HALOTHANE

Introduction

Physical & chemical properties:

• Potent general anesthetic (MAC-0.74%) used via inhalation.
• Volatile heavy liquid.
• Non-explosive & non-inflammable.
• Induction & maintenance anesthetic agent.
• Least pungent.
• Not analgesia.
• 0.01% thymol used as preservative.
• Decomposes when exposed to light.
  • Maintained in amber coloured bottles.
  • Stabilized by addition of thymol.
• Dissolve rubber & corrodes metals.
• Brand Name – Fluothane.

Actions:

• Sensitizes myocardium to catecholamines.
• Coronary vasodilator.
• Potentiates competitive neuromuscular blockers.
• Fall in BP & CO.
• Causes bradycardia.
• Vascular beds of skin & brain directly dilated.

Clinical uses & administration:

• Induction & maintenance of anesthesia without assistance from other CNS depressants.
• Best uterine relaxation.
• Used in closed rebreathing system with precision thermo-stable/thermo-compensated, calibrated vaporizers.

At high levels:

• Cardiac depression (direct myocardial depression by reducing Ca+).
• Cardiac sensitisation to adernaline – Avoid epinephrine.
• Potent bronchial relaxation – Inhalational agent of choice in asthmatics.
• (IV agent of choice – Ketamine).

Dose:

• Induction (Halothane alone)
  • All species: 2.0 – 4.0% of inspired air.
• Maintenance (Halothane alone)
  • Small animals : 0.5 – 1.5%
  • Large animals : 1.0 – 2.0%
• Minimum alveolar concentration/MAC:
  • Humans – 0.76

Pharmacokinetics:

• Rapidly absorbed & excreted through lungs.
• 20% : Biotransformation in liver & excreted through lungs
• 60 %: Excreted unchanged through lungs
• 20 %: Excreted via other routes, either unchanged/metabolized.
• Stored in body for many hours to days.
• Portal flow is maximally reduced.
• Presence of trifluoroacetic acid (TFA) in urine indicates that volatile anaesthetic agent used was Halothane.

Adverse effects:

• Vasodilatation.
• Hypotension.
• Malignant hyperthermia.
• Cardiac arrhythmias.
• Postoperative hepatitis (Methoxyflurane avoided in future) .
  • DOC for these patients – Sevoflurane.
• Centrilobular necrosis in liver, increased SGPT, SGOT, bilirubin (jaundice).
• Post-operative jaundice.
• Post-anesthetic shivering & chills Halothane shakes).
• Postpartum hemorrhage.
• Cardiopulmonary depression.
• Cardiac arrest

Contraindications & precautions:

• Increased CSF & Head injury.
• Hypovolemia.
• Previous history of adverse halothane exposure (Usage within 3 months).
• Cardiac arrhythmia
• Hepatic dysfunction/liver disease.

• Multiple exposures (most important).
• Hypoxia
• Middle age
• Obesity

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