Inhalation Anaesthetics – This book covers the entire syllabus of “Pharmacology” prescribed by BNMC- for diploma in nursing science & midwifery students. We tried to accommodate the latest information and topics. This book is an examination setup according to the teachers’ lectures and examination questions.
At the end of the book, previous questions are given. We hope in touch with the book students’ knowledge will be upgraded and flourish. The unique way of presentation may make your reading of the book a pleasurable experience.
Inhalation Anaesthetics
Pharmacokinetics of inhalation anaesthetics
Inhalation anaesthetics enter and leave the body via the lungs. Anaesthetics can easily cross the polar membrane as they are all lipid soluble molecules. It is therefore the rate of delivery of drug to an the lungs via the inspired air and the bloodstream that determines the overall kinetic behavior of an anaesthetic (induction and recovery).
The main factors that determine the speed of induction and recovery are as follows:
Properties of the anaesthetic agents
a. Blood: Gas partition coefficient
b. Oil: Gas partition coefficient.
Physiological factors
a. Alveolar ventilation rate.
b. Cardiac output.
Blood: Gas partition co-efficient (blood solubility)
The solubility of an anaesthetic in different medium is expressed as partition co-efficient. Partition co-efficient is defined as the ratio of the concentration of the agent (anaesthetic) in two phase (Alveoli: Blood, or Blood: Tissue at equilibrium.
The rate at which this equilibrium is approached depends mainly on the blood: gas partition coefficient (i.e. blood solubility). So in case of induction:
The lower the solubility in blood (i.e. low blood: gas partition coefficient), the faster the process of equilibrium. So faster induction. The higher the solubility in blood (i.e. high blood: gas partition coefficient), the delay the process of equilibrium. So delay induction.
Note: blood acts as a reservoir for the drugs, so that drug does not enter the brain easily until blood reservoir has been filled therefore induction is delayed.
In recovery phase
During induction of anaesthesia the blood is taken up anaesthetic gas from the alveoli. When the anaesthetic is withdrawn the reverse occurs and there is a diffusion flow of gas from the blood into the alveoli. So-
a. Low solubility agent diffuse-out most rapidly, therefore rapid recovery.
b. High solubility agent diffuse-our slowly, therefore delay recovery.
Oil: Gas partition coefficient (i.e. lipid solubility)
Lipid solubility is the main determinates of an anesthetic potency. Agents with high lipid solubility accumulate gradually in body fat. Therefore tends to delay recovery from the effects of anaesthesia.
Inhalation anaesthetic agents
a. Nitrous oxide
b. Halothane
e. Isoflurang
d. Enflurane
e. Methoxyflurane
f. Cyclopropane
g. Ether
Criteria of an ideal inhalation general anaesthetic
An ideal inhalation general anaesthetic agent would be characterized by:
1. Rapid and pleasant induction of, and recovery from the anaesthesia.
2. Rapid changes in the depth of anaesthesia.
3. Adequate relaxation of skeletal muscle.
4. A wide margin of safety and
5. The absence of toxic effects or other adverse properties in normal doses.
Mechanism of action of inhalation G.A
Inhalation anaesthetic
↓
Causes activation of K+ channels
↓
Hyper-polarisation of neurons
↓
Raised threshold value for action potential
↓
Decrease ability to initiate action potential
Pharmacological action of inhalation G.A
A. On CVS: Myocardial depression, Reduce myocardial 02 consumption
B. On respiratory: Increase respiratory rate (except N2O), Respiratory depression
C. Decrease PaCO2, Increase apnoeic threshold, Decrease ventilatory response to hypoxia, Decrease mucocilliary function in airways, Bronchodilatation.
D. On CNS: Increase cerebral blood flow, Amnesia, analgesia
E. On kidney: Decrease GFR. ERPF Increase renal vascular resistant
F. On liver: Decrease hepatic blood flow
G. On uterus: Oxytocic action (nitrous oxide), Tocolytic action (halothane, isoflurane)
Toxicities of inhalation anaesthetics
A. Hepatotoxicity (halothane, chloroform)
B. Nephrotoxicity (methoxyflurane)
C. Malignant hyperthermia.
D. Chronic toxicity:
1. Mutagenecity
2. Carcinogenecity
3. Abortion, congenital abnormality
4. Teratogenicity
5. Hematotoxicity (bone marrow depression by N3O to a prolong exposure)
Concentration effect
The concentration effect may be defined as follows: when higher concentrations of an anaesthetic gases are inhaled. arterial tension increases at a slightly greater rate than it would have if a lesser concentration of the anaesthetic has been inhaled.Second gas effectWhen simultaneous presence of two anaesthetic gases in the lung have been introduced, a rapidly absorbed gas (e.g. N2O) increases the rate of uptake of second anaesthetic gas (e.,q. halothane, enflurane), Clinically the second gas effect and concentration effects are useful during induction of anaesthesia:
1. They increase the rapidity of uptake of a potent inhalational agent (e.g. halothane).
2. Also increase the alveolar concentration of oxygen, thus minimizing hypoxia.
Nitrous Oxide
Characteristics of nitrous oxide
N2O is a general anaesthetic agent of low potency.
1. Non-explosive, nonflammable, slightly sweetish odour gas.
2. It produces light anaesthesia without depressing the respiratory centres.
Advantages of nitrous oxide
1. Rapid induction and recovery because of its low blood: gas partition coefficient.
2. It has strong analgesic action.
3. Very low concentration can cause unconsciousness as well as analgesia.
4. Safe and non-irritating.
Disadvantages of nitrous oxide adioli
1. Low potent inhalation anaesthetic with no muscle relaxant activity.
2. Produce incomplete anaesthesia.
3. Must be used in conjunction with other more potent anaesthetics and muscle relaxants to produce a state of full surgical anaesthesia.
4. Transient post-anaesthetic hypoxia may also occur as large volume of N2O are exhaled.
5. Expensive to buy and transport.
6. Prolong exposure causes bone marrow depression among operating theatre staffs.
Indication of nitrous oxide
1. Maintenance of surgical anaesthesia.
2. Emergency management of injuries.
3. During postoperative physiotherapy.
4. Refractory pain in terminal illness.
Adverse effects of nitrous oxide
1. Postoperative nausea and vomiting.
2. Bone marrow depression, (prolong. repeated exposure): anaemia, Leukaemia.
3. Teratogenic risk
Flurothane (Halothane)
Same drug: Enflurane, Isoflurane.
Advantages and disadvantages of Halothane
Advantages
1. Highly potent inhalational anaesthetic agent.
2. Non-explosive, non-iritant.
3. Fast induction (smooth, rapid)
4. Surgical anaesthesia produced within 2-5 minutes.
5. Induction is rapid.
6. Postoperative nausea and vomiting is low.
7. Best agent for paediatric patient and for patient with bronchial asthma.
Disadvantages
1. Highly lipid soluble: hangover is common,
2. Easily produce cardio-respiratory depression.
3. Severe hepatitis may develop (not with isoflurane)
4. Causes cardiac arrhythmia.
5. Causes hypotension (due to myocardial depression)
Contraindications of Halothane
1. Malignant hyperthermia.
2. Raised cerebrospinal fluid pressure.
3. Unexplained jaundice following previous exposure to halothane,
Adverse effects of Halothane
1. Hepatotoxicity.
2, Cardiac arrhythmia.
3. Hypotension.
Adverse effects:
1. N2O: Postoperative nausea & vomiting. Bone marrow depression, teratogenic risk.
2. Halothane: Hepatotoxicity, Cardiac arrhythmia, Hypotension
Adverse effects:
1. Halothane: Hepatotoxicity, cardiac arrhythmia, hypotension
2. Ether: postoperative nausea and vomiting, laryngeal spasm during induction, postoperative broncho- pneumonia.
Minimum alveolar anaesthetic concentration (MAC)
MAC of an anaesthetic is defined as the concentration (i.e. the percentage of the alveolar gas mixture or partial pressure of the anaesthetic as a percentage of 160 mm of Hg) that results in immobility in 5070 of patients when exposed to a noxious stimulus such as surgical incision. MAC is inversely proportional to the potency of an anaesthetic, e.g
a. More MAC: Less potent (nitrous oxide)
b. Less MAC: High potent (halothane)
Local Anaesthetics Vs General Anaesthetics
| Points | Local | General |
| Site of action | Local nerve- fibres | CNS |
| Mechanism of action | Blocks generation and conduction of nerve impulse. | Directly depress the C.N.S |
| Patient’s consciousness | Present. | Reversible loss of- consciousness |
| Pain sensation and all reflexes | Locally lost | All lost |
| Muscle relaxation | Local | Generalized |
| Pre-anaesthetic Medication | Not needed | Needed |
| Use | During minor operation | During major operation |
| Toxicities | Less, So safer | More, So dangerous |
| Care of vital functions | Not essential | Essential |
| Poor health patients | safer | Risky |
| Use in non-co-operative patient | Not possible | Possible |
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