Confirmation of placement of central line: artery vs. vein

November 26th, 2009
by reuben in vascular access

Excerpted from MSSM M&M teaching points:

* Accidental arterial cannulation is usually benign but should be detected before dilation to avoid significant vessel injury. When performing an ultrasound-guided central line, the point where the needle tip enters the vein must be in the visualized ultrasound field. This may be accomplished by entering the skin with the needle a short distance away from the probe, rather than immediately adjacent to the probe, which will lead to the tip entering the vein out of the vertical plane visualized on the screen. Alternatively, the tip of the needle can be tracked into the vein by sliding the probe along the skin as the needle tip approaches the vein, keeping the tip in the visualized ultrasound field.

* If venous placement is not certain, verification may be performed using a number of techniques.
**A quick blood gas may be helpful if the PaO2/SaO2 values are conclusive; unfortunately blood gas results may lie in between definitively arterial and venous values.
** Transducing the pressure waveform is effective but takes time to set up.
** A brief and definitive technique uses a quick pressure column setup as follows:

1. Insert the guidewire, remove the needle.
2. Slide the conventional angiocath that comes in all central line kits over the wire, remove the wire.
3. Attach an extension set to the angiocath. An extension set comes in the introducer kit for this purpose. For triple lumen kits, ask the nurse to give you an extension set, or use the circular plastic sheath that stores the guidewire as extension tubing.
4. Keeping the tubing parallel to the floor, allow 20-30 cm of blood to fill the tube.
5. Hold the tube straight up to the ceiling. If the angiocath is in a vein, the column of blood will fall back down to the level of the CVP. If the angiocath is in an artery, the column of blood will continue to rise.
6. If the angiocath is in the vein, thread the guidewire, pull out the angiocath and continue Seldinger technique as usual. If the angiocath is in the artery, either remove the angiocath and hold pressure or call vascular surgery for advice.

** To estimate how low a bag of saline needs to be so that it does not overcome arterial blood pressure, use the formula SBP/2 = height in inches. For example, if the patient has an SBP of 70 mmHg, this corresponds to a height of 35 inches. If the fluid bag is hanging more than 35 inches above a patient with an SBP of 70, it will flow into an artery, fooling the unsuspecting observer into thinking the catheter is in a vein. Therefore, to verify venous placement by attaching a bag of saline, the bag of saline must be lower in inches than half of the patient’s systolic blood pressure.

Demonstrations and discussions of full sterile technique, quick pressure column technique, and a number of other central line-related topics can be found on Haru Okuda and Scott Weingart’s central line project page:

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Responses to “Confirmation of placement of central line: artery vs. vein”

  1. Question from resident:

    With regards to the height of the saline, do we use SBP or MAP? Just wanted to clarify.

    My reply:

    Let’s take a patient with a BP of 70/40 mm Hg resulting in a MAP of 50 mm Hg. We’ll also assign him a CVP of 4 mm Hg.

    Note that CVP is variously reported in mm Hg and cm H20 which is confusing. As mercury is 13.6 times as dense as water, the conversion from mm Hg to cm H20 is 1.36. Since 2.54 cm are in one inch, the conversion from mm Hg to inches H20 is .53 or, roughly, half.

    I’m going to convert all pressures in this patient to inches water (which is roughly equivalent to inches blood or inches saline) to illustrate. For this patient

    CVP = 2 inches water
    MAP = 25 inches water
    SBP = 35 inches water

    So, in this patient, for a venous catheter, if a bag of saline is at a height of

    1 inch: no forward flow
    3 inches: continuous forward flow

    For an arterial catheter, if a bag of saline is at at a height of

    20 inches: no forward flow
    40 inches: continuous forward flow

    The question is what would happen if the bag were at a height of 30 inches, i.e. between MAP and SBP. My guess is that either there would be no forward flow (in which case the recommendation should be to use MAP and not SBP) or there would be intermittent forward flow, as in, forward flow during diastole and no flow during systole.

    In any event, the point is that in a hypotensive patient, if the saline bag is a relatively elevated above the bed (which, in a hypotensive patient, it often is, to increase flow), you will see forward flow even if the catheter is in the artery. So, in cases where venous position is not certain, keep the bag initially at a low height, e.g. 15 inches, which will overcome CVP in nearly everyone and be less than MAP in nearly everyone.

  2. Pingback:Critical Care – Central Line Placement | Sinai EM Media Site

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