Respiratory Physiology - Lung Volumes and Spirometry Flashcards
What is spirometry, and what parameters are measured?
Spirometry is a technique to measure the mechanical properties of the respiratory system by measuring flows and volumes
FEV1 - Volume of air forcibly exhaled in 1 second
FVC - Volume of air forcibly exhaled
PEFR - Peak expiratory flow rate
Forced expiratory flow 25-75 (The average flow during the middle 25-75% of FVC)
Patients can be given bronchodilators to assess reversibility, which can be useful in determining between asthma or COPD.
>12% increase in FEV1 is considered reversible
What is reversibility in context of spirometry?
Spirometry is a technique to measure the mechanical properties of the respiratory system by measuring flows and volumes
FEV1 - Volume of air forcibly exhaled in 1 second
FVC - Volume of air forcibly exhaled
PEFR - Peak expiratory flow rate
Forced expiratory flow 25-75 (The average flow during the middle 25-75% of FVC)
Patients can be given bronchodilators to assess reversibility, which can be useful in determining between asthma or COPD.
>12% increase in FEV1 is considered reversible
Explain different lung volumes and capacities
Clinical significance of Closing Volume
What is closing capacity vs closing volume
A capacity is the sum of two or more volumes
VT - Tidal Volume - Volume of gas exhaled during a normal breath at rest
RV - Residual Volume - Volume of gas remaining in the thorax after a full forced expiration
IRV - Inspiratory Reserve Volume - Volume of gas that can be inhaled in addition to a normal TV inhalation
IC - Inspiratory Capacity - (IRV+TV)
ERV Expiratory Reserve Volume - Volume of gas that can be exhaled in addition to a passive tidal exhalation
FRC - Functional residual capacity - The volume of gas in the lungs at end of tidal expiration.
FRC forms the reservoir for oxygen during apnoeic oxygenation.
Supine GA causes a profound decrease in FRC compared to the upright position, as the diaphragm is pushed upwards by the abdominal contents, and respiratory and spinal muscles relax, reducing the AP diameter of the thoracic cage
VC - Vital Capacity - Volume of gas between from full inspiration to full expiration (ERV+VT+IRV)
TLC - Total Lung Capacity - Volume of gas in the thorac at maximal inspiration (RV+VC)
Closing Volume - The volume of gas in the lungs at which the small airways will begin to close, as the balance between elastic recoil of the lung parenchyma, the chest wall and the resulting intrapleural pressure changes, meaning the pressure outside the small airways exceeds the airway pressure, causing the airway to collapse.
It is usually smaller than FRC in healthy young people - airways only begin to collapse in forced expiration below FRC.
Closing volume = FRC when standing at age 65-70
Closing volume = FRC when supine at age 40
Calculated with Nitrogen washout method
Draw and compare a normal vitalograph trace for a single breath with those of obstructive and restrictive pathologies
IMAGE x3
Normal:
Flow is maximal at the start, gradually decreasing in a negative hyperbolic pattern, to an asymptote at the FVC FEV1 is measured at 1 second, and should be 75%
Obstructive:
Flatter curve, with a an asymptote at a lower FVC
The FEV1 is markedly reduced as the rate of exhalation is significantly affected
Restrictive:
Negative hyperbole, similar to normal lungs
FVC is markedly reduced
FEV1 is normal or high, as expiration is less affected, especially early expiration
Draw a spirometry trace of normal tidal volume breathing, and then a maximal inspiratory and expiratory breath for a 75kg person
IMAGE
A capacity is the sum of two or more volumes
For a 75kg person:
TLC - 80ml/kg = 6000ml
Volume of lungs at maximal inspiration
IC = 3000ml (VT of 500ml+IRV of 2500ml)
FRC (37ml/kg) = 3000ml (ERV of 1500ml + RV of 1500ml)
VC (60-70ml/kg) = (ERV+TV+IRV)
VT - Tidal Volume - Volume of gas exhaled during a normal breath at rest
RV - Residual Volume - Volume of gas remaining in the thorax after a full forced expiration
IRV - Inspiratory Reserve Volume - Volume of gas that can be inhaled in addition to a normal TV inhalation
IC - Inspiratory Capacity - (IRV+TV)
ERV Expiratory Reserve Volume - Volume of gas that can be exhaled in addition to a passive tidal exhalation
FRC - Functional residual capacity - The volume of gas in the lungs at end of tidal expiration.
FRC forms the reservoir for oxygen during apnoeic oxygenation.
Supine GA causes a profound decrease in FRC compared to the upright position, as the diaphragm is pushed upwards by the abdominal contents, and respiratory and spinal muscles relax, reducing the AP diameter of the thoracic cage
VC - Vital Capacity - Volume of gas between from full inspiration to full expiration (ERV+VT+IRV)
TLC - Total Lung Capacity - Volume of gas in the thorac at maximal inspiration (RV+VC)
What Volumes cannot be measured using spirometry?
The residual volume, and any capacity including this volume
1. Total Lung Capacity
2. Functional Residual Capacity
RV - Residual Volume - Volume of gas remaining in the thorax after a full forced expiration (Approx 15-20ml/Kg)
How can the residual volume be measured?
Helium dilution doesn’t make sense
- Helium Dilution: A known quantity and concentration of inert gas is delivered to the patient to breathe while inside a box of known volume. The new concentration measured gives the new volume, and therefore the intrathoracic capacity.
- Body plethysmography: Patient sits in a box of known volume, with pressure measurement equipment in the airway and in the box. Boyle’s law states P1V1=P2V2. Thus if the pressure and volume of the box, as well as the pressure of the airway is known, we can calculate intrathoracic volume.
- Nitrogen washout: Patient breathes FiO2 of 1, while the FeN2 is measured until all N2 has been exhaled, giving the total quantity of N2 in the lungs.
Dividing this by the nitrogen concentration on the first exhalation will give the total volume of the thorax
What is lung compliance?
Change in volume per unit pressure. Measured in L/cmH2O.
Lung compliance depends on:
Elastance of lung tissue (Maximal at the FRC, changes as volume increases)
Surface tension in the alveoli
Lung compliance can be:
1. Static (when there is no gas flow, measuring alveolar ‘stretch’
1. Dynamic (during the respiratory cycle, reflects airway resistance during gas equilibration)
What is total thoracic compliance?
Change in volume per unit pressure. Measured in L/cmH2O
Total thoracic compliance includes lung compliance and chest wall compliance (each are usually 150-200ml/cmH2O)
Calculated as reciprocals:1/total thoracic compliance = 1/chest wall compliance + 1/lung compliance1/total thoracic compliacne = 1/200 + 1/200 = 1+1/2001/100 = 100ml/cmH2O
What is hysteresis?
IMAGE - and could do with image on toolkit also
There is Pressure-volume curve on Ventilation page that could be annotated to show this.
Hysteresis describes a parameter which differs depending on whether it is increasing or decreasing. The pressure/volume loop is an example.
When inflating, energy (pressure), is lost to covercome resistance (frictional loss)
Energy (pressure) is also lost due to stretch and recoil of elastic tissues (viscous loss)
Draw and explain a normal flow-volume loop for a vital capacity breath
IMAGE
For all flow-volume loops:
X axis in L, Y axis in L/sec.
Positive area of the graph is expiration, and the negative area is inspiration, with the loop moving in a clockwise direction.
TLC is on the leftmost point of the curve, and RV on the rightmost.
Expiration begins from TLC, with flow peaking when 1/3 of volume has been exhaled, decaying smoothly back to 0 at the end of expiration (RV). Inspiration is -ve on the Y axis, starting from RV, and returning to TLC with a much flatter curve than expiration.
The flow in peak expiration is limited by dynamic airway compression (secondary to intrathoracic pressure), and this cannot be exceeded even in active expiration.
Draw and explain a flow-volume loop for a vital capacity breath in an obstructive lung disease
IMAGE - Normal curve compared with abnormal
For all flow-volume loops:
X axis in L, Y axis in L/sec.
Positive area of the graph is expiration, and the negative area is inspiration, with the loop moving in a clockwise direction.
TLC is on the leftmost point of the curve, and RV on the rightmost.
Expiration begins from TLC, with flow peaking when 1/3 of volume has been exhaled, decaying sharply intially, then flattening out towards 0 at the end of expiration (RV). Inspiration is -ve on the Y axis, starting from RV, and returning to TLC with a much flatter curve than expiration, relatively unchanged from normal.
Both peak and average flows are reduced due to obstructive airways and reduced elastic recoil of lung tissue.
RV may be increased due to gas trapping, TLC may also increase slightly for the same reason
An example would be COPD
Draw and explain a flow-volume loop for a vital capacity breath in a restrictive lung disease
IMAGE - Normal curve compared with abnormal
For all flow-volume loops:
X axis in L, Y axis in L/sec.
Positive area of the graph is expiration, and the negative area is inspiration, with the loop moving in a clockwise direction.
TLC is on the leftmost point of the curve, and RV on the rightmost.
Expiration begins from TLC, with flow peaking when 1/2 of volume has been exhaled, decaying smoothly back to 0 at the end of expiration (RV). Inspiration is -ve on the Y axis, starting from RV, and returning to TLC with a much flatter curve than expiration, relatively unchanged from normal
Dramatically reduced TLC, and thus VC (the entire loop is narrowed). RV is relatively normal.
Flow rate reduced, but appears high compared to the reduced volumes
An example would be pulmonary fibrosis
Draw and explain a flow-volume loop for a vital capacity breath in a fixed large airway obstruction
IMAGE - Normal curve compared with abnormal
For all flow-volume loops:
X axis in L, Y axis in L/sec.
Positive area of the graph is expiration, and the negative area is inspiration, with the loop moving in a clockwise direction.
TLC is on the leftmost point of the curve, and RV on the rightmost.
Expiration begins from TLC, with flow peaking much lower than normal, when 1/2 of volume has been exhaled, decaying slowly back to 0 at the end of expiration (RV). Inspiration is -ve on the Y axis, starting from RV, and returning to TLC with a flatter curve than expiration, but less pronounced than normal.
Relatively normal RV and TLC, and therefore normal VC
Both peak and average flows are reduced according to the remaining airway diameter
An example would be tracheal stenosis
Draw and explain a flow-volume loop for a vital capacity breath in a variable intrathoracic obstruction
IMAGE - Normal curve compared with abnormal
Is my example correct?
For all flow-volume loops:
X axis in L, Y axis in L/sec.
Positive area of the graph is expiration, and the negative area is inspiration, with the loop moving in a clockwise direction.
TLC is on the leftmost point of the curve, and RV on the rightmost.
Expiration begins from TLC, with flow peaking when 1/3 of volume has been exhaled, decaying sharply intially, then flattening out towards 0 at the end of expiration (RV). Inspiration is -ve on the Y axis, starting from RV, and returning to TLC with a much flatter curve than expiration, appearing more peaked than normal.
Relatively normal RV and TLC, and therefore normal VC
Flow easier in inspiration, as negative intrathoracic pressure pulls open the small obstructed airways
Both peak and average flows are severely limited during expiration
An example would be thick secretions
