8 - ABGs and A-Lines

arterial line (a-line) sites and selection (adults)

• advantages of radial artery

• easy access

• good circulation

• low infection risk

arterial line (a-line) sites and selection (adults)

• disadvantages of radial artery

• risk of radial artery occlusion

• must pass Allen’s test

arterial line (a-line) sites and selection (adults)

• advantages of femoral artery

• large vessel

• useful in emergencies

• easy access

arterial line (a-line) sites and selection (adults)

• disadvantages of femoral artery

• high infection risk

• risk of pseudo-aneurysm

• difficult to monitor for ischemia

arterial line (a-line) sites and selection (adults)

• advantages of brachial artery

• larger vessel

• good for high-flow monitoring

arterial line (a-line) sites and selection (adults)

• disadvantages of brachial artery

• poor collateral circulation

• risk of limb ischemia

arterial line (a-line) sites and selection (adults)

• advantages of dorsalis pedis artery

can be used on select patients

arterial line (a-line) sites and selection (adults)

• disadvantages of dorsalis pedis artery

• weak collateral circulation

• hard to palpate

• not ideal for critical patients

arterial line (a-line) sites and selection (neonates and pediatrics)

• advantages of radial artery

common site for neonates

arterial line (a-line) sites and selection (neonates and pediatrics)

• disadvantages of radial artery

Allen’s test must be performed

arterial line (a-line) sites and selection (neonates and pediatrics)

• advantages of brachial artery

alternative site in infants

arterial line (a-line) sites and selection (neonates and pediatrics)

• disadvantages of brachial artery

risk of poor collateral circulation

arterial line (a-line) sites and selection (neonates and pediatrics)

• advantages of umbilical artery

used for NICU patients <7 days old

arterial line (a-line) sites and selection (neonates and pediatrics)

• disadvantages of umbilical artery

• requires sterile insertion

• risk of thrombosis

arterial line (a-line) sites and selection (neonates and pediatrics)

• advantages of dorsalis pedis / posterior tibial artery

alternative in older children

arterial line (a-line) sites and selection (neonates and pediatrics)

• disadvantages of dorsalis pedis / posterior tibial artery

hard to palpate

special considerations for neonate a-line insertion

• heel warmers

  • increases perfusion before puncture

• small needles (25-27G)

  • reduces trauma and hematoma risk

• minimal blood volume withdrawal

• risk of vasospasm with frequent arterial sticks

Allen’s test

test that determines if ulnar artery can supply hand if radial artery becomes occluded

• prevents hand ischemia and necrosis

• must be performed before radial artery catheterization

steps of Allen’s test

1. occlude radial and ulnar arteries while patient makes a fist

2. have patient open hand; palm should be pale

3. release pressure on ulnar artery while still occluding radial artery

4. observe for hand color return within 5-15 seconds

5. test results:

   • positive: normal blood return → safe for cannulation

   • negative: delayed/absent return → avoid radial a-line

contraindications for arterial sticks

• negative Allen’s test

• severe coagulation / thrombocytopenia

• skin infection / cellulitis

• vasopressor-induced vasoconstriction

• arterial occlusion / thrombosis

• active Raynaud’s / severe peripheral artery disease (PAD)

relative contradictions for ABGs and a-lines

• anticoagulation therapy

  • use small-gauge needle (22-25G) and apply pressure to site for at least 5 minutes post-puncture

• severe hypotension/shock

  • femoral artery better in emergencies

• history of vascular surgery/graft in affected limb

• edematous extremity

• previous failed attempt / multiple punctures in same artery

hazards and complications of ABGs and a-lines

• pain

• arteriospam

• air/blood clot emboli

• hematoma

• anaphylaxis from anesthetic

• hemmorhage

• vessel trauma

• arterial occlusion

• vasovagal response

• infection

• needle stick (clinician)

capillary blood gas (CBG)

blood gas usually reserved for neonates/pediatrics when arterial access isn’t available or when frequent monitoring is required

• limitations

  • doesn’t accurately reflect P_aO₂

  • best used for pH, PCO₂, and HCO₃^- when arteries are unavailable

  • not reliable in patients with poor perfusion, hypotension, and vasoconstriction

steps for obtaining CBG sample

1. pre-warm site (heel warmer for 3-5 mins) to increase capillary perfusion

2. clean area with antiseptic solution

3. use lancet to make shallow puncture

4. wipe away first drop to avoid contamination with interstitial fluid

5. allow blood to flow freely into capillary tube; don’t squeeze too much

6. cap and mix sample to prevent clotting

7. apply pressure to puncture site and cover with bandage

indications for a-lines

• frequent ABG sampling

• continuous BP monitoring

• hemodynamic instability

contraindications for a-lines

• failed Allen’s test

• infection at insertion site

• coagulopathy

• severe vasoconstriction

a-line insertion techniques

• angiocath (over-the-needle catheter)

  • insert at 30-45° angle

  • reduce to 10-15° after blood return is seen

• Seldinger (wire-guided insertion)

  • secure cath and connect to transducer

arterial waveform interpretation

• normal waveform phases

1. anacrotic limb

   • ventricular systole

2. dicrotic notch

   • aortic valve closure

arterial waveform interpretation

• abnormal waveforms

3. overdamped

   • underestimates BP

4. underdamped

   • overestimates BP

complications and troubleshooting for ABGs and a-lines

COMPLICATION	CAUSE	MANAGEMENT
infection	poor sterile technique	aseptic insertion daily site monitoring
thrombosis	clot formation	heparin flush assess perfusion
hematoma	poor pressure application	apply firm pressure for 5-10 minutes
ischemia	poor collateral circulation	remove line if needed

pre-analytical errors in ABG sampling

• cause of air bubbles

incomplete syringe filling

pre-analytical errors in ABG sampling

• air bubbles’ effect on ABG

• ↓CO₂

• ↑pH

• variable O₂

pre-analytical errors in ABG sampling

• cause of excess heparin

overdilution of sample

pre-analytical errors in ABG sampling

• excess heparin’s effect on ABG

• ↓pH

• altered CO₂ & O₂

pre-analytical errors in ABG sampling

• cause of blood metabolism

delayed analysis

pre-analytical errors in ABG sampling

• blood metabolism’s effect on ABG

• ↑CO₂

• ↓pH

• ↓O₂

pre-analytical errors in ABG sampling

• cause of venous contamination

poor sampling technique

pre-analytical errors in ABG sampling

• venous contamination’s effect on ABG

• ↓O₂ (false hypoxia)

• ↑CO₂

steps for obtaining ABG sample from a-line

1. verify waveform prior to sampling

2. turn stopcock off to flush bag, discard waste blood (~2-3 mL)

3. attach heparinized syringe, draw sample

4. flush line, ensure waveform normalizes