MECHANISM OF VENTILATION IN RESPIRATORY PHYSIOLOGY

Ventilation

1. MECHANISM OF VENTILATION Pulmonary ventilation ; Mechanism Lung volume Lung capacity flow

2. Lung volume It is useful to divide the total space within the lungs into  volumes  and  capacities . This division allows for assessment of the  mechanical condition  of the lungs, its musculature,  airway resistance  and the effectiveness of gas exchange at the alveolar membrane. These can be determined by simple, cheap and non-invasive tests.

3. Lung volume Volume Description Average Notes Tidal volume Volume that enters and leaves with each breath, from normal quiet inspiration to normal quiet expiration 0.5L (500mls) Changes with pattern of breathing e.g. shallow breaths vs deep breaths Increased in pregnancy Inspiratory reserve volume Extra volume that can be inspired above tidal volume, from normal quiet inspiration to maximum inspiration 2.5L (2500mls) Relies on muscle strength, lung compliance (elastic recoil) and a normal starting point (end of tidal volume) Expiratory reserve volume Extra volume that can be expired below tidal volume, from normal quiet expiration to maximum expiration 1.5L (1500mls) Relies on muscle strength and low airway resistance Reduced in pregnancy, obesity, severe obstruction or proximal (of trachea/bronchi obstruction) Residual volume/reserve volume Volume remaining after maximum expiration 1.50L (150mls) Cannot be measured by spirometry

4. Cont.. Expiratory reserve volume Extra volume that can be expired below tidal volume, from normal quiet expiration to maximum expiration 1.5L (1500mls) Relies on muscle strength and low airway resistance Reduced in pregnancy, obesity, severe obstruction or proximal (of trachea/bronchi obstruction) Residual volume/reserve volume Volume remaining after maximum expiration 1.5L (1500mls) Cannot be measured by spirometry

5. Lung Capacities In describing events in the pulmonary cycle, it is sometimes useful to consider two or more of the volumes together. Such combinations are called pulmonary capacities(lung capacities)

6. Cont.. 1 . The inspiratory capacity equals the tidal volume plus the inspiratory reserve volume . This capacity is the amount of air (≈3500 ml) that a person can breathe in, beginning at the normal expiratory level and distending the lungs to the maximum amount. 2 . The functional residual capacity equals the expiratory reserve volume plus the residual volume. This capacity is the amount of air that remains in the lungs at the end of normal expiration ≈2300 ml).

7. 3 . The vital capacity equals the inspiratory reserve volume plus the tidal volume plus the expiratory reserve volume . This capacity is the maximum amount of air a person can expel from the lungs after first filling the lungs to their maximum extent and then expiring to the maximum extent (≈4600 ml). 4 . The total lung capacity is the maximum volume to which the lungs can be expanded with the greatest possible effort (≈5800 ml ), it is equal to the vital capacity plus the residual volume. Most pulmonary volumes and capacities are usually about 20% to 30% less in women than in men, and they are greater in large and athletic people than in small and asthenic people

8. Capacity Description Expression Average Notes Vital capacity/forced vital capacity Volume that can be exhaled after maximum inspiration ( ie . maximum inspiration to maximum expiration) Inspiratory reserve volume + tidal volume + expiratory reserve volume 4.5L Often changes in disease Requires adequate compliance, muscle strength and low airway resistance Inspiratory capacity Volume breathed in from quiet expiration to maximum inspiration Tidal volume + inspiratory reserve volume 3L Functional residual capacity Volume remaining after quiet expiration Expiratory reserve volume + residual volume 3L Affected by height, gender, posture, changes in lung compliance. Height has the greatest influence. Total lung capacity Volume of air in lungs after maximum inspiration Sum of all volumes 6L Restriction < 80% predicted Hyperinflation > 120% predicted Measured with helium dilution

9. Dead space Dead space difined as a part of respiratory tract , where gaseous exchange does not take place. Air present in the dead space is called dead space air Anatomical (serial) dead space  is the volume of air that never reaches alveoli and therefore never participates in respiration. This includes volume in upper and lower respiratory tract up to and including the terminal bronchioles. Alveolar (distributive) dead space  is the volume of air that reaches alveoli but never participates in respiration. This can reflect alveoli that are ventilated but not perfused , for example secondary to a  pulmonary embolus .

10. Cont.. Area without N2 Volume of Dead space = × expired air Area Area with N2+ without N2 For example, in a subject : 30 Dead space = × 500 = 150mls 70 + 30 Area with nitrogen = 70 sq cm Area without nitrogen = 30 sq cm Volume of air expired = 500 mL

11. Dead space measurement

12. Measuring Volumes and Capacities 1; Simple Spirometry Simple spirometry can measure  tidal volume ,  inspiratory reserve volume  and  expiratory reserve volume . However, it cannot measure residual volume. Measured values are standardised for height, age and sex. Of these, height has the greatest influence upon capacities Process

13. Cont…. The subject breathes from a closed circuit over water. The chamber is filled with oxygen and as they breathe, gas increased and reduces the volumes within the circuit. A weight above the chamber changes height with each ventilation according to the circuit volume. The height is recorded with a pen to reflect the  volume  inspired or expired over  time.

14. Cont… 2; Helium  Dilution Helium dilution is used to measure  total lung capacity . However, it is only accurate if the lungs are not obstructed. If there is a point of obstruction, helium may not reach all areas of the lung during a ventilation, producing an underestimate as only  ventilated  lung volumes are measured.

15. Cont… After quiet expiration, the subject breathes in a gas with a known concentration of helium (an inert gas). They hold their breath for 10 seconds, allowing helium to mix with air in the lungs, diluting the concentration of helium . The concentration of helium is then measured after expiration. The   volume  of air which is ventilated is then calculated according to the degree of dilution of the helium.

16. Cont… 3; Nitrogen  Washout A method for calculating  serial/anatomical dead space  in the conducting airways up to and including the terminal bronchioles (usually 150mL). Process The subject takes a breath of pure oxygen and then exhales through a valve which measures nitrogen levels. At first, pure oxygen is exhaled, representing the dead space volume. This is because the air exhaled never reached the alveoli and so never underwent gaseous exchange.

17. Cont… Then, a mixture of dead space air and alveolar air is expired. This means the detected concentration of nitrogen increases as nitrogen rich air from the dead space reaches the valve . After a few breaths, the lungs are washed out of pure oxygen, meaning that purely alveolar air is expired. The nitrogen levels will reflect that of alveolar air . The levels of nitrogen measured over time can be used to calculate the  anatomical   dead space volume  of the lungs.

18. Vital capacity is the maximum volume of air that can be expelled out of lungs forcefully after a maximal or deep inspiration. Vital capacity includes inspiratory reserve volume, tidal volume and expiratory reserve volume. Normal value is; VC = IRV + TV + ERV = 3,300 + 500 + 1,000 = 4,800 mL.

19. Variation of vital capacity Physiological variations 1 . Sex: In females, vital capacity is less than in males 2. Body built: Vital capacity is slightly more in heavily built persons 3. Posture: Vital capacity is more in standing position and less in lying position 4. Athletes: Vital capacity is more in athletes 5. Occupation: Vital capacity is decreased in people with sedentary jobs. It is increased in persons who play musical wind instruments

20. Pathological variation of vital capacity The vital capacity decrease in the following; 1. Asthima 2. Emphysema 3. Weakness or paralysis of respiratory muscle 4. Pulmonary congestion 5. Pneumonia 6. Pneumothorax 7. Hemothorax 8. Pyothorax 9. Hydrothorax 10. Pulmonary edema 11. Pulmonary tuberculosis

21. measurements of vital capacity Vital capacity is measured by spirometry . The patient is asked to take a deep inspiration and expire forcefully Two important spirometry volumes that can be measured from a Vitalograph are : a) FVC  ( forced vital capacity )  – the maximal volume of air that a subject can expel in one maximal expiration from a point of maximal inspiration b) FEV 1   ( forced expiratory volume in one second ) – the maximal volume of air that a subject can expel in one second from a point of maximal inspiration. The proportion of air that can be exhaled in the first second compared to the total volume of air that can be exhaled is important in assessing for possible airway  obstruction . This proportion is known as the  FEV 1 /FVC ratio . This ratio is important clinically for diagnosis of respiratory conditions.

22. Spirometry .

23. Flow Volume  This plots flow over volume (showing expiratory flow and inspiratory flow as positive and negative values respectively). Important factors to consider when assessing flow-volume curves are as follows: Peak Expiratory Flow Rate (PEFR)  – the  rate  of flow. Vital capacity  – the volume expired, calculated from the X-axis. Shape  of the curve – ‘spooning’ in obstructive disease, small overall loop in restrictive disease

24. Flow volume in lung function test