# Autoregulation and Baroreflex Regulation in Mean Arterial Blood Pressure Maintenance¶

Kyle Iwamoto, Emily Schueppert, Daniel Shannon, Joseph Vallin
Final Project for CBE 30338, Spring 2017

## Introduction¶

The primary methods of blood pressure control are baroreflex and autoregulation. Baroreceptors, located primarily at the carotid sinus and the aortic arch, measure information regarding systemic arterial blood pressure. Baroreceptors can sense change in blood pressure through the expansion and contraction of the arterial walls in response to increased and decreased blood pressure, respectively (Klabunde). This information is sent to the brain stem, where the information is translated into signals that are sent through the parasympathetic (vagus) and sympathetic nervous systems to alter heart rate, venous volume, and arterial resistance (Batzel).The second mechanism of blood pressure control is autoregulation. Autoregulation refers to the process where local vasculature is altered via constriction or dilation to maintain stable blood flow to the desired organ (Batzel). Autoregulation control occurs simultaneously with baroreflex feedback, but acts in opposition to baroreflex control.

Blood pressure regulation is an essential process that occurs regularly when standing and even after traumatic events with significant blood loss (Klabunde). For example, immediately after standing the brain experiences a step decrease in blood flow and pressure. Decreased blood flow to organs, such as the brain, initiates the autoregulatpry response. Nearby blood vessels dilate to provide the brain with a constant blood flow, despite the overall decreased flow. The proportional action taken by autoregulation leads to a significant decrease in blood pressure. Decreased blood pressure leads to a contraction of the arterial walls, which decreases baroreceptor neural firing. Neurons in the brain stem sense the decrease in baroreceptor activity and the effects of autoregulation and respond by increasing sympathetic response and decreasing vagus response. The increase in sympathetic activity produces multiple effects. The resistance in arterioles and capillary beds increases as well as stroke volume due to increased venous volume. Both increased sympathetic and decreased vagus activity increase heart rate. The increased heart rate and stroke volume create an increased cardiac output. Increased vascular resistance and cardiac output help to recover the initial loss of blood pressure (Klabunde).

The baroreflex feedback mechanism is a highly studied control mechanism due to its medical relevance and its dynamic behavior. However, many models neglect the contribution of autoregulation to blood pressure regulation. This project seeks to quantify the effects of baroreflex and autoregulation control on blood pressure due to step changes in cardiac output using systemic arterial pressure models described by Burattini. Using Python simulation and bode plot analysis, the project explores the stability of the model and the influence of the magnitude of the effective baroreflex open-loop gain on the dynamics of the pressure.

## Theory¶

The model proposed by Burattini is shown in Figure 1.

## Conclusions¶

Baroreflex and autoregulation combine to regulate the effects of blood pressure drop due to decreases in cardiac output. Baroreflex control is characterized by minimal offset, but large oscillation, whereas autoregulation is characterized by large proportional control with little oscillation, but significant offset. When combined, the effects of baroreflex on blood pressure regulation are slightly more pronounced than those of autoregulation. Tuning the model provided by Burattini in addition to using the optimization algorithm yielded an optimal effective gain of 1.39, indicating that slightly more baroreflex control than autoregulation control provides the optimal tradeoff between blood pressure offset and instability.

Implementation of control algorithms indicated that proportional-derivative control provides the best fit for this biological control system. PID control eliminated the desired blood pressure offset in response to the cardiac output change, which biologically could lead to hypertension. Proportional only control provided an offset, but only stabilized blood pressure for values of the effective gain less than 1. Proportional-derivative control stabilized blood pressure over a larger range of effective gains, while also yielding a pressure offset proportional to the cardiac output decrease. Additionally, the implementation of a time delay led to increased oscillatory amplitudes, confirming that immediate feedback action by baroreflex and autoregulation are essential for effective blood pressure regulation.

Failure of baroreflex causes detrimental symptoms in patients. Baroreflex failure is commonly caused by injury or radiation to the throat. Baroreflex failure is characterized by surges of severe hypertension and tachycardia, a condition where heart rates increase over 30 bpm when changing from supine to standing positions (Ketch). Baroreflex failure is not as prevalent as its characteristic symptoms because it can be misdiagnosed for common diseases such as anxiety disorders to severe diseases such as pheochromocytoma, an adrenal tumor that causes lethal hypertension (Ketch). However, if properly diagnosed, baroreflex failure can be treated using pharmaceuticals with adrenoreceptor agonists can be used to reduce the frequency of blood pressure surges. Current medical research is focused on the implications of a bionic baroreflex system (Ketch).

## References¶

Baroreceptor human diagram. Digital image. Alila Medical Media. Alila Medical Media, 2017. Web. 30 Apr. 2017.

Batzel, Jerry J. Cardiovasular and Respiratory Systems: Modeling, Analysis, and Control. Philadelphia: Society for Industrial and Applied Mathematics, 2007. Print.

Burattini, Roberto, Piet Borgdorff, and Nico Westerhof. "The Baroreflex Is Counteracted by Autoregulation, Thereby Preventing Circulatory Instability." Experimental Physiology 89.4 (2004): 397-405. The Physiological Society, 6 May 2004. Web. 19 Apr. 2017.

Burattini, Roberto, Piet Borgdorff, David R. Gross, Bruna Baiocco, and Nicolaas Westerhof. Systemic Autoregulation Counteracts the Carotid Baroreflex. Publication. 1st ed. Vol. 38. Ancona, Italy: U of Ancona, 1991. Print. IEEE Transactions on Biomedical Engineering.

Kantor, Jeffery, PhD. Baroreflex as a Linear Control System. Jupyter CBE 30338 Github. Jeffery Kantor, 2017. Web. 18 Apr. 2017.

Ketch, Terry, Italo Biaggioni, Rose Marie Robertson, and David Robertson. "Four Faces of Baroreflex Failure." American Heart Association. American Heart Association, 13 Mar. 2002. Web. 3 May 2017.

Klabunde, Richard E., PhD. "Systemic Vascular Resistance." Cardiovascular Physiology Concepts, Richard E Klabunde PhD. Marian University College of Osteopathic Medicine, 6 Dec. 2016. Web. 19 Apr. 2017.