Acid Base Analysis

Metabolic Acidosis - Anion Gap
¯HCO3-
DDx:
1. Renal failure with uremia
2. Ketoacidosis (IDDM, EtOH withdrawl)
3. Drugs/Poisons
· Methanol - osmolal gap, optic papilledema
· Salicylates - PT, Respiratory alkalosis
· Paraldehyde
· Ethylene Glycol - osmolal gap, ozalate crystals
4. Lactic acidosis (IDDM, renal failure, liver disease, tumors, starvation, sepsis)

Calculations:
Anion Gap = Na - Cl - HCO3 (Increased in >11 ± 2)
¯ D10 HCO3 = ¯ D10 PCO2
PCO2 expected = 1.5 (HCO3) + (8 ± 2)
Osm Estimated = 2Na + Glucose/18 + BUN/2.7 = 285 mOsm
measured - estimated < 10

Metabolic Alkalosis
HCO3-
DDx:
1. Fluid volume contraction (GI tract - vomiting, kidneys, skin, lungs, postoperative ascites)
2. Hypokalemia
3. Hyperaldosteronism, Excess glucocorticoids/mineralcorticoids
4. Bartter's syndrome - K+ wasting, polyuria
5. Ingestion of alkali, bicarbonate, e.g. via antacids
6. Rapid correction of hypercapnea (posthypercapnic metabolic alkalosis)
7. Diuretics - H+ secretion

Calculations:
D10 HCO3 = D7 PCO2

Cl- sensitive - urine Cl- < 15 mEq/L (Diuretics, vomiting, villous adenoma
Cl - resistant - urine Cl- > 15 mEq/L

Metabolic Acidosis - Non Gap
Hyperchloremic
DDx:
¯K+
1. GI HCO3 losses e.g. diarrhea
2. Renal tubular acidosis
· proximal - HCO3 absorption defect
· distal - NH4+ into renal vein, liver converts NH4+HCO3- to urea (spironolactone)

K+
3. Carbonic anhydrase inhibitors (acetazolamide)
4. Ureteral diversions
5. HCl or NH4Cl oral CaCl2
6. Chloride gas exposure, sulfur toxicity
7. Adrenal insufficiency, hypoaldosteronism

Respiratory Alkalosis
¯CO2 pH hyperventilating
DDx:
1. Catastrophic CNS event (e.g. intracerebral hemorrhage)
2. Drug use (salicylates, progesterone)
3. Pregnancy (third trimester)
4. Decreased lung compliance (interstitial fibrosis)
5. Anxiety
6. Cirrhosis (progesterone ® medulla)
7. Sepsis
8. High altitudes

Complications:
· K+ depletion ® cells
· Ca+ depletion
· Left shift of Hb dissociation curve

Calculations:
Acute ¯D10 PCO2 = ¯D1-2 HCO3 (ratio of 5:1)
DpH = 0.08 x (40 - PaCO2)/10

Chronic ¯D10 PCO2 = ¯D5 HCO3 (ratio of 2:1)
DpH = 0.03 x (40 - PaCO2)/10

Respiratory Acidosis
CO2 ¯pH not breathing enough
DDx:
1. CNS depression (sedatives, CNS disease, obesity-hypoventilation)
2. Pleural disease (pneumothorax, pleural effusion)
3. Lung disease (COPD, ARDS, PE, pneumonia)
4. Musculoskeletal (kyphoscoliosis, Guillain Barre, myasthenia gravis, botulism, polymyositis)

Calculations:
Acute D10 PCO2 = D1-2 HCO3 (ratio of 5:1)
DpH = 0.08 x (PaCO2 - 40)/10

Chronic D10 PCO2 = D5 HCO3 (ratio of 2:1)
DpH = 0.03 x (PaCO2 - 40)/10

pH	H+
7.50	32	80%
7.40	40	80%
7.30	50	80%
7.20	63	80%
7.10	79	80%
7.00	100	80%

Normal Values	Male	Female
pCO2	40	36
HCO3-	25	23