During haemodialysis (HD) sessions patients undergo alterations in the extracellular environment mostly concerning plasma electrolyte concentrations pH and volume together with a modification of sympathovagal sense of balance. the same environmental changes. After an overview on how the computational approach has been used in the past to investigate the effect of HD therapy on cardiac electrophysiology the aim of this work has been to assess the current state of the art in human atrial AP models with respect to the MK-1775 HD context. All the published human atrial AP models have been considered and tested for electrolytes volume MK-1775 changes and different acetylcholine concentrations. Most of them proved to be reliable for single modifications but all of them showed some drawbacks. Therefore there is room for a new human atrial AP model hopefully able to physiologically reproduce all the HD-related effects. At the moment work is still in progress in this specific field. 1 Introduction In the last fifteen years the increasing interest towards atrial electrophysiology and atrial fibrillation (AF) together with a greater MK-1775 availability of experimental data led to remarkable developments in human atrial action potential (AP) models [1-6]. As a matter of fact cardiac computational modeling constitutes an efficient tool to investigate the ionic mechanisms involved at cell level and has already been used in a variety of clinical contexts linking patient manifestations to the underlying electrophysiological mechanisms thus providing useful insights into different atrial pathologies including AF especially whenever experimental measurements were lacking or unavailable [6-15]. Haemodialysis (HD) therapy represents a unique model to testin vivoin vivoin vivoextracellular fluid is the interstitial fluid rather than the blood. Therefore it could be questioned whether the plasma electrolyte concentrations are a reliable MK-1775 estimate of the interstitial ones even if this is usually accepted. Indeed the distribution of free ions between vascular and interstitial compartments has been reported to agree with Donnan theory which predicts a theoretical ratio between interstitial and plasma concentrations very close to 1 [32]. 3 Atrial Cell Modeling: Materials and Methods 3.1 Computational Models of Human Atrial AP Starting from the first two human atrial cell models (Courtemanche et al. [1]; Nygren et al. [2]) both published in 1998 four more have been released in the last few years (Maleckar et al. 2009 [3]; Koivum?ki et al. 2011 [4]; Grandi et al. 2011 [5]; Colman et al. 2013 [6]). Hereafter the six models will be referred to using the initial letter of the first and last authors (i.e. CN NG MT KT GB and CZ resp.). All models consist of a set of ordinary differential equations each one representing a specific dynamic process occurring in the cell and the number of equations is related to their complexity: the first models are very simple compared to the most recent ones where a more detailed description of Ca2+ handling and cell compartments is included (see Table 1). Moreover the different parameters and ionic current formulations lead to distinct AP morphologies and properties for example AP duration (APD) and CaT duration (CaTD). Table 1 The human atrial AP models considered in this study and their main properties. Since 1998 several papers comparing atrial model performances have been published mainly concerning CN and NG models which for many years have been the only ones available [33-39]. The two most recent reviews [38 39 compared all models except CZ considering simulations from single cell to whole heart and including both physiological Egf and MK-1775 pathological conditions thus assessing the current state of the art in atrial computational modeling. Therefore the comparison of the peculiar properties of these atrial models exceeds the purpose of this work which rather aims to investigate the acute effects of HD therapy on atrial electrophysiology. The CN and NG models are almost based on the same MK-1775 human atrial data and they share most of the transmembrane ionic current formulations: however CN is developed from the guinea pig ventricular model by Luo and Rudy [40] while NG is usually developed from the atrial rabbit model by Lindblad et al. [41]. The main differences between the two models are related to Ca2+-handling and the CaT is much shorter and with a larger amplitude in NG. As a result their AP shapes are quite different: a spike-and-dome AP for CN and a more triangular one for NG (see Figure 1 pink and blue traces). The MT and KT models are subsequent extensions of NG: the main changes for MT are new formulations for the transient.