NPO
Status and Aspiration
Pulmonary aspiration of gastric contents is a feared but
largely preventable complication of anesthesia.
In a recent closed claims analysis in Great Britain, it accounted for 3%
of all claims and 1/6 of airway-related claims.
Half of these aspiration-related claims involved emergency surgery. For non-emergent surgeries, patients are
asked to fast in order to allow for sufficient gastric emptying time to prevent
aspiration. The 2011 evidence-based ASA
guidelines for fasting time are as follows:
|
ASA fasting guidelines |
|
|
Ingested
material |
Minimum
fasta |
|
Clear
liquidsb |
2 hours |
|
Breast
milk |
4 hours |
|
Infant
formula |
6 hours |
|
Non-human
milk |
6 hours |
|
Light
mealc |
6 hours |
|
a Fasting times
apply to all ages and are intended for healthy patients undergoing elective
procedures. They are not intended for
women in labor. Both
amount and type of food must be considered. |
|
Three components of gastric contents can potentially cause
problems when aspirated into the lungs: particles (causing airway obstruction),
acid (causing chemical burns and inflammation), and bacteria (causing
pneumonia). A good mnemonic to keep in
mind is that 25 mL of a pH 2.5 solution is required to cause a
clinically significant aspiration pneumonitis (although there is evidence that
smaller volumes of less acidic solutions are not without risk).
For patients at higher risk of aspiration (for example,
trauma patients, parturients, or those with severe GERD), rapid-sequence
induction with cricoid pressure is often used to decrease the risk of
aspiration. The classical method of RSI
involves only two drugs, the induction agent and neuromuscular blocker; note
the absence of benzodiazepine or opioid premedication. RSI may be modified in several ways,
including the use of positive pressure ventilation. Traditionally RSI has been performed without
bag-mask ventilation to decrease the risk of gastric insufflation. However, when inspiratory pressures are kept
below 15-20 cm H2O, gastric distention does not typically occur. Especially with patients who are prone to
desaturation when apneic (pregnant, obese, pediatric, or critically ill
patients), one must consider the risks and benefits of using positive pressure
ventilation during RSI. Cricoid pressure
remains the standard of care but is currently a subject of significant
controversy. An observational study of
32 providers in 2010 found that as many as 10 different techniques were used,
and 25% of these providers actually compressed the thyroid cartilage or the
sternocleidomastoid rather than the cricoid, rendering the technique
ineffective at best and dangerous at worst.
When improperly used, cricoid pressure can cause airway compression and
difficulty with intubation, and it does not consistently occlude the esophagus
even when properly used. While it is
intended to reduce the risk of aspiration, it is actually associated with a
decrease in the lower esophageal sphincter tone, which may defeat the purpose
of its usage. As it remains the standard
of care for RSI, a reasonable approach would be to start induction with
properly applied cricoid pressure (10 N or 1 Kg with the patient awake followed
by 30 N or 3 Kg with the patient anesthetized) but have a low threshold to
discontinue it if intubation is difficult.
Board
Review Questions
1)
Nonparticulate antacid administration:
a.
Should be given 3 hours before surgery
b.
Decreases gastric volume
c.
May lead to pulmonary distress if aspiration
occurs
d.
Has a lag time of 1 hour for effectiveness
e.
Is aimed at raising the pH to at least 2.5
Answer: E. The aim of antacid administration is to raise
the pH. It should be given 15 to 30
minutes before surgery. Antacids will
not decrease gastric volume but may actually increase it. Nonparticulate antacids should not cause any
problem if aspirated. There is no lag
time.
Source: Appleton & Lange, The MGH Board Review of Anesthesiology, 5th ed., 1999.
2)
A child requires an emergent exploratory
laparotomy for abdominal trauma due to a motor vehicle collision. He had eaten 2 hours before his
accident. He should:
a.
Always have a rapid sequence induction with
propofol and succinylcholine
b.
Have a nasogastric tube passed to remove gastric
contents before induction
c.
Not be operated on for 6 hours
d.
Have vomiting induced
e.
Be allowed to awaken with the endotracheal tube
in place at the end of the procedure
Answer: E. The child should have an induction that is
most appropriate for his clinical condition.
If there has been a large amount of blood lost, a rapid sequence
induction with propofol and succinylcholine may not be appropriate. Passing a nasogastric tube may not remove all
of the stomach contents. The procedure
should not be delayed to allow the stomach to empty. Once the injury has occurred, the stomach
emptying probably stops, and the contents will still be there 6 hours
later. The child should be allowed to
awaken with the endotracheal tube in place and be extubated once protective
reflexes are intact.
Source: Adapted from Appleton
& Lange, The MGH Board Review of
Anesthesiology, 5th ed., 1999.
References
Apfelbaum JL et al.
Practice guidelines for preoperative fasting and the use of
pharmacologic agents to reduce the risk of pulmonary aspiration: Application to
healthy patients undergoing elective procedures. Anesthesiology
2011; 114(3):495-511.
Brissom P and Brissom M.
Variable application and misapplication of cricoid pressure. J
Trauma 2010 Nov;69(5):1182-4.
Cook TM, Scott S, and Mihai R. Litigation related to airway and respiratory
complications of anaesthesia: an analysis of claims against the NHS in England
1995–2007. Anaesthesia 2010;65:556-563.
El-Orbany M and Connolly LA.
Rapid sequence induction and intubation: Current controversy. Anesth
Analg 2010;110:1318-25.
Englehardt T and Webster NR.
Pulmonary aspiration of gastric contents in anaesthesia. Brit J Anaes 1999;83(3):453-60.