Working lung and respiratory failure

Question 1

Assumption of the one alveolus model

Answer 1

Assumption: the amount of ventilation (V) and the amount of capillary blood flow (perfusion, Q) are good, and the ratio between them (V/Q ratio) is good.

Question 2

Ventilation

Answer 2

Ventilation (per alveolus) is lowest at the lung apex.

Ventilation progressively increases moving lower
down in the lung, and is highest in the base.

Question 3

Mechanistically, the tops of the lungs experience

Answer 3

Lower intrapleural* pressure  
Higher transpulmonary* pressure  
More distended alveoli  
Makes them less compliant  
Leads to less airflow during inspiration

• Intrapleural and transpulmonary pressures are covered in upcoming lectures

Question 4

Perfusion (Q) gradient

Answer 4

PA>Pa>Pv
Pa>PA>pv
Pa>Pv>PA

PA: total (gas) pressure in the alveolus

Pa: blood pressure in the pulmonary artery

Pv: blood pressure in the pulmonary vein

Question 5

V/Q ratio by lung zone

Answer 5

A ratio of 1 = matched. A ratio other than 1 represents a regional mismatch of ventilation and perfusion.

3. 3 - apex
4. 0- midpoint of lung
5. 63 - base of lung

Question 6

Pulmonary vasculature minimise V/Q mismatch

Answer 6

Hypoxic pulmonary vasoconstriction

Other tissues modify blood flow in the opposite direction

Question 7

Hypoxemia

Answer 7

Decreased arterial oxygen pressure (PaO2) which causes a reduction in the delivery of oxygen to tissue (hypoxia)

Question 8

Hypoxaemia from low V/Q

Answer 8

I. V/Q ratio «< 1. (eg .001)
 hypoxaemia via PAO2

II. V/Q ratio = 0. This represents shunt.
 hypoxaemia via reduced PAO2
unable to compensate with increased FIO2

III. IV/Q&raquo_space;> 1 (eg. 100)
 wasted ventilatory efforts

Question 9

Hypoxaemia due to shunt

Answer 9

I. V/Q ratio «< 1. (eg .001)
 hypoxaemia via PAO2

II. V/Q ratio = 0. This represents shunt.
 high PAO2 to PaO2 difference
unable to compensate with increased FIO2

III. IV/Q&raquo_space;> 1 (eg. 100)
 wasted ventilatory efforts

Question 10

Anatomical shunts

Answer 10

Thesbian and bronchial veins

Question 11

Venous admixture

Answer 11

Amount of poorly oxygenated venous blood needed to cause a given drop in A-a pressures. Not to be confused with the term (central) mixed venous blood.

Question 12

Intracardiac shunt

Answer 12

Congenital or acquired heart defects that results in a right to left shunt

Question 13

Intrapulomary shunt

Answer 13

Lung consolidation
Pneumonia
Arteriovenous malformations

Question 14

Hypoventilatory hypoxemia

Answer 14

Drug overdose: narcotics, other depressants, alcohol with impair function of the respiratory control centre

Central nervous system disease

Weakness

Question 15

PaO2 during hypoventilation

Answer 15

PaO2drops with hypoventilation due to a decrease in the V/Q ratio

PaO2 and PAO2 are both decreased
PAO2 = [(Patm – PH20) x FiO2] – (PaCO2/RQ)

Question 16

Diffusion limitation

Answer 16

Gases diffuse across respiratory membrane and through RBCs

Interstitial edema, inflammation, or scarring increase the diffusion distance and slow gas diffusion
reduced PaO2 but normal PAO2

Question 17

Fick’s law: V’gas= D * A * ΔP/T

Answer 17

V'gas= Rate of gas diffusion across permeable membrane
D = Diffusion coefficient of a gas for that membrane
A = Surface area of the membrane
ΔP = Partial pressure gradient of the gas across the membrane
T = Thickness of the membrane

Question 18

Ventilation

Answer 18

Minute ventilation (VE): amount of air, per minute, breathed into the lung
                                 VE = VT (tidal volume) x RR (respiratory rate)

Amount of that air which reaches alveoli and participates in gas exchange is called alveolar ventilation.

Amount of that air which does NOT participate in gas exchange is called anatomical dead space (VD)

Question 19

Dead space

Answer 19

Anatomic dead space: typically, 150 out of 500 mL = an anatomical dead space of ~ 30%

alveolar dead space: normally minimal

Question 20

Respiratory failure always cause

Answer 20

Hypoxia

But not all causes of hypoxia are due to respiratory failure

Question 21

Reduced O2 delivery

Answer 21

Anemic hypoxia

Circulatory hypoxia

Question 22

Reduced ability to utilise O2

Answer 22

Cytotoxic hypoxia

Question 23

Cytotoxic hypoxia

Answer 23

Sepsis

Cyanide: impairs mitochondrial cytochrome oxidase