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Understanding the phlebostatic axis

Iversen, Chelsea MS, RN

doi: 10.1097/01.NME.0000403195.75792.a4
Department: Ask an Expert

Trauma ICU RN • Erie County Medical Center • Buffalo, N.Y.

The author has disclosed that she has no significant relationship with or financial interest in any commercial companies that pertain to this educational activity.

Q: What's the phlebostatic axis and what does it mean when monitoring my patient's vital signs?

A: Venous and arterial catheters provide the most accurate method for hemodynamic monitoring. The nurse must be confident that central venous pressure (CVP) and arterial BP measurements are accurate. The phlebostatic axis is the reference point for zeroing the hemodynamic monitoring device. This reference point is important because it helps to ensure the accuracy of the various pressure readings. Nurses must ensure the accuracy of their hemodynamic monitoring devices.

Figure. Acc

Figure. Acc

Several factors can produce artifacts that decrease the accuracy of pressure monitoring, which establishes a risk of error in patient management. These factors include the length of the pressurized I.V. tubing (4 foot-long tubing is preferred to facilitate accurate placement and zeroing of devices), air bubbles in the system, and incorrect placement of the device stopcock at an inaccurate reference point. Understanding the phlebostatic axis, particularly its importance as a reference point for stopcock placement, helps to ensure accurate readings.

With the head of bed at 60 degrees or less, the phlebostatic axis is located at the fourth intercostal space at the mid-anterior-posterior diameter of the chest wall. This is the location of the right atrium, where the tip of a CVP catheter would lay. Maintaining the device stopcock at the phlebostatic axis will help ensure a proper reading. If the stopcock is positioned below the phlebostatic axis, the readings will be erroneously high; if above the phlebostatic axis, the readings will be erroneously low. Either error can lead to mismanagement of patients and their care.

The device should be zeroed once each day and whenever the accuracy of a reading has been called into question. Zeroing of the device with the patient in an improper position can lead to inaccurate measurements. A backrest position of less than or equal to 60 degrees with the patient's back flat against the surface of the bed won't compromise the accuracy of the reading. Positioning a patient in a 90-degree lateral position is the only acceptable side-lying position that won't affect the reading.

In this case, the reference points for zeroing the pressure monitoring system change. In a right lateral position, at 90 degrees with the backrest flat, the zeroing reference point is the intersection of the fourth intercostal space and midsternum. In a left lateral position, at 90 degrees with the backrest flat, the zeroing reference point is the intersection of the fourth intercostal space and the left parasternal border.

Continuous and accurate hemodynamic measurements may be lifesaving for critically ill patients. Improper use endangers patient outcomes.

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Bridges EJ. Pulmonary artery pressure monitoring: when, how, and what else to use. AACN Adv Crit Care. 2006;17(3):286–303.
    . Cheatham ML. Hemodynamic monitoring: principles to practice.
      . Leeper B. Obtaining valid hemodynamic measures: evidence-based practice.
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