Phenylephrine dosage intra op
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phenylephrine intraoperative hypotension dose
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phenylephrine vasopressor anesthesia hypotension
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I now have comprehensive data from multiple authoritative anesthesia textbooks. Here is a consolidated reference:
Phenylephrine — Intraoperative Dosing
Mechanism
Pure selective α1-adrenergic agonist (noncatecholamine). Causes peripheral vasoconstriction → ↑SVR → ↑MAP. Reflex vagally-mediated bradycardia can reduce cardiac output. No β-activity.
IV Bolus (Intermittent)
| Clinical Context | Dose |
|---|---|
| General adult intraoperative hypotension | 50–100 mcg (0.5–1 mcg/kg) IV per bolus |
| Neuraxial (spinal/epidural) hypotension — general | 50–100 mcg IV bolus |
| Obstetric spinal hypotension (cesarean section) | 40–120 mcg IV bolus |
| Pediatric bolus | 5–20 mcg/kg IV (max single dose 200 mcg) |
| Induction hemodynamic rescue (aortic/vascular surgery) | 50–100 mcg IV bolus |
| Bolus for post-intubation/post-sedation hypotension | 40–100 mcg over 20–30 sec, repeat every 2–5 min as needed |
Onset: Rapid (within seconds IV). Duration: ~15 min per single dose.
IV Continuous Infusion
| Use | Rate |
|---|---|
| Intraoperative hypotension / shock | 100–180 mcg/min (or 0.5 mcg/kg/min), titrate to response |
| Obstetric spinal hypotension prophylaxis | Prophylactic infusion; titrate to maintain MAP; typical starting rate ~25–50 mcg/min, often automated/closed-loop in high-volume obstetric centers |
| Post-CPB / cardiac surgery vasodilation | Per clinical response; used when norepinephrine inadequate |
Preparation
- Commercial ampule: 1% solution = 10 mg/mL
- Standard dilution: 100 mcg/mL (add 1 mL of 1% to 99 mL NS)
- Tachyphylaxis can occur with infusions → may require upward titration
Key Hemodynamic Effects
| Parameter | Effect |
|---|---|
| SVR / MAP | ↑↑ |
| Heart rate | ↓ (reflex bradycardia) |
| Cardiac output | ↓ (reflex, especially at higher doses) |
| β-activity | None |
Special Situations
- Neuraxial (spinal) for cesarean section: Phenylephrine is first-line for spinal hypotension; it maintains uteroplacental pH better than ephedrine in fetal acid-base studies. Norepinephrine is increasingly used as an alternative because it better preserves maternal HR/CO.
- Raised ICP / neurosurgery: Used as a vasopressor to maintain CPP >60 mmHg (typically via infusion with norepinephrine or phenylephrine).
- Pheochromocytoma: Reserved for refractory hypertension management (rarely used intraop for this indication).
- Cardiac surgery (post-CPB): Used for vasodilatory states; many centers have shifted to norepinephrine as primary intraoperative vasoconstrictor.
Contraindications / Cautions
- Severe hypertension
- Ventricular tachycardia
- Severe bradycardia or heart block (reflex bradycardia risk)
- Hypovolemia: correct volume first; phenylephrine is a temporizing measure only
Sources: Morgan & Mikhail's Clinical Anesthesiology 7e (pp. 448, 862); Miller's Anesthesia 10e; Tintinalli's Emergency Medicine (Table 20-1); Harriet Lane Handbook 23e; Miller's Anesthesia 10e (aortic surgery chapter)
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