Notes on Lung Mechanics

Rahul's Noteblog Notes on Pulmonology Notes on Lung Mechanics

Lung Volumes-1:

lung volumes TLC VC RV ERV IRV IC FRC TV

Lung Volumes-2:

NOTE: VC, IC, V_T, and ERV can be measured directly using spirometry.

Tidal Volume:

Amount of air that leaves/enters the lungs during a normal respiratory cycle. 500 ml.

Functional Residual Capacity:

Amount of air at end of passive expiration. 2700 ml.

Inspiratory Capacity:

Amount of air that can be inspired over FRC. 4000 ml.

Inspiratory Reserve Volume:

Amount of air that can be inhaled after normal inspiration. 3500 ml.

Expiratory Reserve Volume:

Additional air that can be expired after normal expiration. 1500 ml.

Residual Volume:

Amount of air inside lung after maximal expiration. 1200 ml.

Vital Capacity:

Amount of air that can be expired after maximal inspiration. 5500 ml.

Total Lung Capacity:

Amount of air in the lung after maximal inspiration. 6700 ml.

Ventilation:

• Total ventilation: amount of air moved in/out of lungs per minute.

• Dead space: these regions contain air but don't participate in gas exchange; two types: anatomical and alveolar; physiologic dead space = anatomical + alveolar.

Muscles of respiration:

• Inspiration: diaphragm and muscles of chest wall.

• Expiration = passive (at rest); muscles of abdominal wall used when active.

• Inspiration = active.

Forces acting on lung:

• Lung recoil: develops as the lung expands; collapses the lung.

• Intrapleural pressure: pressure in thin film between lung and chest wall.

• Pressure lower than atmospheric intrapleural pressure (present during normal breathing): expands the lung.

• Pressure greater than atmospheric intrapleural pressure: compresses the lung.

• If intrapleural pressure is greater than lung recoil: lung expands.

• If intrapleural pressure is less than lung recoil: lung collapses.

• If intrapleural pressure = lung recoil: static lung.

Mechanics of breathing:

Notes:

• Slight negative intrapleural pressure initiates inspiration.

• When intrapleural pressure becomes very negative, the recoil force increases and initiates expiration.

• Alveoli under negative alveolar pressure "want air to enter them," and make pressure 0.

• Alveoli under positive alveolar pressure "want air to out of them," and make pressure 0.

Before inspiration:

• Glottis is open.

• All muscles relaxed.

• This is the neutral or equilibrium point.

• Intrapleural and recoil force are equal and opposite; no air flow out.

During inspiration:

• Intrapleural pressure is negative.

• Airway resistance is also decreasing.

• Alveoli are under negative alveolar pressure.

• The greater the change in intrapleural pressure, the larger the force trying to expand the lung.

• Larger the lung volume, more negative the intrapleural pressure.

End of inspiration:

• When the alveolar pressure is zero again, airflow stops.

Expiration:

• Passive process.

Pneumothorax:

• Intrapleural pressure = atmospheric pressure.

• Recoil pressure = 0 = lung collapses.

• Chest wall expands.

More compliance:

• More air-flow for a given change in pressure.

Less compliance:

• Less air-flow for a given change in pressure.

• Abnormally high compliance: emphysema.

• Abnormally low compliance: fibrosis.

(pressure-volume) Lung inflation curve:

• Steeper the curve, more the lung compliance.

• Compliance increases with age and saline-filled lung.

• Very compliant lungs are easy to inflate.

• In RDS: right-shifted and flatter.

Unstable Alveoli?

Small alveoli are more unstable.

Surfactant:

• Lowers surface tension; lowers recoil force.

• Stabilize alveoli.

• Reduced capillary filtration forces.

• Deficiency = RDS; risk factors = premature birth and maternal diabetes.

• RDS = increased lung recoil, decreased lung compliance, atelectasis, pulmonary edema.

Resistant-volume curve:

• During inspiration, intrapleural pressure and airway resistance is decreasing.

• More negative intrapleural pressure = lower airway resistance.

Vital capacity:

• Maximum air a person can move in a single breath.

• FEV₁ = maximum air moved in 1 sec = ~ 80 % of VC.

Obstructive disease:

• More time required to exhale a smaller volume of air.

• Lung volume ~ same as normal, maybe increased.

• Eg., emphysema.

Restrictive disease:

• Increased recoil.

• Less time required to exhale a really small volume of air.

• Lung volume less than normal.

• Eg., fibrosis.

Flow-volume loops:

• Used to differentiate between obstructive and restrictive diseases.

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