ARTERIAL BLOOD GAS ANALYSIS

ARTERIAL BLOOD GAS ANALYSIS – Definition, Components of ABG, Purpose, Indication, Interfering Factors, General Instructions, Client and Environment Preparation, Equipment Needed, Obtaining Sample by Direct Puncture, Procedure, Post-Procedural Care, Interpretation of ABG and Clinical Symptoms

The term arterial blood gas (ABG) is used to refer to a collection of parameters that reflect an individual’s ability to maintain an internal environment that allows normal cell function. Arterial blood gases are measured to give an indication of adequacy of ventilation and sufficiency of oxygen. They also indicate the body’s ability to maintain a balance between acids and alkalis known as the acid-base status. This is mainly controlled by respiratory and renal mechanisms.

DEFINITION

Arterial blood gas analysis is done by performing an arterial puncture thereby blood sample is collected for analysis.

COMPONENTS OF ABG

• ABG includes measurements of hydrogen ion concentration or pH, base excess (BE), Bicarbonate (SBC) and partial pressure of oxygen and carbon dioxide, PaO₂ and PaCO₂
• Base excess is a measurement that estimates the degree of metabolic acidosis. It refers to the amount of base (alkali) that is needed to restore to a normal 7.4. Negative base excess is referred to as a base deficit (acid surplus)
• Bicarbonate is measured by means of standard bicarbonate measurement

Lung excrete acid in the form of CO₂ tension or partial pressure in arterial blood (PaCO₂) which reflects the alveolar ventilation and measure of acid excretion

The partial pressure of oxygen in the blood (PaCO₂) is not used for acid measurements but reflects the oxygenation of the blood

PURPOSE

• To assess the acid-base status
• To assess the degree of oxygenation of blood and adequacy of alveolar ventilation
• To obtain details of ventilatory status
• To provide active medical and nursing interventions
• ABG determinations are used in the management of clients on mechanical ventilators and during the weaning process from the ventilator

Normal Values

• PH: 7.35 – 7.45
• PCO₂: 35-45 mm Hg
• PHCO₂: 21-28 mEq/L
• PO₂: 80-100 mm Hg
• SaO₂: 95-100%

Indication

• Chronic and restrictive pulmonary disease
• Acute respiratory failure
• Acid-base disturbances
• Pulmonary emboli
• Sleep disorders
• Central nervous system dysfunctions
• Cardiovascular disorders such as congestive heart failure, shunts, and intracardiac atrial or ventricular shunts or both

INTERFERING FACTORS

• Noncompliance with proper collection procedure, including air bubbles in syringe and hemolysis of sample
• Low hemoglobin level
• With continuous intra-arterial blood gas monitoring colt formation at sensor tip, sensor lying against arterial wall and transition periods, when a change in FiO₂

GENERAL INSTRUCTIONS

• The primary nursing responsibility is to protect the client from injury during diagnostic procedure
• Before radial puncture for obtaining an arterial specimen for ABGs, the Allen test should be performed to ascertain adequate ulnar circulation
• Critically-ill clients commonly have femoral or radial arterial catheter system from which blood specimens are drawn
• The nurse is responsible for maintaining errors in ABG analysis due to faulty specimen collection and handling
• Excessive amounts of heparin or an air bubble in the syringe will cause inaccurate results

CLIENT AND ENVIRONMENT PREPARATION

• Explain the procedure to the client thoroughly
• Place the patient in comfortable position
• Before a radial artery puncture is executed or a arterial line is inserted, perform an Allen test to ensure adequate collection to the hand
• Prepare ice and heparinized syringe
• Instruct the client about the arterial puncture, it is painful
• If the client is anxious, hyperventilation may occur, giving false reading because of CO₂ is blown off

EQUIPMENT NEEDED

• One ml or 2 ml disposable syringe
• Disposable needles size 20 gauge
• Leur-lock for syringe
• Heparin 1:1000
• Alcohol swab
• Crushed ice in specimen bag
• Disposable gloves and disposable probes
• Arterial catheter for continuous pressure monitoring
• Waterproof pad

OBTAINING SAMPLE BY DIRECT PUNCTURE

• Use heparinized syringe and aseptic technique
• Arterial puncture may be from the radial, branchial, femoral or dorsalis pedis artery
• Assess distal pulses for vascular insufficiency before and after
• Locate artery by feeling for pulsation with two fingers. Leave gap between fingers and insert needle into artery
• Allow syringe to fill under arterial pressure. Remove needle quickly and press on site for 5 minutes before applying dressing
• When using arterial line, turn off three way tap. Discard 2-10 ml of blood. Allow syringe to fill under arterial pressure
• Sample should be analyzed within ten minutes at room temperature or placed on ice and analyzed within 30 minutes

PROCEDURE

• Arterial blood sample of 5 ml is obtained via an arterial puncture or arterial line
• The radial or femoral artery is usually used in adults, whereas the temporal artery is used in infants
• The procedure is usually performed by a physician or respiratory therapist or nurses in specialized units may perform arterial puncture
• If the radial artery is used, the wrist is hyper-extended and the arm is externally rotated
• Palpate the artery for the point of maximal impulse. Cleanse the site with an alcohol swab
• The needle is inserted at a 45-90 degree angle at the point of maximal pulsation
• Observe the syringe; the plunger will move upward under arterial pressure
• Withdraw the needle and cork the syringe with airtight rubber support
• Roll the syringe between your palms to mix the blood with the heparin. Label the syringe and place it on ice.
• Send the specimen to the laboratory immediately with a requestation slip marked with the client’s temperature, the FiO₂ value and the time
• Care of the continuous intra-arterial blood gas line is similar to care of any arterial line

POST-PROCEDURAL CARE

• Immediately after the needle is withdraw, exert pressure on the arterial site for a minimum of 5 minutes
• If the client is taking anticoagulants, pressure on the site should be maintained for at least 10 minutes

INTERPRETATION OF ABG

Results can be interpreted by looking at each component in turn to build up a complete picture. All ABG should be interpreted with the patient’s physical condition. Doubt about the accuracy should be considered if the result does not fit the clinical picture

CLINICAL SYMPTOMS

Metabolic acidosis: it occurs when there is decreased bicarbonate either due to loss or the use in mopping up excess acid. The loss of bicarbonate can be through the gastrointestinal tract (diarrhea) and urinary tract. Clinically, this will because Kussmaul’s respirations, poor peripheral circulation, raised potassium, cardiac dysrhythmia and drowsiness. The treatment depends on underlying cause and administration of intravenous sodium bicarbonate

Metabolic alkalosis: metabolic alkalosis occurs when bicarbonate is raised due to acid loss or excess alkali. Possibly causes are loss of fluid through vomiting, nasogastric suction, gastrocolic fistula, diarrhea and diuretic abuse. It can also result from hyperaldosteronism and Cushing’s syndrome. Raised alkali levels can result from sodium bicarbonate administration and antacid abuse. The clinical effects are confusion, exaggerated reflexes, tetany and convulsions. Treatment is aimed at the primary cause and replacing potassium.

Respiratory acidosis: it is caused by the retention of CO₂ due to hypoventilation. It can result from drug or alcohol overdose, trauma, tumor, myasthenia gravis, Guillain Barre’s syndrome, tetanus, and organophosphorous poisoning. Respiratory causes include conditions which compromise gas exchange such as severe asthma and emphysema. Respiratory acidosis causes diminished mental state, muscle twitching, sweating, peripheral vasodilation, hypertension and cardiac dysrhythmia. Treatment is by reversal of underlying pathology and mechanical ventilatory support to help blow off excess carbon dioxide.

Respiratory alkalosis occurs when there is decreased carbon dioxide in the blood as a result of hyperventilation. Behavioral causes are voluntary or hysterical hyperventilation, pain and anxiety. It can also occur because of neurological problems such as stroke or tumor, respiratory problems such as pulmonary embolus or high altitude

Respiratory alkalosis causes impaired consciousness, seizures, hypotension, hypokalemia increased muscle tone and tetany. The underlying cause should be treated.

Patients who can breathe spontaneously can rebreathe their own carbon dioxide by breathing into a paper bag.

Overproduction of acid can be caused by diabetic ketoacidosis or lactic acidosis resulting from hypoxia, shock, heart failure or liver disease. Outside sources of acidosis include excess level of salicylates such as aspirin and methanol. Renal failure also can lead to decreased excretion of acids.