Ventilatory frequency variability in spontaneously breathing anaesthetized subjects.

During spontaneous breathing general anaesthesia, inspiration is generally started by a signal related to preceding cardiovascular activity. This phenomenon, 'cardioventilatory coupling', contributes to the variation in ventilatory frequency. However, the detailed, breath-to-breath timing relationship between heart beat and inspiratory onset is complex, with at least four distinct patterns (designated patterns I-IV). These coupling patterns are defined according to the particular breath-to-breath change in: (a) entrainment ratio and (b) coupling interval, the interval between inspiratory onset and the preceding initiating heart beat. We have examined the relationship between coupling and timing of breathing in adult subjects breathing spontaneously during general anaesthesia. The heart rate-ventilatory frequency interaction was explored by identifying the distribution of different coupling patterns in a plot of heart rate vs ventilatory frequency (the HR/f plot) and analysing the variation in breathing frequency during each coupling pattern by differentiating between changes in entrainment ratio from changes in coupling interval. We observed that: (i) coupling patterns are distributed within specific regions of the HR/f plot; (ii) specific patterns of variation in breathing are associated with each coupling pattern; (iii) this variation is a consequence of the balance between changes in entrainment ratio and coupling interval; (iv) coupling was invariably present at low breathing frequencies; and (v) the inverse non-linear relationship between ventilatory frequency and variation is largely a consequence of changing coupling pattern with ventilatory frequency. Coupling explains much of the breath-to-breath variability of ventilatory frequency during anaesthesia, and may be relevant to the phenomena of hypoventilation, central apnoea and ventilatory arrhythmia. A hypothesis concerning the generation of coupling patterns is presented.