How vasopressors actually raise MAP

MAP = Cardiac Output × Systemic Vascular Resistance. Vasopressors raise MAP by increasing SVR, increasing CO, or both. The adrenergic receptor pharmacology determines which pathway each drug takes:

α1 stimulation — arteriolar and venous vasoconstriction, raising SVR and MAP. This is the primary mechanism for most vasopressors
β1 stimulation — increased heart rate and contractility, raising CO and therefore MAP. Useful when low MAP is partly from reduced cardiac output
β2 stimulation — vasodilation. The opposite of what you want. At certain doses, epinephrine's β2 activity partially offsets its α1 effects on peripheral vessels
V1 receptor stimulation (vasopressin) — direct smooth muscle vasoconstriction completely independent of adrenergic pathways. Useful precisely because it doesn't rely on catecholamine receptor responsiveness, which can be downregulated in prolonged septic shock

Why infusions, not boluses: MAP needs to be maintained continuously in haemodynamically unstable patients — not corrected in periodic spikes. Infusions allow dose adjustments in 0.01–0.02 mcg/kg/min increments, with MAP reassessed every 5–15 minutes. Boluses produce pressure peaks followed by troughs. The only clinical setting where vasopressor boluses are standard is spinal-induced hypotension.

The main vasopressors

Norepinephrine (noradrenaline)

First-line — septic and distributive shock

Receptors: α1 dominant, β1 mild, β2 negligible

Effect: ↑↑ SVR → ↑ MAP, minimal HR change at usual doses

Dose: 0.01–0.5 mcg/kg/min, titrated to MAP target

Onset: 1–2 minutes; half-life ~2 min allows responsive titration

Notes: SSC first-line, most extensively studied in septic shock, only vasopressor with consistent mortality advantage over dopamine in RCTs. At high doses (>1 mcg/kg/min) increases digital and mesenteric ischaemia risk. Add vasopressin rather than continuing to escalate NE above 0.25 mcg/kg/min.

Vasopressin (ADH / AVP)

Second-line adjunct — norepinephrine-sparing

Receptors: V1 on vascular smooth muscle — no adrenergic component

Effect: ↑ SVR via adrenergic-independent vasoconstriction

Dose: Fixed 0.03 units/min — not weight-based, not titrated

Notes: Add when NE ≥ 0.25 mcg/kg/min. Purpose is NE dose reduction (typically 20–40%), not independent MAP driving. Cannot titrate up — doses above 0.04 U/min risk coronary and splanchnic ischaemia. VASST trial: safe, non-inferior to NE alone, possible mortality benefit in less severe shock.

Epinephrine (adrenaline)

Third-line ICU / first-line anaphylaxis

Receptors: α1, β1 strong, β2 variable by dose

Effect: ↑ SVR + ↑ CO; raises both HR and contractility significantly

Dose: 0.01–1 mcg/kg/min ICU; 0.3–0.5 mg IM for anaphylaxis

Notes: First-line anaphylaxis (IM, not IV in the field). ICU: third-line or when cardiogenic component needs inotropic support. Elevates lactate via β2-mediated glycogenolysis — this is NOT worsening tissue hypoxia, it's a pharmacological effect. Be careful not to misinterpret rising lactate in patients on epinephrine.

Agent	α1	β1	β2	V1	First-line use	Key risk
Norepinephrine	+++	+	—	—	Septic/distributive shock	Digital ischaemia at high doses
Vasopressin	—	—	—	+++	NE adjunct (add at NE ≥ 0.25)	Coronary/splanchnic ischaemia above 0.04 U/min
Phenylephrine	+++	—	—	—	Spinal-induced hypotension	Reflex bradycardia; ↓ CO in low-output states
Epinephrine	++	+++	++	—	Anaphylaxis; third-line ICU	Tachyarrhythmias; raises lactate (β2 glycogenolysis)
Dopamine	++ (high dose)	++ (mid dose)	—	—	Largely avoided	Higher arrhythmia rate than NE — no mortality advantage
Dobutamine	—	+++	+	—	Cardiogenic shock (inotrope)	Can initially drop MAP via β2 vasodilation

Titration — the step-by-step approach

All vasopressor infusions in the ICU are titrated to a MAP target, not prescribed at a fixed dose. The process:

Set the MAP target — ≥65 mmHg standard; ≥70–80 for chronic hypertensives; ≥80 for TBI; ≥70 for CAD. Document it explicitly
Start norepinephrine at 0.05–0.1 mcg/kg/min via central venous access
Reassess MAP every 5–15 minutes — adjust by 0.01–0.02 mcg/kg/min increments. Wait for the haemodynamic response before adjusting again
Add vasopressin at NE ≥ 0.25 mcg/kg/min — 0.03 units/min fixed. This is an NE-sparing move, not an escalation
Consider hydrocortisone 200 mg/day when MAP remains below target on NE + vasopressin — addresses relative adrenal insufficiency
Monitor for adverse effects throughout — digit colour and perfusion, urine output, lactate trend, ECG for arrhythmias

Vasopressin does not get titrated up: 0.03 units/min is the dose. Full stop. It functions as a norepinephrine-sparing adjunct — adding it allows NE dose reduction. Doses above 0.03–0.04 U/min are associated with coronary artery constriction and mesenteric ischaemia. If MAP is inadequate with NE and vasopressin at standard doses, add hydrocortisone or epinephrine — do not push vasopressin higher.

Why central access is required

Concentrated vasopressor infusions cause intense local vasoconstriction at the infusion site. Peripheral extravasation — even a small amount — causes tissue necrosis that can require surgical debridement or amputation. Central venous access (IJ, subclavian, or femoral CVC) ensures the drug enters a high-flow vessel and is immediately diluted.

In genuine emergencies before central access is established: a short bridge via a large-bore proximal peripheral IV (antecubital, not hand or foot) with dilute concentration (4–8 mcg/mL rather than 32–64 mcg/mL) and close nursing observation is used as a temporary measure only. Transfer to central access as soon as it's available.

Weaning — when and how

Prolonged vasopressor exposure has its own harms: catecholamine cardiomyopathy, peripheral ischaemia, immune dysregulation, accelerated catabolism. Once the underlying cause is controlled and the patient is clearly recovering, wean actively.

Signs the patient is ready: MAP holding at target on current or lower doses for ≥ 2–4 hours, lactate clearing (>10% per 2 hours or absolute <2 mmol/L), urine output ≥ 0.5 mL/kg/h, improving mental status, warm well-perfused extremities.

Wean in reverse initiation order: vasopressin first, then hydrocortisone, then norepinephrine. Reduce NE by 0.02–0.05 mcg/kg/min increments, check MAP 15–30 minutes after each step. Don't stop abruptly — the tonically contracted vascular smooth muscle needs time to reassume normal baseline tone. Abrupt discontinuation causes rebound hypotension.

Complications to monitor actively: Digital ischaemia (pale or cyanotic fingers/toes), mesenteric ischaemia (abdominal pain, bloody stools, rising lactate), tachyarrhythmias (especially with epinephrine or high-dose dopamine), catecholamine cardiomyopathy with very prolonged exposure.

Key takeaways

Vasopressors raise MAP via α1-mediated SVR increase (and β1-mediated CO increase for some agents)
Norepinephrine is first-line in septic shock — most studied, only vasopressor with consistent mortality advantage over dopamine
Vasopressin 0.03 units/min is a fixed-dose NE-sparing adjunct — it does not get titrated upward
Phenylephrine: pure α1, good for spinal hypotension. Avoid in low cardiac output states
Dopamine is largely avoided in septic shock due to higher arrhythmia rate with no mortality benefit vs NE
Epinephrine raises lactate via β2-mediated glycogenolysis — not a sign of worsening hypoperfusion
All infusions titrate to MAP target. Central venous access is required — peripheral extravasation causes tissue necrosis
Wean in reverse order, gradually. Abrupt discontinuation risks rebound hypotension

Sources & references

Surviving Sepsis Campaign Guidelines 2021
De Backer D et al. Comparison of Dopamine and Norepinephrine in the Treatment of Shock. N Engl J Med 2010
Russell JA et al. Vasopressin versus Norepinephrine in Patients with Septic Shock. N Engl J Med 2008 (VASST)
Gordon AC et al. Vasopressin versus Norepinephrine in Patients with Vasodilatory Shock. JAMA 2016 (VANISH)
StatPearls — Mean Arterial Pressure

Medical disclaimer: This article is for educational purposes only and does not constitute medical advice. Clinical decisions should always be made by qualified healthcare professionals based on the complete clinical picture. Always consult current clinical guidelines and institutional protocols.

MAP & Vasopressors