💊 Chapter 52: Inotropes and Vasoactive Medications

Adrenergic Receptors · Inotropy · Vasopressors · Inodilators · Doses · Titration · Weaning · Receptor Physiology

💊 Inotropes and Vasoactive Medications: Optimising Oxygen Delivery

📊 Receptor Physiology
• α1: Vasoconstriction (↑SVR)
• β1: Inotropy, chronotropy
• β2: Vasodilation, bronchodilation
• Dopaminergic: Renal/splanchnic vasodilation
• V1: Vasoconstriction
🫀 Key Drugs & Doses
• Adrenaline: 0.01-0.3 mcg/kg/min (β1 at low, α at high)
• Noradrenaline: 0.01-0.4 mcg/kg/min (α1, β2)
• Dopamine: 3-20 mcg/kg/min (β1 >7, α >10)
• Dobutamine: 3-20 mcg/kg/min (β1 > β2, inodilator)
• Milrinone: 0.25-0.75 mcg/kg/min (PDE3 inhibitor, inodilator)
• Vasopressin: 0.0003-0.002 units/kg/min
🔬 Terminology
• Inotropy: ↑ contractility
• Chronotropy: ↑ heart rate
• Lusitropy: ↑ diastolic relaxation
• Vasopressor: ↑ SVR (α-agonist)
• Inodilator: ↑ contractility + ↓ SVR
🎯 Clinical Selection
• Cold shock (low CO, high SVR) → Adrenaline
• Warm shock (low SVR) → Noradrenaline
• Normotensive low output → Milrinone/Dobutamine
• Cardiogenic shock with hypotension → Adrenaline → add milrinone after BP stable
⚙️ 10 Commandments
1. Use central line (peripheral dilute if emergency)
2. Invasive BP monitoring mandatory
3. Piggyback when changing lines
4. Avoid boluses (except emergency)
5. No other meds through inotrope port
6. Slow weaning, one drug at a time
7. Correct hypovolaemia, acidosis, electrolytes
8. Downregulation = tachyphylaxis after days
9. Extracorporeal circuits may absorb drugs
10. Use combination therapy to limit side effects
⚠️ Adverse Effects
• ↑ myocardial O2 demand
• Arrhythmias
• Vasoconstrictors → skin, mesenteric, renal ischaemia
• High-dose adrenaline → hyperlactataemia (not ischaemia)
DrugReceptorDose (mcg/kg/min)Effects
Adrenalineβ1, α1 (dose-dependent)0.01-0.3Low: inotropy; High: vasoconstriction
Noradrenalineα1, β10.01-0.4Potent vasoconstriction, reflex bradycardia
DopamineDopaminergic, β1, α13-20Low: renal; Mid: inotropy; High: vasoconstriction
Dobutamineβ1 predominant3-20Inodilator, can give peripherally
MilrinonePDE3 inhibitor0.25-0.75Inodilator, lusitropy, thrombocytopenia
VasopressinV1, V20.0003-0.002 U/kg/minVasoconstriction (V1), water retention (V2)

🩺 Step-by-Step: Using Inotropes & Vasoactive Medications

1
Correct modifiable factors BEFORE starting
Optimise volume status (fluids), correct acidosis (pH <7.2 reduces catecholamine efficacy), correct electrolytes (Ca, K, Mg).
2
Choose drug based on haemodynamic phenotype
• Cold shock (narrow pulse pressure, cool extremities, delayed CRT) → Adrenaline
• Warm shock (wide pulse pressure, bounding pulses, flash CRT) → Noradrenaline
• Normotensive low output → Milrinone or Dobutamine
3
Establish central venous access
Use dedicated lumen for inotropes. If no central line in emergency, dilute and give through large peripheral vein. Milrinone and dobutamine are safest peripherally.
4
Invasive arterial monitoring
NIBP is unreliable in shock (may overestimate). Place arterial line for continuous BP monitoring.
5
Start at low dose and titrate to end points
Target: normalising lactate, ScvO2 >70%, UOP >1 mL/kg/h, improving perfusion. BP is not the only target.
6
Use combination therapy
Two drugs with complementary mechanisms (e.g., adrenaline + milrinone) allow lower doses of each, reducing side effects. Avoid high-dose single agent.
7
Weaning strategy
Wean vasopressors (noradrenaline) before inotropes. Wean one drug at a time, slowly. Do not wean if patient on mechanical ventilation with high settings or unstable.
8
Special precautions
Piggyback when changing lines to avoid interruption. Do not give bolus of inotrope (except emergency). No blood draws from inotrope port. Check compatibility when combining drugs.