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 57-year-old man is admitted to the burn unit after he was brought to the emergency room following an accidental fire in his house. His past medical history is unknown due to his current clinical condition. Currently, his blood pressure is 75/40 mmHg, pulse rate is 140/min, and respiratory rate is 17/min. The patient is subsequently intubated and started on aggressive fluid resuscitation. A Swan-Ganz catheter is inserted to clarify his volume status. Which of the following hemodynamic parameters would you expect to see in this patient?

Cardiac output: ↓, systemic vascular resistance: ↑, pulmonary artery wedge pressure: ↓

Cardiac output: ↓, systemic vascular resistance: ↔, pulmonary artery wedge pressure: ↔

Cardiac output: ↑, systemic vascular resistance: ↑, pulmonary artery wedge pressure: ↔

Cardiac output: ↑, systemic vascular resistance: ↓, pulmonary artery wedge pressure: ↔

Cardiac output: ↔, systemic vascular resistance: ↔, pulmonary artery wedge 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

A 34-year-old male is brought to the emergency department by fire and rescue following a motor vehicle accident in which the patient was an unrestrained driver. The paramedics report that the patient was struck from behind by a drunk driver. He was mentating well at the scene but complained of pain in his abdomen. The patient has no known past medical history. In the trauma bay, his temperature is 98.9°F (37.2°C), blood pressure is 86/51 mmHg, pulse is 138/min, and respirations are 18/min. The patient is somnolent but arousable to voice and pain. His lungs are clear to auscultation bilaterally. He is diffusely tender to palpation on abdominal exam with bruising over the left upper abdomen. His distal pulses are thready, and capillary refill is delayed bilaterally. Two large-bore peripheral intravenous lines are placed to bolus him with intravenous 0.9% saline. Chest radiograph shows multiple left lower rib fractures. Which of the following parameters is most likely to be seen in this patient?

Decreased pulmonary capillary wedge pressure

Increased mixed venous oxygen saturation

Decreased systemic vascular resistance

Increased right atrial pressure

Hemodynamic monitoring principles — USMLE Lesson

Hemodynamic Principles - Pressure, Flow, & Pipes

Ohm's Law for Fluids: Blood flow (Q) is driven by the pressure gradient (ΔP) and opposed by resistance (R).
- Formula: $Q = ΔP / R$
Resistance (Poiseuille's Law): Primarily determined by vessel radius (r), length (L), and blood viscosity (η).
- $R hickapprox Lη / r^4$
- 📌 Halving the radius increases resistance 16-fold.

Poiseuille's Law: Flow Rate, Viscosity, Radius, Length

⭐ Arterioles are the principal sites of systemic vascular resistance (SVR) due to their ability to change radius, significantly impacting blood pressure regulation.

Invasive Monitoring - Lines, Waves & Pressures

Arterial Line: For continuous BP monitoring & frequent arterial blood gas (ABG) sampling.
- Waveform: Systolic upstroke, peak pressure, dicrotic notch (aortic valve closure), diastolic runoff.
- Mean Arterial Pressure (MAP) = $⅓(SBP) + ⅔(DBP)$. Target > 65 mmHg.
Central Venous Pressure (CVP): Measures right atrial pressure; a proxy for RV preload.
- Normal: 2-8 mmHg.
- Waveform: a (atrial contraction), c (ventricular contraction/tricuspid bulge), x (atrial relaxation), v (venous filling), y (atrial emptying).
Pulmonary Artery (PA) Catheter:
- Measures PCWP (wedge pressure), a proxy for left atrial pressure.
- Normal PCWP: 6-12 mmHg.

⭐ Cannon a waves on CVP tracing are seen in conditions where the right atrium contracts against a closed tricuspid valve (e.g., complete heart block, ventricular tachycardia).

CVP Waveforms in Cardiac Conditions

The Swan-Ganz - A Float Through The Heart

A pulmonary artery catheter (PAC) providing a continuous, real-time assessment of cardiac function and volume status by measuring pressures as it floats through the right heart.
Catheter Path & Typical Pressures (mmHg):
- Right Atrium (RA): Measures central venous pressure (CVP). Normal: 2-8.
- Right Ventricle (RV): Sharp, pulsatile waveform. Normal: 25/5.
- Pulmonary Artery (PA): Dicrotic notch appears. Normal: 25/10.
- Wedge (PCWP): Balloon inflation occludes a PA branch. Normal: <12.

⭐ PCWP provides an indirect estimate of Left Atrial Pressure (LAP), a key indicator of left ventricular preload and mitral valve function.

Swan-Ganz catheter path and pressure waveforms

Shock Profiles - Decoding The Disaster

Shock Type	CVP/PCWP	CO	SVR
Hypovolemic	↓	↓	↑
Cardiogenic	↑	↓	↑
Distributive	↓	↑ (early)	↓
Obstructive	↑	↓	↑

CO: Cardiac Output
SVR: Afterload

Hemodynamic Profiles of Shock Types

⭐ In early septic shock (distributive), cardiac output is uniquely high while SVR plummets. Mixed venous oxygen saturation (SvO2) is often >70% due to impaired cellular O₂ extraction.

High‑Yield Points - ⚡ Biggest Takeaways

Pulmonary Artery Catheter (PAC) directly measures CVP, PAP, and PCWP, and allows for CO calculation.

PCWP is a reliable estimate of left atrial pressure and left ventricular end-diastolic pressure (LVEDP) in the absence of mitral stenosis.

Cardiac Output (CO) is most commonly determined via thermodilution.

Systemic Vascular Resistance (SVR) is a calculated variable representing afterload, not directly measured.

Mixed venous oxygen saturation (SvO2) reflects the balance of oxygen delivery and consumption.

Differentiating shock states relies on interpreting patterns: cardiogenic shock shows ↑PCWP, ↓CO; septic shock (early) shows ↓SVR, ↑CO.

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