Clinical Chemistry

Arterial Blood Gas

April 02, 2025 by Olivia R

Blood gas analysis is a test to determine the ability of the lungs to circulate oxygen into the blood and remove carbon dioxide from the blood

A. What is Blood Gas Analysis?

Blood gas analysis is a method of examination to measure the levels of oxygen, carbon dioxide, and pH levels in the blood. Blood gas analysis or ABG will focus on the function of the lungs which are the place where oxygen and carbon dioxide are exchanged.

Red blood cells that carry oxygen and carbon dioxide throughout the body are known as blood gases. When blood passes through the lungs, oxygen will enter the blood, while carbon dioxide will exit into the lungs.

B. Purpose of Arterial Blood Gas examination?

If the results of the blood gas analysis (ABG) examination are not good, it shows signs of an imbalance between oxygen, carbon dioxide, and blood pH levels.

The main "gases" in the blood are oxygen and carbon dioxide. They are present in various forms. The partial pressure of these gases varies with the acid-base balance and functions in the lungs (respiratory), kidneys and heart system (cardiovascular).

C. Sample used for Arterial Blood Gas

• Source: Radial, femoral, or brachial artery
• Advantages:
1. Describes oxygenation and ventilation conditions accurately.
2. Directly reflects gas exchange in the lungs.
• Disadvantage:

• Source: Peripheral veins (usually the arm).
• Advantages:
1. Easier to take
2. Useful for assessing acid-base balance (pH, HCO₃⁻, BE)
• Disadvantages:
1. Not accurate for PaO₂ and PaCO₂ (because the blood has passed through the tissue).
2. PaO₂ value is lower, PaCO₂ is slightly higher than arteries

• Source: Fingertip puncture (adults) or heel (infants)
• Advantages:
1. Minimally invasive, suitable for neonates
2. Can be used if arterial access is difficult
• Disadvantages:
1. Risk of interstitial fluid contamination
2. PaO₂ values are less accurate than arteries

D. ABG Sampling Procedure

• Heparin syringe (to prevent clotting)
• 22–25 G needle (for radial artery)
• 70% alcohol cotton (antisepsis)
• Ice cubes (if the sample is not processed immediately)

• Allen's test (for radial artery):
1. i. Ensure adequate collateral circulation by pressing the radial and ulnar arteries, then release the ulnar. If the hand turns red within 5–7 seconds, the radial artery is safe to use
• Disinfect the area with alcohol
• Puncture the artery at a 45° angle (femoral artery) or 60° (radial artery)
• Allow the blood to fill the syringe passively (avoid pulling the plunger to avoid bubbles)
• Immediately close the needle, avoid air bubbles.
• Rotate the syringe to mix the heparin

• Venous blood: Taken like a regular blood test, but without a tourniquet for too long (so as not to affect lactic acid)
• Capillary blood: Puncture deeply (2–3 mm), allow the blood to flow without squeezing (to avoid hemolysis)

E. ABG Sample Handling

• Analyze immediately (within 15 minutes) for optimal results
• if delayed:
1. Store in ice cubes (0–4°C) to slow down blood cell metabolism
2. Maximum 1 hour of storage

F. Parameters Measured in ABG

Here are the main components assessed in ABG:

• Normal value: 7,35-7,45
• pH $lt: 7.35: Acidosis (increased blood acidity)
• pH >: 7.45: Alkalosis (increased blood alkalinity)

• Normal value: 75-100 mmHg
• Indicates how well the blood is oxygenated
• Low PaO2 (<:60 mmHg) indicates hypoxemia (lack of oxygen)

• Normal value: 35-45 mmHg
• ↑ PaCO₂ ($gt: 45 mmHg): Respiratory acidosis (eg in COPD, respiratory failure)
• ↓ PaCO₂ (<: 35 mmHg): Respiratory alkalosis (eg, hyperventilation)

• Normal value: 22–26 mEq/L
• ↑ HCO₃⁻: Metabolic alkalosis (eg, excessive vomiting)
• ↓ HCO₃⁻: Metabolic acidosis (eg, diabetic ketoacidosis)

• Normal value: -2 to +2 mEq/L
• Normal value: -2 to +2 mEq/L
• Normal value: -2 to +2 mEq/L

• Normal value: $gt:95%
• Shows the percentage of hemoglobin bound to oxygen

G. ABG Technology

The main tool used to measure AGD parameters quickly and accurately.

Working Principle

1. Using ion selective electrodes to measure specific parameters
2. Automatic process: sample inserted → analyzer → results come out in 1-2 minutes

Main Components in Bloog Gas Analyzer

• Potentiometry (Ion-Selective Electrode)
1. Used to measure pH, Na⁺, K⁺, Ca²⁺, Cl⁻
2. Example: The pH electrode uses a glass membrane that is sensitive to H⁺ ions
• Amperometry (Clark Electrode for O₂)
1. Principle: O₂ reacts at the cathode to produce an electric current
2. Used to measure PaO₂
• Photometry (Spectrophotometry for Oximetry)
1. Measures oxygen saturation (SaO₂) and hemoglobin
2. Example: CO-oximeter measures HbO₂, Hb, MetHb, and COHb
• Conductometry (Conductivity Measurement)
1. Used in some electrolyte analysis

Brand of Instrument Main Features Measured Parameters

• Point-of-Care Testing (POCT)
1. Portable ABG devices (eg: i-STAT, EPOC) that can be used in the ICU, operating room, or ambulance
2. Advantages: Fast results ($lt:2 minutes), small sample (2-3 drops of blood)

• Automatic Calibration System
1. Modern blood gas analyzers have automatic calibration every 30 minutes to ensure accuracy
2. Uses standard buffer solutions (pH 7.38 and 6.84) and calibration gases (O₂ 12%, CO₂ 5%)

• Quality Control (QC)
1. Use of AGD control solutions (high, normal, low) to verify device performance
2. Example: Radiometer QC Level 1-3

• Optical Sensor (Optode)
1. Uses fluorescence to measure pH, CO₂, and O₂ (example: Nova StatSensor device)
• Microfluidics (Lab-on-a-Chip)
1. Small chip technology that requires very little sample (e.g. Abbott i-STAT)
• Digital Connectivity (LIS/HIS Integration)
1. ABG results are sent directly to the hospital information system (e.g. Siemens Healthineers)

• Modern blood gas analyzers use a combination of electrochemistry, optics, and microfluidics for fast & accurate results
• POCT is increasingly popular for point-of-care testing
• Calibration and QC are key to maintaining instrument accuracy
• Future technologies: wireless sensors, AI integration for automated interpretation