Prevention of Acid Aspiration Syndrome

• Acid aspiration syndrome is known as the aspiration of stomach contents into the lungs.
• If the aspirated material is sufficiently acid with ph lower than 2.5, and there is sufficient volume, the symptoms of aspiration will appear.
• The chemical pneumonia produced is caused by parenchymal inflammatory reaction medicated by cytokines.
• Early signs include: cyanosis, tachycardia, massive pulmonary edema, bronchospasm, hypotension, hypovoleamia with haemoconcentration.
• Cardiac failure may develop later accompanied with increased pulmonary artery pressure, reduced static lung compliance, failing arterial oxygen, metabolic acidosis, infection (not common).

Prophylaxis Therapy

• Histamine (H2) antagonists such as cimetidine, famotidine, nizatidine, and ranitidine and proton pump inhibitors (PPIs) such as dexlansoprazole, esomeprazole, lansoprazole, omeprazole, pantoprazole, and rabeprazole have been shown to be effective in increasing the pH and reduce the volume of gastric contents. 
• H2 antagonists act by blocking H2 receptors of gastric parietal cells, thereby inhibiting the stimulatory effects of histamine on gastric acid secretion.
• On the other hand, PPIs block the ‘proton pump’ of the same cell, thereby inhibiting the stimulatory actions of histamine, gastrin and acetylcholine.
• An oral H2 antagonist must be given 1-2 hours before anaesthesia, and a PPI, 12 hours in advance.
• A recent meta-analysis by Clark et al suggested that ranitidine was superior to PPIs in both reducing gastric fluid volume and acidity. Its use is recommended in patients at risk of aspiration only. It should not be routinely used.
• Oral sodium citrate solution is effective in elevating gastric pH above 2.5, but it does not reduce gastric volume, and is associated with nausea and vomiting
• Prokinetics such as domperidone, metoclopramide, erythromycin, and renzapride promote gastric emptying and in turn should reduce the risk of aspiration. This theory, however, is not supported by a large amount of quality evidence.

References:

1. Nason, K. S. (2015). Acute Intraoperative Pulmonary Aspiration. Thoracic surgery clinics, 25(3), 301-307 retrieved at https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4517287/
2. Pulmonary Aspiration of Gastric Contents 2010 retrieved at http://www.frca.co.uk/Documents/192%20Pulmonary%20aspiration%20of%20gastric%20contents.pdf
3. Shaikh, J. M., Sabbar, S., Aziz, N., Shaikh, N. B., & Akhund, T. (2009). acid aspiration prophylaxis during anaesthesia for Caesarean section: a survey among anaesthetists at Hyderabad. J Ayub Med Coll Abbottabad, 21(4).