Fig. 3
Brain curve (red line), ventilation curve (dashed black line), and metabolic hyperbola (blue line) in a spontaneously breathing patient with a disease affecting the inspiratory flow-generation pathway at the equation of motion level [e.g., restrictive disease (↑Ers), obstructive disease (↑Rrs), dynamic hyperinflation (↑PEE)]. Similar effects are anticipated if the integrity is compromised at higher levels of the inspiratory flow-generation pathway. PaCO2 desired by the brain is 39 mmHg and this corresponds to RCO/min of 6.3 l/min (point 1). In an intact inspiratory flow-generation pathway, the brain and ventilation curves would coincide, resulting in an actual PaCO2 of 39 mmHg. For simplicity, let us assume that the disease acutely compromises the integrity of inspiratory flow-generation pathway and as a result the ventilation curve is moved to the right with a downward slope. Brain curve and metabolic hyperbola are kept constant. Consequently, the RCO/min corresponding to 6.3 l/min decreases actual ventilation to 4.2 l/min (point 2). This decrease in ventilation triggers a gradual rise in PaCO2, stimulating the respiratory centers. RCO/min progressively increases (mainly due to changes in respiratory drive, RCO per breath) along the brain curve in response to the elevated PaCO2. As RCO/min increases, so does actual ventilation along the ventilation curve. A steady state is reached when RCO/min (point 3) yields actual ventilation at the intersection of the ventilation curve and metabolic hyperbola (point 4). At this point, PaCO2 stabilizes at 40 mmHg, and respiratory drive, RCO/min, and ventilation cease increasing as the CO2 stimulus remains constant. Despite ventilatory demands of 9.3 l/min, only 6.2 l/min are met, resulting in a deficit of 3.1 l/min. The respiratory centers activity and ventilatory output are projected to forebrain via the corollary discharge pathway (re-afferent traffic, black arrows) and create the sense of dyspnea. Given the relatively low RCO/min and unmet demands, this patient is unlikely to experience dyspnea, particularly during resting conditions