General anaesthesia is a complex procedure involving:
Pertinent information is the patient's age, weight, medical history, current medications, previous anaesthetics, and fasting time. Usually, the patients are required to fill out this information on a separate form during the pre-operative evaluation. Depending on the existing medical conditions reported, the anaesthetist will review this information with the patient either during the pre-operative evaluation or on the day of the surgery.
Truthful and accurate answering of the questions is important so the anaesthetist can select the proper anaesthetics. For instance, a heavy drinker or drug user who does not disclose their chemical uses could be undermedicated, which could then lead to anaesthesia awareness or dangerously high blood pressure. Commonly used medications such as Viagra can interact with anaesthesia drugs; failure to disclose such usage can endanger the patient.
An important aspect of this assessment is that of the patient's airway, involving inspection of the mouth opening and visualisation of the soft tissues of the pharynx. The condition of teeth and location of dental crowns and caps are checked, neck flexibility and head extension observed. If an endotracheal tube is indicated and airway management is deemed difficult, then alternative placement methods such as fiberoptic intubation may be used.
General anaesthesia can be induced by intravenous (IV) injection, or breathing a volatile anaesthetic through a facemask (inhalational induction). Onset of anaesthesia is faster with IV injection than with inhalation, taking about 10-20 seconds to induce total unconsciousness. This has the advantage of avoiding the excitatory phase of anaesthesia (see below), and thus reduces complications related to induction of anaesthesia. An inhalational induction may be chosen by the anaesthetist where IV access is difficult to obtain, where difficulty maintaining the airway is anticipated, or due to patient preference (e.g. children). Commonly used IV induction agents include propofol, sodium thiopental, etomidate, and ketamine. The most commonly-used agent for inhalational induction is sevoflurane because it causes less irritation than other inhaled gases.
In the 1990s a novel method of maintaining anaesthesia was developed in Glasgow, UK. Called Total IntraVenous Anaesthesia (TIVA), this involves using a computer controlled syringe driver (pump) to infuse propofol throughout the duration of surgery, removing the need for a volatile anaesthetic. Purported advantages include faster recovery from anaesthesia, reduced incidence of post-operative nausea and vomiting, and absence of a trigger for malignant hyperthermia.
Other medications will occasionally be given to anesthetized patients to treat side effects or prevent complications. These medications include antihypertensives to treat high blood pressure, drugs like ephedrine and phenylephrine to treat low blood pressure, drugs like albuterol to treat asthma or laryngospasm/bronchospasm, and drugs like epinephrine or diphenhydramine to treat allergic reactions. Sometimes glucocorticoids or antibiotics are given to prevent inflammation and infection, respectively.
Acetylcholine, the natural neurotransmitter substance at the neuromuscular junction, causes muscles to contract when it is released from nerve endings. Muscle relaxants work by preventing acetylcholine from attaching to its receptor.
Paralysis of the muscles of respiration, ie. the diaphragm and intercostal muscles of the chest requires that some form of artificial respiration be implemented. As the muscles of the larynx are also paralysed, the airway usually needs to be protected by means of an endotracheal tube.
Monitoring of paralysis is most easily provided by means of a peripheral nerve stimulator. This device intermittently sends short electrical pulses through the skin over a peripheral nerve while the contraction of a muscle supplied by that nerve is observed.
The effects of muscle relaxants are commonly reversed at the termination of surgery by anticholinesterase drugs.
3. Blood Pressure Monitoring (NIBP or IBP): There are two methods of measuring the patient's blood pressure. The first, and most common, is called non-invasive blood pressure (NIBP) monitoring. This involves placing a blood pressure cuff around the patient's arm, forearm or leg. A blood pressure machine takes blood pressure readings at regular, preset intervals throughout the surgery. The second method is called invasive blood pressure (IBP) monitoring. This method is reserved for patients with significant heart or lung disease, the critically ill, major surgery such as cardiac or transplant surgery, or when large blood losses are expected. The invasive blood pressure monitoring technique involves placing a special type of plastic cannula in the patient's artery - usually at the wrist or in the groin.
5. Low oxygen alarm - Almost all circuits have a backup alarm in case the oxygen delivery to the patient becomes compromised. This warns if the fraction of inspired oxygen drops lower than room air (21%) and allows the anaesthetist to take immediate remedial action.
6. Circuit disconnect alarm - indicates failure of circuit to achieve a given pressure during mechanical ventilation.
9. EEG or other system to verify depth of anaesthesia may also be used. This reduces the likelihood that a patient will be mentally awake, although unable to move because of the paralytic agents. It also reduces the likelihood of a patient receiving significantly more amnesic drugs than actually necessary to do the job.
Major surgical procedures may require a combination of modalities to confer adequate pain relief. Parenteral methods include Patient Controlled Analgesia System (PCAS) involving morphine, a strong opiate. Here, the patient presses a button to activate a pump containing morphine. This administers a preset dose of the drug. As the pump is programmed not to exceed a safe amount of the drug, the patient cannot self administer a toxic dose.
In the US, up until about 1980 anesthesia was a significant risk, with at least one death per 10,000 times administered. After becoming something of a public scandal, a careful effort was made to understand the causes and improve the results. It is generally believed that anesthesia is now at least ten times safer than it was then. However, there is some controversy about this. In the US, the data is not made public (in fact, the data is not even collected), so the truth is uncertain. The rate for dental anesthesia is reported to be one out of 350,000.
Episodic waveforms in the electroencephalogram during general anaesthesia: a study of patterns of response to noxious stimuli
Jan 01, 2010; SUMMARY Previous studies of the electroencephalogram (EEG) during anaesthesia have identified two distinct patterns of change in...
Predicting Leg-Length Change after Total Hip Arthroplasty by Measuring Preoperative Hip Flexion under General Anaesthesia
Dec 01, 2012; ABSTRACTPurpose. To measure preoperative hip flexion under general anaesthesia in patients with developmental dysplasia of the...
Subsequent general anaesthesia in patients with a history of previous anaphylactoid/ anaphlactic reaction to muscle relaxant
Apr 01, 1999; SUMMARY Of 151 patients with a possible anaphylactoid/anaphylactic reaction to a muscle relaxant investigated over a 20-year...