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Flashcards in Pulmonary Mechanics Deck (36):
1

methods of measuring intrapleural pressure

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

2

typical intrapleural pressure at the (passive) functional residual capacity

-3 to -6 cm H2O

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

3

transpulmonary pressure

the pressure acting to inflate the lungs

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

PA - Pip

4

transthoracic pressure

the force acting on the thoracic wall

Pip - Pb

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

5

transrespiratory pressure

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

PA - PB

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

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

6

compliance

the change in volume with respect to change in pressure

7

elastance

the inverse of compliance in the lungs

8

causes of decreased lung compliance

respiratory distress syndrom ( decrease)

edema (decrease)

atelectasis (decrease)

fibrosis (decrease)

9

causes of increased compliance

age (increase)

emphysema (increase)

increasing body size (increases)

10

Law of Laplace

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

11

implications of Law of Laplace

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

12

regulation of lung fluid balance

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

13

hysteresis

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

14

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

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

15

roles of surfactant in the lungs

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

16

composition of surfactant

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

17

dipalmitoyl lecithin or dipalmitoyl phosphatidyl choline

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

18

How does surfactant contribute to hysteresis?

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

19

the role of surfactant in the first few breaths

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

20

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

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

21

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

the two systems are in series

1/Cresp = 1/Clung + 1/Cchest

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

22

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

the FRC is larger with a normal chest wall

Pip is more negative

23

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

FRC decreases - stiff lungs pull chest to a smaller volume

Pip is more negative

24

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

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

25

Where is ventilation the lowest in the lung and why?

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

26

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

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

27

ventilation at the apex

intrapleural pressure more negative

greater transmural pressure gradient

alveoli are larger and less compliant

less ventialtion

28

perfusion at the apex

lower intravascular pressures

less recruitment

distension

higher resistance

less blood flow

29

ventilation at the base

intrapleural pressure less negative

smaller transmural pressure gradient

alveoli smaller, more compliant

more ventilation

30

perfusion at the base

greater vascular pressures

more recruitment

distension

lower resistance

greater blood flow

31

describe the volume-pressure-flow relationships during a breath

the hashed lines represent the additional pressure generated by airway resistance

32

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

maximum flow decreases because of compression of the airways

33

specific factors affecting resistance

lung volume

bronchial smoth muscle tone

gas characteristics such as desnity or viscosity

34

Why does dependence of airway compression on lung volume occur?

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

35

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

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

36

starling resistor

Pi > Pe > Po

tube will flutter open and closed

flow is continuous, but a function of Pi - Pe