Syringe dead space (revision 8)

Syringe dead space is the volume of liquid that remains inside a syringe and its needle after the plunger has been fully depressed. It occupies the cavity of the needle hub, the lumen of the needle itself and any recess in the face of the plunger tip, and it is not delivered to the injection site.^[1]

The quantity matters for two distinct reasons that are frequently conflated. First, the retained liquid is discarded with the device, so each injection consumes the nominal dose plus the dead-space volume; over the life of a reconstituted vial this reduces the number of doses obtainable. Second, if the dead space is occupied by air rather than liquid at the moment the plunger position is read against the graduations, and that air is subsequently expelled, the volume actually delivered is smaller than the volume indicated. The first effect is a wastage problem and does not alter the delivered dose; the second is a dose error and can be large.^[2]

Magnitudes vary across almost two orders of magnitude between device types. A 1 mL insulin syringe with a permanently attached fine needle has a dead space of a few microlitres, because the plunger tip enters the needle hub and displaces almost all of it. A general-purpose syringe with a detachable needle on a Luer fitting retains of the order of 70–100 µL, most of it in the hub rather than in the needle.^[1][3]

A hypodermic syringe delivers liquid by advancing a plunger through a graduated barrel. The graduations relate to the position of the plunger tip, so the volume expelled is the barrel volume swept between the starting and finishing plunger positions. Any volume distal to the plunger tip at full depression is outside that swept volume and is retained.

Three cavities contribute:

• the hub, the conical or cylindrical space between the end of the barrel and the proximal end of the needle cannula, which in a Luer fitting is a substantial void;
• the needle lumen, the bore of the cannula itself;
• the plunger-tip recess, any concavity in the elastomeric or moulded tip that is not filled by the barrel end when fully advanced.

Terminology in the literature is inconsistent. Dead space, dead volume, residual volume and hold-up volume are all used for the same quantity, and residual volume is also used in a different sense for liquid left in the source container after withdrawal. The two are separate: the liquid left in a vial because the needle cannot reach it, or because the vial geometry traps it, is a container property and is discussed at underfilling and at vial. This article uses dead space for the syringe-and-needle quantity only.^[4]