S2: Integrated Control of Breathing

Question 1

What links pCO2 and pH?

Answer 1

The bicarbonate buffering system

Question 2

What occurs if ratio of HCO3- and CO2 is altered?

Answer 2

As blood pH is proportional to the ratio of HCO3- and CO2 , excessive changes may result from respiratory (CO2) or metabolic (HCO3-) dysfunction.

• Decreased pH = acidosis
• Increased pH = alkalosis

Question 3

What is the HCO3- and CO2 ratio?

Answer 3

pH = [HCO3-]/PaCO2

Question 4

What event occurs in the event of hypoventilation?

Answer 4

• A respiratory acidosis occurs in the event of hypoventilation.
• In the event of chronic hypoventilation, HCO3- reabsorption must increase to maintain normal pH.

Increased alveolar CO2 increases levels of CO2 in the blood will increase the level of H+ in blood decreasing pH. In an healthy individuals, changes in respiratory chemoreceptor feedback and it tells the brain to increase its breathing rate which increases the ventilation and therefore decreases CO2 levels.

Question 5

Name some causes of hypoventilation

Answer 5

1. Airway Obstruction

• Aphyxia, choking
• Obstructive sleep apnoea
• Increased airway resistance (asthma, COPD)

2. Problems with initiation of breathing

• Drug overdose (typically opioids)
• Stroke (e.g. In brain stem)
• Neuromuscular problems
• Motor neurone disease
• Respiratory muscle wasting

Question 6

What occurs in event of hyperventilation?

Answer 6

• A respiratory alkalosis occurs in the event of hyperventilation.
• In the event of chronic hyperventilation, HCO3- reabsorption must decrease to maintain normal pH.

Decreased levels of Co2 in the level will decrease the level of acid in the blood causing an increase in PH. In an healthy individual, changes in respiratory chemoreceptor feedback (decrease) and it tells the brain to slow its breathing rate which decreases the ventilation and therefore increases CO2 levels.

Question 7

Causes of hyperventilation

Answer 7

1. Anxiety e.g. from emotional stimulus.

2. Altitude - increased ventilation to maintain PAO2 and PaO2 (via hypoxic drive).

Question 8

How can hyperventilation cause fainting?

Answer 8

Getting rid of CO2 naturally vasoconstricts blood vessels and in the brain, this causes a risk of fainting.

Bag breathing increases PACO2 and then PaCo2

Question 9

What is metabolic acidosis?

Answer 9

When metabolic acid production exceeds excretion, or due to insufficient [HCO3-] (decreased absorption or increased excretion),
- Unless PaCO2 decreases in proportion to [HCO3-], acidosis with a decrease in pH will occur

Respiratory compensation typically occurs almost instantly, however it may not be sufficient to prevent disturbance in pH (more ventilation).

Question 10

Causes of metabolic acidosis

Answer 10

1. Lactic acidosis (excersize)
2. Lactic acidosis (sepsis)
3. Diabetic ketoacidosis - increase fatty acid release from liver due to insulin deficiency = increase acidic ketone body production
4. Diarrhoea - decreased HCO3- absorption in colon
5. Renal failure - decreased HCo3- Reabsorption in proximal tubule

Question 11

How does lactic acidosis cause metabolic acidosis?

Answer 11

In excersize:

Intense exercise causes insufficient O2 devlivery for aerobic metabolism. Pyruvate is converted into lactate. This increase lactic acid production which causes metabolic acidosis.
Subsequent respiratory compensation can reduce ‘oxygen debt’ and convert lactic acid.

In sepsis:
Infection leads to an pathological immune response which can cause septic shock. This leads to less oxygen being delivered to tissues (hypoxia) and lactic acid production.

Question 12

What is metabolic alkalosis?

Answer 12

When metabolic acid excretion is excess of due to excess [HCO3-] (increase absorption of decrease excretion).
Unless PaCO2 changes in proportion with [HCO3-], alkalosis (↑pH) will occur.

Respiratory compensation typically occurs almost instantly, however it may not be sufficient to prevent disturbance in pH.

Question 13

Causes of metabolic alkalosis

Answer 13

1. Diuretics - changes to H+ and HCO3- reabsorption
2. Vomiting - loss of H+ in stomach acid
3. Antacids - Increase HCO3- consumption

Question 14

How do you interpret ABG results?

Answer 14

PH, PaCO2 and [HCO3-] are the important values you should look at!

Question 15

Interpret ABG results 1:

pH=7.3 (7/35-7.45)
PaO2= 13.5 (11-13.5)
PaCO2=4 (4.9-6.1)
[HCO3-]=18 {22-30)

Answer 15

• PH is low so acidosis
• [HCO3-] is too low –> metabolic acidosis
• Low CO2 is respiratory compensation (low = hyperventilating)
  Diagnosis: Metabolic acidosis with respiratory compensation

Question 16

Interpret ABG results 2:

pH=7.48 (7/35-7.45)
PaO2= 13.5 (11-13.5)
PaCO2=3 (4.9-6.1)
[HCO3-]=34 {22-30)

Answer 16

• High pH –> Alkalosis
• Low CO2 –> Respiratory alkalosis
• High [HCO3-] also increases PH ([HCO3-] is a base)
• Diagnosis: Mixed alkalosis

Question 17

Interpret ABG results 3:

pH=7.3 (7/35-7.45)
PaO2= 13.5 (11-13.5)
PaCO2=7.6 (4.9-6.1)
[HCO3-]=27 {22-30)

Answer 17

• Low PH + excess CO2 = respiratory acidosis

Diagnosis: Respiratory acidosis