or nose (nasotracheal). it is a breathing conduit designed to be placed into the airway of critically injured, ill or anesthetized patients in order to perform mechanical positive pressure ventilation of the lungs and to prevent the possibility of aspiration or airway obstruction. the endotracheal tube has a fitting designed to be connected to a source of pressurized gas such as oxygen. at the other end is an orifice through which such gases are directed into the lungs and may also include a balloon (referred to as a cuff). the tip of the endotracheal tube is positioned above the carina (before the trachea divides to each lung) and sealed within the trachea so that the lungs can be ventilated equally. a tracheostomy tube is another type of tracheal tube; this 2–3-inch-long (51–76 mm) curved metal or plastic tube is inserted into a tracheostomy stoma or a cricothyrotomy incision. tracheal tubes can be used to ensure the adequate exchange of oxygen and carbon dioxide, to deliver oxygen in higher concentrations than found in air, or to administer other gases such as helium, nitric oxide, nitrous oxide, xenon, or certain volatile anesthetic agents such as desflurane, isoflurane, or sevoflurane. they may also be used as a route for administration of certain medications such as bronchodilators, inhaled corticosteroids, and drugs used in treating cardiac arrest such as atropine, epinephrine, lidocaine and vasopressin. originally made from latex rubber, most modern endotracheal tubes today are constructed of polyvinyl chloride. tubes constructed of silicone rubber, wire-reinforced silicone rubber or stainless steel are also available for special applications. for human use, tubes range in size from 2 to 10.5 mm (0.1 to 0.4 in) in internal diameter. the size is chosen based on the patient's body size, with the smaller sizes being used for infants and children. most endotracheal tubes have an inflatable cuff to seal the tracheobronchial tree against leakage of respiratory gases and pulmonary aspiration of gastric contents, blood, secretions, and other fluids. uncuffed tubes are also available, though their use is limited mostly to children (in small children, the cricoid cartilage is the narrowest portion of the airway and usually provides an adequate seal for mechanical ventilation). in addition to cuffed or uncuffed, preformed endotracheal tubes are also available. the oral and nasal rae tubes (named after the inventors ring, adair and elwyn) are the most widely used of the preformed tubes. there are a number of different types of double-lumen endo-bronchial tubes that have endobronchial as well as endotracheal channels (carlens, white and robertshaw tubes). these tubes are typically coaxial, with two separate channels and two separate openings. they incorporate an endotracheal lumen which terminates in the trachea and an endobronchial lumen, the distal tip of which is positioned 1–2 cm into the right or left mainstem bronchus. there is also the univent tube, which has a single tracheal lumen and an integrated endobronchial blocker. these tubes enable one to ventilate both lungs, or either lung independently. single-lung ventilation (allowing the lung on the operative side to collapse) can be useful during thoracic surgery, as it can facilitate the surgeon's view and access to other relevant structures within the thoracic cavity. the "armored" endotracheal tubes are cuffed, wire-reinforced silicone rubber tubes. they are much more flexible than polyvinyl chloride tubes, yet they are difficult to compress or kink. this can make them useful for situations in which the trachea is anticipated to remain intubated for a prolonged duration, or if the neck is to remain flexed during surgery. most armored tubes have a magill curve, but preformed armored rae tubes are also available. another type of endotracheal tube has four small openings just above the inflatable cuff, which can be used for suction of the trachea or administration of intratracheal medications if necessary. other tubes (such as the bivona fome-cuf tube) are designed specifically for use in laser surgery in and around the airway. methods to confirm tube placement an endotracheal tube in good position on cxr. arrow marks the tip. an endotracheal tube not deep enough. arrow marks the tip. no single method for confirming tracheal tube placement has been shown to be 100% reliable. accordingly, the use of multiple methods for confirmation of correct tube placement is now widely considered to be the standard of care. such methods include direct visualization as the tip of the tube passes through the glottis, or indirect visualization of the tracheal tube within the trachea using a device such as a bronchoscope. with a properly positioned tracheal tube, equal bilateral breath sounds will be heard upon listening to the chest with a stethoscope, and no sound upon listening to the area over the stomach. equal bilateral rise and fall of the chest wall will be evident with ventilatory excursions. a small amount of water vapor will also be evident within the lumen of the tube with each exhalation and there will be no gastric contents in the tracheal tube at any time. ideally, at least one of the methods utilized for confirming tracheal tube placement will be a measuring instrument. waveform capnography has emerged as the gold standard for the confirmation of tube placement within the trachea. other methods relying on instruments include the use of a colorimetric end-tidal carbon dioxide detector, a self-inflating esophageal bulb, or an esophageal detection device. the distal tip of a properly positioned tracheal tube will be located in the mid-trachea, roughly 2 cm (1 in) above the bifurcation of the carina; this can be confirmed by chest x-ray. if it is inserted too far into the trachea (beyond the carina), the tip of the tracheal tube is likely to be within the right main bronchus—a situation often referred to as a "right mainstem intubation". in this situation, the left lung may be unable to participate in ventilation, which can lead to decreased oxygen content due to ventilation/perfusion mismatch. special situations emergencies tracheal intubation in the emergency setting can be difficult with the fiberoptic bronchoscope due to blood, vomit, or secretions in the airway and poor patient cooperation. because of this, patients with massive facial injury, complete upper airway obstruction, severely diminished ventilation, or profuse upper airway bleeding are poor candidates for fiberoptic intubation. fiberoptic intubation under general anesthesia typically requires two skilled individuals. success rates of only 83–87% have been reported using fiberoptic techniques in the emergency department, with significant nasal bleeding occurring in up to 22% of patients. these drawbacks limit the use of fiberoptic bronchoscopy somewhat in urgent and emergency situations. personnel experienced in direct laryngoscopy are not always immediately available in certain settings that require emergency tracheal intubation. for this reason, specialized devices have been designed to act as bridges to a definitive airway. such devices include the laryngeal mask airway, cuffed oropharyngeal airway and the esophageal-tracheal combitube (combitube). other devices such as rigid stylets, the lightwand (a blind technique) and indirect fiberoptic rigid stylets, such as the bullard scope, upsher scope and the wuscope can also be used as alternatives to direct laryngoscopy. each of these devices have its own unique set of benefits and drawbacks, and none of them is effective under all circumstances. rapid-sequence induction and intubation laryngoscopes prepared for emergency anaesthesia main article: rapid sequence induction rapid sequence induction and intubation (rsi) is a particular method of induction of general anesthesia, commonly employed in emergency operations and other situations where patients are assumed to have a full stomach. the objective of rsi is to minimize the possibility of regurgitation and pulmonary aspiration of gastric contents during the induction of general anesthesia and subsequent tracheal intubation. rsi traditionally involves preoxygenating the lungs with a tightly fitting oxygen mask, followed by the sequential administration of an intravenous sleep-inducing agent and a rapidly acting neuromuscular-blocking drug, such as rocuronium, succinylcholine, or cisatracurium besilate, before intubation of the trachea. one important difference between rsi and routine tracheal intubation is that the practitioner does not manually assist the ventilation of the lungs after the onset of general anesthesia and cessation of breathing, until the trachea has been intubated and the cuff has been inflated. another key feature of rsi is the application of manual 'cricoid pressure' to the cricoid cartilage, often referred to as the "sellick maneuver", prior to instrumentation of the airway and intubation of the trachea. named for british anesthetist brian arthur sellick (1918–1996) who first described the procedure in 1961, the goal of cricoid pressure is to minimize the possibility of regurgitation and pulmonary aspiration of gastric contents. cricoid pressure has been widely used during rsi for nearly fifty years, despite a lack of compelling evidence to support this practice. the initial article by sellick was based on a small sample size at a time when high tidal volumes, head-down positioning and barbiturate anesthesia were the rule. beginning around 2000, a significant body of evidence has accumulated which questions the effectiveness of cricoid pressure. the application of cricoid pressure may in fact displace the esophagus laterally instead of compressing it as described by sellick. cricoid pressure may also compress the glottis, which can obstruct the view of the laryngoscopist and actually cause a delay in securing the airway. cricoid pressure is often confused with the "burp" (backwards upwards rightwards pressure) maneuver. while both of these involve digital pressure to the anterior aspect (front) of the laryngeal apparatus, the purpose of the latter is to improve the view of the glottis during laryngoscopy and tracheal intubation, rather than to prevent regurgitation. both cricoid pressure and the burp maneuver have the potential to worsen laryngoscopy. rsi may also be used in prehospital emergency situations when a patient is conscious but respiratory failure is imminent (such as in extreme trauma). this procedure is commonly performed by flight paramedics. flight paramedics often use rsi to intubate before transport because intubation in a moving fixed-wing or rotary-wing aircraft is extremely difficult to perform due to environmental factors. the patient will be paralyzed and intubated on the ground before transport by aircraft. cricothyrotomy main article: cricothyrotomy in cricothyrotomy, the incision or puncture is made through the cricothyroid membrane in between the thyroid cartilage and the cricoid cartilage cricothyrotomy kit a cricothyrotomy is an incision made through the skin and cricothyroid membrane to establish a patent airway during certain life-threatening situations, such as airway obstruction by a foreign body, angioedema, or massive facial trauma. a cricothyrotomy is nearly always performed as a last resort in cases where orotracheal and nasotracheal intubation are impossible or contraindicated. cricothyrotomy is easier and quicker to perform than tracheotomy, does not require manipulation of the cervical spine and is associated with fewer complications. the easiest method to perform this technique is the needle cricothyrotomy (also referred to as a percutaneous dilational cricothyrotomy), in which a large-bore (12–14 gauge) intravenous catheter is used to puncture the cricothyroid membrane. oxygen can then be administered through this catheter via jet insufflation. however, while needle cricothyrotomy may be life-saving in extreme circumstances, this technique is only intended to be a temporizing measure until a definitive airway can be established. while needle cricothyrotomy can provide adequate oxygenation, the small diameter of the cricothyrotomy catheter is insufficient for elimination of carbon dioxide (ventilation). after one hour of apneic oxygenation through a needle cricothyrotomy, one can expect a paco2 of greater than 250 mm hg and an arterial ph of less than 6.72, despite an oxygen saturation of 98% or greater. a more definitive airway can be established by performing a surgical cricothyrotomy, in which a 5 to 6 mm (0.20 to 0.24 in) endotracheal tube or tracheostomy tube can be inserted through a larger incision. several manufacturers market prepackaged cricothyrotomy kits, which enable one to use either a wire-guided percutaneous dilational (seldinger) technique, or the classic surgical technique to insert a polyvinylchloride catheter through the cricothyroid membrane. the kits may be stocked in hospital emergency departments and operating suites, as well as ambulances and other selected pre-hospital settings. tracheotomy main article: tracheotomy diagram of a tracheostomy tube in the trachea: 1 - vocal folds 2 - thyroid cartilage 3 - cricoid cartilage 4 - tracheal rings 5 - balloon cuff tracheotomy consists of making an incision on the front of the neck and opening a direct airway through an incision in the trachea. the resulting opening can serve independently as