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Mechanical ventilatory constraints during incremental cycle exercise in human pregnancy: implications for respiratory sensation
Author(s) -
Jensen Dennis,
Webb Katherine A.,
Davies Gregory A. L.,
O'Donnell Denis E.
Publication year - 2008
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1113/jphysiol.2008.158154
Subject(s) - lung volumes , medicine , tidal volume , respiratory system , respiratory physiology , respiratory minute volume , ventilation (architecture) , work of breathing , cardiology , functional residual capacity , respiratory rate , anesthesia , lung , heart rate , blood pressure , mechanical engineering , engineering
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– relationships; and (3) dynamic mechanical ventilatory constraints contributed to respiratory discomfort near the limits of tolerance in late gestation.