Essay On Ventilation Perfusion Ratio Notes

V/Q ratio, what is it?

The local partial pressure of oxygen and carbon dioxide found in the alveoli is determined by the ratio of the amount of air reaching the alveoli (ventilation) to the amount of blood reaching the alveoli (perfusion). This can be quantified by a simple equation which is V/Q and it is the size of this ratio that determines the extent of gas exchange in a single lung unit.

Total ventilation is the volume of air that is moved out of the lungs per unit time and at rest the minute ventilation can be calculated by tidal volume (500ml) x respiratory rate (12) which equals to 6000ml per minute. However only the air that reaches the alveoli participates in gaseous exchange so in order to calculate alveolar ventilation, the air trapped in the conduction system, which is also known as the anatomical dead space as no gaseous exchange occurs, (nose, pharynx, larynx, trachea to the terminal bronchioles) needs to be subtracted from the tidal volume. This gives a value of 350ml and when this is multiplied by the respiratory rate the true ventilation becomes 4200ml/minute. So on average four litres of air is exchanged in the respiratory system per minute. Experimentally if the ventilation rate is increased without a compensatory change in perfusion the alveolar and hence the arterial partial pressure of carbon dioxide decreases whilst the partial pressure of oxygen increase. During hyperventilation, carbon dioxide is exhaled at a faster rate than it is produced in the tissues and this causes partial pressure of carbon dioxide in the body to fall leading to respiratory alkalosis. This eventually leads to a reduction in blood flow which causes dizziness. In contrast, hypoventilation there is a reduction in the volume of carbon dioxide expired per unit time and carbon dioxide begins to accumulate in the blood plasma leading to respiratory acidosis.

The second key component of the ratio is perfusion of the lung which is provided by the pulmonary artery that arises from the right ventricle which then bifurcates and carries deoxygenated blood to each lung. Pulmonary venules then collect the oxygenated blood from the capillary and blood is returned to the left atrium. Unlike the systemic circulation, the pulmonary circulation has low pressure, low resistance but high compliance. Low pressure is essential in regards to the pulmonary circulation as it prevents the formation of pulmonary oedema due to the flux of fluids which is caused by high hydrostatic pressure caused by high blood pressure. Another reason why low pressure is important is because the same cardiac output is pumped only to the top of the lung which has a smaller total resistance compared to the systemic circulation. So decreasing the pressure ensures the same rate of blood flow. In total the perfusion of the lungs equals to 5 litres per minute.

At rest the ventilation perfusion ratio of the whole lung is 4/5 which is 0.8. This ratio determines the partial pressure of gases found in the alveoli. Inspired air has the following partial pressures; Po₂ = 150mmHg, Pc02 = 0mmHg. However due to ventilation perfusion ratio the mean alveolar partial pressure of the two main gases are p02=100mg and pc02= 40mmHg. These values are determined by the balance between the ventilation which replenishes the gases in the alveoli and the removal of gases via diffusion due to the blood flow

Regional differences of V/Q ratio when in an upright position

Due to the force of gravity, the ventilation to perfusion ratio is not the same in all regions of the lungs when a person is standing upright. The changes in ratio are mostly due to regional changes in both ventilation and perfusion. As a result the ratio varies from 0.6 to 3.3 and this leads to a pattern of regional gas exchange.

From the apex to the base of the lung, alveolar ventilation increases slowly when standing upright. The regional differences in ventilation were shown by an imaging technique which involves breathing air containing xenon-133. Due to the low diffusing capacity the xenon remains in the alveoli for a short period of time and when a deep breath is taken the level of radioactivity is measured over different regions of the lung. The result of this experiment shows the maximal regional alveolar volume and it was found that there was a greater radioactivity count at the base of the lung lobe compared to the apex. This proves the ventilation increases from the apex to the base. The reason why ventilation increases is due to gravity which causes the intra pleural pressure to be greatest at the apical zone of the lung (-10cmH20) and lowest at the base of lung (-2.5cmH20). This intra pleural pressure gradient results in different sized alveoli. Compared to other regions of the lung, the alveoli at the apical region have the greatest size and as they are relatively overinflated it results in them having the least compliance. In comparison the alveoli at the base of the lung are the smallest and are relatively underinflated which means they have the greatest compliance. The difference in compliance means that during inspiration the same difference in the intra pleural pressure results in a greater change in volume near the base of the lung than the apex. Therefore alveolar ventilation which is the change of volume per unit of time rather than initial volume increases from the apex to the base.

The second factor that causes regional differences in the V/Q ratio is perfusion. Like ventilation, differences in perfusion down an upright lung is also due to gravity which is similarly shown using xenon133 which is injected whilst a person is holding their breath. When the xenon reaches the lungs via circulation it enters the alveolar air and a lung scan shows the distribution of radioactivity. The result of these experiments shows that perfusion is greater at the base of the lungs compared to the top and the relationship between blood flow and distance up the lung is shown on the side....

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