Pulmonary 2: Lung Mechanics A

Question 1

Describe how intra-alveolar pressure compares to atmospheric pressure inspiration and expiration.

How would they look different on a x-ray?

Answer 1

Air flows down a pressure gradient.

During inspiration, intra-alveolar pressure is below atmospheric pressure. (volume goes up, pressure goes down)

During expiration, intra-alveolar pressure is above atmospheric pressure
(volume goes down, pressure goes up)

Question 2

Using Boyle’s law, describe how pressure changes at a volume of 1/2, 1, and 2 at a fixed temperature.

Answer 2

Boyle’s Law

At a fixed temperature, the volume of a gas is inversely proportional to the pressure exerted by the gas.

Pressure is proportional to 1/V

at Volume =1/2, pressure =2,

at volume =1, pressure=1

at volume =2, pressure =1/2

Question 3

What are the accessory muscles of inspiration and the major muscles of inspiration and expiration?

Answer 3

accessory muscles of inspiration: sternocleidomastoid, scalenus

muscles of active expiration: internal intercostal muscles,

muscles of inspiration: diaphragm, external intercostal muscles

Question 4

Describe the changes before inspiration and during inspiration.

Answer 4

Slide 8.

before- external intercostal muscles are relaxed, diaphragm is relaxed

during- contraction of external intercostal muscles causes bucket-handle-like elevation of ribs which increases side-to-side dimension of thoracic cavity (moves the rib cage out and up)

elevation of ribs causes sternum to move upward and outward, which increases front-to-back dimension of thoracic cavity

lowering of diaphragm on contraction increases vertical dimension of thoracic cavity.

Question 5

Describe the major inspiratory muscles.

What innervates them?
What does stimulation lead to?

Answer 5

diaphragm is stimulated by the phrenic nerve

• accounts for approx. 75% of increase in thorax cavity volume: major inspiratory muscle
• stimulation causes muscle to flatten and move downward; enlarges the cavity in vertical direction

external intercostal muscles are stimulated by the intercostal nerves
-stimulation causes the ribs to move up and outward; enlarges the cavity in both lateral and anteroposterior direction

Question 6

When are the accessory muscles of inspiration used?

Answer 6

used for forced inspiration/ deeper inspiration.

Contraction of the neck muscles (Scalenus and Sternocleiodomastoid) raise the sternum and elevate the first two ribs

Enlarges the upper portion of the thorax
Only used during forceful inspiration, for example during exercise

Muscles of inspiration can generate a maximal pressure of 80-90 mmHg.

Question 7

Describe the process of expiration.

What happens to the thorax when lungs deflate?

Answer 7

Inspiratory muscles relax

Diaphragm relaxation allows the muscle to assume its natural dome shape

Intercostal muscles relax, causing the rib cage to fall down due to gravity

Lungs deflate and thorax expands due to natural recoil

Mostly passive

Question 8

When might you use forced, active expiration? Describe muscles used.

Answer 8

Forced, active expiration requires contraction of expiratory muscles

Only used during active expiration, e.g. exercise
abdominal muscles contraction increases abdominal pressure and pushes the diaphragm upward

Internal intercostal muscles contraction flatten the rib cage by pulling the ribs downward and inward

Question 9

Describe active/passive expiration.

What will contraction of internal intercostal muscles lead to?

Answer 9

Active: contraction of the internal intercostal muscles flattens ribs and sternum, further reducing side-to-side and front-to-back dimensions of thoracic cavity

contraction of abdominal muscles causes diaphragm to be pushed upward, further reducing vertical dimension of thoracic cavity.

passive:
return of diaphragm, ribs, and sternum to resting position on relaxation of inspiratory muscles restores thoracic cavity to preinspiratory size

Question 10

During what phase will internal intercostal muscles contract, and flatten the rib cage by pulling the ribs downward and inward?

Answer 10

forced/active expiration

Question 11

What is a spirometer used for?

Answer 11

to measure lung volumes

slide 13.

Question 12

Graph TLC, FRC, IRV, ERV, FVC, RV, IC, VT and VC on a spirometer.

Answer 12

ERV-expiratory reserve volume; FRC- functional residual capacity; FVC- forced vital capacity; IC- inspiratory capacity; IRV-inspiratory reserve volume; RV-residual volume; TLC-total lung capacity; VC- vital capacity; VT-tital volume

Slides 14-16

Question 13

The air in the lungs is partioned into four volumes (L) and four capacities (L). Describe the four primary non-overlapping volumes:

Tidal volume
Inspiratory Reserve volume
Expiratory Reserve volume
Residual volume

(Capacities are always comprised of two or more lung volumes.)

Answer 13

The lung has four primary non-overlapping volumes:

Tidal volume (Vt=500mL)
Inspiratory Reserve volume (IRV=3000mL)
Expiratory Reserve Volume (ERV=1200mL)
Residual volume (RV=1200mL)

Tidal volume is the change in volume that occurs with cyclic breathing.

Inspiratory and Expiratory Reserve volumes (IRV/ERV) are the volumes that can be in/exhaled in addition to the tidal volume, e.g. during forced inspiration/expiration.

Residual volume (RV) is the volume that remains in the lung even after forced expiration; RV cannot be measured with spirometry.

Question 14

The lung has four secondary overlapping capacities. Show how to calculate each value and explain what each means.

Inspiratory Capacity
Functional residual capacity
vital capacity
total lung capacity

Answer 14

Inspiratory capacity (IC=IRV +Vt=3500mL)

Functional Residual Capacity (FRC=ERV+RV=2400mL)

Vital Capacity (VC=IRV + Vt+ERV=4600mL)

Total lung capacity (TLC=IRV +Vt+ERV +RV=5800mL)

Vital capacity is the maximal amount of air that can be moved from deep expiration to deep inspiration.

Functional residual capacity (FRC) the volume of air in the lungs when all respiratory muscles are relaxed. FRC is best understood as the balance position of the lung-chest wall system. The lung pulls inward and the chest wall springs outward, both at equal force. FRC cannot be measured with a spirometer because RV is a part of FRC.

Inspiratory capacity is the volume that can be inhaled after all respiratory muscles are relaxed (starting at FRC)

Total lung capacity (TLC) is the total volume of air held by the lung. TLC includes both alveoloar volume and dead space volume and is scaled to the size of the person. TLC cannot be measured with a spirometer because RV is a part of TLC.

Question 15

Which values can you not measure with spirometer? How else can you measure them?

Answer 15

FRC cannot be measured with a spirometer because RV is a part of FRC.

TLC cannot be measured with a spirometer because RV is a part of TLC.

Residual volume (RV) is the volume that remains in the lung even after forced expiration; RV cannot be measured with spirometry.

Can measure by 1. helium dilution, 2. body plethysmograph

Question 16

Why might helium dilution underestimate FRC?

Answer 16

because of mal-distribution of poorly ventilated lung regions in diseased lungs.

Question 17

Describe the measurement of FRC by helium dilution technique.

What would it indicate about FRC if helium was diluted?

Answer 17

requires the measurement of helium (in O2 concentration=C1) within a spirometer system of known volume (V1).

At the start of the procedure the patient relaxes to FRC volume (V2), but this lung air contains no helium. Then a valve is opened connecting the patient to the spirometer and the patient inhales and exhales to evenly distribute the helium throughout the lungs and spirometer. The test stops with the patient back at the position of FRC.

C1 x V1 = C2 x (V1+FRC) –> FRC = V1 x (C1-C2)/C2.

The larger the FRC of the patient, the more the initial helium concentration gets diluted. This is known as the dilution principle.

Question 18

What determines the volume of air in the lungs? Define it.

Answer 18

Lung compliance (CL)

• measure of the elastic properties of the lung
• defines as change in lung volume per 1 cm H2O change in the distending pressure (mL/cm H2O)

CL= change in volume/change in pressure

Normal lung: CL is approx 0.2 L/cm H2O.

CL changes with volume (is less distensible at high volumes)

Question 19

How does compliance change at volumes near FRC? What about at volumes way above FRC?

How does FRC change when chest muscles are weak? In presence of airway obstruction?

Answer 19

at lung volumes near FRC, compliance is maximized. Here normal lung compliance is 0.2 L/cm H2O. But at volumes way above FRC, the compliance is lower.

At volumes way above FRC the compliance is lower.

When the chest wall muscles are weak, FRC decreases (lung elastic recoil > chest wall
muscle force). In the presence of airway obstruction,
FRC increases because of premature airway closure,
which traps air in the lung.

Question 20

Graph a pressure volume curve. (Translung pressure against lung volume %TLC). Label TLC, FRC, RV, MV.

Show where inflation/deflation is on the curve.

Answer 20

Slide 18.

By convention lung compliance is the change in pressure in going from FRC to FRC + 1L.

At higher lung volumes, compliance goes down (slope will decrease).

Question 21

Compare saline inflation to air inflation on a graph.

Answer 21

Slide 19.

CL saline is greater than CL air.

Surface tension contributes significantly to the elastic properties of the lung.

Question 22

Compare compliance to specific compliance in three situations all with 5cmH2O pressure.

Situation 1: IL
Situation 2: 0.5L
Situation 3: 0.1L

Answer 22

compliance = lung volume/pressure

specific compliance= lung compliance/lung volume

Situation 1:
compliance: 1L/5cmH2O=0.2
specific compliance= 0.2/1L=0.2

Situation 2:
compliance: 0.5L/5cmH2O=0.1
specific compliance: 0.1/0.5L=0.2

Situation 3:
compliance: 0.1L/5cmH2O=0.02
specific compliance: 0.02/0.1L=0.2

(Comparing the lung of a child with the lung of an adult, the smaller lung will reach 5cm H2O pressure with only 0.1L inflation, but the larger lung will require 1L inflation to reach the same pressure. Compliances are different but both tissues totally normal. Specific (relative) compliance adjusts CL to lung volume.

Question 23

A lung has 5-cm H2O pressure change which results in 1L change in volume. What will happen if half lung is removed to compliance and specific compliance? What about if the lung is reduced by 90%

Answer 23

If half the lung is removed then compliance will decrease, but when corrected for volume of the lung, there is no change (specific compliance).

Even when the lung is reduced by 90% the specific compliance is unchanged.

Question 24

What forces must inflation overcome?

Answer 24

1) viscoelastic properties of the lung parenchyma by stretching elastic and collagen fiber matrices
2) surface tension forces set up between the air/water interface on the alveolar epithelium

(The presence of alveolar surfactant (synthesized and secreted from type II pneumocytes) decreases these surface tension forces, but not to zero.)

Question 25

Graph how lung compliance changes with empheysema and fibrosis.

(Graph translung pressure against vital capacity)

Answer 25

Slide 21.

Question 26

Describe how and why compliance changes in emphysema.

How does FRC change? How does intrapleural pressure change?

Answer 26

compliance is higher than normal because the progression of this disease (mainly attributed to first hand or second hand smoke) the elastin/collagen matrix of the alveolar septa get “eaten” away. Alveoli greatly expand in diameter but decrease in surface area. Consequently, the FRC increases (due to less lung recoil) and the intrapleural pressure becomes less negative (less pull of the lung on the chest wall)

Question 27

Describe how and why compliance changes in fibrosis.

How does FRC change? How does intrapleural pressure change?

Answer 27

Compliance is lower than normal because during the progression of this disease (wide-spread lung scarring due to chronic inflammatory processes, infections, environmental agents etc) the lung recruits fibrotic tissue that not only makes the lung stiffer (less compliant), but also thickens the diffusion pathway for gasses across the alveolocapillary membrane. Accordingly, the FRC decreases (due to more lung recoil) and the intrapleural pressure becomes more negative (more pull of the lung on the chest wall)

Question 28

What determines the volume of air in the lungs?

Answer 28

lung-chest wall interactions

the serous fluid within the intra-pleural space has a total volume of only 10 to 20 mL distributed over the entire surface of the lung (very thin!)

Lung and chest wall move together as a unit = same volume changes.

(balance between elastic properties of the lung and properties of the muscles of the chest wall)

Elastic recoil of the lung is high

Question 29

Describe what would happen to volume of air in lungs without external forces.

Answer 29

without external forces, lung volume would be almost airless (10 % TLC) and chest would expand (spring)

Question 30

Graph the chest wall compliance and lung compliance and then both (respiratory system) compliance. Show expanding/collapsing, FRC, RV and TLC on the graph.

(Pressure against vital capacity).

Answer 30

Slide 24 (and 25)

The pulmonary system has 2 mechanical parts, the lung and the chest wall. Each part has its own unique individual compliance. However, the lung and chest wall function together as a unit as coupled through the serous fluid and intrapleural pressure. So the coupled system has its own compliance, the total compliance (C total).

Question 31

The relaxation pressure-volume curve (single compliance curve slide 24), has 2 components; the lung compliance curve and the chest wall compliance curve.

Where does equilibrium position for the lung/chest wall system reside?

Where does equilibrium for chest wall alone (no lung present) reside?

Answer 31

equilibrium position resides at FRC (35%)

chest wall alone, no lung present is above FRC (60-70% VC) at Pw =0 cmH2O.

equilibrium position for the lung alone (no chest wall present) is below FRC (0% VC-RV) at PL =0cmH2O.

(At FRC the lung pulls inward with a pressure of +5-6cm H2O) which is counterbalanced by the chest wall pulling outward with a pressure of (-5-6 cmH2O). These two pressures cancel each other, bringing the coupled system to its equilibrium volume of FRC.

Question 32

Draw a diagram of pressures that are important in ventilation.

Calculate transpulmonary(translung) pressure, transmural pressure across chest wall, and pressure across respiratory system.

Answer 32

Slide 26 and 27

atmosphere -760 mmHg
airways, lungs

intrapleural pressure -less than 760mmHg (approx. 756 mmHg)

intra-alveolar pressure - 760mmHg

Transpulmonary pressure: PL=PA-P PL = 760-756 mmHg= 4mmHg.
(transmural pressure across chest wall is the difference between plural pressure and atmospheric pressure)

Pressure across respiratory system is the difference between alveolar pressure and atmospheric pressure (=0mmHg when at rest FRC and airways are open)
P (rs)= PL+Pw
= (PA-Ppl) + (Ppl-Pb)
=PA-Pb

Question 33

What is a pneumothorax? How can it be treated?

Answer 33

Slide 29, 30

air/gas in the pleural space, leading to a separation of lung and chest wall

treated with chest tube, under water seal drain (slide 31)

Question 34

Draw pressure volume relationships during a tidal volume breath.

Inspiration/exhalation against volume, pleural pressure, flow, and alveolar pressure.

Answer 34

Slide 32. (explanation on h/o)

pressure gradients are small

intrapleural pressure is always below atmosphere

lungs are always stretched to some degree, even during expiration.

Question 35

What is alveolar pressure?

What is transpulmonary pressure?

What is transmural pressure across chest wall?

What is pressure across the respiratory system?

Answer 35

alveolar pressure= sum of pleural pressure and elastic recoil pressure. Slide 28

transpulmonary pressure is the difference between alveolar pressure and pleural pressure

Transmural pressure across chest wall is the difference between pleural pressure and atmospheric pressure.

Pressure across respiratory system is the difference between alveolar pressure and atmospheric pressure (=0 mmHg when at rest at FRC and airways are open)