a/A ratio

The ratio of arterial oxygen partial pressure (a) to alveolar oxygen partial pressure (A), a measure of oxygen transfer across the lung. This figure is normally greater than 0.9.

Alveolar/Arterial Gradient and Arterial/Alveolar Oxygen Ratio

Synonym/acronym: Alveolar-arterial difference, A/a gradient, a/A ratio.

Common use

To assist in assessing oxygen delivery and diagnosing causes of hypoxemia such as pulmonary edema, acute respiratory distress syndrome, and pulmonary fibrosis.

Specimen

Arterial blood (1 mL) collected in a heparinized syringe. Specimen should be transported tightly capped and in an ice slurry.

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%)

Description

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. This test 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: ventilated alveoli without perfusion, unventilated alveoli with perfusion, or collapse of alveoli and associated blood vessels. Information regarding the alveolar/arterial (A/a) gradient can be estimated indirectly using the partial pressure of oxygen (Po₂) (obtained from blood gas analysis) in a simple 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 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 Po₂ = [(barometric pressure – water vapor pressure) × Fio₂] – [1.25 × Pco₂].

The arterial/alveolar (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

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. 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.

This procedure is contraindicated for

Arterial puncture in any of the following circumstances:
high alert Inadequate circulation as evidenced by an abnormal (negative) Allen test or the absence of a radial artery pulse
high alert 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
high alert Infection at the puncture site carries the potential for introducing bacteria from the skin surface into the blood stream
high alert 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

Indications

Assess intrapulmonary or coronary artery shunting
Assist in identifying the cause of hypoxemia

Potential diagnosis

Increased in

Acute respiratory distress syndrome (ARDS) (related to thickened edematous alveoli)
Atelectasis (related to mixing oxygenated and unoxygenated blood)
Arterial-venous shunts (related to mixing oxygenated and unoxygenated blood)
Bronchospasm (related to decrease in the diameter of the airway)
Chronic obstructive pulmonary disease (related to decrease in the elasticity of lung tissue)
Congenital cardiac septal defects (related to mixing oxygenated and unoxygenated blood)
Underventilated alveoli (related to mucus plugs)
Pneumothorax (related to collapsed lung, shunted air, and subsequent decrease in arterial oxygen levels)
Pulmonary edema (related to thickened edematous alveoli)
Pulmonary embolus (related to obstruction of blood flow to alveoli)
Pulmonary fibrosis (related to thickened edematous alveoli)

Critical findings

N/A

Interfering factors

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.
Values normally increase with increasing age (see monograph titled ‘Blood Gases’).
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.

Nursing Implications and Procedure

Pretest

Positively identify the patient using at least two unique identifiers before providing care, treatment, or services.
Patient Teaching: Inform the patient this test can help to assess respiratory status and identify the cause of the respiratory problems.
Obtain a history of the patient’s complaints, including a list of known allergens, especially allergies or sensitivities to latex or anesthetics.
Obtain a history of the patient’s cardiovascular and respiratory systems, especially any bleeding disorders and other symptoms, as well as results of previously performed laboratory tests and diagnostic and surgical procedures.
Note any recent procedures that can interfere with test results.
Obtain a list of the patient’s current medications, including anticoagulants, aspirin and other salicylates, herbs, nutritional supplements, and nutraceuticals (see Effects of Natural Products on Laboratory Values). Note the last time and dose of medication taken.
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.
Review the procedure with the patient, 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 usually takes 10 to 15 min. The person collecting the specimen should be notified beforehand if the patient is receiving anticoagulant therapy or taking aspirin or other natural products that may prolong bleeding from the puncture site.
If the sample is to be collected by radial artery puncture, perform an Allen test before puncture to ensure that the patient has adequate collateral circulation to the hand. The modified Allen test is performed as follows: extend the patient’s wrist over a rolled towel. Ask the patient to make a fist with the hand extended over the towel. Use the second and third fingers to locate the pulses of the ulnar and radial arteries on the palmar surface of the wrist. (The thumb should not be used to locate these arteries because it has a pulse.) Compress both arteries, and ask the patient to open and close the fist several times until the palm turns pale. Release pressure on the ulnar artery only. Color should return to the palm within 5 sec if the ulnar artery is functioning. This is a positive Allen test, and blood gases may be drawn from the radial artery site. The Allen test should then be performed on the opposite hand. The hand to which color is restored fastest has better circulation and should be selected for specimen collection.
Sensitivity to social and cultural issues, as well as concern for modesty, is important in providing psychological support before, during, and after the procedure.
Note that there are no food, fluid, or medication restrictions unless by medical direction.
Prepare an ice slurry in a cup or plastic bag to have ready for immediate transport of the specimen to the laboratory.

Intratest

Potential complications:
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.
Avoid the use of equipment containing latex if the patient has a history of allergic reaction to latex.
Instruct the patient to cooperate fully and to follow directions. Direct the patient to breathe normally and to avoid unnecessary movement.
Observe standard precautions, and follow the general guidelines in Patient Preparation and Specimen Collection. Positively identify the patient, and label the appropriate specimen container with the corresponding patient demographics, initials of the person collecting the specimen, date, and time of collection. Perform an arterial puncture.
Perform an arterial puncture, and collect the specimen in an air-free heparinized syringe. There is no demonstrable difference in results between samples collected in plastic syringes and samples collected in glass syringes. It is very 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. Apply a pressure dressing over the puncture site. Samples should be mixed by gentle rolling of the syringe 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.

Post-Test

Inform the patient that a report of the results will be made available to the requesting health-care provider (HCP), who will discuss the results with the patient.
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.
Teach the patient breathing exercises to assist with the appropriate exchange of oxygen and carbon dioxide.
Administer oxygen, if appropriate.
Teach the patient how to properly use incentive spirometry or nebulizer, if ordered.
Intervene appropriately for hypoxia and ventilatory disturbances.
Reinforce information given by the patient’s HCP regarding further testing, treatment, or referral to another HCP. Answer any questions or address any concerns voiced by the patient or family.
Depending on the results of this procedure, additional testing may be performed to evaluate or monitor progression of the disease process and determine the need for a change in therapy. Evaluate test results in relation to the patient’s symptoms and other tests performed.

Related Monographs

Related tests include allergen-specific IgE, α₁-antitrypsin, α₁-antitrypsin phenotyping, blood gases, chest x-ray, d-dimer, echocardiography, eosinophil count, fibrinogen, hypersensitivity pneumonitis, IgE, potassium, PFT, and sodium.
See the Cardiovascular and Respiratory systems tables at the end of the book for related tests by body system.