| Question | Answer |
| Pulmonary circulation | Low pressure
Arterial pressure 25/15 mmHg
Circulation has to take entire cardiac output 5 l/min at rest up to 20 l/min during exercise
The pressure only increases slightly as there is a fall in vascular pulmonary resistance |
| Regional blood flow | Inject Xe133 into peripheral vein
First pass through lung it leaves the blood and enters alveoli proportional to blood flow
These counts give regional blood flow
Normalised to volume by rebreathing to equilibrium and taking more counts |
| Theoretical zones of the lung | Zone 1 - PA>Pa>Pv - capillary squashed so no perfusion
Zone 2-Pa>PA>Pv - dynamic changes in flow as capillary opens and closes under pressure
Zone 3 -Pa>Pv>PA - normal scenario where capillary is open
Position of zones varies in cardiac/breathing cycle |
| Hypoxic pulmonary vasoconstriction | Systemic - hypoxia causes vasodilation to match flow to metabolism
Pulmonary - hypoxia causes vasoconstriction to match perfusion to ventilation
Global hypoxic vasoconstriction increases pulmonary pressure and causes right sided heart failure |
| Effect of VQ mismatch | In extreme situations - normal blood flow and normal ventilation but no gas exchange since blood never comes near fresh gas |
| VQ ratios | Low VQ - increased Pco2 and decreased Po2
Hight VQ - increased Po2 and decreased Pco2
Ideal point - correct exchange as alveoli and arterial Po2 and Pco2 equilibrate |
| VQ in a perfect lung | Partial pressure of CO2 in alveolar gas leaving the lung and in the blood leaving the lung is the same |
| VQ in inhomogeneous lungs | Partial pressures of CO2 in alveolar gas and arterial blood are not equal
Increase arterial Pco2 - flow to the lungs will be altered to meet demands
Loss of efficient CO2 production |
| O2 and VQ mismatch | The dissociation curve for O2 is not linear in physiological range
The content of O2 is lowered more by a given Pao2 fall for low VQ units than it is elevated by a rise in Pao2 by high VQ units
Effects of mismatch is worse for O2 than other gases |
| Three compartment model of the lungs | The effects of any VQ distribution in the lung on alveolar and arterial Pco2 and Po2 can be mimicked exactly by a lung with three components
An area of dead space - VQ = infinity
An area of shunt - VQ = 0
Normal area - VQ = 0.8 |
| Causes of VQ mismatch | Pneumothorax
Emphysema
Adult and neonatal respiratory distress syndrome
Asthma
Pneumonia
Pulmonary embolism |
| Pure shunt | An extreme case of VQ mismatch where VQ = 0
Shunt fraction is typically 0.05 but is increased in disease
Low VQ due to shunts is resistant to O2 therapy
Can normally increase effectiveness of V by increasing O2 - no effect in VQ = 0 |
