Pulmonary Mechanics

Question 1

methods of measuring intrapleural pressure

Answer 1

placing a small catheter connected to a pressure measuring device in the intrapleural region

placing a balloon in the thoracic esophagus and measuring the intraesophageal pressure

Question 2

typical intrapleural pressure at the (passive) functional residual capacity

Answer 2

-3 to -6 cm H₂O

this pressure expands the lung and collapses the rib cage by a corresponding amount

Question 3

transpulmonary pressure

Answer 3

the pressure acting to inflate the lungs

usually negative with respect to the alveolar pressure and thus acts to expand the lungs

P_A - P_ip

Question 4

transthoracic pressure

Answer 4

the force acting on the thoracic wall

P_ip - P_b

since pleural presssure is generally negative, it “sucks” the chest wall inward

Question 5

transrespiratory pressure

Answer 5

the potential pressure graident for flow into or out of the alveoli

the difference between alveolar and atmospheric pressure measured with the glottis closed and with respiratory muscles relaxed

P_A - P_B

if the transrespiratory pressure is negative, gas will flow into the alveoli

if transrespiratory pressure is positive, gas will flow out of the alveoli

Question 6

compliance

Answer 6

the change in volume with respect to change in pressure

Question 7

elastance

Answer 7

the inverse of compliance in the lungs

Question 8

causes of decreased lung compliance

Answer 8

respiratory distress syndrom ( decrease)

edema (decrease)

atelectasis (decrease)

fibrosis (decrease)

Question 9

causes of increased compliance

Answer 9

age (increase)

emphysema (increase)

increasing body size (increases)

Question 10

Law of Laplace

Answer 10

P = 2T/R

P is the pressure within a spherical object

T is the tension int he wall of the object

R is its radius of curvature

a smaller bubble would have a greater internal pressure than the larger bubble

constant varies with the geometry of the object, for a sphere it would be 4 instead of 2

Question 11

implications of Law of Laplace

Answer 11

the smaller the radius of curvature, the stronger the inward force resulting from surface tension

bubbles (alveoli) with smaller radii have larger internal pressures

small alveoli will have a tendency to collapse (instability of alveoli)

surface forces tend to pull intersitial fluid into the alveolus

Question 12

regulation of lung fluid balance

Answer 12

because of the additional surface tension, there is more hydrostatic pressures in the capillaries in the alveoli, so this favors fluid movement into the interstitium

surfactant reduces alveolar surface tension, and this helps prevent edema

Question 13

hysteresis

Answer 13

the different relationship between pressure and volume during inflation compared to that during deflation

Question 14

What is the main effect of introducing saline into the lung?

Answer 14

it eliminates the very large (>70m²) fluid-air interface in the alveoli

this abolishes any effects due to surface tension of the fludis lining the alveoli

this makes the lung much more compliant, almost completely abolishing hysteresis

Question 15

roles of surfactant in the lungs

Answer 15

reduces alveolar surface tension and varies it with breathing

preserves alveolar integrity

prevents continuous transudate (edema) from pulmonary capillaries to alveoli

reduces work of breathing

Question 16

composition of surfactant

Answer 16

lipoprotein complex that is approximately 30% protein and 70% phospholipid (mostly dipalmitoyl phosphatidyl choline)

synthesize dnad released from lamellar bodies in type II alveolar epithelial cells

Question 17

dipalmitoyl lecithin or dipalmitoyl phosphatidyl choline

Answer 17

principle active ingredient of surfactant

hydrophilic head, which aligns with the water surface, and a hydrophobic tail that is more compatible with air

released into the alveoli as a result of lung distention or stimulation of beta adrenergic receptors

quickly spreads in a monolayer along the interior lining of the alveoli at the surface of a thin aqueous lining of the alveolus known as the hypophase

Question 18

How does surfactant contribute to hysteresis?

Answer 18

during inflation, the surfactant is broken up and micelles beneath the surface help fill in gaps

this reduces the effect of the surfactant and thus increases surface tension

upon deflation, the expanded layer of surfactant is compressed to a surface solid state, which provides the maximal amoutn of action, reducing the surface tension to almost zero

Question 19

the role of surfactant in the first few breaths

Answer 19

the first breath is very difficult to take as a lot of pressure must build up before the lungs open

once the first few breaths are taken, surfactant is spread across the surfaces, which then makes inspiration much easier

babies with RDS never reach that stage of easy breathing as the lung collapses every time they expire

Question 20

What is the main difference between the complaince of the lung and chest wall?

Answer 20

at normal lung volumes (up to about 60-80% of vital capacity) the chest wall is attempting to expand

reducing the pressure gradient across the chest wall increases thoracic volume

Question 21

describe the relationship between the compliance of the lung and the chest wall

Answer 21

the two systems are in series

1/C_resp= 1/C_lung+ 1/C_chest

at FRC, the total compliance is less than the individual compliances

Question 22

What happens to the FRC when the chest wall is less compliant?

Answer 22

the FRC is larger with a normal chest wall

P_ip is more negative

Question 23

What happens ot the FRC when the lungs are less compliant?

Answer 23

FRC decreases - stiff lungs pull chest to a smaller volume

P_ip is more negative

Question 24

describe the relaxation pressure-volume curve of the lung and chest wall

Answer 24

the compliance curves for the lungs and chest wall are considered together when determining the total compliance of the system

Question 25

Where is ventilation the lowest in the lung and why?

Answer 25

lowest in the apices where the negative intrapleural pressure is the greatest

lung is in the upper portions of the compliance curve, making lung complicance low

therefore, for a given change in intrapleural pressure produced by an inspiratory effory, the volume increase will be small

Question 26

Describe the ventilation of the lung at FRC and at Residual Volume.

Answer 26

at FRC, the base of the lung is open and ventilation is higher there

at RV, the base of the lung is compressed and closed, so ventilation becomes greater at the apex

Question 27

ventilation at the apex

Answer 27

intrapleural pressure more negative

greater transmural pressure gradient

alveoli are larger and less compliant

less ventialtion

Question 28

perfusion at the apex

Answer 28

lower intravascular pressures

less recruitment

distension

higher resistance

less blood flow

Question 29

ventilation at the base

Answer 29

intrapleural pressure less negative

smaller transmural pressure gradient

alveoli smaller, more compliant

more ventilation

Question 30

perfusion at the base

Answer 30

greater vascular pressures

more recruitment

distension

lower resistance

greater blood flow

Question 31

describe the volume-pressure-flow relationships during a breath

Answer 31

the hashed lines represent the additional pressure generated by airway resistance

Question 32

What happens to maximum flow as lung volume decreases and why?

Answer 32

maximum flow decreases because of compression of the airways

Question 33

specific factors affecting resistance

Answer 33

lung volume

bronchial smoth muscle tone

gas characteristics such as desnity or viscosity

Question 34

Why does dependence of airway compression on lung volume occur?

Answer 34

airways are “effectively” tethred to the chest wall

at large lung volumes, tethers are stretched and increased the effective stiffness of the airway - less compressible

diseases such as emphysema increase lung compliance, reduce tethering and increase dynamic compression, thereby increasing flow limitation

Question 35

effort independent (flow limited) portion of the flow-volume curve

Answer 35

arises from dynamic compression of the airways

as expiratory effort increases, Pip increases, thereby increasing airway compression (although U may increase) and limiting flow

the narrowed airway can oscillate and turbulent flow cuase wheezing sounds

Question 36

starling resistor

Answer 36

Pi > Pe > Po

tube will flutter open and closed

flow is continuous, but a function of Pi - Pe