Assuming a normal anatomic dead space of 150 ml and a fixed respiratory minute ventilation of 6 L/min. Which combination of respiratory rate and tidal volume will give the largest alveolar ventilation?

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: 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.

Correct Answer: D

Rationale: Residual volume (RV) is the air remaining after maximal expiration (~1-1.5 L), preventing alveolar collapse and measurable via helium dilution or body plethysmography. Tidal volume (VT, ~500 ml) is normal breath size, not post-forceful exhalation. Expiratory reserve volume (ERV, ~1-1.5 L) is extra air exhaled beyond normal expiration, expelled during forced effort, leaving RV. Vital capacity (VC, ~4-5 L) is the maximum exhailable volume (IRV + VT + ERV), excluding RV. RV's persistence reflects lung elasticity and chest wall limits, ensuring some air stays, distinct from volumes tied to active breathing or maximal efforts, making it the correct term for this residual air critical for maintaining lung structure.

Correct Answer: A

Rationale: Physiological dead space (VDphys) includes anatomic dead space (~150 ml) and alveolar dead space (ventilated, non-perfused alveoli). Pulmonary embolism (PE) blocks pulmonary arteries, cutting perfusion to ventilated alveoli, vastly increasing alveolar dead space (e.g., from near 0 to 150+ ml), raising VDphys significantly. Atelectasis collapses alveoli, reducing ventilation and thus dead space, as unventilated areas don't count. Pneumothorax collapses lung, lowering ventilated volume, not increasing dead space. Bronchoconstriction narrows airways, possibly reducing anatomic dead space slightly, with minimal alveolar effect unless severe. PE's perfusion loss creates the greatest VDphys rise, measurable via Bohr (PaCO2-PECO2), reflecting high V/Q mismatch, a critical gas exchange inefficiency distinguishing it from ventilation-focused conditions.