"Although pregnancy-induced reductions in resting EELV and arterial PCO 2 might be expected to increase both airway resistance and airway hyper-responsiveness, the available research suggests that neither pregnancy nor advancing gestation decreases FEV 1 or peak and mid-maximal expiratory flow rates (Knuttgen and Emerson, 1974; Berry et al., 1989; Garcia-Rio et al., 1997; Jensen et al., 2008b). In fact, resting measures of airway resistance have actually been shown to decrease or improve during pregnancy (Rubin et al., 1956; Gee et al., 1967; Garrard et al., 1978; Jensen et al., 2008b). "
[Show abstract][Hide abstract] ABSTRACT: The healthy human respiratory system has impressive ventilatory reserve and can easily meet the demands placed upon it by strenuous exercise. Several acute physiological adaptations during exercise ensure harmonious neuromechanical coupling of the respiratory system, which allow healthy humans to reach high levels of ventilation without perceiving undue respiratory discomfort (breathlessness). However, in certain circumstances, such as pregnancy, obesity and natural aging, ventilatory reserve becomes diminished and exertional breathlessness is present. In this review, we focus on what is known about the mechanisms of increased activity-related breathlessness in these populations. Notwithstanding the obvious physiological differences between the three conditions, they share some common perceptual and ventilatory responses to exercise. Breathlessness intensity ratings (described as an increased "sense of effort") are consistently higher than normal at any given submaximal power output; and central motor drive to the respiratory muscles is consistently increased, reflecting increased ventilatory stimulation. The increased contractile respiratory muscle effort required to support the increased ventilatory requirements of exercise remains the most plausible source of increased activity-related breathlessness in pregnant, obese and elderly humans. In all three conditions, static and dynamic respiratory mechanical/muscular function is, to some extent, altered or impaired. Nevertheless, breathlessness intensity ratings are not significantly increased (compared to normal) at any given exercise ventilation in any of these three conditions. This strongly suggests that respiratory mechanical/muscular factors, per se, may be less important in the genesis of breathlessness. Moreover, in pregnancy and obesity, we present evidence that effective physiological adjustments exist to counterbalance the potentially negative sensory consequences of the altered respiratory mechanical/muscular function peculiar to these conditions.
"The present study is the first to consider the potential relevance of resting inspiratory capacity recruitment and bronchodilatation (Rubin et al. 1956; Gee et al. 1967; Garrard et al. 1978; Gilroy et al. 1988; Berry et al. 1989; Contreras et al. 1991; Garcia-Rio et al. 1996, 1997) to respiratory sensation during exercise in pregnancy. "
[Show abstract][Hide abstract] ABSTRACT: The aim of this study was to identify the physiological mechanisms of exertional respiratory discomfort (breathlessness) in pregnancy by comparing ventilatory (breathing pattern, airway function, operating lung volumes, oesophageal pressure (P(oes))-derived indices of respiratory mechanics) and perceptual (breathlessness intensity) responses to incremental cycle exercise in 15 young, healthy women in the third trimester (TM(3); between 34 and 38 weeks gestation) and again 4-5 months postpartum (PP). During pregnancy, resting inspiratory capacity (IC) increased (P < 0.01) and end-expiratory lung volume decreased (P < 0.001), with no associated change in total lung capacity (TLC) or static respiratory muscle strength. This permitted greater tidal volume (V(T)) expansion throughout exercise in TM(3), while preserving the relationship between contractile respiratory muscle effort (tidal P(oes) swing expressed as a percentage of maximum inspiratory pressure (P(Imax))) and thoracic volume displacement (V(T) expressed as a percentage of vital capacity) and between breathlessness and ventilation . At the highest equivalent work rate (HEWR = 128 +/- 5 W) in TM(3) compared with PP: , tidal P(oes)/P(Imax) and breathlessness intensity ratings increased by 10.2 l min(-1) (P < 0.001), 8.8%P(Imax) (P < 0.05) and 0.9 Borg units (P < 0.05), respectively. Pulmonary resistance was not increased at rest or during exercise at the HEWR in TM(3), despite marked increases in mean tidal inspiratory and expiratory flow rates, suggesting increased bronchodilatation. Dynamic mechanical constraints on V(T) expansion (P < 0.05) with associated increased breathlessness intensity ratings (P < 0.05) were observed near peak exercise in TM(3) compared with PP. In conclusion: (1) pregnancy-induced increases in exertional breathlessness reflected the normal awareness of increased and contractile respiratory muscle effort; (2) mechanical adaptations of the respiratory system, including recruitment of resting IC and increased bronchodilatation, accommodated the increased V(T) while preserving effort-displacement and breathlessness-VE relationships; and (3) dynamic mechanical ventilatory constraints contributed to respiratory discomfort near the limits of tolerance in late gestation.
The Journal of Physiology 08/2008; 586(Pt 19):4735-50. DOI:10.1113/jphysiol.2008.158154 · 5.04 Impact Factor
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