Arterial Blood Gas (ABG)

What Is an ABG?

"Obtaining a value for arterial PO₂ by analysis of an arterial blood gas is the gold standard for determining oxygenation." — Murray & Nadel's Textbook of Respiratory Medicine

What Does ABG Measure?

• The pH electrode measures the potential difference between a reference solution and the sample at 37 °C.
• The PCO₂ electrode allows CO₂ to react chemically, producing H⁺ ions whose concentration generates a measurable voltage.
• HCO₃⁻ is then calculated from the measured pH and PCO₂ using the Henderson-Hasselbalch equation — it is not directly measured.

Why ABG Over Pulse Oximetry?

Indications

• Respiratory distress or failure
• Acid-base disorders (metabolic or respiratory)
• Monitoring ventilator settings (ICU)
• Pre-operative assessment in significant lung disease
• Suspected CO poisoning or methemoglobinemia
• Confirm hypoxia or hypercapnia suggested by other monitoring
• Guide weaning from mechanical ventilation

How It's Done (Procedure)

Site Selection

• Superficial and easy to palpate
• No adjacent vein or major nerve at risk of injury
• Excellent collateral circulation from the ulnar artery

Avoid the femoral artery for routine sampling (hidden bleeding risk); reserve the brachial artery for emergencies (poor collateral circulation; risk of median nerve damage).

Radial Artery Anatomy

Step-by-Step Technique

• Pre-heparinized ABG syringe (or standard 3 mL syringe flushed with heparin)
• 22–25 gauge needle (23–25 gauge preferred; smaller = fewer complications)
• Antiseptic (chlorhexidine or povidone-iodine + 70% isopropyl alcohol)
• Sterile gloves, gauze
• Crushed ice or ice-water slurry for sample transport
• Optional: 1–2% lidocaine without epinephrine for local anesthesia

Important: Use only a small amount of heparin. Heparin has a very low pH — excess heparin causes falsely abnormal ABG results.

• Insert the needle bevel-up at a 30–45° angle to the skin, directed toward the pulse.
• Apply continuous gentle suction on the plunger while advancing.
• Pulsatile, bright red blood flashback into the hub confirms arterial entry.
• Collect 1–3 mL of blood.

If resistance (bone) is met, withdraw the needle very slowly with continuous suction — the tip may re-enter the arterial lumen on the way out.

• Remove the needle, immediately expel any air bubbles, and cap the syringe.
• Apply firm pressure for at least 5 minutes (longer in anticoagulated patients) to prevent hematoma.
• Label the sample with patient ID, FiO₂, time, and temperature.
• Transport on ice to the lab immediately (metabolic activity of WBCs continues consuming O₂ at room temperature).

Contraindications

• Infection, burn, trauma, or disturbed anatomy at the intended site
• Absent collateral circulation (failed Allen test)
• AV fistula or shunt in the same limb
• Severe coagulopathy (relative)

Complications

Key Pitfalls That Alter Results

• Air bubbles in the syringe → falsely high PaO₂ / low PaCO₂
• Excess heparin → falsely low pH
• Delay in analysis or failure to ice sample → O₂ consumed by WBCs, CO₂ produced → falsely low PaO₂, high PaCO₂, low pH
• Venous contamination → lower PO₂ and pH
• Patient hyperventilating from pain → falsely normal PCO₂ (use lidocaine to reduce this)

Electrocardiogram (ECG)

Definition

"If electrodes are placed on the skin on opposite sides of the heart, electrical potentials generated by the current can be recorded; the recording is known as an electrocardiogram (ECG)." — Guyton & Hall Textbook of Medical Physiology

Normal Cardiac Activation & ECG Waveforms

The Conduction System (what the ECG reflects)

1. SA node (high lateral right atrium) → spontaneous depolarization → spreads across both atria
2. → AV node (physiologic conduction delay)
3. → Bundle of His → Left & Right Bundle Branches → Purkinje fibers
4. → Ventricular muscle depolarization → contraction

Normal ECG Waveforms

Normal ECG. HR ~78 bpm, sinus arrhythmia. Axis ~+60°. PR ~140 ms, QRS ~90 ms, QT ~360 ms.

The 12 Leads

• Limb leads (I, II, III): Bipolar; measure differences between limb electrodes
• Augmented limb leads (aVR, aVL, aVF): Unipolar; compare each limb to a combined reference
• Precordial leads (V1–V6): Unipolar chest leads; define activation across the transverse plane

Systematic Approach to ECG Interpretation

1. Heart rate (normal 60–100 bpm)
2. Rhythm: regular vs. irregular; identify P waves
3. P-QRS relationship
4. Intervals: PR, QRS duration, QT/QTc
5. QRS axis
6. P wave morphology and axis
7. QRS progression in precordial leads (transition point)
8. ST segments (regional groupings)
9. T waves (regional groupings)

ECG Changes in Disease

1. Myocardial Ischemia / Infarction (ACS)

"If the involved coronary artery is totally occluded by a fresh thrombus, the patient's ECG reveals ST segment elevation… If the coronary artery is partially occluded, the ECG does not show ST segment elevation." — Goldman-Cecil Medicine

2. Conduction Abnormalities / Bundle Branch Blocks

3. Arrhythmias

4. Chamber Hypertrophy

5. Pericarditis

• Stage 1: Diffuse concave ST elevation in most leads except aVR and V1 (where there is ST depression); PR depression
• Stage 2–4: ST normalizes → T wave inversion → resolution
• Key distinction from MI: diffuse (not regional), concave shape, PR depression, no reciprocal changes in the usual pattern

6. Electrolyte & Metabolic Changes

7. Other Conditions