A 40-year-old Caucasian male presents to the emergency room after being shot in the arm in a hunting accident. His shirt is soaked through with blood. He has a blood pressure of 65/40, a heart rate of 122, and his skin is pale, cool to the touch, and moist. This patient is most likely experiencing all of the following EXCEPT:

Decreased sarcomere length in the myocardium

A 3-year-old boy is brought to the emergency department with a history of unintentional ingestion of seawater while swimming in the sea. The amount of seawater ingested is not known. There is no history of vomiting. On physical examination, the boy appears confused and is asking for more water to drink. His serum sodium is 152 mmol/L (152 mEq/L). Which of the following changes in volumes and osmolality of body fluids are most likely to be present in this boy?

Increased ECF volume, decreased ICF volume, increased body osmolality

Decreased ECF volume, unaltered ICF volume, unaltered body osmolality

Increased ECF volume, unaltered ICF volume, unaltered body osmolality

Increased ECF volume, increased ICF volume, decreased body osmolality

Decreased ECF volume, decreased ICF volume, increased body osmolality

A 19-year-old man presents to the clinic with a complaint of increasing shortness of breath for the past 2 years. His shortness of breath is associated with mild chest pain and occasional syncopal attacks during strenuous activity. There is no history of significant illness in the past, however, one of his uncles had similar symptoms when he was his age and died while playing basketball a few years later. He denies alcohol use, tobacco consumption, and the use of recreational drugs. On examination, pulse rate is 76/min and is regular and bounding; blood pressure is 130/70 mm Hg. A triple apical impulse is observed on the precordium and a systolic ejection crescendo-decrescendo murmur is audible between the apex and the left sternal border along with a prominent fourth heart sound. The physician then asks the patient to take a deep breath, close his mouth, and pinch his nose and try to breathe out without allowing his cheeks to bulge out. In doing so, the intensity of the murmur increases. Which of the following hemodynamic changes would be observed first during this maneuver?

↑ Mean Arterial Pressure, ↓ Heart rate, ↑ Baroreceptor activity, ↑ Parasympathetic Outflow

↓ Mean Arterial Pressure, ↑ Heart rate, ↑ Baroreceptor activity, ↓ Parasympathetic Outflow

↑ Mean Arterial Pressure, ↓ Heart rate, ↓ Baroreceptor activity, ↑ Parasympathetic Outflow

↑ Mean Arterial Pressure, ↑ Heart rate, ↓ Baroreceptor activity, ↓ Parasympathetic Outflow

↑ Mean Arterial Pressure, ↑ Heart rate, ↑ Baroreceptor activity, ↑ Parasympathetic Outflow

A 32-year-old woman comes to the physician for a screening health examination that is required for scuba diving certification. The physician asks her to perform a breathing technique: following deep inspiration, she is instructed to forcefully exhale against a closed airway and contract her abdominal muscles while different cardiovascular parameters are evaluated. Which of the following effects is most likely after 10 seconds in this position?

Decreased left ventricular stroke volume

Decreased intra-abdominal pressure

Decreased systemic vascular resistance

Increased venous return to left atrium

A 30-year-old woman presents to clinic for a routine checkup. She reports that she is in good health but that she felt short of breath on her hiking and skiing trip to Colorado the week prior. She explains that this was the first time she has gone that high into the mountains and was slightly concerned for the first few days because she felt chronically short of breath. She reports a history of childhood asthma, but this experience did not feel the same. She was on the verge of seeking medical attention, but it resolved three days later, and she has felt fine ever since. What other listed physiological change results in a physiologic alteration similar to that which occurred in this patient?

Increase in concentration of 2,3-bisphosphoglycerate in blood

Increase in partial pressure of water in air

Increase in concentration of dissolved carbon dioxide in blood

Diving physiology for USMLE Step 2 CK: High-Yield Physiology Lessons

Diving Physics - Gas Laws & The Deep

Pressure increases by 1 atmosphere absolute (ATA) for every 10m (33ft) of descent.
Boyle's Law ($P_1V_1 = P_2V_2$): As pressure ↑, gas volume in body cavities (e.g., lungs, middle ear) ↓. Governs barotrauma risk, especially on ascent.
Henry's Law ($P = k_H \cdot C$): ↑ ambient pressure dissolves more nitrogen into tissues. Rapid ascent can cause bubble formation, leading to decompression sickness.
Dalton's Law: Total gas pressure is the sum of its partial pressures. Explains risks of gas toxicity (e.g., O₂, N₂) at depth.

⭐ Nitrogen narcosis ("rapture of the deep"), an anesthetic effect, can occur at depths of 30-40m due to the high partial pressure of nitrogen.

Barotrauma - Squeezes & Strains

Barotrauma is tissue damage from a pressure differential between a gas-filled body space and its surroundings, following Boyle's Law ($P_1V_1 = P_2V_2$).

Feature	Descent ("Squeeze")	Ascent ("Reverse Block")
Mechanism	↑ Ambient pressure → ↓ gas volume. Failure to equalize creates a vacuum.	↓ Ambient pressure → ↑ gas volume. Trapped gas expands, causing strain.
Common Sites	Middle ear, sinuses, mask.	Lungs (pulmonary barotrauma), sinuses, middle ear, GI tract, dental.
Pathology	Mucosal edema, hemorrhage, tympanic membrane rupture.	Alveolar rupture → pneumothorax, pneumomediastinum, subcutaneous emphysema, Arterial Gas Embolism (AGE).

Anatomy of the human ear with labeled parts

Pathogenesis of Arterial Gas Embolism (AGE)

Decompression Illness - Bubbles & Bends

Rapid ascent from a dive causes dissolved nitrogen ($N_2$) to come out of solution, forming bubbles in tissues and blood (Henry's Law), leading to Decompression Sickness (DCS). At depth, high partial pressure of nitrogen ($P_{N_2}$) can also cause Nitrogen Narcosis, an anesthetic effect known as 'Martini's Law' (equivalent to one martini per 10m of depth).

📌 Mnemonic (Risk Factors): DEEP - Dehydration, Exertion, Patent Foramen Ovale (PFO), Deep/long dives.

Type	Systems Affected	Clinical Signs
Type I DCS	Musculoskeletal, Skin, Lymphatic	"The Bends": Deep, aching joint pain (shoulders, elbows). Skin mottling (cutis marmorata), pitting edema.
Type II DCS	Neurologic, Cardiopulmonary	"The Staggers": CNS symptoms (vertigo, ataxia, paralysis). "The Chokes": Substernal chest pain, cough, respiratory distress.

Management:

Immediate: 100% $O_2$ via non-rebreather mask.
Definitive: Recompression with hyperbaric oxygen (HBOT) therapy.

Decompression Sickness and Arterial Gas Embolism

High‑Yield Points - ⚡ Biggest Takeaways

Boyle's law governs barotrauma; as pressure ↑, gas volume ↓, affecting ears, sinuses, and lungs.

Henry's law dictates that ↑ ambient pressure forces more nitrogen to dissolve into tissues.

Rapid ascent causes decompression sickness (“the bends”) as dissolved N₂ forms bubbles in blood and tissues.

Nitrogen narcosis at depth (>100 ft) impairs judgment due to high N₂ partial pressure.

Oxygen toxicity from high FiO₂ at pressure can cause seizures and lung damage.

Treat decompression sickness with recompression and 100% O₂.

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