**Core Concept:** Normal capillary wedge pressure (CWP) is the pressure within the left atrium during ventricular systole. It is a crucial parameter in assessing left ventricular filling pressures in various clinical scenarios, particularly in evaluating heart failure patients.

**Why the Correct Answer is Right:** Normal capillary wedge pressure (CWP) is typically between 5 and 15 mmHg in healthy individuals. A value within this range indicates adequate left ventricular filling and normal ventricular function. It is important to differentiate this from elevated capillary wedge pressure (CWP), which can indicate congestive heart failure or other cardiac disorders affecting left ventricular filling.

**Why Each Wrong Option is Incorrect:**

A. **5 mmHg**: This value is too low and is indicative of low cardiac output and hypovolemia.
B. **18 mmHg**: This value is too high, suggesting increased left atrial pressure, elevated pulmonary capillary wedge pressure (PCWP), or both.
C. **15 mmHg**: This value is within the normal range, but the question asks for a specific value distinguishing normal and elevated capillary wedge pressure.
D. **20 mmHg**: This value is significantly elevated and indicates increased left atrial pressure, congestive heart failure, and impaired left ventricular filling.

**Clinical Pearl / High-Yield Fact:** Elevated capillary wedge pressure (CWP) is a crucial diagnostic indicator in heart failure patients, as it suggests congestive heart failure and impaired left ventricular filling. CWP can be measured invasively via a Swan-Ganz catheter insertion into the pulmonary artery or non-invasively via echocardiography, provided the patient meets the criteria for elevated PCWP.

**Correct Answer:** D. 20 mmHg

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In summary, normal capillary wedge pressure (CWP) is within the range of 5-15 mmHg, while elevated capillary wedge pressure (CWP) is indicative of congestive heart failure and impaired left ventricular filling. This information is crucial for diagnosing heart failure patients and guiding therapeutic interventions, particularly when assessing pulmonary artery pressures using invasive or non-invasive methods.