CPR 48-49 - Airflow, Airways, and Ventilation

Question 1

List the branches of the bronchial tree from beginning to end.

Answer 1

1. Trachea
2. Primary Bronchi
3. Secondary (lobar) Bronchi
4. Tertiary (segmental) Bronchi
5. 4th, 5th, and 6th order Segmental Bronchi (subsegmental bronchi)
6. Bronchioles
7. Terminal Bronchioles
8. Respiratory Bronchioles
9. Alveolar Ducts
10. Alveoli

Question 2

What are the P_A, P_AW, P_IP, P_TM, and P_TA values before respiration, at the beginning and end of respiration, and during forced expiration?

Answer 2

Question 3

Explain the concept of equal pressure points (EPP) and why it is important.

Answer 3

During forced expiration, the pressure is not uniform throughout the airway. It is greatest in the alveoli and steadily lessens down the airway towards the mouth. However, P_Ip is uniform which means that at a certain point along the airway there will be an EPP and the P_TA will be zero (refer to image). At this point and beyond, if there is no cartilage supporting the airway it will collapse. In normal lungs, the EPP occurs in cartilaginous airways. In lungs with high compliance (low elasticity) or an obstruction the EPP occurs in the more distal non-cartilaginous airways and they collapse significantly.

Question 4

How does the length of an airway affect the flow through that airway? Explain

Answer 4

F α 1/L which means that if you double the length (L) of an airway you halve the flow (F) through that airway. This is because you double the resistance to flow when you double the length.

Question 5

How does the radius of an airway affect the flow through that airway? Explain

Answer 5

F α r⁴ which means that if you double the radius (r) of an airway than you increase the flow (F) by (2r)⁴. This is because, by increasing the radius, you are drastically dropping the resistance.

Question 6

Give the equations for laminar flow velocity and resistance.

Answer 6

Question 7

Is airflow into the lungs laminar or turbulent?

Answer 7

It is turbulent in the trachea and laminar in the terminal bronchioles. Airflow is intermediate in the rest of the airways.

Question 8

How does the type of flow pattern (laminar/turbulent) relate to the flow and the pressure gradient?

Answer 8

In laminar conditions, flow is proportionate to the pressure gradient (ΔP). In turbulent conditions, flow is proportionate to the √ΔP. This means that, for turbulent flow, a higher pressure gradient has to be generated to maintain that same flow than laminar flow.

Question 9

Where is airway resistance highest and lowest? Why?

Answer 9

It is highest at the segmental bronchi and lowest at the alveolar ducts/alveoli. Even though the radius of the alveolar ducts is the smallest the cross-sectional area of all the ducts combined is the largest. Refer to image.

Question 10

Why is it important that the conducting portions of the respiratory system moisten the air?

Answer 10

Oxygen diffuses into the fluid that coats the alveolus. If the air we inhaled was constantly dry than that fluid would eventually evaporate off the alveolus and we would develop a gas exchange problem.

Question 11

T/F - the ppO2 in the alveolus is equal to the ppO2 in the atmosphere

Answer 11

False

As the air moistens the ppO2 falls

Question 12

What do “V_E” and “V_T” stand for and what are they normally?

Answer 12

V_E is minute ventilation which is the volume of air move in/out of the lungs per unit time. It is calculated by multiplying the tidal volume, V_T, by the breathing frequency, f.

V_T is normally ~500mL/breath

f is normally 12 breaths/min

V_E, therefore, is normally 6 L/min

Question 13

How is physiological dead space different from anatomical dead space?

Answer 13

Anatomic dead space refers to the conducting regions of the respiratory system. Physiological dead space refers to anatomic dead space and the alveolar spaces that have poor gas exchange.

Question 14

Why is it important to distinguish minute ventilation from alveolar minute ventilation? What is the equation for alveolar minute ventilation?

Answer 14

Minute ventilation consists of alveolar (V_A) and dead space ventilation (V_D). No gas exchange occurs within the dead space so the V_D should be ignored when considering gas exchange ventilation.

V_A = (V_T x f) - (V_D x f)

Question 15

How is anatomical dead space calculated? What is it in a healthy person? What is the amount of wasted ventilation?

Answer 15

Fowler’s Method

A patient is asked to inspire pure O2 and breathe the gas out. The N2 concentration is then recorded. Only gas leaving the respiratory regions will have mixed with the N2 from the FRC of the previous breath. Based on the concentration of N2 expired the volume of respiratory space in the lungs can be measured.

Normal anatomic dead space is 150mL which means there is about 1.8 L/min of wasted ventilation.

Question 16

How is alveolar ventilation measured?

Answer 16

Atmospheric CO2 is essentially zero. Therefore, all expired CO2 must come from a functional alveolar unit. Measuring expired CO2 and comparing it to total expired volume allows the calculation of alveolar ventilation.

C₁V₁ = C₂V₂ therefore V_TF_E = V_AF_A

F_E and F_A are fractional concentrations of CO2 expired and in the alveolus

F_A is equal to arterial ppCO2 (P_aCO₂)

Question 17

How is the partial pressure of CO2 in an alveolus determined?

Answer 17

It is assumed to be equal to the partial pressure of CO2 in the arteries which can be measured via a blood gas test

Question 18

Derive Bohr’s equation for calculating the V_D/V_T ratio using the CO2 expiration test.

Answer 18

Question 19

Is Fowler’s method or Bohr’s method for measuring dead space more accurate? Why?

Answer 19

Bohr’s method is better because Fowler’s method only measures anatomic dead space while Bohr’s method measures physiological dead space.

Question 20

What are the ppO₂ and ppCO₂ in the atmosphere, alveolus, pulmonary vein, and pulmonary artery?

Answer 20

Atmosphere - ppO₂ = 150 mmHg, ppCO₂ = 0 mmHg

Alveolus - ppO₂ = 100 mmHg, ppCO₂ = 40 mmHg

Pulmonary Artery - ppO₂ = 40 mmHg, ppCO₂ = 45 mmHg

Pulmonary Vein - ppO₂ - 100 mmHg, ppCO₂ = 40 mmHg

Question 21

What is the alveolar ventilation equation?

Answer 21

Question 22

What are the primary factors that determine P_AO₂?

Answer 22

• The ppO₂ of the inspired air (ie - altitude or supplemental O₂)
• Metabolic activity - high activity uses O₂ and pushes P_AO₂ down
• Ventilation - increasing ventilation keeps P_AO₂ elevated

Question 23

What is the best indicator of breath frequency and why?

Answer 23

P_ACO₂ because it consistently raises and lowers with a decrease or increase in breath frequency

Question 24

What is the alveolar gas equation?

Answer 24

Question 25

Describe and explain the regional ventilation differences of the lung.

Answer 25

Due to gravity, the weight of the lungs generates more negative P_IP at the apex of the lung. Because of this the alveoli at the top of the lung are more inflated before inspiration begins. Therefore, more air ventialation occurs at the alveoli at the bottom of the lung during inspiration and expiration.

Question 26

What is a lung’s closing capacity and how is it determined?

Answer 26

It is the volume in the lungs at which its smallest airways collapse at the end of a long expiration. The Fowler’s test can be used to determine this. Because the alveoli at the apex of the lung are the last to close and the most poorly ventilated there will be a sharp increase in N₂ expiration when they finally close. Where that sharp N₂ increase occurs on the Fowler’s test graph is where the closing capcity of the lung is

Question 27

Define the following terms:

Eupnea

Hypopnea

Hyperpnea

Tachypnea

Dyspnea

Apnea

Answer 27

• Eupnea - normal ventilation
• Hypopnea - decreased ventilation in response to lowered metabolic CO₂ production
• Hyperpnea - increased ventilation in response to increased metabolic CO₂ production
• Tachypnea - increased frequency of breathing (ventilation may or may not change)
• Dyspnea - shortness of breath, labored breathing
• Apnea - temporary cessation of breathing