Ventilation and Perfusion

Question 1

4 Major Volumes

Answer 1

• Insp Reserve Vol - amount that can be inhaled beyond VT w/ max inspiration
  
  • Normal = 2.5 L
• Tidal Vol - amount inhaled in normal breath; ends at FRC (where P resp syst = 0)
  
  • Normal = 500 mL (.5L)
• Expiratory Reserve Vol -amount that can be forced out of lung beyond normal VT
  
  • Normal = 1.5 L
• Residual Vol - amount of volume remaining in lungs even after max expiration
  
  • Normal = 1.5 L

Question 2

4 Major Capacities

Answer 2

• Insp Capacity - total from beginning of tidal inspiration to max inspiration (VT + IRV)
  
  • Normal = 3 L
• Functional Residual Capacity - volume remaining at end of VT expiration (passive expiration); balanced elastic recoils (RV + ERV)
  
  • Normal = 3 L
• Vital Capacity - volume that can be exhaled w/ max effort after max inhale (IRV + VT + ERV)
  
  • Normal = 4.5 L (all except residual volume)
• Total Lung Capacity - volume in lungs at end of max inspiration (determined by max force from inspiratory muscles and inward elastic recoil of both lungs and chest wall)
  
  • Normal = 6 L (all)

Question 3

Oxygen Cascade

Answer 3

• 160 outside –> 150 in conducting airway –> 100 in alveoli –> 95-98 in arteries
• PO2 of conducting airway is lower than in atmosphere b/c now have water vapor; since all partial pressures must still add up to same total, the individual partial pressure of O2 goes down
• Dec in alveoli relative to PACO2
• Dec in artery from anatomic shunt

Question 4

Alveolar Air Equation

Answer 4

• PAO2 = (PB-Ph20) FiO2 - (PACO2/R)
• Amount of O2 coming in (affected by amount of water vapor and partial pressure of O2) MINUS amount of O2 leaving alveoli (which is related to amount of CO2 in alveoli / R)
• R = .8 b/c more O2 leaves than CO2 leaving alveoli
• FiO2 = .21

Question 5

PA-aO2 Gradient

Answer 5

• Pressure of O2 decreased b/n alveoli and artery (in drawn blood gas) b/c some natural anatomical shunting of deoxy blood
• Normal = 8-12 mmHg
• Can be inc due to vent-perfusion mismatch, inc shunting, diffusion impairment

Question 6

Minute Ventilation

Alveolar Ventilation

Answer 6

• Minute Ventilation (VE*) = VT x RR

- Alveolar Ventilation = Minute Vent - Dead Space Vent = VE* - VD* = VA x RR

Question 7

What 2 Factors Determine PACO2?

Answer 7

1- Rate of CO2 production by tissues - V.CO2

2- Rate at which CO2 is removed from alveoli - or alveolar vent

• SO… PACO2 = K (rate of CO2 prod)/(alveolar vent rate) = K (rate of CO2 prod)/ (VE* - VD*)

Question 8

What does an increased PaCO2 tell you about your patient?

Answer 8

• PACO2 (alveoli) is essentially equal to PaCO2 (arterial)
• So patients can have inc PaCO2 it could be due to inc dead space vent, dec overall minute vent or inc CO2 production by tissues

Question 9

How does alveolar ventilation affect PAO2?

Answer 9

• NOT DIRECT but indirectly affected by PACO2

- Due to rule of partial pressures, if PACO2 inc then PAO2 will dec and vice versa

Question 10

How is ventilation distributed?

Answer 10

• Inc from apex –> base b/c pleural fluid affected by gravity –> more fluid at bases –> less neg pleural pressure at bases
• If less pleural pressure than lower starting volume/ more compliant at bottom of lungs versus at apex where there is more negative pressure and a larger starting residual volume/starts less compliant already
• AKA regional diff in pleural pressure induced by gravity –> regional diff in alveolar ventilation

Question 11

What 3 passive factors affect pulmonary perfusion rate?

Answer 11

1- Lung Volume

- Alveolar vessels are compressed by expansion during inspiration; inc resistance
- Extra-alveolar vessels are relieved during inspiration b/c no more pleural pressure compressing them from outside; dec resistance

2- Cardiac Output

- P=QR so if more Q then must dec PVR
- Dec resistance by recruitment and distention of capillaries and other small vessels

3- Gravity
- More blood flow to bases so vascular distention at bases v. constriction at apex

Question 12

What are the alveolar pressures of O2 and CO2 at high v. low V/Q areas?

Answer 12

• High V/Q ratio = low PACO2 and high PAO2

- Low V/Q ratio = high PACO2 and low PAO2

Question 13

How does V/Q change regionally?

Answer 13

• Both perfusion and ventilation are greater at bases due to gravity BUT they increase at different rates; there is a much greater fluctuation in perfusion than in ventilation
• So… at apex/ non-dependent lung … much lower Q … High V/Q… high PO2/low PCO2
• So… at bases/ dependent lung… much higher Q … low V/Q … low PO2/highCO2

Question 14

What is V/Q Inequality and what are the 3 main results of it?

Answer 14

inc in # high and low V/Q areas (ALWAYS OCCUR TOGETHER)

1- Dec PaO2 (b/c low V/Q areas cannot be comp for)

2- Inc PaCO2 (also due to low V/Q areas - if PO2 goes down there is a reciprocal inc in PCO2)

3- Inc alveolar dead space

Question 15

What drives the inc PaCO2 in V/Q inequality?

Answer 15

Low V/Q areas

If dec in PO2 then inc in PCO2

Question 16

2 Ways to Meas V/Q Inequality

Answer 16

• 1- PA-aO2 (which tells you about low V/Q areas)
  
  • PA-aP2 = PAO2 - PaO2
• 2- VD/VT (Bohr Equation which tells you about high V/Q areas)
  
  • VD/VT = (PaCO2 - PECO2) / PaCO2
  • PECO2 is PCO2 of exhaled gas (meas w/ bag)

Question 17

Low V/Q areas (what do they cause and why?)

Answer 17

SHUNT

• leads to a lower PO2 and thus a reciprocally higher PCO2
• Leads to higher PA-aO2 b/c high V/Q areas may make up for PAO2 at alveoli but does not make up for PO2 in the mixed arterial blood (b/c O2 saturation curve is NOT linear)
• However, high V/Q areas can make up for the elevated PCO2 of low V/Q areas b/c CO2 dissociation curve is linear

Question 18

High V/Q areas (what do they cause? how does this relate to minute ventilation?)

Answer 18

ALVEOLAR DEAD SPACE

• leads to higher PO2 and lower PCO2
• This inc the minute vent needed to maintain same PaCO2
• OR this inc PaCO2 if minute vent stays the same