Respiratory Pressure

RESPIRATORY PRESSURE

Q. 1

Transpulmonary pressure is the difference between:

 A

The bronchus and atmospheric pressure

 B

Pressure in alveoli and intrapleural pressure

 C

Atmosphere and intrapleural pressure

 D

Atmosphere and intraalveolar pressure

Q. 1

Transpulmonary pressure is the difference between:

 A

The bronchus and atmospheric pressure

 B

Pressure in alveoli and intrapleural pressure

 C

Atmosphere and intrapleural pressure

 D

Atmosphere and intraalveolar pressure

Ans. B

Explanation:

Transpulmonary pressure is the pressure difference between alveolar pressure and intrapleural pressure. Before the start of inspiration or at the end of expiration it is about +5cm H2O. Positive transpulmonary pressure keeps the alveoli open.

  • Intrapleural pressure is the pressure between two layers of pleura. It is about -5cm H2O before the start of inspiration or at the end of expiration.
  • Alveolar pressure is the pressure within the terminal air spaces. It is the sum of pleural pressure and elastic recoil pressure of the lung. It is atmospheric before the start of inspiration or at the end of expiration.
  • Transthoracic pressure is the pressure difference between alveolar pressure and pressure at the body surface.

Ref: Fundamentals of Respiratory Physiology By A S Chakrabarty, Page 32


Q. 2

The intrapleural pressure is negative both during inspiration and expiration because:

 A

Intrapulmonary pressure is always negative

 B

Thoracic cage and lungs are elastic structure

 C

Transpulmonary pressure determines the negativity

 D

All

Q. 2

The intrapleural pressure is negative both during inspiration and expiration because:

 A

Intrapulmonary pressure is always negative

 B

Thoracic cage and lungs are elastic structure

 C

Transpulmonary pressure determines the negativity

 D

All

Ans. B

Explanation:

B i.e. Thoracic cage and lungs are elastic structures


Q. 3

Normal Intrapleural Pressure is Negative because

 A

The chestwall and lungs recoil in opposite directions

 B

The surfactant prevents pulmonary collapse

 C

Intrapulmonary Pressure is Negative

 D

Tranplmonary Pressure is Negative

Q. 3

Normal Intrapleural Pressure is Negative because

 A

The chestwall and lungs recoil in opposite directions

 B

The surfactant prevents pulmonary collapse

 C

Intrapulmonary Pressure is Negative

 D

Tranplmonary Pressure is Negative

Ans. A

Explanation:

A i.e. The chest wall and lungs recoil in opposite directions


Q. 4

Negative intrapleural pressure is due to:

 A

Lymphatic drainage of pleura

 B

Uniform distribution of surfactant over alveoli prevents the lungs to collapse

 C

Negative intraalveolar pressure

 D

Presence of cartilage in upper airway

Q. 4

Negative intrapleural pressure is due to:

 A

Lymphatic drainage of pleura

 B

Uniform distribution of surfactant over alveoli prevents the lungs to collapse

 C

Negative intraalveolar pressure

 D

Presence of cartilage in upper airway

Ans. A

Explanation:

A i.e. Lymphatic drainage of pleura

–  Normal negative intrapleural pressure is maintained by opposing elastic recoil forces of chest wall and lungQ (ie the expansile recoil tendency of chest wall and retractile recoil tendency of lung) along with efficient drainage of excessive intrapleural fluid by lymphatic pump and osmotic forces across pleural membrane maintaining a slight suction and only a thin layer of serous fluid between parietal pleural surface of thoracic cavity and visceral pleural surface of lung.

– The basic cause of negative pressure in most tissue spaces of body (including intrapleural cavity) is lymphatic drainage (i.e. pumping of fluid from space by lymphatics)Q.

Intra pleural pressure is negative during the normal breathing cycle (i.e. both during inspiration and expiration) because of elasticity of lung and chest wallQ.


Q. 5

Respiration stops in the last stage of expiration, in forced expiration b/c of:

 A

Respiratory muscle fatigue

 B

Collapse of alveoli

 C

Dynamic compression of airways

 D

Breaking effect of inspiratory muscles

Q. 5

Respiration stops in the last stage of expiration, in forced expiration b/c of:

 A

Respiratory muscle fatigue

 B

Collapse of alveoli

 C

Dynamic compression of airways

 D

Breaking effect of inspiratory muscles

Ans. C

Explanation:

C i.e. Dynamic compression of airways

–                  Increased Airway resistance (Raw) is caused by low (decreased) lung volume such as during forced expirationQ because as the lungs compress, the airways also compress. Airway resistance is also increased if inspired air is more dense or viscous and flow is turbulentQ

Decreased airway resistance is caused by high (increased) lung volume eg during inspiration because expanding lungs exert a traction on airways (trachea bronchial system) causing them to dilate (thereby decreasing resistance). Airway resistance is also decreased if inspired air is less dense or viscous and flow is laminarQ.

–                  Expiatory flow is effort independent and flow limited. Airway resistance is greater during exhalation than during inspiration because of dynamic compression of airwaysQ which stops last stage of forced expiration. Expiration in quiet breathing is passive and requires no muscle activity. Lung recoil pulls the chest back and the airway pressure becomes slightly positive.

Forced Expiration, Equal Pressure Point & Dynamic Airway Compression

– Alveolar pressure (PA) is the sum of leural pressure (Pm) and elastic recoil pressure (Pei).  • and this is the driving pressure for expiratory gas flow. Because alveolar pressure exceeds atmospheric pressure (during expiration), gas begins to flow from alveolus to mouth, through open glottis. As gas flows out of alveoli the transmural pressure across the air way (Pia) decreases. In otherwords, there is a gradual decrease in airway pressure (Paw) from distal (alveoli) to proximal (trachea) respiratory tract. This gradual pressure dissipation is caused by

  1. Expiratory airflow resistance (frictional pressure loss); the major site of resistance along bronchial tree is large bronchi (Berne & Levy) / medium sized bronchi (John West).

ii.As the overall cross sectional area of the airways decreases towards the trachea, gas velocity increases. This acceleration of gas flow decreases the pressure (Paw) and can make flow turbulent (increasing resistance).

iii.As air moves out of lung, the lung volume decreases; which inturn decreases the driving pressure (alveolar pressure – intrapleural pressure) and the airways become narrower (increasing resistance).

– The positive transpulmonary (PL) and transairwa ressure (Pt.)  hold the alveoli and airway open. Because PL = PA – PP and  Pia = Pa, – PPI I; it also means that alveolar pressure (PA) or

 

 

 

– Just beyond the (proximal to) equal pressure point the transairway pressure (Pta) becomes negative. Because Pta = Pressure in the airway expanding it (Paw) – Pressure around

the airway compressing it (i.e. pleural pressure Po)

That is why no amount of expiratory effort will increase the flow further because the higher pleural pressure (which rises with increased expiratory effort) also tends to collapse the airway at the equal pressure point (EPP), just as it tends to increase the gradient for expiratory gas flow (by increasing the pressure gradient b/w alveoli and atmosphere).

– That is how dynamic airway compression limits air flow in normal subjects during a forced expiration and the airflow is independent of total driving pressure. In other words during dynamic compression, flow is determined by transpulmonary pressure (PL = PA — Pri) or alveolar pressure minus pleural pressure (not mouth pressure). Hence the expiratory flow is effort independent, flow limited and has greater airway resistance (than during inspiration).

– Equal pressure point is dynamic (not static). In normal lungs (without disease), the EPP occurs in airways that contain cartilage and thus they resist collapse. As expiration progresses (i.e. lung volume and elastic recoil pressure decreases), the EPP moves distally, deeper into the lung, closer to the alveoli. This occurs because the resistance of airways rises (d/t decrease in radius) as the lung volume falls, and therefore the pressure within airways fall more rapidly with distance from the alveoli. This resistive drop in airway pressure is greater in diseased lung (with airway obstruction secondary to mucus accumulation and inflammation). As a result EPP occurs in small airways that are devoid of cartilage causing them to collapse (premature airway closure). The major site of increased resistance in patients with COPD is in airways 2mm dia.

– Dynamic airway compression may occur in diseased lung at relatively low expiratory flow rates, thus reducing exercise ability. DCA is exaggerated in emphysema because of reduced lung elastic recoil and loss of radial traction on airways.

 

Examples

For example at the start of expiration, pressure inside alveolus (PA) is 0 (i.e. no air flow) and pleural pressure (Po) is – 30 cm H20. So transpulmonary pressure (PL) of + 30 cm (Pi. = PA – PPi = 0 – (-30) = + 30) of water is holding the alveoli open. Because there is no flow , the pressure inside airways (Paw) is also 0 and similarly + 30 cm F120 of transairway pressure holds the airway open (Pt. = Paw – Ppi).

With the contraction of expratory muscles both pleural and alveolar pressure rises + 90 cm each and becomes + 60 cm H20 and + 90 cm 1-120 respectively. So the Pi. remains same but because of higher alveolar pressure (in comparison to atmosphere) airflow begins.

Because of gradual decrease in lung volume (decreasing driving pressure) and expiratory airflow resistance, there is gradual dissipation of airway pressure (Paw). And at equal pressure point (i.e. the point at which the pressure inside the airways equals the pressure outside the airways) the airways become compressed (dynamic airway compression).

Airway Resistance

Highest airway resistance is in medium sized/large bronchi. The smallest airways contribute very little to overall total resistance because in smaller airways

  1. With increase in effective cross sectional area the airflow velocity decreases substantially and flow becomes laminar and
  2. The airway exist in parallel rather than in series. The resistance of

Q. 6

Negative intrapleural pressure is due to ‑

 A

Uniform distribution of surfactant over alveoli

 B

Negative intraalveolar pressure

 C

Absorption by lymphatics

 D

Presence of cartilage in the upper airway

Q. 6

Negative intrapleural pressure is due to ‑

 A

Uniform distribution of surfactant over alveoli

 B

Negative intraalveolar pressure

 C

Absorption by lymphatics

 D

Presence of cartilage in the upper airway

Ans. C

Explanation:

Ans. is ‘c’ i.e., Absorption by lymphatics

  • The pleural pressure is negative, more negative during inspiration, less negative during expiration, but always negative during quiet breathing.

This is because both the thoracic cage and lungs are elastic structures; therefore, both tend to recoil, but in opposite direction. This creates negative intrapleural pressure.

“A negative force is always required on the outside of the lungs to keep the lungs expanded. This is provided by negative pressure in the normal pleural space. The basic cause of this negative pressure is pumping of fluid from the space by the lymphatics (which is also the basis of the negative pressure found in most tissue spaces of the body).”     – Guyton 12/e p 483

So, two important reasons of negative intrapleural pressure are :-

i) Elasticity of lungs and thoracic cage in opposite direction.

ii) Lymphatic drainage of pleural fluid.


Q. 7

Muller’s maneuver is ‑

 A

Forceful expiration against closed glottis

 B

Forceful inspiration against closed glottis

 C

Forceful expiration against open glottis

 D

Reverse of Valsalva’s maneuver

Q. 7

Muller’s maneuver is ‑

 A

Forceful expiration against closed glottis

 B

Forceful inspiration against closed glottis

 C

Forceful expiration against open glottis

 D

Reverse of Valsalva’s maneuver

Ans. D

Explanation:

Ans. is ‘d’ i.e., Reverse of Valsalva’s maneuver

Muller’s maneuvre : – A flexible endoscope is passed through the nose and the patient asked to inspire vigrously with nose and mouth completely closed. Look for collapse of the soft tissues at the level of base of tongue and just above the soft palate. Level of pharyngeal obstruction can be found.

In Valsalva’s maneuver the person expires vigrously with nose and mouth closed completely.

So, Muller’s maneuver is opposite to Valsalva’s maneuver.



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