chapter 11 - respiratory mechanics and lung function

respiratory mechanics

breathing depends on balance of pressures, volumes, and forces between lungs and chest wall

lung function

how well respiratory mechanics work (pressure, volume, and flow)

lungs vs chest wall

lungs naturally want to recoil inward

chest wall naturally wants to spring outward

lungs and chest during inspiration

muscles lift the chest wall and lungs expand

lungs and chest wall during exhalation

lung recoil pulls the chest wall inward

compliance

how stretchy something is

C = change in volume / change in pressure

high vs low compliance

high: lungs inflate easily

low: lungs are stiff or heavy

causes of increased or decreased compliance

increased: emphysema or aging

decreased: fibrosis, atelectasis, edema, ARDS

static compliance

measured when there is no airflow

reflects lung and chest wall elasticity only

normal: 0.2 L/cmH2O

dynamic compliance

measured while air is moving in and out

reflects elastic forces + airway resistance

always equal or lower to static compliance

chest wall compliance and factors

how easily the chest can expand or contract

bone flexibility, muscle strength, and skin/tissue elasticity

elastance

the ability of the lungs or chest wall to return to their original shape after being stretched

E = change in pressure / change in volume

high vs low elastance

high: stiff lungs (more pressure to inflate)

low: floppy lungs (don't recoil well)

how are compliance and elastance related

they are complete opposites

hooke's law

the more pressure (force) you apply, the more something stretches - up to a limit

higher pressure is higher volume (proportional)

pressure gradient

a difference in pressure between two points

air moves from high to low

alveolar pressures

inspiration: below atmospheric

exhalation: above atmospheric

equal: airflow stops

transrespiratory pressure

pressure difference between the alveoli and body surface

total pressure needed to move air into the lungs

transpulmonary pressure

pressure difference between the alveolar space and the pleural space

pressure that keeps the alveoli open

transthoracic pressure

pressure difference between the pleural space and the body surface

represents the pressure needed to move the chest wall in or out

airway resistance with factors

the friction the air meets as it moves through the lungs

affected by radius, velocity, flow pattern, and gas properties

radius affects

the smaller, the harder for gas to pass

decreased by 1/2, flow decreases by 1/16

velocity affects

faster flow increases resistance and slower decreases

airflow patterns

laminar: smooth and parallel = low resistance

turbulent: chaotic and swirling = high resistance

transitional: a mix, often where airways branch

poiseuille's law

how pressure, radius, length, and viscosity affect airflow in the airways

flow affects

increases when pressure rises and radius is increased

decreases when airway length and gas viscosity increases

density and viscosity

density: how heavy the gas is

viscosity: how thick the gas is

heliox

helium + oxygen

helium less dense than oxygen so it flows more easily with less turbulence

heliox helps air bypass narrowed or swollen airways

time constants

tells how long it takes a part of the lungs to fill or empty

time (sec) = resistance X compliance

time constants represented

one: 63%

two: 86%

three: 95%

four: 98%

five: 99%

longer vs shorter time constant

longer (slow): increased resistance or compliance

short (fast): decreased resistance or compliance

restrictive diseases

ARDS or fibrosis

decreased compliance, decreased or normal resistance, short time constant, fast/shallow breathing

obstructive diseases

asthma or COPD

increased compliance, increased resistance, long time constant, slow/deep breathing