This chapter is most relevant to Section F4(ii) from the 2017 CICM Primary Syllabus, which expects the exam candidates to be able to "state the normal values of lung volumes and capacities". Ostensibly, other chapters serve the purposes of Section F4(i), "Explain the measurement of lung volumes and capacities and factors that influence them", but inevitably some of that creeps into this section. It has come up a couple of times in the primary exam:
Both of these questions asked specifically about the FRC, and how it is measured. Nobody seems to care about TLC ERV or IRV. Functional residual capacity, the popular show pony of lung volumes, seems to get all the attention while all the other lung volume compartments are trivialised by the colleges' inattention. Well, this chapter is an effort to return some dignity and respect to the forgotten volumes and capacities of the lung. Moreover, beyond listing the normal values and pretending that they represent real human measurements, one should probably have some understanding of why we care about these volumes and capacities, i.e. their importance in health and disease.
Normal Reference Values for Lung Volumes and Capacities Volume Absolute volume in ml/kg Definition RV (residual volume) 15 The volume of gas remaining in the lung after maximal exhalation ERV (expiratory reserve volume) 15 The volume of gas that can be maximally exhaled from the end-expiratory level during tidal breathing TV (tidal volume) 7 The volume of gas inhaled or exhaled during the respiratory cycle IRV (inspiratory reserve volume) 45 maximum volume of gas that can be inhaled from the end-inspiratory level during tidal breathing. IC (inspiratory capacity) 52 The maximum volume of gas that can be inspired from FRC FRC (functional residual capacity 30 The volume of gas present in the lung at end expiration during tidal breathing VC (vital capacity) 67 The volume change at the mouth between the positions of full inspiration and complete expiration. TLC (total lung capacity) 82 The volume of gas in the lungs after maximal inspiration, or the sum of all volume compartments
In terms of reading material, the most solid peer-reviewed resource for this topic would probably have to be Wanger et al (2005), which also covers the measurement of the volumes and capacities. It appears to be a statement by an ATS/ERS joint task force, outlining the recommendations of these societies; one cannot get any more official than that. For a discussion of the FRC all on its own, one may look to the article by Chandra et al (2013). For the rest, there is scattered information around textbooks and articles, but no single solid source to rely upon.
Static lung volumes and capacities
Quoting directly from the ATS/ERS statement, the definitions of the standard volumes are:
- The volume of gas inhaled or exhaled during the respiratory cycle is called the tidal volume (TV or VT).
- The expiratory reserve volume (ERV) is the volume of gas that can be maximally exhaled from the end-expiratory level during tidal breathing (i.e. from the FRC).
- The inspiratory reserve volume (IRV) is the maximum volume of gas that can be inhaled from the end-inspiratory level during tidal breathing.
- RV refers to the volume of gas remaining in the lung after maximal exhalation (regardless of the lung volume at which exhalation was started).
Those are the four volumes. There are also four capacities. Again, from the ATS/ERS statement, the definitions of the standard capacities are:
- The FRC is the volume of gas present in the lung at end expiration during tidal breathing.
- The maximum volume of gas that can be inspired from FRC is referred to as the inspiratory capacity (IC).
- The vital capacity (VC) is the volume change at the mouth between the positions of full inspiration and complete expiration.
- TLC refers to the volume of gas in the lungs after maximal inspiration, or the sum of all volume compartments.
Note that lung volumes are measurable gas-filled spaces in the lung, whereas capacities are combinations of two or more volumes (where the definition of capacity is the measure of the lungs' ability to hold a gas). It seems that we have come by these terms because a group of eleven senior physiologists got together in 1950 and engraved them indelibly into the psyche of subsequent researchers. The contents of their paper is unfortunately only available only to those who are willing to exhume a physical copy of Federation Proceedings (Volume 9) from the deepest catacombs of their university library, and without the original text it is impossible to determine how these authors defended their choice of terminology.
How it became so engrained is unclear, but it certainly seems carved in stone. By the time a later (1975) revision of lung function nomenclature came around, the authors treated the volume-capacity designation as something well established and beyond argument, whereas a lot of other terminologies were discarded, and a lot of new weird terminologies were invented. This same committee also took upon themselves to define chest findings such as "rhonchi" and "rhales", and not all of those choices were completely non-controversial. "The traditionally trained physician may be surprised, and in some cases appalled, by the recommendations in regard to descriptions of physical findings in the chest", wrote Burrows in his 1975 editorial. Treasured nomenclature such as "sibilant" or "amphoric" breath sounds and "tactile fremitus" were discarded as nonessential and confusing ("their loss will hardly be mourned by most first- and second year medical students" the author gloated) but a whole host of confusing and nonessential neologisms were promoted, like "hypobasemia" and "expiratory retard". In short, when one goes looking for an explanation of why something is called a volume whereas something else is a capacity, one rapidly develops the impression that there was no guiding light of reason to illuminate these decisions.
Reference values of lung volumes and capacities
The CICM trainee would surely be expected to be intimately familiar with this famous diagram which describes the compartments of lung volume.
There are some numbers there, quoted for the trainees because the syllabus document clearly expects them to "state the normal values of lung volumes and capacities". Those exact numbers, needless to say, are almost entirely imaginary, as it would be impossible to give any figure and expect it to be accurate even within a 10% error margin, considering the vast variety of human shapes and sizes. Often, authors resort to throwing some representative values into their textbook diagrams, because of the widespread belief that the use of numbers increases their credibility. For instance, where this appears in the 8th edition of Nunn's, the diagram footer actually makes reference to Dr John Francis Nunn himself, appearing to claim that their diagram accurately represents his personal respiratory performance in 1990 (p. 28, Figure 2.9). Irrespective of how well Dr Nunn's thoracic dimensions represent the approximate human average, it still seems like a weird thing to do. In the diagram above (and the table below) the values were appropriated from Kerry Brandis' The Physiology Viva, mainly because he offers them in terms of ml/kg, indexed to body size. An extra column was added from Garcia-Rio et al (2009) because it probably represents normal values for a 65-year-old male European professor of physiology.
|In ml, for 70kg adult, from Brandis||In ml, for over-65 male,
from Garcia-Rio et al (2009)
|RV||15||1050||1940 (by plethysmography)|
|FRC||30||2100||3140 (by plethysmography)|
Functional Residual Capacity (FRC)
Because of the college examiners' focus on this specific subdivision of the lung volume, it seems like something important to explore. The FRC is composed of ERV and RV, and represents the volume of gas left behind in the chest at the end of expiration during some sort of normal tidal breath. In an anaesthetised patient, one might say that this is the volume of intrathoracic gas measured when the apnoeic patient is disconnected from the ventilator and the alveolar pressure equilibrates with atmospheric pressure.
This volume represents the point at which elastic recoil of the lung (always tending to collapse) is in equilibrium with the elastic recoil of the chest (always tending to expand). This is explored well enough in the chapter on lung compliance, and here it will suffice to say that at FRC the positive pressure of the collapsing lung (5 cm H2O) is balanced with the negative pressure of the chest wall (-5 cm H2O) and so the net pressure is zero.
This gas volume is important physiologically:
- It keeps small airways open. At FRC, the small airways are kept splinted open by the tension of the surrounding lung tissue. If the FRC is reduced below the closing capacity, there will be gas trapping and atelectasis.
- It is representative of compliance. Any decrease in lung compliance (i.e. due to decreased chest wall compliance or due to decreased lung tissue compliance) causes a decrease in FRC (this is developed in greater detail in the chapter on the work of breathing and its components)
- It represents optimal compliance. At FRC, the pressure-volume curve which represents compliance is at its steepest, which means the work of breathing required to inflate the lung from FRC is at its minimum. In other words, ventilating tidal volumes which start and end at FRC is the most energy-efficient form of breathing
- It keeps a gas reserve between breaths. Breathing is an intermittent phenomenon, during two-thirds of which there is no fresh gas entering the chest. If there was no FRC (i.e. hypothetically if the lung collapsed completely during expiration) there would be no gas exchange and the pulmonary circulation would return deoxygenated blood to the left atrium for the majority of the respiratory cycle. This, clearly, is unsatisfactory from the standpoint of ongoing survival. Because some residual gas remains in the lung, gas exchange can carry on during the entire respiratory cycle. The most important implication of this, of course, is during induction of anaesthesia, where one's peri-intubation fiddling time is entirely dependent on the oxygen stores in the FRC.
- It keeps pulmonary vascular resistance at a minimum. The alveolar and extra-alveolar vessels change their resistance characteristics as lung volume changes. It makes sense: at small lung volumes everything is compressed, some of the lung is collapsed and so pulmonary vascular resistance is high because pulmonary arteries are narrowed. As the lung inflates to FRC, arteries can increase in diameter and the resistance decreases. As the lung inflates further, expanding alveoli compress small interalveolar vessels and increase pulmonary vascular resistance again. Ergo, FRC is where pulmonary vascular resistance is at its lowest, representing the bottom of the U-shaped PVR-volume curve first described by Simmons et al in 1961.
- Relationship between FRC and closing capacity influences the development of atelectasis and shunt, as discussed elsewhere.
The normal FRC volume is said to be approximately 30-35ml/kg, or 2100-2400 ml in an average sized person. It varies considerably depending on body size, and obviously changes according to changes in the mechanical properties of the respiratory system. In the event one is ever asked to describe the factors which influence the FRC, these numerous factors could be summarised in a tabulated format:
|Factors which increase FRC||Factors which decrease FRC|
|Factors which influence lung size|
|Increased height||Short stature|
|Male gender||Female gender|
|Age: ratio of FRC to total lung capacity increases, but absolute FRC remains stable
(Wahba et al, 1983)
|Factors which influence lung compliance|
|Increased compliance due to disease, eg. emphysema||Decreased lung compliance due to disease, eg. ARDS|
|Increased end-expiratory pressure, eg. PEEP or auto-PEEP||D|
|Factors which influence chest expansion and chest wall compliance|
|Open chest or mediastinum||Increased intraabdominal pressure:
pregnancy, ascites, abdominal surgery
|Decreased respiratory muscle tone, eg. anaesthesia/sedation|
|Upright position and prone position||Supine and head down position|
|Circumferential burns, chest binder devices (eg. post mastectomy)|
Residual volume (RV)
RV is the amount of gas left at the end of a forceful maximal expiration. To get the lung volume any smaller, one literally needs to squeeze the chest cavity, something actually performed on volunteers by Leith & Mead (1967). Though they unfairly classified subjects over the age of 40 as "old", something to which the rapidly aging author objects to, their results are probably still valid. They confirmed that the RV seems to increase over one's lifespan largely owing to dynamic factors, i.e. airway closure and the increase of the closing capacity, rather than changes in chest wall recoil or lung elasticity. Because the airways had closed, squeezing the chests of older subjects did not result in any additional gas flow (whereas an extra 40 cm H2O caused some air to flow out of the younger subjects). By the same mechanism, this volume increases in obstructive lung diseases such as COPD and asthma.
Apart of diseases which affect closing capacity, RV decreases with any disease that globally decreases all lung volumes, for example idiopathic pulmonary fibrosis and obesity. Interestingly, it appears that in morbidly obese patients, the decrease in RV is measurable, but remains within the normal range of values for height (Lofrese & Lappin, 2018).
Expiratory reserve volume (ERV)
The ERV is the volume of gas that can be maximally exhaled from the FRC volume. This is the volume which is probably the most variable among all the lung volumes, and accounts for most of the changes in FRC related to posture and body habitus. Craig (1960) was able to demonstrate a change of the ERV from 34.2% of VC in the sitting position to 18.4% of VC in the supine position. When FRC decreases in obesity, it is the ERV which is to blame, and there is some evidence that morbidly obese patients have essentially zero ERV, i.e. their tidal volume normally goes all the way down to their residual volume (see below). De Jong et al (2014) mention this as one of the factors which render pre-anaesthetic oxygenation less effective in the morbidly obese.
The tidal volume is defined as the volume of gas inhaled or exhaled during a respiratory cycle. This obviously varies considerably between individuals and therefore between published authors. Reported values vary between ~ 380ml (Tobin et al, 1983) to ~ 810ml (Sorli et al, 1978). The problem is also partly because of the fact that one relies on FRC as the lower margin for where the VT is calculated from, but this depends on the subject being relaxed and at rest, which is a state difficult to achieve when you're trapped in a body plethysmograph. Gilbert et al (1972) demonstrated that the influence of voluntary control on supposedly normal restful ventilation while being measured and monitored tends to increase the tidal volume and decrease the respiratory rate. Most ICU trainees, when asked "what is a normal tidal volume", will probably blurt out "6-8ml/kg", which will have originated from their having overheard somebody talking about lung-protective ARDS ventilation; and it will not be half wrong, falling into the range of 420-560ml for a 70kg person.
Inspiratory reserve volume
The inspiratory reserve volume is the volume you'd inhale if you kept inhaling after a normal tidal inhalation was completed. It obviously falls prey to the same problem of interpreting whether the tidal volume entrained by the laboratory volunteer is "normal" when they are nose-clipped and confined to a small pressurised box.
This is the volume most affected by diseases which decrease the range of movement available to the diaphragm and chest wall. In short, where the chest wall movement or diaphragmatic excursion are restricted, the IRV will be substantially diminished. For instance, this may be seen in severe kyphosis, in the presence of multiple rib fractures, or in the context of severe COPD where the diaphragm is flattened and the chest hyperinflated. In COPD, the IRV is sacrificed when the RV increases (O'Donnell et al, 2017); the investigators found that with disease progression the resting IRV can decrease from ~28% of TLC to about 17%. With dynamic hyperinflation due to tachypnoea causing the RV to increase yet further, in severe COPD the IRV can drop to as low as 5% of the TLC, which demonstrates spectacularly the literal "lack of reserve" such patients have. When they are in respiratory distress and their respiratory rate is in excess of 30, with maximal effort they can increase their tidal volume by perhaps a further 250-300ml.
Age-related changes in lung volumes
The total lung capacity remains relatively stable over the course of one's lifespan. The fraction of the TLC occupied by the FRC increases slightly, as does the RV. This graph is from Stocks & Quanjer (1995):
Size-related changes in lung volumes
Using the predictive equations presented by Stocks & Quanjer (1995), it is also possible to plot graphs relating height to pulmonary volumes and capacities. A clumsy spreadsheet chart derived from these equations is presented below. Note how the TLC increases substantially with increasing height, but the increase in FRC is proportionally smaller.
Effects of obesity on lung volumes
In summary, with obesity all lung volumes decrease, and the worse the obesity, the greater this effect. This was demonstrated abundantly by Jones et al (2006), whose excellent paper must be upheld as some sort of gold standard when it comes to visually presenting information on changes in lung volumes. Unfortunately, their detailed scatter plots spread the data over several graphs. In order to fuse these into one, the author had to take some liberties with their tabulated data (Table 1 on p.829) and construct a crude approximation in a spreadsheet. It was merely a matter of reconstructing absolute volume values from the percentage-of-predicted data. Voila:
Note how the decrease in capacities is almost entirely the result of a change in ERV. The scatter plot for that one is presented below, because it's worth isolating.
The reader is invited to carefully step over the obvious pun and observe the fat part of the 40+ BMI group data distribution. The majority of these people had no ERV whatsoever.