mechanics of breathing dr.m.kannan md da professor and hod department of anaesthesiology tiunelveli...
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MECHANICS OF BREATHINGMECHANICS OF BREATHING
Dr.M.Kannan MD DAProfessor And HODDepartment of AnaesthesiologyTiunelveli Medical College
MECHANICS OF BREATHINGMECHANICS OF BREATHING
Introduction : all about how the lung is supported and moved.
Applications during conduct of GA, Intensive care.
Components of mechanics: 1. Muscles of respiration
2. Compliance
3. Airway resistance
4. Work of breathing
MUSCLESMUSCLES
Diaphragm:
Abnormalities of movement: a. unilateral phrenic n. palsy.
b. Pressure from below
c. Upper abdominal surgery
d. In advanced emphysema
e. In some lung diseases
External intercostals: Paralysis/paresis of intercostals=nerve block,
extra- dural and sub-arachnoid block,
transection
of spinalcord, residual neuro-
muscular blockade,inanition.
::
Accessory muscles: a. The scalene muscles
b. The sternocleidomastoid
c. Anterior serrati
d. The alae nasi
e. small muscles of the head and neck.
Muscles of expiration: Normally passive. Active during exercise ,forced
expiratory manouevers.
Mechanism of respiration
PRESSURE-VOLUME RELATION-CHARACTERISTICSPRESSURE-VOLUME RELATION-CHARACTERISTICS
a) non-linear. Curved.
b) Different for inspiration & expiration.Hysteresis present.
Compliance: Lung compliance+ chest wall compliance
§V/§P(L/cm H2O)
a) §P= Palv-Ppleural(transpulmonary)
b) §P= P pleural-Patm(transmural)
a+b=transthoracic§P=Palv-Patm
Cl= slope of the curve just described.
In the normal range of expanding pressures of -2 to -10 cms of H2O,
the lung compliance=0.2 L/cm of H2O .However at higher expanding
pressures-compliance is less.
Measurement of compliance: pressure in the pleural cavity-measured
by implanting a balloon tipped catheter in the pleural cavity.
Indirectly, oesophageal pressure-can be measured. Alveolar pressure-
pressure at the site of airway opening.
Lung compliance is ↑ in a. emphysema b. with advancing age
(destruction of elastic tissue of the lung.)
Specific compliance:
Though the compliance is ↑ in emphysema- the lungs don’t
function better mechanically.damage to the elastic fibres-gas trapping
occurs-FRC is ↑. The resting ventilation –operating on the flat part of the
pressure-volume curve, so during normal tidal breathing-compliance is less.
This is specific compliance.
Specific compliance= Compliance(L/cm H2O)/Volume of lung at FRC.
Elastic properties of the chest wallElastic properties of the chest wall
Normally, chest wall compliance= lung compliance.
Total compliance= lung compliance+chest wall compliance=0.1 L/cm H2O.
Factors affecting compliance
1. Lung volume
2. Hysteresis
3. Posture
4. Pulmonary blood volume:
Restriction of expansion of the chest:
IPPV-reduces compliance.
a. by causing uneven ventilation
b. by airway obstruction due to accumulation of cellular debris due
to reduced ciliary activity.
GA
Disease states.
Dynamic compliance: First measure peak airway pressure. It is
the pressure measured at the airway opening at end-
expiration,after subtracting applied CPAP.
Cdyn= inspiratory volume/peak airway pressure.
Static compliance: If expiration is prevented from occuring
following inspiration, the respiratory muscles relax-airway
pressure drops.
Cstat= inspiratory volume/plateau airway pressure.
Anaesthetic implicationsAnaesthetic implications
Changes in dynamic compliance without changes in
Cstat=↑ airway resistance.
Alterations in both dynamic and static
compliance=change in lung stiffness.
Surface tensionSurface tension
Laplace’s law: P=2T/R.
Surface tension kept constant, with reducing radius of the alveoli-
retraction pressure would rise-collapse of the smaller alveoli-emptying
into the larger alveoli. This does’nt happen due to the presence of
surfactant.lipoprotein with phospholipid-dipalmitoyl phosphatidyl choline.
Dense molecules in the smaller alveoli-keep the pressure more or less
constant.
Functions:
Application=in neonatal respiratory distress syndrome.
Interdependence:surrounding alveoli exert expanding forces.
Regional differences in ventilation: better ventilation of the base and
dependent lung. Converse true at low lung volumes.
What happens during ageing?
AIRWAY RESISTANCE
>2/3 of the work=spent in overcoming airflow resistance when RR>50/min
Laminar flow: Flow=pir4/8nl Turbulent flow/orificial flow
Applications
.
Airway resistance-contd.Airway resistance-contd. Airway resistance= mouth pressure-intra-pleural pressure/flow
Normal value:0.05-2 cm H2O/l/sec in adults.
Variations:
a. Lung volume
b. Bronchial muscle tone
c. Drugs
d. Posture
The response to↑ airway resistance:
Dynamic compression of airways: EPP describes it.
Abnormal expiratory flow rates.
Tissue resistance
Pulmonary resistance
WORK OF BREATHINGWORK OF BREATHING
Elastic, non-elastic airway work.
Work=pressurexvol
Normal metabolic cost=1-3% of total O2
Consumption
Optimum pattern of ventilation for diseased lungs