Intravenous medications are then administered to the patient to induce a state of general anesthesia. Following induction of anesthesia, loss of consciousness is verified by apnea, loss of response to verbal commands, and loss of the eyelash reflex. Although the patient is asleep, the intravenous medications require an additional period of time to reach their peak effect. During this time, the patient’s lungs are supported with a brief period of bag-mask positive pressure ventilation prior to airway management. Airway management may involve the insertion of a laryngeal mask airway device or similar extraglottic device (see Chapter 8) or insertion of an endotracheal tube (see Chapter 6, 7, 9). Now is the student’s time to shine! Along with establishing intravenous access, bag-mask ventilation of the lungs is one of the most important skills students will acquire during their anesthesia rotation.
The anesthesia machine can be used to provide bag-mask ventilation in an apneic patient. “The circuit” refers to the clear plastic anesthesia tubing that carries oxygen and anesthetic gases from the anesthesia machine to the patient’s airway. It is called a circle breathing circuit because the gas flows in a circle directed by one-way valves. The flow of gas (oxygen) into the circuit is controlled by the gas flow meters. The adjustable pressure release valve (APL) valve controls the pressure at which gas leaves the circuit (provided there is a good seal between the patient’s face and the mask). Prior to induction of anesthesia, ensure the APL valve is fully open (turned counterclockwise). If the APL valve is closed, the patient will not be able to breathe out with a tightly fitting facemask. Ensure the oxygen flow rate is set at ≥ 6 L∙min-1 before applying the facemask. If there is no oxygen flow, the reservoir bag will be collapsed and the patient will experience difficulty breathing.
After induction of anesthesia, a seal must be created between the patient’s face and the anesthesia mask to provide positive pressure ventilation. This is commonly termed ‘bag-mask’ ventilation. Adjust the APL valve by turning the valve clockwise to 20 cmH2O. Airway pressures > 20 cmH2O may result in gastric insufflation and an increase in nausea, vomiting, and potential aspiration (see Chapters 9, 26). Should the reservoir bag become distended and taut, either lift the facemask from the patient to permit passive exhalation or open the APL valve slightly by turning it counterclockwise.
Squeeze the reservoir bag to deliver positive pressure to the patient. When performed correctly, positive pressure will result in oxygen being delivered to the patient’s lungs and the patient’s chest wall will rise. As pressure is released from the reservoir bag, the patient’s chest wall will fall during exhalation, capnography will indicate that CO2 is being exhaled, and the ventilator spirometer will record the volume of gas exhaled by the patient. Common causes of failure of the patient’s chest wall to rise with positive pressure ventilation include a poor facemask seal and / or an upper airway obstruction. Ask your staff anesthesiologist what they do to problem solve when ventilation by facemask fails (see Chapter 6, 8 for maneuvers used to overcome difficulties with facemask ventilation). After insertion of an endotracheal tube or laryngeal mask airway device, ventilation may be controlled using the anesthesia ventilator. The APL valve is only functional during ‘bag-mask’ ventilation and becomes inactive once the ventilator is activated.
Additional resources:
Technique of securing peripheral intravenous access
Preoperative Aestiva anesthesia machine checkout procedure
Chapter 6
Intubation and Anatomy of the Airway
Patrick Sullivan MD, Ashleigh Farrell MD
Learning Objectives
1 To identify indicators that would predict difficulty with bag-mask ventilation (BMV).
2 To identify indicators that would lead to difficult direct laryngoscopy.
3 To describe practical clinical maneuvers to resolve an upper airway obstruction in a patient whose trachea is not intubated.
Key Points
1 The patient’s medical history and clinical exam can help predict difficult bag-mask ventilation as well as difficult tracheal intubation.
2 Bag-mask ventilation and tracheal intubation will be difficult in a small percentage of patients. These patients require special planning.
3 Insertion of an extraglottic airway, such as a laryngeal mask airway device (e.g., LMA™) may be life saving in the “cannot intubate, cannot ventilate” scenario.
4 There are two immediate clinical tools to confirm tracheal intubation: end-tidal carbon dioxide (ETCO 2 ) measurement and visualization of the endotracheal tube (ETT) passing through the glottis.
5 Clinical experience is correlated directly with successful intubation at first attempt. Enlist expert assistance for anticipated difficult intubations.
6 Always have a backup airway plan.
Overview:
Over 25% of anesthetic-related morbidity and mortality arises from complications related to inadequate ventilation resulting from either difficult BMV or difficult tracheal intubation. The goal of the airway assessment is to identify potential problems with the provision, maintenance, and protection of a patent airway and the supply of adequate ventilation following tracheal intubation. Patient assessment requires a specific history and a directed physical examination (see below).
History:
Patients may be aware of past difficulties with anesthesia pertaining to tracheal intubation, and a review of their previous anesthetic records may provide valuable information concerning prior airway management techniques that were either successful or unsuccessful. A history of restricted neck mobility (e.g., cervical spine (C-spine) fusion, degenerative disc disease [DDD]), restricted mouth opening (e.g., temporal mandibular joint disease), cancer of the oropharynx, or radiation to the head and neck are important predictors of a potentially difficult airway.
The mnemonic ‘BONES’ has been used as an aid to identify patients that may be difficult to ventilate using a bag and mask.
Predicting Difficulty with Mask Ventilation:
Factors predicting difficult BMV include elderly, edentulous patients, obesity, history of obstructive sleep apnea, and the presence of a beard. Patients with predictors of difficult BMV may or may not also have predictors of a difficult intubation. Patients with predictors of both a difficult BMV and difficult intubation require special attention and careful planning to avoid a “cannot intubate, cannot ventilate” crisis. Planning would involve having a difficult airway cart immediately available, enlisting experienced assistance for intubation, use of advanced airway equipment tools, and/or consideration for an “awake” intubation with a fiberoptic scope using topical anesthesia and judicious intravenous sedation. See Chapter 7 for a further discussion on advanced airway options for managing a difficult airway.
In a recent review of 50,000 anesthesia cases, the combined frequency of difficult BMV and tracheal intubation was 0.38% or 1 in 266 patients.1 Predictors included the presence of sleep apnea, body mass index (BMI) > 31, Mallampati 3 or 4 view, male, beard, edentulous, restricted thyromental distance, and restricted neck extension. A focused preoperative examination can be used to look for these predictors and identify patients at risk for a “failed airway”. Either “awake” intubation