General anesthesia

Definition:

Anesthesia is defined as a state of induced unconsiousness. There are three components to anesthesia. They are:

1. Analgesia (pain relief)

2. Amnesia (loss of memory)

3. Immobilisation

Drugs used in anesthesia have varying effects on these three areas. Sometimes a combination of drugs will have to be used to optimise the whole process of anesthesia.

What happens when an unanesthetised person is subjected to surgery?

The person undergoing surgery:

1. Takes evasive action

2. Patient will have to endure sever pain and the associated emotional stress

3. There is maximum tension in the skeletal muscles

4. There is an increase in the sympathetic tone causing sweating, tachycardia and hypertension.

5. The patient has a vivid unpleasant memory of the surgery for a long time to follow post operatively.

What are the goals of anesthesia?

The following are the goals of anesthesia:

1. Anesthesia (lack of awareness to surrounding events)

2. Akinesa (the patient is made to lie down still during the surgery)

3. Muscle relaxation

4. Autonomic control (to prevent surges in blood pressure)

General anesthesia:

In general anesthesia drugs are administered parenterally to achieve unconsiousness. Ideally the drug must be safe to administer, without any unpleasantness to the patient, it should provide adequate anesthesia and analgesia. This type of anesthesia has advantages over spinal / regional / local anesthesia.

Advantages of general anesthesia:

1. Patient is not unduly stressed during the procedure

2. Allows complete stillness over prolonged periods of time

3. There is complete control of airway, breathing and circulation

4. Permits surgery in widely separated areas of the body at the same time

5. Can be administered rapidly

6. Can be administered without moving the patient from supine position

Disadvantages of general anesthesia:

General anesthesia has its own disadvantages. They are

1. Requires a special team of a doctor and technicians

2. Requires complex and costly machinery

3. Requires certain amount of preop preparation of the patient

4. Carries a major risk of myocardial infarction or stroke

5. Associated with malignant hyperthermia

Safe and efficient anesthetic practice requires cerified personnel, appropriate drugs and equipment and an optimized patient.

Complications seen following surgery under general anesthesia:

1. Bleeding, nausea and vomiting

2. Fever

3. Dizziness, drowsiness, headache and hoarseness of voice

4. Sore throat

5. Incisional pain

What are the minimum requirements which must be available for safe general anesthesia?

1. An operation theatre of sufficient size with good illumination.

2. Oxygen source (either in cylinders or piped from central supply)

3. A good Boyles apparatus

4. A pulse oxymeter

5. Good collection of anesthetic and emergency medicines

6. A good suction apparatus

7. Cardiac monitor and defibrillator

Preparation of patient before general anesthesia: Careful preoperative preparation of the patient is a must. Ideally the person who is going to give anesthesia should review the patient well before the date of surgery. Patients lung and cardiac status must be carefully evaluated. General disorders like diabetes mellitus / hypertension should be looked for. These conditions must be adequately controlled before taking up the patient for surgery.

The patient's airway anatomy must be carefully reviewed by the anesthetist. The following points must be looked for:

1. Small / receding jaw

2. Prominent upper dentition

3. Short neck

4. Limitations to neck extension due to cervical spondylosis

5. Poor dentition

6. Tumors of oral cavity and throat

7. Facial truma

8. Interdental fixation

Many scoring systems have been evolved to identify patients in whom intubation may be difficult. The most widely used among them is the Mallampati score. It identifies the patient in whom pharynx is not well visualised through open mouth. High Mallampati scores predicts difficult intubation.

Mallampati scoring:

Class I: Entire tonsil clearly visible when mouth is opened

Class II: Upper half of the tonsillar fossa alone is visible when mouth is opened

Class III: Only the hard and soft palates are clearly visible when mouth is opened

Class IV: Only hard palate is visible on mouth opening.

1. Patient should come to the operation theatre on an empty stomach. This is to prevent aspiration of the stomach contents during anesthesia. At least 6 hours of starvation is mandatory before surgery.

2. If the patient is on anticoagulants / antiplatelet drugs it should be stopped atleast one week before surgery

3. Oral hypoglycemics should be avoided on the day of surgery. If the patient is on Metformin it should be discontinued atleast 48 hours prior to surgery. This is done to prevent the development of fatal acidosis during anesthesia.

4. If the patient is on antihypertensives it can be taken on the morning of surgery.

First stage of general anesthesia:

Premedication:

The goal of this stage of the anesthesia process is to have the patient arrive in the operating room in a calm, relaxed frame of mind while causing minimal interference with breathing and cardiovascular status. The commonly used drugs for premedication are the short acting benzodiazepines. Midazolam syrup is commonly given to children to facilitate reduction in anxiety. If facilities are available intravenous administration of Fentanyl or Midazolam can be considered. Drying agents like atropine / Scopalamine can be adminsitered to dry up the oral and tracheal secretions.

Second stage of general anesthesia:

Induction: This second stage of anesthesia is the most critical phase in the whole process. It may even be compared to an aircraft take off. This step transforms the awake patient into an unconsious one. This is achieved by intravenous injection that has a rapid onset of action like the thiopental sodium or Propofol / slow inhalation of anesthetic vapor or a combination of both. During this stage patient also receives a narcotic analgesic like injection fortwin. This has a synergestic effect with thiopental sodium.

The next step is securing the airway. This could be a simple matter of holding the patient's jaw in such a way that normal breathin is not impeded by tongue falling back occluding the larynx. A laryngeal mask or endotrachal tube may also be used to secure the airway.

Indications for endotracheal intubation during general anesthesia:

. Potential for airway contamination from pharyngeal bleeding / stomach contents

. Surgical need for muscle relaxation

. Surgery to the mouth or face

. When a prolonged procedure is anticipated

If the patient needs to be intubated then muscle relaxation is a must. On a short term basis this is achieved by the injection of Scoline (Succinyl Choline chloride). After intubation muscle relaxation is maintained by the use of intermediate or long acting drug like Pavlon (Pancuronium / Vecuronium).

Maintenance phase:

During this phase the patient is maintained on a mixture of oxygen and volatile gas like nitrous oxide. Agent like Halothane can also be used to supplement these drugs in the maintenance phase.

Extubation:

During this phase secretions from the oral cavity are carefully sucked out to prevent aspiration. Injection Neostigmine is given to reverse the neuromuscular blockade created by injection Pavlon. After the patient recovers from general anesthesia the endotracheal tube is carefully removed applying suction through it to clear the trachea of any secretions.

Mallampatti classification to identify difficult airway