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MECHANICS Of breathing/Tests of lung function

Aims:
 How
is breathing affected by changes in pressures,
resistance, compliance and surface tension?
 How
do these factors change in respiratory disease?
 What
tests can be used to assess the function of the
lungs?
 How do you distinguish between an obstructive and
a restrictive respiratory disease ?
2
MECHANICS

PRESSURES
-
pleural, alveolar

RESISTANCE
-
airways

COMPLIANCE
-
ability to expand

SURFACE TENSION -
surfactant
2
PRESSURES
 PLEURAL- Pressure between lungs and chest wall (-ve) =- 5cmH2O




lung elastic recoil pressure
lungs (visceral), chest wall (parietal)
lung tissuel inwards whilst chest wall outwards
I= more -ve; E= less -ve
 ALVEOLAR- pressure within alveoli
 I= PA -ve
 E= PA +ve
 THORAX= -ve pressure pump
 ARTIFICIAL
VENTILATION- +ve pressure at mouth, CPAP,NPPV
 PNEUMOTHORAX-air entry into pleural space due to lung disease or
chest wall injury; causes lung collapse, rib cage springs outward, diaphragm
depressed
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AIRWAYS RESISTANCE
Raw= (mouth pressure-PA)/ airflow


40-50% of resistance in upper airways- nose,
pharynx,larynx
majority of the remainder (40%) in trachea and medium
size bronchi(div. 1-8, 2-4mm), rest in size 1-2mm
INCREASED- smooth muscle contraction, falls in lung
volumes, secretions, obstructive disease, mediators, histamine, air
pollution, PNS (Ach)
DECREASED- smooth muscle relaxation, bronchodilators,
nitric oxide, increase in lung volumes
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COMPLIANCE
Ability
to distend lungs
change of lung volume/change of
pressure
0.2 l/cmH2O
increased- age, emphysema, lung size,
low volumes and low pressures
decreased- oedema, atelectasis, fibrosis,
at high lung volumes and pressures,
high pulmonary venous pressures,
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supine
SURFACE TENSION
 Force- produced at a gas/liquid interface



collapse of alveoli in lungs
pulls fluid from capillaries into alveoli- stiff lungs
smallest alveoli - largest surface tension
 LAPLACE’S
LAW- pressure  1/radius
 SURFACTANT (dipalmitoyl lecithin)- complex lipoprotein

surface tension
produced by type II cells
prevents collapse of alveoli, keeps alveoli dry
normally secreted by 22 weeks in foetus

lack- infant respiratory distress syndrome (RDS)



 RDS- atelectasis, fluid in lungs, decreased compliance
 ARDS - adult RDS
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RESPIRATORY INVESTIGATIONS
 SYMPTOMS-chest
breathlessness
pain, cough, sounds,
 ANATOMICAL-
sputum,CXR, blood,
bronchoscopy, scans (PET, CT, MRI)
 PHYSIOLOGICAL-
skin prick tests, blood gases,
exercise tests, spirometry (LUNG FUNCTION
TESTS)
LUNG FUNCTION TESTS
 TESTS








OF FORCED EXPIRATION
FEV1, FVC, PEF, FEV1/FVC ratio
Effort dependent
Obstructive & Restrictive
TLC, RV, FRC, Raw
GAS TRANSFER- TCO, KCO
FLOW-VOLUME LOOPS
Vitalographs, spirometers
Body plethysmographs

COTES, J.E. (1993). LUNG FUNCTION. Blackwell Scientific Publications.

QUANJER, Ph. H et al (1993). LUNG VOLUMES AND FORCED VENTILATORY FLOWS. EUR RESPIR.
JOURNAL, SUPPL 16, 5-40.
LUNG VOLUMES
SPIROMETER-
SPIROGRAM
 Tidal volume (VT) - 0.5l
Inspiratory capacity(IC)-3.6l
 Inspiratory reserve volume (IRV)- 3.1l
 Expiratory reserve volume (ERV)-1.2l
 Functional residual capacity (FRC)- 2.4l
 vital capacity (VC) - 4.8l
 Residual volume (RV)- 1.2l
 Total lung capacity (TLC) - 6l

VOLUMES
Body size, Age,
 Sex, Muscular Training
 Ethnicity, Diseases
FRC
OF TLC, 10% less in females,  height
 IMPORTANCE:
 40%



Minimum amount of air for gas exchange at all times
Keeps alveoli & small air tubes open after expiration
Provides stability of oxygen pressure (PO2)
 VARIATION:


Obesity, pulmonary fibrosis, kyphoscoliosis, supine
standing, decreased elastic recoil
CONCLUSIONS
 1. The work of breathing is increased by compliance and resistance &
surface tension.
 2. Changes in pleural and alveolar pressures are essential in creating
pressure gradients and thus airflow, inflation and deflation of lungs.
 3. Obstruction of airways(e.g mucus, bronchoconstriction), oedema,
atelectasis and fibrosis resistance, surface tension and compliance of
lungs.
 4. Lung function assessed by anatomical (eg Bronchoscopy, CXR) and
physiological tests (eg blood gases, expiratory tests).
 5. Obstructive diseases-airways obstruction(mucus, smooth muscle
enlargement, airway hypersensitivity), thus FEV1,PEF & FEV1/FVC.
 6. Restrictive diseases-impairment of lung inflation( muscular, rib
cage), thus VC,TLC, same or FEV1/FVC.