Respiratory - Ventilation and dead space

Question 1

What is the formula for tidal volume

Answer 1

Vt = Va + Vd

Va is alveolar volume
Vd is dead space volume

Dead space is air in conducting airways and non-perfused alveoli

Question 2

What is the formula for physiological dead space. What are the different types of dead space

Answer 2

VD (Phys) = VD(Anat) + VD(Alveolar)

Anatomical dead space - VD(Anat)
- Volume of the first 16 generations of the tracheobronchial tree

Alveolar dead space - VD (Alv)
- Total volume of ventilated alveoli that are unable to take part in gas exchange due to impaired perfusion (i.e. V/Q mismatch)

Physiological dead space - VD (Phys)
- Total dead space: the sum of anatomical and alveolar dead space

Question 3

How is anatomical dead space measured - describe this technique

Answer 3

Fowler’s method

1. Tidal volumes of Room Air
2. At end of normal exp. (FRC) the patient takes a vital capacity breath of 100% O2.
3. Slow exhalation into a mouth piece to maximal expiration (to RV)
4. Mouth piece is attached to a spirometer and rapid N2 analyser –> volume and N2 measured and plotted on a graph
5. Plot [N2] % vs volume on a graph with the four phases

Phase 1 - on X -axis –> no N2 expelled as this is gas from conducting airways only

Phase 2 - N2 conc. increasing as now mixed alveolar gas and dead space gas is detected.

Phase 3 - N2 reaches a plateau near the end of expiration as this is now alveolar gas.

Phase 4 - Sudden increase in [N2] as the lower airways close (at CLOSING CAPACITY) and air from the apical alveoli is expelled containing higher N2. (Basal alveoli are more compliant than apical alveoli so they fill first and empty first)

Question 4

What is the normal anatomical dead space in a 70kg Adult male and what factors influence this

Answer 4

150 ml
1/3 of Vt (7 ml/kg)

Factors: –> think of airway diameter

1. Size of lungs
2. Lung volume
   - radial traction at high volumes increases airway diameter and hence VD (Anat)
3. Posture
   - lung volume reduced supine which reduces airway diameter which reduces VD (Anat)
4. Bronchoconstriction reduces airway diameter
5. Bronchodilation increases airway diameter

Question 5

How is alveolar dead space measured

Answer 5

It cannot be measured. It can be calculated from:

VD (Phys) = VD(Anat) + VD(Alveolar)

VD (Anat) - Measured using Fowler’s
VD (Phys) - Measured using Bohr equation
VD (Alv) - Calculated from above

Question 6

What method is used to measure physiological dead space? Describe and derive this method and equation.

Answer 6

The Bohr method/equation

VD = PaCO2 - PetCO2
___ _________________
VT PaCO2

ALL CO2 COMES FROM ALVEOLAR GAS - and thus from alveoli that are bothe ventilated and perfused. NO CO2 comes from the dead space

VT = VD + VA
and
VD = VT - VA

FA = Fractional conc. of Alv CO2
FE = Fractional conc. of Expired CO2
FD = Fractional conc. of dead space CO2

Then,
VT.FE = VD.FD + VA.FA

FD = 0 (No CO2 comes from here)

Therefore,

VT.FE = VA.FA

Substituting: VA = VT - VD

VTFE = (VT - VD)FA

Rearrange

VD = FA - FE
___ _________

VT FA

See above equation

Question 7

What factors determine

1. Anatomical dead space
2. Alveolar dead space

Answer 7

1. Anatomical dead space (think airway diameter)
   - Larger lungs
   - High lung volumes (radial traction)
   - Posture: Supine decrease VD (Anat)
   - Bronchoconstriction
   - Bronchodilatation
2. Alveolar dead space (think reduced perfusion and West zones 1 and 2)
   - Upright posture (West zone 2)
   - Low pulm. pressure (decreased CO) i.e. to West zone 2
   - PEEP and PPV –> cause West zone 1 in lung apices increasing VD (Anat).
   - Pulmonary embolism (thrombus/fat/gas/amniotic fluid)
   - COPD: destroyed alveolar septa –> reduced SA for gas exchange.

Question 8

What are the formulae for:

1. Minute ventilation
2. Alveolar ventilation
3. Dead space ventilation

Answer 8

1. Ve = Vt x RR
2. V’a = Va x RR
3. V’d = Vd x RR

Overall Ve = V’a + V’d

Question 9

How is alveolar ventilation related to PaCO2

Answer 9

PaCO2 = K. Rate CO2 production (ml/min)
                      \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_
                       Alveolar Ventilation (L/min

K = 0.115

Doubling alveolar ventilation will lead to halving of PaCO2 (for a given CO2 production)

Question 10

Patient 1
Vt = 500 ml
RR = 15 /minute
Ve = 7500 mls

Patient 2
Vt = 250 ml
RR = 30/minute
Ve = 7500 mls

Would you expect the PaCO2 to be the same in these two patients? Explain.

Answer 10

NO. Because minute volume (Ve) does not take dead space into account)

Patient 1

VA = VT - VD

VA = 500 - 150
= 350 mls

Therefore
Alveolar ventilation = 350 x 15 = 5250 ml/min

Patient 2

VA = VT - VD

VA = 250 - 150
= 100 mls

Alveolar ventilation = 100 x 30 = 3000 ml/min

Therefore, patient 2 will have lower PaCO2.

Question 11

Plot the graph which demonstrates the relationship between PaCO2 and Alveolar ventilation.

Answer 11

Page 47 Chambers

X-axis Alveolar ventilation L/min: 2,4,6,8,10
Y-axis PaCO2 kPa 2,4,6,8

Hyperbola
Halving and doubling

Question 12

How is the problem associated with mechanical dead space overcome by the Bain circuit and the circle circuit

Answer 12

Bain circuit (Mapelson D)

• Requires high FGF –> used gases are spontaneously flushed along the tubing during expiratory pause.

Circle system

• Soda lime absorbs CO2
• O2 is replenished by FGF

Question 13

In what type of anaesthesia is mechanical dead space particularly important

Answer 13

Paediatric anaesthesia. The additional dead space contributed by the breathing circuit and apparatus is a significantly higher proportion of the dead space compared to adult general anaesthesia.