Pressures, flows, and work of breathing Flashcards
List the relevant flows, pressures and volumes and describe how they are measured
Inspiration
- typically the chest wall and lungs and brought together to form a single unit
- in inspiration, these layers separate
- as a result, intrapleural pressure becomes more negative, as the visceral layer and lung expand, following chest wall expansion
- this trend towards increased negativity can be seen on the graph
- during this time, alveoli becomes more negative
- however as air enters, the alveolar pressure approaches atmospheric pressure as it becomes more open and continuous with the air, and air is sucked in so that pressure equalises
- at this point, when pressures equilibrate, there is no more movement of air (net?)
In expiration:
- the parietal pleura approaches the visceral pleura
- as a result, the intrapleural pressure is less negative
- the lung decreases in size
- positive pressure within the alveoli pushes air out, brings back alveolar pressure to bormal
- at this point, when pressures equilibrate, there is no more movement of air
Flow and Ohm’s Law:
- Flow is the amount of a substance passing a point per unit time
- It can be thought of as volume over time
- Flow within the body is analogous to flow in an electrical current and is governed by Ohm’s law: Resistance = Potential/Current
- In the body, this becomes Flow = Pressure difference/Resistance
Note that the graph of air flow matches the alveolar pressure change graph. This illustrates that the flow is proportional to the pressure gradient in the alveoli, assuming resistance is unchanged.
Note also that the intrapleural pressure is negative or subatmospheric because - the tendency has a tendency to recoil and the chest wall has a tendency to expand.
Static Lung Volumes:
- Static volumes (no flow): only maximal values relevant
- Volumes cannot be broken down any further
- Capacities are greater than or equal to 2 volumes
Functional Residual Capacity (FRC):
- The amount of gas remaining in the lung after tidal expiration
- FRC is the point where the balance between the tendency of the chest wall to spring outwards and the tendency of the lungs to collapse inward is equal
- FRC is reached when in- and expiratory muscles are “relaxed”
- Functions of FRC: minimizes work of breathing (requires less inflaton and therefore less energy expenditure), pulmonary vascular resistance, V/Q mismatch (?), airway resistance, primary oxygen store, prevents atelectasis (failure of part of the lung to expand), prevents collapse, and provides buffer to maintain steady arterial pO2
- this last point is especially important
- without FRC there would be blood not supplied with oxygen during expiration
Measurement of Volume and Flow: Spirometer (Pneumotachograph):
- Apparatus measuring the volume of air expired by the lungs
- Mainly digital these days
- Detects change in pressure and calculates flow, given screen with known resistance
- Based on Ohm’s law: Q = flow = V/t, V = Q * t
- Can measure volumes expired but not residual volume or capacities including RV (e.g. FRC)
Measurement of Residual Volume/Functional Residual Capacity:
- Methods: Helium dilution, Nitrogen washout (These two methods use inhaled inert gases to detect c and v changes and calculate volume), Body plethysmography
List the phases of the respiratory cycle and what defines them
Pulmonary ventilation comprises two major steps: inspiration and expiration. Inspiration is the process that causes air to enter the lungs, and expiration is the process that causes air to leave the lungs (Figure 3). A respiratory cycle is one sequence of inspiration and expiration. In general, two muscle groups are used during normal inspiration: the diaphragm and the external intercostal muscles. Additional muscles can be used if a bigger breath is required. When the diaphragm contracts, it moves inferiorly toward the abdominal cavity, creating a larger thoracic cavity and more space for the lungs. Contraction of the external intercostal muscles moves the ribs upward and outward, causing the rib cage to expand, which increases the volume of the thoracic cavity. Due to the adhesive force of the pleural fluid, the expansion of the thoracic cavity forces the lungs to stretch and expand as well. This increase in volume leads to a decrease in intra-alveolar pressure, creating a pressure lower than atmospheric pressure. As a result, a pressure gradient is created that drives air into the lungs.
The process of normal expiration is passive, meaning that energy is not required to push air out of the lungs. Instead, the elasticity of the lung tissue causes the lung to recoil, as the diaphragm and intercostal muscles relax following inspiration. In turn, the thoracic cavity and lungs decrease in volume, causing an increase in interpulmonary pressure. The interpulmonary pressure rises above atmospheric pressure, creating a pressure gradient that causes air to leave the lungs.
There are different types, or modes, of breathing that require a slightly different process to allow inspiration and expiration. Quiet breathing, also known as eupnea, is a mode of breathing that occurs at rest and does not require the cognitive thought of the individual. During quiet breathing, the diaphragm and external intercostals must contract.
A deep breath, called diaphragmatic breathing, requires the diaphragm to contract. As the diaphragm relaxes, air passively leaves the lungs. A shallow breath, called costal breathing, requires contraction of the intercostal muscles. As the intercostal muscles relax, air passively leaves the lungs.
In contrast, forced breathing, also known as hyperpnea, is a mode of breathing that can occur during exercise or actions that require the active manipulation of breathing, such as singing. During forced breathing, inspiration and expiration both occur due to muscle contractions. In addition to the contraction of the diaphragm and intercostal muscles, other accessory muscles must also contract. During forced inspiration, muscles of the neck, including the scalenes, contract and lift the thoracic wall, increasing lung volume. During forced expiration, accessory muscles of the abdomen, including the obliques, contract, forcing abdominal organs upward against the diaphragm. This helps to push the diaphragm further into the thorax, pushing more air out. In addition, accessory muscles (primarily the internal intercostals) help to compress the rib cage, which also reduces the volume of the thoracic cavity.
ripped off lumen learning
List forced volumes and peak flows,and explain their clinical significance
Forced Volumes & Peak Flow:
- Dynamic volumes important for evaluating airways resistance (RAW)
- Best out of 3 trials ^[in other words, if body can, that’s the value]
- Peak flows: PEF (more sensitive to RAW, early in expiration, typically fast) and PIF (mid-inspiration)
- FEV1 (expired in 1 s, 80% of total expired) and FVC (more dynamic), FEV1 good test of RAW
- note that in obstructive disease, FEV1 takes longer
- to avoid obstruction
- note also that in obstructive disease, naturally positive pressure that exerts compression on airways exacerbates condition (wheeze sounds?)
- Note: FVC > VC (acceleration). Not clinically significant
Discuss the flow-volume loop and describe the important features
Flow Volume Relationship:
- Flow-volume loops provide graphical representation of patient’s respiratory effort and help classify disease
- Maximal efforts required for indicative curves (training)
- Positive flow: expiration, negative flow: inspiration
- Information from FV loops: FVC, TLC, PEF, MEF, MIF
- Clinically more important to understand the value of numbers in different diseases than the numbers themselves
- FEF25 i.e. 25% of TLC expired
- note again PEF is early
- note MEF is very sensitive to OLD due to obstruction in small airways (?)
Explain the pressure-volume work during respiration
Work of Breathing (PV Curve):
- Work = Force x Distance
- In the respiratory system, Work = Pressure x Volume
- Three main types of work of breathing:
- Inspiratory - Elastic and non-elastic work
- Expiratory work
Elastic work:
- 65% of work
- compliance work, to overcome recoil of the lungs
- work against elastic forces stored as potential energy which is used during expiration
- any factor that decreases compliance will increase elastic work
Non-elastic work
- remainder: 35% of work
- work to overcome airway resistance
- non-elastic work lost as heat
- any factor that increases resistance will increase non-elastic work
Expiratory work
- This is the work required to overcome airway resistance during expiration
- it is the same as non-elastic work, in terms of energy requirements
- in normal breathing, this is easily accounted for by the potential energy stored during elastic work
- if airway resistance increases significantly through more energy will be required (recruiting expiratory muscles) and expiration becomes an active process
Note how graph changes in obstructive lung disesae.
Describe differences in obstructive and restrictive disease
Obstructive and Restrictive Disease:
- Obstructive disease: Small airways disease leads:
- to gas trapping
- increased resistance to airflow, increased RV and TLC (air trapped)
- decreased FVC and FEV1
- decreased FEV1/FVC ratio
- decreased MEF
- left shift of curve, indicating higher TLC, and slower downward slope (coving?)
- note PIF also decreased, though not a s dramatic as PEF
- Restrictive disease: Scarring within airways leads to:
- decreased lung compliance
- fall in TLC, FEV1, FVC
- normal or slightly increased FEV1/FVC ratio, in other words, largely unaffected
- right shift of curve, shrunken loop, but shape retained
Describe how work of respiration is optimised and changes with disease
- Resistive work increases with respiratory rate as flow increases
- Elastic work decreases with respiratory rate (time constant of recoil)
- Minimum of total work at ~ eupnoea (12 – 20 bpm)
- Increased elastic work (decreased compliance) shifts minimum total work to a higher rate, characterised by rapid, shallow breathing –“right shift” = restrictive disease
- Increased non-elastic work (increased resistance) shifts minimum total work to a lower rate – “left shift”, to avoid obstruction. Long expiration = obstructive disease
Is this curve indicative of obstructive or restrictive disease? Why?
Is this curve indicative of obstructive or restrictive disease? Why?
