1.2.2 Mechanics of Ventilation

Question 1

What is ventilation?

Answer 1

bulk movement of air into and out of the lungs. O2 is transported into the lungs because O2 is contained in the air that enters the lungs; CO2 moves out because CO2 is present in the air contained in the lungs. This is a convective transport mechanism

Question 2

What is alveolo-capillary diffusion?

Answer 2

transfer of individual molecules of O2 and CO2 across the alveolo-capillary membrane following their partial pressure gradientst. O2 and CO2 move in opposite directions because the gradients are in opposite directions. O2 is higher in alveolar air than in the blood entering the lungs, the opposite occurs with CO2. This is a diffusive transport mechanism

Question 3

What is blood gas transport?

Answer 3

convective transport of the gases by blood. O2 entering blood combines with Hb, CO2 is mainly transformed into bicarbonate.

Question 4

What is capillary-cell diffusion?

Answer 4

transfer of gases across the tissue capillary-cell membrane following their concentration gradients. CO2 is higher in the cells than in the blood entering the arterial side of the capillaries, O2 is higher in the blood than in the cells.

Question 5

The lungs tend to want to do what? What the chest wall tends to want to do what?

Answer 5

The lungs tend to recoil inward and the chest wall tends to expand outwards.

Question 6

What keeps the thorax from separating from the lungs?

Answer 6

The pleural fluid

Question 7

As a result of the chest and lungs wanting to pull away from each other there is a generation of what?

Answer 7

Subatmospheric (negative) pressure in the pleural space

This is transmitted throughout the thoracic structures that exist outside the lungs, for instance the mediastinum, or the large intrathoracic veins which have thin walls. This is the intrathoracic or pleural pressure (PPL)

Question 8

What is P_B?

Answer 8

Atmospheric (barometric) pressure

Question 9

Describe the basic flow of air into the lungs.

Answer 9

Air flows along a pressure gradient established between the airway opening, where the pressure is equal to atmospheric ( barometric pressure; PB), and the pressure at the other end of the airway, the alveolus (PALV).

Question 10

For air to flow in to the lungs what must be true in regards to P_B and P_ALV?

Answer 10

For air to flow in, PB must be higher than PALV.

Question 11

For air to flow out of the lungs what must be true in regards to PB and PALV?

Answer 11

For air to flow out, PALV must be higher than PB.

Question 12

What happens to P_ALV during inspiration and expiration in regards to its relationship to P_B?

Answer 12

Since PB is practically constant, it follows that during breathing the gradient changes as a result of changes in PALV: during inspiration PALV falls below atmospheric; during expiration , PALV rises above atmospheric.

Question 13

Explain this image and what is occurring at each of the steps.

Answer 13

During breathing at rest, the contraction of the diaphragm enlarges the thorax. As the lung is pulled outwards by the contraction of the diaphragm, its tendency to recoil increases, much like when a rubber balloon is inflated. As the volume of the lung increases, the tendency of the lung to recoil also increases, and PPL decreases further below atmospheric.

As the lung expands, PALV falls below PB; this pressure difference produces inspiratory flow. You can think of this as if, at the very first instant of inspiration, the same number of gas molecules are contained in a larger volume; this lowers the pressure of the gas, and makes more molecules flow in from the site of higher pressure (PB). As more gas molecules flow in, PALV tends to return towards PB. At the end of inspiration, PPL is -8 cm H2O, and PALV is equal to PB. The larger PTP at the end of inspiration (+8 cm H2O vs. + 5 m H2O at the beginning) is due to the larger lung volume: as with a rubber balloon, greater pressure differences across the balloon wall are needed to maintain greater volumes and oppose greater recoil tendencies.

Expiration at rest consists of relaxation of the diaphragm. The elasticity of the lung is now unopposed by the contraction of the diaphragm and results in a decrease in lung volume. As lung volume decreases, PALV increases above PB, and the air flows out.

Question 14

How would a respiratory cycle graph look for forced inspiration and expiration compared to normal?

Answer 14

Roughly the same, except for greater amplitudes and slopes of the lines.

Question 15

Compare what occurs in the airways during inspiration vs expiration.

Answer 15

During INSPIRATION, airways distend and resistance falls as the lungs get bigger, in contrast, during EXPIRATION, the airways narrow as the lung get smaller, thereby increasing the resistance. This tends to limit the increase in expiratory airflow during forced breathing.

Question 16

For air to move in and out of the lung what are the two things that need to occur?

Answer 16

1. the lungs and chest have to change in volume. Pressure needs to be applied to overcome the tendency of the lungs to recoil during inspiration, and of the chest to expand during expiration.
2. airflow is generated along the airways. Pressure is needed to overcome the resistance opposed by the airways to the flow of air

Question 17

What is occurring in figures B and C vs A and how could this create problems with airflow?

Answer 17

In order to inflate the balloon, we need to do two separate things: we need to create airflow, i.e. we need to overcome the resistance imposed by the pipe, and we need to stretch the balloon, i.e. we need to overcome the tendency of of the balloon to get smaller

The force applied in B is greater than in A because the pipe is narrower and the resistance it opposes to the flow of air is greater than in A. In C the pipe is of normal size, but the balloon is much stiffer and resists stretch to a greater degree than in A or B, accordingly, greater force than in A has to be applied.

These can manifest themselves as different diseases in patients, which can greatly affect lung function.

Question 18

What two factors determine P_TP?

Answer 18

The elastic characteristic of the lung (compliance)

Lung volume

The greater the tendency of the lung to recoil (low compliance, stiff lung) the greater the P_TP. For any given lung (i.e. a given compliance), the greater the lung volume, the greater P_TP

Question 19

Air flow is determined by?

Answer 19

A pressure difference between PB and PALV. When PB> PALV, air flows in; when PALV>PV, air flows out.

Question 20

P_B-P_ALV depends on what?

Answer 20

1. the magnitude of the flow
2. the resistance to flow presented by the airways.

The higher the flow, or the higher the resistance, the greater the difference PB-PALV.

Question 21

What two factors make up the flow dependent component? Volume dependent component?

How do these relate to the equation P_B-P_PL?

Answer 21

Flow dependent component: P_B-P_ALV

Volume dependent component: P_ALV-P_PL

The formula on the attached image shows the relationship of the flow and volume dependent component to P_B-P_PL

For instance, both at the beginning and at the end of inspiration, PB-PALV is zero, and there is no flow; at these points all of the difference PB-PALV is made up of the “volume-dependent” component; the value PB-PPL is greater at end inspiration because the lung volume is greater.

Question 22

What will occur when a person holds their breathe during inspiration?

Think in regards to PALV, PB, and PPL.

Answer 22

The person holds her breath for a moment without changing the lung volume. During that short period when airflow stops, PALV becomes equal to PB, i.e. the “airflow component” is now zero. All the difference PB-PPL at this moment is made up of the “volume component” and, since lung volume remains the same, PALV-PPL, i.e. the transpulmonary pressure, will also remain unchanged. This means that PPL will move upwards towards PB (blue arrow) . PPL will change by an amount equal the value of the flow component (PB-PALV) when airflow stopped.

In fact, the dotted blue line in the PPL tracing represents the value of PPL that would be necessary to maintain the corresponding lung volumes in the absence of airflow

Question 23

How would an inspiration-expiration graph look for a lung with decreased compliance compared to a normal?

Answer 23

In this case, in order to inflate the lung the same volume, a greater transpulmonary pressure must be applied. Since airway resistance is normal, the flow component of PB-PPL is normal, but the value of PTP or a given lung volume is greater than in the normal lung.

Question 24

How would an inspiration-expiration graph look for a lung with increased airway resistance compared to a normal one?

Answer 24

The flow component of PB-PPL, i.e. PB-PALV is now abnormally large, but, since compliance is normal, PALV-PPL , i.e. the volume component is also normal for any volume.

Question 25

What is the formula for pulmonary compliance?

Answer 25

Question 26

What is the pulmonary compliance for this person?

Answer 26

Question 27

What is the process of measuring pulmonary compliance in a patient?

Answer 27

Usually a balloon is placed in the lower third of the esophagus. The balloon is connected to a pressure transducer via a catheter. The changes in pressure inside the balloon give a good estimate of the changes in pressure pressure inside the thorax, i.e. PPL. Since pressure measurements are made at the moment that airflow has stopped, PALV is equal to PB, and PPL is equal (with opposite sign) to PTP. The patient inhales maximally (to total lung capacity, TLC) and PPL is measured. Then the patient exhales known volumes of air, and PPL is measured at each volume when flow has stopped. This is continued until the patient can not exhale more air (residual volume, RV). The total volume of air is computed and added to the volume remaining in the lung (RV) which is measured by a separate method.

Question 28

What does the slope of this graph represent?

Answer 28

The slope of the curve, change in volume divided by change in transpulmonary pressure, is the compliance of the lung.

Question 29

Where is compliance of the lung lowest? Highest

Answer 29

Lowest - At high lung volumes near total lung capacity Highest - at lung volumes near functional residual capacity (FRC)

Question 30

Do the lungs ever fully empty out?

Answer 30

Even at very high (positive) values of PPL lung volume does not reach zero; i.e. the lungs do not empty completely at maximal expiration. **The volume of air remaining in the lungs at the end of a maximal expiration is the residual volume**, which normally is ~ 20% of TLC.

Question 31

What is hysteresis?

Answer 31

The PV curve obtained when volume is increasing from RV to TLC lies to the right of the curve obtained when the lung is deflated

Question 32

How will changes in compliance affect the pressure-volume curve?

Answer 32

**When compliance increases, inspiration is easy but expiration is hard.** **When compliance decreases, inspiration is hard but expiration is easy.** An increase in lung compliance means that a given change in PTP will be accompanied by a larger change in lung volume than normal. Accordingly, when compliance is increased the PV curve is steeper and shifts to the left. Since its tendency to recoil is decreased, the lung distends easily. This means that for a given lung volume PTP is lower than normal, or, for a given PTP, the lungs are larger than normal. This also means that at any given volume PPL will be closer to PB than in the normal lung. Since the tendency of the lung to recoil is decreased, the lung/chest system adapts to a larger volume. To put it in different words, the tendency of the thorax to expand is now less opposed by the lungs, so FRC tends to increase and/or PPL at FRC tends to be closer to PB. A decrease in pulmonary compliance means that a given change in PTP will produce a smaller change in volume than normal; accordingly the PV curve will be flatter and shifted to the right of the normal curve. Since the lungs have a high recoil tendency, lung volume for a given PTP will be lower than normal. Expiration is not affected since the high recoil tendency actually aids expiration. Compliance may decrease in two main types of conditions: in acute cases, when there is a decrease in lung surfactant (see later). In chronic conditions, lung compliance may decrease because of deposition of collagen fibers in the pulmonary parenchyma (interstitial pneumonias).

Question 33

How do elastic fibers contribute to the elastic properties of the lung?

Answer 33

The presence of elastic fibers in the lung parenchyma. Like elastic rubber bands, these fibers resist stretch when pulled by an increase in lung volume during inspiration, and return to their original size when PTP comes back to its original value. These fibers, anchored between the airways, have a tethering effect and keep the airways open. The same factor that contributes to the elastic lung recoil, also contributes to maintain the airways open.

Question 34

How can destruction of elastic fibers affect the elasticity of the lungs?

Answer 34

Destruction of these fibers, such as in emphysema, leads to decreased elastic lung recoil and increased lung compliance. The lungs do not behave as elastic rubber balloons, but rather like cellophane bags, which are easy to inflate but do not recoil once you stop inflation. The airways become “fluffy” like cellophane cigar wrappers, they distend easily but also collapse easily.

Question 35

How do surface forces in the alveoli contribute to the elastic properties of the lung?

Answer 35

The liquid-air interface in the alveolus helps maintain the elastic properties. ## Footnote The molecules of the liquid phase attract one another more strongly than the molecules of the air phase. The molecules on the surface of the liquid, are attracted strongly by the liquid molecules underneath, but less strongly by the air molecules. This generates a tension in the bubble surface, the surface tension (ST), which tends to make the bubble smaller. To prevent the bubble from collapsing, an opposing pressure (P) that tends to enlarge the bubble, has to be applied. The liquid layer behaves as a small soap bubble that tends to retract due to the tension generated at the interface. The combined effect of these “bubbles in millions of alveoli adds to the recoil tendency generated by the elastic fibers.

Question 36

What is Laplace's Law and what does it dictate?

Answer 36

Laplace’s law dictates that the pressure necessary to maintain a liquid bubble inflated is **directly proportional to ST, and inversely proportional to the radius.** Surface tension (ST) is a property of the liquid. Different liquids have different ST. The liquid lining of the alveoli has a ST that is generally lower than most body fluids, and is not constant. We will see about this later. It is conceptually easy to see that a **bubble made of a liquid with a high ST will have a strong tendency to get smaller; accordingly, it would require a greater counterforce to maintain a given volume.** **Laplace’s law also says that, for a given ST, the pressure needed to keep the bubble inflated increases as the size of the bubble decreases.** This has consequences that will be discussed in the next slide.

Question 37

According to Laplace's law how do required pressures compare to keep a smaller alveolus inflated vs a larger one?

Answer 37

The smaller bubble requires a greater pressure than the large one to stay inflated.

Question 38

How will air move in this image?

Answer 38

Since the alveoli are connected, the smaller one will collapse into the greater one, since air will move from the site of greater pressure to that of lower pressure.

Question 39

How can expiration affect surface tension and Laplace's Law?

Answer 39

This will be exaggerated during a forced expiration. As alveolar volume decreases during a forced expiration, surface tension increases and intra-alveolar pressure has to increase correspondingly. If ST is too high, the pressure that needs to be generated may not be enough to prevent collapse of the smaller alveoli.

Question 40

What is the role of surfactant?

Answer 40

Surfactant is a detergent which tends to lower the surface tension of liquids as most detergents do. Surfactant molecules have a hydrophobic pole and a hydrophilic pole which is immersed in the alveolar fluid lining. Surfactant not only lowers ST in all alveoli, but, since the molecules are closer together in the smaller alveoli, **it tends to reduce ST proportionately more in smaller than in larger alveoli**. **It also promotes uniform expansion in all alveoli: as the alveolus increases in volume, the surfactant molecules spread and surface tension increases, thereby limiting excessive expansion.**

Question 41

What can be the three results of insufficient surfactant?

Answer 41

1. **Compliance of the lung decreases**, because the surface tension of the fluid lining contributes to the elastic recoil. If ST is high, elastic recoil is high, the lungs are stiff and difficult to inflate. 2. **Alveoli will tend to collapse in expiration**, with the smaller alveoli collapsing and inflating the larger ones. Areas of the lung without air are known as atelectasis, and are a major mechanism of impaired gas exchange. 3. **Pulmonary edema**. Stiffer lungs mean that for a given lung volume, PTP will be higher than normal (the PV curve of the lung is shifted to the right) PTP will increase to maintain a given volume. An increase in PTP means a more negative PPL at any given lung volume. This more negative PPL is transmitted throughout the thorax, and results in a more negative pressure in the interstitium surrounding the lung capillaries. This increases the filtration pressure of the capillaries and leads to extravasation of fluid, first into the interstitium, and eventually into the alveolar sacs.