Gururaj

Briefly describe factors affecting Lung Compliance

Resistance to Breathing

Elastic resistance ~ 65%

( of lung and chest wall)

Non-elastic resistance ~ 35%

(frictional resistance to gas flow, inertia

associated with movement of gas and

tissues)

Elastic Resistance

Elastic Recoil of the Lungselastic lung tissue recoils from the chest wall &

results in a sub-atmospheric intrapleural pressure

at FRC, the mean intrapleural pressure ~ 4-5 cmH20 sub-atmospheric

Compliance a measure of the elasticity, or distensibility, of

pulmonary or thoracic tissues for an elastic body, this is given by the relation

between the distending force and length for the lung, this is given by the relationship of

pressure and volume may be measured under static conditions, ie.

zero air flow, or under dynamic conditions units of compliance, V/P = litres/cmH20

Static Lung Compliance the relationship between volume change of lung

and the transpulmonary pressure change, i.e., airway - intrapleural pressure change, under known static conditions (zero airflow)

normal value for a 70 kg adult ~ 200 ml/cmH20 the value decreases as lung volume increases

due to the limitations of the non-elastic components of the lung/chest wall system

Static Lung Compliance : 2 static P/V curves for the lung

sigmoid curvevarying degrees of hysteresisvolume at any given pressure being greater

during deflation

Reasons for Hysteresis

Changes in Surfactant activity: Surface tension greater in inspiration

Stress relaxation: Inherent property of elastic tissues ( crinkled structure of collagen in the lung)

Redistribution of gas: fast and slow alveoli

Static Lung Compliance : 3

Static Lung Compliance : 4 compliance is directly related to lung

volume transpulmonary pressure 1.0 cmH20 will

inflate,two lungs by 0.2 lone lung by 0.1 l

the lung of a neonateabsolute compliance ~ 0.006 l/cmH20specific compliance ~ 0.067

l/cmH20/l.VL

the later being identical to that of an adult lung

Specific Compliance a true measure of the distensibility of lung

tissue defined as,

CS = Lung Compliance = (VPV

Lung Volume

= Lung Compliance FRC

Static Compliance: Factors 1 lung volume

the bigger the lungs the greater the compliance

posturedue to changes in lung volumes? Related to measurement of intrapleural

pressure in supine positiondoes not affect specific compliance

pulmonary blood volumepulmonary venous congestion from any cause

will decrease compliance

Static Compliance: Factors 2 age

many studies have failed to demonstrate any change in compliance when allowing for changes in lung volumes

this is consistent with the notion that most of the elastic recoil is due to surface forces

restriction of chest expansioncauses only temporary changes in compliance

recent ventilatory history

pulmonary disease

Compliance : Disease emphysema

static CL is increased, as is FRChowever, the distribution of inspired gas may be

grossly abnormal, therefore dynamic CL is frequently reduced

asthmaP/V curve is displaced upwards without a

change in CL

the elastic recoil is reduced at normal transmural pressure, thus the FRC is increased

most other types of pulmonary disease decrease the CL, both static & dynamic

Dynamic Compliance : PV Loop

points of flow reversal (zero airflow)

Dynamic Compliance : 2

Dynamic Compliance : 3 measurements made using these points

reflect dynamic compliance in normal lungs at low and moderate

frequencies, dynamic and static lung compliance are approximately equal

however, dynamic CL is less than static CL

at higher frequencies in normal lungsat normal frequencies in abnormal lungs

Dynamic Compliance : 4 pressure equilibrium between applied

pressure and alveolar pressure is not obtainedlung appears artefactually stiffer

the time to fill an alveolus depends on the product of airway resistance and the compliance of the alveolus = the exponential time constant

the higher the airway resistance, or regional lung compliance, the longer to fill a given alveolus

CDynamic : Factors

decreased dynamic lung compliance is seen especially with increased airways resistanceasthma, chronic bronchitis and emphysemaprincipally due to the prolonged time constants

emphysema increases specific lung compliance but, due to its effect on the time constant, produces the phenomenon of frequency dependent compliance

Time Constant numerically, the time required for an exponential

process to reach 63% of its final change alternatively, the time which would be taken to

complete volume change, if the initial rate of volume change (V/t), were maintained

for the lung:

tauCL × RA

Surface Forces and Lung Recoil elastic lung recoil is dependent on,

surface tension : (dynes/cm, SI units = N/m)○ produces > 50% of normal lung recoil

tissue elastic fibres

the recoil pressure of a saline filled lung is lowerdetermined only by the elastic recoil of

pulmonary tissue

Laplace’s Law P = 2T/rWhich one has higher transmural pressure??R1: 0.1 T1: 20

R2: 0.05 T2: 20

Surface Tension

surface active agents, surfactants, exert smaller attracting forces for other molecules

when concentrated at the surface they dilute the molecules of a liquid and lower surface tension

ordinary detergents lower surface tension, however tension does not alter with changes in surface areawith pulmonary surfactant, as the surface area

decreases, so surface tension also decreases

Pulmonary Surfactant

synthesised in type II alveolar cells,

granular pneumocytes

elimination half life : t½ ~ 14 hrs

dipalmitoyl phosphatidyl choline

(DPPC), a phospholipid, is the main

componenthydrophilic and hydrophobic ends,

therefore forms a lipid monolayer

Surfactant : Actions

reduces Ts in alveolireduces lung recoil and work of breathing

stabilises alveoli of variable sizeas surface tension is proportional to surface

areaprevents small alveoli tending to "fill" larger ones

promotes alveolar “dryness”a high Ts tending to draw fluid into alveoli as well

as promoting collapse

Surfactant RDS of new-born

hyperoxia - O2 toxicity of lung

Smoking

gross over distension of alveoli

ARDS

Elastic Recoil: Thoracic

resting volume for thoracic cage

~ FRC + 600-700 ml thoracic cage compliance is calculated

from total compliance of the thoracic cage + the lungs, and from pulmonary compliance when measured simultaneously, where,

1/CTOT = 1/CL + 1/CCW

Elastic Recoil: Thoracic 2 FRC = equilibrium point for both systems

not quite true, as FRC is 400-500 ml above the equilibrium point due to the tonic activity of the diaphragm

thoracic cage compliance is decreased in,kyphoscoliosis, ankylosing spondylitissclerodermamuscle spasticityabdominal distension, obesity

Non-Elastic Resistance

this is composed of,airway flow resistance ~ 80%pulmonary tissue resistance,

or viscous resistance ~ 20%

increases markedly with rapid respiration, or narrowing of the airwaysproportional to the rate of airflowP for a given airflow depends upon whether

the flow is laminar, or turbulent

Laminar Flow Hagen-Poiseuille Equation

. .Q

r P

l

4

8

Pressure gradient = Flow X ResistancePressure gradient = Flow X Resistance

therefore, by rearrangementtherefore, by rearrangement,,

Rl

r

84

.

Turbulent Flow the likelihood of flow becoming turbulent is

predicted by the Reynold's Number

vd

Re

V: velocity, d: diameterV: velocity, d: diameter viscosityviscosity ( (-eta) is relatively less important-eta) is relatively less important

viscosities of respirable gasses do not vary viscosities of respirable gasses do not vary greatly, cf. densities may vary considerablygreatly, cf. densities may vary considerably

densitydensity ( (-rho) decreases flow proportionately-rho) decreases flow proportionately

Reynold’s Number Re < 2000, laminar flow becomes more

likely

Re > 4000 : predominantly turbulent Flow is square front: Fresh gas has to fill the volume of the tube Better at purging the contents of the tube Gas representative at all points

theoretically, the required driving pressure becomes inversely proportional to the fifth power of the tube radius: Fanning equation