Arterial Blood Gases (Indications, Procedure and Interpretation)

Indications

  • Any respiratory distress/failure (acute or chronic)
  • Any severe illness which could lead to an acidotic state e.g.
    • cardiac failure
    • liver failure
    • renal failure
    • hyperglycaemic state e.g. ketoacidosis
    • multiorgan failure
    • sepsis
    • burns
    • poisons/toxins
  • Assessment of response to interventions such as ventilation

Contraindications

  • Infection/lesion at the site
  • Poor collateral circulation (see below)
  • Anticoagulation (relative)

Procedure

  • Prior to sampling
    • Confirm the need and identify any contraindications (for Allen’s test see below)
    • Record details of oxygen therapy and/or respiratory support
      • NB if O2 treatment has recently been altered, delay, if possible, for 20 minutes to allow effect to stabilise
    • Collect the necessary equipment
      • Heparinised syringe with cap
      • +/- 21 gauge (green) or 23 gauge (blue) needle (NB needle may already be attached to preset syringe
      • Alcohol wipe (Clinette)
      • Cotton wool ball
      • Sharps disposal
      • Gloves
  • Patient
    • Introduce yourself and wash hands.  Put on a disposable apron.
    • Check patient name, date of birth and allergies.
    • Explain the procedure, including potential complications (bleeding, bruising, arterial thrombosis, infection, pain) and obtain consent
    • Perform Modified Allen’s test for collateral circulation
      • Ask the patient to clench their fist
      • Occlude both the ulnar and radial arteries by pressing firmly over the lateral and medial aspects of the palmar wrist.
      • Ask the patient to open their hand (it should be white(r))
      • Relieve the ulnar artery (medial) of pressure
        • If colour returns to the hand within 6 seconds, there is adequate circulation.
        • If >6 seconds, this is a relative contraindication to performing an ABG.  Consider the other arm, using another site for arterial puncture, or reconsider the need for blood gases.
        • allens
    • Palpate the radial artery and locate the region of maximum pulsation (ideally, this should be the site for puncture)
  • Puncture
    • Put on gloves and apron
    • Clean the area with an alcohol/chlorhexidine wipe and allow to dry for 30 secs
    • NB ABGs can be particularly painful- consider using local anaesthetic (1ml of 1% lidocaine) at the site of puncture
    • Open the syringe and pre-set the syringe to at least 0.5mls
    • Position the arm- extended at the wrist 20-30° (not more than this, as it can impair arterial flow)
    • Palpate above the intended puncture site (do not contaminate puncture site)
    • Position the bevel of the needle up the way and advance slowly into the artery at a 45° angle
    • Once the needle is in the artery, blood should flash back into the syringe in a pulsatile manor under systolic pressure
      • Dark, non-pulsatile blood that requires manual suction suggests a venous sample.
      • Obtain at least 0.3mls (ideally 0.5-0.8mls)
  • After sampling
    • Remove the needle and apply firm, direct pressure to the sample site for at least 5 minutes (ask a nurse or another colleague to do this if necessary- often patients cannot do this adequately themselves)
      • If the patient is anticoagulated, press for longer (10 minutes is recommended)
    • Dispose of the needle and expel an air from the syringe.
      • A trick is to retract and push the plunger a couple of times to get the bubbles to the top of the syringe
    • Cap the syringe
    • The sample should be sent for analysis as soon as possible.  If it is likely to exceed 10 minutes before analysis, store the sample on crushed ice.
  • If unsuccessful, do not attempt to repeat the test on the same wrist as the first attempt may have induced arterial spasm.

Interpretation

What you get and what is normal:

  • Blood pH (7.35-7.45)
    • From this you can determine whether the patient is acidotic (<7.35) or alkalotic (>7.45)
  • pO2 (10.6-13kPa)
    • NB The pO2 should be roughly 10kPa less than the FiO2 % inspired concentration.  On air (21%) this is 11kPa.  On nasal cannula (24%), this is 14kPa.  On normal and venture masks (24-50%), this can be 14-40kPa.  And on non-rebreather mask (90-100%), it can be 80-90kPa)
    • From this you can determine whether the patient is hypoxic and thus whether the patient has respiratory failure
  • pCO2 (4.7-6kPa)- determines the respiratory component
    • If >6kPa- respiratory acidosis or respiratory compensation for a metabolic alkalosis
      • If combined with hypoxia- type II respiratory failure
    • If <4.7kPa- respiratory alkalosis or respiratory compensation for metabolic acidosis
    • If ‘normal’ (or low) and combined with hypoxia- type I respiratory failure
  • HCO3 (24-30mmol/l)- determines the metabolic component
    • If <22mmol/l- metabolic acidosis or renal compensation for respiratory alkalosis
    • If >26mmol/l- metabolic alkalosis or renal compensation for respiratory acidosis
  • Base Excess (-2 to +2)– determines the metabolic component also – it is defined as the ‘amount’ of H+ ions required to return the pH of the blood to 7.35 if the pCO2 were adjusted to normal.
    • If <-2 (base deficit): metabolic acidosis if combined with low pH
    • If >2 (base excess): metabolic alkalosis if combined with high pH

Other features

  • P(A-a) gradient (1kPa-2kPa)- this is the difference between the partial pressures (concentration) of oxygen in the alveoli (A) and arterial circulation (a). 
    • NB There is some complicated maths involved in calculating the P(A-a) gradient BUT there is a simplified version which gives a fairly representative value:
      • A-a gradient = PAO2-PaO2 = FiO2 – (PaCO2 x 1.2) – PaO2
    • Also note that reference range shown is for young, healthy individuals.  Elderly patients may have a normal P(A-a) gradient of up to 4-5kPa.
    • More than 4kPa suggests a defect of diffusion i.e. an intrinsic (lung) cause; V/Q mismatch or right-left shunt
    • Normal values (in the presence of respiratory failure) suggests CNS/neuromuscular causes
  • Anion Gap (10-18mmol/l)- this is difference between the total number of cations (principally Na and K) and the total number of anions (typically Cl and HCO3).
    • >18mmol/l in a metabolic acidosis suggests an accumulation of acids e.g. in ketoacidosis, lactic acid build up (secondary to sepsis or shock), drug overdoses e.g. aspirin, alcohol
    • Normal values in a metabolic acidosis suggests a loss of bicarbonate or ingestion of acid (hyperchloraemic acidosis) e.g. GI loss (diarrhoea); renal tubular acidosis; addisons; drugs e.g. carbonic anhydrase inhibitors
    • NB Hypoalbuminaemia can cause a low anion gap (otherwise a low anion gap is relatively rare)

For some good clinical examples see here and here

A note on venous blood gases

  • Sometimes, venous blood gases are used to assess carbon dioxide, bicarbonate and pH
    • Note that venous samples cannot be used to determine arterial oxygenation (no correlation)
  • A venous sample may be collected intentionally or (not uncommonly) unintentionally when trying to perform an ABG.
    • If the results come back looking something like
      • pH 7.32; pO2 ~5-6kPa; pCO2 5.8kPa… consider the possibility of venous sample (taken into account the patient state)

Some causes of results

  • Respiratory Alkalosis
    • Anxiety leading to hyperventilation (often accompanied by high O2)
    • PE
    • Salicylate poisoning
      • Note that there is often a mixed picture of respiratory alkalosis and metabolic acidosis (caused by effect on respiratory centre initially then direct acidity of salicylates and consequence of renal tubular acidosis later, respectively)
    • CNS disorders e.g. stroke, SAH, encephalitis
    • Altitude
    • Pregnancy
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