0827 - Perfusion Flashcards
Outline the 4 factors involved in gas diffusion
Membrane thickness - Between RBC and alveolus, decrease only evident at 2-3 x normal thickness (repeated interstitial pneumonias).
Diffusion coefficient - The gas solubility in the membrane - CO2 23x better than O2 which is 2x better than N2. O2 can easily become diffusion-limited.
Total surface area of respiratory membrane - If you lose significant surface area (down to ¼-⅓ of total) - gas exchange comes severely restricted. Aging causes some loss of area, but emphysema and surgery are most common cause.
Pressure gradients - The steeper the pressure gradient, the stronger the driving force for diffusion.
What are the pressure gradients for CO2 and O2?
Pressure gradient drives diffusion
PvO2= 5.33kPa, and PaO2 = 13.33kPa = Pressure gradient = 2.5 x initial value
PvCO2= 6.13kPa, and PaCO2= 5.33kPa = Pressure gradient = 1.15 x initial value
Explain why CO2 diffuses much better than O2
CO2 has a much faster diffusion rate (23x) than O2 due to its increased solubility in the membrane and in the plasma (can dissociate to carbonic acid, maintaining the gradient).
What factors determine perfusion?
Lung circulation is low pressure, and pulsatile even in the capillaries (different to systemic). Pressure is obviously the main driving force for perfusion.
Perfusion can be influenced by vascular resistance (determined largely by capillaries - increase when alveolar pressure increases and vice versa). As not all capillaries are used under resting conditions, can increase perfusion with little change in Resistance.
Also hydrostatic pressure (30cmH2O base to apex) - base is much better perfused than apex.
How do gasses and chemicals modulate perfusion?
Alveolar O2 concentration is the largest modulator of pulmonary flow. Low local PAO2 causes vasoconstriction, and high PAO2 causes vasodilation. Allows body to maintain a good ventilation-perfusion relationship including in times of pathology (block off unventilated areas, avoid shunts).
NO and β-adrenergic also dilators. ɑ-adrenergic and histamine are constrictors.
What are the implications of the ventilation-perfusion relationship for lung function?
Ventilation-perfusion must be matched to ensure optimal function of the lung, and optimal gas exchange. If V/Q >1, it’s hyperventilation, if V/Q<1, it’s hypoventilation.
PaO2 is the most significant driver of V/Q, but many books will say PaCO2 as well.
Mismatch of ventilation and perfusion is the most frequent cause of hypoxaemia in cardiopulmonary disease.
What happens if a lung area is not ventilated?
Alveolar pressures reach those of the blood (hypoventilated). Vessels vasoconstrict due to hypoxia, and bronchi dilate due to increased CO2 - allows the bronchi to dislodge any blockage and resume ventilation.
What happens if a lung area is not perfused?
Alveolar partial pressures reach those of the trachaea (hyperventilated). Vessels vasodilate (due to O2 pressure) to increase perfusion, and bronchi constrict (due to low CO2 pressure) to limit ventilation.
Name 2 anatomical shunts
Bronchial veins, and thebesian (smallest) veins of left myocardium. O2 therapy will not help because this blood does not travel near an alveolus.
Name 2 physiological shunts
Foreign bodies in the bronchus
Atelectasis caused by mucus plugs in airways.
