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". Specifically, the focus here will be on the Functional residual capacity (FRC) because a) it is important physiologically, and b) because the college examiners seem to love asking questions about it. Of the seven or so historical CICM Part One questions on lung volumes, four SAQs discussed the FRC and its measurement. These were:
Therefore, the exam-wise candidate will have a detailed knowledge of the FRC, and to hell with ERV IC and TLC. This is a sensible approach because no other lung volume has such far-reaching influence.
- The FRC is :
- The volume of gas present in the lung at end expiration during tidal breathing
- Composed of ERV and RV
- This is usually 30-35 ml/kg, or 2100-2400ml in a normal sized person
- It represents the point where elastic recoil force of the lung is in equilibrium with the elastic recoil of the chest wall, i.e. where the alveolar pressure equilibrates with atmospheric pressure.
- The measurement of FRC is an important starting point for the measurement of other lung volumes
- The FRC is important because:
- At FRC, the small airway resistance is low.
- At FRC, lung compliance is maximal
- FRC maintains a oxygen reserve which maintais oxygenation between breaths
- At FRC, pulmonary vascular resistance is minimal
- Where closing capacity is greater than the FRC, gas trapping and atelectasis can develop
- The FRC is affected by:
- Factors which influence lung size (height and gender)
- Factors which influence lung and chest wall compliance (emphysema, ARDS, PEEP or auto-PEEP , open chest, increased intraabdominal pressure, pregnancy, obesity, anaesthesia and paralysis)
- Posture (FRC is lower in the supine position)
- If the FRC decreases (say, by 1000ml) the consequences are:
- Decreased lung compliance
- Increased airway resistance
- Increased work of breathing
- Decreased tidal volume and increased respiratory rate
- Decreased oxygen reserves
- Increased atelectasis
- Increased shunt
- Increased pulmonary vascular resistance
- Increased right ventricular afterload
For this sort of topic, one would be best served by a resource which does away with pointless frills and addresses the main point quickly and ideally in some sort of memorable point-form fashion. Hopkins & Sharma (2019) fits some of this description. There is little else out there; nobody has ever published an ode to the FRC for us to refer to. Multiple sources had to be scraped together and remixed to fashion this chapter.
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.
There's a reason this volume was not called called PRC (pointless residual capacity). These specific functions were called upon in Question 2 from the second paper of 2020, for 70% of the marks (i.e. these were the bulk of the question). In short, this gas volume is very important physiologically:
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||Negative end-expiratory pressure|
|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)|
Question 8 from the first paper of 2017 and Question 15 from the second paper of 2010 both asked about what might happen if the FRC decreases by 1000ml. Being able to answer such a question relies on the trainee's ability to know what the FRC does, and extrapolating what might happen if it stops doing that.
Effects of decreased FRC on lung mechanics
Effect of decreased FRC on gas exchange
Effects of decreased FRC on the pulmonary circulation