Lung Function Tests And Oxygen In The Blood

Question 1

Describe how spirometry works

Answer 1

Subject breathes from a closed chamber over water
Volume changes with ventilation
Coupled apparatus with a pen draws a trace on a rotating drum

Question 2

Define inspiratory reserve volume

Answer 2

The extra volume that can be breathed in above the tidal volume

Question 3

Define expiratory reserve volume

Answer 3

The extra volume that can be breathed out after tidal volume

Question 4

What is vital capacity?

Answer 4

Maximum expiration to maximum inspiration

Approx 5L in a typical adult

Question 5

What is the functional residual capacity?

Answer 5

The volume of air left in the lungs at resting expiratory level

Question 6

What is total lung capacity?

Answer 6

Volume of air in the lungs at the end of maximal inspiration
Approx 5.8L in a normal adult

Question 7

Name some factors influencing the vital capacity

Answer 7

Compliance
Force of inspiratory muscles
Airway resistance

Question 8

Describe single breathe spirometry

Answer 8

Breathe out as fast and fully as possible
Typically do 3 tests and take an average
Requires the subject to understand the test

Question 9

What is a vitalograph trace?

Answer 9

Plotting volume expired against time

Question 10

When is flow maximal?

Answer 10

When volume in the lungs is maximal due to the pressure gradient

Question 11

What is FEV1?

Answer 11

The volume expired in the first second

Affected by how quickly air flow slows down if airways are narrowed

Question 12

What is the normal ratio for FEV1:FVC?

Answer 12

> 70%

Question 13

Describe an obstructive disease

Answer 13

Airways are narrowed
Lungs are still easy to fill but resistance increases with expiration
Air will come out more slowly
(FEV1 decreased, FVC relatively normal)

Question 14

If the narrowing of airways is reversible what does this suggest?

Answer 14

Asthma

Question 15

Describe a restrictive disease

Answer 15

Lungs are difficult to fill
Cannot achieve maximal volume of lungs (FVC lowered)
Air will come out at same rate so FEV1 normal
Ratio >70% generally

Question 16

How can we measure PEFR?

Answer 16

Peak flow metre
Cheap and easy to use
Blow into tube as hard and fast as possible
Arrow shoots up scale to an estimate PEFR

Question 17

Describe the flow volume loop of an obstructive disease

Answer 17

Reaches (almost) the PEFR

Then flow rate falls rapidly

Question 18

Describe a restrictive flow volume loop

Answer 18

Shape of flow curve generally the same as normal

Doesn’t reach as high a volume

Question 19

Describe the helium dilution test

Answer 19

Measuring residual volume
Helium isn’t metabolised therefore pt breathes a known volume of gas containing a known conc of helium
Helium conc diluted into the larger volume in the lungs
Measure the conc when we breathe out to work out what volume of air the helium expanded into

Question 20

How do we measure serial/anatomical dead space?

Answer 20

Nitrogen wash out
Subject takes one normal breath of pure oxygen
Breathes out via meter reading the percentage of nitrogen
The volume expired half way between just oxygen and normal concs of nitrogen is the volume of serial dead space

Question 21

What do we use the CO transfer factor for?

Answer 21

Measuring the diffusion capacity of the lung
Not done routinely
Patient inhales to total lung capacity - mixture containing CO
Hold breath for 10 sec
Rate of diffusion of CO estimated

Question 22

Is oxygen soluble in water?

Answer 22

Not very

Question 23

What is the equation for working out the concentration of oxygen dissolved?

Answer 23

Solubility of oxygen x pO2

Question 24

What does the dissociation curve of myoglobin look like?

Answer 24

Rectangular hyperbola.

Saturation = y, partial pressure = x

Question 25

Describe the T state of Hb

Answer 25

Low affinity for oxygen

Difficult for O2 to bind to Hb

Question 26

When the pO2 is low, Hb is in which state?

Answer 26

T state

Question 27

What does the dissociation curve of Hb look like?

Answer 27

Sigmoidal

Question 28

Above what approx kPa is Hb saturated?

Answer 28

9-10 kPa

PO2

Question 29

What is the normal Hb concentration?

Answer 29

2.2 mmol/L

Question 30

What is the normal Hb bound oxygen content?

Answer 30

8.8 mmol/L

Question 31

What can the tissue pO2 not fall below?

Answer 31

3 kPa

Question 32

Describe the Bohr shift

Answer 32

pH affects the affinity of Hb for oxygen
Decreased pH means more H+ bound to Hb
Hb moves into T state
Harder for oxygen to bind
(Decreased pH = increased oxygen dissociation)
Shifts curve to the right

Question 33

Which direction does an increased temperature shift the dissociation curve for Hb?

Answer 33

To the right

Extra oxygen dissociates at higher temperatures

Question 34

Describe a tissue that would have the maximal oxygen unloading

Answer 34

Low pO2
More acidic (lower pH)
Higher temperature

Question 35

What is the effect of 2,3-BPG?

Answer 35

Shifts curve to the right

Decreased affinity of Hb for oxygen

Question 36

When do 2,3 BPG levels increase?

Answer 36

Anaemia

High altitude

Question 37

What happens when carbon monoxide is in the blood?

Answer 37

Very high affinity for Hb
Reacts to form COHb - binds irreversibly
Increases the affinity of unaffected subunits for oxygen therefore oxygen not given up at the tissues
Fatal if HbCO >50%

Question 38

What is the difference between peripheral and central cyanosis?

Answer 38

Peripheral = poor local circulation 
Central = poorly saturated blood in systemic circulation

Question 39

Describe pulse oximetry

Answer 39

Place on end of finger
2 different wavelengths of light are emitted
Detects levels of Hb saturation
Only detects pulsatile arterial blood
(Gives us saturation but cannot tell us how much Hb is present)

Question 40

Where do we commonly take an ABG from?

Answer 40

Radial artery