Working lung and respiratory failure Flashcards
Assumption of the one alveolus model
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.
Ventilation
Ventilation (per alveolus) is lowest at the lung apex.
Ventilation progressively increases moving lower
down in the lung, and is highest in the base.
Mechanistically, the tops of the lungs experience
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
Perfusion (Q) gradient
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
V/Q ratio by lung zone
A ratio of 1 = matched. A ratio other than 1 represents a regional mismatch of ventilation and perfusion.
- 3 - apex
- 0- midpoint of lung
- 63 - base of lung
Pulmonary vasculature minimise V/Q mismatch
Hypoxic pulmonary vasoconstriction
Other tissues modify blood flow in the opposite direction
Hypoxemia
Decreased arterial oxygen pressure (PaO2) which causes a reduction in the delivery of oxygen to tissue (hypoxia)
Hypoxaemia from low V/Q
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»_space;> 1 (eg. 100)
wasted ventilatory efforts
Hypoxaemia due to shunt
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»_space;> 1 (eg. 100)
wasted ventilatory efforts
Anatomical shunts
Thesbian and bronchial veins
Venous admixture
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.
Intracardiac shunt
Congenital or acquired heart defects that results in a right to left shunt
Intrapulomary shunt
Lung consolidation
Pneumonia
Arteriovenous malformations
Hypoventilatory hypoxemia
Drug overdose: narcotics, other depressants, alcohol with impair function of the respiratory control centre
Central nervous system disease
Weakness
PaO2 during hypoventilation
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)
Diffusion limitation
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
Fick’s law: V’gas= D * A * ΔP/T
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
Ventilation
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)
Dead space
Anatomic dead space: typically, 150 out of 500 mL = an anatomical dead space of ~ 30%
alveolar dead space: normally minimal
Respiratory failure always cause
Hypoxia
But not all causes of hypoxia are due to respiratory failure
Reduced O2 delivery
Anemic hypoxia
Circulatory hypoxia
Reduced ability to utilise O2
Cytotoxic hypoxia
Cytotoxic hypoxia
Sepsis
Cyanide: impairs mitochondrial cytochrome oxidase
