Basics of oxygen therapy

Nursing, Oct 2003 by Pruitt, William C, Jacobs, Michael

Find out how to choose the right oxygen delivery device for your patient's condition.

ALTHOUGH COMMON as the air we breathe, oxygen is also a drug that can be dangerous if used improperly. In this article, we'll describe when oxygen therapy is indicated and how to administer supplemental oxygen safely and appropriately. First, let's review some basic oxygen facts.

Air transit

Room air consists of about 21% oxygen, 78% nitrogen, and 1% other gases, so the fractional concentration of oxygen in inspired air (F^sub 1^o^sub 2^) is 21%. By giving supplemental oxygen, you can raise the patients F^sub 1^o^sub 2^ to as much as 100% oxygen.

Oxygen is transported to the tissues in two ways: About 97% of oxygen is bound to hemoglobin, and the other 3% is dissolved in plasma. The arterial blood gas (ABG) analysis measures Pao^sub 2^-the pressure of oxygen dissolved in plasma. A value of 80 to 100 mm Hg is considered normal, but will increase if the patient receives a higher oxygen concentration.

The saturation of hemoglobin with oxygen can be measured via pulse oximetry (Spo^sub 2^). A normal Spo^sub 2^ or Sao^sub 2^ value is above 94%. An Sao^sub 2^ or Spo^sub 2^ value below 90% means the Pao^sub 2^ is below 60 mm Hg, indicating that the patient isn't adequately oxygenated.

When to give supplemental oxygen

Your patient needs supplemental oxygen if she has:

* documented or suspected hypoxemia, or deficient oxygenation of the blood, defined as a Pao^sub 2^ below 60 mm Hg, an Sao^sub 2^ or Spo^sub 2^ below 90%, or either value below the desirable range for her clinical situation

* severe trauma

* acute myocardial infarction

* procedural sedation or general anesthesia.

Assess your patient's need for oxygen therapy by monitoring ABG and Spo^sub 2^ values and assessing her clinical condition regularly. Signs of increasing hypoxia (reduced tissue oxygenation despite adequate perfusion) include increasing tachypnea and dyspnea, skin color changes (pale at first, then cyanotic), increasing tachycardia, hypertension, restlessness, and disorientation.

A patient in severe hypoxia will go from tachycardia to bradycardia and from hypertension to hypotension. She'll become increasingly somnolent, confused, lethargic, and, without treatment, comatose.

In most cases, you'll administer higher concentrations of oxygen as her oxygen need increases. (Do this carefully in patients with chronic obstructive pulmonary disease [COPD].) Monitor your patient closely and wean her from therapy as soon as possible to avoid oxygen toxicity and absorption atelectasis. Watch for blunting of the respiratory drive, especially in patients with COPD. Maintaining an F^sub 1^O^sub 2^ of 40% or less usually will prevent these problems.

Getting the air there

Devices to deliver oxygen to the patient fall into four groups: low flow, conserving, high flow, and enclosures. For details about low flow and high flow devices, see A Closer Look at Some Oxygen Delivery Devices.

Low flow devices such as nasal cannulas, simple face masks, and reservoir masks deliver oxygen at rates below the normal patient inspiratory flow rate of about 30 liters/minute. The patient draws in room air along with the supplemental oxygen, so the true F^sub 1^O^sub 2^ can't be determined and can vary greatly. For example, imagine a patient receiving oxygen at 3 liters/minute via nasal cannula. If she's breathing hard and fast, the supplemental oxygen flow will be diluted by the large amount of room air being drawn in, and a relatively low F^sub 1^O^sub 2^ will result. On the other hand, if she's taking very slow, deep breaths, very little room air will be mixed with the supplemental oxygen and her F^sub 1^O^sub 2^ will be high.

Use a nasal cannula for a patient who has adequate ventilation and tidal volume but needs more oxygen. The nasal cannula gives her more freedom than a mask, which may make her feel claustrophobic. It can also be used for patients who are mouth breathers.

Another low flow device, the simple face mask, is indicated for a patient who needs a little higher concentration. The higher flow rate keeps her from rebreathing exhaled carbon dioxide (CO2).

Reservoir masks come in two types: a partial rebreather mask and a non-rebreather mask. Similar in appearance, both masks have a reservoir bag that fills with oxygen, and the bag must be kept inflated. These are considered low flow devices, but can deliver higher oxygen concentrations than a simple face mask. However, a partial rebreather mask has no one-way valves; a non-rebreather mask has a small one-way valve on the outside of the mask and another inside the mask at the top of the bag where it connects to the mask. These valves allow expired CO2 to leave the mask.

Conserving devices art designed to decrease the oxygen flow needed to provide adequate oxygenation. Patients using home oxygen systems that require periodic refilling (for example, cylinders or liquid oxygen systems) can reduce costs related to the number of home visits needed to replenish their supply. (For information about home oxygen without a mask, see Another Option for Home Oxygen Delivery.)