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Human Physiology · Respiratory system

Lung Volumes, Hypoxia and Respiratory Failure

Lung function tests turn breathing into numbers that guide diagnosis and treatment. This page explains lung volumes and spirometry, how to recognise obstructive and restrictive patterns, the causes and effects of hypoxia, and the two types of respiratory failure.

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Part 4 · Respiratory System · Chapter 20 of 47

Learning objectives

By the end of this page you should be able to:

  • Define the lung volumes and capacities and give typical values.
  • Describe how spirometry is performed and checked for quality.
  • Distinguish obstructive and restrictive patterns on spirometry and flow–volume loops.
  • Classify the types of hypoxia.
  • Distinguish type 1 and type 2 respiratory failure.
  • Summarise the evidence for non-invasive ventilation in hypercapnic COPD.

Why it matters

Physiotherapists read lung function results in almost every respiratory referral, and many perform spirometry or peak flow themselves. Knowing whether a patient’s problem is obstructive or restrictive shapes airway clearance, breathing exercises and exercise prescription.

Recognising respiratory failure, and which type it is, tells you how urgent the situation is and what support the patient needs.

Lung volumes and capacities

Figure 1 · Lung volumes and capacities

Spirometer trace with the four lung volumes and four capacities A trace of quiet breathing followed by a maximal breath in and a maximal breath out. Brackets mark tidal volume 0.5 litres, inspiratory reserve 3.1, expiratory reserve 1.2 and residual volume 1.2. Capacities are inspiratory capacity 3.6, functional residual capacity 2.4, vital capacity 4.8 and total lung capacity 6.0 litres. 0246Volume (L)IRV 3.1TV 0.5ERV 1.2RV 1.2IC 3.6FRC 2.4VC 4.8TLC 6.0VolumesCapacities
Volumes add up to capacities. Residual volume, and so FRC and TLC, cannot be measured by simple spirometry. Typical values for a young adult man.
MeasureDefinitionTypical adult value
Tidal volume (TV)Volume of a quiet breathAbout 0.5 L
Inspiratory reserve volume (IRV)Extra volume that can be inhaled after a quiet breath inAbout 3 L
Expiratory reserve volume (ERV)Extra volume that can be exhaled after a quiet breath outAbout 1.2 L
Residual volume (RV)Volume left after maximal expirationAbout 1.2 L
Functional residual capacity (FRC)ERV + RV; volume at end of quiet expirationAbout 2.4 L
Vital capacity (VC)IRV + TV + ERVAbout 4.8 L
Total lung capacity (TLC)VC + RVAbout 6 L

Values depend on height, age, sex and ancestry, so results are compared with reference equations rather than single numbers [2]. Lying down reduces FRC, because the abdominal contents push the diaphragm up. This is one reason patients desaturate when lying flat after surgery.

Spirometry

In forced spirometry, the patient breathes in fully and then blows out as hard and fast as possible until no more air comes out. The key results are forced vital capacity (FVC), the volume exhaled in the first second (FEV1) and their ratio.

The ATS/ERS technical statement sets standards for equipment, calibration, coaching and acceptable manoeuvres [1]. A poor effort or a slow start makes the result unreliable.

Figure 2 · Reading a spirogram

Forced expiratory volume-time curves for normal, obstructive and restrictive patterns Three curves of exhaled volume against time. The normal curve exhales most of 5 litres within one second. The obstructive curve rises slowly to 3.6 litres with only 1.6 in the first second. The restrictive curve rises quickly but reaches only 3 litres. 1 second0123456012345Time (s)Volume exhaled (L)FEV1FVCRatioNormal4.25.00.84Obstructive1.63.60.44Restrictive2.83.00.92Obstruction: ratio below thelower limit of normal [2]
Obstruction slows the breath; restriction shrinks it. The FEV1/FVC ratio separates the two. Illustrative values; interpret against reference equations [2].
PatternFEV1FVCFEV1/FVCExamples
ObstructiveReducedNormal or reducedBelow lower limit of normal [2]Asthma, COPD, bronchiectasis
RestrictiveReducedReducedNormal or highFibrosis, chest wall deformity, obesity, neuromuscular weakness
MixedReducedReducedReducedCOPD with fibrosis

The interpretive standard recommends defining abnormality by the lower limit of normal from reference equations, rather than fixed cut-offs such as 80% predicted [2]. A low FVC suggests restriction, but only measuring total lung capacity confirms it [2].

Figure 3 · Flow–volume loops

Flow-volume loops for normal, obstructive and restrictive patterns Three loops of flow against volume exhaled. The normal loop has a tall early peak and a straight descending limb. The obstructive loop has a low peak and a concave, scooped descending limb. The restrictive loop has a normal shape but is narrow. Inspiratory limbs are drawn below the line. Normal: tall peak, straight descentObstructive: low peak, scoopedRestrictive: narrowvolume exhaled →outinflow 0
Shape tells the story. The scooped expiratory limb is the signature of small-airway obstruction. Spirometry quality standards determine whether a loop is acceptable [1]. Schematic.

Practical skill · coaching a good blow

Seat the patient upright with a nose clip. Ask for a full breath in, a tight seal, then ‘blast’ out and keep going until told to stop. Repeat until results are repeatable, following the quality criteria [1]. Watch for coughing, early stopping or a slow start.

Hypoxia

Hypoxia means inadequate oxygen at tissue level. Hypoxaemia means low oxygen in arterial blood. Hypoxia can occur without hypoxaemia, for example in anaemia or poor circulation.

TypeProblemPaO2Example
HypoxaemicLow arterial oxygenLowLung disease, altitude, hypoventilation
AnaemicToo little working haemoglobinNormalAnaemia, carbon monoxide
Circulatory (stagnant)Too little blood flowNormalHeart failure, shock, arterial disease
HistotoxicCells cannot use oxygenNormalCyanide poisoning

Signs of hypoxia include breathlessness, a fast heart rate, confusion and restlessness. Cyanosis, a blue tinge, appears late and is harder to see in darker skin and in anaemia.

Respiratory failure

Figure 4 · Two types of respiratory failure

Comparison of type 1 and type 2 respiratory failure Two panels. Type 1 failure has low arterial oxygen with normal or low carbon dioxide and is caused by gas exchange problems such as pneumonia. Type 2 failure has low oxygen with raised carbon dioxide and is caused by inadequate ventilation, as in COPD exacerbations or neuromuscular weakness. Type 1: hypoxaemicPaO2 below 60 mmHg (8 kPa)PaCO2 normal or lowProblem: gas exchangeV/Q mismatch, shunt, diffusione.g. pneumonia, pulmonary oedema,ARDS, pulmonary embolusTreat: oxygen, recruit lung, treat causeType 2: hypercapnicPaO2 below 60 mmHg (8 kPa)PaCO2 above 45 mmHg (6 kPa)Problem: ventilation (the pump)too little alveolar ventilatione.g. COPD exacerbation, neuromuscularweakness, opioids, chest wall diseaseTreat: support ventilation (NIV), treat cause
Ask whether the lung or the pump has failed. Carbon dioxide tells you: it rises only when alveolar ventilation is inadequate.

Respiratory failure is present when the lungs cannot keep arterial gases within safe limits. Type 1 failure is a gas exchange problem: oxygen is low but carbon dioxide is normal or low. Type 2 failure is a pump problem: alveolar ventilation is too low, so carbon dioxide rises as well.

In type 2 failure, rising carbon dioxide makes the blood acidic. In the Cochrane review, acute hypercapnic failure was defined by pH below 7.35 and PaCO2 above 45 mmHg [3].

Non-invasive ventilation

Non-invasive ventilation (NIV) delivers positive pressure through a mask, supporting each breath and increasing alveolar ventilation. Osadnik and colleagues pooled 17 trials in acute hypercapnic failure due to COPD [3].

Figure 5 · Ventilatory support saves lives in hypercapnic COPD

Effects of non-invasive ventilation in acute hypercapnic respiratory failure due to COPD Four results. Hospital mortality was 46 per cent lower, with 12 patients treated to save one. Intubation was 65 per cent lower, with five treated to avoid one. Hospital stay was 3.4 days shorter. Intolerance was more common with non-invasive ventilation. Death in hospital46% lower (RR 0.54)treat 12 to save 1Need for intubation65% lower (RR 0.36)treat 5 to avoid 1Hospital stay3.4 days shorterIntolerance of treatmentmore common with NIVNon-invasive ventilation plus usual care versus usual care; 17 trials, 1,264 patients [3]
Support the pump. Non-invasive ventilation is a first-line treatment for acute hypercapnic failure in COPD [3].

NIV reduced hospital mortality by 46% and the need for intubation by 65% [3]. Benefits were similar on wards and in intensive care, and in milder and more severe acidosis [3]. Intolerance of the mask was more common, which is where good fitting, coaching and positioning matter.

Clinical relevance for the physiotherapist

  • Spirometry. Coach to the quality standard and interpret against limits of normal [1, 2].
  • Obstruction. Airway clearance, inhaler technique and breathing control are priorities.
  • Restriction. Low volumes call for deep breathing, positioning and, where muscle weakness is the cause, cough support.
  • Type 2 failure. Recognise rising CO2: drowsiness, headache, flapping tremor; seek urgent review.
  • NIV. Help with mask fit, positioning and secretion clearance to improve tolerance [3].

Assessment and application

FindingInterpretationAction
FEV1/FVC below lower limit of normalObstructive pattern [2]Assess airway clearance and bronchodilator use
Low FVC, normal ratio, weak coughPossible restriction from muscle weaknessMeasure cough strength; consider lung volume tests [2]
PaO2 52 mmHg, PaCO2 34 mmHgType 1 failureOxygen to target; positioning to improve V/Q
PaO2 55 mmHg, PaCO2 65 mmHg, pH 7.28Acute type 2 failureUrgent medical review; NIV is likely [3]

Common mistakes

  • Accepting a poor-quality spirometry effort [1].
  • Using a fixed 70% or 80% cut-off for everyone, which mislabels older and younger people [2].
  • Diagnosing restriction from a low FVC alone [2].
  • Confusing hypoxia with hypoxaemia.
  • Treating type 2 failure with oxygen alone and ignoring ventilation [3].

Exam preparation

Long answer

Define lung volumes and capacities. Describe spirometry and its use in distinguishing obstructive and restrictive lung disease.

  • Lung volumes and capacities with values (Figure 1).
  • Spirometry technique and quality [1].
  • Volume–time and flow–volume patterns (Figures 2 and 3).
  • Interpretation using lower limits of normal [2].
  • Clinical examples.

Short notes

  • Functional residual capacity.
  • Types of hypoxia.
  • Respiratory failure.
  • Non-invasive ventilation [3].

Viva and practical

  • “Why can’t spirometry measure RV?” — it remains in the lung [1].
  • “How do you define obstruction?” — FEV1/FVC below the lower limit of normal [2].
  • “Type 1 or type 2?” — look at the CO2.
  • OSCE: perform and interpret spirometry [1, 2].

Current evidence

Exam and traditional teaching

Obstruction is defined as FEV1/FVC below 70%, and results under 80% predicted are called abnormal.

What the evidence adds

Current standards use lower limits of normal from reference equations, taking account of variability [2], with strict quality criteria for testing [1].

NIV reduces mortality and intubation in acute hypercapnic COPD [3].

What the student should understand

Good numbers need good testing and sound interpretation. In failure, ask whether the lung or the pump has failed, and support the one that has.

Check yourself

10 questions on this page. Tap one to see the answer and the reasoning.

Q1. Which lung volume cannot be measured by simple spirometry?
  1. (A) Tidal volume
  2. (B) Vital capacity
  3. (C) Residual volume
  4. (D) Inspiratory reserve volume

Answer: (C) Residual volume needs gas dilution or plethysmography; spirometry measures exhaled volumes only [1].

Q2. The 2019 ATS/ERS spirometry statement was written mainly to:
  1. (A) Replace spirometry with imaging
  2. (B) Improve the accuracy, precision and quality of spirometry
  3. (C) Abolish reference values
  4. (D) Remove the need for trained operators

Answer: (B) It set standards to increase accuracy, precision and quality of measurements [1].

Q3. The current ERS/ATS interpretive standard recommends classifying results as abnormal using:
  1. (A) A fixed 80% predicted cut-off
  2. (B) The lower limit of normal from reference equations
  3. (C) The patient’s age alone
  4. (D) Clinical impression only

Answer: (B) Results should be compared with limits of normal from a healthy reference population [2].

Q4. An FEV1/FVC ratio below the lower limit of normal indicates:
  1. (A) Restriction
  2. (B) Obstruction
  3. (C) Normal lungs
  4. (D) Muscle weakness only

Answer: (B) A reduced ratio defines an obstructive pattern under the interpretive standard [2].

Q5. A restrictive pattern on spirometry shows:
  1. (A) Low FEV1/FVC ratio
  2. (B) Normal or high ratio with low FVC
  3. (C) High FVC
  4. (D) Scooped flow–volume loop

Answer: (B) Restriction suggested by spirometry should be confirmed with lung volume measurement [2].

Q6. Type 2 respiratory failure is defined by low PaO2 plus:
  1. (A) Low PaCO2
  2. (B) PaCO2 above about 45 mmHg (6 kPa)
  3. (C) Normal pH always
  4. (D) High haemoglobin

Answer: (B) Hypercapnic failure in the Cochrane review was defined by pH < 7.35 and PaCO2 > 45 mmHg [3].

Q7. In acute hypercapnic failure due to COPD, non-invasive ventilation reduced hospital mortality by about:
  1. (A) 5%
  2. (B) 46%
  3. (C) 90%
  4. (D) It increased mortality

Answer: (B) Risk ratio 0.54, a 46% reduction [3].

Q8. The number needed to treat with NIV to avoid one intubation was about:
  1. (A) 5
  2. (B) 50
  3. (C) 500
  4. (D) 1

Answer: (A) NNTB for intubation was 5 [3].

Q9. Which outcome was worse with NIV?
  1. (A) Mortality
  2. (B) Intubation
  3. (C) Treatment intolerance
  4. (D) Length of stay

Answer: (C) Intolerance was more common with NIV [3].

Q10. Spirometry results should be interpreted:
  1. (A) As exact values without variability
  2. (B) Alongside clinical information and test variability
  3. (C) Only by computer
  4. (D) Without reference to age, sex or height

Answer: (B) Interpretation must consider biological variability and clinical context [2].

Quick review

Key takeaways

  • TLC about 6 L, VC 4.8, FRC 2.4, RV 1.2, TV 0.5.
  • Spirometry gives FVC, FEV1 and their ratio; quality matters [1].
  • Obstruction: low ratio; restriction: low FVC with normal ratio, confirmed by TLC [2].
  • Hypoxia types: hypoxaemic, anaemic, circulatory, histotoxic.
  • Type 1 failure: low O2; type 2: low O2 and high CO2.
  • NIV cuts mortality and intubation in hypercapnic COPD [3].

Further reading

SourceWhy it earns its place
Graham BL et al. — Am J Respir Crit Care Med, 2019How to do spirometry properly [1]
Stanojevic S et al. — Eur Respir J, 2022How to interpret lung function [2]
Osadnik CR et al. — Cochrane Review, 2017NIV in hypercapnic COPD [3]
West JB — Pulmonary Pathophysiology: The EssentialsDisease patterns explained

References

  1. Graham BL, Steenbruggen I, Miller MR, et al. Standardization of spirometry 2019 update. An official American Thoracic Society and European Respiratory Society technical statement. American Journal of Respiratory and Critical Care Medicine. 2019;200(8):e70–e88. doi:10.1164/rccm.201908-1590ST PMID 31613151 Technical statement
  2. Stanojevic S, Kaminsky DA, Miller MR, et al. ERS/ATS technical standard on interpretive strategies for routine lung function tests. European Respiratory Journal. 2022;60(1):2101499. doi:10.1183/13993003.01499-2021 PMID 34949706 Technical standard
  3. Osadnik CR, Tee VS, Carson-Chahhoud KV, Picot J, Wedzicha JA, Smith BJ. Non-invasive ventilation for the management of acute hypercapnic respiratory failure due to exacerbation of chronic obstructive pulmonary disease. Cochrane Database of Systematic Reviews. 2017;7(7):CD004104. doi:10.1002/14651858.CD004104.pub4 PMID 28702957 Cochrane review

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