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Principles of Anesthesia & Pre-Anesthetic Medication - Coggle Diagram
Principles of Anesthesia & Pre-Anesthetic Medication
Overview
Primary Neurophysiologic Effects
Sedation & reduction of anxiety
Lack of awareness & amnesia
Skeletal muscle relaxation
Suppression of undesirable reflexes
Analgesia
General Anesthesia
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
Anesthesia - loss of bodily sensation with or without loss of consciousness
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
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
Anti-Histamines (H1-blocker)
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
Benzodiazepines
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
H2 Blockers
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
Eg:
Fentanyl
Function: To
relieve pain (analgesia)
Anti-Cholinergic Drugs
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
Selection of Anesthesia
Cardiovascular system
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)
Respiratory system
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
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]
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
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
IV infusions
of various drugs may also be incorporated during maintenance phase
Recovery
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
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)
General Anesthetic Agents
Inhalation Gases
Halothane, Isoflurane, Desflurane, Sevoflurane, Nitrous Oxide
Intravenous Drugs
Propofol, Fospropofol, Barbiturates (thiopental, mathohexital), BZD (midzolam, diazepam, lorazepam), Opioids (fentanil, sufentanil, remifentanil), Etomidate, Ketamine, Dexmedetomidine
Stages (Depth) of Anesthesia
Stage 1- Analgesia
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
Stage 2- Excitement
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
4 stages: Analgesia, Excitement, Surgical Anesthesia, Medullary paralysis
Stage 3- Surgical Anesthesia
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
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
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