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concept_blood_gas_analysis [2025/05/13 06:18] – formatting, headings jkohtsconcept_blood_gas_analysis [2025/05/13 06:36] (current) – [1.4 Is there any compensation?] jkohts
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 The PCO₂ is a marker of ventilation. A normal PCO₂ is 35-45mmHg (or 4.7-6.0 kPa). 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 high PCO₂ is > 45 mmHg and implies hypoventilation. 
-  * A low pCO2 is < 35 mmHg and implies hyperventilation.+  * A low PCO₂ is < 35 mmHg and implies hyperventilation.
  
-A venous CO2 is ~5 mmHg higher than an arterial pCO2.+A venous PCO₂ is ~5 mmHg higher than an arterial PCO₂.
  
-**Respiratory acidosis** (pCO2 > 45) is the result of hypoventilation, causes include:+**Respiratory acidosis** (PCO₂ > 45 mmHg) is the result of hypoventilation, causes include:
   * Opioid intoxication   * Opioid intoxication
   * COPD   * COPD
  
-**Respiratory alkalosis** (pCO2 < 35) is the result of hyperventilation, causes include:+**Respiratory alkalosis** (PCO₂ < 35 mmHg) is the result of hyperventilation, causes include:
   * Hypoxia   * Hypoxia
   * Anxiety   * Anxiety
  
  
-==== What is the bicarbonate? ==== +==== - 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.+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 low HCO₃⁻ (< 24 mmol/L) implies a metabolic acidosis. 
-  * a raised HCO3 (> 24 mmol/L) implies a metabolic alkalosis+  * 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. 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? ====+==== - 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. 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**\\ +=== 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 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: A better rule is that:
 +  * In metabolic acidosis, expected PCO₂ = 1.5 [HCO₃⁻] + 8
 +  * In metabolic alkalosis, expected PCO₂ = 0.7 [HCO₃⁻] + 20
  
-  * in metabolic acidosis, pCO2 = 1.5 [HCO3] + 8 
-  * in metabolic alkalosis, pCO2 = 0.7 [HCO3] + 20 
  
-**Metabolic compensation**\\ +=== 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.+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: The following rules can determine the adequacy of metabolic derangement:
  
-In respiratory acidosis+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
  
-  * Acutely, for every rise in 10mmHg of pCO2 the HCO3 rises by 1mmol/L +In respiratory alkalosis: 
-  * Chronically, for every rise in 10mmHg of CO2 the HCO3 rises by 4mmol/L +  * 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
-In respiratory alkalosis +
- +
-  * Acutely, for every fall in 10 mmHg of CO2 the HCO3 falls by 2 mmol/L +
-  * Chronically, for every fall in 10mmHg of CO2 the HCO3 falls by 5 mmol/L+
  
  
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 ===== - Other Useful Information on a Blood Gas ===== ===== - Other Useful Information on a Blood Gas =====
  
-  * pO2 denotes oxygenation of the blood, a pO2 < 60mmHg is concerning for hypoxia.+  * PO₂ denotes oxygenation of the blood, a PO₂ < 60mmHg is concerning for hypoxia.
   * Lactate is often quantified, with normal concentrations < 2mmol/L.   * 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+  * 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%.   * COHb quantifies the percentage of circulating carboxyhaemoglobin. Smoking can be associated with levels up to 10%.
   * MetHb quantifies the percentage of circulating methaemoglobin.   * MetHb quantifies the percentage of circulating methaemoglobin.