Induction of Anesthesia & Stages of Anesthesia
Definition
Transition from an awake to an anaesthetized state.
Stages of anaesthesia
Guedel's classification:
Four stages of anaesthesia
Stage 1
Stage of Induction: is the period between the administration of induction agents and loss of consciousness.
During this stage, the patient progresses from analgesia without amnesia to analgesia with amnesia.
Patients can carry on a conversation at this time.
Stage 2
The excitement stage, is the period following loss of consciousness and marked by excited and delirious activity.
Respiration and heart rate may become irregular.
Uncontrolled movements, vomiting, suspension of breathing, and pupillary dilation.
The combination of spastic movements, vomiting, and irregular respiration may compromise the patient's airway.
Stage 3
Stage of surgical anaesthesia
The skeletal muscles relax
Vomiting stops
Respiratory depression occurs
Eye movements slow and then stop.
The patient is unconscious and ready for surgery.
This stage is divided into four planes:
The eyes roll, then become fixed
Corneal and laryngeal reflexes are lost
The pupils dilate and light reflex is lost
Intercostal paralysis and shallow abdominal respiration occur.
Stage 4
Stage of overdose
Occurs when too much anaesthetic medication is given relative to the amount of surgical stimulation
Patient has severe brainstem or medullary depression, resulting in a cessation of respiration and potential cardiovascular collapse.
This stage is lethal without cardiovascular and respiratory support.
Induction
GA : most commonly in an operating theatre or in a dedicated anaesthetic room adjacent to the theatre .
Anaesthetic agents administered by various routes, including inhalation, injection (intravenous, intramuscular, or subcutaneous), oral, and rectal.
Intravenous injection works faster than inhalation, taking about 10–20 seconds to induce total unconsciousness.
This minimizes the excitatory phase (Stage 2) and thus reduces complications related to the induction of anaesthesia.
Commonly used intravenous induction agents include propofol, sodium thiopental, etomidate, and ketamine.
Inhalational anaesthesia chosen when IV access is difficult to obtain (e.g., children), when difficulty maintaining the airway is anticipated, or when the patient prefers it.
Sevoflurane is the most commonly used agent for inhalational induction
It is less irritating to the tracheobronchial tree than other agents.
Sequence of induction drugs:
Pre-oxygenation to fill lungs with oxygen to permit a longer period of apnea during intubation
Lidocaine for sedation and systemic analgesia for intubation
Fentanyl for systemic analgesia for intubation
Propofol for sedation for intubation
Switching from oxygen to a mixture of oxygen and inhalational anesthetic (Nitrous Oxide)
Laryngoscopy and intubation are both very stimulating and induction blunts the response to these maneuvers while simultaneously inducing a near-coma state to prevent awareness.
Physiologic monitoring
Several monitoring technologies are used
Continuous electrocardiography (ECG or EKG):
Continuous pulse oximetry (SpO2) to detect hypoxaemia.
Blood pressure monitoring:
Anesthetic drug concentration measurement: These monitors include measuring oxygen, carbon dioxide, and inhalational anaesthetics (e.g., nitrous oxide, isoflurane).
A circuit disconnect alarm or low pressure alarm.
Capnography measures the amount of carbon dioxide exhaled by the patient to assess the adequacy of ventilation.
Temperature measurement
ECG monitoring,
Other systems used to verify the depth of anaesthesia. This reduces the likelihood of anesthesia awareness and of overdose.
Airway management
Endotracheal tube is often used: face masks or laryngeal mask airways are also used.
Induction of general anesthesia results in apnea and requires ventillation until the drugs wear off and spontaneous breathing starts. mechanical ventilation can provide ventilatory support during spontaneous breathing to ensure adequate gas exchange.
Eye management
General anaesthesia reduces the tonic contraction of the orbicularis oculi muscle, causing incomplete eye closure; hence to be protected
Keep Ready:
Syringes prepared with medications that are expected to be used during an operation under general anesthesia maintained by sevoflurane gas:
- Propofol, an hypnotic
- Ephedrine, in case of hypotension
- Fentanyl, for analgesia
- Atracurium, for neuromuscular block
- Glycopyrronium bromide (here under trade name Robinul), reducing secretions
Paralysis, or temporary muscle relaxation with a neuromuscular blocker, is an integral part of modern anaesthesia.
The effects of muscle relaxants are commonly reversed at the end of surgery by anticholinesterase drugs. Examples of skeletal muscle relaxants in use today are pancuronium, rocuronium, vecuronium, atracurium, mivacurium, and succinylcholine.
Maintenance
The duration of action of intravenous induction agents : 5 to 10 minutes
Anaesthesia must be maintained.
This is achieved by a mixture of oxygen, nitrous oxide or by administering propofol, through an intravenous catheter.
Inhaled agents are frequently supplemented by intravenous anaesthetics, such as opioids (usually fentanyl or a fentanyl derivative) and sedatives (usually propofol or midazolam)
At the end of surgery, the anaesthetic agents are discontinued. Recovery of consciousness occurs
A novel method of maintaining anaesthesia: called TCI (target controlled infusion), it involves using a computer-controlled syringe driver (pump) to infuse propofol throughout the duration of surgery, removing the need for a volatile anaesthetic
