VQ Matching

Gas exchange between the

alveoli and the pulmonary

capillary blood is by __

diffusion

Diffusion of O2 and CO2 is driven by their ___ across alveolar-capillary barrier

partial pressure gradient

These gradients must be maintained by ___ of the alveoli and ___ of the pulmonary capillaries

ventilation

perfusion

Ventilation (V)

the rate at which gas is passing through the lung

Perfusion (Q)

the rate at which blood is passing through the lung

V/Q Ratio

a measurement that examines how much gas is being moved effectively and how much blood is flowing around the alveoli where gas exchange (perfusion) occurs

indicates matching of blood flow to ventilation

Explain in layman's terms:

The V/Q ratio is a medical math problem that compares how much fresh air (V for Ventilation) gets into your lungs with how much blood (Q for Perfusion) is flowing there to pick up oxygen. Think of it like matching pairs of socks: for your lungs to work perfectly, every bubble of fresh air needs a tiny blood vessel waiting right next to it to swap oxygen. If they don't match up evenly—like having too much air but no blood, or too much blood but no air—it becomes harder for your body to breathe easily.

V/Q ratio can be defined for:

single alveolus

group of alveoli

entire lung

For the normal lung, what is the overall V/Q?

0.8

this is the weighted average — differs in various zones

V/Q ratio determines partial pressures of both ___ and ___ in alveoli and pulmonary capillaries

O2

CO2

A ___ causes air to be trapped in the 'unit' — V/Q = 0

ventilation block

If blood flow is completely, blocked, what happens? - V/Q = infinity

no blood to pick up O2

Apex of Lungs

top of lungs

Apex of lungs has (more/less) ventilation and much (more/less) perfusion

less

much less

The V/Q ratio is ___ in the apex

high

less ventilation

much less perfusion

relatively lots of supply (O2), not enough demand (blood)

How much less ventilation does apex have, as compared to base?

1/3 of base ventilation

How about perfusion?

1/10 of base perfusion

Base of Lungs

bottom

rests on the diaphragm

Base of lungs has (more/less) ventilation and much (more/less) perfusion

more

much more

V/Q ratio for base of lungs is ___

low

plenty of blood flow, relatively speaking, but not enough air

lots of demand, but not enough supply

Breakdown the base/apex ventilation & perfusion comparisons from the POV of the base:

3x more ventilation

10x more perfusion

Clearly, in a normal upright lung, there is ___ ventilation distribution

uneven

Ventilation is greatest near the '___' base of the lung, and progressively gets smaller toward the '___' apex

dependent

independent

What are the causes of this uneven ventilation?

gravity

more negative intrapleural pressure at apex

greater volume of apex alveoli

less compliance of apex alveoli

lower ventilation of apex alveoli

Explain gravity's influence: the 'slinky' effect —>

alveoli in apex of an upright lung are stretched from holding up lower portions of the lung against gravity

The apex is less perfused due to ___

narrow capillaries

= higher PVR

—> less blood available for gas exchange

The apex has ___ pulmonary arterial pressure and ___ capillary hydrostatic pressure

decreased

decreased

The base is more perfused due to ___

wider capillaries

= lower PVR

—> more blood available for gas exchange

Why does it make sense that there is more blood in the lower lung than upper lung?

gravity pulls more blood down

The base has ___ capillary hydrostatic pressure

higher

A normal young adult has a normal V/Q ratio and normal arterial PO2... a patient with ___ is not a normal young adult in terms of health

chronic bronchitis

A patient with CB has a ___ V/Q ratio and ___ arterial PO2, which contribute to ___

low

low

arterial hypoxemia

Extreme of low V/Q ratio — when it is 0 — creates an ___

absolute intrapulmonary (physiological) shunt

How about when V/Q ratio is low (under 0.8 normal) but greater than zero?

normal occurrence at the base of the lung

can occur due to narrowed or blocked bronchi, decreased alveolar

compliance, or increased blood flow relative to ventilation

Shunts create ___

venous admixture

VA is the result of ...

mixing of shunted non-

reoxygenated (unaerated) blood with reoxygenated (aerated) blood distal to

the alveoli

Venous admixture reduces ___

arterial oxygen content

Partial compensation for a shunt is by ___

hypoxic pulmonary vasoconstriction

Hyp Pulm Vasconstrict

partially compensates for a shunt by redirecting blood flow away from poorly ventilated (hypoxic) alveoli toward well-ventilated alveoli

move busses to areas with a lot of people as opposed to areas with no people — the busses want to be filled too!

Venous admixture arises from several sources, both ___ and ___

normal

pathologic

Pathological destruction of the lung parenchyma creates large lung areas with low ___ and more ___

V/Q ratios

venous admixture

When is supplemental O2 beneficial?

in patients with small pathologies

in patients with large pathologies, such as absolute shunts, supplemental O2 is not beneficial

Improvement or lack of improvement of blood gasses is ___ criterion

diagnostic

Hypoxemia

deficient amount of oxygen in the blood

Why does O2 supplement not help with absolute shunts?

O2 pathway is completely blocked off... so more O2 being given would not help

Extreme of high V/Q ratio — it becomes infinity, since you cannot divide by 0:

no blood flow to alveoli

alveolar gas tensions equal to inspired air

creates an alveolar dead space

Extreme of high V/Q occurs in ___

pulmonary embolism

Causes of increased V/Q ratios include:

low cardiac output

blockage of pulm vasculature

emphysema

hyperventilation

Non-extremes of high V/Q is when V/Q ratio is ___, but not infinite

above 0.8

This creates ___ ventilation

wasted

—> increased alveolar dead space

normally occurs at apex of normal upright lung

Changing from a standing to a supine (laying down) position alters V/Q ratios... how? When you lay down...

▪ Posterior lung behaves like the base in standing position

- Low V/Q

▪ Anterior lung behaves like the apex in standing position

- High V/Q

Supine position may be useful in patients who have pathologies which include ___ of portions of the lung

under perfusion

If alveolar ventilation is impaired, the V/Q ratio ___

decreases

Pathologies include:

- Pneumonia

- Bronchitis

- Asthma

If alveolar blood flow to the alveoli is impaired, the V/Q ratio ___

increases

Pathologies include:

- Pulmonary emboli

- Emphysema

As PA of O2 decreases, pulmonary arterioles ___

constrict

blood flow to lungs decreases to match low ventilation

less people at station over time = less busses will be sent to that station

This constriction is called ___

hypoxic vasoconstriction

If large regions of the lung are unventilated, such as in unilateral pneumothorax or a collapsed lung, the volume of shunted blood that bypasses aeration ___

increases