Lung Function Tests Flashcards
What is Spirometry?
Spirometry is the measurement of volume changes over time.
What is Tidal volume, Inspiratory reserve volume, expiratory reserve volume and residual volume?
Tidal volume – volume in and out with each breath
Inspiratory reserve volume – extra volume that can be breathed in over that at rest
Expiratory reserve volume – extra volume that can be breathed out over that at rest
Residual volume – volume remaining after maximal expiration – cannor be measured using spirometry.
What is vital capacity, inspiratory capacity, functional residual capacity and total lung capacity?
Capacities are two volumes added together
• Vital capacity – volume of maximum inspiration after full expiration equal to max inhalation + max expiration
• Inspiratory capacity – biggest breath that can be taken after quiet expiration equal to tidal volume + inspiratory reserve volume
• Functional residual capacity – volume of residual air in lungs at resting expiratory level equal to residual volume + expiratory reserve volume.
• Total lung capacity – volume of gas in the lungs at the end of maximal inspiration
What affects vital capacity?
Inspiration – Compliance of the lungs and the force of inspiratory muscles
Expiration – Airway resistance (increases as expiration proceeds)
What is single breath spirometry?
Single breath spirometry – patient fills their lungs, breathes out as far and as fully as possible, volumes measured by the detector over time measuring how much and how fast the air is breathed out.
What is a vitalograph trace and explain what parts of it are important and what they mean?
This plots volume expired vs time. Initial rapid rise tails to a plateau. This is the forced vital capacity which is the maximum volume that can be expired from full lungs.
FEV1.0 is the volume expired in first second. Affected by how quickly air flow slows down so decreases if airways are narrowed. Ratio between FEV1.0 and FVC is very informative. Normally >70% FVC.
What is an obstructive deficit?
If airways are narrowed – lungs will still fill easily, but resistance will increase in expiration. So air will come out more slowly and FEV1.0 will be reduced but FVC will be relatively normal. This is known as an obstructive deficit. If this is reversible with a bronchodilator, then it suggests asthma.
What is a restrictive deficit?
If the lungs are difficult to fill this could be due to them being stiff, having weak muscles or problems with the chest wall. As a result, there will be less to empty from the lungs so FVC will be less but FEV1.0 should still be > that 70% of the FVC. This is known as a restrictive deficit.
Describe the normal shape of a flow/volume expired graph
From the Vitalograph we can take measurement of flow against volume expired. To begin with flow is very high due to the lungs being at full capacity and all the airways being wide open, little air has been expired at this point. Once maximal flow rate is reached (Peak expiratory Flow rate PEFR) then flow will begin to decrease as more volume is expired and the resistance increases as the tubes get narrower.
What will chronic obstruction/restriction look like on a flow/volume graph?
If there is chronic obstruction in the lungs then the decrease in flow rate will be much faster but will still end up at the same volume at the end.
If there is a restriction the flow volume curve is narrowed, the shape is generally the same but flow greater than normal at comparative volumes after the PEFR.
Describe the helium dilution test
- Helium cannot transfer across the alveolar capillary membrane and is thus contained in the lungs.
- At the end of a normal tidal expiration (when the lung volume = FRC = ERV + RV) a person is connected to a circuit, which connected to a container containing a gas mixture with a known concentration of Helium (C1), container volume is (V1)
- The person continues to breathe into the container (a closed circuit) until equilibrium occurs - for a period of typically 4 to 7 minutes.
- The new concentration of He = (C2)
- C1xV1 = C2 xV2 (V2 = V1 + FRC)
- Since C1, V1, & C2 are all known, FRC can be calculated.
- Residual Volume = FRC minus Expiratory reserve volume (which is measured by spirometry)
What is the carbon monoxide transfer factor?
The Carbon Monoxide Transfer Factor measures the rate of transfer of CO from the alveoli to the blood in ml per minute per kPa (ml/min/kPa). It is a way of measuring the diffusion capacity of the lung, because the amount transferred will depend on how well gas diffusion takes place.
Inhaled CO is used because of its very high affinity for Hb. Since almost all the CO entering the blood binds to Hb, very little remains in plasma so we can assume plasma ppCO is zero.
Therefore, the concentration gradient between alveolar ppCO and capillary ppCO is maintained. As a result, the amount of CO transferred from alveoli to the blood is limited only by the diffusion capacity of the lung.
Describe how a Carbon monoxide transfer factor is done?
• The patient performs a full expiration, followed by a rapid maximum inspiration of a gas mixture composed of air, a tiny fraction of CO and a fraction of an inert gas such as helium.
o Tiny fraction of CO as it is toxic
o Fraction of inert gas to make an estimate of total lung volume
• The breath is held for 10 seconds.
• The patient exhales, and gas is collected mid-expiration, to gain an alveolar sample
• Concentration of CO and inert gas
• From these measurements, the Carbon Monoxide Transfer Factor is calculated
What is the Nitrogen washout test?
Serial (Anatomical) Dead Space is measured by the Nitrogen Washout Test.
How is a Nitrogen washout test done?
- The patient takes a maximum inspiration of 100% oxygen.
- The oxygen that reaches the alveoli will mix with alveolar air, and the resulting mix will contain Nitrogen (there is 79% Nitrogen in air)
- However, the air in the conducting airways (dead space) will still be filled with pure oxygen.
- The person exhales through a one-way vale that measures the percentage of Nitrogen in and volume of air expired
- Nitrogen concentration is initially zero as the patient exhales the dead space oxygen.
- As alveolar air begins to move out and mix with dead space air, nitrogen concentration gradually climbs, until it reaches a plateau where only alveolar gas is being expired
- A graph can be drawn to determine the dead space, plotting Nitrogen % against Expired Volume. Line drawn so area A = area B
