Further Progress in Qualitative Modelling of Cardiac Electrophysiology

A qualitative model of the electrical conducting system of the heart is developed which attempts to represent basic physiology and pathophysiology . State descriptions and state transition rules are used to generate a tree of possible behaviours from a given start state . We demonstrate that without any further knowledge, explosive combinatorial branching occurs, but that qualitative information on state durations can be employed to reduce very significantly the number of behaviours . An initial indication is given as to how the approach could be used to explain observed electrocardiograms (ECGs) in terms of the underlying physiological processes, and the advantages of generating envisionments rather than simulations are presented . 1 . INTRODUCTION Further Progress in Qualitative Modelling of Cardiac Electrophysiology Ian Kirby and Jim Hunter Department of Computing Science, University ofAberdeen, King's College, OLD ABERDEEN, AB9 2UB, Scotland, UK . t This paper explains and discusses an approach to the qualitative modelling of the electrophysiological changes which take place in the human heart as it beats either normally or abnormally ; it builds on work reported in [1] and [2] . Our ultimate objective is to interpret electrocardiograms (ECGs) in terms of the underlying cardiac electrophysiological processes that give rise to them. We will not cover all aspects of ECG interpretation only those that relate to the diagnosis of arrhythmias (disturbances of rhythm) . This excludes, for example, the interpretation of features of the ECG that enable cardiac enlargement and myocardial infarction (heart attack) to be diagnosed . Qualitative modelling [3] describes the state of the process or entity being modelled in non-numerical terms . A common approach, which we take here, is to define a formalism to represent the state of the system at a moment of time or over a interval, and to develop a set of constraints (or rules) which specify what transitions between states are possible . In subsequent sections we will review some necessary background knowledge in cardiac electrophysiology ; we will then develop our formalism for describing electrical state, and derive sets of riles which govern transitions between these states. The use of these rules on their own to develop a tree of behaviours from a given starting state leads to an explosive growth in the number of states explored . We show how knowledge about state duration, expressed qualitatively, can provide very significant constraints on this growth . We then dicuss how such models can be used for ECG interpretation. Simulation proves to be inappropriate in a domain where we are trying to represent continuous cyclic or quasi-cyclic activity, and we demonstrate the advantages of producing envisionments . We conclude by comparing the somewhat better known approach of KARDIO [4] with ours . Ibis is an updated version of the paper "Using Quantitative and Qualitative Constraints in Models of Cardiac Electrophysiology" which appeared in Artificial Intelligence in Medicine, 3 (1991) 41-61 . t phone: (+44 224) 272287 ; FAX: (+44 224 487048); email: ikk@cs . abdn.ac. uk, jhunter@cs.abdn.acuk