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General anaesthesia

General anaesthesia is a state characterised by:

  • Reversible loss of consciousness
  • Absence of pain (analgesia)
  • Blocking of noxious autonomous reflexes
  • Loss of memory (amnesia)
  • Relaxation of skeletal muscles

Multiple drugs are needed to achieve general anaesthesia. Many different drugs are used as general anaesthetics. They’re either inhalation anaesthetics or intravenous anaesthetics, according to the method of delivery. These two types can be combined to exploit the favourable properties of each type while minimizing the unfavourable properties.

There are six stages of achieving general anaesthesia:

  • Preparation
  • Induction – the transfer from wakefulness to narcosis
    • Analgesic first (opioid), then anaesthetic (propofol)
    • Rapid sequence induction (RSI) – for full stomach patients
    • Delayed sequence induction (DSI) – for fasting patients
  • Muscle relaxation
    • In RSI – fast acting relaxants, like succinylcholine or rocuronium
    • In DSI – slow acting relaxants, like atracurium, mivacurium
  • Airway management
    • Face mask, laryngeal mask/i-gel, or intubation
  • Ventilation, monitoring and maintenance of general anaesthesia
  • Reversal and recovery
    • Reversal includes giving antidotes to the muscle relaxant

Preparation

During induction, the patient will not be breathing (apnoea) momentarily, from the administration of anaesthetic to when an airway device is inserted and the patient can be ventilated. To prevent hypoxaemia (desaturation) during this period, the patient is preoxygenated before induction. This involves giving the patient 100% FiO2 oxygen for a few minutes. This replaces the room air (which is 21% oxygen) in the patient's lungs with pure oxygen. When the patient later becomes apnoeic, the oxygen in the lungs acts as a "reserve". Oxygen should be administered with > 10 L/min.

Preoxygenation is usually achieved with a face mask. Non-invasive ventilation should be used for obese or pregnant patients, or those who are critically ill. Preoxygenation should last for approximately 3 minutes or for 8 vital capacity breaths.

Induction

Induction refers to the transition from wakefulness to loss of consciousness. An analgesic (usually an opioid) is usually administered first, followed by the anaesthetic. When the patient is sufficiently anaesthetised and no longer spontaneously breathing, a muscle relaxant is administered to facilitate intubation. When the relaxant is effective, the patient is intubated. From this time until the endotracheal tube is successfully placed, the patient is not breathing; this is the apnoeic period.

During the apnoeic period one can administer oxygen nasally while placing the airway device. This is low-risk and should be used in those with difficult airways (where intubation may take longer than the oxygen reservoir lasts).

Confirm successful tube placement by visualisation of the chest elevating, auscultation of bilateral breathing sounds, and confirming elevated end-tidal CO2 (CO2 in the exhaled air) on the capnograph.

Rapid sequence induction

For patients who are not fasting or vomiting and are therefore at risk of aspiration during ventilation with a face mask.

Maintenance

At this stage the patient has a patent airway device and is anaesthetised. Inhaled or intravenous anaesthetic should be continously administered to maintain anaesthesia. The dose should be continously adjusted so that anaesthesia is not too deep (may cause cardiovascular depression, tissue hypoperfusion, organ dysfunction, and delayed recovery from anaesthesia) and not too shallow (patient may wake up).

Monitoring

The most important monitor during surgery is the anaesthesiologist. It’s important to observe the patient and heed the surgeon’s warnings. The monitoring machines are also used.

Some parameters are monitored in all cases of anaesthesia:

  • SpO2
  • ECG
  • Non-invasive blood pressure
  • FiO2 (the percentage of oxygen in the provided air)
  • EtCO2 (end-tidal CO2)

Other parameters which are optional and can be monitored if necessary:

  • Invasive blood pressure
  • Temperature
  • PaO2
  • Diuresis monitoring
  • Central venous cannulation
  • Nasogastric tube

Depth of anaesthesia

Depth of anaesthesia is usually measured by single-channel EEG-based systems, like the BIS (bispectral index). However, simple measurements like heart rate and blood pressure can give some indication as to whether the anaesthesia is too light and the patient is experiencing pain.

Peripheral muscle relaxants

The effect of peripheral muscle relaxants is evaluated by train-of-four (TOF). The equipment monitors the activity of the adductor pollicis muscle in response to four stimuli of the ulnar nerve.

Note that all sources I can find list the ulnar nerve as the innervator of this muscle, but according to at least three commenters, the examiners are adamant that it’s innervated by the radial nerve. The ABductor pollicis longus is innervated by the radial, but that is not the muscle measured here. All sources say that TOF monitoring involves the adductor pollicis and stimulation of the ulnar nerve. See here and here, for example.

Gas exchange

The EtCO2 is the concentration of CO2 at the end of expiration. It is evaluated by waveform capnography and is another indicator of the ventilation status. If it increases, the patient is hypoventilated.

The ratio between FiO2 and PaO2 is another good indication of the oxygenation status of the patient but requires ABG.

Circulation

The fluid intake (infusions) is monitored and compared to the fluid loss, usually as blood which is suctioned and as urine in the bladder catheter, if present. During long-lasting open surgeries, insensible fluid loss must be taken into account as well.