Dissociative anaesthesia & Ketamine

DISSOCIATIVE ANAESTHESIA

Dissociative Anesthetics include drugs that inhibit pain by cutting off or dissociating the  brain’s perception of pain. The drugs within this category normally will induce a state of  sedation, immobility, amnesia, and marked analgesia.

 The term Dissociative Anesthesia is derived from the strong feeling of dissociation from the  environment that is experienced by the user. This general anesthesia is termed "dissociative" because the patient initially feels consciously detached from the environment before becoming  unconscious.

The dissociative state is marked by sedation (catatonia) ,  amnesia,  immobility (catalepsy)  and marked analgesia

CATATONIA : is a state of apparent unresponsiveness to external stimuli in a person who is apparently awake.

CATALEPSY:  A condition characterized by lack of response to external stimuli and by muscular rigidity, so that the limbs remain in whatever position they are placed. It is known to occur in a variety of physical and psychological disorders, such as epilepsy and schizophrenia, and can be induced by hypnosis, dissociative anaesthetics

Phencyclidine (PCP) was the first drug used for this purpose, but the frequent occurrence of  unpleasant hallucinations and psychological problems soon led to its discontinued legal use.  Ketamine and Ketalar, two analogs of PCP,  and tiletamine ( with zolazepam as “Telazol”) also are considered Dissociative Anesthetics.

KETAMINE

Ketamine is a phencyclidine derivative. Following injection, patients become mentally dissociated or  emoved from the surroundings. This is referred to as dissociative anaesthesia. Ketamine (Ketalar) is  rapidly absorbed after intramuscular and intravenous injection. It is metabolised and excreted in the urine.

Indications

As an induction agent (in place of thiopentone):- · In shocked patients or poor risk patients, · In asthmatics, · In obstetric patients for caesarean sections:;  - It causes less hypotension or foetal depression ;  - It may increase uterine tone- As a sole anaesthetic for minor operations, dilatation and curettage, fractures, diagnostic procedures, burns dressing, debridement (bolusplus infusion or infusion only).  ;; As an infusion with a relaxant anaesthetic

Systemic effects

Central nervous system: the patient is sedated, amnesic and analgesic.It is difficult to detect the point at which sleep commences as patients appear to gaze into space and may not close their eyes for several minutes. The eyelash and corneal reflexes remain. There is dissociation from the surroundings.

Cerebral blood flow and intracranial pressure is increased. Awakening from anaesthesia may take several hours and may be associated with vivid dreams and hallucinations, especially in adult patients.

Cardiovascular system: cardiac output and the systolic and diastolic blood pressures are increased.

Respiratory Effects

Ketamine often causes apneustic breathing (apneustic breathing is characterized by rapid breaths followed by breath-holding on inspiration). Ketamine also causes bronchodilation, which is favorable for example in asthmatic cats.

Other actions of ketamine: Rise in intra-ocular pressure. Eye movements may be noticed. Uterus not relaxed. At 1 mg/kg given slowly over 1 minute it does not depress the foetus, though it has been known to contract the pregnant uterus and cause foetal distress.

Metabolism

In dogs, ketamine is metabolized to active metabolite, nor-ketamine. Nor-ketamine has  10-30% activity of the ketamine and is excreted through kidney. In cats, ketamine is  excreted mostly UNCHANGED through the kidney. Patients with decreased renal  excretion may have prolonged drug effects.

Telazol is the trade name of the combination of tiletamine (dissociative anesthetic) + zolazepam (benzodiazepine); thus is similar to valium + ketamine combinations.  Telazol is useful because it is a small volume to be delivered; which is useful for darting  animals. Telazol should not be used in New Zealand White Rabbits as it causes renal necrosis. The premixed combination is convenient, but the metabolism of each drug

can be different in different species.

Advantages

· Minimal cardiovascular depression, which is good in the shocked patient.

· Minimal depression of pharyngeal and laryngeal reflexes, so the airway is maintained without intubation, provided the patient has had the right amount of ketamine. The airway is not, however protected from aspiration. If the patient is at risk of aspiration a rapid sequence induction should be undertaken.

· Very good analgesic.; · Easy to administer.

Disadvantages

· Emergence reactions: vivid dreams and hallucinations occur, especially in adults. Decrease the incidence of emergence reactions by: - Not using ketamine in nervous patients or those with psychiatric

problems

- Using diazepam with the ketamine - Preventing stimulation in the recovery period

· Hypertension.

· It is difficult to know when a sufficient dose is given. Eyes may be open and there may be spontaneous movement.- possibility of injury  to eyes

· No relaxation of muscles. · Salivation increases.· Respiratory depression.

Precautions with ketamine

- Not to be used in  hypertensive patients, patients with head injuries or nervous patients.

- - Check blood pressure before starting the anaesthetic.

-  Don’t test eyelash reflex as for general anaesthesia. It is not abolished with ketamine.

- If the patient shows purposeless tonic or clonic movements of the limb it does not mean the anaesthesia is too light.