13.4 Pediatric Pulmonary Function Testing and Upright Versus Supine Spirometry

13.4 Pediatric Pulmonary Function Testing and Upright Versus Supine Spirometry

Two content specifications make this section unavoidable. First, the Additional Examination Form Specifications in the PFT Detailed Content Outline set the patient mix at 10 pediatric items and 90 general items on every 100-item form — roughly one scored item in ten involves a child. Second, spirometry is listed in Domain II as item 2 with two explicit sub-parts: (a) standard and (b) upright / supine, and upright/supine comparison reappears as Domain III item 3b.

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Part One: Pediatric Pulmonary Function Testing

Age-Appropriate Expectations

Coaching and Environment

Success depends more on the technologist than on the equipment. Practical measures: schedule when the child is rested and not hungry; explain in concrete terms ("blow out the candles," "make the balloon pop"); demonstrate the maneuver yourself first; use animated incentive software so the child watches a target rather than a number; allow a parent to remain visible; use a smaller-diameter mouthpiece and pediatric-sized filter with appropriately low dead space; and cap the session at roughly 15 minutes before frustration sets in. Praise every effort — a discouraged 4-year-old will not produce a better blow.

Modified Acceptability Criteria for Children Aged $\le$ 6 Years

The 2019 ATS/ERS statement created a separate, more permissive set of criteria for young children, recognizing that their forced expiration is genuinely shorter:

Two acceptable maneuvers may be sufficient in preschool children, where obtaining three is often unrealistic. The technologist's written comment on effort quality carries proportionally more weight in a pediatric report than in an adult one.

Reference Equations and Growth

GLI 2012 equations span ages 3 to 95 continuously, which eliminated the artificial jump that occurred when a child crossed from a pediatric to an adult reference set on a birthday. Height is the dominant predictor and children grow, so predicted values must be recalculated with a freshly measured height at every visit — using a stored height from six months ago is a common and consequential error in a growing child. Z-scores are strongly preferred over percent predicted in pediatrics, because a fixed percent-predicted band means very different things across the growth curve.

Other Pediatric-Relevant Points

• Lung volumes: plethysmography is feasible from about age 5–6 with a transparent booth and a parent in view; claustrophobia is the usual limiting factor. Multiple-breath washout and the lung clearance index (LCI) require only tidal breathing and detect early small-airway disease in cystic fibrosis before spirometry changes.
• DLCO: the standard breath-hold is difficult below school age; hemoglobin adjustment uses the 13.4 g/dL reference for children under 15 years of either sex.
• Bronchodilator response: the ATS/ERS 2022 percent-predicted criterion is particularly useful in children, where a small absolute change is a large percentage of a small baseline.
• Equipment dead space is proportionally much larger relative to a child's tidal volume; use pediatric filters and subtract the correct dead-space volume, or FRC and V$_A$ will be overestimated.

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Part Two: Upright Versus Supine Spirometry

Why Position Changes the Measurement

Moving from sitting to supine displaces abdominal contents cephalad against the diaphragm, reduces functional residual capacity, and increases thoracic blood volume. In a healthy person the diaphragm compensates and vital capacity falls only slightly. When the diaphragm is weak or paralyzed, it cannot resist the abdominal load, and vital capacity falls markedly.

Indications

• Suspected unilateral or bilateral diaphragm paralysis (post-cardiac surgery, phrenic nerve injury, idiopathic).
• Neuromuscular disease surveillance — ALS, myasthenia gravis, muscular dystrophy, high spinal cord injury.
• Unexplained orthopnea with normal cardiac evaluation.
• Morbid obesity, where supine FRC reduction can be dramatic.

Technique

1. Perform a complete, acceptable upright spirometry set first and record position explicitly.
2. Move the patient to a flat supine position (not semi-recumbent) and allow at least 5 minutes of equilibration, because the fluid and volume shifts are not instantaneous.
3. Repeat spirometry using the same device, filter, and nose clip, applying the same acceptability and repeatability criteria.
4. Calculate the percentage fall:

$\% \text{ Fall} = \frac{VC_{\text{upright}} - VC_{\text{supine}}}{VC_{\text{upright}}} \times 100$

Interpretation

Unilateral paralysis typically produces a smaller fall, often in the 10–20% band, and is usually accompanied by a reduction in upright vital capacity of roughly 20–25% from predicted. Correlate with MIP, SNIP, and imaging.

Documentation and Safety

Always record patient position on the report. A supine study performed because the patient could not sit — and not documented as such — will later be trended against upright values and read as a genuine decline. Monitor oxygen saturation during supine testing in patients with significant weakness or obesity, and be prepared to return them upright promptly; the supine position itself can provoke dyspnea in exactly the patients for whom the test is indicated.