Dobutamine

Dobutamine is a synthetic **β₁-adrenergic agonist** with mild β₂ and α₁ effects producing **inotropy > chronotropy** and mild vasodilation. It increases cardiac output, reduces filling pressures, and is ideal for **low-output heart failure** and **post-cardiac-surgery inotropy**. Dose **2–20 mcg/kg/min**, titrated to haemodynamics; combine with norepinephrine if hypotensive.

# 1) Receptor-level action → clinical effects

**Primary receptor targets:**

* **β₁-adrenergic agonist** (predominant) → ↑ **inotropy** (contractility) and modest ↑ **chronotropy** (HR).

* **β₂-adrenergic agonist** (mild) → **vasodilation** (↓ SVR).

* **α₁-adrenergic agonist** (weak) → mild vasoconstriction, balancing β₂ effects.

* **Net effect:** ↑ cardiac output with **slight fall in SVR** and **modest rise in HR**.

**Clinical effects from receptor activity:**

* ↑ myocardial contractility → ↑ stroke volume, ↑ cardiac output.

* Mild ↑ HR (may precipitate tachyarrhythmias in some).

* ↓ LV filling pressures and pulmonary capillary wedge pressure (improved forward flow).

* **Coronary flow** ↑ (secondary to ↑CO).

* **BP**: may rise, fall, or stay unchanged depending on baseline SVR and dose.

* **No significant effect on oxygenation or renal perfusion** beyond increased cardiac output.

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# 2) Vial strength, preparation & basic pharmacokinetics (+ disadvantages)

**Formulation:**

* 250 mg/20 mL vial (12.5 mg/mL).

* Dilute to **250 mg/250 mL** (1 mg/mL) or **500 mg/250 mL** (2 mg/mL) for infusion.

**Pharmacokinetics:**

* **Onset:** 1–2 min (rapid).

* **Peak effect:** 10 min.

* **Duration:** 5–10 min after stopping infusion.

* **Metabolism:** hepatic + tissue **catechol-O-methyltransferase (COMT)** → inactive metabolites.

* **Elimination half-life:** 2–3 min.

* **Excretion:** renal (as metabolites).

**Disadvantages (from above PK/PD):**

* Short half-life → requires **continuous infusion**.

* **Tachyphylaxis** possible with prolonged use.

* ↑ myocardial O₂ consumption → risk of **ischaemia** in CAD patients.

* May cause **tachyarrhythmias** and **hypotension** if hypovolaemic or at high dose.

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# 3) Practical dosing

## A. Anaesthesia / Perioperative

**Indications:** low cardiac output state after CPB, myocardial stunning, heart failure, or to support weaning from bypass.

* **Starting dose:** 2.5 mcg/kg/min.

* **Titrate up:** 2.5–5 mcg/kg/min increments q10–15 min.

* **Typical range:** **2–20 mcg/kg/min** (rarely up to 40 mcg/kg/min in refractory cases).

* **Target:** improved CO/CI, MAP, urine output, lactate trend.

* Reduce if HR >120 bpm, new arrhythmia, or hypotension.

**Example (70 kg):**

* Start 2.5 mcg/kg/min → 10.5 mg/h → **10.5 mL/h** if 1 mg/mL solution.

* Double rate for 5 mcg/kg/min = 21 mL/h.

## B. ICU

**Indications:**

* Low-output heart failure (esp. systolic dysfunction).

* Cardiogenic shock (in combination with vasopressors like norepinephrine).

* Post-cardiac surgery LCOS.

* Bridge to more definitive therapy (IABP, ECMO, LVAD).

**Dosing:** same as above (2–20 mcg/kg/min).

* Start low and titrate to CO, ScvO₂, urine output, and lactate.

* Use **norepinephrine adjunct** if hypotension develops.

* Avoid in severe dynamic LVOT obstruction or hypertrophic obstructive cardiomyopathy (worsens gradient).

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# 4) Special populations — dosing cautions

### Pregnancy

* Crosses placenta; limited data. Use only if benefits outweigh risk (e.g., maternal shock).

* No known teratogenicity.

### Lactation

* Insufficient data; short t½ and poor oral absorption → unlikely harmful.

### Hepatic impairment

* Metabolised by COMT in tissues → minimal hepatic dependence; **no major adjustment needed**.

### Renal impairment

* Excreted as inactive metabolites; **safe in renal failure** but monitor for volume status and arrhythmias.

### Obesity

* Dose based on **actual body weight** (drug acts in plasma, not lipophilic).

* However, **start at lower end** and titrate cautiously for HR and MAP.

### Paediatrics

* **Starting dose:** 1–5 mcg/kg/min IV infusion; titrate q10–15 min up to 20 mcg/kg/min.

* Monitor HR, BP, rhythm closely (more sensitive to tachycardia).

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# 5) Drug interactions (clinically key)

* **β-blockers:** blunt inotropic effect → may require higher doses or switch agent (e.g., milrinone).

* **Volatile anaesthetics:** additive myocardial depression → exaggerated hypotension if combined.

* **MAOIs / COMT inhibitors:** unpredictable pressor or tachycardic responses—avoid or titrate very slowly.

* **Norepinephrine / dopamine:** synergistic for MAP support.

* **Amiodarone or other antiarrhythmics:** increased risk of bradyarrhythmia.

* **Tricyclic antidepressants:** may enhance chronotropy/arrhythmia risk.

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# 6) Significant complications & management

| Complication                             | Mechanism / Features                          | Management                                                              |

| ---------------------------------------- | --------------------------------------------- | ----------------------------------------------------------------------- |

| **Tachycardia / arrhythmia**             | β₁ stimulation                                | Reduce dose; ensure euvolaemia; consider β-blocker (esmolol) if severe. |

| **Hypotension**                          | β₂-mediated vasodilation esp. in hypovolaemia | Fluid resuscitation; add vasopressor (norepi).                          |

| **Myocardial ischaemia**                 | ↑ O₂ demand                                   | Reduce dose; nitrates; ensure adequate coronary perfusion pressure.     |

| **Tolerance (tachyphylaxis)**            | β-receptor desensitisation                    | Alternate with other inotropes (milrinone).                             |

| **Eosinophilic myocarditis (very rare)** | Hypersensitivity after prolonged use          | Stop drug; supportive care; steroids if severe.                         |

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