Introduction
Look at the patient and see and predict what should be going on based on clinical suspicion.
Then look at the blood gas.
"Gases" get confusing if you don't note (scribble down) each finding as you work through the printout.
VBG vs ABG
VBGs used in EDs in preference to ABGs.
Correlation good, apart from pO2.
pCO2 correlates well at normal ranges. Correlation worsens with sepsis/shock/rising pCO2.
Normal ranges
pH = 7.4 (7.35-7.45)
PaCO2 = 4.7 - 6.0 kPa.
HCO3- = 24 mmol/l (22-26)
BE = 0 (±2)
Approach
- Note (scribble down) if the patient has an acidaemia or an alkalaemia?
- What is the primary acidosis or alkalosis?
- If it is a Metabolic Acidosis (MA):
- Is the anion gap wide (R[raised] AGMA) or not (N[normal] AGMA)?
- If RAGMA check delta gap [see below] (to see if another problem)?
- Is the respiratory compensation as expected [see below]?
- If it is a metabolic alkalosis
- Is the respiratory compensation as expected? [See below]
- If it is a respiratory acidosis:
- Is the metabolic compensation as expected?[see below]
- If the anion gap is wide there is probably a concurrent RAGMA
- 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 as 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?
Acidosis
Anion gap : K+ + Na+ - Cl- - HCO3 = 8 - 16 mmol/l
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
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 XS
- Analytical error: ↑Na+, hyperlipidaemia
In lone metabolic acidosis:
Expected PaCO2 = (1.5*bicarbonate) + 8
or
In mmHg .... 7.xx rule (mmHg) : 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
- Acute Resp. acidosis - Δ HCO3- = 1 mmol/l for each 1 kPa change in pCO2
- Chronic Resp. acidosis - Δ HCO3- = 4 mmol/l for each 1 kPa change in pCO2
Alkalosis
Causes of respiratory alkalosis:
- CNS diseases
- Anxiety / hysteria
- Hypermetabolic states
- Hepatic insufficiency
- Assisted ventilation
- Pregnancy/Altitude/Exercise
- Hypoxia
- Toxins
Metabolic compensation for resp. alkalosis
Respiratory acidosis and alkalosis are characterized by a primary change in pCO2. Secondary physiological compensation is seen in blood gas results as a change (Δ) in bicarbonate concentration (↑ in the case of acidosis and ↓ in alkalosis). So expect:
- Acute Resp. alkalosis - Δ HCO3- = 2 mmol/l for each 1 kPa change in pCO2
- Chronic Resp. alkalosis - Δ HCO3- = 3 mmol/l for each 1 kPa change in pCO2
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
Other compensations
Corrected K+
= Each pH ↓, K+ should ↑ by from 5mmol/L by 0.5 mmol/L
E.g. Expect K+ for pH 7.2 = 5.0 + 2 = 6.0 mmol/L
Corrected Na+
= Na+ + (Glucose -5)/3
A-a oxygen gradient [P(A-a)O2]
Approx. FiO2 on O2 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 |
Approx. FiO2 on O2 flow rate nasal cannulae
Flow rate l/min | 1 | 2 | 3 | 4 | 5 | 6 |
---|---|---|---|---|---|---|
FiO2 | 25 | 29 | 33 | 37 | 41 | 45 |
PaO2 and age
The normal value for the partial pressure of arterial oxygen (PaO2) irrespective of age is >10.6 kPa.
The normal PaO2 for a given age can be predicted from:
- Seated PaO2 = 13.8 kPa - 0.27 x age
- Supine PaO2 = 13.8 kPa - 0.42 x age
If PaO2 is < 10.7 kPa, the patient has arterial hypoxemia.
Causes of a low PaO2
- Shunt (pulmonary, cardiovascular)
- Ventilation perfusion imbalance
- Diffusion block
- Hypoventilation
- ↓ PiO2
- Low mixed venous oxygen content