This page is read only. You can view the source, but not change it. Ask your administrator if you think this is wrong. ====== 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]] ---- ===== Interpretation ===== <font inherit/inherit;;#c12b04;;inherit>**What is the pH?**</font> 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. \\ \\ \\ <font inherit/inherit;;#c12b04;;inherit>**What is the pCO2?**</font> The pCO2 is a marker of ventilation. A normal pCO2 is 35-45mmHg. * A high pCO2 is > 45 mmHg and implies hypoventilation * A low pCO2 is < 35 mmHg and implies hyperventilation 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> ---- ===== 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