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wikitox:arterial_blood_gases [2024/04/24 00:32] – kharriswikitox:arterial_blood_gases [2025/02/17 00:33] (current) – kharris
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-Link to [[:wikitox:3.2.1.1.1_acidosis_teaching_resources|Acidosis Teaching Resources]] +====== Arterial Blood Gases ======
- <font 36px/inherit;;#c12b04;;inherit>Arterial Blood Gases</font>+
  
-----+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]]
  
-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]]+----
  
-What is the pH?+===== Interpretation ===== 
 + <font inherit/inherit;;#c12b04;;inherit>**What is the pH?**</font>
  
 A normal pH is 7.35 to 7.45 A normal pH is 7.35 to 7.45
  
-•   acidaemia = low pH < 7.35\\ +  * acidaemia = low pH < 7.35 
-•   alkalaemia = high pH > 7.45+  * alkalaemia = high pH > 7.45
  
-The venous pH is ~ 0.05 lower than arterial pH. +The venous pH is ~ 0.05 lower than arterial pH. \\  \\  \\ 
- + <font inherit/inherit;;#c12b04;;inherit>**What is the pCO2?**</font>
-What is the pCO2?+
  
 The pCO2 is a marker of ventilation. A normal pCO2 is 35-45mmHg. The pCO2 is a marker of ventilation. A normal pCO2 is 35-45mmHg.
  
-•   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 ~ 5mmHg higher than an arterial pCO2. A venous CO2 is ~ 5mmHg higher than an arterial pCO2.
  
-Respiratory acidosis (pCO2 > 45) is the result of hypoventilation, causes include:\\ +Respiratory acidosis (pCO2 > 45) is the result of hypoventilation, causes include:
- Opioid intoxication\\ +
- COPD+
  
-Respiratory alkalosis (pCO2 < 35) is the result of hyperventilation, causes include:\\ +  * Opioid intoxication 
- Hypoxia\\ +  * COPD
- Anxiety+
  
-What is the bicarbonate?+Respiratory alkalosis (pCO2 < 35) is the result of hyperventilation, causes include: 
 + 
 +  * Hypoxia 
 +  * Anxiety 
 + <font inherit/inherit;;#c12b04;;inherit>**What is the bicarbonate?**</font>
  
 The bicarbonate is a marker for the metabolic acid-base status of a patient. A normal HCO3 is 22 - 26, but we tend to use 24 for calculations. The bicarbonate is a marker for the metabolic acid-base status of a patient. A normal HCO3 is 22 - 26, but we tend to use 24 for calculations.
  
-•   a low HCO3 (< 24) implies a metabolic acidosis\\ +  * a low HCO3 (< 24) implies a metabolic acidosis 
-•   a raised HCO3 (> 24) implies a metabolic alkalosis+  * a raised HCO3 (> 24) 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 more 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 more than 3) implying a metabolic alkalosis. \\  \\ 
- + <font inherit/inherit;;#c12b04;;inherit>**Is there any compensation?**</font>
-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.
- + <font inherit/inherit;;#c12b04;;inherit>//Respiratory compensation//</font>
-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,    pCO2 = 1.5 [HCO3] + 8\\ +
-- in metabolic alkalosis, pCO2 = 0.7 [HCO3] + 20+
  
-Metabolic compensation+  * in metabolic acidosis, pCO2 = 1.5 [HCO3] + 8 
 +  * in metabolic alkalosis, pCO2 = 0.7 [HCO3] + 20 
 + <font inherit/inherit;;#c12b04;;inherit>//Metabolic compensation//</font>
  
 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.
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 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 10mmHg of pCO2 the HCO3 rises by 1mmol/L \\ +
-Chronically, for every rise in 10mmHg of CO2 the HCO3 rises by 4mmol/L+
  
-In respiratory alkalosis\\ +  * Acutely, for every rise in 10mmHg of pCO2 the HCO3 rises by 1mmol/L 
-Acutely, for every fall in 10 mmHg of CO2 the HCO3 falls by 2 mmol/L \\ +  * Chronically, for every rise in 10mmHg of CO2 the HCO3 rises by 4mmol/L
-Chronically, for every fall in 10mmHg of CO2 the HCO3 falls by 5 mmol/L+
  
-\\ +In respiratory alkalosis 
-What kind of a Metabolic Acidosis is present?+ 
 +  * 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 
 + 
 +---- 
 + 
 +===== Types of Metabolic Acidosis =====
  
 The anion Gap is the measured cations minus the measured anions and reflects any unmeasured anions. The normal value is < 12. The anion Gap is the measured cations minus the measured anions and reflects any unmeasured anions. The normal value is < 12.
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 When you have a metabolic acidosis, you need to measure this value, as it helps determine what sort of metabolic acidosis exists. 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**
-A high anion gap metabolic acidosis (HAGMA) occurs when AG is > 12+
  
 The causes of HAGMA can be grouped into The causes of HAGMA can be grouped into
  
-•   Lactic acidosis\\ +  * Lactic acidosis 
-•   Ketoacidosis\\ +  * Ketoacidosis 
-•   Renal failure\\ +  * Renal failure 
-•   Toxins (eg toxic alcohols)+  * Toxins (eg toxic alcohols)
  
-Some prefer difficult to remember mnemonics such as CAT MUDPILES +Some prefer difficult to remember mnemonics such as **CAT MUDPILES** 
- +  * **C**arbon monoxide, cyanide 
-       Carbon monoxide, cyanide\\ +  * **A**lcohol, alcohol ketoacidosis 
-     Alcohol, alcohol ketoacidosis\\ +  * **T**oluene 
-     Toluene\\ +  * **M**etformin, methanol 
-     \\ +  * **U**raemia 
-     Metformin, methanol\\ +  * **D**iabetic ketoacidosis 
-     Uraemia\\ +  * **P**aracetamol, propylene glycol, pyroglutamic acid 
-     Diabetic ketoacidosis\\ +  * **I**ron, isoniazid 
-     Paracetamol, propylene glycol, pyroglutamic acid\\ +  * **L**actic acidosis 
-     Iron, isoniazid\\ +  * **E**thylene glycol 
-     Lactic acidosis\\ +  * **S**alicylates 
-     Ethylene glycol\\ +**A normal anion gap metabolic acidosis (NAGMA) occurs when AG ≤ 12**
-     Salicylates +
- +
-A normal anion gap metabolic acidosis (NAGMA) occurs when AG ≤ 12+
  
 A NAGMA is due to either bicarbonate loss or chloride gain. A NAGMA is due to either bicarbonate loss or chloride gain.
  
-There is another difficult to remember mnemonic (USED CARP)+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 
 + <font inherit/inherit;;#ffffff;;inherit>d</font>
  
-       Ureterostomy\\ +----
-     Small bowel fistula\\ +
-     Extra chloride\\ +
-     Diarrhoea +
- +
-       Carbonic anhydrase inhibitors\\ +
-     Adrenal insufficiency\\ +
-     Renal tubular acidosis\\ +
-     Pancreatic 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.+===== Other useful information on a blood gas =====
  
-•   Electrolytes such as sodium (Na+), potassium (K+) and chloride (Cl-) are usually reported 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.
  
-•   COHb quantifies the percentage of circulating carboxyhaemoglobin. Smoking can be associated with levels up to 10%. 
  
-•   MetHb quantifies the percentage of circulating methaemoglobin.