Pulmonary Gas Exchange and Neuro-sensory Abnormalities in Patients With Idiopathic Pulmonary Fibrosis and Mild Mechanical Restriction. Implications for Dyspnea and Exercise Intolerance

Idiopathic pulmonary fibrosis (IPF) is a progressive fibrotic interstitial lung disease characterized by bi-basilar sub-pleural honeycombing, septal thickening, and traction bronchiectasis. Patients with IPF, even in mild cases, have a reduced exercise capacity which is strongly associated with exertional breathlessness (dyspnea). Our previous work in IPF has shown that dyspnea during exercise is associated with increased inspiratory neural drive (IND) compared with healthy controls. High IND, in turn, is related to a combination of 1) reduced ventilatory efficiency (i.e. increased ventilation relative to carbon dioxide production (V̇E/V̇CO2)); 2) abnormal dynamic breathing mechanics (blunted tidal volume (VT) and critically low inspiratory reserve volume (IRV)), especially in more advanced disease, and; 3) impaired pulmonary gas-exchange (i.e. diffusion limitation and arterial hypoxemia).

Preliminary work from our laboratory in patients with IPF but only mild restriction (total lung capacity (TLC) >70% predicted) demonstrated elevated IND and dyspnea during exercise, when compared to healthy age- and sex-matched controls. The increased IND appeared to be largely the result of the excess ventilation (high V̇E/V̇CO2), as dynamic respiratory mechanics (VT and operating lung volumes) during exercise were similar to healthy controls, when accounting for ventilation. Importantly, these patients showed only minor decreases in arterial O2 saturation. These data suggest that patients with mild forms of IPF have significant exertional dyspnea, secondary to reduced ventilatory efficiency (high V̇E/V̇CO2), although the exact mechanisms of elevated V̇E/V̇CO2 in mild IPF remains unclear.

Increased chemosensitivity has been linked to elevated V̇E/V̇CO2 in cardiopulmonary diseases. It is reasonable to postulate that persistent V̇A/Q̇ mismatch with elevated total physiological dead space and possible sympathetic over-excitation may alter central medullary chemoreceptor characteristics in patients with IPF, at least partially explaining elevated exercise V̇E/V̇CO2. Pulmonary microvascular abnormalities may also be a key contributor to the increased dead space and V̇E/V̇CO2 during exercise in IPF. Patients with IPF and mild mechanical restriction have relatively preserved gas transfer between the alveoli and capillaries, even in fibrotic lung regions with interstitial thickening. This suggests that regional capillary hypoperfusion in IPF with mild restriction, despite a relatively preserved alveolar-capillary interface, may lead to V̇A/Q̇ mismatch (specifically an increased proportion of high V̇A/Q̇ lung units), which would increase total physiologic dead space and V̇E/V̇CO2. The relative contribution of increased chemosensitivity and/or pulmonary microvascular abnormalities to elevated exercise V̇E/V̇CO2 in patients mild IPF has not been determined and are the primary focus of this study.

Treatment options for dyspnea management in IPF are limited. Recent work from the INSTAGE trial showed that a combination of nintedanib (anti-fibrotic) and sildenafil (pulmonary vasodilator) showed minimal improvement in dyspnea. However, improvements in physical activity and gas-exchange in patients with IPF following 8-week treatment of inhaled nitric oxide (iNO), a selective pulmonary vasodilator have been demonstrated in other, more recent studies. Since patients with mild forms of IPF are thought to have a relatively intact capillary bed but a relatively high physiological dead space due to attenuation of regional pulmonary perfusion, inhaled selective vasodilation may be more beneficial than in advanced disease with fixed microvascular destruction. This is supported by recent work demonstrating a reduced V̇E/V̇CO2 (reflecting a decrease in physiological dead space) and dyspnea during exercise in patients with mild chronic obstructive pulmonary disease with minimal or no emphysema. Importantly, arterial O2 saturation was normal throughout exercise and unaffected by iNO, which suggests no deleterious effects of iNO on overall gas-exchange. The reduction in V̇E/V̇CO2 during exercise with iNO suggests that iNO increases pulmonary microvascular perfusion heterogeneity, leading to improved V̇A/Q̇ matching, reduced dead space and therefore a lower ventilation for a given metabolic demand.

As an exploratory outcome, we will determine whether iNO improves V̇A/Q̇ and reduces dead space and attendant dyspnea, in patients with IPF and mild mechanical restriction. Moreover, this would clearly establish if partially reversible vascular dysfunction contributes to V̇A/Q̇ mismatch, elevated V̇E/V̇CO2, inspiratory neural drive and dyspnea exists in non-hypoxemic patients with IPF and minimal mechanical abnormalities.