0820 - Breathing Pressures and Airflow - RM Flashcards
Discuss different types of spirometers Outline how flows, volumes, and pressures are measured Illustrate the phases of the respiratory cycle and what determines them Define forced volumes and peak flows Recognise some important features in a flow-volume loop Explain the P-V work during respiration
What are the four different types of spirometry?
Bell Spirometer - Bell filled with air, immersed in water. Oldest and most accurate (direct volume measurement), and allow metabolic studies but cannot measure flow and very expensive.
Pneumotachograph - Modern and able to measure flow by passing air across a screen with known resistance. Based on Ohm’s law, is cheap, with a dynamic response. Need to be recalibrated regularly, and require a separate computer for calculations.
Body Plethysmograph - Contain whole body and used to measure residual lung volume and total lung capacity. Capable of measuring all respiratory pressures if intrapleural pressure is known.
Helium dilution can also measure RV and TLC via C1V1=C2V2 method.
How are flows measured during breathing?
PIF, PEF, FEV1, FVC
Flows (litres per second) are best measured by a pneumotachograph. Peak Inspiratory Flow is normally greater than peak expiratory flow, which is more sensitive to airway resistance. Forced Expiratory Volume in 1 second (FEV1) is a good test of airway resistance, and forced vital capacity (expire for 10 seconds) is generally greater than vital capacity.
How are respiratory volumes measured?
TV, IRV, ERV, RV (volumes measured directly)
IC, EC, VC, FRC, TLC (capacities of 2 or more volumes)
Volumes can be measured in several ways but are most accurately and easily measured by a bell spirometer if it is available. However, residual volume (and total lung capacity) must be measured either by a body plethysmograph or by a helium dilution method (C1V1=C2V2).
How are respiratory pressures measured/calculated?
Barometric pressure (taken as zero) Intrapleural pressure measured by oesophogeal pressure transducer.
Alveolar pressure measured by body plethysmograph. (Used to track flow)
Translung (transpulmonary) pressure = alveolar-intrapleural (used to track volume)
Transthoracic pressure = intrapleural-barometric
Respiratory System Pressure = alveolar-barometric =translung+transthoracic.
Explain the phases of the respiratory cycle
FRC (End-expiration) - No Flow. Alveolar Pressure is 0, and Intrapleural pressure (-5) and translung pressure (+5) cancel each other out.
Inspiration - Muscles contract, expanding the thoracic cavity and lowering intrapleural pressure and translung pressure (slight lag). This creates negative alveolar pressure, causing air to flow into lungs. Flow is determined by airway resistance and lung compliance.
End-Inspiration - No Flow. The thorax is no longer expanding, so intrapleural pressure and translung pressure once again cancel each other out (though more negative/larger magnitude - greater potential space with same ‘volume’).
Expiration - Muscles relax, reducing size of thoracic cavity. This raises intrapleural pressure (though it remains negative), and translung pressure, and leads to positive alveolar pressure, causing air to flow out of the lungs. Alveolar Pressure tracks air flow, translung pressure tracks volume.
Draw the key pressure, volume, and flow features of the respiratory cycle
Define Peak Expiratory Flow
The maximum flow rate achieved during expiration.
Define Peak Inspiratory Flow
The maximum flow rate achieved during inspiration.
Define FEV1
The volume of air expired during the first second of a forced expiration.
Define FVC
The volume of air expired during a period of forced expiration - peak inspiration to end-forced expiration. Will be greater than vital capacity due to acceleration (inertia).
What is at A-F on this diagram?
A - Total Lung Capacity
B - Peak Expiratory Flow Rate (should be within 20% of total volume)
C - Forced Expiratory Flow Rate at 25% of VC (FEF 25)
D - FEF 50 (switches from being effort-dependent to effort-independent expiration and flow is now determined by airway resistance and recoil (compliance))
E - FEF 75
F - Residual Volume (end-expiratory volume) (some books will reverse the X axis scale)
Explain P-V work during respiration (tidal and forced)
During TV breathing, all effort takes place during inspiration - you use energy both to inspire and to ‘load’ the thoracic cavity for elastic recoil (Total Work = Inspiratory Work + Elastic Work) - there is no additional work required during expiration (Total Work = Expiratory Work - Elastic Work).
In larger volumes (when you’re puffed), elastic recoil does not fully cover the expiratory work, and additional work from muscles is required (you need to actively breathe out).
Minimum total respiratory work takes place at RR of around 15.
