A nurse is monitoring manometer for CVP value from a central line inserted via the brachial vein. CVP line inserted in a patient in ICU on face mask with oxygen. For correct reading she should record the value at?

1. A

   She should disconnect the manometer just before reading

2. B

   Either is correct

3. C

   End expiration

4. D

   End inspiration

Appeared in: AIIMS Rishikesh & Jodhpur NO - 2017

Explanation

• The most accurate CVP reading is obtained at the end of expiration.
• During end-expiration, intrathoracic pressure is at its baseline (closest to atmospheric pressure), minimizing its effect on the CVP value.
• This provides a consistent and reliable measurement that accurately reflects the patient's right atrial pressure and fluid status.

Why Other Options Were Wrong

• Option A: Disconnecting the manometer is a procedural error. It breaks the sterile, closed system, prevents a reading from being taken, and poses a risk of air embolism or infection.
• Option B: This is false. The respiratory cycle causes significant fluctuations in intrathoracic pressure, which directly and predictably alters CVP readings. Therefore, the timing of the measurement is critical for accuracy and consistency.
• Option D: During spontaneous inspiration, the chest cavity expands, creating negative intrathoracic pressure. This negative pressure is transmitted to the central veins, artificially lowering the CVP reading and resulting in a falsely low value.

Related Visual

Clinical Relevance

• Nursing practice connection: Prioritize focused assessment, early escalation, and real-time monitoring when managing Hemodynamic Monitoring: Central Venous Pressure (CVP) Measurement in acute care settings.
• Accurate CVP monitoring is crucial for guiding fluid resuscitation in critically ill patients, preventing both under-resuscitation (leading to organ hypoperfusion) and over-resuscitation (leading to pulmonary edema and heart failure).
• Consistency is key. All nurses caring for the patient must measure the CVP at the same point in the respiratory cycle (end-expiration) to ensure that trends in the data are reliable and not just artifacts of measurement technique.
• What if the patient was on a mechanical ventilator? The principle remains the same: measure at end-expiration. However, with positive pressure ventilation, inspiration artificially increases the CVP. End-expiration (when ventilator-applied pressure is lowest) still provides the most accurate baseline reading.

How to Approach the Question

• First, identify the core of the question: it asks for the correct timing of a CVP measurement in relation to breathing.
• Recall the physiology of spontaneous respiration. Remember that inspiration creates negative pressure in the chest, while expiration is a passive process where pressure returns to a neutral baseline.
• Consider how pressure changes in the chest would affect a pressure reading from the right atrium (CVP). Negative pressure will pull the reading down, while positive pressure would push it up.
• Deduce that the most stable and true reading will be obtained when the influence of breathing is minimal, which is at the end of expiration.
• Evaluate the other options. Disconnecting equipment is a procedural error, not a measurement technique. 'Either' is incorrect because the timing does matter. This leaves 'end expiration' as the only logical and physiologically sound answer.

Concept Tested & Keywords

• Concept Tested: Hemodynamic Monitoring: Central Venous Pressure (CVP) Measurement
• Stem keywords: CVP, central line, manometer, monitoring
• Lead-in keywords: correct reading
• Clinical cues: Patient in ICU on face mask with oxygen (indicates spontaneous breathing)

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