Respiratory medicine · Patient guide
Supine vital capacity (VC), the sitting-versus-lying spirometry test that detects diaphragm weakness.
Supine VC compares the volume you can breathe sitting up with the volume you can breathe lying flat. A drop of more than 20% points to a weak diaphragm. It is the core respiratory test for ALS, myasthenia gravis, muscular dystrophy, and diaphragmatic paralysis.
Key facts
What supine vital capacity is, in six lines.
The essentials — what it measures, who runs it, and where it sits alongside MIP/MEP and SNIP.
- 01
Definition
The vital capacity sitting up is compared with the vital capacity lying flat. The difference between the two positions is the signal for a weak diaphragm.
- 02
15–20 minute spirometry test
A short, non-invasive spirometry protocol performed in clinic — sitting VC first, five minutes of rest supine, then supine VC.
- 03
Fall > 20% sitting-to-supine indicates diaphragm weakness
A drop in VC greater than 20% between sitting and lying flat is the accepted threshold for significant diaphragmatic dysfunction.
- 04
Complements MIP/MEP and SNIP
Sits alongside maximal inspiratory and expiratory pressures and sniff nasal inspiratory pressure — the four measures together characterise neuromuscular respiratory function.
- 05
Foundation of NIV timing
A key input into when to start non-invasive ventilation in neuromuscular disease — the postural fall predicts nocturnal hypoventilation.
- 06
Delivered by respiratory physiologist
Performed and quality-assured by a respiratory physiologist to ATS/ERS standard, with consultant respiratory interpretation.
How the test is done
From consultation to report — the seven steps, in order.
A respiratory physiologist performs the measurements to ATS/ERS standard, with consultant interpretation.
- 01
Before
Respiratory / neuromuscular consultation
A consultant respiratory or neurology-linked physician takes the history — bulbar symptoms, orthopnoea, prior FVC, disease stage.
- 02
Before
Sitting spirometry
Baseline seated vital capacity is recorded to ATS/ERS acceptability and repeatability criteria.
- 03
The test
5 min rest supine
You lie flat for five minutes to allow the diaphragm to work against the weight of the abdominal contents before the second measurement.
- 04
The test
Supine spirometry
Vital capacity is repeated lying flat, using the same technique and quality standards as the seated measurement.
- 05
After
% fall calculated
The postural fall in VC is calculated as a percentage of the sitting value — the primary diagnostic result.
- 06
After
Structured report
A written report with sitting VC, supine VC, % fall, MIP/MEP and SNIP correlation and a clear interpretation for you and your team.
- 07
After
Structured plan
If diaphragm weakness is confirmed, the same team arranges NIV assessment, cough augmentation review and structured neuromuscular follow-up.
What it shows
When supine VC is the right test.
The measurements the test produces, the conditions it identifies, and the red-flag finding that changes the plan.
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Sitting VC
Baseline upright vital capacity — the reference against which the supine fall is measured.
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Supine VC
Vital capacity lying flat — reveals ventilation loss when the diaphragm bears the weight of the abdomen.
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% fall (sitting → supine)
The postural drop expressed as a percentage — the primary result and diagnostic threshold.
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MIP/MEP correlation
Read alongside maximal inspiratory and expiratory pressures to characterise inspiratory and expiratory muscle strength.
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SNIP correlation
Sniff nasal inspiratory pressure — a diaphragm-specific test that supports and cross-checks the supine VC finding.
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Diaphragmatic paralysis
A very large postural fall — often 40% or more — is characteristic of unilateral or bilateral diaphragmatic paralysis.
-
Neuromuscular respiratory failure
ALS, myasthenia gravis, muscular dystrophy and Guillain-Barré all cause ventilatory failure detectable by the supine drop.
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Red flag: > 30% fall + FVC < 50% — urgent NIV
A postural fall above 30% with an FVC below 50% predicted warrants urgent non-invasive ventilation assessment.
Treatment options
What supine VC findings unlock in treatment.
The interventions a confirmed diaphragm weakness makes possible — from NIV to disease-modifying pathways.
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NIV / BiPAP
Bi-level non-invasive ventilation is the mainstay intervention for confirmed diaphragm weakness and nocturnal hypoventilation.
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Cough augmentation (MI-E)
Mechanical insufflation-exsufflation supports airway clearance when expiratory muscle strength is inadequate.
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Diaphragm pacing
A neuro-stimulator option for selected patients with intact phrenic nerves — most commonly in high spinal-cord injury and selected ALS cases.
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Respiratory physio
Specialist respiratory physiotherapy to preserve chest-wall compliance, teach breath-stacking and support secretion clearance.
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Vaccination
Influenza, pneumococcal and COVID-19 vaccination reduce the risk of respiratory decompensation in neuromuscular disease.
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Home mechanical ventilation
For advanced ventilatory failure — home NIV programme with structured equipment support and follow-up.
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Respiratory follow-up
Serial supine VC, MIP/MEP and SNIP at set intervals track disease trajectory and refine the timing of intervention.
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MDT review
Complex neuromuscular cases — ALS, DMD, post-polio, post-COVID — are discussed in a multidisciplinary meeting before major decisions.
Red flags
When supine VC moves from routine to urgent.
Findings and conditions that trigger urgent NIV assessment — not routine outpatient follow-up.
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> 30% VC fall + FVC < 50%
A postural fall above 30% combined with an FVC below 50% predicted mandates urgent NIV assessment — not routine outpatient review.
-
ALS with bulbar involvement
Amyotrophic lateral sclerosis with bulbar features has accelerated ventilatory decline — supine VC should be repeated at short intervals.
-
DMD with hypercapnia
Duchenne muscular dystrophy with rising CO₂ signals advanced ventilatory failure and mandates NIV initiation.
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Post-polio
Late-onset respiratory failure decades after polio — the supine drop can be the first quantitative signal of decline.
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Diaphragmatic paralysis
Unilateral or bilateral diaphragmatic paralysis produces very large postural falls and warrants urgent respiratory review.
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Myasthenic crisis
Acute or subacute myasthenia gravis exacerbation with respiratory involvement — an emergency, not an outpatient problem.
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Guillain-Barré with respiratory failure
Ascending weakness reaching the respiratory muscles is a critical-care indication — supine VC helps quantify the trajectory.
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Post-COVID diaphragm weakness
An emerging indication — persistent diaphragmatic dysfunction after severe COVID-19 that benefits from targeted assessment and NIV.
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Central sleep apnoea with hypoventilation
Central patterns combined with hypoventilation demand paired sleep-study and supine VC assessment before treatment.
Sources
The guidelines this guide is built on.
The four consensus and society guidelines that shape UK practice on neuromuscular respiratory assessment and NIV.
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British Thoracic Society. Guideline for non-invasive ventilation in acute respiratory failure.
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European Respiratory Society. Statements on respiratory muscle assessment.
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American Thoracic Society. Standardisation of spirometry and respiratory muscle testing.
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NICE. Motor neurone disease: assessment and management (NG42).
Last reviewed 2026-07-30 · Next review 2027-07-30 · 4-minute read
Frequently asked
Everything patients ask about supine VC.
Quick answers on what supine VC measures, how it differs from standard spirometry, and what happens next.
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What is a supine vital capacity (VC) test?
A short spirometry protocol that compares vital capacity sitting upright and lying flat. The postural difference — expressed as a percentage fall — is the diagnostic signal. A fall greater than 20% indicates significant diaphragm weakness, and a fall above 30% with a low FVC often warrants urgent non-invasive ventilation assessment.
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Why measure vital capacity lying down?
When you lie flat, the diaphragm has to work against the weight of the abdominal contents. A healthy diaphragm handles this with little change in vital capacity. A weak diaphragm cannot, and the vital capacity drops. The supine measurement therefore isolates diaphragmatic function in a way that seated spirometry alone cannot.
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Who should have a supine VC test?
Patients with suspected neuromuscular respiratory weakness — motor neurone disease (ALS), myasthenia gravis, muscular dystrophy, post-polio syndrome, Guillain-Barré syndrome, phrenic nerve injury and suspected unilateral or bilateral diaphragmatic paralysis. It is also used to quantify post-COVID diaphragm weakness.
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How is supine VC different from a standard spirometry test?
Standard spirometry is performed sitting upright and measures overall airflow. Supine VC adds a second measurement lying flat and focuses specifically on the postural change — the diaphragm-specific signal. The two tests answer different clinical questions.
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What happens after the test?
A respiratory physiologist quality-assures the traces and a consultant respiratory physician writes a structured report with sitting VC, supine VC, the percentage fall and interpretation alongside MIP/MEP and SNIP. If diaphragm weakness is confirmed, the same team arranges NIV assessment and structured follow-up.
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What are the red-flag results?
A postural fall greater than 30% with an FVC below 50% predicted, ALS with bulbar involvement, DMD with hypercapnia, myasthenic crisis, Guillain-Barré with respiratory failure and confirmed diaphragmatic paralysis all warrant urgent non-invasive ventilation assessment rather than routine follow-up.
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In practice, in London
What supine vc looks like on the ground in London
With supine vc, the London question is usually about report turnaround and the radiologist reading it — not whether the scan is available. On the NHS, supine vc typically sits behind a triage step and a wait that can stretch from a few weeks into months. In London’s private sector, the same appointment often lands within days. That speed matters when symptoms are disrupting work, sleep, or a plan you’d already committed to — and it’s the single most common reason people call us in the first place.
A typical private booking for supine vc in London starts with a consultant conversation — sometimes in person on Harley Street or Marylebone, sometimes on video if that suits better. Any imaging or diagnostics happen at a nearby CQC-registered facility, and reports usually land within 24 to 72 hours. The whole loop, from first call to written report, is often done inside a fortnight. For supine vc specifically, the difference between a routine report and a sub-speciality read is where private care earns its keep.
Fit matters more than people expect. For supine vc, the right consultant depends on what you actually need — a second opinion, a definitive diagnosis, a bridge into treatment, or reassurance that nothing’s being missed. We match on that, not on who has the biggest brochure. If a test isn’t the right next step, we’ll say so before you book anything.