Can Oxygen Hurt?

通过Mike McEvoy, PhD, RN, CCRN, REMT-P

EMS providers are taught that oxygen is a drug with tremendous ability to help. For years, the ABCs of medicine have included administration of supplemental oxygen to a wide variety of patients. Recently, oxygen has been found to worsen certain medical conditions such as strokes. There are suggestions in other studies that oxygen may be harmful to patients with cardiac related problems as well. The time has come to rethink the old adage that “Oxygen never hurt anyone.”

One way to avoid problems from inappropriate use of oxygen is to administer it only when needed. The 2000 Advanced Cardiac Life Support (ACLS) protocols call for administration of oxygen based on measurement of a patient’s oxygen saturation, citing dangers of inappropriate use.¹Measurement of oxygen saturation can be done with a technology called pulse oximetry. Over the past decade, this technology has found its way into the fire service as an assessment tool. Pulse oximetry was originally developed for monitoring patients during and after surgery. It was so successful in preventing complications, deaths, and associated lawsuits that it has become a standard vital sign recorded by anesthesia providers. Oximetry is now routinely used in critical care units, emergency departments, and special procedure units in hospitals and clinics.

Questions have existed about the value of pulse oximetry to the prehospital patient and the EMS provider. Today, new evidence on the dangers of oxygen use as well as our tight economic times favor its use. A Canadian study published last year estimated that oximeters would save $2,324 per ambulance per year in oxygen costs.²Several studies have demonstrated that up to 60% of patients given oxygen by prehospital providers do not require supplemental O₂. Perhaps more importantly, prehospital providers have been shown to consistently not give oxygen to 29 to 40% of patients who really require it when they rely on signs and symptoms alone.³

Pulse oximetry uses two light-emitting diodes (LEDs)–one red and the other infrared–transmitted through the patient’s skin, usually a finger or toe. Special probes are available for use on an earlobe, nose, or forehead as well. The LEDs shine through the nail, skin tissue, venous blood, and arterial blood to a photo detector located opposite the LEDs. Oxygenated blood cells absorb more infrared light while deoxygenated blood cells absorb more red light. The photo detector measures that amount of each light absorbed through a process called spectrophotometry and, using that information, the pulse oximeter computes the oxygen saturation. This number is usually displayed as SpO₂%.

Normally, SpO₂is between 95 and 100%. Oxygen is usually administered to patients with saturations below 95%. Saturations below 90% are serious, because blood cells tend to hold onto oxygen molecules rather than release them to the body’s tissues at these lower levels. Current ACLS protocols suggest oxygen administration for patients with saturations below 90%. Oxygen saturations below 70% are life threatening.

What exactly can an oximeter can tell you? First, it gives you some sense of how loaded the blood cells are with oxygen molecules. Second, it can indicate a trend in your patient’s blood oxygen saturation levels that may help you to assess the effects of your treatment.

血氧计有很大的局限性。没有马chine or number can replace good patient assessment. Yet recent data shows that pulse oximeters boost our assessment capabilities by picking up on patients with extremely low oxygen levels that we would not detect otherwise. Here are some limitations of pulse oximetry:

• Inadequate Perfusion–low blood pressure or poor blood flow may cause false readings. Most oximeters produce an error message when this condition exists. If poor perfusion is suspected, compare the oximeter waveform to a palpated pulse or an ECG signal. Consider changing the probe site to one with better blood flow. Remember that probes that are too tight can constrict an extremity and cause poor perfusion.
• Nail Polish–black, blue, and green nail polish all produce falsely low SpO₂readings. Other colors of nail polish have been shown not to affect oximetry readings. If in doubt, remove the nail polish or change the probe site. Keep some nail polish remover wipes with your oximeter. An alternative is turning the oximeter probe so that the LED light shines sideways through a finger (underneath the nailbed).
• Hyperbilirubinemia and Dark Skin Pigmentation–jaundice and dark skin colors have been shown to have no effect on the accuracy of oximeter readings.
• Ambient Lighting–oximeter probes cost several hundred dollars because they emit and sense infrared light. Sunlight has been shown to interfere with oximeter readings, and most oximeters produce an error message when excessive ambient light is detected. Indoor fluorescent and xenon arc lighting can both produce a falsely lowered oximeter reading. If ambient light interference is suspected, cover the probe site with a towel, blanket, or washcloth.
• Sickle Cell Anemia–no effect on the accuracy of oximeter readings.
• Carboxyhemoglobenemia–a great deal of research has been focused on oximetry in patients with elevated blood levels of carbon monoxide (CO). It is believed that carbon monoxide (which binds to hemoglobin just like oxygen) is erroneously sensed as oxygen by oximeters. Oximeter models vary in their ability to differentiate between oxyhemoglobin and carboxyhemoglobin (i.e., blood cells saturated with oxygen versus carbon monoxide). If your oximeter can’t tell the difference between hemoglobin saturated with oxygen versus hemoglobin saturated with carbon monoxide, then a patient with CO poisoning will have a falsely elevated SpO₂reading. Because of this, you should not rely on an oximeter reading in patients who are known or suspected to have elevated blood levels of CO. This would include smoke inhalation, burns near the face or occurring in a closed area, and heavy cigarette smokers who have smoked within the past couple hours. Any patient with suspected CO poisoning should receive 100% oxygen. Although the oximeter may not be useful, it has been used to rule out CO poisoning in patients who present with a low SpO₂reading initially. Additionally, in patients with known exposure to high CO levels, a low SpO2 reading would be quite an ominous finding.
• Methemoglobinemia–a very rare genetic condition or use of methylene dyes in a hospital or outpatient clinic may cause elevated levels of methemoglobin. Oximeter readings will be falsely lowered and fixed between 80 and 85% despite any changes in oxygen therapy. Prehospital providers should note that methemoglobinemia is almost always induced by drug therapy and has a near zero likelihood of being encountered in the field.
• Cyanide Poisoning–most literature reports normal oximeter readings with cyanide poisoning. Cyanide kills by interfering with the movement of oxygen from the blood into the tissues (which occurs after the point of oxygen measurement by the oximeter).

Like any medical device, a protocol is essential to effectively implement the use of pulse oximetry. Such a protocol should take into account the limitations of pulse oximetry and reflect current medical practice.

In summary, oxygen can no longer considered a harmless drug. Pulse oximetry is a useful assessment tool for the EMS provider to determine when supplemental oxygen is needed. Oximetry can also save money in unnecessary oxygen administration and perhaps prevent injury to patients in circumstances when oxygen administration could potentially be more harmful than helpful.

References

1. Circulation. 2000; 102 (suppl I): I-209.
2. Howes DW, Field B, Leary T, Jones GR, Brison RJ. “Justification of pulse oximeter costs for paramedic providers.” Prehospital Emergency Care. 4(2):151-5, 2000 Apr-Jun.
3. Brown LH, Manring EA, Kornegay HB, Prasad H. “Can prehospital personnel detect hypoxemia without aid of pulse oximeters?” Am Journal of Emergency Medicine. 14(1):43-4, 1996 Jan.

---

Mike McEvoy, Ph.D., RN, CCRN, REMT-P, is the EMS coordinator for Saratoga County, New York. A former forensic psychologist, he now works in the Cardiac Surgical ICU at Albany Medical Center and teaches at Albany Medical College in NY. He is a paramedic for Clifton Park-Halfmoon Ambulance Corps and medical advisor for West Crescent Fire Department. He presently serves as a member of the New York State EMS Council and the State Emergency Medical Advisory Council and is the EMS director on the Board of the New York State Association of Fire Chiefs.