1. Acid-Base homeostasis
Normal- pH- 7.35-7.40-7.45
pCO2- 36-40-44 mm Hg
HCO3- 22-24-26 mmol/L

2. Body’s response
Buffering, in seconds
Lungs- change in arterial pCO2, in minutes
Kidneys- change in HCO3 excretion, in hours/days

3. Buffers
Extracellular- HCO3 & NH3
Intracellular- proteins & PO4

4. Approach to A-B disorders
 Initial clinical assessment- s/s
 Acid-base diagnosis
 Arterial pH
 Pattern of HCO3 & pCO2
 Additional clues in other investigations
 Compensation
 Formulation
 Confirmation by further tests
 Clinical diagnosis

5. Systematic evaluation
 History necessary for possible defect
 Get ABG, electrolytes, RFT
 pH<7.35- acidosis; >7.45- alkalosis
 Respiratory disorder- CO2 changes
 Increase- acidosis; Decrease- alkalosis
 Metabolic disorder- HCO3 changes
 Decrease- acidosis; Increase- alkalosis

6. Evaluation- contd.
 Clues-
 High anion gap- metabolic acidosis
 Hyperglycemia +/- urine ketones- DKA
 Hyperchloremia- normal anion-gap acidosis
 Elevated creatinine- consider ketoacidosis
 Elevated BUN/Cr- uremic acidosis/pre-renal ARF
 Hypokalemia &/or hypochloremia- metabolic alkalosis

7. Evaluation- contd.
 Compensation- predictable physiologic consequence of the primary
disturbance
 Compensation follows the direction of CO2/HCO3 change
 Compensation brings pH towards normal, but never overshoots
• Respiratory acidosis- CO2 increase- HCO3 increase
acute- 1 mEq/L for 10 mm Hg rise in PaCO2- max. 38
chronic- 4 for 10- max. 45
 Respiratory alkalosis- CO2 decrease- HCO3 decrease
acute- 2 for 10- max. 18
chronic- 5 for 10- max. 15
 Metabolic acidosis- HCO3 decrease- CO2 decrease
last 2 digits of pH or one and a half+8- max. 15
 Metabolic alkalosis- HCO3 increase- CO2 increase
point 7+20- max. 55

8. Anion gap (AG)
 Na + unmeasured cations (UC) =
Cl + HCO3 + unmeasured anions (UA)
 UC- Ca, Mg, K, IgG
 UA- albumin, PO4, SO4, organic acids
 AG = UA-UC = Na-(Cl+HCO3)
 Normal AG- 9 ± 3 mEq/L
 Helps classify metabolic acidosis
UrinaryAG = UA-UC = (Na+K)-Cl
To differentiate hyperchloremic/normal AG metabolic acidosis

9. Acid-Base disorders
Simple- one defect with
appropriate compensation
Mixed- more than one defect

10. Respiratory acidosis
 Hypoventilation
 CNS- depression, trauma,
Pickwickian syndrome, polio, tetanus
 Nerve/muscle- GBS, myasthenia,
myopathies, snake venom
 Lung- ARDS, aspiration, edema,
pneumothorax, restrictive lung disease,
COPD
 Airway- asthma, broncho/laryngospasm
 Inadequate mechanical ventilation

11. Respiratory alkalosis
 Hyperventilation
 CNS- head injury, CVA
 Psychogenic
 Pain, fever, stress
 Lung- PE, pneumonia, asthma, edema
 High altitude
 Progesterone in pregnancy, cytokines in sepsis,
toxins in CLD
 Iatrogenic

12. Metabolic acidosis
 High anion gap- added acids
 Ketoacidosis- DM, alcoholic, starvation
 Lactic acidosis- shock, extreme exertion
 ARF/CRF
 Toxins- ethylene glycol, methanol, salicylates
 Normal anion gap- hyperchloremic-
bicarbonate loss
 Renal tubular acidosis
 Severe diarrhea, GI fistulas, drainage of biliary/pancreatic
secretions

13. Metabolic alkalosis
 Chloride depletion- U Cl<10 mmol/L-
saline responsive
 Vomiting
 Diuretics
 K depletion- U Cl>20 mmol/L-
saline resistant
 Hyperaldosteronism
 Cushing’s syndrome
 Bartter’s syndrome
 Other
 Laxative abuse
 Severe hypoalbuminemia

14. Example 1
 Poorly controlled diabetic presents with polyuria
 Na-136, K-4.8, Cl-101, Cr-0.09
 Urine- 2+ ketones, 4+ glucose
 ABG- pH-7.26, pCO2-16, HCO3-7.1
 pH- acidosis
 Low HCO3- metabolic acidosis
 Compensation- 1.5x7.1+8=18.5 mm Hg- appropriate
 Anion gap- 136-(101+7)= 28- increased
 Final A-B Dx- high anion gap metabolic acidosis

15. Example 2
 Middle aged man with diarrhoea for 2 days
 Na-134, K-2.9, Cl-113, Cr-0.3
 ABG- pH-7.31, pCO2-33, HCO3-16
 pH- acidosis
 Low HCO3- metabolic acidosis
 Anion gap- 134-(113+16) = 5- normal
 Compensation- 1.5x16+8 = 32- appropriate
 Final A-B Dx- normal anion gap metabolic acidosis

16. Example 3
 Young female, on postop. Morphine & oxygen,
noted drowsy the next day
 ABG- pH-7.16, pCO2-61.9, HCO3-21.2
 pH- acidosis
 Increased pCO2 & decreased HCO3- mixed acidosis-
predominant respiratory acidosis
 Compensation- 24+22/10 = 26- mild metabolic acidosis
 Final A-B Dx- mixed acute respiratory & mild metabolic acidosis
 Electrolytes, glucose, RFT, lactate would have helped

17. Example 4
 Elderly woman with weakness & areflexia with poor oral intake
 Na-145, K-1.9, Cl-86
 ABG- pH-7.58, pCO2-49, HCO3-44.4
 pH- alkalosis
 High HCO3- metabolic alkalosis
 Compensation- 0.7x44.4+20 = 51- appropriate
 Final A-B Dx- severe metabolic alkalosis
 Emergency management of low K

18. Example 5
 Elderly woman with CHF on diuretics, admitted with lobar
pneumonia
 ABG- pH-7.64, pCO2-32, HCO3-33
 pH- alkalosis
 Low pCO2 & high HCO3- mixed alkalosis
 Compensation-
For low CO2- HCO3 = 20- inappropriate
For high HCO3- pCO2 = 43- inappropriate
 Final A-B Dx- mixed metabolic & respiratory alkalosis

19. Example 6
 Elderly man had cardiac arrest  resuscitated
 ABG- pH-6.85, pCO2-82, HCO3-14
 pH- acidosis
 High pCO2- respiratory acidosis
 Low HCO3- metabolic acidosis
 Final A-B Dx- mixed respiratory & metabolic acidosis
 Electrolytes, RFT, lactate would have helped

20. Example 7
 Elderly man with CHF, admitted with vomiting for 5 days, was
hyperventilating
 Na-127, K-5.2, Cl-79, BUN-55, Cr-0.38, glucose-98
 ABG- pH-7.58, pCO2-21, HCO3-19
 pH- alkalosis
 Low pCO2- respiratory alkalosis
 Anion gap- 29- high anion gap metabolic acidosis
 Compensation- 24-10=14- actual 19- met. Alkalosis
 Final A-B Dx- triple disorder- R.al & M.al & high AG M.ac