Study Notes: Pressors in Shock

What to expect:

• Describe the receptors involved in vascular tone control.

• List the effects of common pressors on systemic vascular resistance (SVR) and cardiac output (CO).

• Choose appropriate pressors in hypotension and shock.

Control of Vascular Tone

• Vasoconstrictors: Epinephrine, norepinephrine, angiotensin II, vasopressin (ADH), sympathetic norepinephrine, endothelin.

• Vasodilators: ANP, BNP, bradykinin, histamine, nitric oxide, adenosine.

Catecholamines

• Endogenous: Dopamine → Norepinephrine → Epinephrine (biosynthetic chain from phenylalanine).

• Synthetic: Dobutamine, phenylephrine (not true catecholamine), isoproterenol.

• Structural differences → different hemodynamic effects.

Receptor Actions

• α1: Vasoconstriction → ↑ SVR.

• β1: Inotropy (↑ contractility) & chronotropy (↑ HR) → ↑ CO.

• β2: Vasodilation → ↓ SVR.

• (α2, β3 not clinically relevant here).

Key Catecholamines

• Norepinephrine: Strong α1, minor β1, no β2 → ↑ SVR, mild ↑ CO.

• Epinephrine: Strong β1, β2; minor α1 → ↑ CO, vasodilation at low doses; at high doses α1 dominates → ↑ BP.

Dose-Dependent Effects

• Receptor selectivity changes with dose (e.g., epinephrine, dopamine).

Classification of Pressors

• Vasopressors (↑ SVR): Phenylephrine.

• Inotropes (↑ CO): Dobutamine, milrinone (PDE inhibitor).

• Both: Norepinephrine, epinephrine, dopamine.

Clinical Use by Shock Type

• Septic shock:

  • 1st line → Norepinephrine.

  • Alternative → Phenylephrine (only if arrhythmias, high CO, or other agents fail).

  • Refractory → Add vasopressin or epinephrine.

• Cardiogenic shock:

  • Traditional → Dobutamine ± dopamine.

  • Alternative → Norepinephrine ± dobutamine (controversial, post-SOAP II trial).

• Hypovolemic shock:

  • Primary → IV fluids (pressors not routine).

  • Pressors only if collapse imminent while fluids infusing.

• Mixed/uncertain shock:

  • Norepinephrine often chosen; sometimes epinephrine or dopamine.

Special Notes

• Phenylephrine caution: Pure vasoconstrictor; avoid if myocardial dysfunction suspected.

• SOAP II Trial (1700 pts, dopamine vs norepi):

  • No overall mortality difference.

  • Cardiogenic subgroup → norepi seemed better (less arrhythmia risk).

  • Flawed design (high dopamine dose, mixed use of dobutamine).

  • Bottom line: Trial shouldn’t change cardiogenic shock management.

Evidence Summary

• No clear mortality benefit of one pressor over another.

• Choice depends on:

  • Clinical scenario (shock type).

  • Physician experience.

  • Side effects, drug interactions.

  • Availability & cost.

Key Takeaway:
Pressors are selected based on underlying shock physiology. Norepinephrine is the most widely used first-line agent, but no single drug has proven mortality.