Arterial Blood Gas Analysis

Arterial blood gas analysis measures the acid base state and the gas exchange of arterial blood. A blood gas analyser reports pH, the partial pressures of carbon dioxide (PaCO2) and oxygen (PaO2), and derives bicarbonate, base excess and oxygen saturation. Most analysers also report haemoglobin, electrolytes, glucose and lactate, and a co-oximeter measures carboxyhaemoglobin (COHb) and methaemoglobin (MetHb). Results are available in about 2 minutes at the bedside.

It is the primary test for respiratory failure, metabolic acidosis and alkalosis, poisoning and the response to oxygen and ventilation. It separates a primary process from its compensation and identifies mixed disorders that a single value hides.

Acute respiratory failure and acid base disorders are among the most frequent reasons for intensive care and high dependency admission. Arterial blood gases are taken in a large share of emergency admissions with breathlessness, sepsis or altered consciousness.

Indications. Acute breathlessness, hypoxia, exacerbation of chronic obstructive pulmonary disease, suspected carbon dioxide retention, shock, sepsis, diabetic ketoacidosis, suspected poisoning, cardiac arrest, kidney failure, unexplained altered consciousness, monitoring of ventilation, and preoperative assessment in severe lung disease.

Specimen. Arterial blood from the radial, brachial or femoral artery into a heparinised syringe with all air bubbles expelled, or an arterial line. Analyse within 15 minutes at room temperature (within 30 minutes if on ice in a glass syringe). Record the inspired oxygen fraction (FiO2), temperature and whether the patient was at rest. Allow 5 minutes after a change in oxygen therapy. Venous blood is adequate for pH, bicarbonate and lactate in most patients but not for oxygenation.

Reference values are for arterial blood at sea level. Conversion: kPa multiplied by 7.5 gives mmHg.

pH. The negative logarithm of hydrogen ion concentration. Range: 7.35 to 7.45 (hydrogen ion 35 to 45 nmol/L). Lowered (acidaemia): respiratory or metabolic acidosis. Raised (alkalaemia): respiratory or metabolic alkalosis. A normal pH does not exclude a disorder, since mixed disorders can cancel.

The pH defines the direction of the dominant process. Values under 7.20 and over 7.60 are life threatening.

PaCO2. Reflects alveolar ventilation. Range: 4.7 to 6.0 kPa (35 to 45 mmHg). Raised: hypoventilation (chronic obstructive pulmonary disease, opioid or sedative excess, neuromuscular weakness, obesity hypoventilation, chest wall disease, severe asthma, fatigue), and compensation for metabolic alkalosis. Lowered: hyperventilation (pain, anxiety, hypoxia, pulmonary embolism, sepsis, salicylate poisoning, pregnancy, hepatic failure, high altitude), and compensation for metabolic acidosis.

PaO2. Reflects oxygenation. Range: 10.0 to 13.3 kPa (75 to 100 mmHg) on air, falling with age. Lowered: hypoventilation, ventilation perfusion mismatch, shunt, diffusion limitation, low inspired oxygen (altitude). Under 8.0 kPa (60 mmHg) defines respiratory failure.

Bicarbonate and base excess. Bicarbonate range: 22 to 26 mmol/L. Base excess range: minus 2 to plus 2 mmol/L. Lowered bicarbonate or a negative base excess: metabolic acidosis, or renal compensation for chronic respiratory alkalosis. Raised bicarbonate or a positive base excess: metabolic alkalosis, or renal compensation for chronic respiratory acidosis.

Oxygen saturation and haemoglobin derivatives. Arterial saturation range: 95 to 100%. COHb range: under 3% in non-smokers and up to 10% in heavy smokers. MetHb range: under 1.5%. Raised COHb indicates carbon monoxide poisoning, and raised MetHb indicates drug or toxin exposure (dapsone, nitrites, local anaesthetics) or congenital disease.

Alveolar arterial gradient and P/F ratio. The alveolar arterial gradient (A-a) separates hypoventilation (normal gradient) from intrapulmonary causes (raised gradient). The expected value is about age divided by 4, plus 4 mmHg. The ratio of PaO2 (mmHg) to FiO2 (as a fraction) under 300 indicates impaired oxygenation, and under 200 and under 100 indicate moderate and severe acute respiratory distress syndrome when other criteria are met.

Lactate and electrolytes. Lactate and anion gap are read with the acid base picture. Anion gap is sodium minus chloride minus bicarbonate, normal 8 to 12 mmol/L when potassium is excluded.

Stepwise interpretation. Check pH for acidaemia or alkalaemia, then PaCO2 for a respiratory cause and bicarbonate for a metabolic cause, then test whether compensation is appropriate, and finally compare the anion gap and the change in bicarbonate.

Four primary disorders and expected compensation.

• Respiratory acidosis (high PaCO2): bicarbonate rises by about 1 mmol/L for each 1.3 kPa (10 mmHg) rise in PaCO2 acutely, and 3.5 to 4 mmol/L chronically.
• Respiratory alkalosis (low PaCO2): bicarbonate falls by about 2 mmol/L for each 1.3 kPa fall acutely, and 4 to 5 mmol/L chronically.
• Metabolic acidosis (low bicarbonate): expected PaCO2 in mmHg is 1.5 times bicarbonate plus 8, give or take 2. A PaCO2 higher than expected indicates a coexisting respiratory acidosis, and lower indicates a coexisting respiratory alkalosis.
• Metabolic alkalosis (high bicarbonate): PaCO2 rises by about 0.7 mmHg for each 1 mmol/L rise in bicarbonate.

High anion gap metabolic acidosis (over 12 mmol/L). Lactic acidosis, ketoacidosis (diabetic, alcoholic, starvation), kidney failure, and toxins (salicylate, methanol, ethylene glycol, metformin, pyroglutamic acid with chronic paracetamol use). A change in the anion gap greater than the fall in bicarbonate indicates an additional metabolic alkalosis, and a smaller change indicates an additional normal gap acidosis.

Normal anion gap (hyperchloraemic) metabolic acidosis. Diarrhoea, ileostomy and pancreatic fistula, renal tubular acidosis, large volume saline infusion, acetazolamide, early kidney disease, and ureterosigmoidostomy.

Metabolic alkalosis. Vomiting and nasogastric loss, diuretics, severe hypokalaemia, mineralocorticoid excess, milk alkali syndrome, and recovery from hypercapnia. A urinary chloride under 20 mmol/L indicates chloride responsive causes (vomiting, diuretics past), and over 20 mmol/L indicates mineralocorticoid excess or active diuretic use.

Respiratory failure. Type 1: PaO2 under 8.0 kPa with normal or low PaCO2 (pneumonia, pulmonary embolism, pulmonary oedema, acute respiratory distress syndrome). Type 2: PaO2 under 8.0 kPa with PaCO2 over 6.0 kPa (chronic obstructive pulmonary disease, neuromuscular disease, opioid excess). In chronic carbon dioxide retainers a target saturation of 88 to 92% avoids worsening hypercapnia.

Mixed disorders. Examples include a near normal pH with a high anion gap acidosis and metabolic alkalosis (vomiting with ketoacidosis), and respiratory alkalosis with metabolic acidosis (salicylate poisoning, sepsis).

Critical values and findings needing immediate action.

• pH under 7.20 or over 7.60.
• PaO2 under 8.0 kPa (60 mmHg), or under 6.7 kPa (50 mmHg) at any FiO2.
• PaCO2 over 8.0 kPa (60 mmHg) with pH under 7.30, or any rising PaCO2 with drowsiness.
• Bicarbonate under 10 mmol/L, or lactate over 4 mmol/L.
• COHb over 25%, or over 10% in pregnancy, or any raised value with neurological features or myocardial ischaemia.
• MetHb over 20% with symptoms, or saturation that does not improve with oxygen.

Interferences and pitfalls.

• Air bubbles in the syringe bring PaO2 towards 20 kPa and lower PaCO2.
• Delayed analysis at room temperature in plastic syringes lowers pH and PaO2 and raises PaCO2 through cell metabolism, and very high white cell or platelet counts accelerate this.
• Excess liquid heparin dilutes the sample and lowers PaCO2 and bicarbonate.
• Venous blood has a pH about 0.03 lower, a PaCO2 about 0.8 kPa (6 mmHg) higher and a bicarbonate about 1 mmol/L higher than arterial blood.
• Pulse oximetry overestimates saturation in carbon monoxide poisoning, and reads about 85% in methaemoglobinaemia whatever the true saturation.
• Results are corrected to 37 °C, and temperature corrected values are used only if the laboratory requires them.
• Altitude, anaemia and shunt alter the oxygen content without altering PaO2 in the same way.
• Analyser lactate can read falsely high in ethylene glycol poisoning.

Special groups.

• Neonates: cord arterial pH under 7.0 with base deficit of 12 mmol/L or more supports significant intrapartum hypoxia. Normal PaO2 in the first hours is lower.
• Children: respiratory rate and compensation differ, and capillary samples are often used.
• Pregnancy: PaCO2 is about 4.0 kPa (30 mmHg) with compensated respiratory alkalosis, so a normal adult value is abnormal.
• Older adults: PaO2 falls with age, and bicarbonate compensation may be limited.
• Altitude: PaO2 and PaCO2 are lower, with chronic mountain adaptation.
• Regions: carbon monoxide poisoning from indoor charcoal and biomass cooking, and salicylate poisoning, are important in some settings.

Follow up tests. Anion gap, osmolar gap, lactate, blood ketones, urea and electrolytes, glucose, urinary chloride and pH, chest radiograph, CT pulmonary angiography, ECG, salicylate and paracetamol levels, toxic alcohol levels, carboxyhaemoglobin and methaemoglobin by co-oximetry, spirometry, and repeat blood gas after intervention.