Respiratory: Pulmonary Ventilation: Volumes, Flows, Dead Space and Preoxygenation

Question 1

Draw and label a spirometry trace

Question 2

Tidal Volume

Answer 2

the volume of gas which is inhaled or exhaled during the course of a normal breath

TV V_T, ml

6-8ml/kg

Question 3

Residual Volume

Answer 3

the volume of gas that remains in the lungs after a maximal forced expiration

RV, ml

20ml/kg

Question 4

Inspiratory reserve volume

Answer 4

the volume of gas that can be further inhaled aftee the end of a normal tidal inhalation

IRV, ml

20-40ml/kg

Question 5

Expiratory reserve volume

Answer 5

the volume of gas that can be further exhaled after the end of a normal todal exhalation

ERV, ml

20ml/kg

Question 6

Capacity

Answer 6

the sum of two or more lung volumes

Question 7

Vital capacity

Answer 7

The volume of a gas inhaled when a maximal expiration is followed immediately by a maximal inspiration.

The sum of the ERV, IRV and TV

VC, ml

60ml/kg

Question 8

Functional residual capacity

Answer 8

The volume of gas that remains in the lungs after a normal tidal expiration

The sum of the ERV+RV

FRC, ml

35-45ml/kg

Key functions:

1. O2 resevoir
2. prevention of airways collapse
3. optimal compliance
4. optimal pulmonary vascular resistance

Question 9

Closing volume

Answer 9

the volume of gas over and above residual volume that remains in the lungs when the small airways begin to close (ml)

Calculated by measuring the nitrogen concentration in expired gas after a single breath of 100% O2 (nitorgen wash out)

Question 10

Closing capacity

Answer 10

the lung capacity at which the small airways begin to close.

It is a combination of residual volume and closing volume (ml)

Increases with age

reaches standing FRC at 70 years and supine FRC at 40 years

Question 11

Total lung capacity

Answer 11

90ml/kg

Question 12

What volumes/capacities can not be measured?

Answer 12

RV, FRV, TLC

Can be measured by helium dilution or body plethysmography

Question 13

Water-Sealed Spirometer

Answer 13

• breathing into a closed chamber that is partially submerged in water
• brathing in and out displaces the water
• movement recored by pen on moving paper
• cant measure residual volume

Question 14

Dry Spirometer

Answer 14

• bellows driven recording device e.g. a vitalograph
• set of bellows which are attached to a pen
• as patient inhales and exhales the bellows collapse and expand moving the pen

Question 15

Body Plethysmography

Answer 15

• The subject is placed in an airtight box
• air pressure (P1) and volume (V1) within the box are measured
• subject then inhales and exhales to a particular lung volume (normally FRC) through a mouthpiece
• shutter drops across the breathing tube
• subject continues to make respiratory efforts against the closed shutter
• chest volume increase -> box air volume decrease
• Boyles law
  
  • PV = k
• pressue in box increases (P2)
• P1 x V1 = P2 x (V1-change in lung volume)
• FRC:
  
  • Initial airway pressure x initial lung volume = inspiratory airway pressure x inspiratory volume of chest
  • Where initial lung volume = FRC, and inspiratory volume of the chest = change in lung volume + FRC
  • This measurement of FRC (unlike helium dilution) includes lung units that are collapsed or with poor air entry

Question 16

Helium Dilution

Answer 16

• subject breathes air containing a known concentration of helium in a closed system containing a spirometer
• CO2 produced during the test is absorbed by soda lime and replaced with oxygen
• helium is distributed throughout the subjects lungs (although not into obstructed lung units) and the equipment
• Helium is used because of its very low solubility, which means a minimal amount is lost through absorption into the bloodstream

Question 17

Nitrogen Washout

Answer 17

• subject breathes 100% oxygen from the end of a normal expiration through a closed breathing circuit connected to a spirometer
• After several minutes the nitrogen concentration and volume of gas within the equipment is measured
• The amount nitrogen initially present = FRC x 79% (atmospheric nitrogen concentration)
• poorly or non-ventilated lung units will not be included in this measurement

Question 18

FRC: oxygen resevoir

1. Typical FRC volume?
2. How much oxygen when breathing air?
3. Oxygen reserve when breathing air?
4. How much oxygen when pre-oxygenating with 100% O2?
5. What is the oxygen reserve when pre-oxygenating?
6. When is oxygen reserve reduced

Answer 18

1. 2500 mls
2. Alveolar concentration O2 in air at sea level 15%: 2500 x 0.15 = 375 ml of O2
3. With typical oxygen consumption of 250 mls/min, this equates to 375/250 = 1.5 minutes, or 90 seconds of oxygen reserve
4. alveolar oxygen content can be increased to around 90%. FRC now contains 2500 x 0.90 = 2250 ml of O2.
5. The oxygen reserve is now 2250/250 = 9 minutes.
6. O2 reserve reduced in
   
   1. smaller FRC eg obese
   2. high O2 demand eg children, sepsis

Question 19

Prevention of Airway Collapse

1. when does airway closure occur?
2. what is closing capacity
3. what is the CC in healthy patients?
4. What can cause the FRC to reach CC?
5. how can FRC be maintained above CC in the anaethetised patient?

Answer 19

1. Airway closure occurs when the lung volume equals the closing capacity
2. Closing capacity (CC) is the sum of closing volume + residual volume
3. In young, fit patients CC is always less than the FRC, so no airway collapse occurs during normal tidal volume ventilation
4. smoking, asthma, age
5. by applying PEEP

Question 20

1. Draw the compliance curve

Answer 20

1. FRC an equilibrium is reached between two opposing forces - the tendency of the lungs to collapse and the tendency of chest wall to spring out. FRC normally corresponds with a point on the steepest part of the curve =lung compliance is greatest. If the FRC is reduced (eg. restrictive lung disease) the patient breathes from a point of the curve that is flatter. This is associated with a fall in compliance and an increase in the work of breathing.

Question 21

1. What is compliance
2. What is lung compliamce?
3. Whats is static compliance?

Answer 21

1. compliance is the volume change per unit change in pressure (ml.cmH2O or L.kPa)
2. add compliances (reciprocals)
   
   1. 1/C_Total=(1/C_chest)+(1/C_lung)
3. Static compliance is the compiance of the lung measured when all gas flow has ceased
4. Dynamic compliance is the compliance of rhe lung measured during the respiratory cycle when gas flow is still ongoing

Question 22

Optimal Pulmonary Vascular Resistance

1. when is PVR lowest?
2. What is resistance?
3. What is lung resistance?

Answer 22

1. PVR is lowest at FRC allowing optimal pulmonary blood flow
2. the pressure change per unit change in flow (cmH2O.L^-1.sec^-1 or kPa.L^-1.sec^-1
3. Add resistances (normal intergers): total resistance= chest wall resistance+lung resistance

Question 23

What factors affect FRC?

Answer 23

Increase FRC:

• height
• male gender
• asthma
• emphysema
• IPPV

Decrease FVC:

• obesity
• anaesthesia
• supine
• kyphoscoliosis
• pulmonary fibrosis

No effect:

• age (but can increase CC)

Question 24

What is dead space?

How does dead space affect avleolar ventilation?

What constituents of deadspace are there in an anaesthetised and ventilated patient?

What is physiological deadspace?

Answer 24

• Dead space is the volume of inspired air that does not take part in gas exchange
• makes up around 30% of normal tidal volume ventilation
• Alveolar Ventilation (VA) = Tidal Volume Ventilation (VT) – Dead Space Ventilation (VD)
• constituents of dead space:
  
  • Apparatus: the volume of any external equipment, such as HMEFs, mainstream capnometry, face masks etc
  • Anatomical: gas in the larger, conducting airways
  • Alveolar: gas from alveoli that are poorly perfused (with high V/Q ratios)
• Physiological Dead Space = Anatomical Dead Space + Alveolar Dead Space

Question 25

Fowler’s Method

Answer 25

* measures anatomical dead space * subject breathes air normally and then takes a vital capacity inspiratory breath of 100% O2 from the end of normal expiration * Exhaled N2 is then measured throughout a slow maximal expiration down to RV * Line at 0 N2: This is anatomical dead space of the conducting airways. It contains no N2, but only the O2 from the vital capacity inspiration. * First upward slope: mix of alveolar gas and dead space gas. The horizontal distance from start of expiration to midpoint of the slope represents the volume of anatomical dead space * Plateau: N2 from the alveoli. This nitrogen was present in the alveoli prior to the inspiration of the O2. The gradient of this slope is determined by the alveolar time constants, and is increased in obstructive lung disease. * End of line: epresents closing capacity. During normal tidal breathing, the alveoli in the upper parts of the lung are best ventilated. When the vital capacity breath of O2 was taken most of this entered the smaller lower alveoli. Thus the upper parts of the lung contained normal, N2containing air, whilst the lower parts contained the inspired O2. At closing capacity the lower airways collapse and the exhaled gas comes from the upper alveoli instead. Thus the N2 concentration rises.

Question 26

The Bohr equation

Answer 26

* calculates physiological dead space ration * assumes all expired CO2 comes from alveolar gas * Alveolar CO2 Concentration x Alveolar Ventilation = Mixed expired CO2Concentration x Tidal Volume * V_D/V_T=(P_aCO₂-P_ECO₂)/P_aCO₂ * 0.8 in mammals

Question 27

Flow-volume loop

Answer 27

1. after max inspiration exhales from TLC. Maximal recoil -\> exposive expiratoery flow (=peak expiratory flow) 2. expiratory fow decreases glradually as lung volume returns to RV 3. inspiration starts, does not reach instant maximal flow as inspiratory respiraotry muscle contraction is slower than pure elastic recoil. Maximal flow reached at midpoint of VC

Question 28

Flow-volume loop in obstructive disease

Answer 28

* RV is increased due to gas trapping * TLC is raised due to hyperexpansion of the lungs * Left shift of loop * inspiratory and expiratory flows are decreased (expiratory to a greater extent) * Elastic lung tissue is lost -\> flow at the start of expiration is not as explosive as usual * small airways collapse rapidly resulting a very early decline in flow -\> scalloped out appearance of expiratory limb.

Question 29

Flow-volume loop in restrictive lung disease

Answer 29

* both RV and TLC are reduced (right shift) * shape of the inspiratory and expiratory limbs is near normal * smaller flow rates due to less elastic recoil of lungs

Question 30

Flow-volume loop in fixed upper airway obstruction

Answer 30

* Lung volumes are unchanged * obstruction affects both inspiratory and expiratory flow rates * results in flow loops that are flattened

Question 31

Flow-volume loops in variable extrathoracic airway obstruction

Answer 31

* eg. unilateral vocal cord paralysis * Lung volumes are unchanged * During inspiration the paralysed vocal cord is drawn inwards, resulting in a reduction in inspiratory flow * Expiratory flow is unaffected as the paralysed vocal cord is easily blown aside

Question 32

Flow-volume loops in variable intrathoracic airway obstruction

Answer 32

* eg tracheomalacia below the thoracic inlet * Lung volumes are unchanged * During inspiration the trachea is held open by the negative intrapleural pressure, resulting in a near normal inspiratory flow loop * During expiration the lack of support to the trachea results in tracheal collapse and a reduction in expiratory flow