01The blood-pressure equation
Everything you do to blood pressure runs through one relationship:
BP = CO × SVR = (HR × SV) × SVR, where stroke volume depends on preload, contractility, afterload.
So the four levers on BP are heart rate, preload, contractility, afterload. Change one and the others react — e.g., give phenylephrine (raises SVR) to a hypotensive, tachycardic patient and the BP rises, which reflexively slows the heart. Keeping this equation in your head turns confusing vital-sign changes into a working diagnosis.
The goal of vasoactive drugs is to keep HR and BP within ~20% of the pre-op baseline (tighter, ~10%, for critically ill patients).
02The adrenoceptors
Knowing what each receptor does tells you what each drug will do:
| Receptor | Where | Agonist effect |
|---|---|---|
| α1 | Peripheral vessels | ↑ SVR (vasoconstriction) |
| α2 | Nerve terminals / CNS | Inhibits norepinephrine release (sedation, analgesia) |
| β1 | Heart | ↑ HR and contractility |
| β2 | Lungs / vascular smooth muscle | Bronchodilation; vasodilation |
03Pressors & antihypertensives
Pressors (raise BP):
| Drug | Action | Typical bolus | Dilute to |
|---|---|---|---|
| Phenylephrine ("Neo") | α1 (direct) | 50–100 mcg | 100 mcg/mL (double dilution) |
| Ephedrine | Mixed α/β (indirect) | 5–10 mg | 5 mg/mL (single dilution) |
| Epinephrine | α + β (dose-dependent) | ~10 mcg (titration) | 10 mcg/mL |
Phenylephrine is great when HR is fine or high (it'll bring HR down); ephedrine when the HR is also low; epinephrine is the "big gun" — start low and titrate.
Antihypertensives (lower HR/BP):
| Drug | Action | Starting dose | Duration |
|---|---|---|---|
| Esmolol | β1 (pure) | 10 mg IV q3 min | ~10 min (very short) |
| Labetalol | α + β | 5–10 mg IV q5 min | hours |
| Metoprolol | β1 selective | 2.5 mg IV q2 min | hours |
04Diluting safely
Vasoactive drugs come very concentrated — giving them undiluted can kill someone. Do one dilution at a time, don't multitask, and LABEL the syringe with the final concentration. If you lose track, throw it out and start over.
The key rule: for every 1 mL of drug you put into 9 mL of saline, you move the decimal one place left (a 1-in-10 dilution).
- Single dilution (e.g., ephedrine): 1 mL of 50 mg/mL + 9 mL NS → 5 mg/mL.
- Double dilution (e.g., phenylephrine, epinephrine): repeat it. 10 mg/mL → 1 mg/mL → 0.1 mg/mL = 100 mcg/mL.
- 100 mL-bag method (fast, avoids syringe swaps): pull 1 mL of saline out of a 100 mL bag, inject 1 mL of 10 mg/mL phenylephrine → 100 mcg/mL in the bag.
- Giving a concentrated pressor without diluting — a tiny volume can be a massive, dangerous dose.
- Labeling several diluted syringes at the end instead of one at a time — the setup for a swap.