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.