Physiology of The Respiratory System II: Lung Function Tests

The assessment of lung volumes is important in dignosing respiratory disease and monitoring progression. Spirometry is used to measure lung volumes. It is important to know the definition of each lung volume in order to be able to intepret spirometry findings and their relevance:

• Tidal Volume (TV): air breathed in and exhaled during quite respiration
• Inspiratory reserve volume (IRV): maximum volume of air that can be inspired on top of normal inspiration
• Expiratory reserve volume (ERV): maximum amount of air that can be forcefully expired after normal expiration
• Functional residual capacity (FRC): volume of gas left in the lungs after expiration during normal breathing. This can be determined using the helium dilution method. This involves the patient breathing normally from a spirometerfilled with a known volume of helium and air thus as they breath in and out, the helium is diluted into the air that is left in the lungs 

FRC = (initial helium concentration of spirometer) x Volume/(final helium concentration) 

• Residual volume (RV): volume remaining after maximal expiration. It can't be measured directly but is calculated as RV= FRC - ERV
• Total lung capacity (TLC): the sum of all volumes plus the residual volume
• Vital capacity (VC):volume of air expelled from maximal inspiration to maximal expiration

Dead space
The concept of dead space is important to grasp as this is the volume of air which does not take part in gas exchange. The are two types:

1. Anatomical: the volume of gas which does not mix with air in the alveoli. It can be determined using Fowler's method. This involves the patient breathing through a tube connected to a nitrogen analyser. The patient initial takes a single breath of pure oygen, holds their breath for several seconds and breathes out. This will determine deadspace as only the alveoli will have maximal concentrations of nitrogen whilst the higher up airways will have purer concentrations of oxygen as they did not participate in gas exchange. Thus if a curve is drawn, air initially expired will not have nitrogen as it is part of the anatomical deadspace whilst nitrogen concentrations will increase as alveolar air is expired.
2. Physiological: this is the volume of gas that reaches the alveoli but due to a lack of perfusion does not take part in gs echange. It can be determined using the Bohr equation

Volume of deadspace= Volume expired CO2(1-(Fraction of expired CO2/Fraction of alveolar CO2))

Alveolar Ventilation rate
This is the rate at which gas exchange occurs in the alveoli.
Alveolar ventilation rate= (TV-dead space) x Respiratory rate

Peak Expiratory Flow rate
This is a cheap and simple test that can be performed at the bedside. A patient is asked to take a maximal inspiration and then blow out as fast as possible into the peak flow meter. It is useful in assessing the severity of asthma attacks and monitoring treatment.

Closing Capacity
This is the volume of the lungs at which airways at the base of the lung start to close. It is normally 10% of vital capacity and can be assessed by getting the patient to breath a maximal inspiration of 100% O2 then expiring fully through a nitrogen analyser. A graph can be plotted which will show 4 phases:

1. Pure dead space is exhaled so its 100% oxygen
2. a mixture of deadspace and alveolar gas (increasing concentration of nitrogen)
3. pure alveolar gas (reaches a plateu)
4. abrupt increase in nitrogen as airways at the base of the lung close and therefore not participating in gas exchange so the expired air is coming from the apex which has received less oxygen thus the nitrogen is more concentrated.

 Factors that affect the closing capacity include increasing age, supine position and anaesthesia which increase it.

Diffusion Capacity
This tests the diffusion capacity of the alveolar membrane and pulmonary vasculature. It is measured by inhaling small amounts of carbon monoxide and measuring its levels in the blood.Diffusion capacity is most commonly reduced in pulmonary oedema (as diffusion distance is increased) and emphysema (causes loss of alveolar surface area).

Flow-Volume and Volume-Time Curves
These can be plotted using spirometry results and are important because certain pathological processes such as obstructive lung disease cause typical curves.

Well that's it for now from me. Watch out for the third and final respiratory physiology tutorial. By the way if there are any specific topics you'd like us to cover/discuss just leave a comment/send an email and we'll get on to it.

Amel