We investigate nonlinear instabilities in human heart rate variability. We focus on phenomena with characteristic, easily recognizable features which are well known in physics. In the past we were able to show two groups of evidence. The first was an ever expanding roster of such cases of heart rate variability pathology which exhibit type I intermittency. This phenomenon occurs in those dynamical systems which have come close to a saddle-node bifurcation. The second were observations of homoclinic orbits and the gluing bifurcation in measured heart rate variability. We present here two cases of 24-hour recordings of human heart rate which exhibit special, regular patterns. We show that period-1, period-2 orbits and homoclinic orbits may be found in return maps formed using this data. Using a pair of coupled modified van der Pol--Duffing oscillators, we are able to model the behavior of the sino-atrial node and of the atrio-ventricular node (elements of the conduction system of the heart) in such a way as to obtain orbits similar to those measured during the sino-atrial block in a human.

PACS numbers: 05.45.--a, 82.40.Bj, 87.19.Hh