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AIRWAY V

THE USE OF PHARMACOLOGIC AGENTS IN AIRWAY MANAGEMENT


Garry V. Walker, M.D.


Pharmacologic agents play an important role in airway management. The proper use of these agents represents only a portion of the pertinent issues regarding the airway. Safe airway management consist of airway evaluation, identification of the difficult airway, selection of the most successful plan of management, and a reasonable and promptly available alternative plan. Intravenous, topical and infiltrative anesthetic agents are the armamentarium for airway management. Thus a good understanding of commonly used agents is essential. Certain situations warrant the use of intravenous induction agents while others call for their complete avoidance. The old adage to "don't burn any bridges" in anesthesia jargon usually means "don't give a muscle relaxant unless securing the airway is a certainty". In other words don't burn a bridge you can't leap back across. A discussion of the properties and use of sedative/hypnotic, local anesthetics, and muscle relaxants follows. The concept of rapid sequence induction and a few case scenarios will be addressed.


Specific Agents:

Sedatives-Hypnotics: rapid onset of unconsciousness (0.5 to 1 minute) and short duration of action

Thiopental

_ a barbiturate

_ 2-5 mg/kg

_ increased venous capacitance decreased preload CO and BP

_ Hypovolemic patients and those with poor cardiac reserve are prone to hypotension with induction

_ Increased HR

_ Useful as induction agent and brief sedation

_ Respiratory depression can be significant


Methohexitol _ a barbiturate

_ 1-2mg/kg

_ similar effects as Thiopental but shorter duration

_ less lipid soluble thus 4X hepatic clearance versus thiopental


Etomidate _ imidazole derivative

_ 0.1-0.3mg/kg

_ minimal effect on hemodynamics

_ pain on injection

_ may activate seizure in patients with focal epilepsy

_ good choice in patients with ventricular dysfunction and/or ischemic heart disease patients


Ketamine _ phencyclidine derivative

_ 1-3mg/kg _ maintenance of spontaneous ventilation and upper airway skeletal muscle tone

_ potent analgesic and amnestic agent with bronchodilator effects

_ increased systemic and pulmonary pressures due to sympathetic nervous system stimulation

_ direct myocardial depressant effect unmasked in critically ill, catecholamine depleted patients


Propofol _ an isopropyl phenol

_ 1-2.5mg/kg _ more rapid and complete awakening compared to barbiturates

_ reduces blood pressure more than thiopental

_ exaggerated hemodynamic effects in hypovolemic patients

_ pain with injection into non-antecubital, small vein


Antianxiety agents:


Amnestic, anticonvulsant, hypnotic, and sedative effects. Useful sedation for procedures and toleration of mechanical ventilation.


Benzodiazepines
Midazolam _ sedation: 0.5-1 mg increments _ induction: 0.15-0.35 mg/kg

_ amnestic: 50 mcg/kg

_ mild vasodilatation

_ respiratory depression: increased in the elderly and when combined with narcotics

_ onset 1 - 2 mins. and recovery 30 - 120 mins.


Lorazepam _ sedation: 0.5 mg - 2 mg

_ as with Midazolam, dose dependent respiratory depression


Diazepam _ sedation: 2 - 10mg (adults)

_ onset 1 -2 mins. and recovery 2 - 4 hrs.



Muscle relaxants FULL VENTILATORY SUPPORT IS MANDATORY


Succinylcholine:

_ depolarizing muscle relaxant

_ indications: facilitate tracheal intubation, provide skeletal muscle relaxation during surgery or mechanical ventilation

_ intubation: 0.5-1.5 mg/kg

_ onset: 60 seconds

_ recovery time: 3-10 minutes

_ Cardiac dysrhythmia including sinus bradycardia, junctional rhythm, and sinus arrest have been reported. May cause tachycardia in adults and bradycardia in children

_ significant hyperkalemia may result in patients with skeletal muscle myopathy, neurologic deficits, prolonged bed rest, multiple trauma, major burns (safe within first 24 hours of burn)

_ risk of hyperkalemia peaks at 7-10 days post burn, neurologic injury or multiple trauma

_ Use in children should be reserved for emergency intubation or instances where immediate securing of the airway is necessary.

_ Increases intraocular, intragastric and intracranial pressure


Rocuronium _ rapid onset, intermediate duration, steroidal relaxant

_ tracheal intubation: 0.6-1.0 mg/kg

_ onset time to intubation: 60-90 seconds

_ recovery time: ~20 minutes

_ modest increase in heart rate

_ useful in patients requiring rapid intubation in whom succinylcholine is contraindicated


Vecuronium _ steroidal analogue of pancuronium

_ intubation: 0.08-1 mg/kg

_ onset: ~ 3 minutes

_ duration: 20-35 minutes

_ hemodynamically benign


Pancuronium _ Intubation: 0.1 mg/kg

_ onset: 3-5 minutes

_ duration: 60-90 minutes

_ 10-15% increase in HR, arterial BP and CO


Local Anesthetics:

Lidocaine _ goals: topical anesthesia to increase patient comfort, control hemodynamics and facilitate tracheal intubation via oral or nasal routes.

_ Amide local anesthetic, metabolized by the liver.

_ elective nasotracheal intubation: 3-5 cc of 2% lidocaine in an atomizer inhaled into each nares or 4 cc of 4% lidocaine nebulized into the oropharynx; both techniques completely anesthetize the vocal cords in ~ 5 minutes

_ Intravenous route may be effective in blunting the response to laryngoscopy and intubation


Rapid Sequence Induction (RSI) :

Is it safe?

definition:

Classic RSI is the use of a rapid onset sedative hypnotic agent i.e. sodium thiopental, followed by succinylcholine in the presence of continuous cricoid pressure. The patient is adequately preoxygenated and ventilation is not assisted until the trachea is protected by a cuffed endotracheal tube. Modified RSI involves the use of the above mentioned induction agents but the patient may be ventilated through cricoid pressure. RSI can be dangerous in patients with severe cardiac disease or hypovolemia. The use of a hemodynamically benign induction agent, i.e. Etomidate, makes RSI safer in these patients. The need to assess airway anatomy and anticipate the ease of mask ventilation is especially important in patients undergoing RSI.


Case Scenarios:


Trauma patient with an unstable C-spine:

options:

_ awake, sedated nasotracheal intubation facilitated by topical anesthesia. Cervical spine protection throughout procedure

_ rapid sequence induction with cricoid pressure and cervical traction by an informed assistant

_ Medical ICU patient with end-stage cardiac failure and respiratory distress

_ the use of sedative/hypnotic agents in patients with systemic hypotension may be associated with a poor outcome

_ a high incidence of mortality exist in these patients even when sedative/hypnotics are avoided. The loss of sympathetic hyperactivity and the decreased preload associated with mechanical ventilation may explain this phenomenon.

_ Medical ICU patient with end-stage cardiac failure and respiratory distress/insufficiency pO2, pCO2

_ Sympathetic hyperactivity BP and HR

_ Ventilatory support (intubation and mechanical ventilation)

_ decreased sympathetic activity

_ decreased BP and HR

_ Replacement of an endotracheal tube in a patient with an unstable cervical spine

_ topical laryngeal anesthesia and avoidance of muscle relaxants if possible may be the safest approach

_ In-line axial traction if utilizing laryngoscopy



References:
Barash, Clinical Anesthesia, 1st edition


Wood and Wood, Drugs and anesthesia, 2nd edition


Dailey et al, The Airway, Emergency Management, Chapter 14, Pharmacologic Aids in Airway Management, 1992


Schwartz, Matthay, Cohen: Death and other complications of emergency airway management in critically ill adults, Anesthesiology, V 82, No 2, Feb 1995



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