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: NIMHANS Nursing Officer - 2021

Explanation

• The correct time to measure Central Venous Pressure (CVP) is at the end of expiration.
• During spontaneous breathing, intrathoracic pressure is least negative and most stable at the end of expiration.
• Measuring at this point minimizes the artifact caused by respiratory effort, providing the most accurate reflection of right atrial pressure (preload).
• This ensures consistency and reliability in hemodynamic monitoring, which is crucial for making clinical decisions about fluid management.

Why Other Options Were Wrong

• Option A: The manometer must be connected to the patient's central line and leveled correctly (at the phlebostatic axis) to obtain a pressure reading. Disconnecting it would make measurement impossible.
• Option B: CVP values fluctuate significantly throughout the respiratory cycle. Measuring at random points (either inspiration or expiration) would lead to inconsistent and inaccurate data, potentially causing incorrect clinical interventions.
• Option D: During spontaneous inspiration, the diaphragm contracts and the chest expands, creating negative intrathoracic pressure. This negative pressure is transmitted to the great veins and right atrium, causing the CVP reading to be artificially low.

Related Visual

Clinical Relevance

• Nursing practice connection: Prioritize focused assessment, early escalation, and real-time monitoring when managing Timing of Central Venous Pressure (CVP) Measurement in acute care settings.
• Accurate CVP measurement is fundamental for guiding fluid therapy. A falsely low reading (taken during inspiration) might lead to giving unnecessary fluids, risking fluid overload. A falsely high reading could lead to withholding necessary fluids.
• The standard point for measurement (end-expiration) should be identified on the monitor's waveform, which often displays a graphic of the respiratory cycle alongside the CVP trace.
• What if? If the patient was on a mechanical ventilator (positive pressure ventilation), the rule remains the same: measure at end-expiration. In this case, inspiration involves positive pressure that falsely raises the CVP, while end-expiration represents the lowest, most stable pressure in the cycle (the PEEP level).

How to Approach the Question

• First, identify the core of the question: it asks for the correct timing to measure CVP in a spontaneously breathing patient.
• Recall the basic physiology of breathing. In spontaneous breathing, inspiration creates negative pressure in the chest, while expiration is a passive return to a neutral baseline.
• Consider how pressure changes in the chest would affect a pressure reading from the right atrium (CVP). Negative pressure during inspiration will artificially lower the CVP reading.
• To get a true reading, you need to measure when the effect of breathing is minimal. This occurs at the end of expiration, when the respiratory muscles are relaxed and intrathoracic pressure is stable.
• Evaluate the options based on this principle: 'End inspiration' is wrong because it gives a falsely low reading. 'Disconnecting the manometer' is procedurally incorrect. 'Either is correct' is wrong because of the significant fluctuation. Therefore, 'End expiration' is the only correct choice.

Concept Tested & Keywords

• Concept Tested: Timing of Central Venous Pressure (CVP) Measurement
• Stem keywords: CVP, manometer, central line, correct reading, face mask
• Lead-in keywords: record the value at
• Clinical cues: Patient is on a face mask, which indicates spontaneous breathing.

Question ID

QoW1SHQ2Ve6JIKL9AmlnA6

Copy ID

Reference Book

E6 Nursing Fundamentals Taylor p. 764-766