Evaluation and Analysis of Acid-Base Disorders

Acid-Base Analysis, What do You Need? Blood gas (pH, CO2) Serum chemistry (Na, Cl, HCO3) Calculator 30 seconds

ABG: 7.40 / 40 / 80 / 24 pH PaCO2 PaO2 HCO3

pH= 7.40 (7.35 - 7.45) PCO2 = 40 (35 - 45) HCO3 = 24 (22 - 26) Acid-Base Normals: pH= 7.40 (7.35 - 7.45) PCO2 = 40 (35 - 45) HCO3 = 24 (22 - 26)

Acidemic vs. Alkalemic pH < 7.35 = Acidemic pH > 7.45 = Alkalemic

Rule 1 Look at the pH. Whichever side of 7.40 the pH is on, the process (CO2, HCO3) 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: CO2 = Acid HCO3 = Base CO2 =  pH (acidemia)  CO2 =  pH (alkalemia) HCO3 = Base  HCO3 =  pH (alkalemia)  HCO3 =  pH (acidemia)

Four Primary Disorders: PCO2 < 35 = respiratory alkalosis PCO2 > 45 = respiratory acidosis HCO3 < 22 = metabolic acidosis HCO3 > 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!

Simple Acid-Base Disorders

Example # 1: Blood gas: 7.50 / 29 / 22 Alkalemic Low PCO2 is the primary (respiratory alkalosis) No metabolic compensation = acute process Acute Respiratory Alkalosis

Acute Respiratory Alkalosis

Example # 2: Blood gas: 7.25 / 60 / 26 Acidemic Elevated CO2 is primary (respiratory acidosis) No metabolic compensation= acute process Acute Respiratory Acidosis

Acute Respiratory Acidosis

Example # 3: Blood gas: 7.34 / 60 / 31 Acidemic Elevated CO2 is primary (respiratory acidosis) Metabolic compensation has occurred = chronic process Chronic Respiratory Acidosis with Metabolic Compensation* *true metabolic compensation takes 3 days (72hrs)

Chronic Respiratory Acidosis with Metabolic Compensation

Example # 4: Blood gas: 7.50 / 48 / 36 Alkalemic Elevated HCO3 is primary (metabolic alkalosis) Respiratory compensation has occurred = acute /chronic ? Metabolic Alkalosis with Respiratory Compensation* *Respiratory compensation takes only minutes

Metabolic Alkalosis with Respiratory Compensation

Example # 5: Blood gas: 7.20 / 21 / 8 Acidemic Low HCO3 Is primary (metabolic acidosis) Respiratory compensation is present Metabolic Acidosis with Respiratory Compensation

Anion Gap (AG): The calculated difference between the positively charged (cations) and negatively charged (anions) electrolytes in the body: AG= Na+ - (Cl- + HCO3 -) 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 Excess AG = Total AG – Normal AG (12) Excess AG + measured HCO3 = > 30 or < 23?

Mixed Acid-Base Disorders

Remember the Rules Look at the pH: (< or > 7.40?) whichever caused the shift (CO2 or HCO3) is the primary disorder Calculate the anion gap: if AG  20 there is a primary metabolic acidosis (regardless of pH or HCO3) Calculate the excess anion gap, add it to HCO3: Excess AG = Total AG – Normal AG (12) Excess AG + HCO3 = ? 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 CO2 is primary (respiratory alkalosis) Partial metabolic compensation for chronic condition? AG = 22 (primary metabolic acidosis) Excess AG (AG – 12) + HCO3 = 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) + HCO3 = 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 CO2 is primary (respiratory alkalosis) AG = 30 (primary metabolic acidosis) Excess AG (AG – 12) + HCO3 = 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 CO2 and low HCO3- both primary (respiratory acidosis and metabolic acidosis) AG = 30 (metabolic acidosis is anion gap type) Excess AG + HCO3 = 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 HCO3- primary (metabolic acidosis) AG= 25 (metabolic acidosis is anion gap type) Excess AG + HCO3 = 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