Acid base disorders
1. What is normal pH?
pH = 7.38 - 7.42
[H+] = 40 nM/L for a pH of 7.4
PaCO2 = 40 mm Hg
[HCO3] = 24 meq/L
2. What is the definition for acid base disorder?
Acid base disorder is considered present when there is abnormality in HCO3 or PaCO2 or pH.
3. What does acidosis or alkalosis refer to?
Acidosis and alkalosis refer to in-vivo derangement's and not to any change in pH.
4. What does acidemia or alkalemia refer to?
Acidemia (pH < 7.38) and Alkalemia (pH >7.42) refer to derangement's of blood pH.
5. Which organs are key players in maintaining acid base balance?
Kidney, Respiratory system and Central nervous system play a key roles in maintaining the acid base status.
6. What are the primary acid base disorders?
Primary acid base disorders
7. When would you consider metabolic acidosis?
Metabolic acidosis: loss of [HCO3] 0r addition of [H+]
8. When would you consider metabolic alkalosis?
Metabolic alkalosis: loss of [H+] or addition of [HCO3]
9. When would you consider respiratory acidosis?
Respiratory acidosis: increase in pCO2
10. When would you consider respiratory alkalosis?
Respiratory alkalosis : decrease in pCO2
11. What are the required lab values and historical information you need to assess acid base disorders?
Recquired lab values/information
12. What are anions? List the anions?
13. What are cations? List the cations?
14. What is anion gap?
Anion gap (AG)
Electrochemical balance: the total anions are the same as total Cations. An imbalance between Sodium, Chlorides and Total CO2.is measured as anion gap.
- Sum of Cations minus anions
- Use the measured total CO2 from venous blood as HCO3
- anion gap is an artifact because some anions are not measured
- gap is mainly due to unmeasured proteins, phosphates and sulfates
- Normal anion gap is 8-12 meq/L (Varies from Lab to Lab)
15. How do you calculate anion gap?
For practical purposes anion gap is calculated using only Sodium, Chlorides and Total CO2.((140-(104+24)) = 12.
16. What are the compensatory measures for acid base disorders?
Respiratory regulation of pCO2 is intermediate (12-24 hours)
Renal regulation of [H] and [HCO3] occurs more slowly (several days)
17. What is difference between bicarbonate value reported in arterial blood gases and bicarbonate reported in electrolytes?
calculated using the H-H equation
From arterial blood
measured from venous blood
includes bicarbonate and dissolved carbon dioxide
runs slightly higher than calculated value from nomogram
18. What is the metabolic compensation for acute respiratory acidosis? How do you assess whether it is appropriate?
Hypoventilation: Respiratory acidosis
19. What is the metabolic compensation for chronic respiratory acidosis? How do you assess whether it is appropriate?
20. What is the metabolic compensation for acute respiratory alkalosis? How do you assess whether it is appropriate?
- The expected fall in bicarb for acute decrease in PaCO2 (Buffering) is 0.2 x (normal PaCO2-observed PaCO2)
21. What is the respiratory compensation for chronic respiratory alkalosis? How do you assess whether it is appropriate?
22. What is the respiratory compensation for acute metabolic acidosis? What are the sensors and effectors for this response?
When it comes to metabolic acid base disorders we usually do not think in terms of acute and chronic. Most of them are chronic.
Acidosis increases respiratory drive, alveolar ventilation and gets rid of Carbonic acid.
Respiratory system can never completely compensate for a metabolic defect.
Respiratory compensation attempts to maintain pH in a reasonable range.
23. What is the respiratory compensation for chronic metabolic acidosis? How do you assess whether it is appropriate?Metabolic acidosis
Estimation of expected PaCO2 for a given acidic pH also enables us to determine whether respiratory compensation is appropriate.
Compensation is never complete. If the pH is normal there is probably a superimposed second acid base disturbance.
24. What is the respiratory compensation for acute metabolic alkalosis? How do you assess whether it is appropriate?
When it comes to metabolic acid base disorders we usually think in terms of acute and chronic. Most of them are chronic.
25. What is the respiratory compensation for chronic metabolic alkalosis? How do you assess whether it is appropriate?Metabolic alkalosis
26. What is bicarbonate gap?
The bicarbonate gap
27. Explain renal mechanism for regulation of acid base disorders?
28. What are the common causes for high anion gap metabolic acidosis?
Increased anion gap
Either due to addition or reduced excretion of acid
reduced excretion of inorganic acids
retention of sulphates and phosphates
impaired net acid excretion (ATN)
impaired ammonia excretion (Chronic renal failure)
accumulation of organic acids
lactic acidosis (impaired cellular respiration with anaerobic glycolysis)
29. What are the common causes for normal anion gap metabolic acidosis?
Normal anion gap
Either due to loss or failure to generate bicarbonate
abnormally high bicarbonate loss
Kidney fails to reabsorb
renal tubular acidosis
Kidney fails to regenerate
extra renal loss of bicarbonate
acidifying salts have been added
30. What are the common causes for metabolic alkalosis?
loss of hydrogen ions from the body
net rate of renal bicarbonate generation is greater than normal
increased delivery of sodium to distal tubule (Loop diuretics)
Rapid correction of ventilation in a patient with chronic CO2 retention (posthypercapnic alkalosis)
In severe alkalosis
31. What are the common causes for respiratory acidosis?
Decreased alveolar ventilation
acute respiratory acidosis (normal bicarbonate)
respiratory muscle paralysis
acute airway obstruction
Chronic respiratory acidosis (increased bicarbonate)
Obesity hypoventilation syndromes
End stage restrictive pulmonary disease
32. What are the common causes for respiratory alkalosis?
Acute hyperventilation (Light headedness, paresthesias, circumvoral numbness, tingling of extremities and tetany.)
anxiety (breath with a paper bag)
Diffuse interstitial fibrosis
33. What are the factors that stimulate kidney to excrete acid?
Excretion of acid stimulated by
34. What are the factors that inhibit kidney to excrete acid?
Excretion of acid inhibited by
35. How do you determine whether an acid base disorder is simple or mixed?
Single acid base disturbance: the change in concentration of one anion is balanced by a reciprocal change in one other anion.
Mixed acid base disturbance: the anion patterns are more complex
36. Explain CSF barrier for acid base disorders. How does it come into play clinically?
Blood brain barrier
Freely permeable to CO2
responses occur instantaneously
lag in equilibrating with bicarbonate
early stages of metabolic acidosis 2-3 hour lag in respiratory response
hyperventilation may persist even after correction of metabolic acidosis
37. Give me your step by step approach to interpreting acid base disorders.
Approach to interpreting Acid-base disturbance
What is the [HCO3]
What is the PaCO2
38. Describe the buffer system.
Buffering-Bicarbonate is in both ICF and ECF and participates in buffering capacity
- almost entirely through bicarbonate whose concentration highest of all buffers
- small contribution from phosphate
- Hemoglobin can directly buffer protons
- H+ entry into RBC matched by exit of Na and K+
39. Describe how kidney excretes acid.
Excretion of Acid
Excretion of free [H]
40. What is the relationship between Ventilation and pCO2
Interrelationship between acute changes in CO2 and pH
This formula is usable because in the range of pH values we usually deal with, there is nearly linear relationship between pH and pCO2.
41. How do you calculate the hydrogen ion concentration?