Integrated Control of Breathing

Question 1

How does respiratory acidosis occur in the event of hypoventilation?

Answer 1

1. In the event of hypoventilation (decreased ventilation), there is a decreased ventilation / exchange rate.
2. This increases P_ACO₂, which increases P_aCO₂, which results in hypercapnia.
3. This increases H+ concentration in the blood, which decreases the pH (acidosis).
4. The body’s chemoreceptors detect both the increased P_ACO2 and the decreased pH.
5. They then signal to increase the respiratory rate to counterbalance this.

In the event of chronic hypo-ventilation, HCO₃^- excretion must decrease to maintain normal pH. So the kidneys compensate and retain more HCO₃^-.

Question 2

What are some causes of hypoventilation?

Answer 2

• It can be caused by airway obstruction.
• This can manifest in several ways: -
  
  • Asphyxia, chocking, etc.
  • Obstructive sleep apnoea.
  • Increased airway resistance (as in asthma and COPD).
• It can also be caused by problems with the initiation of breathing. Here the airway is clear.
• This can happen in several ways: -
  
  • Drug overdose (typically opioids).
  • Stroke.
  • Neuromuscular problems (such as motor neuron disease, or respiratory muscle wasting).

Question 3

How does respiratory alkalosis occur in the event of hyperventilation?

Answer 3

1. In the event of hyperventilation, there is an increased ventilation rate.
2. This decreases P_ACO₂, which decreases PaCO₂, resulting in hypocapnia.
3. This decreases the H+ concentration in the blood, which increases the pH (alkalosis).
4. The body’s chemoreceptors detect the decreased P_ACO₂ and the increased pH.
5. They then signal to decrease the respiratory rate to counterbalance this.

In the event of chronic hyperventilation, HCO₃^- excretion must increase to maintain normal pH. So the kidney will not reabsorbe a much HCO₃^-.

Question 4

What is another cause of respiratory alkalosis?

Answer 4

Altitude.

1. As you tarvael to regions of high altitude the pressure of the surrounding atmosphere decreases.
2. The fraction of the gases in the atmosphere doesnt change much. (So O2 is still 21%, CO2 ~ 0%)
3. This casues the PaO2 to decrease at higher altitudes.
4. So you are beathing in less O2 and the body responds by increasing the level of ventilation.
5. As you try to get in more O2 by increased ventilation the PaCO2 level decreases (As you end up expelling more as you inrease breathing rate).
6. This causes the pH to become less acidic and more alkaline causing respiratory alkalosis.

Question 5

What is metabolic acidosis?

Answer 5

• It is when the metabolic acid production exceeds excretion, or it can occur due to insufficient HCO₃^- concentrations.
• Unless the PaCO2 changes in proportion with the HCO3- concentration (in the opposite direction), acidosis will occur.
• Respiratory compensation typically occurs almost instantly; however, it may not be sufficient enough to prevent a disturbance in pH.

Question 6

What are some of the causes of metabolic acidosis?

Answer 6

• LACTIC ACIDOSIS (SEPSIS):
  
  • Infection.
  • Pathological immune response.
  • Septic shock.
  • Decreased O2 delivery to tissues (hypoxia).
  • Increased lactic acid production and decreased pH
  • Leads to metabolic acidosis.
  • There is respiratory compensation to increase O2 delivery and increase lactic acid conversion.
• DIABETIC KETOACIDOSIS:
  
  • Increased fatty acid released from liver due to insulin deficiency.
  • This causes increased (acidic) ketone body production.
• DIARRHOEA:
  
  • e.g. decreased HCO3- absorption in the colon.
• RENAL FAILURE:
  
  • e.g. decreased HCO3- reabsorption in proximal tubule.

Question 7

What is metabolic alkalosis?

Answer 7

• It is when metabolic acid excretion is excessive, or it can occur due to excess HCO₃^- concentration.
• Unless the PaCO2 changes with the HCO3- concentration, alkalosis will occur.
• Again, respiratory compensation typically occurs almost instantly; however, it may not be sufficient to prevent a disturbance in pH.

Question 8

List some causes of metabolic alkalosis.

Answer 8

• DIURETICS:
  
  • Changes to H+ and HCO₃^- reabsorption.
• ANTACIDS:
  
  • Increase HCO₃^- consumption.
• VOMITING:
  
  • Loss of H+ in stomach acid.

Question 9

What are the diferent types of acid - base disorders and their compensatory mechanisms?

Answer 9

Question 10

Interpret these ABG results for acid-base disorders:

• pH: 7.30 (7.35-7.45)
• PaCO2: 4.0 (4.9 - 6.1)

Answer 10

• The pH is low, indicating acidosis.
• The PaCO2 is low, indicating hyperventilation. However, this doesn’t produce acidosis, so it indicates respiratory compensation.
• The [HCO3-] is low. This does produce acidosis, so it indicates metabolic acidosis.
• This patient has metabolic acidosis with respiratory compensation.

Question 11

Interpret these ABG results for acid-base disorders:

• pH: 7.48 (7.35-7.45)
• PaCO2: 3.0 (4.9-6.1)

Answer 11

• The pH is high, indicating alkalosis.
• The PaCO2 is low, indicating hyperventilation. This produces alkalosis, thus indicating respiratory alkalosis.
• The [HCO3-] is low. However, this doesn’t produce alkalosis, so this indicates metabolic compensation.
• This patient has respiratory alkalosis with metabolic compensation.

Question 12

Interpret these ABG results for acid-base disorders:

• pH: 7.1 (7.35-7.45)
• PaCO2: 8.0 (4.9-6.1)

Answer 12

• The pH is low, indicating acidosis.
• The PaCO2 is high, indicating hypoventilation. This produces acidosis, indicating respiratory acidosis.
• The [HCO3-] is low. This does produce acidosis, so it indicates metabolic acidosis.
• The patient has mixed metabolic and respiratory acidosis.

Question 13

During exercise does PaCO2 increase or decrease?

Answer 13

• In healthy individuals, PaCO2 does not increase during exercise (in fact it may decrease slightly).
• This is because ventilation (so more CO2 is removed from the body while taking in more O2) increases before the extra CO2 produced by muscles can increase PaCO2.