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--- wikitox:arterial_blood_gases [2018/09/01 09:00] – created - external edit 127.0.0.1
+++ wikitox:arterial_blood_gases [2025/02/17 00:33] (current) – kharris
@@ Line 1: / Line 1: @@
-Link to [[:wikitox:3.2.1.1.1_acidosis_teaching_resources|Acidosis Teaching Resources]]
+====== Arterial Blood Gases ======
+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]]
 ----
-====== Arterial Blood Gases ======
+===== Interpretation =====
+ <font inherit/inherit;;#c12b04;;inherit>**What is the pH?**</font>
-Arterial blood gases should be taken on any patients who appear to have respiratory depression or are unconscious, and on patients who have taken [[:wikitox:2.1.11.9.2.1_tricyclic_antidepressants|tricyclic antidepressants]].
+A normal pH is 7.35 to 7.45
-The presence of CO<sub>2</sub> retention, particularly if the pCO<sub>2</sub> is rising progressively, may be an indication for ventilation.
+  * acidaemia = low pH < 7.35
+  * alkalaemia = high pH > 7.45
-The presence of acidosis is usually secondary to some other effect of the poison:
+The venous pH is ~ 0.05 lower than arterial pH. \\  \\  \\
+ <font inherit/inherit;;#c12b04;;inherit>**What is the pCO2?**</font>
-  * respiratory depression leading to respiratory acidosis
+The pCO2 is a marker of ventilation. A normal pCO2 is 35-45mmHg.
-  * seizures or shock leading to lactic acidosis
-However, the acidosis will occasionally be due to a direct effect on metabolism from specific drugs leading to a high [[:wikitox:anion_gap_acidosis|anion gap acidosis]].
+  * A high pCO2 is > 45 mmHg and implies hypoventilation
+  * A low pCO2 is < 35 mmHg and implies hyperventilation
-Acidosis can increase the toxicity of a number of drugs:
+A venous CO2 is ~ 5mmHg higher than an arterial pCO2.
+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:
+  * 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.
+  * a low HCO3 (< 24) implies a metabolic acidosis
+  * 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. \\  \\
+ <font inherit/inherit;;#c12b04;;inherit>**Is there any compensation?**</font>
+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>
+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, 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.
+The following rules can determine the adequacy of metabolic derangement:
+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 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.
+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
+ <font inherit/inherit;;#ffffff;;inherit>d</font>
+----
-  * acidosis significantly increases toxicity of tricyclic antidepressants
+===== Other useful information on a blood gas =====
-  * acidosis significantly increases CNS [[:wikitox:2.1.1.4_salicylates|salicylate]] concentrations
-  * acidosis directly increases the risk of arrhythmias with a wide range of [[:wikitox:cardiotoxic_drugs|cardiotoxic drugs]]
-Thus, in the management of poisonings, it is important to prevent acidosis and correct it if it occurs with [[:wikitox:bicarbonate|sodium bicarbonate]] and/or [[:wikitox:alkalinisation|hyperventilation]] as appropriate.
+  * 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.