Pulse Oximeter and CO-oximeters

In the event of carbon monoxide poisoning, pulse oximeters are unable to detect any abnormalities in the oxygenation status of a patient. Because of this, a pulse co-oximeter would be necessary to delineate the actual gas components of the blood.

Carboxyhemoglobin

Normally, carbon monoxide (CO) can be found in small amounts in the atmosphere. But due to pollution and burning of fossil fuels, the air will contain significant quantities of CO. When inhaled (such as getting trapped inside burning structures, or tobacco smoking), CO will accumulate in lethal amounts. Carbon monoxide will bind with hemoglobin component of the red blood cell to form carboxyhemoglobin. Compared to O₂, CO has a higher affinity to hemoglobin which means hemoglobin would prefer binding with CO. Consequently, there wouldn’t be enough oxygen to be delivered to the body cells and the person might suffer from severe hypoxia which could eventually lead to death.

The fact that pulse oximeters are isolated to one type of hemoglobin gives a lot of advantages for pulse co-oximeters. The former only uses two wavelengths of light that could not differentiate carboxyhemoglobin from oxyhemoglobin. Hence, with a pulse oximeter, the SpO₂ of a person would remain normal despite low oxygen levels.

On the other hand, pulse CO-oximeter is a telemetry unit that analyzes multiple wavelengths of light to accurately measure the total amount of hemoglobin including oxyhemoglobin, carboxyhemoglobin, methemoglobin, reduced hemoglobin, and total hemoglobin. It yields fast and reliable results that facilitate timely diagnosis and counseling with the physician. Pulse CO-oximeters utilize all 7 wavelengths to acquire and quantify data on blood constituents. Certain models can also read through the tissues during motion and low perfusion.

Pulse oximetry and CO-oximetry are not tests, but a methodology. They do not generate all the information required to determine the total respiratory status. They cannot be a substitute for arterial blood gas and other tests for respiratory function. However, they may be used for monitoring and early identification of signs of hypoxia and certain cardiopulmonary problems.

Sources of Errors in Pulse Oximeter Readings

Pulse oximetry is just one of the many commonly used monitoring devices in patient care. But this doesn’t spare each device from errors which may be caused by technical problems, or poor understanding of its principles and limitations.

Falsely Low Readings

There are many possible sources of error in pulse oximetry readings. A falsely low result may be due to the following factors:

1.) Poor peripheral arterial perfusion and low blood pressure (hypotension)

The device is intended to calculate variations in light absorbance produced by pulsatile blood. In this way, changes in light absorbance because of the skin and surrounding tissues are eliminated. Any factor that decreases arterial perfusion or vascular pulsations may greatly reduce the capacity of the pulse oximeter to identify and evaluate the light transmissions, hence, inaccuracy of measuring the arterial oxygen saturation. One good example is vaso-occlusive crisis of sickle cell anemia. Physical factors may also include getting a blood pressure reading on the same arm where the fingertip pulse oximeter is attached.

2.) Methemoglobinemia

Considerable levels of methemoglobin could cause readings to fall below towards 85%.

3.) Intravenous dyes

In medicine, a dye called methylene blue, which also used to treat methemoglobinemia, indigocarmine, and indocyanine green may cause momentary disruption in the oximetry readings up to 65%.

4.) Opaque nail polish

Opaque nail polish and synthetic fingernails may cause an interference with the absorption of light.

5.) High intensity light sources

While the instrument is being used, avoid exposure of the probe and the measuring site to intense bright light as this may lower the spot checks.

Falsely High Readings

1.) Carbon monoxide poisoning

Carbon monoxide has a higher affinity to hemoglobin compared to oxygen. When it binds to hemoglobin, it produces carboxyhemoglobin which is inefficient in delivering oxygen to various tissues. Carboxyhemoglobin absorbs much light as oxygen and registers a falsely high oxygen saturation.

Other Error Sources

In some cases, motion can cause considerable changes in the pulse oximeter readings. Nowadays, pulse oximeters can already read through motion that reduces the interference caused by exaggerated movements.

Pulse Oximeter – Device Limitations

In some situations, the pulse oximeter cannot be used as an alternative to other diagnostic studies. Basically, it measures the real-time amount of oxygen dissolved in the blood as well as the pulse rate. However, along with these capabilities are misconceptions about the use of the device and limitations which can sometimes cause inaccurate readings.

Not as an Alternative to ABG

The pulse oximeter cannot be used as an overall substitute to arterial blood gas test. It only measures the amount of oxygen in the blood at the present time, without the pH of the blood, bicarbonate level, and carbon dioxide levels, and therefore it cannot determine the acid-base balance in the body.

Disease Process

Readings may also be inaccurate in some disorders. Because it cannot measure carbon dioxide, a person may still experience respiratory acidosis despite excellent oxygen saturation. This may be due to retained carbon dioxide in the system. However, there are newer devices that have incorporated a pulse CO-oximeter with them that can differentiate oxygen from carbon dioxide.

Oximetry can only read the percentage of oxygen bound to hemoglobin. Severe anemia may lead to inaccurate readings due to the insufficient amount of hemoglobin in the blood despite adequate oxygen concentrations. All the red blood cells may be bound with oxygen so the spot checks are usually normal, but the person may still suffer from tissue hypoxia. Anemia must be addressed first before dealing with the low amount of oxygen delivered to the cells.

Technical Limitations

Because the pulse oximeter uses two wavelengths of light (red and infrared), it can only identify hemoglobin and oxyhemoglobin. It cannot distinguish between oxyhemoglobin (oxygen bound to hemoglobin in red blood cells) from carboxyhemoglobin from carbon monoxide poisoning or heavy smoking. Carboxyhemoglobin may cause falsely high results when present in large amounts. Methemoglobinemia is also a known factor for desaturation as methemoglobin (a form of hemoglobin) cannot bind with oxygen.

Despite these, there is no doubt in the efficacy of the pulse oximeter. When a health worker or a person using it is aware of its limitations and proper usage, only then the device will be considered a useful tool in monitoring one’s health.

Readings and Reliability of Pulse Oximeter Results

For many years, there is no doubt that the pulse oximeter is a revolutionary innovation in patient care because of its unique advantage on continuously monitoring the cardiac and respiratory status of a person.

Oxygen saturation is defined as the amount of oxygen dissolved in the blood, specifically the amount bound to hemoglobin, expressed in percentage. Human blood typically consists of four types of hemoglobin: oxyhemoglobin (O2Hb), reduced hemoglobin (Hb), methemoglobin (MetHb), and carboxyhemoglobin (COHb). The pulse oximeter is designed to measure the amount of oxyhemoglobin, but if methemoglobin and carboxyhemoglobin are present in large quantities, the results would become erroneous.

Interpretation of the Results

After properly wearing the probe, the device will display a number that will indicate the amount of oxygen saturation. A healthy human has a normal blood oxygen saturation value (SpO2) between 95 to 100% (value varies depending on literature used). Should the results go below 90%, the person will be given supplementary oxygen, and he/she will be further evaluated for signs of marked hypoxia.

Accuracy

There are particular limitations to the accuracy of the pulse oximeter to predict oxygen saturation. The accuracy of pulse oximeters vary extensively, most likely due to the different methods or algorithms utilized in calculating the light signals. Generally, oximeters are capable of generating precise readings within oxygen saturations ranges of 80 to 100% and less accurate below 80%.
On rare occasions, the readings may not be accurate. As mentioned under the page dedicated to limitations, clinical conditions may give rise to inaccurate readings, and therefore should be taken always in consideration. The probes may even become a factor in the accuracy, such as in the case of surgery or peripheral arterial occlusion where fingertip probes may have lower accuracy than ear probes.

In some situations, oximeters might have problems. But similar to most monitoring equipment, the results should at all times be viewed in view of the person’s medical condition. One should not neglect a reading that indicates that the person is starting to be hypoxic.