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 64-year-old man with longstanding ischemic heart disease presents to the clinic with complaints of increasing exercise intolerance and easy fatigability for the past 2 weeks. He further states that he has been experiencing excessive daytime somnolence and shortness of breath with exertion. His wife adds that his shortness of breath is more in the recumbent position, and after approximately 2 hours of sleep, after which he suddenly wakes up suffocating and gasping for breath. This symptom is relieved after assuming an upright position for more than 30 minutes. The vital signs are as follows: heart rate, 126/min; respiratory rate, 16/min; temperature, 37.6°C (99.6°F); and blood pressure, 122/70 mm Hg. The physical examination reveals a S3 gallop on cardiac auscultation and positive hepatojugular reflux with distended neck veins. An electrocardiogram shows ischemic changes similar to ECG changes noted in the past. An echocardiogram reveals an ejection fraction of 33%. Which of the following best describes the respiratory pattern abnormality which occurs in this patient while sleeping?

Prolonged lung-to-brain circulation time

Decreased central hypercapnic ventilatory responsiveness

Increased pulmonary artery pressure

Increased partial pressure of oxygen

A 55-year-old man presents to the physician for the evaluation of excessive daytime sleepiness over the past six months. Despite sleeping 8–9 hours a night and taking a nap during the day, he feels drowsy and is afraid to drive. His wife complains of loud snoring and gasping during the night. His blood pressure is 155/95 mm Hg. BMI is 37 kg/m2. Oropharyngeal examination shows a small orifice and an enlarged tongue and uvula. The soft palate is low-lying. The examination of the nasal cavity shows no septal deviation or polyps. Examination of the lungs and heart shows no abnormalities. Polysomnography shows an apnea-hypopnea index of 20 episodes/h. The patient is educated about weight loss, exercise, and regular sleep hours and duration. Which of the following is the most appropriate next step in management?

Continuous positive airway pressure

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

Sleep effects on respiration for USMLE Step 2 CK: High-Yield Physiology Lessons

Sleep Stages - A Dreamy Descent

Hypnogram of sleep stages and awakenings

Sleep Architecture: The predictable cycling between NREM and REM sleep stages, typically 4-5 cycles per night. Visualized on a hypnogram.
NREM (Non-Rapid Eye Movement) Sleep: Constitutes 75-80% of total sleep.
- N1: Transition to sleep; light sleep.
- N2: Majority of sleep time; features sleep spindles and K-complexes.
- N3: Slow-wave sleep (SWS); deepest, most restorative stage.
REM (Rapid Eye Movement) Sleep: Makes up 20-25% of sleep; associated with dreaming.

⭐ REM sleep is characterized by muscle atonia, which includes most accessory muscles of respiration, making breathing predominantly diaphragmatic.

Physiological Changes - The Sleeping Lung

Overall, minute ventilation ($V_E = V_t \times RR$) decreases during sleep. The pattern of breathing and chemoreceptor sensitivity varies significantly between NREM and REM stages, impacting gas exchange.

Parameter	Wakefulness	NREM Sleep	REM Sleep
Minute Ventilation ($V_E$)	Normal	↓ (stable)	↓↓ (irregular)
Tidal Volume ($V_t$)	Normal	↓	↓↓
Respiratory Rate (RR)	Normal	Regular	Irregular/Variable
Hypercapnic Drive	Normal	↓	↓↓ (blunted)
Hypoxic Drive	Normal	↓	↓↓ (blunted)
Upper Airway Resistance	Normal	↑	↑↑ (variable)

-   Upper airway muscle (e.g., genioglossus) tone ↓, leading to ↑ resistance.
-   REM sleep features atonia of intercostal muscles, making breathing diaphragm-dependent.

⭐ The ventilatory response to both hypoxia and hypercapnia is most significantly blunted during REM sleep, increasing the risk of desaturation.

Sleep Apnea - When Breathing Stops

Recurrent episodes of apnea (cessation of airflow) or hypopnea (reduced airflow) during sleep, leading to oxygen desaturation.

Obstructive Sleep Apnea (OSA): Most common type. Characterized by upper airway collapse despite persistent respiratory effort.
Central Sleep Apnea (CSA): Defined by the absence of respiratory effort, stemming from reduced central nervous system drive.
Diagnosis: Polysomnography measures the Apnea-Hypopnea Index (AHI).
- AHI Severity: Mild 5-15, Moderate 15-30, Severe >30 events/hour.

⭐ In OSA, there is continued (often exaggerated) respiratory effort against an obstructed upper airway, whereas in CSA, both airflow and respiratory effort cease.

📌 Mnemonic: For OSA think 'Obstruction': Effort Present, Airflow Absent.

Obstructive Sleep Apnea Polysomnography

Clinical Correlates - The Zzz-Sleuth

Polysomnography (PSG): The definitive diagnostic test for sleep-related breathing disorders.
- Integrates multiple measurements:
  - Electroencephalogram (EEG): Brain waves (sleep staging).
  - Electrooculogram (EOG): Eye movements (REM sleep).
  - Electromyogram (EMG): Muscle tone (atonia in REM).
  - Respiratory airflow/effort & O₂ saturation.
Epworth Sleepiness Scale: A questionnaire to quantify daytime sleepiness; a score >10 is considered sleepy.
Management:
- CPAP (Continuous Positive Airway Pressure): Provides constant pressure to keep the airway open.
- BiPAP (Bilevel Positive Airway Pressure): Delivers higher pressure on inhalation and lower pressure on exhalation.

⭐ Polysomnography is the gold standard for diagnosing sleep-related breathing disorders, as it simultaneously measures brain waves (sleep stage), eye movements, muscle activity, respiratory airflow/effort, and oxygen saturation.

Polysomnography setup for sleep respiration study

High-Yield Points - ⚡ Biggest Takeaways

During NREM sleep, minute ventilation ↓ due to reduced metabolic rate and chemosensitivity.

REM sleep brings irregular, shallow breathing and profound muscle atonia, especially of the upper airway, increasing collapse risk.

Ventilatory responses to hypoxia and hypercapnia are significantly blunted, most severely in REM.

Sleep induces a state of relative hypoventilation, causing a physiological ↑ in PaCO2.

Upper airway resistance increases during all sleep stages.

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