In which scenario is the compliance of lungs not greater than the others: when compared to the compliance of lungs and thorax together, in infants versus adults, when filled with saline versus air, or in standing versus recumbent position?

Lung Compliance is NOT Greater in Infants Than in Adults

The correct answer is option 2: lung compliance is NOT greater in infants than in adults. In fact, infants have significantly lower lung compliance compared to adults, while the other three scenarios correctly describe situations where lung compliance is greater.

Analysis of Each Option

Option 1: Lung Compliance vs. Lung-Thorax Combined Compliance ✓ TRUE

• Lung compliance alone is indeed greater than the compliance of the lung-thorax system combined 1, 2
• When the chest wall is added to the lung system, the combined compliance is lower than lung compliance alone because compliances in series add reciprocally 3
• At functional residual capacity (FRC), the inward elastic recoil of the lungs opposes the outward recoil of the chest wall, creating a system with lower overall compliance 2

Option 2: Infant vs. Adult Lung Compliance ✗ FALSE - THIS IS THE EXCEPTION

• Infants have LOWER lung compliance than adults, not higher 4
• In younger children, the chest wall is nearly 3 times more compliant than the lungs due to immature bone ossification 4
• This creates an imbalance that lowers overall respiratory system compliance even in healthy infants 4
• The high chest wall compliance relative to lung compliance is an inherent characteristic of newborn mammals 4
• During the first 2 years of life, as bones ossify, chest wall compliance falls linearly until equilibrating with the lungs 4

Option 3: Saline-Filled vs. Air-Filled Lungs ✓ TRUE

• Lung compliance is greater when filled with saline than with air (based on general medical knowledge of surface tension effects)
• When lungs are filled with saline, surface tension forces at the air-liquid interface are eliminated
• This removes a major component of elastic recoil, substantially increasing compliance
• This principle is demonstrated in classic physiologic experiments comparing pressure-volume curves of saline-filled versus air-filled lungs

Option 4: Standing vs. Recumbent Position ✓ TRUE

• Lung compliance is greater in the standing position compared to recumbent (based on general medical knowledge)
• In the supine position, abdominal contents push against the diaphragm, reducing lung volumes and compliance
• Gravitational effects on blood distribution also affect regional lung compliance
• Studies in quadriplegics demonstrate that position significantly affects respiratory mechanics 5

Clinical Implications

The misconception about infant lung compliance has important clinical consequences:

• Infants require comparatively more negative intrapleural pressures to maintain sufficient tidal volumes for gas exchange 4
• This mechanically disadvantageous state makes infants more susceptible to respiratory distress 4
• The compliant chest wall in infants allows paradoxical inward motion during inspiration, especially during REM sleep when intercostal muscle tone is reduced 4
• Understanding these differences is critical for recognizing respiratory distress in pediatric patients 4