Acid Base Analysis

VBG ® ABG
pH +0.04
pCO2 -6

Henderson-Hasselbach Equation
H+ = 24 x pCO2/HCO3

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

Normal Values
Male
Female
pCO2
40
36
HCO3
25
23
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