Aa Gradient

A-a Gradient

the difference between the partial pressure of oxygen in the alveoli (PAO₂) and the partial pressure of oxygen in the arterial blood (PaO₂)

Explain PA and Pa:

PA is how much oxygen gets into your lungs when you breath in

Pa is how much of that lung oxygen actually gets into your blood vessels

Blood at the end of the ___ has the highest Pa of O2

pulmonary capillary

Why?

deoxygenated blood enters the lungs with very low Pa of O2, since it has no oxygen in it

at the end of the lungs, at the pulmonary capillaries point, blood is fully oxygenated and ready to leave to go back to the heart

fully oxygenated = highest Pa of O2

Why does blood in the peripheral arteries have lower Pa of O2?

oxygen is let out to tissues along the length of the vessel, leaving only some leftover oxygen by the time the more peripheral arteries are left

Venous Admixture

describes the mixing of oxygenated blood with deoxygenated blood

Blood involved in VA is considered "___"

shunted

The shunted blood creates the difference between ___ and ___

PA of O2

Pa of O2

This difference is defined as the ___

A-a Gradient

Is a little bit of venous admixture common in healthy people?

yes

not all blood in the lungs gets re-oxygenated... most does, but there is always a slight discrepancy

if the discrepancy is larger, there is evidently an issue

What is always a bit higher PA of O2 or Pa of O2?

PA

blood in lungs is always a bit more oxygenated than blood in arteries, as lungs are closer to source of air

A-a gradient is used to diagnose the source of ___ — abnormally low level of oxygen in the blood

hypoxemia

Hypoxia has many causes, often ___, and can cause tissue hypoxia as the blood is not supplying enough oxygen to the body

respiratory disorders

Normally, O2 equilibrates across the alveolar-pulmonary capillary barrier, and the A-a gradient is close to ___

zero

basically, oxygen normally does go from lungs (to heart) to body — respiration! duh

Which factors decrease Pa of O2? — how much oxygen gets to the body, essentially

high altitude

hypoventilation

diffusion defect, like fibrosis

V/Q defect

right-to-left shunt

Which of these also increase the A-a gradient?

diffusion defect

V/Q defect

right-to-left shunt

What does it mean to decrease Pa of O2?

less oxygen gets into the lungs , so less oxygen gets into the bodily tissues

What does it mean to increase the A-a gradient?

prevent oxygen that has already reached the lungs from reaching the bodily tissues

Is venous admixture an actual amount?

no, it is a calculated amount

Is arterial O2 content reduced AND CO2 arterial content increased as per venous admixture?

yes

___: a portion of the cardiac input or blood flow that is diverted or rerouted

Shunt

Physiologic Shunt: a small fraction of the pulmonary blood flow bypasses the ___

alveoli

some of the blood that heads to the lungs — or is supposed to head to the lungs — doesn't actually head to the lungs

There are ___ categories of physiologic shunts

2

1. Right-to-Left Shunt

abnormal blood flow from right to left heart

doesn't go thru pulmonary artery to get to lungs, just crosses over to L side

2. True Shunt

blood which enters the systemic arterial system without going through ventilated areas of the lung

pulmonary and extra pulmonary subcategories

P vs. EP

P in lungs — bronchial veins

EP in heart — thebesian veins

EP and RtoL shunts seem almost the same...

they pretty much are

EP describes WHERE, RtoL explains HOW... these terms cross over a lot

Pathological Shunt.. a whole new category, alongside physiologic shunt

blood goes through lungs but does NOT get oxygenated resulting in refractory hypoxemia

Physiologic vs. Pathologic

Phys — blood from heart doesn't reach the lungs

Path — blood gets to lungs, but doesn't pick up oxygen — or picks up oxygen, but can't bring it back to the heart/body afterwards

Pathological shunt examples:

alveoli with impaired diffusion — fibrosis, edema

congenital heart disease with R-L shunt

pulmonary arteriovenous malformations

vascular lung tumors

perfusion of non-ventilated alveoli — bronchial obstruction

Let's talk more about R-L shunt, which is ___

intrapulmonary

Shunting of blood from the right heart to the left heart can occur if there is a defect in the ___ between the right and left ventricles

wall

In the right-to-left shunt, hypoxemia always occurs because ...

a significant fraction of the

cardiac output is not delivered to the lungs for oxygenation

In R-to-L shunt, the portion of the cardiac output delivered to the lungs for oxygenation is "___" by the low O2-shunted blood

diluted

How about L-to-R shunts... are these more or less common than R-to-L?

more

Do L-to-R shunts cause hypoxemia?

no

What are some causes of L-R shunts?

patent ductus arteriosus

traumatic injury

What is the most pertinent effect of L-to-R shunts?

oxygenated blood from the lungs is added to the right heart without being delivered to the tissues

Well... how does L to R shunt not cause hypoxemia then???

the blood — a majority — that does make it out to the bodily tissues instead of heading back to the R heart is fully oxygenated

Does the size of the shunt matter?

yes — quantifies the extent to which venous blood bypasses oxygenation in the pulm caps of the lung

the size of the shunt is expressed as % of cardiac output

Are small shunts always present?

yes

Pa of O2 will always be slightly less than PA of O2

Give the shunt equation, which surmises venous admixture:

Qs divided by Qt

Qs

blood flow through R-to-L shunt

Qt

total cardiac output

V and Q were briefly mentioned earlier... what are they?

V — the air that reaches the alveoli

Q — the blood that reaches alveoli via the capillaries

Pulmonary Capillaries

surround the thin tissue membranes of the alveoli; site of gas exchange between blood that flows from heart to lungs and back to the heart

What crosses these pulmonary capillaries?

oxygen

blood flows straight from heart, to lungs, back to heart.. the stream doesn't stop; it is guided by the heart's beats

oxygen quickly makes its way into the fast flowing stream of blood, which flows through pulmonary capillaries that sit in between the pulmonary vein and pulmonary artery

like passengers quickly hopping onto a fast moving train

Equilibrated blood in pulmonary circulation has a Pa of O2 of ___ mmHg. In peripheral arteries, the Pa of O2 is ___ mmHg

104

100

Low Pa of O2 levels will induce ___

hypoxemia

This has been a lot! Let's quickly go back to the basics of A-a gradient and define it in a more qualitative way — A-a gradient normally ___ with age

increases

healthy young person = under 20

elderly person = 30

elderly people get less oxygen from lung to body — bodies are less efficient at gas transfer/exchange

Generally, increased A-a gradient implies pathology in the form of a ___

shunt

Let's now take a quick detour to discuss Pa of CO2 in this context... high Pa of CO2 means ___

hypoventilation

not enough CO2 is exhaled

Low Pa of CO2 means ___

hyperventilation

too much CO2 is being eliminated

Ok, cool, now back to A-a gradients... these can be caused by mismatch in the previously mentioned ___

V/Q

difference in blood level and oxygen level that reaches the lungs

V/Q Mismatch can be caused by these maladies:

pulmonary edema

atelectasis

pneumonia

obstructive lung disease, like COPD

pneumothorax

Oxygen Therapy

the administration of oxygen to increase the supply of oxygen to the lungs

ex: increased diffusion of O2 across thickened alveolar walls

does not help w/ R-to-L shunts

Oxygen therapy helps to determine if the shunt is primarily from ....

a V/Q defect, a diffusion

defect, or an anatomic shunt

Correcting hypoxemia involves ___

increase in PA of O2 and Pa of O2

Decreasing hypoxemia involves ___

relieving symptoms of hypoxia

Low O2 is obviously bad... is too much O2 bad?

yes

causes toxicity by formation of reactive oxygen species, which can damage tissues

What are respiratory causes of low Pa of O2?

Pulmonary right-to-left shunt

(increased A-a gradient)

V/Q mismatch

(increased A-a gradient)

Diffusion barrier (increased A-a gradient)

Hypoventilation

(increased PaCO2 with a normal A-a gradient)

What are non-respiratory causes of low Pa of O2?

Cardiac right-to-left shunt (increased A-a gradient, does not respond to increased FiO2)

Low mixed venous O2 saturation

(increased A-a gradient due to increased extraction of O2)

Decreased FiO2, or decreased PB

(normal A-a gradient)

What is the role of Pa of CO2?

regulates ventilation

small increases in PCO2 significantly increase ventilation

Hypoxia with normal A-a gradient:

high altitude

hypoventilation

Hypoxemia with increased A-a gradient:

Acute respiratory distress syndrome

Atelectasis

Infection

Interstitial lung disease

Pneumonia

Pulmonary embolism