Ventilation Perfusion Ratio Notes

V/Q ratio, what is it?

-local partial pressure of oxygen + carbon dioxide found in alveoli is determined by the ratio of ventilation to blood flow- the V_A/Q_A ratio determines the gas exchange in any single alveoli

-however ventilation and perfusion in a single alveolus is difficult to measure, so it is generalised to the whole lung which is V/Q

Ventilation/perfusion require matching- over ventilation wastes energy by drawing in more o2 than can be carried by the blood whilst underventilation prevents blood carrying enough oxygen to the tissue

Alveolar ventilation

-total ventilation: volume of the air moved out of the lungs per unit of time

The minute ventilation can be calculated by tidal volume x respiratory rate = 6000ml/litre. However the alveolar ventilaton rate (the amount of ventilator air that goes to alveoli for exchange process) is less than this because all the 500ml doesn’t go to the physiological alveolar for exchange purposes-some air becomes trapped in the conduction system where no gas exchange occurs- nostril-terminal bronchi-anatomical dead space-roughly 150ml. So 350 ml in total in the alveoli, so true ventilation is 4200ml/minute. So average 4L of air exchanged in respiratory system per minute.

-alveolar ventilation is the volume of fresh air per minute that actually reaches the alveoli/ volume of alveolar air that reaches the atmosphere

-When ventilation is increased without increase in perfusion- alveolar and hence arterial pressure of co2 decreases, partial pressure of o2 increases, as seen in a metabolic hyperbola = Hyperventilation- C02 exhaled faster than is produced- respiratory alkalosis- dizziness

-Hypoventilation – reduction in volume of c02 expired per unit time- c02 accumalates- respiratory acidosis

-ventilation is mostly controlled by chemoreceptors- mostly central, then peripheral- respiratory acidosis/ metabolic acidosis- kussmaul breathing

Perfusion of the lung:

-pulmonary circulation has low pressure (arterial pressure 25/15mmHg), low resistance but high compliance- pulmonary circulatory system handles the same cardiac output as the systemic circulation but it needs to pump blood only to the top of the lung- it also needs to be low pressure to avoid the consequences of starling forces-otherwise lead to oedema-blood flow-5L per minute

-the pulmonary artery arises from the Right ventricle, carries relatively deoxygenated blood- the pulmonary arterioles are less muscular and resting tone is low- low resistance- they also have very thin walls- high compliance

V/Q RATIO

-so ideal ventilation perfusion ratio is 4/5 which is 0.8

-inspired air: P02=150mmHg, PC02=0, however the average alveolar partial pressure of O2=100mmHg-determined by balance between ventilation (replenishment of the oxygen) and its removal by blood flow, alveolar partial pressure of C02= 40mmHg which is balance between ventilation and removal by blood flow

-regional differences of V/Q in the upright human lung causes a pattern of regional gas exchange

Physiological V/Q mismatch

-however there a differences in the ventilation and blood flow down an upright lung-resulting in different V/Q ratios in different regions of the lung. As we go down an upright lung

- ventilation increases slowly -from the top to bottom of the lung- due to gravity, intrapleural pressure is greatest at the apical zone of the lung, leading to the greatest size alveoli –relatively overinflanted -with the least compliance, ventilation is smaller. Lower region of the lung, least intrapleural pressure, smallest alveoli, greatest compliance, greatest ventilation- During inspiration the same change in intrapleural pressure produces a large change in volume near the base than near the apex- change in volume per unit time and not the initial volume that determines ventilation

-The compliance curve shows the relationship between negative pressure outside the lung and the volume when inspiring. At the apex, greatest negative pressure, the volume of the alveoli are greatest. Gradient of the line is shallow, least compliance- less ventilation

Experiment to show how volume changes with intrapleural pressure: imaging technique, subject breathes air containing Xe133-low diffusing capcity-during a short period it remains almost entirely within the alveoli. When maximal inspiratory effort radioactivity is detected-shows maximal alveolar volume- greater radioactivity count at the base of the lung lobe compared to the apex In order to normalise these values per ml of lung tissue the subject rebreathes the xenon to and fro to get it evenly dispersed through the alveoli. The initial counts are divided by second counts to get ventilation per ml of lung.

Blood flow increases more rapidly- gravity antagonises flow to the upper portion of the lung but assisting flow to the lower zone. This leads to a greater perfusion pressure in the lower lobe. Also resistance in the blood vessels is greatest in the upper region of the lung due to negative transmural pressure causing compression of the capillaries as alveolar pressure is greater in the upper region.

-regional blood flow differences are seen when injecting radioactive Xenon 133 into the peripheral vein. First pass through lung, xenon leaves blood and enters alveoli in proportion to the blood flow. These counts give regional blood flow.

-ventilation rises from the apex to the base and perfusion rises more steeply- the V/Q ratio decreases down an upright lung

-This has led to the lung being divided into three theoretical zones

a) Apex (zone1), ventilated not perfused : P_A (alveolar pressure) > P_pa (pulmonary artery pressure) > P_V (pulmonary vein pressure) – negative transmural pressure across the pulmonary capillary crushes it and reduces blood flow: ventilation perfusion ratio is highest at the top of the lung (blood flow in minimal) – so at the apex, of the lung, alveolar P02, PC02 closely approaches their values in inspired air and as oxygen and carbon dioxide transport across...

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