Minute Respiratory Volume (MRV)

1. Minute Respiratory Volume (MRV) • Definition: it is total volume of new air that enters respiratory passages per minute • Formula: Minute Resp. Volume= VTx Resp. rate • Value: 500x12= 6000ml/min or 6L/min

2. 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

5. 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

6. LUNG COMPLIANCE (Cont..) • 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) .

7. Compliance diagram of lungs

8. Compliance diagram of saline filled lungs

11. 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

12. 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

13. 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 • Lung elastic tissue---1/3 • Surface tension in alveloi---2/3 It 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

14. 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.

15. Surfactant Composed of • Surfactant apoproteins, • Phospholipids e.g. Dipalmityollecithin • Calcium ions • During inspiration water molecules move apart & expiration close to each other

16. Law of LaPlace Pressure = 2×Surface tension __________ Radius of alveolus

17. 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.