Hypoventilation

Question 1

What are some stimuli for ventilation?

Answer 1

Increased CO2
Significantly decreased O2
Increased metabolic activity (ventilation matched to O2 consumption and CO2 production)
Metabolic acidosis
Anxiety

Question 2

What kind of acid-base imbalance is produced by anxiety?

Answer 2

Anxiety leads to hyperventilation which is excessive for O2 consumption and CO2 production
Results in a respiratory alkalosis

Question 3

What is hypoventilation?

Answer 3

Situation where rate of alveolar ventilation is not meeting metabolic requirements for O2 consumption and CO2 production

Question 4

What is the acid-base abnormality in acute vs. chronic hypoventilation?

Answer 4

Acute: respiratory acidosis
Chronic: compensatory metabolic alkalosis

Question 5

List 5 causes of hypoventilation

Answer 5

Reduced respiratory centre activity (e.g. drugs)
Neuromuscular disease (e.g. MND)
Chest wall deformity (gross)
Obesity (gross)
Sleep disordered breathing

Question 6

What are the 3 forms of sleep disordered breathing?

Answer 6

Obstructive sleep apnoea
Central sleep apnoea
Obesity hypoventilation syndrome

Question 7

What is the difference between obstructive and central sleep apnoea?

Answer 7

Obstructive: abnormality of upper airways
Central: abnormality relates to brainstem

Question 8

What causes obstruction in OSA?

Answer 8

Airway muscles relax (especially in REM)
Throat is usually already narrowed due to obesity or tonsils
Tongue falls backwards

Question 9

What is the cycle of events in OSA?

Answer 9

Snoring in light sleep
Complete obstruction in deep sleep
Reduced blood O2, increased CO2, other stimuli
Brain “wakes” to lighter sleep
Muscles contract, airway opens, breathing recommences
Back into deep sleep, obstructs
N.B. Occurs more than 10x every hour

Question 10

How is OSA managed?

Answer 10

Nasal CPAP
Mandibular advancement splint
Surgery

Question 11

What are the consequences of chronic severe sleep apnoea? In what other conditions does this occur?

Answer 11

Re-setting of the respiratory centre causing day-time hypoventilation
Chronic hypoxia, hypercapnoea and compensated respiratory acidosis
Can occur with other conditions in which there is less ventilation during sleep than when awake (e.g. severe COPD)

Question 12

Why shouldn’t you administer supplemental O2 to patients with chronic hypercapnoea?

Answer 12

In chronic hypercapnoea, patients are dependent on the hypoxic drive to breath
Administering supplemental O2 gets rid of this drive and causes acute hypoventilation
Should be given incrementally if O2 saturation is <90%, just to bring patient back to normal baseline for them (e.g. 92%)