Rimawi, Ramzy Husam MD

Consequences of pulmonary aspiration can be biphasic with dissimilar sequelae. An early phase, coined pneumonitis, involves an inflammatory reaction to gastrointestinal contents with a pH typically less than 2.4 resulting in rapid clinical deterioration and recovery typically within 48 hours. The later phase is the progression to an acute respiratory distress syndrome or bacterial aspiration pneumonia (BAP) as a result of a large-volume aspiration banking a sizeable pathogen load from the gastrointestinal tract or oral cavity. Distinguishing pneumonia from pneumonitis is notoriously difficult as history, physical examination, laboratory and radiographic findings cannot reliably predict BAP or distinguish it from chemical pneumonitis. Diagnosing early onset pneumonia in comatose patients after cardiac arrest is arduous given the limitations in diagnostic tools caused by therapeutic hypothermia and postcardiac arrest syndrome. In comatose trauma patients, the diagnosis can also be made challenging by underlying chest and pulmonary damage.

In this issue of Critical Care Medicine, Lascarrou et al (¹) use telescopic plugged catheter (TPC) sampling in patients with aspiration syndrome to assess whether negative cultures can allow for safe discontinuation of antibiotics in mechanically ventilated comatose patients. TPC cultures were positive in 43 (46.7%) and negative in 49 (53.3%) of the 92 included patients. Sixteen (32.7%) of the 49 patients continued antibiotic therapy for other infectious concerns. Of the remaining 33 patients (67.3%), antibiotics were discontinued. A lung infection subsequently developed in only two patients (6%), one of whom had a positive TPC culture on repeat sampling, whereas the other did not have repeat sampling. It may also be possible that the lung infection that developed may have been due to recurrent aspirations. There were no significant differences in clinical, laboratory, or radiographic evidence, mechanical ventilation duration, length of stay, or mortality among the bacterial and nonbacterial aspiration groups. This report is unique in that it reignites the use of TPC in aspiration processes, encourages safe antimicrobial discontinuation in patients with negative TPC cultures, and challenges the recommendations of recent randomized trials that have recommended prophylactic antibiotics in cerebrovascular accident patients to reduce the risk of BAP (²).

Although there are numerous methods available to assist the clinician in diagnosing BAP, many have drawbacks. Microbiologic cultures may be contaminated by the oropharyngeal flora and upper airway, regardless of whether it is obtained via expectoration, orotracheal aspiration, nasotracheal aspiration, or bronchoalveolar lavage (BAL). Although there were no reported consequences of fiberoptic bronchoscopy in this article, bronchoscopy still poses additive risks and may also allow for contaminated secretions to be aspirated from the suction channel. Direct needle transtracheal aspiration is invasive and carries a risk of barotrauma, principally in patients undergoing mechanical ventilation. Biomarkers, including procalcitonin, pepsin, and amylase, have been studied. Serum procalcitonin had a poor diagnostic value in differentiating BAP from aspiration pneumonitis (³). Tracheal aspirate α-amylase concentrations are reportedly moderate and inaccurate in diagnosing microaspiration when compared with tracheal secretion or BAL pepsin, which may potentially be used to suggest aspiration in patients with moderate reflux or abnormal swallow though further evidence is needed (⁴).

Studies analyzing TPC are scarce and dated to nearly 2 decades ago. Wimberley et al (⁵) first introduced TPC in 1979 to attain lower airway samples with minimal risk of contamination by avoiding upper airway flora. While blinded introduction of the TPC through an endotracheal tube is possible, most institutions that use TPC sampling perform fiberoptic bronchoscopy. Combining BAL with TPC yields a sensitivity of 88% and specificity of 100% (⁶). Quantitative cultures of TPC samples should be obtained in patients mechanically ventilated, and a positive sample is typically defined as 10² cfu in the presence of antibiotic receipt within 24 hours and 10³ cfu in the absence of antibiotics. In patients with low risk of BAP, antibiotics should safely be discontinued without TPC sampling. In patients with moderate risk of BAP, antibiotics can be safely discontinued if TPC is negative. In patients with a high concern of BAP and negative TPC culture, while antibiotics may be discontinued, a repeat TPC may be prudent. Patients with positive TPC should be treated for BAP.

Despite the recent randomized trials encouraging prophylactic antibiotics in patients with cerebrovascular accident, this remains controversial as other reports, like that of Lascarrou et al (¹), have also discouraged antibiotic prophylaxis in comatose patients, including out-of-hospital cardiac arrest patients (⁷). Studies have failed to show a reduction in mortality or mechanical ventilation duration with antibiotic prophylaxis in this cohort of patients. Although a delay in antibiotic therapy during sepsis has been associated with increased morbidity and mortality, antibiotic prescription after aspiration should be reserved to patients with a high clinical suspicion for BAP and bacteriologically documented pneumonia as unnecessary antibiotic prescription can result in avoidable adverse effects and antimicrobial resistance. Real-time multiplex polymerase chain reaction can further facilitate stewardship efforts more promptly with a high sensitivity and specificity (⁸).

Clinical suspicion alone does not suggest infection, as demonstrated in this article where only 43 (47%) of the 92 patients had positive TPC cultures. The presence of fever, tachycardia, increased secretions or oxygen requirement, and/or radiographic changes within 48 hours of an aspiration event does not necessarily suggest an infectious process has occurred. Lascarrou et al (¹) illustrated the lack of any clinical or laboratory variable that dependably predicted BAP. Furthermore, witnessing an aspiration, even during intubation, does not suggest infection, as evidence has shown healthy people with no history of dysphagia frequently aspirate during sleep with no clinical consequences (⁹).

Controversy in distinguishing pneumonia from pneumonitis in an effort to safely discontinue antibiotics may still remain as TPC is not routinely available in the United States. Auspiciously, Lascarrou et al (¹) have shown that antibiotics can safely be withheld without the need for TPC sampling in comatose patients without a suspected BAP. This may further strengthen our efforts to reduce healthcare costs, morbidity, mortality, and antimicrobial resistance.

REFERENCES

antibiotics; aspiration; deescalation; pneumonia; stewardship