Ventilation- Perfusion Relatioships

Question 1

What is the typical partial pressures of O2 and CO2 at the beginning of pulmonary circulation? End-capillary?

Answer 1

O2- 40 mmHg, CO2- 45 mmHg; O2- 100 mmHg and CO2- 40 mmHg

Question 2

What are the two extremes of ventilation-perfusion mismatch?

Answer 2

Alveolar dead space and intrapulmonary shunts

Question 3

If there is more ventilation than perfusion what is the effect on arterial CO2 and O2?

Answer 3

PO2 will increase and PCO2 will decrease

Question 4

What is the normal V/Q ratio?

Answer 4

1

Question 5

If the V/Q ratio is decreased, what will happen to PO2?

Answer 5

It will decrease

Question 6

What is represented by the alveolar plateau phase of the closing volume test?

Answer 6

The plateau represents high resistance pathways which get less of the pure inhaled oxygen and therefore have more nitrogen. The plateau is the slow gradual emptying of those slow alveoli

Question 7

What is the relationship between high resistance small airways and nitrogen washout time?

Answer 7

The more highly resistant airways, the longer it will take to washout the nitrogen

Question 8

How can pulmonary blood flow be tested?

Answer 8

Pulmonary angiograms, or lung scans using labeled macroaggregates of albumin

Question 9

What vessels contribute to the normal anatomic shunt?

Answer 9

Part of bronchial venous drainage, thebesian veins (coronary a. into L. ventricle) and pleural veins all enter left-sided arterial blood

Question 10

What is the physiologic shunt?

Answer 10

It corresponds to physiologic dead space and consists of anatomic shunts plus intrapulmonary shunts

Question 11

What are the two classes of intrapulmonary shunts which describe the severity?

Answer 11

Absolute shunts and shunt-like states

Question 12

What are absolute intrapulmonary shunts?

Answer 12

When mixed venous blood perfusing pulmonary capillaries associated with totally unventilated or collapsed alveoli

Question 13

What is a intrapulmonary shunt-like state?

Answer 13

Alveolar-capillary units with low V/Q that have low PO2

Question 14

What is the Shunt Equation and for what is it used?

Answer 14

Says the ratio of shunt flow (flow going by unventilated alveoli) to the total cardiac output is equal to (end-capillary oxygen content minus arteriole oxygen contents) (contents in ml O2/100 ml blood) divided by (end cap oxygen content minus mixed venous oxygen content); Used to quantify how big a physiologic shunt is

Question 15

What is the problem with shunts?

Answer 15

Venous blood is going directly into arterial circulation, decreasing pO2 and causing hypoxemia if severe enough

Question 16

True or false: True shunts can be compensated for using high FIO2?

Answer 16

False

Question 17

What is the complete opposite of intrapulmonary shunts?

Answer 17

Alveolar dead space

Question 18

Is arterial PO2 equal to alveolar PO2 normally?

Answer 18

No- arterial PO2 is always a little lower

Question 19

Why is arterial PO2 typically a bit lower than the calculated ideal alveolar PO2?

Answer 19

1- the normal anatomic shunt; 2- there is normally some degree of V/Q mismatch in some parts of the lung; 3-some fluid collects in lower parts of the lung making oxygen diffusion more difficult

Question 20

What is the typical normal value for the PO2/FIO2 ratio? What values indicate problems?

Answer 20

Nearly 500; Less than 2 or 3 hundred indicates serious trouble

Question 21

Where does the most ventilation take place in the lung? Why?

Answer 21

Lower parts because intrapleural pressure is more negative at top and so there is greater transmural pressure differences and so the alveoli are bigger and less compliant

Question 22

Where in the lung does the most perfusion take place? Why?

Answer 22

In the lower lung because there is greater pressure in the vessels at the bottom leading to more distension and recruitment

Question 23

In the very lowest part of the lungs what is the V/Q ratio? Why?

Answer 23

Less than one because while there is more ventilation in the lower lung, there is way more perfusion in the lower lungs

Question 24

Where in the lung is PO2 highest? PCO2 highest?

Answer 24

PO2 is highest in the uppermost segment of the lungs; PCO2 is highest in lowest segments of the lung

Question 25

Where in the lung is the diffusion gradient for PO2 and PCO2 the greatest?

Answer 25

Upper parts of the lung