This page is read only. You can view the source, but not change it. Ask your administrator if you think this is wrong. ====== Blood Gas Analysis ====== Blood gas analysis is readily available and provides useful information about the acid-base status of a patient. The following is a simplified approach to interpreting arterial blood gas analysis. A more comprehensive resource is found at [[http://www.anaesthesiamcq.com/AcidBaseBook/ABindex.php|http://www.anaesthesiamcq.com/AcidBaseBook/ABindex.php]] ===== - Interpretation ===== ==== - What is the pH? ==== A normal pH is 7.35 to 7.45. * Acidaemia: low pH < 7.35 * Alkalaemia: high pH > 7.45 The venous pH is ~ 0.05 lower than arterial pH. ==== - What is the PCO₂? ==== The PCO₂ is a marker of ventilation. A normal PCO₂ is 35-45mmHg (or 4.7-6.0 kPa). * A high PCO₂ is > 45 mmHg and implies hypoventilation. * A low PCO₂ is < 35 mmHg and implies hyperventilation. A venous PCO₂ is ~5 mmHg higher than an arterial PCO₂. **Respiratory acidosis** (PCO₂ > 45 mmHg) is the result of hypoventilation, causes include: * Opioid intoxication * COPD **Respiratory alkalosis** (PCO₂ < 35 mmHg) is the result of hyperventilation, causes include: * Hypoxia * Anxiety ==== - What is the Bicarbonate (HCO₃⁻)? ==== The bicarbonate is a marker for the metabolic acid-base status of a patient. A normal HCO₃⁻ is 22-26 mmol/L, but we tend to use 24 mmol/L for calculations. * A low HCO₃⁻ (< 24 mmol/L) implies a metabolic acidosis. * A raised HCO₃⁻ (> 24 mmol/L) implies a metabolic alkalosis. The base excess gives similar information with a normal BE being −3 to +3. With a low base excess (BE less than −3) implying a metabolic acidosis and a raised base excess (BE greater than +3) implying a metabolic alkalosis. ==== - Is there any compensation? ==== Both the lungs and kidneys adapt to compensate for acid-base disturbances in an attempt to bring the pH closer to normal. The adequacy of this compensation should be assessed. === Respiratory Compensation === A quick rule is that the PCO₂ should roughly equal the last two digits of the pH value. This only works within a pH range of 7.1-7.6. A better rule is that: * In metabolic acidosis, expected PCO₂ = 1.5 [HCO₃⁻] + 8 * In metabolic alkalosis, expected PCO₂ = 0.7 [HCO₃⁻] + 20 === Metabolic Compensation === Renal metabolic compensation occurs quickly via intracellular buffering, and more slowly via the kidney, where under normal conditions, HCO₃⁻ is absorbed and H⁺ is secreted in varying amounts. The following rules can determine the adequacy of metabolic derangement: In respiratory acidosis: * Acutely, for every rise in 10 mmHg of PCO₂ the HCO₃⁻ rises by 1 mmol/L * Chronically, for every rise in 10 mmHg of PCO₂ the HCO₃⁻ rises by 4 mmol/L In respiratory alkalosis: * Acutely, for every fall in 10 mmHg of PCO₂ the HCO₃⁻ falls by 2 mmol/L * Chronically, for every fall in 10mmHg of PCO₂ the HCO₃⁻ falls by 5 mmol/L ===== - Types of Metabolic Acidosis ===== The Anion Gap is the difference between measured cations and measured anions. It reflects any unmeasured anions. The normal value is < 12 mmol/L. It is calculated by the equation Na⁺ − (Cl⁻ + HCO₃⁻) When you have a metabolic acidosis, you need to measure this value, as it helps determine what sort of metabolic acidosis exists. **A high anion gap metabolic acidosis (HAGMA) occurs when AG is > 12** The causes of HAGMA can be grouped into * Lactic acidosis * Ketoacidosis * Renal failure * Toxins (eg toxic alcohols) Some prefer difficult to remember mnemonics such as **CAT MUDPILES** * **C**arbon monoxide, cyanide * **A**lcohol, alcohol ketoacidosis * **T**oluene * **M**etformin, methanol * **U**raemia * **D**iabetic ketoacidosis * **P**aracetamol, propylene glycol, pyroglutamic acid * **I**ron, isoniazid * **L**actic acidosis * **E**thylene glycol * **S**alicylates **A normal anion gap metabolic acidosis (NAGMA) occurs when AG ≤ 12** A NAGMA is due to either bicarbonate loss or chloride gain. There is another difficult to remember mnemonic (**USED CARP**) * **U**reterostomy * **S**mall bowel fistula * **E**xtra chloride * **D**iarrhoea * **C**arbonic anhydrase inhibitors * **A**drenal insufficiency * **R**enal tubular acidosis * **P**ancreatic fistula ===== - Other Useful Information on a Blood Gas ===== * PO₂ denotes oxygenation of the blood, a PO₂ < 60mmHg is concerning for hypoxia. * Lactate is often quantified, with normal concentrations < 2mmol/L. * Electrolytes such as sodium (Na⁺), potassium (K⁺) and chloride (Cl⁻) are usually reported on a blood gas * COHb quantifies the percentage of circulating carboxyhaemoglobin. Smoking can be associated with levels up to 10%. * MetHb quantifies the percentage of circulating methaemoglobin. CKG Edit