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