Blood gases



Approach

Look at the patient see and if you can predict what should be going on based on clinical suspicion. Then look at the blood gas .

Normal ranges

pH = 7.4 (7.35-7.45)

PaCO2 = 40 mmHg (35-45)

HCO3- = 24 mmol/l (22-26)

BE = 0

  1. Is it an acidaemia or an alkalaemia?
  2. What is the primary acidosis or alkalosis?
  3. If it is a metabolic acidosis
    1. Is the anion gap wide?
    2. IIs the respiratory compensation as expected?
  4. If it is a metabolic alkalosis
    1. Is the respiratory compensation as expected?
  5. If it is a respiratory acidosis
    1. Is the metabolic compensation as expected?
    2. If the anion gap is wide there is probably a concurrent wide anion gap metabolic acidosis
  6. If it is a respiratory alkalosis
    1. Is the metabolic compensation as expected?
  7. If the pH is normal check the anion gap, the base excess, and the PaCO2 , there may be hidden disturbances
  8. Check the PaO2 and the A-a oxygen difference. is it what is expected given the FiO2?
  9. Check the Hb, the glucose, and the electrolytes
  10. Does the gas fit the patient?

Anion gap (AG) = ( Na+ ) - ( HCO3- + Cl- )

Upper limit normal is about 15 using (Na+) - (HCO3- + Cl-)

More than 20 definitely abnormal

Causes of a wide anion gap

  • Ketoacidosis
  • Lactic acidosis
  • Rhabdomyolysis
  • Non-ketotic hyperosmolar coma
  • Uraemia (or other Organic acidosis)
  • Haemoconcentration
  • Hypomagnesaemia
  • Hypocalcaemia
  • Hypokalaemia
  • IgA myeloma
  • Lactate or Citrate
  • Acetate

Causes of a low anion gap

  • Haemodilution
  • Hypoalbuminaemia
    • Adjusted anion gap = observed anion gap + 0.25 (normal albumin - observed albumin): Where albumin concentrations are in g/l
  • IgG myeloma
  • Hypercalcaemia
  • Hypermagnesaemia
  • Hyperkalaemia
  • Lithium intoxication
  • Analytical error: hypernatraemia, hyperlipidaemia

Acidosis

Anion gap : K+ + Na+ - Cl- - HCO3 = 8 - 16 mmol/l

Metabolic Acidosis and Anion Gap

Metabolic acidosis
with a high anion gap

Metabolic acidosis
with a normal AG

C

CO (or Cyanide)

   

A

Alcohol

   

T

Toluene

P

Potassium sparing diuretics

   

A

Acetazolamide

M

Methanol

R

Rhabdomyolysis /RTA

U

Urea

A

Alimentation feeding

D

DKA (and AKA)

M

Mineral Acids

P

Paraldehyde / Phenformin

E

Enterostomy

I

Iron / isoniazid

D

Diarrhoea

L

Lactic acidosis

I

Intestinal fistula

E

Ethanol/Ethylene glycol

C

Cholestyramine

S

Salicylate/Solvents
/Starvation

   

Delta gap

  • If a wide-anion-gap metabolic acidosis is the only disturbance, then the change in value of the anion gap should equal the change in bicarbonate (ie) ↑ AG = ↓ HCO3-
  • The delta gap = increase AG - decrease HCO3-
  • For purposes of calculation take normal AG as 12 and normal HCO3- as 24
  • Shortcut calculation: Δ AG - Δ HCO3- = (AG -12) - (24 - HCO3-) = Na+ - Cl- - 36
  • If the delta gap is < -6 there is also a non-anion gap metabolic acidosis.
  • Other causes of a delta gap < -6 are a respiratory alkalosis (with compensating non-anion gap acidosis), or a low anion gap state
  • If the delta gap > +6 there is a concurrent metabolic alkalosis.
  • Other causes of a delta gap > +6 are respiratory acidosis (with compensating metabolic alkalosis), or a non-acidotic high anion gap state

Expected PaCO2 = (1.5*bicarbonate) + 8
OR
PaCO2 = decimal digits of pH ± 5 mmHg

Respiratory compensation for metabolic acidosis

Causes of respiratory acidosis

Disorders of gas exchange

Acute:

  • Asthma
  • Bronchiolitis
  • Pneumonia
  • Pulmonary oedema
  • Laryngospasm
  • Foreign body aspiration
  • Mechanical ventilation

Chronic:

  • COPD
  • Prolonged pneumonia

Respiratory muscle abnormalities

Acute:

  • Chest wall trauma
  • Tension pneumothorax
  • Aminoglycosides
  • Familial periodic paralysis

Chronic:

  • Muscle weakness
  • Myasthenia gravis
  • Poliomyelitis
  • Amyotrophic lateral sclerosis
  • Kyphoscoliosis
  • Pickwickian syndrome

Respiratory centre abnormalities

Acute:

  • Opiates
  • Sedatives
  • General anaesthesia
  • Cardiac arrest

Chronic:

  • CNS abnormalities

Metabolic compensation for respiratory acidosis

  • In acute respiratory acidosis - For every 10 mmHg increase in PaCO2 the bicarb should rise 1 mmol/l (max 30)
  • In chronic respiratory acidosis - For every 10 mmHg increase in PaCO2 the bicarb should rise 4 mmol/l (max 36)

Alkalosis

Causes of respiratory alkalosis

  • CNS diseases
  • Anxiety / hysteria
  • Hypermetabolic states
  • Hepatic insufficiency
  • Assisted ventilation
  • PregnancyAltitude
  • Exercise
  • Hypoxia
  • Toxins

Metabolic compensation for respiratory alkalosis

  • In acute respiratory alkalosis - For every 10 mmHg decrease in PaCO2 the bicarbonate should fall 2 mmol/l (min 18)
  • In chronic respiratory alkalosis - For every 10 mmHg decrease in PaCO2 the bicarbonate should fall 5 mmol/l (min 12)

Causes of metabolic alkalosis

Saline responsive
(urine chloride < 10 mmol/l ) .. hypovolaemia

  • Diuretics
  • Vomiting, NG suctioning
  • Following respiratory acidosis
  • Exogenous alkalis
  • Contraction alkalosis

Saline unresponsive . normovolaemia

  • Hyperaldosteronism ( primary, secondary, exogenous )
  • Cushing's syndrome
  • Severe hypokalaemia

Unclassified

  • Milk alkali syndrome
  • Metabolism of organic anions
  • Massive blood transfusion
  • Nonparathyroid hypercalcaemia

Respiratory compensation for metabolic alkalosis

Expected PaCO2 = (0.9 x bicarb) + 9 OR Expected PaCO2 = decimal digits of pH ± 5 mmHg


PaO2

Predicted PaO2 when breathing air = 109 - (0.43*age) in mmHg

$= Also cause an increased A-a oxygen difference

Causes of a low PaO2

  • Left - right shunt (pulmonary, cardiovascular) $
  • Ventilation perfusion imbalance $
  • Diffusion block $
  • Hypoventilation
  • Decreased PiO2
  • Low mixed venous oxygen content

A-a oxygen difference [P(A-a)O2]

  • PAO2 is estimated from the alveolar gas equation
  • PaO2 is measured in the blood gases
  • P(A-a)O2 at sea level = (FiO2 x 713) - 1.2(PaCO2) - PaO2 (Drop the 1.2 correction factor if FiO2 > 0.6)
  • The predicted A-a oxygen difference is 5-15 mmHg in youth, 15-25 mmHg in the elderly
  • The A-a oxygen difference can be 10-110 mmHg for individuals breathing >60% oxygen
  • The A-a oxygen difference should be measured with the patient upright

Approximate FiO2 based on oxygen flow rate in a rigid mask:

Flow rate l/min 4 6 8 10 12  15
FiO2 0.35 0.5 0.55  0.6  0.65 0.7

Approximate FiO2 based on oxygen flow rate with nasal cannulae

Flow rate l/min 1 2  3 4  5 6
FiO2 25 29  33  37 41 45

Content by Dr Ian Higginson, Dr Íomhar O' Sullivan 20/04/2002. Reviewed by Dr ÍOS 20/04/2004, 18/05/2005, 13/05/2007. Last review Dr. ÍOS 19/09/14.