Respiratory load compensation. I. Role of the cerebrum

Ventilatory and perceptual responses to cycle exercise in obese women

The main purpose of this study was to examine the relative contribution of respiratory mechanical factors and the increased metabolic cost of locomotion to exertional breathlessness in obese women. We examined the relationship of intensity of breathlessness to ventilation (V̇e) when exertional oxygen uptake (V̇o2) of obesity was minimized by cycle exercise. Eighteen middle-aged (54 ± 8 yr, mean ± SD) obese [body mass index (BMI) 40.2 ± 7.8 kg/m2] and 13 age-matched normal-weight (BMI 23.3 ± 1.7 kg/m2) women were studied. Breathlessness at higher submaximal cycle work rates was significantly increased (by ≥1 Borg unit) in obese compared with normal-weight women, in association with a 35–45% increase in V̇e and a higher metabolic cost of exercise. Obese women demonstrated greater resting expiratory flow limitation, reduced resting end-expiratory lung volume (EELV)(by 20%), and progressive increases in dynamic EELV during exercise: peak inspiratory capacity (IC) decreased by 16% (0.39 liter) of the resting value. V̇e/V̇o2 slopes were unchanged in obesity. Breathlessness ratings at any given V̇e or V̇o2 were not increased in obesity, suggesting that respiratory mechanical factors were not contributory. Our results indicate that in obese women, recruitment of resting IC and dynamic increases in EELV with exercise served to optimize operating lung volumes and to attenuate expiratory flow limitation so as to accommodate the increased ventilatory demand without increased breathlessness.

Inspiratory loading elicits aberrant fMRI signal changes in obstructive sleep apnea

We hypothesized that neural processes mediating deficient sensory and autonomic regulatory mechanisms in obstructive sleep apnea (OSA) would be revealed by responses to inspiratory loading in brain regions regulating sensory and motor control. Functional magnetic resonance imaging (fMRI) signals and physiologic changes were assessed during baseline and inspiratory loading in 7 OSA patients and 11 controls, all male and medication-free. Heart rate increases to inspiratory loading began earlier and load pressures were achieved later in OSA patients. Comparable fMRI changes emerged in multiple brain regions in both groups, including limbic, cerebellar, midbrain, and primary motor cortex. However, in OSA subjects, altered signals appeared in primary sensory thalamus and sensory cortex, supplementary motor cortex, cerebellar cortex and deep nuclei, cingulate, medial temporal, and insular cortices, right hippocampus, and midbrain. Signal delays occurred in basal ganglia. We conclude that areas mediating sensory and autonomic processes, and motor timing, are affected in OSA; many of these areas overlap regions of previously demonstrated gray matter loss.

Identification of human brain regions underlying responses to resistive inspiratory loading with functional magnetic resonance imaging

Compensatory ventilatory responses to increased inspiratory loading are essential for adequate breathing regulation in a number of pulmonary diseases; however, the human brain sites mediating such responses are unknown. Midsagittal and axial images were acquired in 11 healthy volunteers during unloaded and loaded (30 cmH2O; 1 cmH2O = 98 Pa) inspiratory breathing, by using functional magnetic resonance imaging (fMRI) strategies (1.5-tesla MR; repetition time, 72 msec; echo time, 45 msec; flip angle, 30 degrees; field of view, 26 cm; slice thickness, 5 mm; number of excitations, 1; matrix, 128 x 256). Digital image subtractions and region of interest analyses revealed significantly increased fMRI signal intensity in discrete areas of the ventral and dorsal pons, interpeduncular nucleus, basal forebrain, putamen, and cerebellar regions. Upon load withdrawal, certain regions displayed a rapid fMRI signal off-transient, while in others, a slower fMRI signal decay emerged. Sustained loading elicited slow decreases in fMRI signal across activated regions, while second application of an identical load resulted in smaller signal increases compared to initial signal responses (P < 0.001). A moderate inspiratory load is associated with consistent regional activation of discrete brain locations; certain of these regions have been implicated in mediation of loaded breathing in animal models. We speculate that temporal changes in fMRI signal may indicate respiratory after-discharge and/or habituation phenomena.