Rationale: Aspiration of infective subglottic secretions causes ventilator-associated pneumonia (VAP) in mechanically ventilated patients. Mechanisms underlying subglottic colonization in critical illness have not been defined, limiting strategies for targeted prevention of VAP.
Objectives: To characterize subglottic host defense dysfunction in mechanically ventilated patients in the ICU; to determine whether subglottic mucin contributes to neutrophil phagocytic impairment and bacterial growth.
Methods: Prospective subglottic sampling in mechanically ventilated patients (intubated for four or more days), and newly intubated control patients (intubated for less than 30 min); isolation and culture of primary subglottic epithelial cells from control patients; laboratory analysis of host innate immune defenses.
Measurements and main results: Twenty-four patients in the ICU and 27 newly intubated control patients were studied. Subglottic ICU samples had significantly reduced microbiological diversity and contained potential respiratory pathogens. The subglottic microenvironment in the ICU was characterized by neutrophilic inflammation, significantly increased proinflammatory cytokines and neutrophil proteases, and altered physical properties of subglottic secretions, including accumulation of mucins. Subglottic mucin from ICU patients impaired the capacity of neutrophils to phagocytose and kill bacteria. Phagocytic impairment was reversible on treatment with a mucolytic agent. Subglottic mucus promoted growth and invasion of bacterial pathogens in a novel air-liquid interface model of primary human subglottic epithelium.
Conclusions: Mechanical ventilation in the ICU is characterized by substantial mucin secretion and neutrophilic inflammation. Mucin impairs neutrophil function and promotes bacterial growth. Mucolytic agents reverse mucin-mediated neutrophil dysfunction. Enhanced mucus disruption and removal has potential to augment preventive benefits of subglottic drainage.
Keywords: critical care; mucoproteins; neutrophils; phagocytosis; ventilator-associated pneumonia.