Syringe Pump Infusion Technique
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Direct answer
A syringe pump pushes a plunger at a programmed rate in mL/h, so safe use rests on one conversion: rate (mL/h) = dose (microgram/kg/min) × weight (kg) × 60 ÷ concentration (microgram/mL). For a standard noradrenaline preparation of 8 mg in 100 mL (80 microgram/mL) in a 70 kg patient at 0.1 microgram/kg/min, the pump runs at 0.1 × 70 × 60 ÷ 80 = 5.25 mL/h. The pump is loaded with the syringe clamped true, the line primed and purged away from the patient, an anti-siphon valve in the circuit to prevent free flow, and the occlusion pressure limit set so clots signal early without bursting. Smart pumps add dose-error reduction software — drug libraries with soft limits that warn and hard limits that refuse — and patient-controlled analgesia pumps add a demand dose, lockout interval (commonly 5-10 minutes) and a maximum per hour, with the iron rule that only the patient presses the button. The classic dangers are the post-occlusion bolus (pressurised fluid released at clot displacement) and the unlabelled line, both prevented by procedure rather than vigilance.
What you must remember
- The master equation: rate mL/h = dose (microgram/kg/min) × weight (kg) × 60 ÷ concentration (microgram/mL); for milligram/kg/h doses, drop the 60.
- Worked noradrenaline: 8 mg in 100 mL = 80 microgram/mL; 70 kg at 0.1 microgram/kg/min = 5.25 mL/h — and doubling the dose doubles the rate, so titration arithmetic is linear and fast at the bedside.
- Worked propofol: neat 10 mg/mL at 3 mg/kg/h in a 70 kg patient = 210 mg/h = 21 mL/h, the everyday TIVA calculation that must be reproducible under exam pressure.
- Occlusion alarm logic: pressure limits (commonly 100-300 mmHg settings) detect blocked lines; when an occlusion clears, the stored elastic energy releases a bolus — aspirate before flushing, and never silence the alarm and simply restart.
- Dead space discipline: at low rates, drug reaches the patient only after the carrier flow traverses the line's dead volume — start vasopressors through short, dedicated lines, never piggy-backed through a slow-running distant lumen.
- PCA configuration: demand dose (for example morphine 1 mg), lockout 5-10 minutes, and a 1-hour maximum; a background infusion is used only in select palliative or opioid-tolerant patients, and family members never press the button.
Setting up a noradrenaline infusion, end to end
A septic patient needs noradrenaline. The preparation is standardised: 8 mg into 100 mL of saline — 80 microgram/mL — labelled with drug, concentration, time and preparer. The syringe loads into the pump, checked seated (a half-seated syringe delivers a fraction of the displayed rate), the line primed and purged into a receiver, never toward the patient, and the drug library entry selected rather than a generic mL/h mode: weight entered, dose in microgram/kg/min, hard limits configured.
The infusion starts at 0.05 microgram/kg/min — 2.6 mL/h by the master equation — through a dedicated central lumen, because peripheral extravasation of a vasopressor causes digit-threatening ischaemia. Ten minutes later the mean pressure sags: the dose doubles to 0.1, the rate to 5.25 mL/h — every titration is multiplication, not re-derivation. When the pump alarms occlusion, aspirate first, flush gently, then restart; jamming the flush forward is the exact mechanism of the accidental bolus. On weaning day the rate falls to 1-2 mL/h, where dead space dominates, and the line is shortened or the concentration changed.
Where students slip
The equation is memorised but the units are mangled: candidates multiply by 60 when the dose is already per hour, or mix microgram and milligram between dose and concentration. Papers plant the trap in the dilution — a "500 microgram/mL" preparation halves rates compared with the standard, and the candidate who memorised one rate instead of the formula fails the variant. The second slip is conceptual: believing the displayed rate equals delivery — seating errors, occlusions and dead space all break that equality, which is why the anti-siphon valve, the aspiration-first rule and dedicated lumens exist. PCA questions fail on the lockout concept, and the "family must not press" rule is a patient-safety answer examiners explicitly reward; the free-flow catastrophe is prevented by a mechanical valve, not by attention.
Frequently asked questions
How is a syringe pump rate calculated from a dose?
Rate in mL/h = dose in microgram/kg/min × weight in kg × 60 ÷ concentration in microgram/mL; for doses already per hour, omit the 60.
What is the standard noradrenaline concentration and its rate at 0.1 microgram/kg/min for 70 kg?
Commonly 8 mg in 100 mL (80 microgram/mL); the rate is 0.1 × 70 × 60 ÷ 80 = 5.25 mL/h.
What is the post-occlusion bolus and how is it prevented?
When an occluded line releases, elastic energy stored in the tubing delivers a sudden drug bolus; prevention is aspirating before flushing, appropriate occlusion pressure limits, and treating alarms as patient events rather than clearing them.
Why do low infusion rates delay drug delivery?
The drug must first fill the line's dead volume carried by the carrier flow, so at very low rates minutes pass before concentration reaches the patient — managed with short, dedicated lines and higher carrier flows for potent drugs.
How is a PCA pump configured?
With a demand dose (for example 1 mg morphine), a lockout interval of 5-10 minutes, and a maximum dose per hour; only the patient presses the button, and background infusions are exceptional.