Pulse transit time measured by photoplethysmography improves the accuracy of heart rate as a surrogate measure of cardiac output, stroke volume and oxygen uptake in response to graded exercise

Heart rate (HR) is a valuable and widespread measure for physical training programs, although its description of conditioning is limited to the cardiac response to exercise. More comprehensive measures of exercise adaptation include cardiac output ((Q) over dot), stroke volume (SV) and oxygen uptake ((V) over dotO(2)), but these physiological parameters can be measured only with cumbersome equipment installed in clinical settings. In this work, we explore the ability of pulse transit time (PTT) to represent a valuable pairing with HR for indirectly estimating (Q) over dot, SV and (V) over dotO(2) non-invasively. PTT was measured as the time interval between the peak of the electrocardiographic (ECG) R-wave and the onset of the photoplethysmography (PPG) waveform at the periphery (i.e. fingertip) with a portable sensor. Fifteen healthy young subjects underwent a graded incremental cycling protocol after which HR and PTT were correlated with (Q) over dot, SV and (V) over dotO(2) using linear mixed models. The addition of PTT significantly improved the modeling of (Q) over dot, SV and (V) over dotO(2) at the individual level (R-1(2) = 0.419 for SV, 0.548 for (Q) over dot, and 0.771 for (V) over dotO(2)) compared to predictive models based solely on HR (R-1(2) = 0.379 for SV, 0.503 for (Q) over dot, and 0.745 for (V) over dotO(2)). While challenges in sensitivity and artifact rejection exist, combining PTT with HR holds potential for development of novel wearable sensors that provide exercise assessment largely superior to HR monitors

Chest Wearable Apparatus for Cuffless Continuous Blood Pressure Measurements Based on PPG and PCG Signals

n/

Clinically feasible model-based methods to guide cardiovascular fluid therapy in the ICU

Fluid therapy is one of the most commonly used clinical procedures employed in the ICU to treat circulatory shock. Approximately 30% of ICU patients receive a fluid therapy at some stage during their stay, with 20% of patients reviving it on the day of admission. By increasing the total volume in the circulation, clinicians aim to stimulate an increase in cardiac output, helping restore or maintain adequate organ and tissue perfusion. However, only ≈50% of patients receiving a fluid therapy will have also have the desired increase in cardiac output. Furthermore, excessive fluids have been strongly associated with worsened patient outcome and can negate the effects of earlier successful treatment. Therefore, knowledge of a patient’s fluid responsiveness, prior to administering of treatment is essential for safe treatment. Current, clinically used, indices of fluid responsiveness have a number of inherent limitations restricting their applicability or invalidating their use altogether. A recently developed model of the cardiovascular system showed a new index, model based stressed blood volume, to be a potential improvement over currently available indices, but required measurements from inside the cardiac chambers and central arteries for identification, which are not available in standard ICU care. This thesis develops a series of novel methods for estimating the required cardiovascular waveforms required for model identification from currently available clinical measurements. Thus, developing a clinically feasible model-based method to guide fluid therapy in the ICU. The first part of this work introduces the relevant physiology of the cardiovascular system is introduced along with the principles of the circulation driving flow to and from the heart. Next, a brief overview of current indices of fluid responsiveness is given, highlighting their advantages and limitations. Stressed blood volume, in the context of fluid responsiveness, is also introduced. The three-chamber lumped parameter model of the cardiovascular system is then introduced. Each component of the model and all model equations, including the equations governing model dynamics and equations for initial parameter estimation, are detailed. An initial study showing the implementation of the three chambered model is performed to highlight the clinical utility of identified parameters in assessing changes in patient condition. This study was performed on several porcine endotoxin experiments and provided the reference model parameters used to compare the final, clinically feasible method. A subsequent study is performed using the first method introduced to achieve clinical feasibility by removing the requirement of direct cardiac measurements by estimating left ventricle pressure and volume from aortic pressure. Because aortic pressure is also not available in standard ICU environments, the next section of work aimed to develop a method of estimating central pressure. A tube-load model of the arterial system was used to develop and test a novel method of estimating central pressure from commonly available peripheral pressure measurements, via an arterial transfer function, and nothing else. The transfer corresponding flow waveforms were then used, with an initial calibration measurement, to estimate stroke volume in the porcine endotoxin experiments. The final part of his thesis begins by consolidating all developed methods to identify the three-chambered model parameters from an anticipated clinically available subset of measurements from the porcine experiments. Finally, the model, and associated methods for estimating the required inputs, are used to assess fluid responsiveness in patient data obtained from the Christchurch hospital ICU in New Zealand. Overall, this thesis provides a method of identifying the parameters of model of the cardiovascular system model from a minimal, currently available, set of clinical measurements. Model-based stressed blood volume, identified using the three-chambered model, has the physiological basis, animal study validation, and now clinical feasibility, to be considered a potential diagnostic tool in the ICU. The intrinsic relation between stressed blood volume, perfusion pressure and fluid responsiveness means the methods proposed in this thesis may significantly improve a clinicians ability to safely and effective guide fluid therapy

Nichtinvasive Kreislaufüberwachung von Dialysepatienten mittels Pulstransitzeit

Die Pulstransitzeit (PTT) bezeichnet die Laufzeit einer Pulswelle zwischen proximalen und distalen Messpunkten. Oftmals wird die PTT aus Elektrokardiogramm und Pulsplethysmogramm bestimmt. Die PTT ist ein Indikator für Blutgefäßwandsteifigkeit. Mit zunehmendem Blutdruck oder steiferen Gefäßen steigt die Gefäßwandspannung, die Pulswelle ist schneller und die PTT sinkt. Ziel dieser Arbeit ist die Untersuchung, inwieweit sich die Kreislauffunktion von Dialysepatienten während der Dialyse im Hinblick auf problematische Blutdruckabfälle mittels der PTT überwachen lässt.The Pulse Transit Time (PTT) is the time it takes for a pulse wave to travel from proximal to distal measurement points. The electrocardiogram/pulse plethysmogram is often used as a proximal/distal pulse wave. The PTT is an indicator of blood vessel stiffness. The blood vessel wall tension increases with increasing blood pressure. As a result, the pulse wave moves faster and the PTT decreases. The aim of this thesis is to investigate to what extent the circulatory function of a patient can be monitored during dialysis using the PTT with regard to problematic blood pressure drops