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  1. Normal blood ph (7.35 – 7.45) (LQ 2012)
  2. Buffers provide defense to the pH and include HCO3, phosphates, and protein buffers.
  3. Regulation of pH depends on three main regulatory processes that include Chemical buffering, regulation of pCO2 by the respiratory system, and the regulation of HCO3 by the kidneys.
  4. The lungs eliminate CO2, which is the principal end product of acid metabolism.
  5. The role of the kidneys is to retain HCO3 and eliminate protons of nonvolatile acids.

Normal Values Of Blood Gases With Definitions

  1. PaO2. It is the partial pressure of oxygen in the blood. Normal values are 100 mmHg and 40 mmHg in the artery and mixed venous blood respectively.
  2. PaCO2. It is partial pressure of CO2 in the blood. Normal values are (40mmHg in artery and 45 in venous blood).
  3. SO2. It is the amount of oxygen carried on the surface of the hemoglobin molecules present in the plasma. Normal values are 98% saturation in the arterial blood and 75% saturation in the mixed venous blood.
  4. HCO3 Normal values are (20-28 mmol/L).
  5. PAO2. It is the partial pressure of oxygen in the alveolar gas. (A-a) O2 gradient. It is the pressure difference between the alveolar and the arterial oxygen pressures that drives gas exchange between the alveoli and the arterial blood. Normal values are 5-25 mmHg on room air and up to 150mmHg at a FiO2 of 1.0.
  6. Anion Gap. It is the difference between the unmeasured anions and the unmeasured cations of the body. Normal plasma anion gap is 6 to 12 [AG = (Na) – (Cl + HCO3)] (Ref: Hari.18th ed., Pg- 365 & 2976)
  7. Osm (Serum Osmolality). It is measured by the formula: 2Na + glucose (mg%)/18 + BUN(mg%)/2.8. Normal serum osmolality is 275-290 mOsmol/kg.
  8. Cl- levels. Normal values range from 97-110 mmol/L.
  9. Na+. Normal values range from 135-145 mmol/L.
  10. K+. Normal values range from 3.3-4-9 mmol/L.
  11. iCa. ionized calcium. Normal values range from 1.13-1.28 mmol/L (4.5 to 5.3 mg%)

Delta Gap

  1. This is simply the difference in the calculated anion gap (not including potassium) and a normal anion gap of 12. The delta gap is useful in assessing the bicarbonate levels and thus is considered a HCO3 equivalent.
  2. The Delta Ratio (∆/∆)
    i. The delta ratio is sometimes used in the assessment of elevated anion gap metabolic acidosis to determine if a mixed acid base disorder is present.
    ii. Delta ratio = ∆ Anion gap/∆ [HCO3-]

Delta ratio

Assessment Guidelines

  < 0.4

Hyperchloremic normal anion gap acidosis

  < 1

High AG & normal AG acidosis

  1 to 2

Pure Anion Gap Acidosis
Lactic acidosis: average value 1.6
DKA more likely to have a ratio closer to 1 due to urine ketone loss

  > 2

High AG acidosis and with metabolic alkalosis


High AG acidosis with respiratory acidosis

Basic fundamental

  1. Acidosis occur by two mechanism: a. Gain of acid b. Loss of alkali
  2. Alkalosis occur by two mechanism: a. Loss of acid b. Gain of alkali


CO2 is an acid, being controlled by lungs. HCO3 is an alkali, being controlled by kidneys.


Any disturbance of CO2 leads to respiratory acidosis / alkalosis


Any disturbance of HCO3 leads to metabolic acidosis/ alkalosis


Primary alteration

Compensatory response


Metabolic acidosis

plasma HCO3



Metabolic alkalosis

plasma HCO3



Respiratory acidosis



HCO3 re-absorption by the kidney

Respiratory alkalosis



HCO3 re-absorption by the kidney

Anion Gap

  1. Unmeasured anions in the body are anionic proteins, phosphate, sulfate, and organic anions.
  2. Unmeasured actions in the body include calcium, magnesium and potassium. In equilibrium state:
  3. Normal plasma anion gap = 14 ± 4 (MCQ)

Decreased anion gap

  1. Increased unmeasured cations:
    1. Hyperkalemia
    2. Hypercalcemia
    3. Multiple Myeloma
  2. Decreased unmeasured anions:
    1. Hypoalbuminemia due to Nephrotic syndrome

Common Causes of Acid Base Disturbances (Ref. Hari. 18th ed., Pg- 365, table 47.4)

  1. Metabolic Acidosis
    1. Increased anion gap acidosis
      1. 1). Lactic acidosis 2). DKA (LQ 2012) 3). Starvation ketoacidosis 4). ARF and CRF (LQ 2012)
      2. Poisoning
        1. Salicylates
        2. Methanol
        3. Ethylene glycol
    2. Non anion gap (hyperchloremic) metabolic acidosis
      1. RTA (LQ 2012)
      2. Diarrhea
      3. Interstitial nephritis
      4. Glue sniffing (LQ 2012)
    3. Anion gap less than normal is seen in
      1. Multiple myeloma
      2. Hyperparathyroid
  2. Metabolic Alkalosis
    1. Vomiting
    2. Diuretic therapy
    3. Hyperaldosteronism
    4. Cushing syndrome
    5. Little syndrome
    6. Bartter syndrome
  3. Respiratory Acidosis
    1. Respiratory centre inhibition E.g. sedatives, CNS lesions (stroke, encephalitis) oxygen therapy in chronic hypercapnic patients.
    2. Nerve damage outside CNS. E.g. Guillain Barre syndrome.
    3. Chest wall disorder. E.g. kyphoscoliosis
    4. Airway obstruction. E.g. COPD
  4. Respiratory Alkalosis
    1. CNS disorders. E.g. meningitis, hysteria
    2. Metabolic disorders. E.g. sepsis, high altitude
    3. Pulmonary disorders. E.g. asthma, pulmonary embolism
    4. Drugs. E.g. salicylate excess

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