Integrated Control of Breathing

Question 1

How does respiratory acidosis occur in the event of hypoventilation?

Answer 1

In the event of hypoventilation, there is a decreased ventilation/ exchange rate. This increases PACO2, which increases PaCO2, which results in hypercapnia. This increases H+ concentration in the blood, which decreases the pH (acidosis).
The body’s chemoreceptors detect both the increased PACO2 and the decreased pH. They then signal to increase the respiratory rate to counterbalance this.

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. 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

In the event of hyperventilation, there is an increased ventilation rate. This decreases PACO2, which decreases PaCO2, resulting in hypocapnia. This decreases the H+ concentration in the blood, which increases the pH (alkalosis).
The body’s chemoreceptors detect the decreased PACO2 and the increased pH. They then signal to decrease the respiratory rate to counterbalance this.

Question 4

What is metabolic acidosis?

Answer 4

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 5

List some causes of metabolic acidosis.

Answer 5

• LACTIC ACIDOSIS (EXERCISE): intense exercise, insufficient O2 delivery, increased lactic acid production, metabolic acidosis, subsequent respiratory compensation to reduce ‘oxygen debt’ and convert lactic acid
• LACTIC ACIDOSIS (SEPSIS): infection, pathological immune response, septic shock, decreased O2 delivery (hypoxia), increased lactic acid production and decreased pH, metabolic acidosis, respiratory compensation to increase O2 delivery and increase lactic acid conversion
• DIABETIC KETOACIDOSIS: increased fatty acid released from liver due to insulin deficiency, increased (acidic) ketone body production
• DIARRHOEA: e.g. decreased HCO3- absorption from the colon
• RENAL FAILURE: e.g. decreased HCO3- reabsorption in proximal tubule

Question 6

What is metabolic alkalosis?

Answer 6

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 7

List some causes of metabolic alkalosis.

Answer 7

• DIURETICS: changes to H+ and CO3- concentration
• ANTACIDS: increase HCO3- consumption
• VOMITING: loss of H+ in stomach acid

Question 8

Interpret these ABG results for acid-base disorders:

pH: 7.30 (7.35-7.45)
PaCO2: 4.0 (4.9 - 6.1)
[HCO3-]: 18 (22-30)

Answer 8

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 9

Interpret these ABG results for acid-base disorders:

pH: 7.48 (7.35-7.45)
PaCO2: 3.0 (4.9-6.1)
[HCO3-]: 18 (22-30)

Answer 9

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 10

Interpret these ABG results for acid-base disorders:

pH: 7.1 (7.35-7.45)
PaCO2: 8.0 (4.9-6.1)
[HCO3-]: 18 (22-30)

Answer 10

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.