Airway maintenance in anesthesia (Proceedings)
The respiratory system is one of the most important physiologic systems that the anesthetist can affect during an anesthetic event. If respiration ceases, a patient can die within minutes. Most anesthesia involves the use of volatile inhalant anesthetics that must be delivered through the respiratory tract. This means that a patent and normally functioning airway is essential to ensure uptake of inhalant and maintenance of an adequate anesthetic place. The exchange of oxygen and carbon dioxide also relies on the normally functioning airway and respiratory system. Understanding how to maintain, monitor, and troubleshoot a patient's airway are essential jobs of the veterinary anesthetist.
The first step is to choose the method of providing inhalant anesthetic and oxygen to the patient. There are several options each with advantages and disadvantages. Providing inhalant by mask is technically the easiest strategy. It is important to keep the head and neck extended so that air flow is not compromised. A mask also offers the advantage of being useful for both induction and maintenance of anesthesia as well as providing oxygen to patients that are receiving intravenous anesthetics. Higher oxygen flow rates are necessary compared to the flow rates needed for intubated patients. The mask diaphragm must be tight fitting to the animals muzzle to prevent loss of gas and exposure of staff to waste gas pollution. The smallest mask possible minimizes dead space and re-breathing of carbon dioxide. The disadvantages of this type of anesthetic maintenance are that it is difficult to provide positive pressure ventilation to the patient and it is impossible to protect the airway from aspiration.
The endotracheal (ET) tube is the most efficient choice for administration of inhalant. When the tube is properly place and the cuff is inflated the anesthetist is able to provide inhalant as well has 100% oxygen to the patient. The ET tube also allows the anesthetist to maintain a patent airway, protect the airway from foreign material, allow application of efficient positive pressure ventilation, and to protect personnel from exposure to waste gases.Anatomy of the ET tube
The three main parts are the ET tube, the cuff system (consisting of the pilot line and the cuff itself) and an adaptor that connects to the anesthetic circuit. The size of the ET tube is most often measured as internal diameter in millimeters. Some tubes are labeled in the French system. The French size is the external diameter in millimeters multiplies by 3. Many tubes also have a radio opaque line that allows the tube to been easily seen on radiographs. Many tubes have centimeter markings that allow the anesthetist to judge length in order to avoid endo-bronchial intubation.
A Murphy tube is the most common type of ET tube. This type of tube has a "Murphy eye" at the end of the tube that helps prevent occlusion by foreign material. A Magill ET tube looks very similar to a Murphy tube but lacks the Murphy eye. Cole tubes are used primarily in birds and reptiles. These species have tracheas with complete tracheal rings and are predisposed to tracheal necrosis and rupture if cuffed ET tubes are used. A Cole tube is a type of un-cuffed ET tube that has a "shoulder" that sits in front of the larynx to help provide a seal without the use of a cuff.
Wire reinforced ET tubes are used primarily when the positioning of patient for a procedure or surgery could cause a normal ET tube to bend and kink preventing movement of gas. These tubes have a wire that is embedded in the wall of the tube and runs the length of the tube in a circular fashion that prevents it kinking when bent. Often, these tubes are too long for many small patients and do not lend themselves to being cut due to the wire that is embedded in the tube and connected to the adaptor. If the tube is long, it is important to make sure that the animal is not endo-bronchially intubated.