Principles of Anesthesia & Pre-Anesthetic Medication

Overview

Primary Neurophysiologic Effects

General Anesthesia

Anesthesia - loss of bodily sensation with or without loss of consciousness

Reversible state of CNS depression, resulting in loss of response to & perception of external stimuli - patient will be unconscious, unable to respond to pain signals, no reflex

GA usually uses a combination of inhalation gases & intravenous drugs

Sedation & reduction of anxiety

Lack of awareness & amnesia

Skeletal muscle relaxation

Suppression of undesirable reflexes

Analgesia

Pre-Anesthetic Medications

They are adjuncts to anesthesia & becomes components to achieve a balanced anesthesia

Eg: BZD, H2 Blocker, Anti-histamines, Anti-emetics, Opioids, Anti-cholinergic drugs, NMBs

Anti-Histamines (H1-blocker)

Benzodiazepines

H2 Blockers

Anxiety interferes with patient comfort, increases stress hormone production, gastric secretions, initial anesthetic requiremnets & preoperative procedure difficulty (i.e: iv placement)

Children may have high anxiety levels thus, giving lack of cooperation

Function: To relieve anxiety & facilitate amnesia

Given orally 1-2 hours before surgery, give small effect on cardiorespiratory function but large doses can interfere with the speed & quality of recovery

Eg: Midazolam & Diazepam

Function: To prevent allergic reactions by blocking H1-receptor

Some subs used in preoperative period carry a certain risk of releasing histamine & triggering an allergic reaction.

Eg: Diphenhydramine

Some things that induce allergy: latex, antibiotics, anesthesia induction agents & radiocontrast material

Patients who have acid reflux are at risk of acid aspiration under GA

Aspiration of acidic gastric fluid can cause pneumonitis with the syndrome of progressive dyspnoea, hypoxia, bronchial wheeze

Function: To **prevent gastric acid secretion (reduce gastric acidity) by blocking action of histamine on parietal cells of stomach

H2 blockers help to increase gastric pH & lower gastric acid volume & reduce the risk of acid aspiration

Eg: Famotidine & Ranitidine

Anti-Emetics

Function: To prevent aspiration of stomach contents & post-surgical nausea & vomiting

Postoperative nausea & vomiting (PONV) is the 2nd most common complaint

Eg: Ondansetron (5-HT3 antagonist)

Opioids

Anti-Cholinergic Drugs

Eg: Fentanyl

Function: To relieve pain (analgesia)

Eg: Glycopyrrolate

Function: To prevent bradycardia & secretion of fluids into the respiratory tract

Neuromuscular Blocking Agents

Eg: Tubocurrarine

Function: To facilitate intubation & to achieve muscle relaxation

When administered concurrently -> pre-anesthetic medication lower the dose of anesthetic required to maintain the desired level of surgical anesthesia

However, co-administration may as well: (1) enhance undesirable anesthetic effects (eg: hypoventilation) (2) produce negative effects that are not observed when each drug is given individually

Selection of Anesthesia

Cardiovascular system

Respiratory system

Overview

During pre-operative planning, efficient anesthetic regime was chosen on nature of surgery or diagnostic procedure as well as on patient's physiologic, pathologic & pharmacologic state

Status of organ systems are always taken into consideration (1) CVS (2) Respiratory (3) Liver & Kidney (4) Nervous (5) Other [pregnancy]

If a hypotensive episode develops during anesthesia, the reduced perfusion pressure may be followed by ischemic injury to tissues

Inhalation anesthetics such as halothane may sensitize the heart to catecholamines -> increase risk of arrhythmias

Anesthetic agents suppress CV function to varying degrees (Eg: propofol, iv agent caused markedly fall in BP)

Inhaled anesthetics depress the respiratory system & acts as bronchodilators while IV anesthetics & opioids may suppress respiration

This effect on respiratory function may affect the ability for adequate ventilation & oxygen supply during surgery & post-operatively

Patients with asthma & ventilation or perfusion abnormalities -> may complicate control of inhalation anesthetics

Liver & Kidney

Release of fluoride, bromide & other metabolic products of halogenated hydrocarbons can affect these organs esp the metabolites that accumulate over repeated administration over short period of time

Both organs influence the long-term distribution & clearance & may also become the target organs for toxic effects, thus their status are important to be considered

Nervous system

Eg: Propofol causes CNS depression

Neurologic disorders (eg: epilepsy, myasthenia gravis, neuromuscular disease & compromised cerebral circulation) or patient history of malignant hyperthermia influences the selection of anesthetics

Others (Pregnancy)

BZDs should not be used routinely during labor, because of resultant temporary hypotonia & altered thermoregulation in newborn

Special precaution for use in pregnant women: Anesthetics may affect organogenesis in fetus during early pregnancy; Transient use of nitrous oxide -> aplastic anemia in the unborn child; Oral clefts among child of women who received BZD during early pregnancy (2-fold increased risk of oral cleft)

Stages of GA

Maintenance

Recovery

Induction

Adults normally induced with IV anesthetic (eg: propofol produces unconsciousness within 30-40 seconds);;; children w/o IV access, inhalatio induction is used with non-pungent agents (eg: halothane or sevoflurane)

Additional inhalation and/or IV drugs of selected combination may be given to produce the desired depth of surgical anesthesia (Stage III)

Induction depends on how fast the effective concentrations of anesthetic drugs reach brain

Period of time from onset of administration to the development of surgical anesthesia

This often includes co-administration of IV skeletal muscle relaxant to facilitate intubation & muscle relaxation (eg: rocuronium, vecuronium or succinylcholine)

Patient's vital signs & response to various stimuli are monitored continuously throughout surgery by carefully balancing amount of drug inhaled and/or infused with the depth of anesthesia

During this period, anesthesia is commonly maintained by administration of volatile anesthetics - for good control over the depth of anesthesia

A period of sustained surgical anesthesia

Opioids (eg: fentanyl) often used for pain relief along with inhalation agents, because the latter are not good analgesics

General Anesthetic Agents

Inhalation Gases

Intravenous Drugs

Propofol, Fospropofol, Barbiturates (thiopental, mathohexital), BZD (midzolam, diazepam, lorazepam), Opioids (fentanil, sufentanil, remifentanil), Etomidate, Ketamine, Dexmedetomidine

Halothane, Isoflurane, Desflurane, Sevoflurane, Nitrous Oxide

IV infusions of various drugs may also be incorporated during maintenance phase

Recovery is the reverse of induction

Redistribution from site of action underlies recovery (rather than metabolism of the anesthetic)

Post-operatively, the anesthetic mixture is withdrawn & the patient is monitored for the return of consciousness

Recovery depends on how fast the anesthetic diffuses from the brain

Monitoring continues until normal physiologic function is achieved (eg: spontaneous respiration, acceptable BP & HR etc)

A period from discontinuation of administration of anesthesia until consciousness & protective physiologic reflexes are regained

If skeletal muscle relaxants have not been fully metabolized, reversal agents may be used

Patients are also observed for delayed reactions eg: respiratory depression from opioids administered for post-operative pain control

Stages (Depth) of Anesthesia

Stage 1- Analgesia

Stage 2- Excitement

4 stages: Analgesia, Excitement, Surgical Anesthesia, Medullary paralysis

Stage 3- Surgical Anesthesia

These stages were based on the observation of the inhaled anesthetics

4 sequential stages, based on increasing depth of CNS depression (GUEDEL'S SIGN)

Stage 4- Medullary Paralysis

Patient progresses from conscious to drowsy

Amnesia & reduced awareness of pain will occur as stage II is approached

Interference with sensory transmission in the spinothalamic tract lead to loss of pain sensation

A rise & irregularity in BP & respiration

There is also some risk of laryngospasm

To shorten or eliminate this stage, a rapid acting agent such as propofol is given iv before inhalation anesthesia is administered

Patient experience delirium & possibly combative behavior

Relaxation of skeletal muscle with eventual loss of spontaneous movement occur in this stage

IDEAL STAGE FOR SURGERY

Regular respiration

Continuous careful monitoring is required to prevent undesired progression to stage IV

Gradual loss of muscle tone & reflexes as CNS is further depressed

Severe depression of respiratory & vasomotor centers

If no measures taken to maintain circulation & respiration, -> DEATH

Local Anesthesia

Esters linkage - procaine, chloroprocaine, tetracaine, cocaine

Amides linkage - lidocaine, bupivacaine, ropivacaine, mepivacaine, prilocaine

Blocks pain sensation to specific (localized) areas of the body without loss of consciousness