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Respiratory therapists routinely administer supplemental oxygen to hundreds of thousands of patients every year in the emergency room and preoperative and postoperative settings. But do they all need it?

Do we administer so much oxygen to patients because it improves their care, or is it because of fear of litigation? Do we reason that "it can't hurt," and "if a little is good, more is better?"

While it's appreciated that an arterial oxygen level that's extremely low can be detrimental and that very high oxygen levels sustained for long periods can be equally devastating, much less is known about the effects of supplemental oxygen administration. Most information that exists comes from studies conducted before pulse oximeters were regularly available.

During that era, the only way to access oxygenation was to obtain an arterial blood gas. If a patient displayed worrisome symptoms, a blood gas had to be an obtained and analyzed before the clinicians knew if a patient was hypoxemic. That may have taken an hour or more.

Considering those circumstances, it's understandable why studies done at that time promoted giving patients extra oxygen "just in case." But is that philosophy still needed now considering that pulse oximetry is essentially the fifth vital sign and performed on almost all patients (either before or after pain)? With oxygen saturation so readily available, do we need the extra time that giving patients prophylactic oxygen may allow, and does this extra time delay diagnosis of the underlying problem?

For example, all patients will have an increase in arterial saturation when given oxygen. If they hypoventilate, administrating supplemental oxygen could slow detection. Who benefits in this situation? Could administering prophylactic oxygen mask the cause of patients' hypoxemia?

Arterial Oxygen Level
There's an assumption that administration of supplemental oxygen will improve the patient's outcome. To date, however, little research exists on the effects of supplemental oxygen given to patients. That being the case, it makes sense to look at work done in the critical care environment, where a great deal of research has been done involving oxygen delivery.

Modern medicine allows practitioners to manipulate patient's arterial oxygen levels through a variety of techniques such as simple oxygen delivery devices to sophisticated mechanical ventilators. But the question is does this ability to augment arterial oxygen levels allow for increased survival? Decades of research on adult respiratory distress syndrome have shown that there are many therapies that can improve oxygenation but very few that actually change outcomes.

For this reason, a common endpoint of early ARDS research was change in oxygenation. High-frequency jet ventilation, inhaled nitric oxide, extracorporeal circulation membrane oxygenation and liquid ventilation have all been shown to improve oxygenation in our sickest patients, but unfortunately none of these therapies has been shown to improve survival.

The only large multicenter controlled study done that showed a change in outcome in ARDS was the ARDSNet study. This study found a 25-percent decrease in mortality by limiting tidal volume size, plateau pressures and oxygen percentage.  

Does Oxygen Improve Outcomes?
Oxygen has been shown to positively affect outcomes in some situations. Treatment in near-term infants with hypoxemia is a good example. Oxygen is the most powerful pulmonary vasodilator known. This physiologic fact allows newborn infants to transition from fetal circulation to normal extra-uterine circulation. The key to this transition is oxygen.

The same physiology is the key to treating persistent fetal circulation. In this condition, near-term infants who suffer from hypoxemia can revert back to fetal circulation and shunt blood away from the lungs due to high pulmonary artery pressures. Oxygen is the primary treatment. The key to improvement with these children is an elevated arterial oxygen level, which helps to dilate the pulmonary artery and allow blood to flow through the lungs instead of around them.

Another efficacious use of oxygen is while a patient is ruled in or out for an acute myocardial infarction. The American Heart Association recommends administering supplemental oxygen to any patient who's being ruled out for an acute myocardial infarction and has oxygen saturation less than 90 percent. The association also gives a class II recommendation that all patients ruled out for an AMI receive supplemental oxygen.

Another valid use of oxygen would be for pre-oxygenation of a patient prior to an invasive procedure where the patient will be purposely rendered apneic, as when given to a paralytic prior to an attempt at intubation. Pre-oxygenation allows more time to elapse without desaturation during a procedure when it's required that the patient be apneic. 

Is Routine Administration OK?
Current standards of care for treating many conditions call for the administration of supplemental or additional oxygen. Is this wise? Why shouldn't practitioners routinely administer supplemental or additional oxygen?

The best argument questioning routine oxygen administration was made by John Downs, MD, at the 2002 Donald F. Eagan Scientific Lecture during the 48th AARC International Symposium in Tampa, Fla.¹ Dr. Downs is a legend in the field of aesthesia and critical care medicine.

His argument centered on literature that questioned the protective affect of pre-oxygenation in the critical care environment and suggested that routine use of prophylactic oxygen can mask underlining problems and delay recognition and treatment of hypoventilation and shunt. In examining underlining causes of acute hypoxemia, he points to only one as requiring treatment with oxygen.

Generally, humans can tolerate arterial oxygen saturation levels of 80 percent to 90 percent. At roughly 80 percent oxygen saturation, tissue and cells don't actually receive much more oxygen than when the arterial saturation is more than 90 percent. The flat point of the oxygen dissociation curve starts at approximately 80 percent. An oxygen saturation of 90 percent represents an arterial oxygen level of 60 mm Hg. (See oxygen dissociation curve below.)

An important point to remember is that the PaO₂ only accounts for 3 percent of the oxygen in our blood. The other 97 percent is bound to hemoglobin. Super-saturating that 3 percent only goes so far. Another point to consider is that perfusion may be more important than oxygen level in maintaining end organ function.

Too Much Oxygen
The key to damage caused by excessive oxygen levels is the PaO₂ and the length of exposure. Adults exposed to high levels of oxygen for long periods can develop pulmonary fibrosis, and divers may suffer from acute episodes of central nervous system oxygen toxicity. This condition can be caused by a rapid assent, and the incidence of it is directly proportional to the concentration of oxygen in their breathing mixture and the period of time they're exposed to it.

Retinalopathy of prematurity (ROP) is an eye disease that can cause blindness in premature infants. ROP is caused by exposure to excessive amounts of oxygen, and it's associated with infant arterial oxygen levels of greater than 80 mm Hg. High arterial oxygen levels cause constriction of blood vessels in the retina. With prolonged exposure the blood vessels become necrotic, and new vessels form. The new vessels are small and are prone to hemorrhaging, which causes scarring behind the retina.

This process can result in very poor vision or blindness to the infant, but thankfully this only tends to happen in extremely premature infants who were born before 30 weeks of gestation.

In addition to leading to ROP, excessive supplemental oxygen can contribute to bronchopulmonary dysplasia (BPD) in premature infants.

Questions to Ask
Textbooks contain many indications for administering supplemental oxygen, and rarely do we question whether such practices could be harmful to patients. Maybe we should.

Excessive oxygen exposure to premature infants can harm vision and contribute to the development and worsening of BPD. Caregivers may consider limiting prophylactic use of oxygen to situations where a patient will be purposely made apneic.

Ideally, oxygen should only be administered when a patient is shown to clinically need it, and the level given should be the lowest level that derives acceptable clinical outcomes. Unexplained causes of hypoxemia need to be thoroughly investigated to find the source; that may lead to treatment of the cause rather than the symptom.

Reference
1. Downs JB. Has oxygen administration delayed appropriate respiratory care?  Fallacies regarding oxygen therapy. Respir Care. 2003;48(6):611-20.

A respiratory therapist for 20 years, Ryan Grueber, MHA, RRT-NPS, has worked in various academic centers in different roles. He's now the value analysis facilitator for University of Missouri Health Care in Columbia.