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General anesthesia

• Usually it involves the following :
• analgesia
• amnesia
• loss of consciousness
• inhibition of sensory and autonomic reflexes
• and skeletal muscles relaxation.

• Usually we can't produce all these effects with a single drug without causing major side effects ;therefore; in practice of general anesthesia ; use combination of different drugs .

Drugs that are used in practice of G.A. include Is a drug of choice in treatment acute form of angina pectorisit is potent vasodilator ,it relaxes smooth muscles of blood vessels ( on venous side more than arterial side ), lead to decrease in preload and after load on the heart lead to reduce myocardial oxygen and energy demand On venous side lead to decrease venous return to heart, lead to decrease ventricular volume and consequently lead decrease left ventricular diastolic pressure.On arterial side lead to decrease arterial blood pressure lead to reduction ejection time by this decrease heart work and decrease oxygen and energy demand and overcome on the anginal pectoris, beside increase oxygen supply to heart by increase coronary vasodilatation
• Drugs used before surgery (pre-operative ) .
The principle aims are to provide the following :
A)Sedation :-
We started with them as soon as possible ,because anxious
patients with high levels of adrenalin and raised BMR is more
liable to cardiac arrhythmias ,and more difficult to be
anesthetized, include Benzodiazepines e.g. Diazepam
;Promethazine (antihistamine),chloral-hydrate(in children) .
B) analgesia :-
they are used when there's an existing pain or as a supplement to
an anesthetic agent having low analgesic effect like Morphine and
Pethidine (Opioids),NSAIDs, can be used with caution .


Drugs that are used in practice of G.A. include
C) inhibition of parasympathetic nervous system to produce
the following
1-decrease salivation and bronchial secretion ,which might increased by some irritant anesthetic agents .
2- decrease any tendency to bronchospasm .
3- decrease reflex bradycardia and hypotension ;all this can achieve by using Atropine and Scopolamine
D) for stomach ; as prophylaxis against aspiration pneumonia caused by aspiration of acid content of the stomach ,can use the following either :
- A single dose of an antacid .
- H2 receptor blockers (as Ranitidine, Cimetidine ,… )
- Proton pump inhibitors like: Omeprazole, Lansoprazole, ……)

The principle aims are to provide the following

1- loss of consciousness .
2-analgesia .
3- muscular relaxation .
Typical G.A. is consist of :-
1)Induction ; achieved by short acting Thiopentone I.V. with Suxamethionium ,if endotracheal intubation is intended, Ketamine, Propofol, or Etomidate.
2) maintenance :- achieved by
a- Nitrous oxide + volatile agent as Halothane or Ether .
b- Nitrous oxide + I.V. analgesia (as Morphine or Pethidine ) +NM. Blockers (Tubocurarine ).
2- Drugs used during surgery :-


3) Drugs used after surgery to ensure that :-

a) the effects of NM blocking agents and opioids induced

respiratory depression have either worn off or reversed by an antagonist Neostigmine .
b) re-gaining of consciousness .
c) relief of pain ( by NSAIDs or by Opioids ) .
d) post operative nausea and vomiting treated :-
1- Cyclizine, Metoclopramide, or Ondansetron.
2- Dexamethasone, also reduce incidence of post operative
nausea and vomiting
3- Combination of the above in sever cases

Mechanism of action of G.A.

• The principle site of action of G.A. at the neuronal
lipid membranes , interfering with the movement
of cat ions through ion channels , mainly sodium
and potassium channels which are associated with
action potential .
• Types of G.A. :-
2 types ; inhalation and I.V. agents


Pharmacokinetic of Inhalation agents
The level of anesthesia is correlated with the tension of the anesthetic drugs in the brain tissue and this is depend on the development of tension gradient from high partial pressure delivered to the alveoli ,then through the blood to the brain and other tissues.
The gradient depends on physical properties of the anesthetic and the tissues, as well as on physiological functions (pulmonary ventilation and pulmonary blood flow)

Factors that affect the uptake and distribution of anesthesia

1) Solubility ;
anesthetic gas which has high solubility in blood (as Ether ) when given at a steady concentration will provide slow induction because the blood act as a reservoir for the drug ; so that it doesn’t enter the brain easily until the blood reservoir has been filled ;while agents with low solubility as( Nitrous oxide , Halothane or Sevoflurane),will provide rapid induction because the reservoir is small and the gas is available to pass to the brain sooner .
2) Anesthetic concentration (tension) in inspired air ;this affect the maximum tension that could be achieved in alveoli and the rate of increase in tension in the arterial blood .

Factors that affect the uptake and distribution of anesthesia

3) pulmonary ventilation ;
the rate of raise in the anesthetic conc. In arterial blood is dependent on the rate and depth of ventilation ; hyperventilation that is induced by mechanical control of respiration will increase speed of induction ; hypoventilation induced by opioids will decrease the speed of anesthesia (onset of action ) .
4) pulmonary blood flow ;
an increase in the pulmonary blood flow will slow the rate of rise the arterial tension especially for gases with high blood solubility . This occur because in this case there is increase of the amount of blood needed to be saturated (slow induction)

Elimination:-

The major rout of elimination is inhalation of
elimination depend on the lungs and is affected by
1)solubility :-
agents with high solubility → slow elimination (slow recovery ), while insoluble agents are eliminated at faster rate which may cause ( diffusion hypoxia ) because insoluble agents displace oxygen by fast accumulation in alveoli .
2) duration of exposure to anesthesia .
3) pulmonary blood flow .
4) ventilation .


Effects of G.A.
1) Sympathetic nervous system ;
stimulation is produced by Ether , Cyclopropane and Nitrous oxide ;while it is inhibited by halogenated anesthetics as Halothane .
2)Parasympathetic nervous system ;
Cyclopropane and Halothane are vagal stimulation that lead to bradycardia .
3)Cardiovascular system :-
Heart → all G.A. cause (dose related ) negative inotropic effect because of the membrane stabilizing action on the cells of the heart ,but there are difference between G.A. e.g. Halothane is more than Ether ; G.A. ;causes sensitization of the heart to the catecholamine ;so any exogenous catecholamine may lead to cardiac arrhythmia ..
Cyclopropane and Halothane are the most powerful negative inotropic effect .

Effects of G.A.

On blood pressure :-
Ether lead to increase the blood pressure due to its ability to liberate catecholamine .
Halothane ,Enflurane, Isoflurane lead to decrease the mean arterial blood pressure .
Halothane and Enflurane decrease the blood pressure due to decrease cardiac output ;while Isoflurane due to decrease systemic resistance with little effect on C.O.
Respiratory system :-
All inhaled anesthetics are respiratory system ;G.A. ,decrease tidal volume and increase respiratory rate except Nitric oxide and Ether .
Isoflurane is the most potent respiratory depressant ;they depress mucocilliary function of airways → lead to lung collapse and respiratory infection .
They have a bronchodilator effect especially Halothane .

Effects of G.A.

On renal system :-
They decrease GFR . and decrease the effective blood flow through the kidney ;so they will decrease renal function .
On liver:-
They decrease the hepatic blood flow ;Halothane is hepatotoxic (sever hepatitis, may be fetal
On uterine smooth muscles:-
Nitric oxide have little effect on uterine smooth muscles ;Halothane ,Isoflurane and Enflurane are potent muscle relaxants ,but they may cause postpartum hemorrhage and they are may be used in intrauterine fetal manipulation .
Miscellaneous effects :-
They cause nausea and vomiting especially Ether and Cyclopropane .
They decrease temperature due to hypothalamic depression .
Halothane cause malignant hyperthermia and acidosis .



رفعت المحاضرة من قبل: Ali Haider
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