
- Nail polish - The darker the polish, the more likely that the SpO2 reading will be inaccurate. Blue, black, and green polishes cause the most problems. If unable to remove polish, place the probe on an ear lobe, a toe, or position the probe sideways on the finger, rather than across the nail bed (you may need to tape the sensor in place).
- Intravenous dyes can reduce readings by absorbing light, so when intravenous dyes have been given, caregivers should check what the half-life of the dye is.
Abnormal hemoglobins – Pulse oximeters cannot differentiate between different forms of saturated hemoglobin. Oximetry measures the percentage of hemoglobin that is saturated by oxygen. But oxygen carriage also depends on the amount of hemoglobin available. Hemoglobin levels should be considered. Medical staff may wish to obtain a blood sample for lab measurement. If very low the medical staff may wish to prescribe blood to restore oxygen-carrying capacity as in the case of anemia.
Carbon monoxide – Carbon monoxide concentrations in air, and therefore in human blood, are normally insignificant, but significant levels will follow smoke inhalation (fires, cigarettes, traffic exhaust). Hemoglobin affinity for carbon monoxide is twenty times that of hemoglobin’s affinity for oxygen so hemoglobin carries carbon monoxide (carboxy-hemoglobin) in preference to oxygen. Carboxyhemoglobin prevents oxygen binding to hemoglobin, yet being bright red, causes over reading of oxygen saturation (often 100%). Pulse oximetry should be avoided where significant amounts of carbon monoxide have been inhaled until levels have fallen. Significant carboxyhemoglobin levels usually occur in patients admitted from a fire. Cigarette smoking can cause over-readings up to four hours after smoking a cigarette.
Hypercapnia – As well as supplying the body with oxygen, breathing oxygen removes carbon dioxide, a waste product of metabolism. Oximetry measures the saturation of hemoglobin by oxygen, but does not indicate carbon dioxide carriage. Carbon dioxide is used in the production of carbonic acid, the main intravascular acid. Hypercapnia contributes significantly to respiratory acidosis. Nurses should observe patients’ breathing pattern – rate and depth. Poor breathing patterns may mean that blood carbon dioxide remains high. Any concerns should be reported to the medical staff so that appropriate drugs, such as respiratory stimulants, may be prescribed. Nurses should continue to monitor closely the patients’ respiratory pattern, as well as oximetry.
Hemoglobinopathies, such as sickle cell disease, can alter the shape and the function of erythrocytes, causing over or under reading. When reporting measurements, nurses should state the patients’ diagnosis.
Mechanical problems - Like any device, oximeters are not foolproof. You should always periodically compare the pulse reading with the patient’s actual heart rate to be certain they match. Also correlate pulse oximetry readings with ABG results.
If there are any consistencies or readings do not match assessment findings, check for equipment malfunction by putting the sensor on your own finger.
Patient EducationPulse oximetry is a means for monitoring patients’ conditions. Like any monitoring, it is an adjunct to care, which should remain focused on the person and not the machine. Oximetry may be started in an emergency situation, but if time allows and the patient is unfamiliar with oximeters, the nurse should explain what is being measured and why, and answer any questions. Placing the probe on your own finger first can reassure the patient that it is not painful.
Pulse oximetry has provided many clinical areas with a simple, reliable and relatively inexpensive means to monitor the respiratory functions of patients, detecting problems long before cyanosis becomes visible. However monitoring equipment can lull caregivers, and patients, into a false security. Oximeters are sometimes introduced into clinical areas without staff being given sufficient information to understand fully how to interpret the information they provide. Caregivers using oximetry should be aware of their limitations, so that individual measurements can be placed in a more meaningful context of the whole patient.
As the use of pulse oximetry continues to expand, more and more caregivers will use it in their day-to-day care. Understanding the physiology will help you treat the patient, not the numbers. And that will make both you and your patient breathe a little easier.