minute respiratory volume (mrv)

Minute Respiratory Volume (MRV)

• Definition: it is total volume of new air that enters respiratory passages per minute

• Formula: Minute Resp. Volume= VT x Resp. rate

• Value: 500x12= 6000ml/min or 6L/min

Alveolar Ventilation

Definition :The rate at which new air reaches alveoli and other gas exchange areas is called alveolar ventilation

• Alveolar ventilation per minute is Rate of alveolar ventilation• Formula: VA=Freq× (VT - VD)

• VA is the volume of alveolar ventilation per minute• Freq. is the frequency of respiration per minute• VT is the tidal volume

• VD is the physiologic dead space volume• Calculation: VA= 12 × (500 - 150)

VA= 12 × (350)

VA= 4200 ml/ min

LUNG COMPLIANCE• Expansibility of lungs per unit

increase in Trans pulmonary pressure

• It is determined by elastic forces• Formula: ∆V ____________

∆P• Combined compliance of thorax

and lungs is 110ml/cm of H2O• Compliance of lung alone =

Average 200ml/cmH2O• ELASTANCE: It is reciprocal of

compliance. i.e. ∆P/∆V

• Elastic forces of the lungs.

(1) recoil forces of the lung’ elastic tissue (1/3 of total forces)

(2) forces caused by surface tension of the fluid that lines the inside walls of the alveoli and other lung air spaces (2/3 of total forces)

.

LUNG COMPLIANCE (Cont..)

Compliance diagram of lungs

Compliance diagram of saline filled lungs

• High Compliance – – Emphysema, – old age.

• Decreased Compliance :It means lungs are difficult to

expand.– Pulmonary edema, – fibrosis – pneumothorax, – scaring of lungs in T.B. – thickening of pleura, – absence of surfactant in new born

Work of Breathing

• Inspiration - active process, so work is done

• Energy consumed (work done) during inspiration – 3-5% of total energy used by body

• During exertion - ↑ ventilation – both inspiration and expiration – active, energy utilized upto 50 times more than at rest

TYPES OF WORK OF BREATHING

1. COMPLAINCE WORK OR ELASTIC WORK (65%):that required to expand the lungs against the lung and chest elastic forces

1. Lung elastic tissue---1/32. Surface tension in alveloi---2/3It increases in RDS.

2. AIRWAY RESISTANCE WORK (28%): – that required to overcome airway resistance to movement of air into the

lungs.– Medium and large sized airways offer most of the airway resistance not

terminal bronchioles.– It is increased in asthma and COPD.

3. TISSUE RESISTANCE WORK (7%):– that required to overcome the viscosity of the lung and chest wall structures– It is increased in restrictive lung diseases

Surfactant• Lipoprotein mixture in thin fluid layer on the interior

of alveoli • Secreted by alveolar type II cells• Surface tension inversely proportional to

concentration of surfactant

• Functions of surfactant: – Prevents collapse of lungs– Stabilize size of alveoli– Surfactant increases lung compliance.– Surfactant helps to keep lungs expanded. – Surfactant also helps to keep the alveoli dry and prevent development

of pulmonary edema.

Surfactant

Composed of • Surfactant apoproteins, • Phospholipids e.g.

Dipalmityollecithin• Calcium ions

• During inspiration water molecules move apart & expiration close to each other

Law of LaPlace

Pressure = 2×Surface tension __________ Radius of alveolus

respiratory distress syndrome (RDS)• During intrauterine life surfactant formation begins at 30th week and it

can be detected in amniotic fluid. • Pre-mature infants do not produce enough surfactant• the pressure of -20 to -30 mm of Hg will be required to keep the lungs

expanded• Amnicentisis can be performed and in that fluid we can estimate the

surfactant concentration.• Surfactant secretion is stimulated by:

– glucoorticoids, – epinephrine, – thyroxine

• Deficiency occurs in:– premature babies, – babies of hypothyroid,– diabetic mothers. – Smoking decreases surfactant.