ABG

Arterial Blood Gas (ABG) Interpretation and Application

Objectives

  • Recognize normal values for arterial blood gas (ABG) levels:

    • pH

    • PaCO2

    • SaO2

    • HCO3-

  • Interpret and plan care for arterial blood gas abnormalities.

  • Discuss the respiratory and metabolic mechanisms and their role in controlling the body’s acid-base balance.

  • Identify clinical manifestations of each acid-base imbalance in the body.

  • Interpret and analyze basic arterial blood gas values related to a patient’s condition.

What are Arterial Blood Gases (ABGs)?

  • Definition: A measurement of systemic gas exchange useful in assessing respiratory status and acid-base balance of a patient.

  • Acid-base balance: The balance of acids and bases that are measured as pH

    • Normal range: 7.35 to 7.45

    • Acidosis: pH < 7.35

    • Alkalosis: pH > 7.45

  • Regulation:

    • The lungs (respiratory system) and kidneys (metabolic system) help regulate acid-base balance.

Major Buffer System

  • Bicarbonate–carbonic acid buffer system: Major extracellular fluid buffer system.

  • Kidney function: Regulate bicarbonate in extracellular fluid (ECF).

  • Lung function: Under control of the medulla, regulate CO2, and subsequently affect carbonic acid in ECF.

Components of an ABG Reading

  • Commonly assessed components of arterial blood gas and their indications:

    • pH (7.35 – 7.45): Measurement of the hydrogen ion concentration - an indication of blood’s acidity or alkalinity.

    • PaCO2 (35 – 45 mmHg): Reflects adequacy of ventilation of the lungs.

    • HCO3- (22-26 mEq/L): Reflects the activity of the kidneys in retaining or excreting bicarbonate.

    • PaO2 (80-100 mmHg): Reflects the body’s ability to pick up oxygen from the lungs.

    • SaO2 (95% - 100%): Ratio of hemoglobin oxygen content to potential maximum oxygen carrying capacity of hemoglobin (often measured by pulse oximetry).

Types of Acid-Base Imbalances

Respiratory Effects

  • Respiratory Acidosis:

    • Conditions: COPD, CHF, respiratory depression (overdose), airway obstruction.

    • Result: Increased CO2 retention.

    • Clinical factors: Sleep apnea, CNS depressants, severe pneumonia, pulmonary edema, overdose, atelectasis, pneumothorax, severe obesity, muscular dystrophy, multiple sclerosis, myasthenia gravis.

  • Respiratory Alkalosis:

    • Conditions: Anxiety, panic, hyperventilation, asthma (acute).

    • Result: CO2 depletion.

    • Clinical factors: Panic attack, stimulated respiratory center (i.e., stroke, septicemia, brain injury), liver failure, panic disorder, hypoxemia.

Metabolic Effects

  • Metabolic Acidosis:

    • Conditions: Gain of fixed acid (e.g., diabetic ketoacidosis, diarrhea, renal failure).

    • Clinical factors: Starvation, salicylate poisoning, propylene glycol toxicity.

  • Metabolic Alkalosis:

    • Conditions: Loss of strong acid or gain of base (e.g., vomiting, nasogastric suctioning, diuretic therapy).

    • Clinical factors: Hypokalemia, hyperaldosteronism, Cushing’s syndrome.

Interpreting Arterial Blood Gases

  • pH Interpretation:

    • Normal: 7.35 – 7.45

    • Acidotic: < 7.35

    • Alkalotic: > 7.45

  • PaCO2 Interpretation:

    • Normal: 35 – 45 mmHg

    • Acidotic: > 45 mmHg

    • Alkalotic: < 35 mmHg

  • HCO3- Interpretation:

    • Normal: 22 – 26 mEq/L

    • Acidotic: < 22 mEq/L

    • Alkalotic: > 26 mEq/L

Compensation Mechanism

  • Types of Compensation:

    • Uncompensated: No help.

    • Partially Compensated: Some help present.

    • Fully Compensated: Total help, with pH returning to normal.

  • When there is an acid-base imbalance, one system compensates for the other to restore balance.

Steps to Interpret ABGs

  1. Patient Assessment: Evaluate clinical manifestations of the patient.

  2. Analyze pH: Determine if it is normal, acidotic, or alkalotic.

  3. Determine the Cause: Assess both CO2 and HCO3- to find the system causing the imbalance.

  4. Check Compensation: Look at the component that is not causing the problem to see if there's partial or complete compensation.

  5. Oxygen Assessment: Assess the PaO2 and O2 saturation; abnormal values indicate hypoxemia.

Goals of ABG Analysis

  • With the given lab values, determine if the interpretation is:

    1. Acidosis or Alkalosis

    • Metabolic or Respiratory

    1. Compensation Level

    • Fully Compensated

    • Partially Compensated

    • Uncompensated

Practice Problems for ABG Interpretation

  1. Practice Problem #1:

    • pH=7.26 | PaCO2=32 | HCO3=18

  2. Practice Problem #2:

    • pH=7.44 | PaCO2=30 | HCO3=21

  3. Practice Problem #3:

    • pH=7.1 | PaCO2=40 | HCO3=18

Allen Test

  • Purpose: Assess blood flow in the ulnar and radial arteries.

  • Procedure:

    • Depress radial and ulnar arteries while the person opens and closes fist to observe blood return.

  • Interpreting the result:

    • Normal: Blood returns via ulnar artery.

    • Occluded: Ulnar artery shows no blood return.

Notes on Arterial Puncture

  • Detailed instructions on how to perform an arterial puncture are essential, as successful collection of arterial blood for analysis is critical in evaluating respiratory and metabolic function.