How to Crush ABGs

Stepwise Approach w/ interpretations found below. You won’t need to look anywhere else unless you want to do the Stewart Acid Base Approach.

Step 1: Is there alkalemia or acidemia present?

pH < 7.35 acidemia
pH > 7.45 alkalemia

This is usually the primary disorder
Remember: an acidosis or alkalosis may be present even if the pH is in the normal range (7.35 – 7.45)

You will need to check the PaCO2, HCO3-, anion gap, and Albumin

Step 2: Is the disturbance respiratory or metabolic?

What is the relationship between the direction of change in the pH and the direction of change in the PaCO2?

In primary respiratory disorders, the pH and PaCO2 change in opposite directions; in metabolic disorders the pH and PaCO2 change in the same direction.

If Respiratory

  • Acidosis
    • Chronic => for every PaC02 increase of 10 mmhg, ph drops by .03
    • Acute => for every paC02 increase of 10 mmhg, ph drops by .08
  • Alkalosis
    • Chronic => Decrease of PaC02 by 10 mmHg, pH increase by .03
    • Acute => Decrease of PaC02 by 10 mmHg, pH increase by .08

Step 3: Is there appropriate compensation for the primary disturbance?

Usually, compensation does not return the pH to normal (7.35 – 7.45). If the observed compensation is not the expected compensation, it is likely that more than one acid-base disorder is present.

Metabolic Acidosis: PaC02 = 1.5 (HC03) + 8 +/- 2
Acute Respiratory Acidosis: Increase in HC03 = Change in PaC02/10 +/- 3
Chronic Respiratory Acidosis (3-5 Days) Increase in HC03 = 3.5 (Change PaC02/10)

Metabolic Alkalosis: Increase in PaC02 = .6 (Change in Bicarb)
Acute Respiratory Alkalosis: Decease in Bicarb = 2 (Change PaC02/10)
Chronic Respiratory Alkalosis: Decrease in Bicarb = 5(Change in PaC02/10) to 7(Change in PaC02/10)

Step 4: Calculate the anion gap

(if a metabolic acidosis exists):
AG = Na – (Cl +Bicarb) = 12 +/- 2
A normal anion gap is approximately 12 meq/L. Correct for albumin.
Correct for Alb: (2.5) (4-pts Alb) + AG. Super important in the Unit.
If the anion gap is elevated, consider calculating the osmolar gap in compatible clinical situations.

Elevation in AG is not explained by an obvious case (DKA, lactic acidosis, renal failure)! Toxic ingestion is suspected
OSM gap = measured OSM –
(2[Na+] – glucose/18 – BUN/2.8)
The OSM gap should be < 10!

Step 5: Are there other disturbances with AGMA?

Calculate Delta/Delta Ratio: (AG-12)/(24-Serum HC03)
Delta/Delta >6: AGMA + Metabolic Alk
Delta/Delta <6 AGMA + NAGMA

OR

Correct for Bicarb = measured serum bicarb + (AG-12)
>24 => underlying metabolic alkalosis
<24 => underlying NAGMA

Selected etiologies of respiratory acidosis
Airway obstruction, COPD, asthma, other obstructive lung disease, CNS depression, OSA, Neuromuscular impairment, Ventilatory restriction

Increased CO2 production: shivering, rigors, seizures, malignant hyperthermia, hypermetabolism, increased intake of carbohydrates, incorrect mechanical ventilation settings 

Selected etiologies of respiratory alkalosis
CNS stimulation: fever, pain, fear, anxiety, CVA, cerebral edema, brain trauma, brain tumor, CNS infection, Hypoxemia or hypoxia: lung disease, profound anemia, low FiO2

Stimulation of chest receptors: pulmonary edema, pleural effusion, pneumonia, pneumothorax, pulmonary embolus, Drugs, hormones: salicylates, catecholamines, medroxyprogesterone, progestins
Pregnancy, liver disease, sepsis, hyperthyroidism, incorrect mechanical ventilation settings

Selected etiologies of metabolic acidosis
Elevated anion gap
: Methanol, Uremia, DKA, alcoholic ketoacidosis, starvation ketoacidosis, Paraldehyde toxicity, INH, Lactic acidosis, Ethanol (elevated osmol gap) or ethylene glycol intoxication (elevated osmol gap), Salicylate intoxication

Normal anion gap: will have increase in [Cl-], GI loss of HCO3-, Diarrhea, ileostomy, proximal colostomy, ureteral diversion
Renal loss of HCO3-: proximal RTA, carbonic anhydrase inhibitor
Renal tubular disease: ATN, Chronic renal disease, Distal RTA, Aldosterone inhibitors or absence, NaCl infusion, TPN, NH4+ administration

Mixed Disorders:
Respiratory acidosis with metabolic acidosis:
Cardiac Arrest, Tox, Multi-organ failure

Respiratory alkalosis with metabolic alkalosis: Cirrhosis w/ diuretics, Pregnancy w/ vomiting, overventilation of COPD
Respiratory Acidosis w/ Metabolic Alkalosis: COPD w/ diuretics, vomiting, NG suction, severe hypokalemia
Respiratory alkalosis w/ metabolic acidosis: normal Ph: Sepsis, Aspirin Tox, RF w/ CHF and pna, advanced liver disease
Metabolic Acidosis with Metabolic Alkalosis: Normal PH: Uremia or ketoacidosis w/ vomiting, NG suction, diuretics

Sources: The ICU Book; thoracic.org; UptoDate

2 thoughts on “How to Crush ABGs”

  1. Regarding the osmolar gap and tox:
    remember, once the the parent compound is metabolized, the osmolar gap normalizes and the anion gap rises (acidic metabolites)
    so, early ingestions of ethylene glycol, methanol (or metabolism blocked by ethanol and/or fomepizole): wide osmolar gap, normal anion gap
    late ingestions: normalizing osmolar gap and wide anion gap

  2. Regarding the osmolar gap and tox:
    remember, once the the parent compound is metabolized, the osmolar gap normalizes and the anion gap rises (acidic metabolites)
    so, early ingestions of ethylene glycol, methanol (or metabolism blocked by ethanol and/or fomepizole): wide osmolar gap, normal anion gap
    late ingestions: normalizing osmolar gap and wide anion gap

Leave a Reply

Your email address will not be published. Required fields are marked *