Presentation on theme: "Evaluation and Analysis of Acid-Base Disorders. Acid-Base Analysis, What do You Need? Blood gas (pH, CO 2 ) Serum chemistry (Na, Cl, HCO 3 ) Calculator."— Presentation transcript:
Rule 1 Look at the pH. Whichever side of 7.40 the pH is on, the process (CO 2, HCO 3 ) that caused it to shift that way is the primary abnormality. Principle: The body does not fully compensate for a primary acid-base disorder
Keep It Simple: CO 2 = Acid – CO 2 = pH (acidemia) – CO 2 = pH (alkalemia) HCO 3 = Base – HCO 3 = pH (alkalemia) – HCO 3 = pH (acidemia)
Four Primary Disorders: PCO 2 < 35 = respiratory alkalosis PCO 2 > 45 = respiratory acidosis HCO 3 < 22 = metabolic acidosis HCO 3 > 26 = metabolic alkalosis – Can have mixed pictures with compensation – Can have up to 3 abnormality simultaneously (1 respiratory + 2 metabolic) – The direction of the pH will tell you which is primary!
Anion Gap (AG): The calculated difference between the positively charged (cations) and negatively charged (anions) electrolytes in the body: AG= Na + - (Cl - + HCO 3 - ) Normal AG = 12 ± 2 (10 – 14)
Rule 2 Calculate the anion gap. If the anion gap is 20, there is a primary metabolic acidosis regardless of pH or serum bicarbonate concentration Principle: The body does not generate a large anion gap to compensate for a primary disorder (anion gap must be primary)
Rule 3 Calculate the excess anion gap (total anion gap – normal anion gap) and add this value to the measured bicarbonate concentration: – if the sum is > than normal bicarbonate (> 30) there is an underlying metabolic alkalosis – if the sum is less than normal bicarbonate (< 23) there is an underlying nonanion gap metabolic acidosis 1. Excess AG = Total AG – Normal AG (12) 2. Excess AG + measured HCO 3 = > 30 or < 23?
Remember the Rules 1. Look at the pH: ( 7.40?) whichever caused the shift (CO 2 or HCO 3 ) is the primary disorder 2. Calculate the anion gap: if AG 20 there is a primary metabolic acidosis (regardless of pH or HCO 3 ) 3. Calculate the excess anion gap, add it to HCO 3 : Excess AG = Total AG – Normal AG (12) Excess AG + HCO 3 = ? If sum > 30 there is an underlying metabolic alkalosis If sum < 23 there is an underlying nonanion gap metabolic acidosis
Example # 1 Blood gas: 7.50 / 20 / 15 Na= 140, Cl = 103 Alkalemic Low CO 2 is primary (respiratory alkalosis) Partial metabolic compensation for chronic condition? AG = 22 (primary metabolic acidosis) Excess AG (AG – 12) + HCO 3 = 25 (no other primary abnormalities) Respiratory Alkalosis and Metabolic Acidosis The patient ingested a large quantity of ASA and had both centrally mediated resp. alkalosis and anion gap met. Acidosis associated with salicylate overdose
Example # 2 Blood gas: 7.40 / 40 / 24 Na= 145, Cl= 100 pH normal AG = 21 (primary metabolic acidosis) Excess AG (AG – 12) + HCO 3 = 33 ( underlying metabolic alkalosis) Metabolic Acidosis and Metabolic Alkalosis This patient had chronic renal failure (met. acidosis) and began vomiting (met. alkalosis) as his uremia worsened. The acute alkalosis of vomiting offset the chronic acidosis of renal failure = normal pH
Example # 3 Blood gas 7.50 / 20 / 15 Na= 145, Cl = 100 Alkalemic Low CO 2 is primary (respiratory alkalosis) AG = 30 (primary metabolic acidosis) Excess AG (AG – 12) + HCO 3 = 33 (underlying metabolic alkalosis) Respiratory alkalosis, Metabolic Acidosis and Metabolic Alkalosis This patient had a history of vomiting (met. alkalosis), poor oral intake (met. acidosis) and tachypnea secondary to bacterial pneumonia (resp. alkalosis)
How Many Primary Abnormalities Can Exist in One Patient? Three primary abnormalities is the max because a person cannot simultaneously hyper and hypoventilate One patient can have both a metabolic acidosis and a metabolic alkalosis – usually one chronic and one acute
Example # 4 Blood gas: 7.10 / 50 / 15 Na= 145, Cl= 100 Acidemic High CO 2 and low HCO 3 - both primary (respiratory acidosis and metabolic acidosis) AG = 30 (metabolic acidosis is anion gap type) Excess AG + HCO 3 = 33 (underlying metabolic alkalosis) Respiratory Acidosis, Metabolic Acidosis and Metabolic Alkalosis This is an obtunded patient (resp. acidosis) with a history of emesis (metabolic alkalosis) and lab findings c/w diabetic ketoacidosis (metabolic acidosis w/ gap)
Example # 5 Blood gas: 7.15 / 15 / 5 Na= 140, Cl= 110 Acidemic Low HCO 3 - primary (metabolic acidosis) AG= 25 (metabolic acidosis is anion gap type) Excess AG + HCO 3 = 18 (underlying nonanion gap metabolic acidosis) Anion Gap and Nonanion gap Metabolic Acidosis Diabetic ketoacidosis was present (anion gap met. acidosis). Patient also had a hyperchloremic nonanion gap met. acidosis secondary to failure to regenerate bicarbonate from ketoacids lost in the urine.
Conclusions: To do accurate acid-base evaluations you need both blood gas and serum chemistry Use a systematic approach Remember the 3 rules “normal” blood gases may not be normal It is important to identify all the underlying acid- base in order to appropriately treat the patient