A anestesia é uma das partes fundamentais para se dar início a uma cirurgia.
Sendo esta uma área em que tenho interesse pessoal, hoje decidi deixar aqui um artigo sobre anestesia geral.
O artigo está em inglês, mas explica um pouco de como funciona a anestesia geral e quais as drogas mais utilizadas.
Abraço
F. Ferreira
"Anesthesia, GeneralAuthor: Saifee Rashiq, BM BS, MSc (Epid) DA (UK), FRCPC, Associate Professor, Director, Division of Pain Medicine, Department of Anesthesiology and Pain Medicine, University of Alberta, Edmonton, Canada
Contributor Information and Disclosures
Updated: May 4, 2007
General ConsiderationsAnesthesia is the process by which a patient is rendered able to undergo surgery. Surgery was, of course, commonly performed before any means was available to spare the patient any part of the experience. It takes little imagination to realize that an unanesthetized person enduring a surgical wound exhibits the following:
Evasive action
Severe pain and emotional distress
Maximum tension in skeletal muscles
Massive increase in sympathetic tone causing sweating, tachycardia, and hypertension
Vivid and unpleasant memory of the event forever
The goals of anesthesia thus include the following:
Anesthesia (lack of awareness of surrounding events)
Akinesia (keeping the patient still to allow surgery to take place)
Muscle relaxation (to enable access through muscles to bones and body cavities)
Autonomic control (to prevent dangerous surgesin hemodynamics)
General anesthesia General anesthesia uses drugs administered systemically to render the patient unaware of anything that is being done to or around him or her. It must be safe, not threatening or unpleasant to the patient, allow adequate surgical access to the operative site, and cause as little disturbance as possible to internal homeostatic mechanisms. A point worth noting is that general anesthesia, as opposed to local or regional anesthesia, may not always be the best choice. The anesthesiologist selects the optimal technique for any given patient and procedure.
Attributes of general anesthesia include the following: Advantages Makes no psychological demand of the patient
Allows complete stillness for prolonged periods of time
Facilitates complete control of the airway, breathing, and circulation
Permits surgery to take place in widely separated areas of the body at the same time
Can be used in cases of sensitivity to local anesthetic agent
Can be administered without moving the patient from the supine position
Can be adapted easily to procedures of unpredictable duration or extent
Usually can be administered rapidly
Disadvantages Requires the involvement of an extra set of healthcare providers
Requires complex and costly machinery
Requires some degree of preoperative patient preparation
Usually associated with some degree of physiological trespass
Carries the risk of major complications including death, myocardial infarction, and stroke
Associated with less serious complications such as nausea or vomiting, sore throat, headache, shivering, and delayed return to normal mental functioning
Associated with malignant hyperthermia, a rare, inherited muscular condition in which exposure to some (but not all) general anesthetic agents results in acute and potentially lethal temperature rise, hypercarbia, metabolic acidosis, and hyperkalemia
A given patient's risk for complications as a direct result of general anesthesia is small but depends largely on his or her medical comorbidities.
Death attributable to anesthesia is said to occur at rates of less than 1:10,000, but these are average figures incorporating both elective and emergency patients with all types of physical conditions. Minor complications occur at predicable rates, even in previously healthy patients. The frequency of symptoms during the first 24 hours following ambulatory surgery is as follows:
Bleeding, vomiting, nausea - Less than 5%
Fever - 5-15%
Dizziness, headache, drowsiness, hoarseness - More than 15%
Sore throat - 25%
Incisional pain - 30%
An excellent recent review of the literature concerning anesthesia-related morbidity and mortality will be of interest to readers wanting more information.1
Preparation for General AnesthesiaSafe and efficient anesthesia practice requires certified personnel, appropriate drugs and equipment, and an optimized patient.
These requirements need to be adapted to the context; no one would criticize a trauma surgeon attending a patient trapped in the wreckage of a motor vehicle accident for administering a bolus dose of intravenous (IV) ketamine at the roadside to amputate a limb and free the victim. Such a general anesthetic, given in an uncontrolled fashion by an individual with no anesthesia training, would be completely inappropriate for an elective surgical procedure.
Minimum requirementsMinimum infrastructure requirements for general anesthesia include a well-lit space of adequate size, a source of pressurized oxygen (either piped in or from cylinders), an effective suction device, and equipment to continuously monitor heart rate and rhythm, blood pressure, oxygen saturation, and temperature.
Additional monitoring requirements exist in certain jurisdictions. Beyond this, some equipment is needed to deliver the anesthetic agent. This may be as simple as needles and syringes if the drugs are to be administered entirely intravenously, but, in most circumstances, this means the availability of a properly serviced and maintained anesthetic gas delivery machine.
An array of routine and emergency drugs, including supplies of Dantrolene sodium (the specific treatment for malignant hyperthermia), airway management equipment, a cardiac defibrillator, and a recovery room staffed by properly trained individuals completes the picture.
Preparing the patientThe patient should be adequately prepared. The most efficient method is for the patient to be reviewed by the person responsible for giving the anesthetic well in advance of the surgery date.
Persons without concomitant medical problems may need little more than a quick medical review and the opportunity to discuss anesthetic questions or concerns. Those with comorbidity in general should be optimized for the procedure.
Patients with diabetes, coronary artery disease, chronic bronchitis and emphysema, and other chronic aliments should be stable. The question of whether such a disease state is under optimal control is usually a simple matter of good clinical judgment and can be determined by anyone who asks the appropriate questions. However, a few areas exist in which preoperative review by an anesthesiology provider can predict and prevent major adverse events. Foremost amongst these is a careful examination of the patient's airway anatomy. Identification of 1 or more of these anomalies may indicate that management of the patient's airway might prove difficult under anesthesia.
Airway managementPossible or definite difficulties with airway management include the following:
Small or receding jaw
Prominent maxillary teeth
Short neck
Limited neck extension
Poor dentition
Tumors of the face, mouth, neck, or throat
Facial trauma
Interdental fixation
Hard cervical collar
Halo traction
Various scoring systems have been created using orofacial measurements to predict difficult intubation. The most widely used is the Mallampati score, which identifies patients in whom the pharynx is not well visualized through the open mouth. High Mallampati scores predict difficult intubation with good but not perfect accuracy.
Often, such histories describe factors such as prolonged postoperative vomiting or slow emergence, which, while important, do not cause undue concern for the patient at hand.
Of much greater concern is a history of high temperature under anesthesia or any form of anesthesia complication that resulted in death or the necessity for intensive care.
When suspicion of an adverse event is high but a similar anesthetic technique must be used again, obtaining records from other institutions may be necessary.
Other requirementsThe necessity to come to the operating room with an empty stomach is well known to health professionals and the lay public alike.
While aspiration of food or fluids into the lungs during anesthesia is a serious complication, do not forget that depriving the patient of fluid is not without risk, particularly in the case of small children. The aim is to strike a reasonable balance between safe anesthetic care and dehydration.
Most anesthetists would agree that solid food should be avoided for 6 hours and clear fluids for up to 4 hours prior to the induction of anesthesia, but even anesthesia specialty societies do not agree absolutely on these intervals.
With a few exceptions, patients should continue to take regularly scheduled medications up to and including the morning of surgery.
Obvious exceptions include the following:
Discontinue anticoagulants, including aspirin, in good time to avoid increased surgical bleeding.
Avoid oral hypoglycemics on the day of surgery and manage blood glucose using intravenous dextrose and insulin.
Metformin is an oral hypoglycemic agent that is associated with the development of profound and occasionally irreversible metabolic acidosis under general anesthesia. Discontinue it 48 hours prior to the surgery date.
Since monoamine oxidase inhibitors are associated with anesthetic drug interactions, discontinue them prior to surgery if possible.
Recent catastrophes under anesthesia have focused attention on the interaction between nonprescribed medications and anesthetic drugs, including interactions with vitamins, herbal preparations, traditional remedies, and food supplements. Good information on the exact content of these preparations is often hard to obtain.2
The routine performance of standard batteries of preoperative tests is no longer justified. These waste resources and time and expose the patient to needless venipuncture and radiation. From the anesthesia point of view, the most efficient route is to have the anesthesia provider order his or her own tests, based on the known comorbid conditions, drug history, and surgical plan. The routine chest radiograph is particularly egregious, as it rarely yields information that might alter the anesthetic management in patients without known cardiopulmonary disease.
The Process of AnesthesiaPremedication: The first stage of a general anesthetic This stage, which is usually conducted in the surgical ward or in a preoperative holding area, is something of a throwback to the early days of ether and chloroform anesthesia when drugs such as morphine and scopolamine were routinely administered to make the inhalation of these highly pungent vapors more tolerable.
The goal of this stage of the anesthesia process is to have the patient arrive in the operating room in a calm, relaxed frame of mind while causing minimal interference with breathing and cardiovascular status.
For many patients, this step is either unnecessary or impractical because of the way in which operations are scheduled.
The most commonly used premedicants are short-acting benzodiazepines. Midazolam syrup is often given to children to facilitate calm separation from their parents prior to anesthesia. In anticipation of surgical pain, nonsteroidal anti-inflammatory drugs or acetaminophen can be administered preemptively, and when a history of gastroesophageal reflux exists, H2 blockers and antacids are administered. An unresolved debate in the anesthesia community is whether to administer preoperative and intraoperative beta-blockers to those with risk factors for coronary disease to prevent myocardial infarction. A large international trial is currently under way to address this question.
Where appropriate facilities are available, an excellent alternative to oral premedicants is the titration of small doses of intravenous fentanyl and midazolam by a nurse in the preoperative holding area.
Drying agents (eg, atropine, scopolamine) are now only administered routinely in anticipation of a fiberoptic endotracheal intubation.
The patient is transferred to the operating table and baseline vital signs are obtained.
Induction: The patient is now ready for induction, which is usually the most critical part of the anesthesia process.
In many ways, induction of general anesthesia is analogous to an airplane taking off. It is the transformation of a waking patient into an anesthetized one.
This can be achieved by intravenous injection of induction agents (drugs that work rapidly, such as thiopental and propofol), by the slower inhalation of anesthetic vapors from a face mask, or a combination of both.
For the most part, contemporary practice dictates that adult patients and most children be induced with intravenous drugs, this being a rapid and minimally unpleasant experience for the patient. However, the arrival of sevoflurane, a new and well-tolerated anesthetic vapor, has led to something of a renaissance of elective inhalation induction of anesthesia in adults.
In addition to the induction drug, most patients receive an injection of narcotic analgesic. A wide range of synthetic and naturally occurring narcotics with different properties is available. Induction agents and narcotics work synergistically to induce anesthesia. In addition, anticipation of events that are about to occur, such as endotracheal intubation and incision of the skin, generally raises the blood pressure and heart rate of the patient. Narcotic analgesia helps preempt this undesirable response.
The next step of the induction process is the securing of the airway. This may be a simple matter of manually holding the patient's jaw such that his or her natural breathing is unimpeded by the tongue or may demand the insertion of a prosthetic airway device such as a laryngeal mask airway or endotracheal tube. A variety of factors are considered when making this decision. The major issueis whether the patient requires an endotracheal tube.
Indications for endotracheal intubation under general anesthesia include the following:
Potential for airway contamination (full stomach, gastroesophageal [GE] reflux, gastrointestinal [GI] or pharyngeal bleeding)
Surgical need for muscle relaxation
Predictable difficulty with endotracheal intubation or where anesthetist's access to the airway during the case will be difficult (lateral or prone position)
Surgery of the mouth or face
Prolonged procedure anticipated
Not all surgery requires muscle relaxation. In this context, only the major muscle groups of the thorax and abdomen are considered.
If surgery is taking place in these areas, then in addition to the induction agent and narcotic, an intermediate or long-acting muscle relaxant drug is administered. This paralyzes muscles indiscriminately, including the muscles of breathing. Therefore, the patient's lungs must be ventilated under pressure, necessitating an endotracheal tube.
Persons who, for anatomic reasons, are likely to be difficult to intubate are usually intubated electively at the beginning of the procedure, using a fiberoptic bronchoscope or other advanced airway tool (Hagberg, 2002). This prevents a situation in which attempts are made to manage the airway with a lesser device, only for the anesthetist to discover that oxygenation and ventilation are inadequate. At that point during a surgical procedure, swift intubation of the patient can be very difficult, if not impossible.
Maintenance phase: At this point, the drugs used to initiate the anesthetic are beginning to wear off, and the patient must be kept anesthetized with a maintenance agent.
For the most part, this refers to the delivery of anesthetic gases (more properly termed vapors) into the patient's lungs. These may be inhaled as the patient breathes himself or delivered under pressure by each mechanical breath of a ventilator.
The maintenance phase is usually the most stable part of the anesthesia. However, understanding that anesthesia is a continuum of different depths is important. A level of anesthesia that is satisfactory for surgery to the skin of an extremity, for example, would be inadequate for manipulation of the bowel.
Appropriate levels of anesthesia must be chosen both for the planned procedure and for its various stages. In complex plastic surgery, for example, a considerable period of time may elapse between the completion of the induction of anesthetic and the incision of the skin. During the period of skin preparation, urinary catheter insertion, and marking incision lines with a pen, the patient is not receiving any noxious stimulus. This requires a very light level of anesthesia, which must be converted rapidly to a deeper level just before the incision is made. When the anesthesia provider and surgeon are not accustomed to working together, good communication (eg, warning of the start of new stimuli, such as moving the head of an intubated patient or commencing surgery) facilitates preemptive deepening of the anesthetic. This maximizes patient safety and, ultimately, saves everyone's time.
As the procedure progresses, the level of anesthesia is altered to give the minimum amount necessary to ensure adequate anesthetic depth. Traditionally, this has been a matter of clinical judgment, but new processed EEG machines give the anesthetist a simplified output in real time, corresponding to anesthetic depth. These devices have yet to become universally accepted as vital equipment.
If muscle relaxants have not been used, inadequate anesthesia is easy to spot. The patient will move, cough, or pupillary obstruct his airway if the anesthetic is too light for the stimulus being given.
If muscle relaxants have been used, then clearly the patient is unable to demonstrate any of these phenomena. In these patients, the anesthesiologist must rely on careful observation of autonomic phenomena such as hypertension, tachycardia, sweating, and capillary dilation to decide that the patient requires a deeper anesthetic.
This requires experience and judgment. It is from failure to recognize such signs that tragic and highly publicized cases of awareness under anesthesia are caused.
Excessive anesthetic depth, on the other hand, is associated with decreased heart rate and blood pressure, and, if carried to extremes, can jeopardize perfusion of vital organs or be fatal. Short of these serious misadventures, excessive depth results in slower awakening and more side effects.
As the surgical procedure draws to a close, the patient's emergence from anesthesia is planned. Experience and close communication with the surgeon enable the anesthesiologist to predict the time at which the application of dressings and casts will be complete.
In advance of that time, anesthetic vapors have been decreased or even switched off entirely to allow time for them to be excreted by the lungs.
Excess muscle relaxation is reversed using specific drugs and adequate long-acting narcotic analgesic to keep the patient comfortable in the recovery room.
If a ventilator has been used, the patient is restored to breathing by himself and, as anesthetic drugs dissipate, the patient emerges to consciousness.
Emergence is not synonymous with removal of the endotracheal tube or other artificial airway device. This is only performed when the patient has regained sufficient control of his or her airway reflexes.
Anesthesia Drugs in Common UseA number of choices exist for every aspect of anesthetic care; the way in which they are sequenced probably depends more on the personal preference of the person administering them.
Induction agents For 50 years, the most commonly used induction agents were rapidly acting, water-soluble barbiturates such as thiopental, methohexital, and thiamylal.
These drugs are still commonly in use today, have an enormous record of safety and reliability, and are economical.
More recently, propofol, a nonbarbiturate intravenous anesthetic, has displaced barbiturates in many anesthesia practices.
The use of propofol is associated with less postoperative nausea and vomiting and a more rapid, clear-headed recovery.
In addition to being an excellent induction agent, propofol can be administered by slow intravenous infusion instead of vapor to maintain the anesthesia.
Among its disadvantages are the facts that it often causes pain on injection and it is prepared in a lipid emulsion, which if not handled using meticulous aseptic precautions, can be a medium for rapid bacterial growth.
Anesthesia can also be induced by inhalation of a vapor. This is how all anesthetics were once given and is a common and useful technique in uncooperative children. It is reemerging as a choice in adults. Halothane and sevoflurane are the most commonly used drugs for this purpose.
Traditional narcotic analgesics Morphine, meperidine, and hydromorphone are widely used in anesthesia as well as in emergency rooms, surgical wards, and obstetric suites.
In addition, anesthesia providers have at their disposal a range of synthetic narcotics, which, in general, cause less fluctuation in blood pressure and are shorter acting. These include fentanyl, sufentanil, alfentanil, and remifentanil. Remifentanil is the newest drug in this class and has such a short duration of action that it must be administered as a continuous infusion.
Muscle relaxants come in many varieties Succinylcholine, a rapid-onset, short-acting depolarizing muscle relaxant, is the drug of choice when rapid muscle relaxation is needed.
For decades, anesthetists have used it extensively despite a number of predictable and unpredictable adverse effects associated with its use.
The search for a drug that replicates its onset and offsets speed without its adverse effects is the holy grail of muscle relaxant research.
Other relaxants have durations of action ranging from 15 minutes to more than 2 hours.
Older drugs in this class were often associated with changes in heart rate or blood pressure, but the newer ones are devoid of these adverse properties.
Muscle relaxants generally are excreted by the kidney, but some preparations are broken down by plasma enzymes and can be used safely in partial or complete renal failure.
Pancuronium is an established drug that is still in widespread use because of its low cost and familiarity, especially in intensive care units; rocuronium, mivacurium, and cisatracurium are more likely to be used by contemporary anesthesiologists.
Anesthetic vapors These are highly potent chlorofluorocarbons, which are delivered from precision vaporizers directly into the patient's inhaled gas stream. They may be mixed with nitrous oxide, a much weaker but nonetheless useful anesthetic gas.
The prototype of modern anesthetic vapors is halothane. It has an unparalleled track record of safety and efficacy, though it is associated with rare but devastating hepatic necrosis to a greater extent than other agents.
In the 1980s, it was displaced by isoflurane and enflurane, agents that were cleared from the lungs faster and thus were associated with more rapid anesthetic emergences.
In the late 1990s, 2 new vapors became very popular, desflurane and sevoflurane. These drugs are much more maneuverable than their predecessors and are associated with much more rapid emergence.
Intense commercial interest is present in anesthesia drug research, and the continuous introduction of new and better drug products for many years to come seems inevitable.
http://emedicine.medscape.com/article/1271543-overview"
