The Mechanical Effects of Expiratory Airflow Limitation on Cardiac Performance in Cystic Fibrosis

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Abstract
The influence of altered lung mechanics on cardiac performance in cystic fibrosis (CF) was examined in an analysis of cardiorespiratory performance during exercise. The stroke volume (SV) response to exercise, derived from an indirect Fick (CO2) determination of cardiac output during submaximal steady-state exercise served as the index of cardiac performance. The relationship between exercising SV and forced expiratory volume in one second (FEV1), inspiratory duty cycle during exercise (T1/Ttot) and exercising systemic arterial oxygen tension (PaO2) was examined in 15 studies involving 11 individuals with CF. The subjects were 12 to 25 yr of age and the FEV1 ranged from 15 to 72% predicted. Although a relationship existed between the SV expressed as a percentage of the mean predicted from height and gender (SV%) and PaO2 (r = 0.61, p < 0.02), a more striking relationship was noted between SV% and either FEV1 or T1/Ttot (r = 0.79 and 0.73, p < 0.002). A multiple regression analysis of SV% identified an element of the relationship between FEV1 or T1/Ttot and cardiac performance which was distinct from the influence of the coexisting hypoxia (p .ltoreq. 0.05). The limitation in the SV response to exercise was noted to be in part reversible, coincident with an improvement in ventilatory mechanics that had occurred in 2 of the 4 persons in whom repeated studies were available. These observations suggest that in individuals with CF and a moderately severe degree of expiratory airflow limitation, there is a mechanical element of cardiopulmonary interdependence.