Which of the following physiologic changes decreases pulmonary vascular resistance (PVR)?

Inhaling the entire vital capacity (VC)

Inhaling the inspiratory reserve volume (IRV)

Exhaling the entire vital capacity (VC)

Exhaling the expiratory reserve volume (ERV)

Breath holding maneuver at functional residual capacity (FRC)

A 25-year-old previously healthy woman is admitted to the hospital with progressively worsening shortness of breath. She reports a mild fever. Her vital signs at the admission are as follows: blood pressure 100/70 mm Hg, heart rate 111/min, respiratory rate 20/min, and temperature 38.1℃ (100.6℉); blood saturation on room air is 90%. Examination reveals a bilateral decrease of vesicular breath sounds and rales in the lower lobes. Plain chest radiograph demonstrates bilateral opacification of the lower lobes. Despite appropriate treatment, her respiratory status worsens. The patient is transferred to the intensive care unit and put on mechanical ventilation. Adjustment of which of the following ventilator settings will only affect the patient’s oxygenation?

Tidal volume and respiratory rate

A 21-year-old man is admitted to the intensive care unit for respiratory failure requiring mechanical ventilation. His minute ventilation is calculated to be 7.0 L/min, and his alveolar ventilation is calculated to be 5.1 L/min. Which of the following is most likely to decrease the difference between minute ventilation and alveolar ventilation?

Decreasing the physiologic dead space

Increasing the partial pressure of inhaled oxygen

Decreasing the affinity of hemoglobin for oxygen

Increasing the respiratory depth

Increasing the respiratory rate

A 35-year-old woman volunteers for a study on respiratory physiology. Pressure probes A and B are placed as follows: Probe A: between the parietal and visceral pleura Probe B: within the cavity of an alveolus The probes provide a pressure reading relative to atmospheric pressure. To obtain a baseline reading, she is asked to sit comfortably and breathe normally. Which of the following sets of values will most likely be seen at the end of inspiration?

Probe A: -6 mm Hg; Probe B: 0 mm Hg

Probe A: 0 mm Hg; Probe B: -1 mm Hg

Probe A: -4 mm Hg; Probe B: 0 mm Hg

Probe A: -4 mm Hg; Probe B: -1 mm Hg

Probe A: -6 mm Hg; Probe B: -1 mm Hg

A 72-year-old man with coronary artery disease comes to the emergency department because of chest pain and shortness of breath for the past 3 hours. Troponin levels are elevated and an ECG shows ST-elevations in the precordial leads. Revascularization with percutaneous coronary intervention is performed, and a stent is successfully placed in the left anterior descending artery. Two days later, he complains of worsening shortness of breath. Pulse oximetry on 3L of nasal cannula shows an oxygen saturation of 89%. An x-ray of the chest shows distended pulmonary veins, small horizontal lines at the lung bases, and blunting of the costophrenic angles bilaterally. Which of the following findings would be most likely on a ventilation-perfusion scan of this patient?

Normal perfusion with bilateral ventilation defects

Matched ventilation and perfusion bilaterally

Normal ventilation with multiple, bilateral perfusion defects

Normal perfusion with decreased ventilation at the right base

Increased apical ventilation with normal perfusion bilaterally

A 62-year-old man is brought to the emergency department with a 2-day history of cough productive of yellowish sputum. He has had fever, chills, and worsening shortness of breath over this time. He has a 10-year history of hypertension and hyperlipidemia. He does not drink alcohol or smoke cigarettes. His current medications include atorvastatin, amlodipine, and metoprolol. His temperature is 38.9°C (102.0°F), pulse is 105/min, respirations are 27/min, and blood pressure is 110/70 mm Hg. He appears in mild distress. He has rales over the left lower lung field. The remainder of the examination shows no abnormalities. Leukocyte count is 15,000/mm3 (87% segmented neutrophils). Arterial blood gas analysis on room air shows: pH 7.44 pO2 68 mm Hg pCO2 28 mm Hg HCO3- 24 mEq/L O2 saturation 91% An x-ray of the chest shows a consolidation in the left lower lobe. Asking the patient to lie down in the left lateral decubitus position would most likely result in which of the following?

Decreased ventilation of the left lung

Increased perfusion of right lung

A 52-year-old woman presents to the emergency department with breathlessness for the past 6 hours. She denies cough, nasal congestion or discharge, sneezing, blood in sputum, or palpitation. There is no past history of chronic respiratory or cardiovascular medical conditions, but she mentions that she has been experiencing frequent cramps in her left leg for the past 5 days. She is post-menopausal and has been on hormone replacement therapy for a year now. Her temperature is 38.3°C (100.9°F), the pulse is 116/min, the blood pressure is 136/84 mm Hg, and the respiratory rate is 24/min. Edema and tenderness are present in her left calf region. Auscultation of the chest reveals rales over the left infrascapular and scapular region. The heart sounds are normal and there are no murmurs. Which of the following mechanisms most likely contributed to the pathophysiology of this patient’s condition?

Increased right ventricular preload

Secretion of vasodilating neurohumoral substances in pulmonary vascular bed

Decreased physiologic dead space

Decreased alveolar-arterial oxygen tension gradient

Ventilator strategies addressing V/Q for USMLE Step 2 CK: High-Yield Physiology Lessons

V/Q Mismatch - The Great Divide

Goal: To improve arterial oxygenation by matching ventilation to perfusion, primarily by recruiting atelectatic lung tissue and optimizing gas distribution.

Key Strategies:
- PEEP: Recruits collapsed alveoli (improves low V/Q), increases FRC. Titrate to best compliance/oxygenation.
- Recruitment Maneuvers (RMs): Sustained high pressure (e.g., 30-40 cmH2O for 30-40s) to open atelectatic lung.
- Prone Positioning: Gravitationally shifts perfusion to better-ventilated dorsal lung regions, improving V/Q matching in ARDS.

⭐ PEEP primarily addresses intrapulmonary shunt. By recruiting alveoli, it can improve oxygenation but may increase dead space if it overdistends healthy lung regions, impairing their perfusion.

Ventilator Levers - Twisting the Knobs

Oxygenation Knobs (Correcting ↓ PaO₂):
- FiO₂ (Fraction of Inspired Oxygen): Primary lever. Directly ↑ alveolar oxygen ($P_A O_2$). Titrate to keep $SpO_2$ > 90%, aiming for FiO₂ < 0.6 to avoid oxygen toxicity.
- PEEP (Positive End-Expiratory Pressure): Recruits collapsed alveoli, ↑ FRC, and improves V/Q matching. Crucial for intrapulmonary shunts (e.g., ARDS).
Ventilation Knobs (Correcting ↑ PaCO₂):
- Respiratory Rate (RR): Main driver of minute ventilation. ↑ RR to blow off more CO₂.
- Tidal Volume ($V_T$): Volume per breath. Secondary lever for CO₂ control.

Compliance curve: alveolar recruitment and lung function

⭐ High-Yield: For ARDS, use low tidal volume ventilation (4-8 mL/kg of ideal body weight). This lung-protective strategy is proven to reduce mortality by minimizing ventilator-induced lung injury (VILI).

Strategic Maneuvers - The Lung Savers

Goal: Recruit alveoli, improve oxygenation, and prevent Ventilator-Induced Lung Injury (VILI).
Positive End-Expiratory Pressure (PEEP):
- Keeps alveoli open at end-expiration, ↑ functional residual capacity (FRC).
- Recruits collapsed alveoli, converting shunt (V/Q = 0) to functional units.
- Optimal PEEP balances improved $PaO_2$ against risk of overdistension & barotrauma.
Prone Positioning:
- Improves V/Q matching by redirecting ventilation to better-perfused dorsal lung regions.
- Reduces compression of lung bases by the heart and abdominal contents.
Low Tidal Volume Ventilation (LTVV):
- Core of lung-protective strategy; targets 4-6 mL/kg ideal body weight.
- Minimizes volutrauma (overstretching alveoli).

⭐ Driving Pressure ($P_{plat} - PEEP$) is a key mortality predictor in ARDS. Aim for a driving pressure $< extbf{15}$ cm H₂O.

V/Q mismatch is the most common cause of hypoxemia; ventilator strategies aim to improve this relationship.

PEEP is the cornerstone, recruiting collapsed alveoli and increasing functional residual capacity (FRC).

Recruitment maneuvers (sustained high pressure) can open atelectatic lung regions.

Prone positioning improves oxygenation by redirecting perfusion to better-ventilated dorsal lung zones.

In unilateral lung disease, placing the "good lung down" optimizes V/Q matching.

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