Understanding the Three Principal Goals of Clinical Airway Management-Juniper Publishers
Juniper Publishers-Journal of Anesthesia
Introduction
There are three main goals of clinical airway
management-appropriate oxygenation, appropriate ventilation, and
protection of the airway from injury. Let's briefly look at each of
these goals.
Oxygenation
Oxygenation is controlled via the concentration of oxygen (fraction of inspired oxygen - Fi02)
delivered to the patient, although "PEEP" adjustment can be equally
important to improve oxygenation in ventilated patients with acute lung
injury (PEEP or positive end expiratory pressure, is the minimum lung
distending pressure over expiration during positive pressure
ventilation; it is usually set between 2 and 5 cm H2O in
patients with normal lungs). The minimum oxygen concentration used
during general anesthesia is usually 0.3 (30%) and can be increased to
1.0 (100%) by decreasing the concentration air administered (or of
nitrous oxide (N2O) in patients where this is used during general anesthesia). As a rough rule one adjusts FI02
(and PEEP in specialized settings) to keep arterial oxygen saturation
above 94% (using a pulse oximeter) or keeping the arterial oxygen
tension (PaO2) between 100 and 150 mm Hg in patients where arterial lines are available for arterial blood gas analysis.
Ventilation
In spontaneous ventilation (negative pressure
ventilation), negative pressure inside the lungs from diaphragmatic
flattening draws in air. It is important that clinicians recognize when a
patient is not adequately ventilating; reasons could include inadequate
respiratory effort (e.g., from excessive opioids, partial or complete
airway obstruction (e.g., from airway edema) or both. If the patient is
not breathing adequately one generally starts with a simple maneuver
such as a chin lift or jaw thrust to help open the airway, with positive
pressure ventilation with a bag-mask device being the next step if this
intervention proves to be ineffective. Concurrently, in cases of
suspected airway obstruction, the clinician physician must take measures
to alleviate the obstruction. Prolapse of the tongue into the posterior
pharynx due to loss of tone in the submandibular muscles is a frequent
cause in unconscious patients. While a chin lift or jaw thrust is often
sufficient adequate chest ventilation, some cases require that an
artificial airway be placed (discussed later). Also, if one hears
"gurgling" with breathing the oropharynx should be suctioned.
With positive pressure ventilation (PPV) gas is
forced into the lungs using a positive pressure source such as a manual
resuscitator or an automatic ventilator. PPV is often facilitated with
muscle relaxation ("paralytics") but it is not generally necessary. With
conventional ventilators, ventilation is determined by adjusting two
parameters: tidal volume (TV) and respiratory rate (RR). To ventilate a
typical patient using a ventilator, start with TV=7- 10ml/kg and
RR=10/min and then adjust according to obtained end-tidal CO2 levels (ETCO2) (obtained via capnography) or from arterial carbon dioxide tension (PaCO2)
measurements. On some older anesthesia machines the tidal volume
delivered depends on the total fresh gas flow (FGF), often set between 1
and 6 liters/min (flows of 1-2 liters/min are most economical).
Protection of the Airway from Injury
A final important goal of clinical airway management is preventing lung injury that may result from various causes such as [1] gastric contents spilling into the lungs (aspiration pneumonitis) [1], [2] retention of secretions that may lead to pneumonia, or [3]
partial lung collapse (atelectasis). The prevention of aspiration in
unconscious patients (generally those under general anesthesia or
patients with a head injury) is usually achieved by using a cuffed
endotracheal tube; unintubated patients may develop deadly aspiration
pneumonitis and ARDS (adult respiratory distress syndrome) [2]
if stomach contents spill into the lungs (especially if the pH is
<2.5 or volume >25ml). Patients at risk of aspiration with the
induction of general anesthesia are usually managed with either a rapid
sequence induction (RSI) or with awake intubation.
Finally, note that lung ventilation itself can sometimes be the cause of lung injury ("ventilator-associated lung injury") [3,4].
Numerous studies have proven that imprudent lung ventilation can cause
inflammatory damage to the lungs from repetitive closing and reopening
of the alveoli, barotrauma (trauma from excessive pressure), and
volutrauma (trauma from excessive lung expansion). Even worse, induced
systemic inflammatory changes from imprudent ventilation may even cause
dysfunction or failure in other organs.
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