Ventilatory and peripheral O2 saturation oscillations during spontaneous and voluntary periodic breathing: a spectral analysis approach

It has been hypothesized that the periodic breathing (PB) pattern often observed in chronic heart failure (CHF) patients (pts) originates from the instability of the feedback loop controlling ventilation, due to the prolonged circulation time of these pts. To test this hypothesis, we studied the relationship between the instantaneous tidal volume (ITV) and O₂ saturation at the ear (SpO₂) in 24 CHF pts during sustained episodes of PB and 13 healthy subjects during voluntary PB. Some basic modeling assumptions about the respiratory control system were made and, following the hypothesis, a set of expected results formulated. The relationship between SpO₂ and ITV was analyzed by bivariate spectral analysis and the phase shift and time delay at the PB frequency estimated. The ITV oscillation in CHF patients always mirrored and anticipated the contemporary oscillation of SpO₂ with a phase shift of -145±19° and a delay of -3.3 s. The oscillation of SpO₂ was also a delayed version of the ITV, with a lung-to-ear circulation time (LECT) of 27.9±6 s. The SpO₂ oscillation of healthy subjects was simply a delayed version of the ITV due to LECT. The phase shift was 123±37°, with a corresponding LECT of 15.5±5 s. Provided the ventilatory open loop gain of CHF patients with PB is ⩾1, the results of this study are in good agreement with theoretical expectation and support the instability hypothesis of PB in these pts