In which of the following pathological states would the oxygen content of the trachea resemble the oxygen content in the affected alveoli?

Foreign body obstruction distal to the trachea

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 68-year-old man comes to the emergency room with difficulty in breathing. He was diagnosed with severe obstructive lung disease a few years back. He uses his medication but often has to come to the emergency room for intravenous therapy to help him breathe. He was a smoker for 40 years smoking two packs of cigarettes every day. Which of the following best represents the expected changes in his ventilation, perfusion and V/Q ratio?

Low ventilation, normal perfusion and low V/Q ratio

Normal ventilation, low or nonexistent perfusion and infinite V/Q ratio

Medium ventilation and perfusion, V/Q that equals 0.8

Higher ventilation and perfusion with lower V/Q ratio

Lower ventilation and perfusion, but higher V/Q ratio

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

Ventilation-perfusion ratio concept for USMLE Step 3: High-Yield Physiology Lessons

V/Q Ratio - The Lung's Balancing Act

The V/Q ratio is a measure of the efficiency of gas exchange between the alveoli and the pulmonary capillaries. It quantifies the matching of ventilation to perfusion.

Ventilation (V): The amount of gas reaching the alveoli.
- Normal alveolar ventilation ≈ 4 L/min.
Perfusion (Q): The blood flow through pulmonary capillaries.
- Normal pulmonary blood flow ≈ 5 L/min.
Calculation: The ratio is expressed as $V/Q = (Alveolar Ventilation) / (Pulmonary Blood Flow)$.
The overall V/Q ratio for a healthy lung is approximately 0.8.

⭐ In an upright lung, gravity creates regional V/Q differences: the apex is better ventilated (↑ V/Q), while the base is better perfused (↓ V/Q).

V/Q Extremes - Shunt vs. Dead Space

📌 Shunt = No ventilation; Dead space = Destroyed perfusion.

Feature	Shunt (V/Q → 0)	Dead Space (V/Q → ∞)
Pathophysiology	Perfusion without ventilation	Ventilation without perfusion
V/Q Value	$V/Q \to 0$	$V/Q \to \infty$
Gas Exchange	No gas exchange; venous admixture	No gas exchange; wasted ventilation
PaO₂/PaCO₂	↓PaO₂, ↑PaCO₂	↓PaO₂, ↑PaCO₂
Response to 100% O₂	Poor/no correction (refractory)	Corrects hypoxemia
Clinical Examples	ARDS, pneumonia, atelectasis	Pulmonary embolism, emphysema

⭐ A key feature of a shunt is an elevated A-a gradient that does not correct with 100% O₂ supplementation, as the oxygen cannot reach the blood in unventilated alveoli.

Lung Zones - Gravity's Playground

In an upright lung, gravity causes both ventilation (V) and perfusion (Q) to increase from the apex to the base.
However, the increase in perfusion is much steeper than the increase in ventilation.
Apex (Zone 1):
- ↓ V and ↓↓ Q → High V/Q ratio (>3.0)
- Represents physiologic dead space (wasted ventilation).
Base (Zone 3):
- ↑ V and ↑↑ Q → Low V/Q ratio (<0.6)
- Represents physiologic shunt (wasted perfusion).

Lung zones and blood flow based on pressure gradients

⭐ The overall V/Q ratio for a healthy lung is approximately 0.8, reflecting the weighted average of all zones.

Clinical V/Q - Disease Detective

↑ High V/Q (Dead Space): Ventilation without perfusion. Think wasted ventilation.
- Examples: Pulmonary Embolism, Emphysema, Foreign body.
↓ Low V/Q (Shunt): Perfusion without ventilation. Think wasted perfusion.
- Examples: Pneumonia, Pulmonary Edema, Asthma, Chronic Bronchitis.
Compensation: Lungs use hypoxic vasoconstriction to shunt blood from poorly ventilated areas to better-ventilated ones, optimizing the V/Q match.

⭐ The normal V/Q ratio is ~0.8, as perfusion is slightly greater than ventilation overall.

High‑Yield Points - ⚡ Biggest Takeaways

The normal V/Q ratio is approximately 0.8.

The apex of the lung has a high V/Q (~3.0), representing physiologic dead space.

The base of the lung has a low V/Q (~0.6), representing a physiologic shunt.

Dead space (V/Q → ∞) is ventilation without perfusion (e.g., pulmonary embolism).

Shunt (V/Q → 0) is perfusion without ventilation (e.g., airway obstruction).

Both V/Q mismatch extremes result in hypoxemia.

Hypoxemia due to shunt is characteristically not corrected by 100% O₂.

Continue reading on Oncourse

CONTINUE READING — FREE

or get the app

App Store|Google Play