Ventilation Perfusion Abnormalaties

Question 1

How does intrapleural pressure vary throughout the lung?

Answer 1

There is a higher (less negative) pressure towards the base of the lungs, this helps balance out the increased weight of the bases of the lungs.

Question 2

How does compliance vary throughout the lung?

Answer 2

As long as the bases remain partially inflated (not collapsed) there is greater compliance at the bases. This is as they have a low resting volume.

(Balloon analogy)

Question 3

Describe how perfusion varies across the different ‘west zones’ of the lung?

Answer 3

Zone 1:where alveolar pressure is higher than arterial and venous pressure (PA>Pa>Pv). Describes capillaries compressed in the apices providing no flow. This is not normally present but will occur with IPPV or reduced pulmonary arterial pressure. This area is alveolar dead space

Zone 2: where the alveolar pressure is lower than the arterial but higher than the venous pressure (Pa>PA>Pv). Blood flow is therefore dependent on the gradient between alveolar and pulmonary arterial pressure. In diastole, particularly in conditions of hypovolemia, pulmonary arterial pressure may also be lower than the alveolar pressure, which means flow would only occur during systole

Zone 3: where both arterial and venous pressure is higher than alveolar (Pa>Pv>PA). Alveolar pressure does not play much of a role unless it exceeds pulmonary venous pressure (IPPV with very high pressures). Blood flow to this zone exceeds the blood flow to all the other zones.

Zone 4: where the interstitial pressure is higher than alveolar and pulmonary venous pressure (but not pulmonary arterial pressure). This occurs in atelectatic or oedematous lung at the very base of the chest cavity, where interstitial fluid pressure exceeds pulmonary venous pressure. As a result, blood flow is governed by the gradient between arterial and interstitial pressures.

Question 4

What are the 2 overarching factors which effect PAO2

Answer 4

1. Rate of Oxygen removal from the blood (oxygen consumption)
2. Rate of oxygen delivery (alveolar ventilation)

Question 5

Define shunt?

Answer 5

Shunt is blood passing from the arterial to the venous system without passing through ventilated lung.

Question 6

Where does shunt normally occur (non pathologically)?

Answer 6

1. Bronchial arteries draining directly into pulmonary veins
2. Coronary venous blood draining into the left ventricle via Thebesian veins

Question 7

Derive the shunt equation Qs/Qt= X?

Drawing a diagram may help.

Answer 7

Start by drawing a diagram of a partially ventilated lung in terms of flow and 2 content.

Qn.CcO2 + QsCvO2 = Qt.CaO2

Qn= perfusion normal lung Qs= shunt perfusion Qt=total perfusion

Qn = Qt-Qs

(Qt.-Qs).CcO2 + QsCvO2 = Qt.CaO2

Qt.CcO2 - Qs CcO2 + QsCvO2 = Qt.CaO2

Qt.CcO2 - Qt.CaO2= Qs.CcO2 - Qs.CvO2

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

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

CCO2 must be estimated from the alveolar gas equation and haemoglobin-O2 dissociation curve.

There is no change in PaCO2 with shunt due to the reflex increase in ventilation.

Question 8

What is V/Q scatter?

Answer 8

This describes that there is difference in V/Q matching throughout the lung. For example:

At the lung base the V/Q ratio < 1 due to perfusion > ventilation i.e. shunt

At the lung apex the V/Q ratio >1 due to ventilation > perfusion i.e. alveolar dead space

Question 9

Describe the situations in which you get the following V/Q ratios: 1, >1, <1,∞ and 0?

Answer 9

V/Q = 1.0: A lung unit with well-matched gas and blood flows will have a V/Q ratio close to 1.0, i.e. for every unit of blood flow it will receive a unit of gas flow.

V/Q < 1.0: Good blood flow but insufficient ventilation; generally seen in the bases of lung (West’s Zone 4)

V/Q >1.0: Excellent ventilation but poor blood flow; West’s Zone 1.

V/Q = ∞: Lung units which receive no blood flow, i.e. “true” dead space

V/Q = 0: Lung units which receive no ventilation, i.e. “true” shunt

Question 10

Comapring the bases and the apices of the lungs is there a greater difference in ventilation or perfusion?

Answer 10

Apices well ventilated but poorly perfused.

Bases well perfused but poorly ventilated.

There is a greater difference in perfusion, than in ventilation.

Question 11

Will the PAO2 be greater at the bases or the apicies of the lungs?

Answer 11

They will be greater at the apices, if you consider the determinants of PAO2:

O2 delivery via alveolar ventilation
O2 removal from the blood

The Apices are better ventilated.
The bases are more perfused therefore more O2 is exchanged and removed from the Alveoli.

Question 12

What is the A-a gradient?

Answer 12

It is the difference between the Alveolar and arterial partial pressure of oxygen.

PAO2 - PaO2 = A-a gradient

Question 13

What is a normal A-a gradient?

Answer 13

~4mmHg

Question 14

What can cause an increased A-a gradient?

Answer 14

• Venous admixture or shunt (may have normal PAO2 but due to lack of perfusion to underperfused area will not translate into the PaO2)
• V/Q scatter: the effect of blood flowing through a heterogeneously ventilated/perfused lung. Consider underventilated but overperfused alveoli which will contribute a lot of hypoxic blood. Blood flowing through overventilated but underperfused units will never be able to compensate, as its volume is lower.
• Congenital heart disease causing a right to left shunt (Think patient S)
• Increasing FiO2 in the presence of shunt (will increase PAO2 further, making A-a gradient larger)
• Increased Cardiac output: Blood travels through the alveoli faster then oxygen can diffuse into them; this is seen in the exercise-induced hypoxia of pulmonary fibrosis patients.

Other small print reasons: reduced PaCO2 due to hyperventilation (Dalton’s gas law, PAO2 will increase), Cold temp will not effect PAO2 but will reduced PaO2 as more O2 will dissolve reducing the partial pressure

Good article: https://derangedphysiology.com/main/cicm-primary-exam/required-reading/respiratory-system/Chapter%20135/oxygen-tension-based-indices-oxygenation

Question 14

What can cause an increased A-a gradient?

Answer 14

Main reasons:
* * Venous admixture or shunt may have normal PAO2 but due to lack of perfusion to underperfused area will not translate into the PaO2. Low V/Q.
* V/Q scatter: the effect of blood flowing through a heterogeneously ventilated/perfused lung. Consider underventilated but overperfused alveoli which will contribute a lot of hypoxic blood(alveolar dead space high V/Q). Blood flowing through overventilated but underperfused units will never be able to compensate, as its volume is lower.

Other reasons:
* Congenital heart disease causing a right to left shunt (Think patient S)
* Increasing FiO2 in the presence of shunt (will increase PAO2 further, making A-a gradient larger)
* Increased Cardiac output: Blood travels through the alveoli faster then oxygen can diffuse into them; this is seen in the exercise-induced hypoxia of pulmonary fibrosis patients.

Other small print reasons: reduced PaCO2 due to hyperventilation (Dalton’s gas law, PAO2 will increase), Cold temp will not effect PAO2 but will reduced PaO2 as more O2 will dissolve reducing the partial pressure

Good article: https://derangedphysiology.com/main/cicm-primary-exam/required-reading/respiratory-system/Chapter%20135/oxygen-tension-based-indices-oxygenation