Gas Transport Part 1

Question 1

PiO2 Equation

Partial pressure of inspired oxygen

Answer 1

PiO2= (PB-PH2O) x FiO2

PB= barometric pressure
PH2O= water vapor pressure
FiO2= fraction inspired oxygen

Question 2

T or F: For a gas, it’s partial pressure is directly proportional to it’s concentration.

Answer 2

True

Question 3

At sea level with room air, the PiO2 is ___.

Answer 3

PiO2= (760 mmHg-47 mmHg) x 0.21
PiO2= 150 mmHg

Question 4

Alveolar Gas Equation

Answer 4

PAO2= PiO2 – PaCO2/RQ

PAO2= alveolar partial pressure oxygen
PiO2= partial pressure inspired oxygen
PaCO2= arterial partial pressure carbon dioxide
(measured from ABG)
RQ= respiratory quotient = 0.8
(this is the number of CO2 molecules produced depending on diet; it’s a given value)

Question 5

At sea level with room air, the PAO2 is ___.

Answer 5

PAO2=150 mmHg- 40mmHg/0.8
PAO2= 150 mmHg- 50 mmHg
PAO2= 100 mmHg

Question 6

Why do we subtract water vapor pressure (PH2O) from barometric pressure (PB) when calculating partial pressure of inspired oxygen (PiO2)?

Answer 6

With every breath, inspired gas is humidified at 37 degrees Celsius in the upper airway, adding water vapor

Question 7

Why is alveolar partial pressure of oxygen (PAO2) less than the partial pressure of inspired oxygen (PiO2)?

Answer 7

In alveoli, inspired gases are mixed with residual alveolar gas from previous breaths, and oxygen is taken up into the blood and carbon dioxide is added to the alveoli

Question 8

At sea level, room air is ___ oxygen (FiO2).

Answer 8

21%

Question 9

Diffusion of carbon dioxide across the alveolar-capillary membrane is ___ rapid than that of oxygen diffusion.

Answer 9

20x more

Question 10

At higher elevations, PiO2 and PAO2 will ___.

Answer 10

both decrease because the barometric pressure (PB) is less

Question 11

The Alveolar-arterial oxygen gradient (A-a O2) is normally ___, but progressively increases with age up to ___.

Answer 11

15 mmHg
20-30 mmHg

A quick way to estimate:
Age/3

Question 12

At sea level with room air, the normal range for PaO2 is ___.

Answer 12

60-100 mmHg

A quick way to estimate:
PaO2= 102- (age/3) where age/3 represents the Alveolar-arterial oxygen gradient

Question 13

At FiO2 of 1.0, the PaO2 increases to ___.

Answer 13

550 mmHg

This is useful to calculate PaO2 at any FiO2. For example, at FiO2 of 0.5 the PaO2= 550 mmHg x 0.5= 275 mmHg

Question 14

Define Hypoxemia

Answer 14

PaO2 < 60 mmHg

desaturation (<90%) occurs quickly with PaO2 below 60 mmHg

Question 15

The most common mechanism for hypoxemia is ___.

Answer 15

An increased Alveolar-arterial oxygen (A-a O2) gradient ( > 15 mmHg)

Question 16

List the 4 causes of an increased Alveolar-arterial oxygen (A-a O2) gradient

Answer 16

1. )R-L shunting: blood is not participating in gas exchange
2. ) V/Q Mismatch
3. ) Diffusion Defect: impaired gas exchange across membrane
4. ) Mixed Venous Tension: increased CO, increased O2 consumption, low Hgb concentration

Question 17

List the 5 mechanisms of hypoxia

Answer 17

Normal A-a oxygen gradient:

1. ) Low inspired Oxygen
2. ) Hypoventilation

Increased A-a oxygen gradient:

3. ) V/Q mismatch
4. ) Shunt
5. ) Diffusion Abnormality

Question 18

For hypoxia in the lungs, the pulmonary response is ___.

Answer 18

Hypoxic pulmonary vasoconstriction-vasoconstriction of arteries going to poorly oxygenated alveoli in order to shift blood towards well oxygenated alveoli. Works well when hypoxia is focal, causes problems when hypoxia is global.

Question 19

PEEP improves oxygenation and prevents atelectasis by ___.

Answer 19

expanding alveoli during expiration allowing for better gas exchange

Question 20

We normally have ___ cmH2O of PEEP from the closing of the epiglottis at the end of expiration.

Answer 20

5

Question 21

Minute Ventilation (MV)=

Answer 21

RR x VT

RR= respiratory rate
VT= tidal volume

Normally 5 L/min at rest

Question 22

What are the 3 distribution areas of ventilation for tidal volume?

Answer 22

1. ) Physiologic Dead Space: alveoli not participating in gas exchange (ventilated but not perfused)
2. ) Anatomic Dead Space: the conducting branches of the tracheobronchial tree not participating in gas exchange
3. ) Alveolar Ventilation: alveoli participating in gas exchange (ventilated and perfused)

Question 23

Alveolar Minute Ventilation (MVA)=

Answer 23

VA x RR

VA= VT-VD

VA=alveolar tidal volume
RR= respiratory rate
VT=tidal volume
VD= dead space volume

Alveolar minute ventilation is the ONLY ventilation that will remove carbon dioxide from the body (not total minute ventilation)

Question 24

T or F: PACO2-ETCO2 gradient is normally less than 5 mmHg and represents dilution of alveolar gas with CO2-free gas from non-perfused alveoli.

Answer 24

True