Ventilatory frequency variability in spontaneously breathing anaesthetizedsubjects

During spontaneous breathing general anaesthesia, inspiration is generally started by a signal related to preceding cardiovascular activity. This phen omenon, 'cardioventilatary coupling', contributes to the variation in venti latory frequency. However, the detailed, breath-to-breath timing relationsh ip between heart beat and inspiratory onset is complex, with at least four distinct patterns (designated patterns I-IV). These coupling patterns are d efined according to the particular breath-to-breath change in: (a) entrainm ent 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 spont aneously during general anaesthesia. The heart rate-ventilatory frequency i nteraction was explored by identifying the distribution of different coupli ng patterns in a plot of heart rate vs ventilatory frequency (the HR/f plot ) and analysing the variation in breathing frequency during each coupling p attern by differentiating between changes in entrainment ratio from changes in coupling interval. We observed that: (i) coupling patterns are distribu ted within specific regions of the HR/f (ii) specific patterns of variation in breathing are associated with each coupling pattern; (iii) this variati on is a consequence of the balance between changes in entrainment ratio and coupling interval; (iv) coupling was invariably present at low breathing f requencies; and (v) the inverse non-linear relationship between ventilatory frequency and variation is largely a consequence of changing coupling patt ern with ventilatory frequency. Coupling explains much of the breath-to-bre ath variability of ventilatory frequency during anaesthesia, and may be rel evant to the phenomena of hypoventilation, central apnoea and ventilatory a rrhythmia. A hypothesis concerning the generation of coupling patterns is p resented.