Blood gases
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
- Is it an acidaemia or an alkalaemia?
- What is the primary acidosis or alkalosis?
- If it is a metabolic acidosis
- Is the anion gap wide?
- IIs the respiratory compensation as expected?
- If it is a metabolic alkalosis
- Is the respiratory compensation as expected?
- If it is a respiratory acidosis
- Is the metabolic compensation as expected?
- If the anion gap is wide there is probably a concurrent wide anion gap metabolic acidosis
- If it is a respiratory alkalosis
- Is the metabolic compensation as expected?
- If the pH is normal check the anion gap, the base excess, and the PaCO2 , there may be hidden disturbances
- Check the PaO2 and the A-a oxygen difference. is it what is expected given the FiO2?
- Check the Hb, the glucose, and the electrolytes
- 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
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) increase AG = decrease 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
Acidosis
Causes of wide anion gap metabolic acidosis
- Uraemia
- Ketoacidosis
- Diabetic ketoacidosis
- Starvation ketoacidosis
- Alcoholic ketoacidosis
- Lactic acidosis(causes)
- Exogenous poisoning
- Methanol ( formic acid )
- Ethylene glycol ( oxalic acid )
- Salicylate
Causes of normal anion gap metabolic acidosis
- Loss of bicarbonate
- Diarrhoea
- Ureterosigmoidostomy
- Cholestyramine therapy
- Proximal Renal Tubular acidosis
- Renal insufficiency
- Acetazolamide
- Inability to excrete hydrogen
- Obstructive uropathy
- Pyelonephritis
- Hypoaldosteronism
- Distal Renal tubular acidosis
- Ingestion of ammonium chloride
- Hyperalimentation
Expected PaCO2 = (1.5*bicarbonate) + 8
OR
PaCO2 = decimal digits of pH +/- 5 mmHg
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 ooedema
- 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 |
