Lecture 13 - Ventilation - perfusion mismatch Flashcards
Pulmonary ventilation
Movement of air into and out of lungs
Pulmonary gas exchange
Diffusion of oxygen from the lungs into the blood
Diffusion of CO2 from the blood to the lungs
Pulmonary perfusion
Flow of blood through the pulmonary capillaries surrounding the alveoli
Alveolar ventilation
RR x (Tidal volume - anatomical dead space)
Normal V/Q
1
Ventilation (CO) - 4900 ml/min
Perfusion - 4900 ml/min
CO
CO= SV x HR
Low pulmonary capillary pO2
Low ventilation
Hypoxic vasoconstriction
Decreases perfusion
Diverts blood to well ventilated alveoli
Maintained VQ
Bronchoconstriction
Diverts air to better perfused alveoli
Mild therefore in some disease states poorly ventilated alveoli still have significant perfusion
Gas compositions in normal alveolus
pO2 - 13.3 kPa
pCO2 - 5.3 kPa
Inadequate ventilation
V is less than Q
V/Q less than 1
CO2 increases as less breathed off
O2 decreases - hypoxaemia
Causes of V/Q less than 1
Pneumonia - acute inflammatory exudate Asthma COPD Respiratory distress syndrome Pulmonary oedema
Pulmonary embolism
V
- Obstruction in e.g. left upper lobar branch of a pulmonary artery
- Blood that would of perfused the LUL redirected to rest of pulmonary circulation
- Increased perfusion of normal lung requires extra ventilation
- Hyperventilation
- If not sufficient, V
Pulmonary embolism V > Q
- Obstruction in e.g. left upper lobar branch of a pulmonary artery
- Blood that would of perfused the LUL redirected to rest of pulmonary circulation
- Alveoli of LUL ventilated normally as bronchus is patent
- Less gas exchange as less blood supply V>Q therefore increased CO2
How to calculate pO2 in mixed blood of normal and affected alveoli
Half way between saturations corresponding pO2
pO2
dissolved O2 once all Hb bound
