1. Acid Base Disorders
Farah Adibah Kasmin

2. OUTLINE
• Introduction
• Definitions
• Regulatory Mechanisms
• Acid Base Disorders
• Causes
• Diagnosis

3. Introduction
• Acid-base homeostasis critically affects the tissue and organ
performance.
• Both acidosis and alkalosis can have severe and life threatening
consequences.
• It is the nature of the responsible condition that determines the
prognosis.

4. Definitions
• An acid is a substance that can release or donate H+.
• A base is a substance that can combine with or accept H.
• Acid base balance : maintenance of normal pH within the body
systems.
• pH is a logarithmic measure of hydrogen ion concentration.
pH= -log10 [H+]
• Normal body pH : 7.35 - 7.45
• Acidosis < 7.35 alkalosis >7.45

5. Definitions
• The pH of a solution is determined by the pKa or partial acidity
constant and the ratio of the concentration of the conjugate base
to acid.
pH= pKa + log [A-]
[HA]
(Henderson-Hasselbalch equation)

6. Regulation of acid base balance
• The body blood’s pH is strictly regulated between 7.35 – 7.45.
• Minor changes in the body acidity will affect the protein stability
and biochemical process.
• Avoiding acidemia and alkalemia by tightly regulation of H+ is
essential for normal cellular function.

7. Regulatory Mechanisms
• Buffer system
– 1st line defense
• Respiratory
• Renal

8. Rates of Correction
• Buffers function almost instantaneously
• Respiratory mechanisms take several minutes
to hours
• Renal mechanisms may take several hours to
days

9. Buffering
• The concentration of free hydrogen is controlled by buffers
which acts as hydrogen sponge.
• When [H] is low (high pH) , hydrogen sponges release hydrogen
and increase the free H conc.
• When [H] is high (low pH), hydrogen sponges engulf the free
hydrogen and decrease the free H conc.
• The major Hydrogen buffers are Bicarbonate, inorganic
phosphate and plasma protein.

11. Respiratory Mechanism
• Exhalation of carbon dioxide.
• Powerful, but only works with volatile acids such
as carbonic acid.
• Doesn’t affect fixed acids like lactic acid.
• Body pH can be adjusted by changing rate and
depth of breathing.

12. Respiratory Mechanisms
• Arterial PCO2 stimulates chemorecptors in the medulla
oblongata.
• An elevated arterial blood PCO2 is a stimulus to increase
ventilation leading to increased expiration of CO2 hence
increase blood pH.
• Conversely, a drop in blood PCO2 inhibits ventilation; the
consequent rise in blood [H2CO3] reduces the alkaline shift
in blood pH.

13. Renal regulation
• The role of kidney is to maintain plasma HCO3 concentration and
there by pH regulation.
• The kidneys regulate HCO3 by:
1. Excretion of H ions by tubular secretion.
2. Reabsorption of filtered bicarbonate ions.
3. Production of new HCO3 ions.

14. Mechanism of HCO3
- Reabsorption and
Na+ - H+ Exchange In Proximal Tubule and LOH

15. HCO3
- Reabsorption and H+ Secretion in
Distal and Collecting Tubules

18. Causes of Acid Base Disorders
Metabolic
Acidosis Anion
Gap
“MUDPILERS”
Metabolic
Acidosis Non-
Gap
“HARDUPS”
Acute Resp.
Acidosis
“anything
causing
hypoventilation”
Metabolic
Alkalosis
“CLEVERPD”
Respiratory
Alkalosis
“CHAMPS”
•Methanol
•Uremia
•DKA/Alcoholic
ketoacidosis
•Paraldehyde
•Isoniazid
•Lactic acidosis
•Ethanol
•Renal
failure/Rhabdo
•Salicylates
•Hyperalimentation
•Acetazolamide
•Renal Tubular
Acidosis
•Diarrhea
•Uretero-Pelvic
shunt
•Post-hypocapnia
•Spironolactone
•CNS depression
•Airway
obstruction
•Pulmonary
edema
•Pneumonia
•Hemo/Pneumo
thorax
•Neuromuscular
•Contraction
•Licorice
•Endocrine
(Conn/Cushing
/Bartters)
•Vomiting
•Excess alkali
•Refeeding
•Post-
hypercapnia
•Diuretics
•CNS disease
•Hypocapnia
•Anxiety
•Mech. Ventilation
•Progesterone
•Salicylates
•Sepsis

19. Anion Gap
• The anion gap is the difference in the
measured cations (positively charged ions)
and the measured anions (negatively charged
ions) in serum or urine.
• It is calculated as :
[Na+] − ([Cl−] + [HCO3−])
• Anion gap is calculated when attempting to
identify the cause of metabolic acidosis.

20. Diagnosis of Acid Base Disorder
1. Determine the primary disturbance:
– Acidemia or Alkalemia: look at the pH
< 7.40 = acidemia
> 7.40 = alkalemia
– Respiratory or Metabolic: look at HCO3 and CO2
HCO3 = primary metabolic acidosis
pCO2 = primary respiratory acidosis
and vice versa for alkalosis

21. Diagnosis of Acid Base Disorder
2. Primary Metabolic Disturbance:
o Calculate anion gap : Na – (Cl + HCO3)
o Normal = 12 +/- 2
o If gap is >20 then there is primary metabolic acidosis regardless of
pH or bicarb.
o Helps narrow differential with a anion gap or non- anion gap metabolic
acidosis

22. Diagnosis of Acid Base Disorder
4. Assess appropriate respiratory compensation
for metabolic disorder:
o Respiratory compensation is fast
o Winters formula:
Expected pCO2 = (1.5 * HCO3) + 8 (+/-2)
o If measured pCO2 is
< expected then co-existing resp. alkalosis
> expected then co-existing resp. acidosis

23. Clinical correlation: Example 1
• A 15 year old boy is brought from examination hall in apprehensive
state with complain of tightness of chest.
pH 7.54 HCO3 21 mEq/L PaCO2 21 mm of hg

24. Example 1 : Analysis
• pH is high so patient has alkalosis.
• Low PaCo2 is suggestive of respiratory alkalosis.
• HCO3 is also low suggestive of compensation (follows same direction
rule)
• Expected acute compensation (fall in HCO3) in respiratory alkalosis.
• So the patient has primary respiratory alkalosis due to anxiety.

25. Example 2
• A patient with poorly controlled IDDM missed his insulin for 3 days.
pH 7.1
mEq/l
HCO3 8 mEq/l PaCO2 20 mmhg Na 140
CL 106 mEq/l and urinary ketones +++

26. Example 2: Analysis
• pH is low so patient has acidosis. Low HCO3 is suggestive of
metabolic acidosis. PaCO2 is also low suggestive of compensation.
• AG is 26 (AG=Na-(Cl+HCO3)=140-(106+8)=140-114=26, which is high.
• So high AG Metabolic Acidosis. Presence of urinary ketones suggests
presence of diabetic ketoacidosis.
• So the patient has high anion gap metabolic acidosis due to DKA

27. Example 3
• A patient with severe diarrhea, c/o difficulty in breathing (due to
muscle weakness).
pH 7.1 HCO3 14 mEq/l PaCO2 44 mmhg K 2.0 mEq/l

28. Example 3: Analysis
• pH is low so patient has acidosis.
• Low HCO3 is sign of metabolic acidosis.
• PaCO2 is expected to reduce due to compensation. However actual
PaCO2 is high, which is indicates presence of associated respiratory
acidosis.
• Very low K causing weakness of respiratory muscles is the cause of
respiratory failure leading to respiratory acidosis. So this patient has
mixed disorder ,metabolic acidosis with respiratory acidosis.

29. Thank you.