Physiology for MRCEM Primary

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Respiratory physiology

Ventilation perfusion

[box type=”download”] VA/Q mismatch types  Effects of dead space and shunts on ventilation / perfusion gradients  Shunts as lung regions with relatively low blood PO2 and high PCO2  Effects of ventilation / perfusion mismatch upon arterial gas composition  Limitations of additional inspired oxygen [FiO2] upon shunts  Potential gravitational clinical effects upon lung function in the elderly [/box]

Ventilation–perfusion matching
At rest, total alveolar ventilation (VA) is similar to total pulmonary capillary perfusion (Q), or about 5 L/min.
In a right to left shunt for example, ventilation is zero and VA/Q = 0;
When an embolism blocks a pulmonary artery, perfusion in that part of the lung is zero, and VA/Q = ∞.
Regions of the lung that have a VA/Q value much greater than unity have excessive ventilation, and blood derived from them will have a high PO2 and a low PCO2 (dead space effect).
Regions with a VA/Q value much less than unity have a shunt or venous admixture effect; there is some gas exchange, but the blood has a lower than normal PO2 and a higher than normal PCO2.

Effect of ventilation–perfusion mismatch on arterial gases.
Although regions of high VA/Q produce blood with a high PO2, this is not too significant increase in O2 content, as the haemoglobin is already close to saturation at the normal PO2.
From low VA/Q zones, especially if PO2 < 8 kPa, blood will have significantly reduced O2 content.
As a result, the combined blood from regions with high and low V/Q will have a low O2 content and a low PO2, even if total ventilation and perfusion are matched for the whole lung.
The CO2 content is less severely affected, because overventilated areas can lose extra CO2 and partly compensate for underventilated areas.
Moreover, a rise in PCO2 will stimulate breathing via the chemoreceptors, allowing CO2 to be corrected, or even overcorrected if PO2 is sufficiently low.
Significant V/Q mismatching will therefore usually result in arterial blood with a low PO2 but normal or low PCO2.
High flow O2 will improve oxygenation in regions of low V/Q, but is not useful for shunts, as shunted blood cannot come in contact with alveolar O2.
Hypoxic pulmonary vasoconstriction reduces the severity of VA/Q mismatch by diverting blood from the affected region to well-ventilated areas.

Effect of gravity
The blood pressure at the base of the lungs is greater than that at the apex because of gravity and the flow is therefore increased.
Conversely, blood flow at the apex may be reduced.
Gravity also affects the intrapleural pressure, which is thus less negative at the base than at the apex.
Alveoli at the base are therefore less expanded and thus have more potential for expansion during inspiration.
As a result, ventilation is greatest at the base of the lung.
Although the effects of gravity on perfusion and ventilation partly cancel each other out, ventilation is less affected than perfusion, so that VA/Q is highest at the apex of the lung and lowest at the base.
In the young, this relatively small variation has little effect on blood gases, but in the elderly, it may contribute to a low PO2.

[box type=”download”] Anatomical right to left shunts  Existence in health  Basis of clinical effects of large shunts in lung and / or heart disease[/box]

Right to left shunts
Oxygenated blood from the lungs is polluted by venous blood (Part of the venous effluent of the bronchial via pulmonary vein and coronary circulations via Lt ventricle).
These anatomical right to left shunts account for <2% of cardiac output.
Larger shunts can occur in disease when regions of the lung are not ventilated (e.g. lung collapse, pneumonia), or due to congenital heart malformations.
For a 20% shunt, the 80% of blood will have normal arterial O2 and CO2 contents of 200 and 480 mL/L, respectively, whilst the 20% will have normal venous values of 150 and 520 mL/L, respectively.
So, the blood will contain (200 × 0.8) + (150 × 0.2) = 190 mL/L of O2, and (480 × 0.8) + (520 × 0.2) = 488 mL/L of CO2.
From the dissociation curves, it can be seen that this results in a fall in PO2 from 13 to 9 kPa, whereas PCO2 rises only marginally from 5.3 to 5.5 kPa.
This stimulates the chemoreceptors and increase ventilation, so that arterial PCO2 returns to normal.
However, increased ventilation cannot increase blood O2 content (Hb is already saturated).
Thus, right to left shunts commonly result in a low arterial PO2 but a normal or low PCO2.