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Chapter 122: Arterial Blood Gas and Placement of A-line

Joseph J. Miaskiewicz

INTRODUCTION

Critically ill patients require arterial blood gas (ABG) analysis to assess oxygenation and ventilation due to limitations of noninvasive oximetry measurements. Below a pO2 of 60 mm Hg corresponding to an O2 saturation of 80%, the oxyhemoglobin saturation curve is steep and large changes in oximetry may mean small changes in oxygenation. Below this level oximetry may not correlate with oxygenation, and an arterial blood gas (ABG) should be obtained (Table 122-1).

Table 122-1Obtaining an Arterial Sample and Placement of an Arterial Line
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Table 122-1 Obtaining an Arterial Sample and Placement of an Arterial Line
  ABG A-Line
Indications

In hospitalized medical patients, an ABG is primarily obtained to confirm the severity and likely cause of the disturbance

  • Level of oxygenation, especially in settings when the oximeter measurements are thought to be unreliable or difficult to obtain

  • Need for intubation: refractory hypoxemia (pO2 < 55 on 100% O2 NRB mask) or hypercapnic respiratory failure (pCO2 > 55 with acidemia pH < 7.25)

  • Severity metabolic acidosis and adequacy of respiratory compensation when ↑ work of breathing

  • Contribution of ↑pCO2 versus other causes in somnolent patient

Usually in the ICU setting for

  • Frequent ABG sampling

  • Continuous blood pressure monitoring in use of inotropic or vasopressor agents
Contraindications

Impaired collateral circulation

  • Raynaud

  • Thromboangiitis obliterans

  • Cyanosis

 Impaired collateral circulation
Preparation Allen test: occlusion of the radial and ulnar arteries by firm pressure while the fist is clenched followed by opening of the hand and release of the arteries one at a time to assess adequacy of returning blood flow to the hand

Assess collateral circulation with Allen test

Avoid brachial and femoral arteries (inadequate collateral supplies)

Technical Tips

The radial artery at the wrist best site (near the surface, relatively easy to palpate, and stabilize with good ulnar collateral supply)

Apply local anesthetic with 1% lidocaine in the conscious patient

Immobilize hand on a wrist board or towel and dorsiflex wrist

Same as for ABG

If lose ability to palpate pulse, likely arterial spasm precluding successful cannulation. Wait until subsides or choose another site

If unsuccessful, apply pressure for several minutes to avoid hematoma formation (which will make subsequent attempts more difficult) and consider use of ultrasound to visualize vessel

Reassess perfusion of hand after placement
Complications

Transient obstruction of blood flow may ↓ arterial flow in distal tissues unless adequate collateral arterial vessels available in the setting of

  • Spasm

  • Intraluminal clotting

  • Bleeding and hematoma formation

Remove catheter immediately if any sign of vascular compromise

Use nondominant hand preferred

By measuring both oxygenation and ventilation ABG analysis assesses the effects of the cardiopulmonary system in oxygen delivery. ABG analysis directly measures the pH, pCO2, and pO2. The normal range for the pH is between 7.36 and 7.44 corresponding to a normal range of 36 to 44 torr for ...

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