Aetiology
The cause of aspiration can be classified according to the underlying mechanism, which can be due to:
Aspiration of food, drink, oral secretions, or other substances:
Swallowing dysfunction (e.g., stroke, dementia, epilepsy, multiple sclerosis, Parkinson’s disease, motor neuron disease, cardiorespiratory disease, head and neck cancers)
Impaired conscious level
Substance misuse (e.g., acute alcohol intoxication, opioid toxicity)
During general anaesthesia (or other oropharyngeal procedures) or in the intensive care unit. These are common scenarios; the patient may present asymptomatically, or with bronchospasm, hypoxia, cough, dyspnoea, fever, and even respiratory failure from non-cardiogenic pulmonary oedema[11][12]
Gastrointestinal (GI) disease (e.g., upper GI surgery, hiatal hernia), conditions that affect gastric emptying (e.g., obesity, pregnancy), and oesophageal abnormalities (e.g., dysmotility, strictures, fistulas, gastroparesis [which can be due to diabetes])
Poor cough (e.g., upper airway problems such as pharyngeal pouches and vocal cord palsies, neuromuscular disease)
Increased severity of illness
Following upper gastrointestinal studies with barium. Aspiration of barium sulfate can cause aspiration pneumonitis, which can present with respiratory distress
Head and neck trauma. The patient may acutely aspirate blood in this scenario
Reflux/vomiting:
Substance misuse (e.g., acute alcohol intoxication, opioid toxicity)
General anaesthetic or oropharyngeal procedures
Recumbent position during enteral feeding
Polytrauma
Head and neck cancers.
Among critically ill patients, the major risk factors for aspiration include:[15]
Documented previous episode of aspiration
Decreased level of consciousness (Glasgow coma scale score <9 or a high level of sedation)
Neuromuscular disease, or congenital or acquired structural abnormalities of the aerodigestive tract
Endotracheal intubation
Vomiting
Persistently high gastric residual volume
Supine positioning.
Additional risk factors include the presence of a nasoenteric tube, intermittent feeding, abdominal/thoracic surgery or trauma, inadequate nursing staff, large size or diameter of feeding tube in children (not covered in this topic), malpositioning of the feeding tube, and transport.
In the perioperative period, factors that increase the likelihood of aspiration include:[3]
A high urgency of surgery
Difficult airways
Inadequate depth of anaesthesia
Use of the lithotomy position
Gastrointestinal problems such as delayed gastric emptying, gastro-oesophageal reflux, ileus, or bowel obstruction
Impaired conscious level
Increased severity of illness
Obesity.
Certain drugs reduce the lower oesophageal sphincter pressure and promote gastro-oesophageal reflux in anaesthesia and disease states, and thereby increase the risk for aspiration. These drugs include atropine, glycopyrrolate, dopamine, sodium nitroprusside, ganglion blockers, thiopental, tricyclic antidepressants, beta-adrenergic stimulants, halothane, enflurane, opioids, and propofol.
Pathophysiology
Older age (particularly >70 years) is a risk factor for aspiration because it is associated with a higher prevalence of cerebrovascular and degenerative neurological disease than younger age groups. These conditions can cause dysphagia and an impaired cough reflex, both of which increase the risk for aspiration of food, liquids, and barium contrast.[8][16]
Anticholinergics, antipsychotics, or anxiolytics may also impair the cough reflex and/or swallowing.
Tachypnoea, which can be caused by various medical conditions, alters the co-ordination between the action of swallowing and respiration, and increases the risk of aspiration.[17]
The lower and upper oesophageal sphincter tone is decreased in GORD, by certain drugs (anticholinergics, antipsychotics, and anxiolytics), and by indwelling endotracheal, tracheostomy, gastric, or nasogastric tubes. This decrease in sphincter tone increases the risk of gastric content aspiration.[3]
During the perioperative period, the loss or impairment of protective laryngeal reflexes increases the risk of anaesthesia-related pulmonary aspiration. In addition, the supine position during diagnostic, surgical, and dental procedures aligns the trachea and the oropharynx, which also increases the risk of aspiration.[11]
Altered physiological states such as pregnancy, gastrointestinal disorders, and diabetes mellitus are associated with delays in the rate of gastric emptying, which increase the gastric volume.
Pregnancy increases the risk of aspiration because progesterone decreases the tone of the lower oesophageal sphincter and delays gastric emptying, and there is an increased intra-abdominal pressure secondary to the gravid uterus. Aspiration pneumonitis is called Mendelson’s syndrome in pregnancy and can have deleterious effects secondary to lower pH of gastric contents.[18]
Aspiration of different gastric materials may have different effects. Low pH (acidic) material leads to neutrophil-predominant lung injury. There is apoptosis of type I alveolar epithelium caused by direct acid contact, and release of pro-inflammatory mediators secondary to activation of capsaicin-sensitive (TRPV-1) receptors. Alternatively, aspiration of small non-acidified gastric particles (SNAP) or bacteria directly stimulates alveolar macrophages to release innate immune/pro-inflammatory mediators through activation of polyanionic scavenger receptors or Toll-like receptors. Regardless of the type of aspiration material, there is an acute inflammation in the lung characterised by neutrophil infiltration, alveolar haemorrhage, intra-alveolar and interstitial oedema, and impairment of alveolar fluid clearance. This is followed by a repair process characterised by scavenging of alveolar detritus by macrophages and proliferation of type II alveolar epithelial cells.[19] Complex gastric contents, which are a combination of gastric food particles, bacterial products, cytokines, and acid, are referred to as CASP (combined acid and small food particles). Aspiration of CASP may exacerbate damage due to the synergistic effect of acid and small gastric particles.
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