Norepinephrine titration: changing the dose
A 39.6 lb dog is on a norepinephrine CRI from a 4 mg in 250 mL NaCl bag (final concentration 16 µg/mL) at 0.05 µg/kg/min. MAP is only 58 mmHg and you want to titrate up to 0.2 µg/kg/min. What's the new pump rate (mL/hr)?
Hint
Convert weight first. The starting dose is a distractor. When you titrate, you recalculate from the new dose. Same math, new number.
Another hint
(1) lb → kg via 2.2 lb/kg. (2) Pump rate (mL/hr) = (dose × weight × 60) ÷ bag concentration. The 60 is min → hr. Bag concentration is already given in µg/mL so no further conversion.
Show worked answer
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Convert the patient's weight from lb to kg.
$$\frac{39.6 \,\cancel{lb}}{2.2 \,\cancel{lb}/kg} = 18 \,kg$$ -
Compute µg/min at the new dose.
$$0.2 \,\tfrac{\mu g}{\cancel{kg}\cdot min} \times 18\,\cancel{kg} = 3.6 \,\tfrac{\mu g}{min}$$ -
Convert µg/min to µg/hr.
$$3.6 \,\tfrac{\mu g}{\cancel{min}} \times 60\,\tfrac{\cancel{min}}{hr} = 216 \,\tfrac{\mu g}{hr}$$ -
Divide by bag concentration to get mL/hr.
$$\frac{216 \,\cancel{\mu g}/hr}{16 \,\cancel{\mu g}/mL} = 13.5 \,\tfrac{mL}{hr}$$ -
Shortcut for next time: at this bag concentration and weight, mL/hr ≈ (dose in µg/kg/min) × 67.5. Useful for fast bedside titration once the first dose is set up.
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Why norepinephrine raises blood pressure: it's a potent α-1 agonist with modest β-1 activity. The α-1 effect is what drives the pressor response (peripheral vasoconstriction → increased systemic vascular resistance). The β-1 effect adds modest inotropy. Unlike dopamine, norepi has minimal effect at dopaminergic receptors. This is why it's preferred for vasodilatory shock where the primary problem is loss of vascular tone.
Set the pump to 13.5 mL/hr to deliver 0.2 µg/kg/min.