Unfavorable mechanical effects of heat and moisture exchangers in ventilated patients

Objective:To investigate the mechanical effects of artificial noses. Setting: A general intensive care unit of a university hospital. Patients: 10 patients in pressure support ventilation for acute respiratory failure. Interventions: The following three conditions were randomly tested on each patient: the use of a heated humidifier (control condition), the use of a heat and moisture exchanger without filtering function (HME), and the use of a combined heat and moisture exchanger and mechanical filter (HMEF). The pressure support level was automatically adapted by means of a closed-loop control in order to obtain constancy, throughout the study, of patient inspiratory effort as evaluated from airway occlusion pressure at 0.1 s (P-0.1). Patient's ventilatory pattern, P-0.1, work of breathing, and blood gases were recorded. Measurements and main results: The artificial noses increased different components of the inspiratory load: inspiratory resistance, ventilation requirements (due to increased dead space ventilation), and dynamic intrinsic positive end-expiratory pressure (PEEP). The additional load imposed by the artificial noses was entirely undertaken by the ventilator, being the closed-loop control of P-0.1 effective to maintain constancy of patient inspiratory work by means of adequate increases in pressure support level. Conclusions: The artificial noses cause unfavorable mechanical effects by increasing inspiratory resistance, ventilation requirements, and dynamic intrinsic PEEP. Clinicians should consider these effects when setting mechanical ventilation and when assessing patients' ability to breathe spontaneously.