Lung volumes & capacities

Why this matters to the anaesthetist

You must define every volume/capacity, state typical adult values, explain spirometry vs helium dilution/body box for RV, and reason how posture, anaesthesia, pregnancy, obesity and disease move FRC relative to closing capacity.[1]

One-liner: TV+IRV+ERV+RV make TLC; FRC = ERV+RV is the resting volume where elastic recoil of lung and chest wall balance; under anaesthesia FRC falls toward CC → airway closure and shunt. [1]

The four volumes

The four capacities (sums)

FRC — the star of anaesthesia

Definition: volume at end of quiet expiration; equilibrium between lung inward recoil and chest wall outward recoil. [1]

Why it matters: [1]

• O2 store during apnoea (with VC preoxygenation you fill FRC with O2).
• Keeps airways open if FRC > closing capacity.
• Falls with: supine posture, anaesthesia (especially GA + muscle relaxation), obesity, pregnancy, abdominal surgery, ascites, laparoscopic insufflation.
• Rises with: PEEP/CPAP, upright posture, some obstructive disease (air trapping). [1]

Closing capacity / closing volume

Closing capacity (CC) = closing volume + RV — volume at which dependent airways begin to close. [1]

CC rises with age and smoking; FRC falls when supine/anaesthetised → CC may exceed FRC → cyclic airway closure → absorption atelectasis, shunt, impaired gas exchange. [1]

Measurement methods

Obstructive pattern: ↑RV, ↑TLC possible, ↓FEV1/FVC. Restrictive: ↓TLC, ↓VC, FEV1/FVC normal/high. [1]

Minute ventilation and dead space links

VE = TV × RR. Alveolar ventilation VA = (TV − VD) × RR. Volumes set the stage for V/Q leaf content — mention anatomic VD ~2 mL/kg. [1]

Factors shifting volumes (exam table)

Apnoea oxygenation teaching

Preoxygenate to FETO2 > 0.9 fills FRC with O2 → prolongs safe apnoea. High-flow nasal oxygen can extend apnoea by providing ongoing O2 to FRC (THRIVE concepts). Desaturation faster in children/obese/pregnant/ill because O2 store small and VO2 high. [1]

Numbers board

• TV ~500 mL; RV ~1200 mL; FRC ~30 mL/kg; TLC ~6 L
• VC ≥15 mL/kg classic extubation/ventilatory reserve teaching threshold (context-dependent)
• FEV1/FVC <0.7 suggests obstruction (guideline nuances exist) [1]

[1] [1] [1] [1]

Graph scripts

Draw stacked volume bar and bracket capacities. [1]

Draw FRC vs CC with age — lines cross in older supine patients. [1]

Spirometry traces: obstructive scooped exhalation vs restrictive small volumes. [1]

Extended viva dialogue

Examiner: Define FRC and give two reasons it falls under anaesthesia. [1]

Candidate: FRC is the lung volume at end of quiet expiration, ERV plus RV. It falls when supine because the abdomen displaces the diaphragm, and under general anaesthesia with muscle relaxation the chest wall and diaphragm lose tone so the equilibrium volume drops. [1]

Examiner: Why does that cause hypoxia? [1]

Candidate: If FRC falls below closing capacity, dependent airways close, alveolar gas is absorbed, atelectasis and shunt increase, and the oxygen store for apnoea shrinks. [1]

Clinical synthesis: If you can draw the stack and reason FRC versus CC, you own lung volume vivas. [1]

Helium dilution principle

Subject breathes known He concentration in closed circuit; final He dilution calculates FRC (C1V1 = C2V2). Does not see gas in bullae poorly communicating — plethysmography includes trapped gas (Boyle’s law box). [1]

Preoxygenation maths intuition

FRC ~30 mL/kg O2-filled after denitrogenation; VO2 ~3 mL/kg/min → rough apnoea time scales with store/consumption, shortened by shunt, low FRC, high VO2 (child, pregnant, septic). [1]

Worked SAQ

SAQ: Define FRC and explain changes under general anaesthesia (7 marks)

FRC is the lung volume at end of quiet expiration, equal to ERV plus RV, set by balance of lung and chest wall recoil. Under general anaesthesia, especially with muscle relaxation and supine posture, FRC falls toward or below closing capacity, promoting airway closure, atelectasis, shunt and faster desaturation during apnoea. PEEP and head-up positioning help restore end-expiratory volume. [1]

Body plethysmography Boyle’s law

Patient pants against closed shutter; box and alveolar pressure–volume changes estimate intrathoracic gas volume including trapped gas: P1V1 = P2V2. Why TLC in emphysema is higher by box than by dilution. [1]

Dynamic hyperinflation link

In obstruction, incomplete emptying raises end-expiratory volume above relaxed FRC — intrinsic PEEP. Anaesthesia and PPV interact with this physiology in COPD. [1]

Extended viva add-on

Examiner: Why can’t spirometry measure FRC? [1]

Candidate: Spirometry only measures exhaled or inhaled volumes from the breathing path. Residual volume remains in the lung after maximal expiration and cannot be seen without gas dilution or plethysmography; FRC includes residual volume. [1]

Primary exam expansion — dense examiner pack

Definitions with spirometry pattern (commit)

RV and FRC need helium dilution, N2 washout, or body plethysmography — spirometry alone cannot measure them. [1]

FRC — the anaesthetic centre of gravity

FRC is volume at end of quiet expiration: balance of inward lung recoil and outward chest wall recoil. Oxygen store for apnoea; determines atelectasis propensity; alters airway resistance and compliance (volumes on steep part of curve if low). Factors reducing FRC: GA induction, supine/Trendelenburg, obesity, pregnancy, laparoscopy, abdominal compartment, ascites, thoracic banding, inadequate PEEP. [1]

Closing capacity and airway closure

Closing capacity (CC) = closing volume + RV. When FRC falls below CC, dependent airways close in normal tidal breathing → shunt-like effect, impaired gas exchange. CC rises with age; FRC falls when supine — elderly supine spontaneously may encroach; GA makes it worse. Why PEEP/CPAP and head-up help. [1]

Preoxygenation maths intuition

After denitrogenation, alveolar O2 fraction high; FRC store ≈ FRC × FAO2. Consumption VO2 ≈ 3 mL/kg/min. Apnoea time roughly store/VO2, shortened by low FRC (obesity, pregnancy), high VO2 (sepsis, pregnancy, child), airway obstruction, and inadequate denitrogenation. High-flow nasal oxygen / THRIVE extends safe apnoea by ongoing O2 delivery and some CO2 clearance — sister clinical topics. [1]

GA effects on volumes (classic list)

1. Loss of diaphragmatic/end-expiratory muscle tone → FRC ↓ ~20%. 2. Atelectasis in dependent lung within minutes on 100% O2. 3. Cephalad diaphragm in supine. 4. Blood volume shift. 5. Airway closure tendency ↑. Recruitment: PEEP, sigh, head-up, avoid prolonged pure O2 when safe, NM blockade carefully with ventilation strategy. [1]

Obstructive versus restrictive patterns

Flow–volume loops: scooped expiration obstructive; tall narrow restrictive — draw in viva. [1]

Dead space volumes link

Anatomical dead space ~2 mL/kg; apparatus dead space added by connectors. Alveolar ventilation VA = (TV − VD) × RR. Small TV high RR strategies may ventilate mostly dead space — EtCO2 and PaCO2 diverge with high VD/VT (shock, PE). [1]

Body plethysmography Boyle link

Patient pants against closed shutter; P1V1=P2V2 estimates intrathoracic gas volume including trapped gas — may exceed dilution FRC in obstruction (trapped gas detected). [1]

SAQ: FRC under general anaesthesia (7–8 marks)

Define FRC (1). Determinants recoil balance (1). List factors reducing FRC under GA and positioning (3). Consequences for oxygenation/atelectasis (2). Mitigation PEEP/position/FiO2 (1). [1]

Viva

Q: Why measure FRC not just VC? A: Gas exchange and apnoea store depend on end-expiratory volume including RV component. Q: Why do elderly desaturate faster when supine under GA? A: CC approaches/exceeds FRC; plus lower FRC from GA. Q: Can spirometry give RV? A: No — needs dilution/plethysmography methods. [1]

High-yield viva battery and numbers lock-in

Capacity arithmetic

• IC = TV + IRV
• FRC = ERV + RV
• VC = IRV + TV + ERV
• TLC = VC + RV = IC + FRC
• Minute ventilation = TV × RR
• Alveolar ventilation = (TV − VD) × RR [1]

FRC modifiers list (memorise 8)

GA, supine, Trendelenburg, obesity, pregnancy, laparoscopy/↑IAP, abdominal pain/binders, inadequate muscle tone/paralysis without PEEP, pulmonary fibrosis (low volumes overall), emphysema (high FRC/TLC — contrast). [1]

Preoxygenation and apnoea extension tools

Tight mask FiO2 1.0, end-tidal O2 >0.9 target teaching, head-up if possible, CPAP/NIV in obese, nasopharyngeal oxygen/THRIVE during apnoea, avoid premature suction desaturation cycles, careful RSI timing. [1]

Full viva dialogue (additional)

Examiner: Why does FRC fall under general anaesthesia? [1]

Candidate: Loss of end-expiratory diaphragmatic and chest wall muscle tone allows the balance of recoils to settle at a lower volume; the supine posture and cephalad diaphragm shift compound this, and absorption atelectasis develops especially with high FiO2, further reducing ventilated FRC. [1]

Examiner: Define closing capacity and its anaesthetic importance. [1]

Candidate: Closing capacity is the lung volume at which dependent airways begin to close during expiration. If FRC falls below closing capacity during tidal breathing, airways close, V/Q mismatch and shunt increase, and oxygenation falls — common in elderly supine patients under anaesthesia. [1]

Exam traps

• Claiming spirometry measures RV/FRC alone.
• Forgetting units on 30 mL/kg FRC estimate.
• Saying PEEP only 'opens alveoli' without FRC language.
• Mixing obstructive high TLC with restrictive low TLC patterns. [1]

Examiner synthesis paragraph

Lung volumes become anaesthetic the moment you treat FRC as the oxygen store and the atelectasis set-point rather than a spirometry curiosity. Under general anaesthesia FRC falls, closing capacity encroaches, dependent airways close, and shunt rises — faster still in the elderly, obese and pregnant. Preoxygenation fills FRC with oxygen; PEEP, head-up posture and recruitment defend it; apnoea time is store divided by consumption. Quote the capacity arithmetic, know that RV and FRC need dilution or plethysmography, and contrast obstructive high TLC with restrictive low TLC on a flow–volume loop. [1]

References

1. [1]Harris BR. The Impact of Patient Positioning on the Compromised Respiratory System Respir Care, 2026.PMID 42300995
2. [2]Bouvet L, et al. Effect of high-flow nasal preoxygenation on anesthetic preparation time for general anesthesia in emergency caesarean delivery: a prospective randomized cross-over simulation study Minerva Anestesiol, 2026.PMID 42206896
3. [3]Task Forces on Pulmonary Function Testing (CACP). [Expert consensus on optimizing the multidisciplinary clinical pathways and management models for pulmonary function testing] Zhonghua Jie He He Hu Xi Za Zhi, 2026.PMID 41912385
4. [4]Al-Qerem W, et al. Applicability of the Global Lung Initiative 2022 Reference Equations on a Sample of Healthy Adolescents in Jordan Children (Basel), 2026.PMID 42194139
5. [5]Tabbah K, et al. Spirometric Lung Function Among Smokers and Non-Smokers: A Cross-Sectional Study in University Students Int J Environ Res Public Health, 2026.PMID 42354240
6. [6]Chidini G, et al. Effects of Noninvasive Respiratory Support on Ventilation Distribution During Spontaneous Breathing Sedation in Preschool/School-Aged Children: An Electrical Impedance Tomography Study Paediatr Anaesth, 2025.PMID 40119601