Alveolar/Arterial Gradient and Arterial/Alveolar Oxygen Ratio
Synonym/Acronym
Alveolar-arterial difference, A/a gradient, a/A ratio.
Rationale
To assist in assessing oxygen delivery and diagnosing causes of hypoxemia, such as pulmonary edema, acute respiratory distress syndrome, and pulmonary fibrosis.
Patient Preparation
There are no food, fluid, activity, or medication restrictions unless by medical direction. Indicate the type of oxygen, mode of oxygen delivery, and delivery rate as part of the test requisition process. Wait 30 min after a change in type or mode of oxygen delivery or rate for specimen collection.
Normal Findings
Method: Selective electrodes that measure PO2 and PCO2.
| Alveolar/arterial gradient | Less than 10 mm Hg at rest (room air); 20–30 mm Hg at maximum exercise activity (room air) | | Arterial/alveolar oxygen ratio | Greater than 0.75 (75%) |
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Study type: Blood [arterial] collected in a heparinized syringe; related body system: Respiratory system. It is important that no room air be introduced into the collection container because the gases in the room and in the sample will begin equilibrating immediately. The end of the syringe must be stoppered immediately after the needle is withdrawn from the puncture site; follow the facility’s safety protocol regarding sharps. Apply a pressure dressing over the puncture site. Samples should be mixed by gentle rolling of the syringe between the hands to ensure proper mixing of the heparin with the sample, which will prevent the formation of small clots leading to rejection of the sample. The tightly capped sample should be placed in an ice slurry immediately after collection. Information on the specimen label should be protected from water in the ice slurry by first placing the specimen in a protective plastic bag. Promptly transport the specimen to the laboratory for processing and analysis.
A test of the ability of oxygen to diffuse from the alveoli into the lungs is of use when assessing a patient’s level of oxygenation. The A/a gradient and a/A ratio can help identify the cause of hypoxemia (low oxygen levels in the blood) and intrapulmonary shunting that might result from one of the following three situations:
- Collapse of alveoli and associated blood vessels
- Unventilated alveoli with perfusion
- Ventilated alveoli without perfusion
alveolar/arterial (A/a) gradient
The A/a gradient increases as the concentration of oxygen the patient inspires increases. If the gradient is abnormally high, either there is a problem with the ability of oxygen to pass across the alveolar membrane or oxygenated blood is being mixed with nonoxygenated blood (e.g., arterial-venous shunts). Information regarding the A/a gradient can be estimated indirectly using the partial pressure of oxygen (PO2) (obtained from blood gas analysis) in a mathematical formula: A/a gradient = PO2 in alveolar air (estimated from the alveolar gas equation) - PO2 in arterial blood (measured from a blood gas).
An estimate of alveolar PO2 is accomplished by subtracting the water vapor pressure from the barometric pressure, multiplying the resulting pressure by the fraction of inspired oxygen (FIO2; percentage of oxygen the patient is breathing), and subtracting this result from 1.25 times the arterial partial pressure of carbon dioxide (PCO2). The gradient is obtained by subtracting the patient’s arterial PO2 from the calculated alveolar PO2: 
Alveolar PO 2 = [(barometric pressure - water vapor pressure) × FIO 2] - [1.25 × PCO 2].
a/A ratio
The a/A ratio is not dependent on FIO2; it does not increase with a corresponding increase in inhaled oxygen. For patients on a mechanical ventilator with a changing FIO2, the a/A ratio can be used to determine if oxygen diffusion is improving. The a/A ratio reflects the percentage of alveolar PO2 that is contained in arterial PO2. It is calculated by dividing the arterial PO2 by the alveolar PO2: a/A = Pa O2/PA O2.
Contraindications
Arterial puncture in any of the following circumstances:
Inadequate circulation as evidenced by an abnormal (negative) Allen test or the absence of a radial artery pulse.
Significant or uncontrolled bleeding disorder, as the procedure may cause excessive bleeding; caution should be used when performing an arterial puncture on patients receiving anticoagulant therapy or thrombolytic medications.
Infection at the puncture site carries the potential for introducing bacteria from the skin surface into the bloodstream.
Congenital or acquired abnormalities of the skin or blood vessels in the area of the anticipated puncture site such as arteriovenous fistulas, burns, tumors, vascular grafts.
Factors That May Alter the Results of the Study
- Specimens should be collected before administration of oxygen therapy or antihistamines.
- The patient’s temperature should be noted and reported to the laboratory if significantly elevated or depressed so that measured values can be corrected to actual body temperature.
- Exposure of sample to room air affects test results.
- Prompt and proper specimen processing, storage, and analysis are important to achieve accurate results. Specimens should always be transported to the laboratory as quickly as possible after collection. Delay in transport of the sample or transportation without ice may affect test results.
Before the Study: Planning and Implementation
Teaching the Patient What to Expect
- Discuss how this test can help to assess respiratory status and identify the cause of respiratory problems.
- Explain that an arterial blood sample is needed for the test and advise rest for 30 min before specimen collection.
- Address concerns about pain, and explain that an arterial puncture may be painful. The site may be anesthetized with 1% to 2% lidocaine before puncture.
- Inform the patient that specimen collection takes about 10 to 15 min.
Potential Nursing Actions
- Ensure an ice slurry is prepared and available for specimen transport.
- Perform an Allen test before radial artery puncture to ensure that the patient has adequate collateral circulation to the hand. The modified Allen test is described in the study titled “Blood Gases.”
- Avoid burned, scarred, or tattooed areas for specimen collection.
Safety Considerations
- Ensure that the person collecting the specimen is notified beforehand if the patient is receiving anticoagulant therapy or taking aspirin or other natural products that may prolong bleeding from the puncture site.
- Assess if assistance will be required to keep the patient from moving during specimen collection.
After the Study: Implementation & Evaluation Potential Nursing Actions
Avoiding Complications
- Consider that samples for A/a gradient evaluation are obtained by arterial puncture, which carries a risk of bleeding, especially in patients with bleeding disorders or who are taking medications for a bleeding disorder.
- Apply pressure to the puncture site for at least 5 min in the unanticoagulated patient and for at least 15 min in a patient receiving anticoagulant therapy.
- Observe/assess puncture site for bleeding or hematoma formation.
- Apply pressure bandage, monitor circulation.
Treatment Considerations
- Consider respiratory and oxygenation assessment, including respiratory rate, rhythm, work of breathing, hypoxic evaluation, and pulse oximetry to monitor oxygen status.
Safety Considerations
- Institute bleeding precautions for patients who are on coagulation therapy.
Nutritional Considerations
- Note that nutritional status may be compromised for those patients with poor respiratory status.
Clinical Judgement
- Consider how adequate oxygenation is the foundation for body system homeostasis and how altered oxygenation can cause rapid deterioration and death.
Follow-Up and Desired Outcomes
- Demonstrates how to use controlled breathing to improve breathing patterns.
- Acknowledges the importance of continuous oxygen to support homeostasis.
- Achieves and maintains optimal gas exchange.