What is expected in a premature baby with IRDS? T=alveolar surface tension, C=lung compliance, PaO2=arterial PO2?

Correct Answer: B

Rationale: Infant respiratory distress syndrome (IRDS), or hyaline membrane disease, occurs in premature infants due to insufficient surfactant production by immature type II alveolar cells. Surfactant lowers alveolar surface tension (T), facilitating lung expansion. In IRDS, reduced surfactant leads to increased surface tension, causing alveoli to collapse (atelectasis) after each breath. This high tension decreases lung compliance (C), as the lungs become stiffer and harder to inflate, requiring greater pressure for ventilation. Consequently, collapsed alveoli impair gas exchange, reducing arterial oxygen partial pressure (PaO2) below normal (hypoxemia), often to levels like 50-60 mmHg instead of the typical 75-100 mmHg. The correct combination increased T, decreased C, decreased PaO2 reflects the pathophysiology of IRDS, where surfactant deficiency drives a cascade of respiratory challenges. Other combinations, like increased compliance or unchanged PaO2, contradict the condition's mechanics, where stiff lungs and poor oxygenation are hallmark features.

Correct Answer: D

Rationale: Respiratory minute ventilation (VE) is tidal volume (VT) × respiratory rate (RR), fixed here at 6 L/min (6000 ml/min). Alveolar ventilation (VA) is the air reaching alveoli for gas exchange: VA = (VT - VD) × RR, where anatomic dead space (VD) is 150 ml. For 200 ml at 30/min: VE = 200 × 30 = 6000 ml/min, VA = (200 - 150) × 30 = 50 × 30 = 1500 ml/min. For 300 ml at 20/min: VE = 300 × 20 = 6000 ml/min, VA = (300 - 150) × 20 = 150 × 20 = 3000 ml/min. For 400 ml at 15/min: VE = 400 × 15 = 6000 ml/min, VA = (400 - 150) × 15 = 250 × 15 = 3750 ml/min. For 600 ml at 10/min: VE = 600 × 10 = 6000 ml/min, VA = (600 - 150) × 10 = 450 × 10 = 4500 ml/min. The 600 ml at 10/min yields the highest VA (4.5 L/min), as larger VT maximizes air past the fixed VD, despite lower RR. Higher rates with smaller VT waste more ventilation in dead space, reducing VA efficiency, making the deeper, slower pattern optimal.

Correct Answer: D

Rationale: Pneumothorax occurs when air enters the pleural space, disrupting negative intrapleural pressure (~-4 mmHg), causing lung collapse and chest wall expansion. The thorax's diameter increases as the chest wall springs outward due to its elastic recoil. Venous return decreases because positive pleural pressure compresses the vena cava, reducing preload, especially in tension pneumothorax. Vital capacity (VC) drops as the collapsed lung reduces expirable volume (e.g., from 4-5 L to much less). However, lung compliance (C = ΔV / ΔP) doesn't increase it's a lung property (stiffness), not directly altered by pneumothorax. The collapsed lung's volume change per pressure is irrelevant, as it's deflated; compliance may appear effectively zero, but the lung tissue itself isn't more compliant. Increased compliance misrepresents pneumothorax's mechanics, where the issue is pressure loss, not lung elasticity, making this the untrue statement.

Correct Answer: B

Rationale: Alveolar ventilation (VA) is the air reaching alveoli for gas exchange: VA = (VT - VD) × RR, where VT (tidal volume) = 650 ml, VD (dead space) = 150 ml, and RR (respiratory rate) = 15 breaths/min. Calculate: VT - VD = 650 - 150 = 500 ml per breath. VA = 500 ml × 15 = 7500 ml/min = 7.5 L/min. Verify: FRC (3 L) = ERV (1.5 L) + RV (1.5 L), and TLC (8 L) = FRC + IC (VT + IRV), consistent but not needed for VA. Total ventilation (VE) = VT × RR = 650 × 15 = 9750 ml/min = 9.75 L/min, with 2.25 L/min as dead space ventilation (150 × 15), leaving 7.5 L/min as VA. This matches option B, reflecting effective gas exchange volume, critical for oxygenation and CO2 removal, aligning with standard respiratory calculations.

Correct Answer: C

Rationale: Physiological dead space (VD) is the air not participating in gas exchange, calculated via the Bohr equation: VD/VT = (PaCO2 - PECO2) / PaCO2, where VT is tidal volume (600 ml), PaCO2 is arterial PCO2 (40 mmHg), and PECO2 is mixed expired PCO2 (28 mmHg). Compute: VD/VT = (40 - 28) / 40 = 12 / 40 = 0.3. Thus, VD = 0.3 × 600 = 180 ml. Cross-check: alveolar ventilation (VA) = 4.3 L/min = (VT - VD) × RR. Assuming RR = 10/min (a reasonable resting rate), VA = 4300 ml/min ÷ 10 = 430 ml/breath, so VT - VD = 430, VD = 600 - 430 = 170 ml, close to 180 ml with rounding. The 180 ml fits directly from Bohr, reflecting both anatomical (~150 ml) and alveolar dead space, aligning with data where CO2 dilution indicates 30% of each breath is ineffective, a key metric for ventilatory efficiency.