In a 24‑year‑old woman with left‑sided thoracic scoliosis who performed submaximal spirometry showing vital capacity ≈71% of predicted, forced vital capacity ≈72% of predicted, and FEV1/VC ratio ≈117% of predicted, does this indicate a restrictive lung defect rather than asthma, and what further tests and management steps are recommended?

Interpretation: True Restrictive Defect, Not Asthma

This spirometry demonstrates a true restrictive ventilatory pattern caused by thoracic scoliosis, not asthma or obstructive airway disease. The preserved—indeed supranormal—FEV₁/VC ratio of 98% (117% of predicted) combined with reduced vital capacity (71% predicted) and FVC (72% predicted) definitively excludes obstruction and points to chest-wall restriction. 1

Why This Is Restriction, Not Asthma

The FEV₁/VC Ratio Rules Out Obstruction

• Obstruction requires an FEV₁/VC ratio below the 5th percentile (z-score < –1.64). 1 Your patient's ratio is 98%, which is 117% of predicted—well above normal and completely incompatible with asthma or any obstructive disease. 1
• In asthma, air-trapping and early airway closure reduce the FEV₁/VC ratio; here it is elevated, indicating that expiratory flow is preserved relative to lung volume. 2, 1
• The flow rates (MEF 75, MEF 50, MEF 25, MMEF) are all normal or above normal (90–117% predicted), further excluding obstruction. 1

Scoliosis Causes Chest-Wall Restriction

• Left-sided thoracic scoliosis mechanically restricts lung expansion by deforming the rib cage, stiffening the chest wall, and impairing respiratory muscle function. 3, 4, 5
• Even mild scoliosis (Cobb angle < 35°) can reduce FVC below 80% predicted in 13–42% of adolescents, and the reduction correlates more strongly with chest-wall deformity than with inspiratory muscle strength. 4, 5
• Vital capacity is the most sensitive routine test for respiratory muscle weakness and chest-wall restriction, reflecting the combined effect of mechanical load and muscle performance. 3

The Pattern Fits Classic Restriction

• Reduced VC and FVC with a normal or elevated FEV₁/VC ratio (> 85–90%) is the spirometric hallmark of restriction. 1, 6
• The steep, convex flow-volume curve (inferred from normal flow rates) supports restriction rather than the concave, scooped curve seen in obstruction. 1

Common Pitfall: Submaximal Effort vs. True Disease

Was the Test Performed Correctly?

• Simultaneous reductions in FEV₁ and FVC with a preserved ratio are the most common manifestation of poor inspiratory or expiratory effort. 1 The nurse's concern about technique is valid and must be addressed first.
• However, in a patient with known scoliosis, reduced volumes with a supranormal ratio are entirely consistent with true chest-wall restriction, not just poor effort. 4, 5, 7
• Key quality indicators to verify:
  • Was forced expiratory time ≥ 6 seconds? 1
  • Were at least three acceptable maneuvers obtained with FVC reproducible within 150 mL? 1
  • Does the flow-volume curve show a smooth, maximal effort without cough or early termination? 1

If Effort Was Adequate

• The pattern is genuine restriction caused by scoliosis. 4, 5, 7
• No further testing is needed to exclude asthma; the supranormal FEV₁/VC ratio already does so. 2, 1

If Effort Was Submaximal

• Repeat spirometry with careful coaching. 1
• Even if volumes remain low, a persistently elevated FEV₁/VC ratio in a patient with scoliosis still indicates restriction, not obstruction. 1, 6

What Further Testing Is Needed?

To Confirm True Restriction: Measure Total Lung Capacity

• Spirometry alone cannot definitively diagnose restriction; a reduced VC predicts low TLC in only ~50% of cases. 1, 8
• Body plethysmography is mandatory to confirm restriction (TLC < 5th percentile, z-score < –1.64). 1, 8
• Do not use single-breath alveolar volume (VA) from DLCO testing; in any setting it can underestimate TLC by up to 3 L and is unreliable for diagnosing restriction. 2, 1

To Assess Functional Impact: DLCO and Respiratory Muscle Strength

• A normal DLCO would exclude parenchymal lung disease (interstitial fibrosis, etc.) and confirm that volume loss is purely mechanical. 2, 1
• Maximal inspiratory pressure (MIP) and maximal expiratory pressure (MEP) quantify respiratory muscle strength; in scoliosis, FVC correlates more closely with chest-wall deformity than with MIP, but severe weakness (MIP < 60 cm H₂O) predicts worse outcomes. 3, 4, 5, 9

Bronchodilator Testing Is Not Indicated

• A bronchodilator response (≥ 12% and ≥ 200 mL increase in FEV₁ or FVC) would indicate reversible obstruction and asthma. 2, 1
• However, with an FEV₁/VC ratio of 98%, obstruction is already excluded, so bronchodilator testing adds no diagnostic value. 2, 1
• If you perform it anyway and volumes improve, the improvement reflects better effort or reduced air-trapping from submaximal baseline testing, not asthma. 2

Management Algorithm

Step 1: Verify Test Quality

• Review the spirometry report for forced expiratory time, reproducibility, and flow-volume curve morphology. 1
• If quality is acceptable, proceed to Step 2.
• If quality is poor, repeat spirometry with meticulous coaching; if the pattern persists, it is likely genuine. 1

Step 2: Confirm Restriction with Plethysmography

• Order body plethysmography to measure TLC, RV, and RV/TLC. 1, 8
• If TLC < 5th percentile: true restriction is confirmed; proceed to Step 3. 1
• If TLC is normal: the reduced VC reflects submaximal effort or a borderline variant; clinical correlation is needed. 1, 8

Step 3: Assess Severity and Functional Impact

• Measure DLCO to exclude parenchymal disease. 2, 1
• Measure MIP and MEP to quantify respiratory muscle strength. 3, 9
• Severity grading (based on FVC or TLC % predicted): 1
  • Mild: ≥ 60% predicted
  • Moderate: 50–59% predicted
  • Severe: < 50% predicted
• At FVC 72% predicted, this is mild restriction, but in scoliosis, even mild defects can progress and warrant monitoring. 4, 5, 10

Step 4: Monitor and Intervene

• Serial VC measurements every 6–12 months track progression; a declining trend predicts worsening respiratory function and may prompt surgical or supportive intervention. 3, 10
• Nocturnal BiPAP or non-invasive ventilation should be considered if symptoms develop (dyspnea, morning headache, hypercapnia) or if FVC falls below 50% predicted. 9, 11
• Surgical correction of scoliosis may stabilize or improve lung function in progressive curves, though outcomes are variable and depend on age, curve severity, and underlying neuromuscular disease. 10, 11

Key Caveats

• Do not diagnose asthma based on reduced volumes alone. Asthma requires a low FEV₁/VC ratio or documented bronchodilator reversibility; neither is present here. 2, 1
• Do not label this as "mixed obstruction-restriction" without measuring TLC. Spirometry cannot diagnose mixed disease; plethysmography is mandatory. 1
• Do not assume poor effort explains everything. In a patient with visible chest-wall deformity, reduced volumes with a supranormal ratio are physiologically consistent with true restriction. 4, 5, 7
• Postural change in VC (supine vs. upright) is not relevant here. A fall > 30% suggests diaphragmatic paralysis, not chest-wall restriction from scoliosis. 3