Pulmonary aspiration under anesthesia is a potentially catastrophic event characterized by the entry of gastric contents into the tracheobronchial tree, leading to chemical pneumonitis, obstruction, or infection. Although relatively rare, its consequences—including hypoxia, acute respiratory distress syndrome (ARDS), and even death—require a thorough understanding of the mechanism and risk of aspiration under anesthesia.
The primary mechanism increasing the risk of aspiration during anesthesia is the suppression of protective airway reflexes. Under normal conditions, reflexes such as the laryngeal adductor reflex and the cough reflex are essential in preventing the entry of foreign material into the lungs. However, general anesthesia diminishes these reflexes, especially during induction and emergence, the most vulnerable periods for aspiration events (1). Drugs such as propofol and volatile anesthetics reduce upper airway muscle tone, impair coordination of swallowing and breathing, and decrease glottic reflex sensitivity.
Aspiration can be classified as passive or active. Passive aspiration results from regurgitation of gastric contents due to decreased lower esophageal sphincter (LES) tone, a side effect of many anesthetic agents. Muscle relaxants also contribute by eliminating esophageal peristalsis and diaphragmatic tone. The resulting pressure gradient between the stomach and esophagus facilitates retrograde flow of contents. Conversely, active aspiration may occur during vomiting, a reflex response occasionally induced by light anesthesia or opioid administration (2).
Patient-specific factors also play an important role. Delayed gastric emptying, common in diabetic gastroparesis, obesity, pregnancy and renal failure, increases the volume and acidity of gastric contents, exacerbating damage in the event of aspiration. Interestingly, aspiration has been documented even in fasted, healthy individuals, challenging the traditional reliance on nil per os (NPO) status as the sole preventive strategy (3).
Epidemiological data from the Australian Incident Monitoring Study, which analyzed 133 cases of aspiration under anesthesia, highlight the timing and circumstances under which aspiration is most likely to occur. The study found that more than 60% of events occurred during induction, particularly when airway protection by endotracheal intubation was delayed or failed. In several cases, standard preoperative fasting guidelines were followed, suggesting that factors such as LES dysfunction or unrecognized delayed gastric emptying may override the protection assumed by fasting protocols (4).
Preventive measures can be divided into pharmacologic and procedural categories. Medications such as H2-receptor antagonists, proton pump inhibitors, and prokinetics are used preoperatively to reduce gastric volume and acidity. However, a meta-analysis suggests that while these reduce the severity of aspiration under anesthesia, they do not completely interrupt the mechanism underlying the risk of it occurring (5). Procedural approaches, such as rapid sequence induction (RSI), aim to secure the airway promptly with endotracheal intubation while minimizing the time during which unprotected regurgitation can occur. However, the use of cricoid pressure, once a standard component of RSI, remains controversial due to conflicting evidence regarding its efficacy and its potential to distort airway anatomy, making intubation more difficult.
In conclusion, aspiration under anesthesia results from a confluence of physiological compromise, pharmacological effects, and patient-specific risk factors. Anesthetic management must include comprehensive preoperative risk stratification, evidence-based pharmacologic prophylaxis, and vigilant airway management techniques. Despite adherence to fasting and procedural protocols, no single strategy can completely eliminate the risk of aspiration, underscoring the need for multimodal prevention and real-time clinical judgment.
References
- Robinson M, Davidson A. Aspiration under anaesthesia: risk assessment and decision-making. Contin Educ Anaesth Crit Care Pain. 2014;14(4):171-175. doi:10.1093/bjaceaccp/mkt053
- Metheny NA. Risk factors for aspiration. JPEN J Parenter Enteral Nutr. 2002;26(6 Suppl):S26-S33. doi:10.1177/014860710202600605
- Hayashi R, Maeda S, Hideki T, Higuchi H, Miyawaki T. Pulmonary Aspiration During Induction of General Anesthesia. Anesth Prog. 2020;67(4):214-218. doi:10.2344/anpr-67-02-03
- Kluger MT, Short TG. Aspiration during anaesthesia: a review of 133 cases from the Australian Anaesthetic Incident Monitoring Study (AIMS). Anaesthesia. 1999;54(1):19-26. doi:10.1046/j.1365-2044.1999.00642.x
- Nason KS. Acute Intraoperative Pulmonary Aspiration. Thorac Surg Clin. 2015;25(3):301-307. doi:10.1016/j.thorsurg.2015.04.011