general anesthesia
When you get general anesthesia, you're "put under," which means that you're totally unconscious and immobilized. You "go to sleep" and don't feel, sense or remember anything that happens after the drugs begin to work on your system.
It's not completely clear exactly how general anesthetics work, but the current accepted theory is that they affect the spinal cord (which is why you end up immobile), the brain stem reticular activating system (which explains the unconsciousness) and the cerebral cortex (which results in changes in electrical activity on an electroencephalogram).
Major, complex surgeries that require a long period of time to perform typically require general anesthesia. Patients may be under for just a few hours for a knee replacement, or as many as six hours for something more complicated, such as heart bypass surgery.
If you're preparing for a surgery requiring general anesthesia, you'll typically meet with the anesthesiologist to give him or her your medical history. This is important because people certain with conditions might require special care under anesthesia -- a patient with low blood pressure might need to be medicated with ephedrine, for example. Patients who are heavy drinkers or drug users also tend to react differently to anesthesia. During this meeting, you'll be instructed not to eat for several hours before surgery. It's possible for someone under general anesthesia to aspirate, or breathe in, the contents of the stomach.
When you're under general anesthesia, you'll be wearing a breathing mask or breathing tube, because the muscles become too relaxed to keep your airways open. Several different things are continuously monitored while you're under -- pulse oximetry (oxygen level in the blood), heart rate, blood pressure, respiratory rate, carbon dioxide exhalation levels, temperature, the concentration of the anesthetic and brain activity. There's also an alarm that goes off if your oxygen level drops below a certain point.
There are four stages of general anesthesia:
- During the first stage, induction, the patient is given medication and may start to feel its effects but hasn't yet fallen unconscious.
- Next, patients go through a stage of excitement. They may twitch and have irregular breathing patterns or heart rates. Patients in this stage don't remember any of this happening because they're unconscious. This stage is very short and progresses rapidly to stage three.
- During stage three, the muscles relax, breathing becomes regular and the patient is considered fully anesthetized.
- Stage four anesthesia isn't a part of the regular process. This is when a patient has received an overdose of drugs, which can result in heart or breathing stoppage, brain damage or death if swift action isn't taken.
We'll look at the drugs administered during general anesthesia, as well as recovery, next.
General anaesthesia (or general anesthesia) is a state of unconsciousness and loss of protective reflexes resulting from the administration of one or more general anaesthetic agents. A variety of medications may be administered, with the overall aim of ensuring hypnosis, amnesia, analgesia, relaxation of skeletal muscles, and loss of control of reflexes of the autonomic nervous system. The optimal combination of these agents for any given patient and procedure is typically selected by an anaesthesiologist or another provider such as an anaesthesiologist assistant or nurse anaesthetist, in consultation with the patient and the medical or dental practitioner performing the operative procedure.
Risks General anaesthesia
General anesthesia is usually safe for healthy people. The following people may have a higher risk of problems with general anesthesia:
- People who abuse alcohol or medications
- People with allergies or a family history of being allergic to medicine
- People with heart, lung, or kidney problems
- Smokers
Ask your doctor about these complications:
- Death (rare)
- Harm to your vocal cords
- Heart attack
- Lung infection
- Mental confusion (temporary)
- Stroke
- Trauma to the teeth or tongue
- Waking during anesthesia (rare)
Anesthesia awareness
Estimates vary, but about 1 or 2 people in every 1,000 may wake up briefly while under general anesthesia. In most cases, the person is simply aware of his or her surroundings and doesn't feel any pain. However, some people experience excruciating pain and develop long-term psychological problems.
The following factors appear to make this phenomenon — also called unintended intraoperative awareness — more likely:
- Emergency surgery
- Long-term use of anticonvulsants, opiates, tranquilizers or cocaine
- Heart or lung problems
- Daily alcohol use
Before general anesthesia
Before you undergo general anesthesia, a medical professional specially trained to deliver anesthetics will talk with you and may ask questions about:
- Your health history
- Prescription medications, over-the-counter medications and herbal supplements you take
- Allergies
- Your past experiences with anesthesia
The information you provide will help the anesthesia specialist choose the drugs that will work best and be safest for you.
During general anesthesia
In most cases, the anesthesia is started with medication delivered through an intravenous line in your arm, but sometimes it can be started with a gas that you breathe from a mask. For example, children who are afraid of needles may prefer to go to sleep with a mask. Once you are asleep, a tube may be inserted into your mouth and down your windpipe to ensure you get enough oxygen and to protect your lungs from blood or body secretions, such as from your stomach. In some cases this breathing tube isn't needed, which reduces your chance of a sore throat after surgery.
A member of the anesthesia care team monitors you continuously during your procedure, adjusting your medications, breathing, temperature, fluids and blood pressure as needed. Any abnormalities that occur during the surgery are corrected by administering additional medications, fluids and, sometimes, blood transfusions.
After general anesthesia
When the surgery is complete, the anesthesia drugs are discontinued, and you gradually awaken either in the operating room or the recovery room. You'll probably feel groggy and a little confused when you first awaken. Other common side effects include:
- Nausea
- Vomiting
- Dry mouth
- Sore throat
- Shivering
- Sleepiness
Physiologic monitoring
Monitoring involves the use of several technologies to allow for a controlled induction of, maintenance of and emergence from general anaesthesia.
- Continuous Electrocardiography (ECG): The placement of electrodes that monitor heart rate and rhythm. This may also help the anaesthetist to identify early signs of heart ischaemia.
- Continuous pulse oximetry (SpO2): The placement of this device (usually on one of the fingers) allows for early detection of a fall in a patient's haemoglobin saturation with oxygen (hypoxaemia).
- 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.
- Agent concentration measurement - Common anaesthetic machines have monitors to measure the per cent of inhalational anaesthetic agent used (e.g. sevoflurane, isoflurane, desflurane, halothane etc.). The monitors also usually measure nitrous oxide and oxygen percentages and could give a MAC level.
- 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 minimum alarm setting and allows the anaesthetist to take immediate remedial action.
- Circuit disconnect alarm or low pressure alarm indicates failure of circuit to achieve a given pressure during mechanical ventilation.
- Carbon dioxide measurement (capnography)- measures the amount of carbon dioxide expired by the patient's lungs in per cent or mmHg, mmHg is usually used to allow the anaesthesia provider to see more subtle changes in CO2. It allows the anaesthetist to assess the adequacy of ventilation
- Temperature measurement to discern hypothermia or fever, and to aid early detection of malignant hyperthermia.
- 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.
