L9 Mechanism of sleep apnea and pathophysiology of hypoxemia & hypoxia

Question 1

What is hypopnea ?

Answer 1

It is significant reduction of air flow for 10 seconds or longer
Either :
- more than 50% reduction
Or
- less than 50% reduction + 4% desaturation and more

Question 2

What is RERA ( respiratory effort related arousal ) ?

Answer 2

When the body is trying with effort to breath for more than 10 seconds that will wake the patient up but it is not fulfilling apnea or hypopnea criteria

Question 3

What is OSA ?

Answer 3

It is the most common sleep-related breathing disorder that is caused by the collapse of upper airway during lying down

Question 4

What are the types of OSA ?

Answer 4

1. Excessive daytime sleepiness OSA syndrome

2. OSA- hypopnea syndrome

Question 5

What is the spectrum of sleep related breathing disorder ?

Answer 5

1. Snoring
2. Upper airway resistance syndrome
3. Obstructive sleep apnea ( hypopnea then apnea ) —

Question 6

What are the factors that determine transmural pressure of upper airways ?

Answer 6

1. Pressure of moving airways

2. Neuromuscular tone

Question 7

What is bernoulli law ?

Answer 7

Fluids that move faster posses and exert less pressure

Question 8

What are the body features that increase the risk of having sleep related breathing disorder ?

Answer 8

1. High BMI
2. High neck circumference
3. High cranial index
4. Low facial index

3+4 are only considered for white Americans not African Americans ( they are already more prone )

Question 9

What is cranial index ?

Answer 9

It is width/length

Low CI = long head = low risk of OSA
High CI = short head = high risk

Question 10

Why upper airways are collapsing ?

Answer 10

For two reasons : either

1. The dilating muscles are weak and have low tone
2. Patho-anatomical defect that will lead to this narrowing and require more forces to open them

Question 11

What is the cycle of OSA ?

Answer 11

1. Hypopnea-apnea
2. Body do efforts to breath but can’t
3. Arousal ( wake up )
4. Muscle activity increased and airways dilated
5. Over ventilation
6. Return to sleep
7. Increase in UA resistance and decrease in muscle tone
   8 . UA narrowing
8. Hypopnea - apnea

Question 12

What is the importance of EEG ?

Answer 12

It will show you when the patient is awake and when he is asleep
Sleep = low frequency, high amplitude
Awake =high frequency, low amplitude ( desynchronized )

Question 13

What are the main types of sleep apnea? What is the difference between them ?

Answer 13

1. Obstructive sleep apnea ( our subject )
   - it is due an obstruction ( collapse ) of upper airways with body efforts to breath
2. Central sleep apnea
   - the body is unable to breath due to central problem without any effort to open them

Question 14

How OSA is causing cardiovascular diseases ?

Answer 14

1. The hypoxia —

Question 15

What is severity classification of sleep apnea ?

Answer 15

Apnea-hypopnea index (AHI ):
Mild ( 5-14)
Moderate (15-30)
Severe ( more than 30 ) 
Times Per Hour

Question 16

What are the main types of hypoxia ?

Answer 16

1. Hypoxic hypoxia ( which is caused by hypoxemia )
2. Circulatory ( stagnant ) hypoxia —> due defects in CVS
3. Anemic hypoxia ( blood has less capacity to carry oxygen )
4. Histological hypoxia ( when the respiratory enzymes are being destroyed by poisons such as cyanide , the cells won’t be able to produce ATP so they will die )

Question 17

Why hypercapnia is not necessarily associated with hypoxia and hypoxemia ?

Answer 17

As Co2 has high solubility and ability to cross the respiratory membrane so not necessarily accumulate

Question 18

What are the main causes of hypoxemia ( hypoxic hypoxia ) ?

Answer 18

1. High amplitude ( low available oxygen )
2. Hypoventilation ( improve with pure oxygen )
3. Diffusion impairment ( improve with pure oxygen )
4. Ventilation/perfusion mismatch
5. Abnormal shunt (won’t improve with pure oxygen )

Question 19

Why Pure hypoventilation can not cause major hypoxemia?

Answer 19

As to reduce 1 mmHg of oxygen there is a need to have a 1mmHg increase in Co2
However Co2 is readily diffuse across membrane so won’t accumulate easily

Question 20

What are the causes of diffusion impairment ?

Answer 20

• increased thickness
• sarcoidosis
• asbestosis
• interstitial pneumonia
• interstitial fibrosis
• smaller surface area
• emphysema
• interstitial lung disease
• pulmonary vessel occlusion
• free fluid
• pulmonary edema
• poisonous gases ( nitrogen dioxide , phosgene)

Question 21

What are the 3 tests that you do them in case of increases AaDo2 ?

Answer 21

1. DLCO —