Chemical and Physical Properties
- Halogenated compound chemically.
- Volatile, so kept in sealed bottles
- Light-sensitive, so kept in amber coloured bottles
- Interaction – rubber and plastic tubing
a. Potent general anesthetic
b. MAC – 0.75, thus lower MAC value, more drug potency
c. Blood Gas partition coefficient – 2.3
d. Induction & recovery is slower to cause change in partial pressure due to high lipid solubility
e. Metabolism – normally excreted in form of trifluoroacetic acid. 20-40% is metabolized in liver.
Around 60-80% is cleared out unchanged by lungs
In some patients a toxic intermediate compound is formed, trifluoroacetylchloride. This:
1. Binds hepatic cellular proteins, responsible for hepatitis esp. on cell membrane, damaging hepatic cells
2. In a number of patients, against these trifluoroacetylated proteins (TFA proteins), body forms antibodies, leading to fulminant hepatic disease.
Depressant effect on myocardial contractility, leading to decreased blood pressure.
But does not allow reflex tachycardia due to blood pressure lowering by suppressing baroreceptor reflex, instead causes bradycardia. Due to:
- Depressant effect on SA node
- Vagal stimulation
- It sensitizes myocardium to catecholamines so anxious patients or in pheochromocytoma or exogenous epinephrine, chances of cardiac arrhythmias.
Thus adrenaline is never used in patients anesthetized with halothane.
For treatment of vagal stimulation:
II. Beta-blockers to prevent cardiac arrhythmias
Redistribution of blood flow; also leads to redistribution of blood, increasing cerebral blood flow and blood flow to skin, it suppresses auto regulation in renal, splanchnic and cerebral vasculature.
When blood pressure is decreased, decreased flow to these areas occurs, with no auto regulation.
It does not affect coronary and pulmonary circulation.
2. Respiratory system
a. Pulmonary ventilation
Increases rate but tidal volume is decreased. Respiration becomes shallow.
b. CO2 / O2 response
Suppresses body response to carbon dioxide excess, by acting on chemoreceptor zone in medulla. Also suppresses hypoxic response by affecting peripheral chemoreceptor zone.
c. Muco-ciliary function
Mucociliary functions are suppressed, sometimes stops -atelectasis
d. Bronchodilator – status asthmatics
Bronchodilator effect so given in refractory cases of asthma.
a. Cerebral Blood Flow
Increases cerebral blood flow
Decreases rate of metabolism in CNS.
c. Intra-cranial pressure
Increases intra-cranial pressure, so halothane is not given in patients of head injury, cerebral edema, space occupying lesion, and tumor
Has depressant effect on EEG.
Decrease renal blood flow by suppressing auto regulation.
Concentrated urine is produced
The effect is transient and the patient goes back to normal.
Suppresses auto regulation of splanchnic blood flow, decreasing it, which is of no significant clinical effect.
6. Skeletal muscles
Relaxes skeletal muscles.
Relaxes uterus so can be used for manipulation of fetus, when positional changes are done perinatally. It is not given during delivery and can delay labor as may relax the uterus. It can be used after delivery for retained placenta as need to relax uterus.
Potent general anesthetic for:
1. Maintenance anesthesia– 0.5-1%
2. Induction – 2-4%
It has now been replaced by newer safer drugs. But are still used:
a. In children
b. And because of low cost
1. Halothane shake / Shivering
During recovery phase, patient complains of shivering known as halothane shake. The exact mechanism is not known.
2. CVS / Respiration
Depressant effect on CVS and respiration.
Chronic toxicity studies have shown no carcinogenic or mutagenic property. Epidemiological studies show increased chances in patients working in theatres, but there is no official data.
Immune response of the body against trifluorated compounds. As a result of which hepatic cells are damaged, leading to severe hepatitis.
- Electrolyte imbalance
- Receiving enzyme agents or drugs
- Generalized weakness
Occurs several days after administration of halothane.
Those already exposed to halothane are at a higher risk of developing hepatitis.
- Deranged LFTs
- Trifluoroacetylated proteins in serum
No specific treatment. In severe cases like liver failure, liver transplant is required.
4. Malignant Hyperthermia
Also occurs with succinyl choline.
a. Ryanodine Receptors (RyRI)
It is a autosomal genetic disease where there is gene mutation responsible for ryanodine receptors. Because of this increase release of calcium through these channels from sarcoplasmic reticulum occurs, leading to increased calcium levels in sarcoplasm of skeletal muscles.
b. L-type Ca+2 channels
In some cases, mutation is seen in genes for L-type of calcium channels. Result is the same.
Clinically patient shows:
- Severe muscular rigidity
- Increased heart rate
- Signs and symptoms of hyperkalemia and acidosis
- Deranged electrolytes
- Increased free calcium in skeletal muscles
- Confirmed by tests on skeletal muscle biopsy
- In vitro caffeine halothane contracture test
- Dantrolene, which decreases calcium release
- Symptomatic treatment of fever
Restoration of electrolyte balance
Not an analgesic
Variable muscle relaxation
Induction smooth and rapid
Sensitizes heart to catecholamines
Non – inflammable
Compatible with soda lime
Shivering during recovery
Less incidence of post-operative nausea/ vomiting
Does not cause laryngospasm
Easier endotracheal intubation due to relaxation of masseter muscles
Reacts with rubber equipment