Clinical significance of a spiral phenomenon in the plot of CO₂ output versus O₂ uptake during exercise in cardiac patients

Abstract

A spiral phenomenon is sometimes noted in the plots of CO₂ output (VCO₂) against O₂ uptake (VO₂) measured during cardiopulmonary exercise testing (CPX) in patients with heart failure with oscillatory breathing. However, few data are available that elucidate the clinical significance of this phenomenon. Our group studied the prevalence of this phenomenon and its relation to cardiac and cardiopulmonary function. Of 2,263 cardiac patients who underwent CPX, 126 patients with a clear pattern of oscillatory breathing were identified. Cardiopulmonary indexes were compared between patients who showed the spiral phenomenon (n = 49) and those who did not (n = 77). The amplitudes of VO₂ and VCO₂ oscillations were greater and the phase difference between VO₂ and VCO₂ oscillations was longer in the patients with the spiral phenomenon than in those without it. Patients with the spiral phenomenon also had a lower left ventricular ejection fraction (43.4 ± 21.4% vs 57.1 ± 16.8%, p <0.001) and a higher level of brain natriuretic peptide (637.2 ± 698.3 vs 228.3 ± 351.4 pg/ml, p = 0.002). The peak VO₂ was lower (14.5 ± 5.6 vs 18.1 ± 6.3, p = 0.002), the slope of the increase in ventilation versus VCO₂ was higher (39.8 ± 9.5 vs 33.6 ± 6.8, p <0.001), and end-tidal PCO₂ both at rest and at peak exercise was lower in the patients with the spiral phenomenon than in those without it. In conclusion, the spiral phenomenon in the VCO₂-versus-VO₂ plot arising from the phase difference between VCO₂ and VO₂ oscillations reflects more advanced cardiopulmonary dysfunction in cardiac patients with oscillatory breathing.