Gas Exchange

Question 1

P_gas

Answer 1

Partial pressure of a gas.

• The contributing pressure of one gas in a mixture of gases
• P_gas = % gas × pressure of total gas mixture

Question 2

Room Air P_O2

Answer 2

P_O2 = 21% × (760 mmHg - 47 mmHg) ⇒ 150 mmHg

(47 mmHg accounts for water vapor and is excluded)

Question 3

P_AO₂

Answer 3

P_O2 in the alveolus

Question 4

P_aCO₂

Answer 4

P_CO2 in the artery

Question 5

⩒_gas

Answer 5

Minute production or consumption of a gas.

⩒_O2 ⇒ oxygen consumption per minute

⩒_CO2 ⇒ carbon dioxide production per minute

Question 6

R

Answer 6

“Respiratory exchange ratio”

• Ratio of CO₂ exhaled by the lungs divided by O₂ taken up by the lungs
• R = ⩒_CO2/⩒_O2

Question 7

RQ

Answer 7

“Respiratory quotient”

• Ratio of CO₂ produced by metabolism divided by O₂ consumed by cellular metabolism
• Normal = 0.8

Question 8

Steady State

Answer 8

R = RQ

(Normal about 0.8)

Question 9

V_T

Answer 9

Tidal volume

Question 10

⩒_E

Answer 10

Minute ventilation = V_T × RR

Question 11

V_D

Answer 11

Dead space volume

Question 12

V_A

Answer 12

Alveolar volume

Question 13

⩒_A

Answer 13

Minute alveolar ventilation = V_A × RR

Question 14

Hemoglobin

Oxygen Saturation

Answer 14

Amount of oxygen bound to Hb in the blood, expressed as a percentage of the maxiumal binding capacity.

%Sat = [oxygenated Hb] / ( [oxygenated Hb] + [deoxygenated Hb] )

Question 15

Oxygen Content

Answer 15

Amount of O₂ bound to Hb + Amount of O₂ dissolved in plasma

O₂ content = (1.36 x [Hb] x %Sat ) + (0.003 x P_aO₂)

Question 16

O₂ Content

Effects of Hb

Answer 16

As [Hb] increases, O₂ content at a given P_O2 increases.

Question 17

Alveolar CO₂

Determinants

Answer 17

Question 18

P_ACO2

Effects of Ventilation

Answer 18

Hypoventilation ⇒ ↑ P_ACO2

Hyperventilation ⇒ ↓ P_ACO2

↑ P_ACO2 with hypoventilation >>> ↓ P_ACO2 associated with hyperventilation

Question 19

CO₂ Retention

Causes

Answer 19

1. Hypoventilation
   
   • ↓ alveolar ventilation with ↔︎ CO₂ production
2. Ventilation/Perfusion (V/Q) Mismatch
   
   • ↓ in CO₂ in high V/Q region <<< ↑ in CO₂ in low V/Q region
   • High V, low Q region ⇒ well-ventilated blood <<< Low V, high Q region ⇒ poorly ventilated blood

Question 20

Alveolar O₂

Determinants

Answer 20

Given by the alveolar air equation

Question 21

Alveolar-Arterial Gradient

(A-a gradient)

Answer 21

A-a gradient = P_AO2 - P_aO2

P_AO2 given by alveolar air equation

P_aO2 measured by sampling arterial blood gas

Normal A-a grandient on RA is 5-10 mmHg.

Question 22

Arterial Blood Gas

Procedure

Answer 22

1. Confirm patency of ulnar and radial arteries with Allen test
2. Find radial artery pulse
3. Insert a small guage needle attached to heparinized syringe @ ~ 90° angle
4. Expel all air from syringe
5. Send sample on ice

Question 23

Hypoxemia

Causes

Answer 23

1. Hypoventilation
   
   • ↓ Alveolar ventilation ⇒ ↑ CO₂ ⇒ ↓O₂
2. V/Q mismatch
   
   • Well-oxygenated blood from high V/low Q <<< Poorly-oxygenated blood from low V/highQ
   • Blood from high V area already fully saturated so little additional oxygen content from ↑ P_aO2
3. Shunt
   
   • R→L shunt adds blood that does not participate in gas exchange
4. Diffusion impairment
   
   • Loss or thickening of diffusion barrier affects O₂ > CO₂
5. Decreased P_IO2 (altitude)

Question 24

Hypoventilation

Causes

Answer 24

1. Impaired respiratory drive
   
   • CNS issue ⇒ tumor, stroke, hemorrhage
   • CNS depressant drugs
   • Obesity hypoventilation syndrome
2. Mechanical problems
   
   • Kyphoscoliosis
   • Diaphragm paralysis
3. Neurological diseases
   
   • Poliomyelitis ⇒ loss of anterior horn cell function
   • ALS ⇒ loss of UMN function
   • Spinal cord compression/trauma
   • Phrenic nerve damage
4. NMJ diseases
   
   • Myasthenia gravis
   • Botulism
   • Toxins ⇒ cholinestererase inhibitors, organophosphates

Question 25

Hypoventilation

Diagnosis

Answer 25

Hypoxia always associated with ↑ P_CO2

Underlying lung normal ⇒ normal A-a gradient

Question 26

V/Q Mismatch

Causes

Answer 26

1. Chronic obstructive pulmonary diseases
   
   • Bronchitis, emphysema
   • CF
2. Asthma
3. PE
4. PNA

Question 27

V/Q Mismatch

Diagnosis

Answer 27

• Often associated with ↑ P_CO2
• Always has ↑ A-a gradient
• Hypoxia corrects with O₂
• Often dx by diagnosis underlying disease
  
  • PFT
  • CXR
• Research ⇒ inert gas method to measure V/Q

Question 28

R → L Shunt

Causes

Answer 28

1. Intracardiac shunts
   
   • PFO or ASD
   • VSD
2. Pulmonary AVM
3. Diseases that fill alveoli with fluid
   
   • CHF
   • PNA
   • ARDS

Question 29

R → L Shunt

Diagnosis

Answer 29

• Always has ↑ A-a gradient
• Hypoxia does not correct with O₂
• ECHO w/ bubble study
• Cardiac cath
• Perfusion lung scan w/ simultaneous scan of lung and systemic organ
• Shunt equation (don’t memorize)

Question 30

Diffusion Impairment

Causes

Answer 30

1. Loss of surface area for gas exchange
   
   • Emphysema
   • Pulmonary fibrosis
   • Lung resection
2. Thickening of alveolar/capillary membrane
   
   • Pulmonary fibrosis
   • Pulmonary edema

Question 31

Diffusion Impairment

Diagnosis

Answer 31

Measure diffusion capacity for carbon monoxide

(Diffusion limited gas)

• Inhale known [CO]
• DL_CO is proportional to difference between inspired and expired CO
• DL_CO is proportional to D_O2

Question 32

Hypoxia

Evaluation

Answer 32