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Renal Week 2 > Lecture 20: Respiratory Acid-Base Disorders > Flashcards

Lecture 20: Respiratory Acid-Base Disorders Flashcards

1
Q

What determines pCO2?

A

Determined by ventilation
Hyperventilation = lower pCO2
Hypoventilation = higher pCO2

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2
Q

What are normal values of pH, PCO2 and bicarb?

A

pH = 7.4
pCO2 = 40 mmHg
bicarb = 24 mEq/L
control of ventilation primarily in medullary respiratory center
Chemoreceptors in medulla, carotid and atrium that respond to pH, PCO2 and PO2
Tissue is buffered first (through bicarb buffer system), and then bicarb excretion is adjusted

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3
Q

What is hypocapnia?

A

Known as hypocarbia

A state of REDUCED carbon dioxide in the blood

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4
Q

What causes respiratory alkalosis?

A

Hyperventilation

Decrease pCO2

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5
Q

What causes respiratory acidosis?

A

Hypoventilation

Increased pCO2

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6
Q

Why is there a division between acute and
Chronic compensation for respiratory
Alkalosis?

A

Because there are two phases to alkalosis,
First is tissue buffering
Second is bicarb excretion
Figure on the right shows hypocapnia patient
Vs normal control
Patient that has reduced pCO2 maintains bicarb levels for a little while before falling significantly by 13th hour (important to determine whats acute and what’s chronic)

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7
Q

What is the compensation for acute respiratory alkalosis?

A

2 mEq of HCO3 for every -10 mEq of pCO2

Occur in minutes

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8
Q

What is the compensation for chronic respiratory alkalosis?

A

-5 mEq of bicarb for every -10 mEq of pCO2
Occurs in days
Compensation is so good that the pH can completely correct

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9
Q

What is the only primary acid-base abnormality that can correct to normal pH?

A

Chronic respiratory alkalosis
Thus, if you see normal pH with abnormal CO2 and bicarb levels, then the patient either has chronic respiratory alkalosis or mix acid/base disorder

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10
Q

What are the associated problems of respiratory alkalosis?

A
  1. Parasthesias, numbness, tetany
    • decreased calcium and increased neuromuscular excitability
  2. Dizziness, confusion (cerebral vasospasm)
    • decreased intracranial pressure
    • also decreased blood flow to brain
  3. Chronic respiratory alkalosis is generally asymptomatic due to compensation
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11
Q

What is the pathophysiology of respiratory alkalosis?

A 
Determined by equation:
PCO2 = VCO2/VA * K 
VCO2 = metabolic production of CO2
VA = alveolar ventilation
K = constant

Take home point: PCO2 = 1/VA or PCO2 is determined by alveolar ventilation
Thus, increased ventilation (hyperventilation) means DECREASE in PCO2

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12
Q

What does carbohydrate loading do to PCO2 levels?

A

Increased carb intake = increased metabolic production of CO2 (VO2) which can lead to increase in PCO2…thus there must be a compensatory in VA (alveolar ventilation)

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13
Q

What are the causes of respiratory alkalosis?

A
  1. Pain
  2. Anxiety
  3. fever
  4. exercise
  5. hypoxia
  6. liver disease
  7. pregnancy
  8. Drugs
  9. excessive mechanical ventilation
  10. sepsis
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14
Q

What do liver disease and pregnancy have in common that lead to respiratory alkalosis?

A

Both conditions will increase levels of estrogen and progesterone
Liver disease will increase estrogen/progesterone because that is where those hormones are broken down

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15
Q

What are the drugs that cause respiratory alkalosis?

A
  1. Aspirin
  2. Theophylline
  3. Progesterone
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16
Q

What is theophylline?

A

A methylxanthine drug used for respiratory diseases like COPD

17
Q

What is the treatment of respiratory alkalosis?

A

Usually unnecessary
Sedation, analgesia, antipyretics
Treat underlying problem
In respiratory alkalosis complicating mechanical ventilation, adjusting settings may not be enough because you are not treating underlying cause

18
Q

What are the characteristics of respiratory acidosis?

A

Less breathing
Hypoventilation
Increased pCO2 which leads to increased bicarb compensation

19
Q

What is the compensation for respiratory acidosis?

A

Acutely = +1 bicarb for every +10 in pCO2
-occurs in minutes
Chronically = +3.5 for ever +10 in pCO2
-occurs in days and weeks

20
Q

What are the associated problems of respiratory acidosis?

A 
Usually clinically important (unlike respiratory alkalosis)
Patient = confusion and obtundation (like the person took opiates)
Increased cerebral vasodilation that then leads to increased intracranial pressure
	-blurred vision, headache, restlessness
Respiratory alkalosis (hyperventilation) is important in defense against metabolic acidosis
21
Q

What are the components of alveolar ventilation?

A 
Vt = Va + Vd
Vt = total ventilation
Va = alveolar ventilation
Vd = dead-space ventilation (gas that never gets to alveoli)
22
Q

Thus, knowing that Va = Vt – Vd, how do you decrease alveolar ventilation?

A
  1. decreased total ventilation = due to opiate overdose

2. Increased dead space ventilation = COPD

23
Q

What are the acute causes of respiratory acidosis?

A
  1. sedative drug OD (like opiates and benzodiazepines)
  2. acute exacerbations of respiration disease
  3. O2 in patients with chronic hypercapnia
24
Q

What are the chronic causes of respiratory acidosis?

A
  1. emphysema
  2. other severe lung, chest, neuromuscular diseases like Guillain Barre syndrome
  3. obstructive sleep apnea
  4. obesity hypoventilation

Once can also have acute on chronic respirator acidosis = patient with emphysema that gets sedative drug OD

25
Q

How do we treat respiratory acidosis?

A

Sedative overdoses treated with naloxone and ventilators (like the iron lung)
Requires urgent treatment

26
Q

How do we treat patients with highest levels of pCO2?

A

By eliminating fat and carbs from diet, but only in desparate cases

27
Q

How does one determine if there are mixed acid-base disorders?

A

5 different combinations
A. Respiratory alkalosis with metabolic acidosis
B. Respiratory alkalosis with metabolic alkalosis
C. Respiratory acidosis with metabolic alkalosis
D. Respiratory acidosis with metabolic acidosis
E. Meetabolic acidosis with metabolic alkalosis

28
Q

What are the characteristics of respiratory alkalosis with metabolic acidosis?

A

pH 7.34, pCO2 = 25, bicarb = 15
1.5*15 = 22 + 8 = 30  expected value for pCO2
However, lowered pCO2 = respiratory alkalosis
Due to aspirin OD, sepsis + cirrhosis

29
Q

What are the key characteristics of respiratory acidosis with metabolic acidosis?

A

pH = 7.19, pCO2 = 45, bicarb = 18
1.5*18 + 8 = 30.5  expected value for pCO2
High pCO2 = respiratory acidosis
Low bicarb = metabolic acidosis
Caused by cardiac arrest, prolonged COPDexacerbation, COPD + sepsis

30
Q

What are the key characteristics of respiratory acidosis with metabolic alkalosis?

A

pH = 7.34, pCO2 = 60, bicarb = 36
Normal pCO2 = 40 so pCO2 is +20
-pCO2 + 20 means that bicarb should compensate chronically by +7 (since it’s +3.5 for every +10 of PCO2 in respiratory ACIDOSIS)
-thus you know that bicarb is too high, thereby suggesting metabolic alkalosis
Caused by COPD, diuretics, post hypercapneic alkalosis

31
Q

What are the key characteristics of respiratory alkalosis with metabolic alkalosis?

A

pH = 7.63, pCO2 = 30, HCO3 = 32
Low pCO2 so bicarb should decrease in order to compensate, but since neither pCO2 and biarb is compensating, it suggests BOTH alkalosis and metabolic alkalosis
Caused by early sepsis + gastric drainage, pain + diuresis

32
Q

How does one analyze an acid base problem?

A
  1. is this an acidemia or an alkalemia?
  2. Is the primary process respiratory or metabolic
  3. Is there appropriate compensation
  4. What are the possible causes
  5. What’s the diagnosis and therapy?

Renal Week 2 (12 decks)

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