This notion suggests that there should be some sort of bell-curve-like Gaussian distribution of flow values for V̇ and Q̇. Classically, this spread of V/Q values is described as "V/Q scatter". One may expect a broad range of theoretically possible V/Q ratios, spanning from zero (at the collapsed bases where V̇ = 0) to infinity (in the completely anaemic apices where Q̇ = 0).
some regions receive good ventilation and poor blood flow, whereas other regions are rich in blood supply but receive little air. However, as mentioned in the paragraphs above, there are some differences in the blood and gas supply to different lung regions, i.e. All of the lung units would get exactly the same volume of blood and air per minute, and the lung would be perfused in a perfectly homogeneous fashion. If one needed to represent this distribution, one could probably do so by plotting the flow (whether blood or gas) against V/Q ratio, and in this magical perfect lung it would look like this: In an ideal scenario where blood flow is perfectly matched to ventilation, the V/Q ratio would end up being 1.0, and under these conditions, theoretically the gas exchange should be ideal. The proper use of diacritical marks being a dying art in this lawless post-apostrophe hellscape, even Respiratory Care have given up and officially resorted to the use of undotted V/Q, because who could possibly give a fuck.Īnyway. Without any further digression, using the abovementioned abbreviations we arrive at the familiar representation of ventilation and perfusion matching, the V̇/Q̇ ratio. Although it is also strictly speaking a form of "volume over time" and could also be represented with V̇, physiologists have t raditionally used Q̇ because that is an abbreviation of the French quantité or the Latin quantitas, both languages which were dominant in the physics literature of the 18th and 19th centuries. By a similar convention, blood flow is usually represented by an overdotted capital Q̇. Matching of regional ventilation and perfusion: the V̇/Q̇ ratioīy convention, ventilation is represented in physiology textbooks as V̇, borrowing the diacritical overdot above the "V" from Newton's notation because it represents a time derivative (i.e not just volume but volume over time). All of the original articles quoted by Petersson & Glenny are paywalled, but their content is more interesting than educational, and one could easily just stick to that one paper.
The best article to discuss ventilation and perfusion matching would have to be Petersson & Glenny (2014), mainly because it has the distinction of being free for all readers. In a normal young person, this "scatter" spans a V/Q range between 0.6 and 3.0.The distribution of lung units along a spectrum of V/Q ratios is referred to as "V/Q scatter".In a normal lung, most of the lung units will have V/Q ratios close to 1.0.V/Q = 0: Lung units which receive no ventilation, i.e.V/Q = ∞: Lung units which receive no blood flow, i.e.V/Q 1.0: Excellent ventilation but poor blood flow West's Zone 1.
for every unit of blood flow it will receive a unit of gas flow.
To maintain some attachment to the structure of the college syllabus, any talk of gas exchange will be avoided here as much as possible. Fundamental concepts relevant to V/Q relationships and the factors which affect these are discussed in another chapter, and so the focus here will be on explaining what V/Q matching and mismatching is, and where it can be expected to occur. No written questions have ever interrogated this knowledge, leaving the candidates to make up their own mind as to whether this section merits their attention and it would be quite reasonable for the pragmatic trainee to abandon it in favour of topics which are better represented in the exams. One would have preferred to also use this chapter to discuss in detail the effects of V/Q mismatch on gas exchange, which is definitely in the past exam papers - but that would overlap with the subject of Section F6(viii), "explain the effect of ventilation-perfusion mismatch on oxygen transfer and carbon dioxide elimination".
This chapter is most relevant to Section F6(ii) from the 2017 CICM Primary Syllabus, which expects the exam candidates to be able to "explain ventilation-perfusion matching and mismatching".