Syringes and Needles

The basic parts of a syringe are the barrel, plunger, and tip. The barrel is a tube that is open at one end and tapers into a hollow tip at the other end. The plunger is a piston-type rod with a slightly cone-shaped top that passes inside the barrel of the syringe. The tip of the syringe provides the point of attachment for a needle. The volume of solution inside a syringe is indicated by graduation lines on the barrel. Graduation lines may be in milliliters or fractions of a milliliter, depending on the capacity of the syringe. The larger the capacity, the larger the interval between graduation lines.

There are three common types of syringe tips: Slip-Tip®, Luer-Lok®, and eccentric. Slip-Tips® allow the needle to be held on the syringe by friction. The needle is reasonably secure, but it may come off if not properly attached or if considerable pressure is used. Luer-Lok® tips incorporate a collar with grooves that lock the needle in place. Eccentric tips, which are off-center, are used when the needle must be parallel to the plane of injection such as in an intradermal injection.

Syringes come is different sizes ranging from 1 to 60 ml. As a rule, select a syringe whose capacity is the next size larger than the volume to be measured. For example, a 3 ml syringe should be selected to measure 2.3 ml, or a 5 ml syringe to measure 3.8 ml. In this way, the graduation marks on the syringe will be in the smallest possible increments for the volume measured. Syringes should not be filled to capacity because the plunger can be easily dislodged.

The Cornwall syringe is used when many repetitions of filling a syringe to the same volume is needed. It is a manual device that has a two-way valve that attaches to both the syringe and the solution to be drawn into the syringe. Each time the grip is released, the syringe fills with solution. Each time the grip is compressed, the syringe volume is expelled. There are electronic versions of this basic design concept.

Needles

A needle has three parts, the hub, the shaft, and the bevel. The hub is at one end of the needle and is the part that attaches to the syringe. The shaft is the long slender stem of the needle that is beveled at one end to form a point. The hollow bore of the needle shaft is known as the lumen. Disposable needles should always be used when preparing admixtures as they are presterilized and individually wrapped to maintain sterility.

Needle size is designated by length and gauge. The length of a needle is measured in inches from the juncture of the hub and the shaft to the tip of the point. Needle lengths range from 3/8 inch to 3 1/2 inches; some special use needles are even longer. The gauge of a needle, used to designate the size of the lumen, ranges from 27 (the finest) to 13 (the largest).

There are two considerations when choosing a needle size; the viscosity of the solution, and the nature of the rubber closure on the parenteral container. Needles with larger lumens should be used for viscous solutions. Smaller gauge needles are preferred if the rubber closure can be cored easily. Coring is when a needle punctures or tears a piece of the rubber closure and the piece then falls into the container and creates particulate material contamination.

Vascular Access Needle Gauge Recommendations