Pathophysiology - Sinclair

Question 1

How much Oxygen and Nitrogen is there in inspired air?

Answer 1

21% Oxygen

78% Nitrogen

Question 2

How does ambient air differ once it enters the respiratory tract and becomes inspired air?
• how does this change calculations for PO2?

Answer 2

Ambient air is Rapidly warmed and saturated with 47mmHg of H2O

PO2 inspired = (Pb - Ph2o) x Fgas

*You must subtract 47mmHg from whatever the barometic pressure is

Question 3

What is the formula for minute ventilation?

Answer 3

Ve = tidal volume x respiratory rate = Vt x f

Question 4

If you take in 500mL of fresh air in a tidal breath, what is the volume of fresh air delivered to the alveoli?
• Why must we account for this?

Answer 4

Fresh Delivered air = 500mL - anatomic dead space

•Must account for this because alveolar gas remains in the anatomic dead space and re-enters the alveoli with the next breath

Question 5

What causes Alveolar Dead Space?

Answer 5

• Alveoli that are ventilated but NOT perfused are useless in gas exchange

Question 6

What is the physiologic dead space?

Answer 6

Vphysio = Vanatom + Valv

Question 7

Differentiate Minute Ventilation and Alveolar Ventilation.

• what is Alveolar ventilation AKA?

Answer 7

Minute Ventilation:
• Vt•f

Alveolar Ventilation:
• (Vt - Vdphysio)•f

Alveolar Ventilation aka Effective Ventilation

Question 8

What percentage of the Tidal Volume is physiologic dead space?

Answer 8

25-30% of Vt = dead space

typically this is 150-180mL

Question 9

T or F: INCREASED Vdalv (alveolar dead space) is always pathologic

Answer 9

TRUE - paradigm example = pulmonary embolism

Question 10

What happens to the alveolar dead space when you excercise and why?

Answer 10

• Increased Tidal Volume

* massively INCREASED perfusion

Question 11

What does alveolar gas equation tell us?

• what is the equation?

Answer 11

Tells us the PAO2 (we normally just estimate this as PaO2, but this may not be the case in lung pathology)
• Tells you if lung disease is due to hypoxemia or not

Question 12

What is the only way we can remove CO2 from the body?

Answer 12

Ventillation

Question 13

What is the Tissue O2 consumption?

Answer 13

~250ml/min

Question 14

• ***what is the COMPLETE ALVEOLAR GAS EQ?

* **What are the typical values in this equation?

Answer 14

PAO2 = PIO2 - (PaCO2/R)

OR

PAO2 = (Pb - Ph2o)•FiO2 - (PaCO2/R)

Typically:
• Alveolar O2 = 100mmHg
• Respiratory Exchange ratio is typically = 0.8
•Arterial CO2 = 40 mmHg

Question 15

What defines Hyperventilation?

• how does this affect PAO2?

Answer 15

• Breathing in O2 in excess of metabolic needs

* Increased Breathing increases PAO2

Question 16

What is the A - a difference?
• how would you go about calculating this on the basis of obtainable values?
**What is a normal A-a difference

Answer 16

A - a difference = difference in O2 saturation in alveoli and in the blood
• Should be close to 0 if diffusion is happening efficiently

How to obtain:
• PaO2 obtained by direct measure
• Calculate PAO2 with eqn. = PAO2 = PiO2 - (PaCO2/R)

***A-a difference should usually be (Age+4)/4

Question 17

WHAT ARE THE 5 CAUSES OF HYPOXEMIA?
• break these down into categories
• what is the importance of this categorization?

Answer 17

1. NORMAL A-a DIFFERENCE => No Lung Disease
   • Decreased PiO2
   • Hypoventilation
2. WIDENED A-a DIFFERENCE => Lung Disease
   • Diffusion Limitation
   • R to L Shunt
   • Ventilation/Perfusion (V/Q) mismatch

Question 18

How does the A-a gradient change as you climb in altitude?

Answer 18

It’s unchanged because there is no lung disease, both the alveolar and arterial PO2 will decrease proportionally

Question 19

T or F: in someone who has overdosed on heroin, their arterial PO2 is a good predictor of alveolar PO2.

Answer 19

True, since there is no intrinsic lung disease PAO2 and PaO2 should be similar

Question 20

What is the least common cause of hypoxemia with increased A-a gradient?
• Can an abnormal Diffusion Capacity Test differentiate between the mechanisms of this disease?

Answer 20

Diffusion Limitation => anything that affects membrane Surface Area or Thickness

NO - abnormal diffusion capacity test will not differentiate what is causing this problem

Question 21

How far must the diffusion of O2 drop for it to become clinically relevant?

Answer 21

0. 75s is the normal time an RBC spends inside a pulmonary capillary
1. 25s is the normal time for equilibration

Therefore, diffusion must be decreased to 1/3 of normal for hypoxemia to result

CONVERSELY, if transit time increases by 3x you run the risk of hypoxemia

Question 22

Why do people with diffusion defects get out of breath when they start walking around, but are fine at rest?

Answer 22

Diffusion of Oxygen is limited (greater than 0.25 seconds needed for equilibration) THEN you get up and move and DECREASE transit time of RBCs and HYPOXEMIA results

Question 23

T or F: a DIFFUSION DEFECT can be corrected with 100% O2.

Answer 23

True

Question 24

Differentiate a Pulmonary and Cardiac Shunt.

• how does supplemental O2 affect patients with these problems?

Answer 24

Pulmonary:
• Perfused areas are NOT VENTILATED

Cardiac:
• ASD/VSD allows mixing of Right sided oxygenated blood with Left side deoxygenated blood

SUPPLEMENTAL O2 has NO EFFECT IN PATIENTS with this issue aka the A-a gradient will STILL BE HUGE

Question 25

If someone is put on 100% O2, how should their PaO2 change?

• how will this differ in a patient with shunting?

Answer 25

Normal:
• Low PaO2 ==> PaO2 ~ 600 mmHg

Shunting:
• Low PaO2 ==> PaO2 ~ 80 mmHg

Question 26

What is the respiratory exchange ratio in a patient that is on 100% oxygen?

Answer 26

Respiratory Exchange Ratio:

1.0 when on supplemental O2

Question 27

What Pulmonary Diseases are associated with a LOW V/Q mismatch?

Answer 27

• COPD (asthma)
• Pulmonary Edema
• SHUNT = extreme case = 0

Question 28

What Pulmonary Diseases are associated with a HIGH V/Q mismatch?

Answer 28

• Pulmonary Embolism = infinity in DEAD SPACE

Question 29

WHAT IS THE MOST COMMON CAUSE OF HYPOXEMIA?

• does it correct with supplemental O2?

Answer 29

V/Q mismatch = most common cause

• YES, this corrects with O2