Pulmonary 2: Lung Mechanics A Flashcards
Describe how intra-alveolar pressure compares to atmospheric pressure inspiration and expiration.
How would they look different on a x-ray?
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)
Using Boyle’s law, describe how pressure changes at a volume of 1/2, 1, and 2 at a fixed temperature.
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
What are the accessory muscles of inspiration and the major muscles of inspiration and expiration?
accessory muscles of inspiration: sternocleidomastoid, scalenus
muscles of active expiration: internal intercostal muscles,
muscles of inspiration: diaphragm, external intercostal muscles
Describe the changes before inspiration and during inspiration.
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.
Describe the major inspiratory muscles.
What innervates them?
What does stimulation lead to?
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
When are the accessory muscles of inspiration used?
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.
Describe the process of expiration.
What happens to the thorax when lungs deflate?
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
When might you use forced, active expiration? Describe muscles used.
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
Describe active/passive expiration.
What will contraction of internal intercostal muscles lead to?
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
During what phase will internal intercostal muscles contract, and flatten the rib cage by pulling the ribs downward and inward?
forced/active expiration
What is a spirometer used for?
to measure lung volumes
slide 13.
Graph TLC, FRC, IRV, ERV, FVC, RV, IC, VT and VC on a spirometer.
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
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.)
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.
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
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.
Which values can you not measure with spirometer? How else can you measure them?
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
Why might helium dilution underestimate FRC?
because of mal-distribution of poorly ventilated lung regions in diseased lungs.
Describe the measurement of FRC by helium dilution technique.
What would it indicate about FRC if helium was diluted?
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.
What determines the volume of air in the lungs? Define it.
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)
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?
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.
Graph a pressure volume curve. (Translung pressure against lung volume %TLC). Label TLC, FRC, RV, MV.
Show where inflation/deflation is on the curve.
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).
Compare saline inflation to air inflation on a graph.
Slide 19.
CL saline is greater than CL air.
Surface tension contributes significantly to the elastic properties of the lung.
Compare compliance to specific compliance in three situations all with 5cmH2O pressure.
Situation 1: IL
Situation 2: 0.5L
Situation 3: 0.1L
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.
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%
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.
What forces must inflation overcome?
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.)
