Respiratory - Dr Podcast and In a box

Question 1

What is closing capacity

Answer 1

Closing volume + Residual volume

It is the lung volume at which airways close

Question 2

When is closing capacity increased and when do airways close during normal breathing

Answer 2

Closing capacity increases with:

mnemonic is CLOSE

Chronic Bronchitis (COPD)
LV Failure
Old Age (CC > FRC at 45 supine and 60 upright)
Smoking cigarettes
Emphysema

If CC > FRC then airway closure occurs during normal breathing

Question 3

In what circumstances in closing capacity equal to FRC

Answer 3

Neonates
Infants
Supine person aged 40
Standing person aged 65

Once CC > FRC –> airway closure and gas trapping occurs during normal breathing

Question 4

What is FRC

Answer 4

Functional Residual capacity is the volume of air remain in the lungs at the end of a normal tidal expiration.

It is the balance point between the tendency of the lungs to collapse inward and the chest wall to spring outward.

FRC = RV + ERV and is 30 ml/kg in a supine patient

Standing 3000ml
Supine 2100 ml
(Reduces by 900 ml when supine)

Question 5

Tabulate factors that increase and decrease FRC

Answer 5

Increase:

1. Position: Supine to erect
2. Size: Increased height (with increased lung volume)
3. Elasticity: Decreased elastic recoil (emphysema, increasing age)
4. Iatrogenic: PEEP/IPPV

Decrease:

1. Position: Erect to supine
2. Size: Obesity
3. Intra-abdominal pressure: Gas/Fat/Pregnancy/Acute abdomen etc
4. Lung disease (fibrosis / oedema / ARDS/ atalectasis) Anaesthesia

Question 6

What is the formula for dead space, including all the types

Answer 6

VD Phys = VD Anat + VD Alv

VD Anat (Fowler’s method)

Question 7

What is Fowler’s method for the measurement of anatomical dead space?

Answer 7

Single breath N2 washout.

1. Vt room air
2. At FRC –> VC breath of 100% O2 inhaled
3. Expire into mouthpiece until RV.
4. Spirometer measures volume air expired + rapid N2 analyser, measures conc. of N2 expired.

FOUR PHASES
Phase 1 –> Gas from VD anat (only O2, no N2)
Phase 2 –> Mix VD anat + Alv gas
(Midpoint of this curve is taken as VD anat where area A = B)
Phase 3 –> expired N2 reaches a plateau. All gas is now alv. gas (upwards slant plateau)
Phase 4 –> Sudden increase Nitrogen at closing capacity

Question 8

Why does the sudden increase of N2 in phase 4 of the Fowler’s method indicate the patient’s closing capacity

Answer 8

Basal alveoli = more compliant during inspiration. Therefore 100% O2 inhaled preferably inflates these alveoli instead of the apical alveoli. At the start of expiration, the process reverses and the basal alveoli empty first. When the lower airways close, the N2 rich gas from the apical alveoli is exhaled, resulting in sudden increase in expired N2 concentration

Question 9

What are the 3 functions of FRC

Answer 9

1. O2 reservoir (prevent drop PAO2 during expiration)
2. Prevent alveolar collapse / atalectasis / WOB
3. Optimal compliance and PVR at FRC

Question 10

Why is FRC of crucial importance to anaesthetists

Answer 10

1. Apnoea on induction –> reservoir for O2 during apnoea

2. Small airway closure: If FRC falls below certain volume (the CC), small airways close –> V:Q mismatch and hypoxaemia

Question 11

Which lung volumes and capacities cannot be measured by spirometry and why. What methods can be used to measure them?

Answer 11

RV and therefore: FRC and TLC cannot be measured by spirometry as by definition RV is the volume of air that remains in the lungs after maximal expiration.

Methods used to measure:

1. Gas dilution techniques (Helium)
2. Multiple breath Nitrogen washout
3. Body plethysmography

Question 12

How does the gas dilution method for measurement of FRC/TLC work

Answer 12

1. Helium does not cross into body via alv - cap mem.
2. Therefore, any reduction in He concentration is attributed to distribution in the lung rather than absorption.
3. At FRC (end tidal expiration) known [He] opened to patient who is allowed to breath for equilbration.
4. New concentration is measured

C1V1 = C2 (V1 + FRC)

Question 13

Why does the gas dilution method underestimate lung volumes?

Answer 13

relies on He equilibrating in the lungs which it can only do if all the airways are patent (e.g. air trapping in COPD)

Question 14

What are the advantages of plethysmography over as dilution techniques

Answer 14

Takes into account all gas in the lung, including any gas trapped behind closed airways.

Question 15

What principle is body plethysmography based on. Describe this principle and write and equation for it

Answer 15

Boyle’s law: At constant temperature, the volume of a fixed mass of gas is inversely proportional to its absolute pressure:

PV = k
Pressure = k/ Volume

Question 16

How does a body plethysmograph work

Answer 16

PV = k (Boyles law)

1. Patient inside airtight box with mouth piece + shutter + pressure transducer.
2. Shutter closes suddenly before tidal inspiration (at end tidal expiration).
3. Patient inspires against closed mouth piece. Resp. effort increases A-P diameter of thoracic cage, increasing lung volume. the gas remaining in the lung expands –> change in volume in the box will cause change in pressure as box is air tight (PV = K)
4. Pressure change read by box manometer
5. P1 and V1 known. P2 known and V2 can be calculated.

BOX
P1V1 = P2V2
V2 = V1 - delta V

So delta V can be calculated because all other variables are known.

PATIENT LUNGS
P3 - pressure at mouth piece before shutter closure
V3 - is FRC

P4 - Pressure at mouth after inspiration
V4 - is FRC + delta V

As delta V is known

P3 x FRC = P4 x (FRC + delta V)

All variables except FRC are known

Question 17

‘what is the multiple breath N2 washout used for and how is it carried out

Answer 17

Its used to calculate FRC
Does not account for gas trapping and may underestimate FRC

1. Patient breaths normal air
2. At FRC: changed to 100 % O2.
3. N2 analyser measures conc. N2 throughout
4. All expired gas collected until N2 expired = 1%

Then

C1 x V1 = C2 x V2

C1 - initial fractional [N2]
V1 - FRC

C2 - final fractional [N2]
V2 - total expired N2 volume

Question 18

What is closing capacity

Answer 18

the lungs are affected by gravity.

Upright –> lung parenchyma stretched at the apices and compressed at the dependent bases. This means that airway radius is reduced at the bases. Therefore, the basal airways are the first to be compressed during active expiration –> VQ mismatch. The lung volume at which this occurs is called the closing capacity = RV + CV.

Question 19

What would happen if CC exceeds FRC

Answer 19

Airway closure and VQ mismatch during tidal breathing –> hypoxaemia

Question 20

When might CC exceed FRC

Answer 20

Decreased FRC

• Postion: supine
• Size: Small (paediatrics)
• Lung volume reduced: Intrabdominal pressure (Pregnancy / obesity / acute abdomen / ascites etc)
• Lung disease (Fibrosis / oedema / ARDS)

Increased CC
- CC increases with age encroaching FRC at 45 when supine and at 60 when standing

Question 21

How does encroachment of CC on FRC affect work of breathing?

Answer 21

Increased WOB as closed alveoli require more energy to open as the exist at a disadvantageous point on the lung compliance curve