What are the different vasopressors (vasoactive medications that constrict blood vessels) and inotropes (medications that increase cardiac contractility) used in critical care, and when are they indicated?

Comprehensive Guide to Vasopressors and Inotropes in Critical Care

Norepinephrine should be considered the first-line vasopressor for most shock states, particularly distributive shock, while dobutamine is the preferred first-line inotrope for cardiogenic shock with adequate blood pressure. 1

Vasopressors Overview

First-Line Agents

1. Norepinephrine

   • Dosing: 0.01-3 μg/kg/min
   • Indications: First-line for distributive shock (sepsis), persistent hypotension after fluid resuscitation
   • Mechanism: Potent α1-adrenergic effects with moderate β1 effects
   • Target: MAP ≥65 mmHg
   • Advantages: Reduces incidence of organ failure, less tachycardia than dopamine 1

2. Vasopressin

   • Dosing: 0.01-0.04 units/min (fixed dose, not weight-based)
   • Indications: Adjunct to norepinephrine in refractory shock, reduces norepinephrine requirements
   • Mechanism: V1 receptor-mediated vasoconstriction independent of adrenergic pathways
   • Pharmacokinetics: T½ ≤10 minutes, steady state reached in 30 minutes 2
   • Special consideration: May reduce renal replacement therapy requirements 1

3. Phenylephrine

   • Dosing: 0.5-9 μg/kg/min
   • Indications: Salvage therapy, afterload-dependent states (aortic stenosis, mitral stenosis)
   • Mechanism: Pure α1-agonist without β effects
   • Caution: Should be reserved for specific situations where tachycardia must be avoided

Inotropes Overview

1. Dobutamine

   • Dosing: 2-20 μg/kg/min
   • Indications: First-line for cardiogenic shock with adequate blood pressure, low cardiac output states
   • Mechanism: Predominantly β1 effects with mild β2 and minimal α effects
   • FDA indication: Short-term treatment of cardiac decompensation due to depressed contractility 3
   • Caution: May cause hypotension due to peripheral vasodilation

2. Epinephrine

   • Dosing: 0.01-0.5 μg/kg/min
   • Low dose (0.01-0.1 μg/kg/min): Predominantly β effects (inotropic)
   • High dose (>0.1 μg/kg/min): Increasing α effects (vasopressor)
   • Indications: Alternative to dobutamine in cardiogenic shock, dobutamine-refractory low cardiac output
   • Caution: Metabolic side effects (lactic acidosis, hyperglycemia) limit routine use 1

3. Milrinone

   • Dosing: 0.375-0.75 μg/kg/min (loading dose: 50 μg/kg over 10 min)
   • Indications: Right ventricular failure, pulmonary hypertension, β-blocker overdose
   • Mechanism: Phosphodiesterase III inhibitor causing increased cAMP, vasodilation and inotropy
   • Advantage: Non-catecholamine, no tachycardia
   • Caution: Long half-life, profound hypotension, requires dose adjustment in renal failure

Shock-Specific Approach

Distributive Shock (Septic Shock)

1. First-line: Norepinephrine after adequate fluid resuscitation 1
2. If persistent hypotension: Add vasopressin (up to 0.03 units/min)
3. If evidence of myocardial depression: Add dobutamine or switch to epinephrine
4. Refractory shock: Consider hydrocortisone 200-300 mg/day 1

Cardiogenic Shock

1. With adequate blood pressure: Dobutamine as first-line inotrope
2. With hypotension and tachycardia: Norepinephrine + dobutamine
3. With hypotension and bradycardia: Consider dopamine
4. Afterload-dependent states (aortic stenosis): Phenylephrine or vasopressin 1

Pulmonary Arterial Hypertension with RV Failure

1. Preferred inotropes: Dobutamine, milrinone, or epinephrine
2. Key principle: Maintain systemic vascular resistance (SVR) greater than pulmonary vascular resistance (PVR)
3. Adjunct therapy: Consider inhaled nitric oxide (iNO) at 20 parts per million 1

Hemorrhagic Shock

1. Primary treatment: Fluid resuscitation and hemorrhage control
2. Transient vasopressor support: For life-threatening hypotension until definitive control
3. Consideration: Vasopressin may improve blood pressure without increasing blood loss 1

Clinical Pearls and Pitfalls

• Avoid routine use of inotropes without evidence of cardiac dysfunction 1
• Monitor for tachyarrhythmias with all catecholamine agents
• Target the lowest effective dose of vasopressors to minimize adverse effects
• Maintain MAP ≥65 mmHg as a general target, but individualize based on patient's baseline
• Vasopressin dosing is fixed (not weight-based) and should not exceed 0.04 units/min 2
• Dobutamine should not be used beyond 48 hours if possible, as safety data is limited 3
• Intubation in pulmonary hypertension can precipitate hemodynamic collapse; consider pre-emptive iNO and vasopressors 1

Monitoring Parameters

• MAP ≥65 mmHg
• Urine output ≥0.5 mL/kg/hr
• Central venous oxygen saturation (ScvO2) ≥70%
• Lactate clearance
• Clinical signs of improved perfusion

By understanding the pharmacologic properties and hemodynamic effects of each agent, clinicians can optimize therapy while minimizing adverse effects in critically ill patients requiring hemodynamic support.