Mechanics of breathing (compliance)

Question 1

What is meant by minute ventilation?

Answer 1

It is the amount of air that enters or (exits) the lung per minute

• Ve = Vt (tidal volume) * F (breathing frequency)

Question 2

What is meant by alveolar ventilation?

Answer 2

• The amount of fresh air that reaches the alveoli per minute
• Va (alveolar ventilation) = (tidal volume “Vt” - Dead space volume “Vd”) * F (frequency of breath)
• The equation can be rewritten as:

Alveolar Ventilation (L/min) = Produced CO2 * Constant / Partial pressure of CO2 in the alveoli or the artery

Question 3

What is the link between the amount of CO2 produced, alveolar CO2 partial pressure & alveolar ventilation?

Answer 3

• The higher the amount of CO2 produced the more ventilation will occur (If the rate of CO2 production doubles alveolar ventilation will double “In an attempt to maintain constant arterial and alveolar CO2 pressure, like during exercise”)
• The more you ventilate, the less the Partial pressure of CO2 in the alveoli and artery (If alveolar ventilation doubles the PaCO2 will decrease by half)
• PaCO2 = CO2 produced / alveolar ventilation

Question 4

How can we measure the partial pressure of alveolar oxygen (PAo2), knowing the (PAco2)?

Answer 4

PAo2 (Partial pressure of oxygen in the alveoli) = Partial pressure of inspired oxygen (Pio2) - Partial pressure of alveolar CO2 (PAco2) / “R” Respiratory exchange ratio (CO2 production / O2 consumption “0.8”)

Question 5

What is the relation between the PAo2 and PAco2?

Answer 5

If we increase the partial pressure of CO2 in the alveoli the partial pressure of O2 in the alveoli will decrease

• PAo2 = Pio2 - PAco2 / R

Question 6

What are the physical factors that influence pulmonary ventilation?

Answer 6

1) Lung compliance

2) Alveolar surface tension

3) Airway resistance

Question 7

What is meant by lung compliance?

Answer 7

• Compliance is a measure of how the volume changes as a result of pressure change, thus lung compliance describes the change in lung volume for a given change in pressure
• Compliance = change in volume/change in pressure

Question 8

What are the major factors that influence compliance?

Answer 8

1) Elastic recoil of the tissue

2) Surface tension of the alveoli

• The more elastic tissue, the more the tendency to snap back, the greater the elastic recoil force “but the lower the compliance”

Question 9

What is meant by hysteresis?

Answer 9

The difference in compliance in a lung between inspiration and expiration, which is caused by the surface tension (reduced by the surfactant).

• As during inspiration it takes some effort to start expanding the lungs, which is not present during expiration
• 2/3 of the work is required to overcome the surface tension while only 1/3 is required to overcome the elastic recoil

Question 10

What is the relationship between the compliance of the lung and chest wall?

Answer 10

• The lung tends to recoil while the chest wall tends to expand, both of them have a different compliance and a combined compliance
• When the compliance of both is added it is less than the compliance of individual ones
• The functional residual capacity pint is where the lung recoil force = chest expanding force
• Volume above FRC will increase the tendency of lung and chest wall collapse, volume below FRC will increase the tendency of chest wall and lung expansion

Question 11

What conditions change the compliance of the lung and chest wall?

Answer 11

1) Emphysema (increases compliance)

2) Fibrosis (decreases compliance)

3) Pulmonary vascular congestion (decreases compliance)

4) Pneumonia (decreases compliance)

5) Pleural effusion (decreases compliance)

6) Decreased surfactant (decreases compliance)

7) Obesity (decreases compliance)

8) Spine deformity (decreases compliance)

Question 12

How does emphysema change the compliance of the chest wall and lungs?

Answer 12

In emphysema lungs will have less elastic tissue, making it easier to stretch, increasing the compliance, and making the compliance graph more vertical

• In emphysema, the patient will have a higher equilibrium point, breathing at a higher FRC

Question 13

How does fibrosis change the compliance of the chest wall and lungs?

Answer 13

• Lungs will have more elastic tissue, making it harder to stretch and the compliance graph will be more horizontal
• Lower FRC breathing at a lower volume

Question 14

What is meant by the surface tension?

Answer 14

• It is the force of attraction of water molecules in the air into each other, forcing structures to come into close proximity

Question 15

What are the forces that try to keep the alveoli open?

Answer 15

1) Transmural pressure gradient

2) Pulmonary surfactant (opposes alveolar surface tension)

3) Alveolar interdependence (neighboring alveoli work together to maintain their stability and prevent collapse)

Question 16

What are the forces that promote the alveoli to collapse?

Answer 16

1) Elasticity of the stretched pulmonary connective tissue

2) Alveolar surface tension

Question 17

What will happen to the pressure in the alveoli when the radius is reduced?

Answer 17

Using Laplace law (P = 2 * Surface tension/radius) the lower the radius the higher the pressure, creating a pressure gradient between the alveoli of a small diameter and the one with a bigger diameter, shifting the air from the small alveoli to the large one which might lead to the collapse of the alveoli

• This is where the surfactant comes into play where it reduces the surface tension, and thus the pressure, it reduces the surface tension in smaller alveoli more than the bigger ones “Subhanallah”

Question 18

Which type of cell produces the surfactant?

Answer 18

It is synthesized from the pulmonary capillary via type 2 pneumocytes from lipids

Question 19

What is the function of the surfactant?

Answer 19

• It reduces the surface tension, promoting:

1) Stability of the lung (prevents the small alveoli from emptying into larger ones, thus preventing their collapse)

2) Reduces the inflation pressure and work of breathing

3) Helps to keep the lungs dry

Question 20

What are the different causes of reduced surfactant?

Answer 20

1) Neonatal respiratory distress syndrome (hyaline membrane disease)

2) Adult respiratory distress syndrome

3) Post-cardio-pulmonary bypass

4) Pulmonary embolism

5) Oxygen toxicity and other toxic agents

Question 21

What is meant by alveolar interdependence?

Answer 21

• It is one of the forces that helps keep the alveoli open
• The alveoli are closely packed and share common walls with each other, If one alveolus starts to shrink or collapse (due to surface tension or other factors), the surrounding alveoli pull on it through the shared walls.
• This pulling force from neighboring alveoli prevents the collapsing alveolus from closing completely and helps it stay open.

Question 22

What are the types of airflow that occur in the lungs?

Answer 22

1) Turbulent flow (causes airway resistance)

• In the large airways like the trachea and bronchi

2) Transitional flow

3) Laminar flow

• In the small peripheral airways

Question 22

What is meant by airway resistance and what determines it?

Answer 22

• It is the frictional resistance that must be overcome for air to move in and out
• It is primarily found in the upper airways as the diameter of the lower airway is much bigger than that of the upper airways
• Airway resistance is inversely proportional to the fourth power of radius which is reduced by:

1) Bronchoconstriction

2) Mucus buildup

3) Inflammation

4) Edema

• Increasing the airway resistance will increase the work needed for breathing

Question 22

How to calculate the airway resistance?

Answer 22

Raw (total airway resistance “cm H2O/L/sec)) = pressure difference between the mouth and alveoli (Pmouth - Palveoli “mmHg”) / airflow (ml/min)

• Raw = (Pmouth - Palveoli) / Airflow

Question 23

What are the factors that determine the airway resistance?

Answer 23

• According to Poiseuille’s law:

R = 8 * n * l / pie * r^4

• The radius is the most important determinant (radius decreases by half, resistance increases by 16 folds, while if the radius doubles resistance decreases by 1/16 of the original number)
• For ex: Decreased Paco2 (alkalosis) 🡪 as a reflex will cause an increase in resistance by bronchoconstriction 🡪 Decreased Flow 🡪 Keeps CO2 in the body

Question 24

Which airway structure has the lowest radius?

Answer 24

The medium sized-bronchi, especially during expiration where they become smaller

Question 25

What are the factors that produce bronchoconstriction?

Answer 25

• Increased resistance due to decreased radius, increasing the resistance to flow

1) Pathological factors:

• Allergy-induced spasm of the airway caused by:

1) Slow-reactive substance of anaphylaxis

2) Histamine

2) Physical blockage of the airways caused by:

1) Excess mucus
2) Edema of the wall
3) Airway collapse

3) Physiological control factors

• Neural control (parasympathetic stimulation M3 receptor)

4) Local chemical control:

• Decreased CO2 concentration

Question 26

What are the factors that produce bronchodilation?

Answer 26

• Increased radius, and decreased resistance to flow

1) Physiological control factor

• Neural control (sympathetic stimulation “minimal effect”, B2 receptor)

2) Hormonal control

• Epinephrine

3) Local chemical control

• Increased CO2 concentration

Question 27

What is the effect of the lung volume on the airway resistance?

Answer 27

Increased lung volume will decrease the airway resistance (bigger diameter), while the compline decreases and elastic work increases

• People with restrictive lung disease will try to increase their lung volume by raising their arms above their head

Question 28

What happens during forced expiration?

Answer 28

• In a forced expiration, a person deliberately and forcibly breathes out
• The only time the intrapleural pressure should be positive, compressing the airway increasing the airway resistance
• Any problem with the airway resistance it will be more difficult to exhale than inhale
• Cough is an example of airway compression caused by forced exhalation, against a closed glottis
• The lung does not collapse as the transmural pressure (difference between intrapulmonary & intrapleural pressures) is positive

Question 29

What is the relationship between the lung volume and airflow?

Answer 29

• The X-axis represents the lung volume while the Y-axis represents the flow
• At rest, the flow-volume curve is very small
• You have inspiration and expiration, you start expiring with the TLC and then reach the peak at the PEF, then you start expiring at a lower rate till you reach the RV, once the RV is reached you start inspiring reaching the PIF and then inspiring slowly till you reach the TLC
• The first part in forced expiration is effort dependant it depends on the force u put in, the last part is effort independent it depends on the elastic recoil and resistance of the airway
• You can calculate the FRC from TLC - RV

Question 30

What happens to the flow volume loops in obstructive and restrictive lung diseases?

Answer 30

1) Obstructive:

• Difficulty in expiration, where the air will remain in the lungs, hyperinflation them, shifting the loop to the right, increasing the RV, TLC, & decreasing the PEF & PIF
• FEV1/FVC <80%

2) Restrictive:

• The loop is shifted toward the right, as the lung elastic tissue is damaged not permitting much air into the lungs, reducing all measures (TLC, RV, PIF, PEF)
• FEV1/FVC >80%

Question 31

Describe the airway compression during forced expiration

Answer 31

• Transmural pressure = (Paw - Pip)
• Before Inspiration, the transairway pressure is positive 5 (0 - -5)
• During Inspiration, the Intrapleural pressure becomes more negative while the intrapulmonary pressure becomes slightly negative giving a total of +6
• At the end of inspiration the intrapleural pressure becomes -12 and intrapulmonary is equal to the atm (0) giving a total of +12
• During forced expiration the Intrapleural pressure becomes positive and the transmural pressure becomes negative forcing air out of the lung and collapsing the lungs

Question 32

What is meant by equal pressure point and when does it occur?

Answer 32

• It is when the pressure inside the airway is equal to the pressure outside the airway (pleural pressure), establishing a driving pressure in the airway, dividing the airway into downstream and upstream segments
• The pressure drive during expiration is created by the difference between the alveolar pressure and the intra-pleural pressure the upstream being from the alveoli to the EPP point and the downstream being from the EPP to the mouth
• During forced expiration airways are subjected to collapse due to compression from the EPP to the trachea
• The EPP occurs in the cartilaginous airway in healthy individuals and thus no collapse occurs, in disease the EPP moved upstream closer to the alveoli due to obstruction or loss of elastic recoil
• The lung elastic recoil influences the equal pressure point. A, In healthy lungs, elastic recoil adds 10 cm H2O pressure, to produce an alveolar pressure of 40 cm H2O at the beginning of a forced expiration. As a result, the equal pressure point (EPP) is established in the larger airways. Airway collapse is minimal in large airways because cartilage supports the airways. B, A loss of elastic recoil causes the EPP to shift downward and to be established in the small airways. In emphysematous lungs, the elastic recoil pressure is low, and little recoil pressure is added to the alveolus. As a result, the EPP is shifted downward and is established in the smaller airways. The small airways are more easily compressed because they are thin and lack cartilaginous support.

Question 33

What is the effect of elastic recoil on expiratory airflow?

Answer 33

• In healthy individuals, they have elastic recoil that will make the EPP in the large airways, and thus minimal airway collapse
• In diseased individuals they lose the elastic recoil, shifting the EPP downwards into the small airways, and airway collapse

Question 34

What is meant by positive end-expiratory pressure?

Answer 34

• It is a measure that prevents the tendency to develop regional atelectasis, preventing the alveolar pressure from returning to zero at the end of expiration
• PEEP is given for patients with emphysema to raise the intra-alveolar pressure which prevents lung collapse

Question 35

What happens in forced exhalation in emphysema, compared to a normal individual?

Answer 35

• In a person with normal lungs, the forced expiration makes the pressures in the lungs “alveolar pressure” (+35) and airways (+25) very positive, Contraction of the expiratory muscle also raises intrapleural pressure to positive values (+20)
• In emphysema lung compliance increases due to diminished elastic tissue, reducing the alveolar and airway pressures, the alveoli will still have a greater pressure than the intrapleural pressure, however the large airways will collapse increasing the airway resistance
• pursing the lips will create a high resistance at the mouth raising the airway pressure preventing the collapse of the large airways, which might be done by patients with emphysema