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concept_blood_gas_analysis [2025/05/13 05:58] – removed - external edit (Unknown date) 127.0.0.1concept_blood_gas_analysis [2025/05/13 06:36] (current) – [1.4 Is there any compensation?] jkohts
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 +====== 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.
 +