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