Air Hunger Far Exceeds Dyspnea Sense of Effort during Mechanical Ventilation and a Weaning Trial

Rationale: No systematic investigation into dyspnea in patients receiving prolonged ventilation (.21 d) after recovering from critical illness has been published. Objectives: We sought to determine the magnitude, nature, and pathophysiological basis of dyspnea during an unassisted-breathing trial in patients receiving prolonged ventilation. Methods: Dyspnea intensity and descriptor selection were investigated in 27 patients receiving prolonged ventilation during a 60-minute unassisted-breathing trial. Pressure–time product, respiratory mechanics, and Ptc_CO2 were also measured. Measurements and Main Results: Of 10 patients who reported dyspnea during assist-control ventilation, 9 (90.0%) selected “Not getting enough air” to characterize dyspnea. VT setting was lower in dyspneic than in nondyspneic patients (480.0 vs. 559.4 ml), P, 0.046. During the unassisted-breathing trial (n = 26), patients developed increases in dyspnea (P, 0.01) and Ptc_CO2 (P, 0.01) but no change in V^_ E. Dyspnea score was strongly linked to Ptc_CO2 (P, 0.012) and airway resistance (P, 0.013) but not respiratory work (although pressure–time product was almost three times higher than normal). At 60 minutes into the trial, 83.3% of patients selected “Not getting enough air” on its own or in combination with “Too much effort” to describe discomfort, whereas only 16.7% selected “Too much effort” on its own (P, 0.001). Across the dyspnea spectrum, patients chose “Not getting enough air” overwhelmingly over other descriptor options (P, 0.001). Conclusions: Patients developed increases in dyspnea and Ptc_CO2 but unchanged V^_ E and work of breathing during an unassisted-breathing trial; patients selected air-hunger descriptors overwhelmingly over excessive effort. The observations support the belief that air hunger results from heightened respiratory center stimulation combined with the incapacity to increase V^_ E.