1.2.2 Mechanics of Ventilation Flashcards
What is ventilation?
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
What is alveolo-capillary diffusion?
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
What is blood gas transport?
convective transport of the gases by blood. O2 entering blood combines with Hb, CO2 is mainly transformed into bicarbonate.
What is capillary-cell diffusion?
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.
The lungs tend to want to do what? What the chest wall tends to want to do what?
The lungs tend to recoil inward and the chest wall tends to expand outwards.
What keeps the thorax from separating from the lungs?
The pleural fluid
As a result of the chest and lungs wanting to pull away from each other there is a generation of what?
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)
What is PB?
Atmospheric (barometric) pressure
Describe the basic flow of air into the lungs.
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).
For air to flow in to the lungs what must be true in regards to PB and PALV?
For air to flow in, PB must be higher than PALV.
For air to flow out of the lungs what must be true in regards to PB and PALV?
For air to flow out, PALV must be higher than PB.
What happens to PALV during inspiration and expiration in regards to its relationship to PB?
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.
Explain this image and what is occurring at each of the steps.
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.
How would a respiratory cycle graph look for forced inspiration and expiration compared to normal?
Roughly the same, except for greater amplitudes and slopes of the lines.
Compare what occurs in the airways during inspiration vs expiration.
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.
For air to move in and out of the lung what are the two things that need to occur?
- 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.
- airflow is generated along the airways. Pressure is needed to overcome the resistance opposed by the airways to the flow of air
What is occurring in figures B and C vs A and how could this create problems with airflow?
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.
What two factors determine PTP?
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 PTP. For any given lung (i.e. a given compliance), the greater the lung volume, the greater PTP
Air flow is determined by?
A pressure difference between PB and PALV. When PB> PALV, air flows in; when PALV>PV, air flows out.
PB-PALV depends on what?
- the magnitude of the flow
- the resistance to flow presented by the airways.
The higher the flow, or the higher the resistance, the greater the difference PB-PALV.
What two factors make up the flow dependent component? Volume dependent component?
How do these relate to the equation PB-PPL?
Flow dependent component: PB-PALV
Volume dependent component: PALV-PPL
The formula on the attached image shows the relationship of the flow and volume dependent component to PB-PPL
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.
What will occur when a person holds their breathe during inspiration?
Think in regards to PALV, PB, and PPL.
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
How would an inspiration-expiration graph look for a lung with decreased compliance compared to a normal?
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.
How would an inspiration-expiration graph look for a lung with increased airway resistance compared to a normal one?
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.
What is the formula for pulmonary compliance?
What is the pulmonary compliance for this person?
What is the process of measuring pulmonary compliance in a patient?
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.
What does the slope of this graph represent?
The slope of the curve, change in volume divided by change in transpulmonary pressure, is the compliance of the lung.
Where is compliance of the lung lowest? Highest
Lowest - At high lung volumes near total lung capacity
Highest - at lung volumes near functional residual capacity (FRC)
Do the lungs ever fully empty out?
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.
What is hysteresis?
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
How will changes in compliance affect the pressure-volume curve?
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).
How do elastic fibers contribute to the elastic properties of the lung?
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.
How can destruction of elastic fibers affect the elasticity of the lungs?
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.
How do surface forces in the alveoli contribute to the elastic properties of the lung?
The liquid-air interface in the alveolus helps maintain the elastic properties.
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.
What is Laplace’s Law and what does it dictate?
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.
According to Laplace’s law how do required pressures compare to keep a smaller alveolus inflated vs a larger one?
The smaller bubble requires a greater pressure than the large one to stay inflated.
How will air move in this image?
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.
How can expiration affect surface tension and Laplace’s Law?
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.
What is the role of surfactant?
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.
What can be the three results of insufficient surfactant?
- 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.
- 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.
- 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.
