Ventilation and Perfusion

Question 1

What does V/Q represent?

Answer 1

Alveolar ventilation / pulmonary capillary blood flow

Question 2

What happens if the V/Q increases?

Answer 2

The delivery of oxygen and relative to its removal will increase, as will the removal of carbon dioxide relative to its delivery

This means the O2 in the alveolar increases and the CO2 decreases

Question 3

What happens if the V/Q decreases?

Answer 3

The removal of oxygen relative to its delivery will increase and the delivery of carbon dioxide relative to its removal will increase

This mean the O2 in the alveolar decreases and the CO2 increases

Question 4

What is a right-to-left shunt?

What happens to gas exchange?

What’s it’s V/Q?

Answer 4

The airway supplying the unit has become completely occluded and as time goes on the air trapped in the alveolus equilibrates by diffusion with the gas dissolved in mixed venous blood.

No gas exchange occurs and any blood perfusing this alveolar-capillary unit will leave exactly as it has entered

It has Zero V/Q

Question 5

What is alveolar dead space?

Answer 5

The alveolar is being ventilated but not perfused

Question 6

What happens to an alveolar that is not being perfused?

What is it’s V/Q?

Answer 6

No oxygen entering the alveolar can diffuse into the blood and no CO2 in the blood can diffuse into the alveolar

Gas is basically the same as inspired air

Its V/Q is infinite

Question 7

What is a physiological shunt?

Answer 7

Consists of the anatomic shunts plus the intrapulmonary shunts

intrapulmonary shunts can be anatomic or shunt-like states

Correlates with the physiological deadspace

Question 8

What is an absolute shunt?

Whats the affect on V/Q?

What is a shuntlike state?

Whats the affect on V/Q?

Answer 8

Mixed venous blood perfusing pulmonary capillaries associated with totally unventilated or collapsed alveoli constitutes an absolute shunt (like the anatomic shunts) because no gas exchange occurs as blood passes through the lung.

Absolute pulmonary shunt - V/Q = 0

Areas of low ventilation-perfusion ratios in which alveoli are under-ventilated and/or over perfused

Shunt-like state - V/Q = low

Question 9

What is an anatomic shunt?

Answer 9

Where systemic venous blood enters the left ventricle without having entered the pulmonary vasculature

Theres a normal anatomical shunt that accounts for <5% of CO, it includes thebesian veins, pleural veins, and bronchial veins.

Question 10

What constitutes and absolute shunt?

Answer 10

Mixed venous blood perfusing pulmonary capillaries associated with totally unventilated or collapsed alveoli constitutes an absolute shunt (like the anatomic shunts) because no gas exchange occurs as blood passes through the lung.

Question 11

What is being looked at in the nitrogen washout test?

Answer 11

The patient inhales some nitrogen which should decrease from 80% to 2% within 7 minutes. If it takes longer than this then the patient has high resistance pathways

Question 12

What is the alveolar plateau closing volume test?

What is the CC point?

Answer 12

Trying to find the CC point (closing capacity point), when the plateau increases sharply the alveoli in the lower lung have closed and the air rushes out of the alveoli in the top

Alveoli in the bottom are more compliant and get more O2, alveoli in the top are less compliant and contain more N2O

The CC point is the when the volume of air in the lungs causes the alveoli to collapse

Question 13

What is a drawback of the alveoli plateau closing volume test?

Answer 13

Cannot use on patients with high airway resistance, causes the alveolar plateau to be steep which makes it hard to determine CC point

The steeper your plateau is, means those alveoli are getting less O2

Question 14

What’s a way to extend the N2 washout test?

Answer 14

Inject person with histamine which increases airway resistance, N2 may last in lungs up to 6 months

Question 15

What are common ways to test for V/Q mismatch which are nonuniform gas distribution?

Answer 15

Alveolar plateau of closing volume test
Nitrogen washout
Trapped gas
Single breath of 133Xe
99mTc-labeled DTPA  (diethylene triamine pentaacetic acid)

Question 16

What are common ways to test for V/Q mismatch which are nonuniform pulmonary blood flow?

Answer 16

Pulmonary angiogram

Lung scans which can be:
131I-labeled MAA (macroaggregates of albumin)
99mTc-labeled MAA***
133Xe (for perfusion scans, VQ matching)

Question 17

How can a pulmonary angiogram be preformed?

Answer 17

Insert a swan-ganz and inject some dye and look for where dye doesn’t go

Question 18

How are lung scans preformed?

Answer 18

Radioactive labeled MAA (macro aggregates of albumin)- they’re too big to get through the capillaries= they’re like emboli, so wont really move, but you can see where they don’t go

Question 19

What are additional ways of testing for V/Q mismatch?

Answer 19

Physiologic shunts : shunt equation
Anatomic
Intrapulmonary
Absolute
Shuntlike states  (low V/Q)

Physiologic dead space : Bohr equation
Anatomic
Alveolar : (a-A)Dco2

(A-a)Do2

Lung scans after inhaled and infused markers

Multiple inert gas technique

Question 20

What is the concept of the Shunt Equation?

Answer 20

Conceptually divides all alveolar-capillary units into two groups: those with well-matched ventilation and perfusion and those with ventilation-perfusion ratios of zero

Question 21

What is the Shunt equation

Answer 21

(Qs/Qt) = (CcO2 - CaO2)/(CcO2 - CvO2)

Qs = Shunt flow - Represents the amount of blood flow per minute entering the systemic arterial blood without receiving any oxygen
Qt = Total Pulmonary blood flow

CcO2 = End-capillary oxygen content
CaO2 = Oxygen content of arterial blood
CvO2 = Oxygen content of mixed venous blood

Question 22

What is the concept of the Bohr equation?

Answer 22

Permits the determination of the sum of the anatomic and the alveolar dead space: Physiological dead space

Simple concept: Any measurable volume of carbon dioxide must come from alveoli that are both ventilated and perfused because there are negligible amounts of carbon dioxide in inspired air

If arterial PaO2 is greater than PECO2 (Mixed expired partial pressure of carbon dioxide or End-tidal CO2) there person has dead space

This is cause CO2 can’t leave the blood at the blocked off capillary sites

Question 23

What is the Bohr equation?

Answer 23

VdCO2/Vt = (PaCO2 - PECO2)/PaCO2

VdCO2 = Dead space for CO2 (physiological dead space)
Vt = Tidal Volume
PaCO2 = Arterial partial pressure O2
PECO2 = Mixed expired partial pressure of carbon dioxide (End-tidal CO2)

Question 24

What are some causes of an increased alveolar-arterial Oxygen Partial-Pressure Gradient?

Answer 24

Increased right - to left shunt

• Anatomic
• Intrapulmonary

Increased ventilation-perfusion mismatch

Impaired diffusion

Increased inspired partial pressure of oxygen

Decreased mixed venous partial pressure of oxygen

Shift of oxyhemoglobin dissociation curve

Question 25

What are the regional differences in the ventilation-perfusion ratios of the lungs?

Answer 25

The lower lung is both better perfused and ventilated
In the lower lung, it is perfused much more than ventilated
In the upper lung, it is ventilated more than perfused
On the graph, where perfusion and ventilation intersect is where the current V/Q is
Upper lung has higher V/Q - higher O2 and low CO2
Lower lung has lower V/Q - lower O2 and higher CO2
Lower lungs have less negative intrapleural pressure and lower transpulmonary pressure making them more compliant
Upper lungs have more negative intrapleural pressure so higher transpulmonary pressure, making alveoli larger and less compliant