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 Lungs
 elastic 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,
dV/dP = 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 curve
 varying degrees of hysteresis
 volume 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 l
 one lung by 0.1 l

the lung of a neonate
 absolute compliance
 specific compliance
~ 0.006 l/cmH20
~ 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 =
(dV/dP)/V
Lung Compliance =
Lung Volume
=
Lung Compliance
FRC
Static Compliance: Factors 1

lung volume
 the bigger the lungs the greater the compliance

posture
 due to changes in lung volumes
 ? Related to measurement of intrapleural
pressure in supine position
 does not affect specific compliance

pulmonary blood volume
 pulmonary 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 expansion
 causes only temporary changes in compliance

recent ventilatory history

pulmonary disease
Compliance : Disease



emphysema
 static CL is increased, as is FRC
 however, the distribution of inspired gas may be
grossly abnormal, therefore dynamic CL is
frequently reduced
asthma
 P/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 lungs
 at normal frequencies in abnormal lungs
Dynamic Compliance : 4

pressure equilibrium between applied
pressure and alveolar pressure is not
obtained
 lung 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
resistance
 asthma, chronic bronchitis and emphysema
 principally 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 (dV/dt), were maintained
 for the lung:

t (tau) = CL × 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 lower
 determined only by the elastic recoil of
pulmonary tissue
Laplace’s Law
P = 2T/r
Which one has higher transmural pressure??
R1: 0.1
R2: 0.05
T1: 20
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 area

 with 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 :

dipalmitoyl phosphatidyl choline
(DPPC), a phospholipid, is the main
component
t½ ~ 14 hrs
 hydrophilic and hydrophobic ends,
therefore forms a lipid monolayer
Surfactant : Actions

reduces Ts in alveoli
 reduces lung recoil and work of breathing

stabilises alveoli of variable size
 as surface tension is proportional to surface
area
 prevents 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

Smoking

gross over distension of alveoli

ARDS
- O2 toxicity of lung
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 spondylitis
 scleroderma
 muscle spasticity
 abdominal 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 airways
 proportional to the rate of airflow
 dP for a given airflow depends upon whether
the flow is laminar, or turbulent
Laminar Flow

Hagen-Poiseuille Equation
 . r . dP
4
 =
Q
8l
Pressure gradient = Flow X Resistance
therefore, by rearrangement,

R =
8l
. r
4
Turbulent Flow

the likelihood of flow becoming turbulent is
predicted by the Reynold's Number
Re =


vd

V: velocity, d: diameter
viscosity (-eta) is relatively less important
 viscosities of
respirable gasses do not vary
greatly, cf. densities may vary considerably

density (-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