Syringe dead space: difference between revisions
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| 1 | {{Infobox concept | 1 | {{Infobox concept |
| 2 | | name = Syringe dead space | 2 | | name = Syringe dead space |
| + | 3 | | subtitle = Residual volume | |
| 3 | | image = syringe.svg | 4 | | image = syringe.svg |
| 4 | | caption = Dead space lies distal to the plunger tip at full depression: the hub cavity, the needle lumen and any recess in the plunger face. | 5 | | caption = Dead space lies distal to the plunger tip at full depression: the hub cavity, the needle lumen and any recess in the plunger face. |
| ⋮ | ⋮ | ||
| 70 | === Effect of needle length === | 71 | === Effect of needle length === |
| 71 | Doubling cannula length from 12.7 mm to 25.4 mm doubles the lumen volume, taking a 25G needle from 0.67 to 1.34 µL. Against a hub of 60–70 µL this is immaterial. Needle length is selected on anatomical grounds — 4, 6 and 8 mm pen needles and 12.7 mm syringe needles are the common lengths for [[Subcutaneous injection|subcutaneous administration]] — and not on dead-space grounds.{{r|iso7864}} | 72 | Doubling cannula length from 12.7 mm to 25.4 mm doubles the lumen volume, taking a 25G needle from 0.67 to 1.34 µL. Against a hub of 60–70 µL this is immaterial. Needle length is selected on anatomical grounds — 4, 6 and 8 mm pen needles and 12.7 mm syringe needles are the common lengths for [[Subcutaneous injection|subcutaneous administration]] — and not on dead-space grounds.{{r|iso7864}} |
| + | 73 | ||
| + | 74 | == Consequences for delivered dose and for vial yield == | |
| + | 75 | The two consequences are separable and are treated separately here because the distinction is the most common error in secondary discussion of the topic. | |
| + | 76 | ||
| + | 77 | '''Wastage.''' When a syringe is filled by drawing liquid through the needle, the hub and lumen fill first and remain full. Setting the plunger tip to a graduation then places the nominal volume in the barrel ''in addition'' to the dead-space volume already drawn. Full depression delivers the barrel volume — the dose is correct — and discards the dead-space volume with the device. The loss is to vial yield, not to the dose. | |
| + | 78 | ||
| + | 79 | '''Dose error.''' If an air pocket equal to the dead space sits in the hub when the graduation is read, the barrel contains that much less liquid than the graduation indicates. Expelling the air before injection then leaves a shortfall equal to the air volume. The proportional error is the dead-space volume divided by the intended dose. | |
| + | 80 | ||
| + | 81 | A worked case. A vial containing 5.0 mg of peptide is reconstituted with 2.00 mL of diluent, giving 2.50 mg·mL⁻¹. An intended dose of 0.50 mg corresponds to 0.20 mL. | |
| 72 | 82 | ||
| 73 | == References == | 83 | == References == |