Heart rate fluctuations of lower frequencies than the respiratoryrhythm but caused by it.

In conscious adult rabbits, "classical" respiratory sinus arrhythmia does not occur because the respiratory frequency (RF) always exceeds half the heart rate (HR). However, slow HR fluctuations, not synchronous with the respiratory rhythm but affected by it, occur systematically. We have shown that these can be calculated by using aliasing rules. During general anaesthesia, when the RF decreases so that respiratory frequency is less than half the heart rate, classical respiratory sinus arrhythmia occurs and can be greatly reduced by vagal blockade. The slow HR fluctuations which are not synchronous with the respiratory rhythms, but are affected by it are mainly vagally mediated, because vagal blockade virtually eliminates them.