Respiratory Pressure

• Is pressure within lung, especially within alveoli.
• Difference between alveolar pressure & atmospheric pressure – 
• Responsible for airflow into & out of lungs (alveoli).

VARIATIONS WITH RESPIRATORY PHASES:

• Usually, air flows from higher to lower pressure area.

1. Alveolar pressure is positive (more) in relation to atmospheric pressure –

• Air is expired.
• Alveolar pressure: +1 to +2 cm water at peak of expiration.

2. Alveolar pressure is negative (less) in relation to atmospheric pressure.

• Air is inspired.
• Alveolar pressure: -1 to -2 cm water at peak of inspiration. 

3. Alveolar pressure is zero (equal to atmospheric pressure) –

• Indicates airflow cessation at end of inspiration & expiration.

4. Muller’s maneuver:

• During “Forceful Inspiratory Effort” against a closed glottis producing intrapulmonary pressure as low as -80 mm Hg.

5. Valsalva’s maneuver:

• During “Forceful expiration” against a closed glottis producing an intrapulmonary pressure as much as +100 mm Hg.

INTRAPLEURAL/PLEURAL PRESSURE:

• Lung is covered with visceral pleura.
• Thoracic cage is lined on inside with parietal pleura.
• “Pleural cavity” – Very narrow space between two layers of pleura.
• Pressure between two pleural surfaces.

VARIATIONS WITH RESPIRATORY PHASES:

• During quiet breathing, intrapleural pressure fluctuates between -3.8 mm Hg (-5.0 cm water) to -6.0 mm Hg (-8.0 cm water) 
• Intrapleural pressure is negative during quiet expiration (-3.0 mm Hg).
• More negative (-6.00 mm Hg) during normal inspiration.
• Becomes positive only on forceful expiration.

REASON FOR NEGATIVE PLEURAL PRESSURE:

• Pleural pressure is more negative during inspiration & less negative during expiration.
• Yet, always negative during quiet breathing.

2 major reasons:

• Because both thoracic cage & lungs are elastic structures & both tend to recoil but in opposite direction.
• Lymphatic drainage of pleura.
• Maintains layer of pleural fluid very thin between parietal & visceral pleura.
• Thining of pleural fluid layer is necessary for negative intrapleural pressure as it resists separation of two pleural layers in opposite directions.

TRANSPULMONARY/TRANSMURAL PRESSURE (PT) ACROSS LUNGS:

• Difference between alveolar pressure (intra-alveolar pressure/PA) & pleural pressure (intrapleural pressure/PPL).
• I.e., Pressure difference between inside & immediately outside lung.
• Is a measure of “Elastic recoil pressure”.
• I.e., Elastic forces in lungs tending to collapse lungs at each instant of respiration.

Lungs tend to recoil inwards & require positive transpulmonary pressure.

• I.e., Relatively more alveolar or lesser pleural pressure, to increase its volume.
• Lung volume increases along with transpulmonary pressure (PT).

During quiet breathing –

• Transpulmonary pressure is always positive (more than atmospheric).
• Hence, lungs always have some expansion.

More expanded at end of inspiration:

• Alveolar pressure = Zero.
• Intrapleural pressure = (-8.0) H2O.
• Hence, Transpulmonary pressure = 0 – (- 8.0) = + 8 cm H2O.

Less expanded at end of expiration:

• Alveolar pressure is zero.
• Intrapleural pressure = (-5.0) H2O.
• Hence, Transpulmonary pressure = 0 – (- 5.0) = 5cm H2O.
• During quiet breathing, airways remain always open.
• Because transmural pressure remains always positive.
• During forced expiration, intrapleural pressure becomes strongly positive.
• Causing negative transmural pressure.
• This causes dynamic compression of airway.
• Because positive transmural pressure is necessary for airway patency maintenance.
• Airway dynamic compression causes expiratory flow limitation.
• I.e., Beyond a limit increased expiratory efforts do not produce further increase in flow.

FORCES MAINTAINING LUNG & AIRWAY PATENCY:

• Negative (subatmospheric) intrapleural pressure.
• Positive alveolar/airway pressure
• Thus, positive transmural pressure.

FORCES COLLAPSING (COMPRESSING) AIRWAY/ALVEOLI:

• Positive intrapleural pressure.
• Negative pressure in alveoli/airways.
• Hence, negative transmural pressure.