Respiratory Physiology Lecture 3 Part 14: Lung Compliance

Question 1

How is lung compliance measured?

Answer 1

presence or absence of airflow

Question 2

What is lung compliance?

Answer 2

Lung compliance, or pulmonary compliance, is a measure of the lung’s ability to stretch and expand (distensibility of elastic tissue).

• A measure of the elastic properties of the lungs and a measure of how easily the lungs can expand
• The magnitude of the change in lung volume (ΔV) produced by a given change in the transpulmonary pressure (ΔPTP)
• The slop measured in the pressure-volume curve

Question 3

In clinical practice what can you measure for lung compliance?

Answer 3

• static compliance
• dynamic compliance

Question 4

Static compliance of the lung

Answer 4

change in volume for any given applied pressure

• Represents lung compliance measured during periods of no gas flow, such as during an inspiratory/expiratory pause

Question 5

How is static compliance deterimined?

Answer 5

Static compliance of the lung is determined by the P/V slope when measured at FRC (That is, at the end of an expiratory event).

• Test done by having patient breathe in maximally and then let out a little bit of air at which LV and TP are measured and continues in intervals and build the curve

Question 6

Result of high compliance or low compliance

Answer 6

• High compliance: Associated with loss of alveolar tissue so less gas exchange available; floppy lungs so small change in pressure result in large changes in volume
• Low compliance: more collagen making it stiffer

Question 7

Dynamic Compliance

Answer 7

the compliance of the lung at any given time during actual movement of air.

• Represents pulmonary compliance during periods of gas flow, such as during an inspiration (When transpulmonary pressure continuously changes)
  
  • It reflects not only lung stiffness but also the airway resistance, against which distending forces have to act
• Is always less than or equal to static lung compliance
• Falls when either lung stiffness or airway resistance increases

Question 8

High compliance

Answer 8

High compliance indicates a pliable lung (one with low elastic recoil) and can be thought of as a grocery bag – this is the case often seen in emphysema.

Question 9

Low compliance

Answer 9

Low compliance indicates a stiff lung (one with high elastic recoil) and can be thought of as a thick balloon – this is the case often seen in fibrosis.

Question 10

Where is compliance highest and lowest

Answer 10

Compliance is highest at moderate lung volumes, and much lower at volumes which are very low or very high

Question 11

The compliance of the lungs demonstrates _______

Answer 11

lung hysteresis

Question 12

Hysteresis

Answer 12

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

• Defines the difference between the inflation and deflation compliance paths

Question 13

Purpose of hysteresis

Answer 13

It exists because a greater pressure difference is required to open a previously closed (Or narrowed) airway than to keep an open airway from closing

Question 14

Hysteresis in healthy person

Answer 14

Changes in pressure and lung volume of normal healthy person, or lung compliance, can see that changes in pressure and volume are minimal (orange circle)

• Normal breathing: pressure-volume loop has much less hysteresis

Question 15

Steps of the pressure-volume relationship and hysteresis

Answer 15

1. Stable V_L (lung volume)
2. Opening of airways
3. linear expansion of open airways
4. Limit of airway inflation

Question 16

1. stable V_L of the pressure-volume relationship & hysteresis

Answer 16

At low lung volumes, it is difficult to pop open an almost completely collapsed airway; rising P_TP (transpulmonary pressure) has little effect on V_L

• Collapsed lung: reinflate lungs by increasing transpulmonary pressure

Question 17

2. Opening of airways of the pressure-volume relationship and hysteresis

Answer 17

The first increases in V_L reflect the popping open of the proximal airways, followed by their expansion and recruitment of others

Question 18

3. Linear expansion of open airways of the pressure-volume relationship in hysteresis

Answer 18

When all the airways are open, making P_IP (intrapleural pressure) more negative by chest wall expansion inflates the lungs and increases VL in a linear 50 fashion

Question 19

4. Limit of airway inflation of the pressure-volume relationship & hysteresis

Answer 19

At high V_L lungs compliance decreases (stiffer)