ABG interpreter

pH

PaCO2 (mmHg)

Bicarbonate

PaO2 (mmHg)

Interpretation

Calculate the anion gap (if a metabolic acidosis exists):

Sodium (mEq/L)

Chloride (mEq/L)

Bicarbonate (mEq/L)

Albumin (mg/dl)

Measured serum osm (mmol/kg), optional

BUN (mg/dl) , optional

glucose (mg/dl), optional

Ethanol (mg/dl), optional

Interpretation

Respiratory acidosis with metabolic acidosis

Cardiac arrest, intoxication, multiorgan failure

Respiratory alkalosis with metabolic alkalosis

Cirrhosis with diuretics, Pregnancy with vomiting, Overventilation of COPD

Respiratory acidosis with metabolic alkalosis

COPD with diuretics, vomiting, NG suction, severe hypokalemia

Respiratory alkalosis with metabolic acidosis

Sepsis, Salicylate toxicity, Renal failure with HF, pneumonia, Advanced liver disease

Metabolic acidosis with metabolic alkalosis

Uremia or ketoacidosis with vomiting, NG suction, diuretic

Selected etiologies of respiratory alkalosis

CNS stimulation: fever, pain, fear, anxiety, CVA, cerebral edema, brain trauma, brain tumor, CNS infection

Hypoxia: lung disease, profound anemia, low FiO2

Stimulation of chest receptors: pulmonary edema, pleural effusion, pneumonia, pneumothorax, pulmonary emboli

Drugs, hormones: salicylates, catecholamines, medroxyprogesterone, progestins

Pregnancy, liver disease, sepsis, hyperthyroidism

Incorrect mechanical ventilation setting

Selected etiologies of respiratory acidosis

Airway obstruction

- upper

- lower: COPD, asthma, others

CNS depression

Sleep disorder breathing (OSA or OHS)

Ventilatory restriction

Increase CO2 production: shivering, rigor, seizures, malignant hyperthermia, hypermetabolism, increase intake of carbohydrates

Incorrect mechanical ventilation setting

Selected causes of metabolic alkalosis

Hypovolemia with Cl- depletion

GI loss of H+: Vomiting, gastric suction, villous adenoma, diarrhea with chloride-rich food

Renal loss H+: Loop and thiazide diuretics, post-hypercapnia (After institution of mechanical ventilation)

Hypervolemia Cl-expansion

Renal loss of H+: edematous states (heart failure, cirrhosis, nephrotic syndrome)

hyperaldosteronism, hypercortisolism, excess ACTH, exogenous steroids, hyperreninemia

Severe hypokalemia, renal artery stenosis, bicarbonate administration

Selected etiologies of elevated anion gap metabolic acidosis

Methanol intoxication

Uremia

DKA, alcoholic ketoacidosis, starvation ketoacidosis

Paraldehyde toxicity

Isoniazid

Lactic acidosis :
- Type A: tissue ischemia
- Type B: Altered cellular metabolism, MALA (metformin-associated lactic acidosis)

Ethanol or ethylene glycal intoxication (frequently associated with an osmolar gap)

Salicylate intoxication

Selected etiologies of normal anion gap metabolic acidosis

GI loss of HCO3

- Diarrhea, ileostomy, proximal colostomy, ureteral diversion

Renal loss of HCO3

- proximal RTA

- carbonic anhydrase inhibitor (acetazolamide)

Renal tubular disease

ATN

Chronic renal disease

Distal RTA

Aldosterone inhibitor or absence

NaCl infusion, TPN, NH4+ admistration

Internal consistency of the values using the Henderson-Hasselbach equation
(If there is a discrepancy between the pH and the [H+], the ABG may not be valid.)

[H+] = 24(PaCO2) =
[HCO3-]

PH

Approximate [H+] (nmol/L)

7.00 100

7.05 89

7.10 79

7.15 71

7.20 63

7.25 56

7.30 50

7.35 45

7.40 40

7.45 35

7.50 32

7.55 28

7.60 25

7.65 22

Test is valid

or

PCO2 < 30

HCO3 >30

Is there alkalemia or acidemia present?

Acidosis

pH < 7.35

or

PCO2 ≥45

HCO3 ≤ 15

Normal pH

7.35-7.45

and

PaCO2 35-45

HCO3 16-28

Alkalosis

pH > 7.45

or

PCO2 ≤35

HCO3 ≥28

Is the disturbance respiratory or metabolic?

Respiratory acidosis pH PaCO2

Respiratory alkalosis pH PaCO2

Metabolic acidosis pH PaCO2

Metabolic alkalosis pH PaCO2

Is there appropriate compensation for the primary disturbance?

Metabolic acidosis PaCO2 = (1.5 x [HCO3-]) +8

Acute respiratory acidosis Increase in [HCO3-]= ∆ PaCO2/10

Chronic respiratory acidosis Increase in [HCO3-]= 3.5(∆ PaCO2/10)

Metabolic alkalosis Increase in PaCO2 = 40 + 0.6(∆HCO3-)

Acute respiratory alkalosis Decrease in [HCO3-]= 2(∆ PaCO2/10)

Chronic respiratory alkalosis Decrease in [HCO3-] = 5(∆ PaCO2/10)

Calculate the anion gap (if a metabolic acidosis exists)

A standard anion gap is approximately 12 meq/L AG= [Na+]-( [Cl-] + [HCO3-]

In patients with hypoalbuminemia, the normal anion gap is lower than 12 meq/L; the “normal” anion gap in patients with hypoalbuminemia is about 2.5 meq/L lower for each 1 gm/dL decrease in the plasma albumin concentration.

If the anion gap is elevated, calculate 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 +Ethanol/4.6)
- The OSM gap should be < 10.

If there is an increased anion gap, evaluate the ratio of the change in the anion gap (∆AG) to the shift in bicarbonate concentration (∆[HCO3-]), expressed as ∆AG/∆[HCO3-]

This ratio should lie between 1.0 and 2.0 to indicate that uncomplicated anion gap metabolic acidosis is present.

If it falls outside this range, another metabolic disorder may be present:

if ∆AG/∆[HCO3-] < 1.0, a concurrent non-anion gap metabolic acidosis is likely

if ∆AG/∆[HCO3-] > 2.0, a concurrent metabolic alkalosis is likely to be present.