6Rule 1Look 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
7Keep It Simple: CO2 = Acid HCO3 = Base CO2 = pH (acidemia) CO2 = pH (alkalemia)HCO3 = Base HCO3 = pH (alkalemia) HCO3 = pH (acidemia)
8Four Primary Disorders: PCO2 < 35 = respiratory alkalosisPCO2 > 45 = respiratory acidosisHCO3 < 22 = metabolic acidosisHCO3 > 26 = metabolic alkalosisCan have mixed pictures with compensationCan have up to 3 abnormality simultaneously (1 respiratory + 2 metabolic)The direction of the pH will tell you which is primary!
20Anion 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)
21Rule 2Calculate the anion gap. If the anion gap is 20, there is a primary metabolic acidosis regardless of pH or serum bicarbonate concentrationPrinciple: The body does not generate a large anion gap to compensate for a primary disorder (anion gap must be primary)
22Rule 3Calculate 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 alkalosisif the sum is less than normal bicarbonate (< 23) there is an underlying nonanion gap metabolic acidosisExcess AG = Total AG – Normal AG (12)Excess AG + measured HCO3 = > 30 or < 23?
24Remember the RulesLook at the pH: (< or > 7.40?) whichever caused the shift (CO2 or HCO3) is the primary disorderCalculate 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 alkalosisIf sum < 23 there is an underlying nonanion gap metabolic acidosis
25Example # 1 Blood gas: 7.50 / 20 / 15 Na= 140, Cl = 103 AlkalemicLow 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 AcidosisThe patient ingested a large quantity of ASA and had both centrally mediated resp. alkalosis and anion gap met. Acidosis associated with salicylate overdose
26Example # 2 Blood gas: 7.40 / 40 / 24 Na= 145, Cl= 100 pH normalAG = 21 (primary metabolic acidosis)Excess AG (AG – 12) + HCO3 = 33 ( underlying metabolic alkalosis)Metabolic Acidosis and Metabolic AlkalosisThis 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
27Example # 3 Blood gas 7.50 / 20 / 15 Na= 145, Cl = 100 AlkalemicLow 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 AlkalosisThis patient had a history of vomiting (met. alkalosis), poor oral intake (met. acidosis) and tachypnea secondary to bacterial pneumonia (resp. alkalosis)
28How Many Primary Abnormalities Can Exist in One Patient? Three primary abnormalities is the max because a person cannot simultaneously hyper and hypoventilateOne patient can have both a metabolic acidosis and a metabolic alkalosis – usually one chronic and one acute
29Example # 4 Blood gas: 7.10 / 50 / 15 Na= 145, Cl= 100 AcidemicHigh 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 AlkalosisThis is an obtunded patient (resp. acidosis) with a history of emesis (metabolic alkalosis) and lab findings c/w diabetic ketoacidosis (metabolic acidosis w/ gap)
30Example # 5 Blood gas: 7.15 / 15 / 5 Na= 140, Cl= 110 AcidemicLow 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 AcidosisDiabetic 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.
31Conclusions:To do accurate acid-base evaluations you need both blood gas and serum chemistryUse a systematic approachRemember the 3 rules“normal” blood gases may not be normalIt is important to identify all the underlying acid-base in order to appropriately treat the patient