Taoufik Wassar

Reduced-order modelling of transient flow in transmission lines using distributed lumped parameters

Abstract Developed in this paper are mathematical models capturing the one-dimensional underdamped dynamics of confined fluid flow within cylindrical transmission lines. The resulting models are rational transfer functions with coefficients that are explicit functions of the fluid properties and line geometry. Unlike a traditional lumped-parameter approach, the accuracy of the fluid resonant frequencies predicted by the proposed models is precise and not a function of transmission line axial discretisation. Therefore, model order (complexity) is solely a function of the number of desired modes, which in turn influences pressure and flow predictions. The results are applicable to both laminar and turbulent flow. To develop the models, a distributed lumped-parameter approach is employed. Specifically, a quasi-steady state friction approximation is used within the governing partial differential equations. The solution to the linearised ordinary differential equations produces three transcendent transfer functions that are approximated using finite-order rational transfer functions. The parameters of resulting transfer functions are then modified to capture the second-order effects. A fluid power design example using the proposed model is provided to illustrate the utility of these models.

Physician-Directed Versus Computerized Closed-Loop Control of Blood Pressure Using Phenylephrine in a Swine Model

BACKGROUND: Vasopressors provide a rapid and effective approach to correct hypotension in the perioperative setting. Our group developed a closed-loop control (CLC) system that titrates phenylephrine (PHP) based on the mean arterial pressure (MAP) during general anesthesia. As a means of evaluating system competence, we compared the performance of the automated CLC with physicians. We hypothesized that our CLC algorithm more effectively maintains blood pressure at a specified target with less blood pressure variability and reduces the dose of PHP required. METHODS: In a crossover study design, 6 swine under general anesthesia were subjected to a normovolemic hypotensive challenge induced by sodium nitroprusside. The physicians (MD) manually changed the PHP infusion rate, and the CLC system performed this task autonomously, adjusted every 3 seconds to achieve a predetermined MAP. RESULTS: The CLC maintained MAP within 5 mm Hg of the target for (mean ± standard deviation) 93.5% ± 3.9% of the time versus 72.4% ± 26.8% for the MD treatment (P = .054). The mean (standard deviation) percentage of time that the CLC and MD interventions were above target range was 2.1% ± 3.3% and 25.8% ± 27.4% (P = .06), respectively. Control statistics, performance error, median performance error, and median absolute performance error were not different between CLC and MD interventions. PHP infusion rate adjustments by the physician were performed 12 to 80 times in individual studies over a 60-minute period. The total dose of PHP used was not different between the 2 interventions. CONCLUSIONS: The CLC system performed as well as an anesthesiologist totally focused on MAP control by infusing PHP. Computerized CLC infusion of PHP provided tight blood pressure control under conditions of experimental vasodilation.

Automatic Control of Arterial Pressure for Hypotensive Patients using Phenylephrine

Abstract Developed in this paper is an automated closed-loop system that regulates the target blood pressure and maintains haemodynamic stability in hypotensive patients using the vasopressor drug phenylephrine. First, experimental studies are conducted on several healthy swine to identify dynamic mathematical models that quantify and predict blood pressure response to infusion of vasopressors. A first-order plus time-delay model structure has been selected to capture mean arterial pressure (MAP) response of patient’s subject to drug injection. Intra- and inter-patient variabilities of the model parameters have been identified and characterized. Then, an anti-windup proportional integral controller is designed, taking patient response variation in account. A nonlinear stochastic simulation environment has been developed to investigate the controller under diverse scenarios. Finally, automatic control of blood pressure is applied for the treatment of eight anesthetized swine subjected to hypotension induced by standard haemorrhage, spinal cord injury, and vasodilator injection. Results from clinical evaluations show that the proposed automated closed-loop control system is able to keep MAP near target and its performance is superior to that of manual control of infusion.

Model-based design and analysis of a subsea high integrity pressure protection system (HIPPS)

Developed in this paper is a model-based procedure for the design and analysis of a subsea High Integrity Pressure Protection System (HIPPS). Offering an alternative to the conventional design requiring the use of pipelines rated to the well shut-in pressure, the installation of HIPPS with low pressure-rated pipelines downstream of its location enables considerable savings in the project CAPEX making the development of high-pressure-high-temperature (HPHT) wells with a long tieback cost-effective without compromising the safety. The proposed design procedure is based on a low-dimensional two-phase flow transient model to evaluate the effect of the HIPPS on the subsea architecture. A case study demonstrates that the installation of HIPPS can result in savings up to 42% of the total pipelines procurement cost if compared to the conventional approach.

Heart assist devices: Modeling and diagnostics

In this paper, adaptive stead-state models are presented for axial flow-pumps used as heart assist devices. These models predict flow rate and power consumption of the pump based on the pressure differential (head) and impeller speed. The developed models are identified using system identification techniques on data obtained from a mock circulatory loop. The mock circulatory experiments include physiologic conditions ranging from a healthy heart function to heart failure. The online adaptive nature of these models is used to estimate effective blood viscosity in real-time. Additional mock circulatory loop experiments are performed to emulate flow obstruction at the pump outlet. Results show that the coefficients from the adapted models can be used to detect, identify and estimate the fault.

Automatic Control of Mean Arterial Pressure during Trauma Resuscitation using Closed-Loop Vasopressor Therapy

Hemodynamic stabilization of combat casualties with hemorrhagic shock often requires fluid resuscitation. However, rapid acting vasopressors are being used with increasing frequency to help maintain adequate perfusion of the brain and other vital organs, especially when hemodynamic stability is not maintained despite infusion of fluids. In this paper, a computerized decision support and fully autonomous closed-loop system to regulate the target blood pressure and to maintain hemodynamic stability is proposed. Two closed-loop algorithms using phenylephrine are designed and examined: anti-windup proportional integral control and adaptive internal model control. For analysis, design and evaluation dynamic mathematical models are identified to quantify mean arterial pressure response to phenylephrine using animal experiment data. A simple first-order time-delayed model is proposed. The controllers are first evaluated in a simulation environment, then implemented and validated in several animal experimental studies. Automatic control of blood pressure with anti-windup proportional integral control approach is used for the treatment of 15 anesthetized swine subjected to hypotension induced by standard hemorrhage, spinal cord injury, and sodium nitroprusside. From simulations and experimental responses it is found that the proposed automatic closedloop control systems keeps mean arterial pressure near target.

Modeling and Diagnostics of Heart Assist Devices

Presented are online adaptive models for ventricular assist devices (VADs). Such devices are used to assist failing hearts or in the case considered here to create a total artificial heart. Adaptive models are developed to estimate cardiac output (CO) and power consumption of the VAD. These parameters are critical to physicians during patient care as well as in diagnosing VAD operation. The online adaptive nature of these models will be used to estimate effective blood viscosity in real-time and to create a mechanism whereby specific VAD diagnostics, important to robust CO delivery, can be identified, isolated and estimated. The experiments conducted were ex-vivo in a mock circulation loop in which the precise nature of the working fluid properties can be controlled and measured.Copyright