Quantal ventilatory variability during spontaneous breathing anaesthesia

Background. Cardioventilatory coupling is the triggering ofinspiratory onset by preceding cardiac activity. We have observedtwo forms of coupling with a bimodal (‘quantal’)variation of respiratory period. Methods. We investigated the variables of inspiratory duration(TI), expiratory duration (TE), and tidal volume (VT) whererespiratory period variation was bimodal. In 25 anaesthetizedspontaneously breathing subjects we took 11 samples of recordingwhere the variation of respiratory period was quantal. Results. In eight of these epochs the variation in respiratoryperiod was associated with fluctuations in the number of heartbeats per breath (entrainment ratio) with a constant time intervalbetween inspiration and the immediately preceding heart beat(coupling interval), which we define as pattern II coupling.During pattern II coupling, the quantal variations in respiratoryperiod were entirely caused by variation in TE, with no associatedchanges in either TI or VT. The other three epochs with quantalvariations in respiratory period were observed in pattern IIIcoupling, where an alternating fluctuation in both entrainmentratio and coupling interval occurs. During pattern III coupling,quantal fluctuations were observed in TE, TI, and VT. Implications. Cross correlation analysis suggested that whenpattern III was present, TI was dependent upon the precedingTE, which differs markedly from traditional views on the interactionbetween inspiratory and expiratory duration. VT was linearlyrelated to TI, and so could also be determined by the precedingTE during this type of coupling. Br J Anaesth 2003; 91: 184–9