2.2 Winged Infusion Sets & Difficult Vein Access

Winged Infusion Sets & Difficult Vein Access

Phlebotomy Technicians routinely encounter patients with compromised, fragile, rolling, or inaccessible veins. Standard Evacuated Tube System (ETS) equipment with rigid 21-gauge needles can easily puncture through delicate vessels or cause thin-walled veins to collapse under full vacuum pressure. Mastering specialized equipment—specifically Winged Infusion Sets (butterflies) and syringe assemblies—along with advanced stabilization techniques, is vital for managing difficult venous access.

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Section 1: Winged Infusion Set (Butterfly) Mechanics

A Winged Infusion Set, commonly called a butterfly needle, consists of a short needle (1/2 to 3/4 inch long) anchored between two flexible plastic wings, connected to a length of thin, flexible tubing (5 to 12 inches) ending in a Luer-lock adapter or ETS hub attachment.

Gauge Sizes & Indications

Butterfly needles are manufactured in higher gauge sizes (smaller needle diameters), typically 23-gauge (light blue wings) and 25-gauge (dark blue wings).

• Primary Indications:
  • Small, fragile, or thinned veins in geriatric or pediatric patients.
  • Superficial dorsal hand veins.
  • Veins prone to rolling or collapsing.
  • Patients with restrictive contractures or severe arthritis preventing arm hyperextension.

Insertion Mechanics & Angle

Because hand and delicate forearm veins lie very close to the skin surface, the insertion angle for a butterfly set must be much shallower than a standard needle:

• Insertion Angle: 10 to 15 degrees relative to the skin surface with the needle bevel facing up.
• Visual Flash Signal: As the needle tip enters the vein lumen, blood immediately enters the transparent tubing, producing a visible "flash of blood." This confirms entry into the vessel before an evacuated tube is engaged.

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Section 2: The Critical Light Blue Discard Tube Protocol

One of the most frequent preanalytical errors on the ASCP exam involves the improper use of a butterfly needle when drawing a Sodium Citrate (Light Blue top) tube for coagulation studies (e.g., Prothrombin Time [PT] / International Normalized Ratio [INR], Activated Partial Thromboplastin Time [aPTT]).

The Tubing Air Space Problem

The interior lumen of winged infusion tubing contains dead air space (typically 0.5 mL of air). If a sodium citrate tube is connected directly to the butterfly tubing as the first tube in the draw:

1. The 0.5 mL of air in the tubing is pulled into the vacuum tube first.
2. This air displaces 0.5 mL of blood volume, resulting in an underfilled tube.
3. Sodium citrate tubes contain a fixed volume of liquid anticoagulant (3.2% sodium citrate). Underfilling the tube alters the mandatory 9:1 blood-to-anticoagulant ratio.
4. Diagnostic Impact: Excess free citrate binds calcium in the testing reagent, producing falsely prolonged coagulation times (falsely high PT/INR or aPTT results).

The Correct Discard Protocol

When using a butterfly set, if a light blue tube is ordered first (or if no blood cultures/red tubes precede it), you must draw a discard tube first:

• Discard Tube Selection: Use a non-additive tube (plain red top) or another light blue tube as a discard.
• Execution: Attach the discard tube and allow blood to flow into it just until the tubing is filled with blood and air is purged. Remove the discard tube (it does not need to fill completely) and discard it. Then attach the primary light blue tube, which will fill completely to its proper indicator mark, preserving the 9:1 ratio.

[Butterfly Needle] ---> (0.5 mL Air in Tubing) ---> [Discard Tube (Purges Air)] ---> [Light Blue Tube (Achieves 9:1 Ratio)]

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Section 3: Difficult Vein Access Strategies

When standard antecubital veins are absent or compromised, phlebotomists must employ specialized physical and technical adjustments.

Physical Site Enhancement Techniques

• Warming the Site: Apply a warm, moist towel or commercial heat pack (warmed to no more than 42°C) to the arm for 3 to 5 minutes. Heat causes localized vasodilation, expanding vein diameter up to 7 times.
• Gravity Assistance: Allow the arm to hang downward over the side of the chair for 1 to 2 minutes to increase venous pooling.
• Gentle Massage: Massage the arm gently from wrist upward toward the antecubital fossa to encourage venous filling. Never slap the skin, as this causes localized inflammation and invalidates analyte testing.

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Section 4: Syringe Collection & Safety Transfer Devices

When drawing from extremely delicate or collapsing veins, a syringe system allows the phlebotomist to manually control the amount of vacuum suction applied to the vessel wall.

Syringe Draw Mechanics

1. Select a syringe size matched to the total volume needed (e.g., 5 mL or 10 mL syringe).
2. Break the Plunger Seal: Before attaching the needle, move the syringe plunger back and forth to loosen the rubber seal and purge air.
3. Perform the venipuncture using a syringe needle or butterfly-syringe adapter.
4. Once blood appears in the syringe hub, gently pull back on the plunger. Pull slowly and steadily. Pulling too forcefully creates high negative pressure, collapsing the vein or causing red blood cell destruction (hemolysis).

Safe Transfer to Evacuated Tubes

Never use a standard syringe needle to manually puncture evacuated tube stoppers while holding the tubes in your hand. This is a severe needle-stick hazard and causes cell lysis.

[Syringe with Blood] ---> [Attach Safety Transfer Device] ---> [Push Tube Into Device] ---> [Vacuum Pulls Blood Automatically]

• Safety Transfer Device Protocol:
  1. Activate the syringe needle's safety shield and detach the needle into a sharps container.
  2. Thread the syringe barrel directly onto an OSHA-approved Blood Transfer Device.
  3. Insert evacuated collection tubes into the transfer device in the standard Order of Draw.
  4. Never press the syringe plunger. Allow the internal vacuum of each evacuated tube to pull the blood naturally from the syringe. Depressing the plunger forces blood under high pressure through the internal needle, ruining the sample via hemolysis.