Student: Jorge Mariscal-Harana
1st supervisor: Jordi Alastruey, King’s College London
2nd supervisor: Spencer Sherwin, Imperial College London
3rd supervisor: Peter Charlton
The blood pressure (BP) waveform carries valuable information for the diagnosis and treatment of cardiovascular disease and plays an important role in conditions such as hypertension. The BP waveform is produced by the propagation of pulse waves, which distend and contract arteries (eg they produce the pulse that we can feel on our wrist). As a result, BP changes in time and space throughout the cardiovascular system.
Knowledge of the BP waveform in systemic or pulmonary arteries proximal to the heart is clinically more relevant than in peripheral vessels, because the left and right ventricles are directly exposed to proximal pressures. However, direct proximal BP measurements are only possible invasively using pressure-sensing wires or catheters advanced from a more peripheral vessel such as the femoral or brachial artery in the systemic circulation. On the other hand, magnetic resonance imaging (MRI) provides non-invasive, accurate measurements of proximal flow waveforms and, as both BP and flow waveforms are generated by the propagation of the same pulse wave, it is theoretically possible to calculate proximal BP waveforms in the aorta from the following non-invasive data: a peripheral BP waveform measured using applanation tonometry (eg at the carotid artery), and arterial geometry, proximal flow waveforms and luminal displacements (distensibility) measured using MRI. This calculation can be achieved using our existing nonlinear 1-D code and pulse wave analysis tools[1]. In large pulmonary arteries, non-invasive calculation of ‘absolute’ proximal BP is not feasible since non-invasive peripheral measures of BP are not possible. However, it is theoretically possible to calculate relative changes in proximal BP from MRI geometry, flow and distensibility data.
We have previously shown the ability of the nonlinear 1-D formulation to capture the main features of pulse waveforms in the systemic circulation with reasonable accuracy and computational cost by comparison against in vivo (rabbit)[2], experimental[3] and 3-D numerical data[4]. In all these studies we were able to directly measure all the properties of the 1-D model. However, this is more challenging in the clinic, where some model parameters such as the mechanical properties of arteries cannot be directly measured and have to be indirectly estimated. Moreover, our existing code does not have a friendly user interface and is difficult to set up in non-Linux environments, which limits its applicability by the clinical community.
The aim of this project is to develop a 1-D model methodology that integrates patient-specific MRI and tonometry data to calculate the BP waveform at any arterial site in the systemic circulation that can be MR imaged. We will also investigate how to integrate MRI data only to calculate relative changes in BP in large pulmonary arteries. Our methodology will be developed and tested using, first synthetic BP and flow waveforms generated numerically at the same arterial sites where accurate clinical measurements are feasible, and then using clinical measurements acquired in volunteers and patients. The numerical approach will allow us to investigate the effect that errors in the clinical data have on the calculated BP. We will verify our theoretical results using invasive measurements of BP waveforms in patients.
References:
[1]. Alastruey, Parker, Sherwin (2012). Arterial pulse wave haemodynamics. In Anderson S (Ed.), 11th International Conference on Pressure Surges, Chapter 7, 401–442 (ISBN: 978-1-85598-133-1).
[2]. Alastruey, Hunt, Weinberg (2009). Modelling pulse wave propagation in the rabbit systemic circulation to assess the effects of altered nitric oxide synthesis. J Biomech 42:2116–2123, 2009.
[3]. Alastruey, Sherwin et al. (2011). Pulse wave propagation in a model human arterial network: Assessment of 1-D visco-elastic simulations against in vitro measurements. J Biomech 44:2250–2258, 2011.
[4]. Xiao, Alastruey, Figueroa (2014). A systematic comparison between 1-D and 3-D hemodynamics in compliant arterial models. Int J Numer Meth Biomed Eng 30:204–231.