Chapter 47

Infection of paranasal sinus cavities is a well-recognized cause of fever in mechanically ventilated patients.17 Easy to detect in maxillary sinuses, the infectious process also frequently involves ethmoid, frontal, and sphenoid sinuses8 where the diagnosis is more difficult to establish.9 Infectious sinusitis represents an important reservoir of bacteria10 that may disseminate into the respiratory tract11 and intracranially.12 In contrast to community-acquired sinusitis, ventilator-associated sinusitis is often clinically silent in sedated critically ill patients and may be underdiagnosed if not systematically screened for in the presence of fever of unknown origin. In the absence of diagnosis and appropriate treatment, bacteremia,2,5 ventilator-associated pneumonia,13 and life-threatening complications, such as orbital infection,14 meningitis, mastoiditis, cerebral abscess, or thrombosis of the sinus cavernosus, may result.12 Early detection and treatment of infectious maxillary sinusitis significantly reduces the incidence of ventilator-associated pneumonia and may decrease intensive care mortality.11

### Classical Hypotheses

Located at the entry of the respiratory system, paranasal sinuses and the nose serve to humidify, filter, warm, and sense inspiratory gas.15,16 A number of other presupposed physiologic roles have not yet received firm scientific confirmation.17 Among them, the most popular are that paranasal sinuses help lighten the bones of the skull, improve the resonance of the voice, serve as a sound box for opera singers, increase the surface area of the olfactory membrane, serve as shock absorbers in mechanical impacts, protect against high pressure in the nasal region when sneezing, act as thermal insulators of the brain, and promote facial growth and architecture. More simply, paranasal sinuses might be considered as evolutionary relics or faults, whose form results from the influence of the forces created during the act of chewing.

### Recent Hypothesis: The Role of the Regional Production of Nitric Oxide

Large amounts of nitric oxide are produced in human paranasal sinuses18,19 and permanently released in the upper airways through the different ostia that link antral cavities to the nostrils.20 As shown in Figure 47-1, an inducible form of nitric oxide synthetase is present in cilia and microvilli of the maxillary sinus epithelium of healthy volunteers.18,19 Significant nitric oxide concentrations are also found in the exhaled gas of guinea pigs, pigs, rhesus monkeys, rats, rabbits, horses, and Asian elephants, all species possessing open paranasal sinuses.2024 Interestingly, seals and baboons, which do not have open paranasal sinuses, do not exhale nitric oxide,25,26 thus confirming that pneumatic facial cavities play a critical role in the nasal production of nitric oxide in mammals. Humming, by accelerating gas exchange in sinus cavities, markedly increases exhaled nasal nitric oxide27 whereas moderate exercise has an opposite effect.28 Nitric oxide produced in the paranasal sinuses likely has an important role in host defense against inhaled pathogens, in the optimization of ventilation–perfusion ...

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