Pulmonary Physiology Topics

Question 1

Define Intrapleural pressure (PPL)

Answer 1

• Less than atmospheric pressure (negative)
• Helpful to think of Pip as the intrathoracic pressure—the pressure everywhere in the thorax except in the lumens of blood vessels, lymphatics, or airways.

Question 2

At rest, what is intrapleural pressure (PPL)? Inspiration?

Answer 2

At rest, intrapleural pressure (PPL) is near -5 cm H2O

-During inspiration, volume increases, Ppl decreases to near -8 cm H2O; Because of coupling between lungs and chest wall, lungs expand as thorax expands

Question 3

What happens to Alveolar Pressure as the thoracic cavity expands?

Answer 3

As thoracic cavity increase in size, alveolar pressure (PALV) – pressure within alveoli – decreases

• At rest, Patm = PALV= 0 cm H2O
• At end of inspiration, PALV now -1 cm H2O
• Due to increase in alveolar size

=>This causes air to enter the lungs!!!

Question 4

Define Transpulmonary pressure

Answer 4

•Ptp= PALV – Ppl= 5 cm H2O at rest (difference between alveolar pressure and pleural pressure at any point)

Question 5

What is the equation for minute ventilation

Answer 5

VE = VT x frequency

ex: Patient’s respiratory volume is 14 breaths per min (RR) and VT is 500 ml per breath. What is minute ventilation?14 breaths/min x 500 ml/breath = 7,000 ml/min or 7 L/min

Question 6

Physiologic Dead Space equation

Answer 6

=Vt x ((PaCO2 - PeCO2)/PaCO2)

Question 7

Examples of clinical scenarios of dead space:

Answer 7

• Lung disease such as emphysema will lead to trapping of air and decreased surface area available for diffusion (increased dead space)
• Ventilation/perfusion mismatch (increases)
• Mechanical ventilation – tubing increases dead space volume
• Hypoxia – decreases dead space due to bronchoconstriction and vasoconstriction
• Anesthesia – bronchodilation increases dead space volume.

Question 8

What is the equation for minute alveolar ventilation?

Answer 8

Subtracting dead space volume from tidal volume:
•VALV = VT – VDS
•500 ml – 150 ml = 350 ml

Minute alveolar ventilation is now alveolar ventilation x frequency
V(dot)ALV = VALV x f

Question 9

Define Hysteresis

Answer 9

Hysteresis is the term used to describe the difference between inspiratory and expiratory compliance. Lung volume at any given pressure during inhalation is less than the lung volume at any given pressure during exhalation.

Question 10

What happens to lung compliance during emphysema? Fibrosis? Age?

Answer 10

Emphysema - increases compliance as it destroys alveolar septal tissue that normally opposes lung expansion [harder to move air out – air trapping occurs]

Fibrosis - lowers compliance, more change in pressure required for change in volume; obesity too

Age - compliance increases with age as elasticity decreases with age due to the loss of elastin and increased collagen

Question 11

What is the concept of alveolar interdependence?

Answer 11

It is the concept that alveoli hold each other open; losing some will alter forces that normally counter collapse