Abg sampling

Reading File

Finding Sources

Reading File

Finding Sources

Reading File

Finding Sources

Reading File

I now have comprehensive information to give a thorough answer on ABG sampling. Let me compile this.

Arterial Blood Gas (ABG) Sampling

What is ABG and Why It Matters

ABG analysis is the gold standard for assessing:

Oxygenation (PaO2)
Ventilation (PaCO2)
Acid-base status (pH, HCO3-)

It directly measures pH, arterial PCO2, and arterial PO2; bicarbonate (HCO3-) is then calculated via the Henderson-Hasselbalch equation.

Normal values:

Parameter	Normal Range
pH	7.35 - 7.45
PaCO2	35 - 45 mmHg
HCO3-	21 - 27 mEq/L
PaO2	80 - 100 mmHg (room air)

Indications

Acute respiratory decompensation (hypoxia, hypercapnia)
Metabolic emergencies (DKA, sepsis, shock)
Titration of vasoactive drugs (nitroprusside, norepinephrine)
Perioperative monitoring in major surgery
Baseline correlation with noninvasive monitors (SpO2, EtCO2) in critically ill patients
Trauma and post-cardiac arrest resuscitation monitoring

Venous blood gas (VBG) can substitute for most parameters except PO2 - venous pH runs ~0.03 lower and venous PCO2 runs ~5-9 mmHg higher than arterial.

Roberts and Hedges' Clinical Procedures in Emergency Medicine, p. 461

Site Selection

Site	Notes
Radial artery (preferred)	Superficial, easy to palpate, no adjacent nerve/vein, excellent collateral from ulnar artery
Brachial artery	Poor collateral - avoid unless no other option
Femoral artery	Avoid for routine sampling; use only when others inaccessible
Dorsalis pedis / posterior tibial	Acceptable alternatives
Umbilical arteries	Neonates only

The radial artery is preferred because its superficial location is easy to palpate and puncture, and there are no immediately adjacent nerves or veins that may be injured or accidentally punctured. - Roberts and Hedges, p. 438

Allen Test (Pre-Procedure)

Before radial artery puncture, assess collateral ulnar circulation:

Ask patient to clench fist; compress both radial and ulnar arteries
Patient opens hand - palm will blanch
Release ulnar artery while keeping radial occluded
Normal (negative): color returns within 5 seconds
Abnormal (positive): pallor persists >5-15 seconds - suggests inadequate collateral flow - consider alternate site

Roberts and Hedges' Clinical Procedures in Emergency Medicine, p. 1527-1531

Contraindications

Strict (absolute):

Inadequate circulation to the limb
Raynaud's syndrome or Buerger's disease
Full-thickness burns over the site

Relative:

Anticoagulation / coagulopathy (caution, not absolute)
Skin infection at puncture site
Previous surgery in the area
Atherosclerosis / inadequate collateral flow
After thrombolysis (only if data cannot be obtained any other way)

Equipment

For adult percutaneous puncture:

Pre-heparinized ABG syringe (1000 IU/mL heparin - fill dead space only, collect ~3 mL blood)
20-23 gauge needle
Antiseptic swab + sterile gauze
Local anesthetic (lidocaine - reduces hyperventilation artifact)
Ice (if analysis delayed >5 minutes)

For infants/children: 23-gauge butterfly needle (25-gauge in newborns)

Technique (Radial Artery Puncture)

Explain procedure; obtain consent
Perform modified Allen test
Position wrist in supination with slight extension (dorsiflex 30-60°, may use a rolled towel)
Clean site with antiseptic; allow to dry
Optional: infiltrate 0.5 mL lidocaine subcutaneously (reduces pain-induced hyperventilation)
Palpate radial pulse with index/middle finger of non-dominant hand
Insert needle at 30-45° angle bevel up, aimed at strongest pulse point
Advance slowly - pulsatile blood flashback confirms arterial entry
Allow syringe to fill (2-3 mL) - ideally by arterial pressure alone, not aspiration
Withdraw needle; apply firm pressure for minimum 5 minutes (longer if anticoagulated)
Expel any air bubbles immediately; cap syringe
Mix gently to prevent clotting; label and send to lab on ice if delayed

Sample Handling - Critical Errors to Avoid

Error	Effect on Result
Air bubble in syringe	PO2 elevated; PCO2 unchanged
Excess heparin	PCO2 lowered; pH unchanged
Delayed analysis (>20 min)	PCO2 elevated, pH lowered, PO2 variable
Failure to note FiO2 at time of sampling	Misinterpretation of oxygenation

Roberts and Hedges' Clinical Procedures in Emergency Medicine, p. 1287

Always record: FiO2, patient position, activity level, and time of sampling alongside the result.

Complications

Hematoma (most common)
Arterial thrombosis / embolism
Vasospasm (can cause device entrapment if cannulated)
Infection
Ischemia (especially if collaterals inadequate)
Arteriovenous fistula
Pseudoaneurysm
Compression neuropathy (from hematoma)
Rarely: compartment syndrome

When ABG Cannot Be Avoided vs. Alternatives

Scenario	Alternative
Monitoring oxygenation	SpO2 (pulse oximetry) - but unreliable on supplemental O2 for detecting hypoventilation
Monitoring CO2	End-tidal CO2 (capnography)
pH and PCO2 trending	VBG via central line (venous pH ≈ arterial pH - 0.03)
Continuous monitoring	Indwelling arterial catheter with biosensor

Key caveat: Pulse oximetry loses all ability to detect hypoventilation in patients receiving supplemental oxygen, because the arterial PO2 is pushed far up on the flat portion of the oxyhemoglobin dissociation curve. In these patients, ABG or capnography is essential. - Murray & Nadel's Textbook of Respiratory Medicine, p. 956

Sources: Roberts and Hedges' Clinical Procedures in Emergency Medicine; Murray & Nadel's Textbook of Respiratory Medicine; Tintinalli's Emergency Medicine

Abg sampling pics

Loading Image

Finding Sources

Loading Image

Here are all the key ABG sampling images from Roberts and Hedges' Clinical Procedures in Emergency Medicine:

1. Radial Artery Anatomy - Site Selection

Radial artery anatomy showing median nerve, ulnar artery, radial artery, ulnar nerve and palmar arch

The radial artery is preferred - superficial, no adjacent nerve or vein, and well collateralised by the ulnar artery via the palmar arch.

2. The Allen Test (Pre-Procedure)

Allen Test - 3 step illustration showing compression of radial and ulnar arteries, blanching, then release of ulnar

Step 1: Occlude both arteries + fist pump. Step 2: Relax hand - should blanch. Step 3: Release ulnar only - color returns in <5-10 seconds = adequate collateral = safe to proceed.

3. Arterial Puncture Technique (Radial Artery) - Step by Step

Arterial Puncture Radial Artery - 6 step illustrated guide showing wrist positioning, local anesthetic, needle insertion at 30°, syringe filling, pressure, and capping sample

Step 1: Dorsiflex wrist, clean, palpate pulse. Step 2: Optional lidocaine wheal. Step 3: Insert needle bevel-up at 30°. Step 4: Let syringe fill passively from arterial pressure. Step 5: Apply firm pressure 3-5 min. Step 6: Expel air, cap, send on ice.

4. Arterial Cannulation - Guidewire (Seldinger) Technique

Arterial Cannulation Guidewire Technique - 4 steps showing needle insertion, catheter held, guidewire threaded, catheter advanced over wire

Used when repeated sampling is needed. Step 1: Insert needle, look for pulsatile flashback. Step 2: Remove needle, hold catheter. Step 3: Thread guidewire freely. Step 4: Advance catheter over wire, secure to skin.

5. Arterial Cannulation - Arrow Kit (Commercial Seldinger)

Arterial Cannulation Arrow Kit - 4 steps showing actuating lever insertion, guidewire advancement, catheter threading, and transducer attachment

The Arrow kit combines the needle and guidewire into one integrated system - confirm flashback, advance wire via actuating lever, thread catheter, attach transducer tubing.

6. Arterial Pressure Monitoring Setup

Arterial pressure monitoring system showing saline bag, manometer, Pressurveil, continuous flush device connected to patient and oscilloscope

Once cannulated: pressurised saline bag (250-300 mmHg) delivers 3 mL/hr flush via the continuous flush device. Transducer must be kept at the level of the patient's heart.

This is a shared conversation. Sign in to Orris to start your own chat.