Anesthesia
Anesthesia is a state of temporary induced loss of sensation or
awareness. It may include analgesia (relief from or prevention
of pain), paralysis (muscle relaxation), amnesia (loss of memory),
or unconsciousness. A patient under the effects of anesthetic drugs is
referred to as being anesthetized.
An analgesic or painkiller is any member of the group of drugs used to
achieve analgesia, relief from pain.
Analgesic drugs act in various ways on
the peripheral and central nervous systems. Analgesics
include paracetamol , the nonsteroidal anti-inflammatory drugs (NSAIDs)
such as the Diclofenac, and opioid drugs such as morphine and Fentanyl.
Sedation is the reduction of irritability or agitation by administration
of sedative drugs, generally to facilitate a medical procedure or
diagnostic procedure. Examples of drugs which can be used for sedation
include propofol,thiopental, etomidate and midazolam.
Muscle relaxant is a drug that affects skeletal muscle function and
decreases the muscle tone . Neuromuscular blockers act by interfering
with transmission at the neuromuscular end plate and have no central
nervous system (CNS) activity. They are often used during surgical
procedures and in intensive care and emergency medicine to cause
temporary paralysis.
Types Of Anesthesia
There are three main categories of anesthesia, each having many forms and uses.
They are:



General
Regional (Neuroaxial)
Local
In general anesthesia, unconscious and have no awareness or other sensations.
There are a number of general anesthetic drugs - some are gases or vapors
inhaled through a breathing mask or tube and others are medications introduced
through a vein.
In regional anesthesia, anesthesiologist makes an injection near a cluster of
nerves to numb the area of body that requires surgery. Patient may remain
awake, or may be given a sedative, either way Pt. do not see or feel the actual
surgery taking place. There are several kinds of regional anesthesia; the two
most common are spinal anesthesia and epidural anesthesia.
In local anesthesia, the anesthetic drug is usually injected into the tissue to
numb just the specific location of body requiring minor surgery.
General anaesthesia
Preanaesthetic evaluation
Prior to a planned procedure, the anaesthetist reviews medical history. Key
factors in this evaluation are the patient's age, medical and surgical history,
current medications, and fasting time. Thorough and accurate answering of the
questions is important so that the anaesthetist can select the proper drugs and
procedures.
An important aspect of pre-anaesthetic evaluation is an assessment of the
patient's airway, involving inspection of the mouth opening and visualisation of
the soft tissues of the pharynx. The condition of teeth and location of dental
crowns are checked, and neck flexibility and head extension are observed.
Premedication
Prior to administration of a general anaesthetic, the anaesthetist may administer
one or more drugs that complement or improve the quality or safety of the
anaesthetic.
Midazolam and Diazepam a benzodiazepine characterized by a rapid onset and
short duration, is effective in reducing preoperative anxiety . Anaesthesiologists
may administer an antiemetic agent such as ondansetron, or dexamethasone to
prevent postoperative nausea and vomiting.
Induction
General anaesthesia is usually induced in operating theatre. Most general
anaesthetics are induced either intravenously or by inhalation. Intravenous
injection works faster than inhalation, taking about 10–20 seconds to induce total
unconsciousness. Commonly used intravenous induction agents include
propofol, thiopental, etomidate and ketamine . Inhalational anaesthesia may be
chosen when intravenous access is difficult to obtain (e.g., children). Sevoflurane
is the most commonly used agent for inhalational induction, because it is less
irritating to the tracheobronchial tree than other agents.
Physiologic monitoring
Several monitoring technologies allow for a controlled induction , maintenance
and emergence from general anaesthesia.
1. Continuous electrocardiography ECG .
2. Continuous pulse oximetry (SpO2): To detect Hypoxemia
3. Blood pressure monitoring
4. Capnography measures the amount of carbon dioxide exhaled by the
patient
5. Temperature measurement to detect hypothermia or fever.
Maintenance
The duration of action of intravenous induction agents is generally 5 to 10
minutes, after which spontaneous recovery of consciousness will occur. In order
to prolong unconsciousness for the required duration (usually the duration of
surgery), anaesthesia must be maintained. This is achieved by allowing the
patient to breathe a controlled mixture of oxygen, sometimes nitrous oxide, and a
volatile anaesthetic agent .
Recovery
Recovery is the return of concious after the cessation of general anaesthetics
Local Anesthetics
Local and regional anesthesia and analgesia techniques depend on a group of
drugs—local anesthetics—that transiently inhibit sensory, motor, or autonomic
nerve function, or a combination of these functions, when the drugs are
injected or applied near neural tissue.
Although regional anaesthesia has side effects and complications , it is an
excellent option for many patients, especially those with major co-morbidities
such as significant heart and lung disease.
The duration of action of an LA is related to the extent of protein binding at the
site of action and factors that affect removal of the drug from the site, e.g.
blood supply. The speed of onset of an LA depends on the local availability of
unionized free base. This can be improved by increasing the concentration of
the LA . The acidic, low-pH environment surrounding infected areas, e.g.
abscesses, impairs the action of LAs.
Despite the expense, the use of ultrasound machines is established practice in
many countries and improves block quality, decreases LA doses, and reduces
complication rates. A detailed knowledge of the anatomy , current resuscitation
skills, and the willingness to accept that they will need to be committed to
deliver the best results are essential for the practitioner.
Indication of endotracheal intubation
1- Patients with full stomach
2- Head and neck surgery
3- Upper airway bleeding
4- After giving muscle relaxant
5- Abnormal position (prone position)
Regional anaesthesia (NEURAXIAL ANESTHESIA)
Regional anaesthesia is ideal for many operations, in particular those on the
limbs and lower abdomen. For those who do not wish to be fully awake for
surgery, sedation can also be used. For many other operations, regional
analgesia can complement GA and provide lasting and effective post-operative
pain relief .
Spinal and epidural anaesthesia are the techniques of choice for a Caesarean
section for the vast majority of women . Regional anaesthesia can be
accomplished with basic equipment.
Spinal Anesthesia
The mechanism of action of spinal anesthesia is due to the effect of local
anesthetic agents on the individual nerve roots, effects that depend on the size
and myelin content of nerve fibers, concentration of the local anesthetic agent,
and duration of contact between the nerve root and the local anesthetic agent.
Cardiovascular Effects
The loss of sympathetic activity that accompanies a spinal anesthetic results in
vasodilation below the level of blockade, and the associated venodilation
decreases cardiac preload and, therefore, cardiac output and bradycardia. The
net effect of these physiologic changes can manifest as a profound decrease in
systemic blood pressure.
Contraindications to Spinal Anesthesia
Absolute contraindications include patient refusal, coagulation abnormalities,
severe
hypovolemia,
1- Patient
refusal increased intracranial pressure, infection at site of
injection,
and severe
stenotic aortic and mitral valvular heart disease
2- coagulation
abnormalities
3- severe hypovolemia
4- increased intracranial pressure
5- infection at site of injection
6- severe stenotic aortic and mitral valvular heart disease