What is an artificial heart?
An artificial heart is a prosthetic device that can take on the function of a human heart when necessary. The function of an artificial heart can be modeled as a process similar to those developed in chemical engineering applications. Several components of the human body serve as the process system that can be analysed when an artificial heart is used.
How does the natural human heart work?
Working to understand the various complex mechanisms and processes within the human body can potentially be useful in inciting innovation in biomedical fields. Because such systems are so intricate, it is necessary to pare down some of these and find ways to isolate them for individual study. It is also important to understand the functionality of the natural human heart, and how an artificial replacement works to mimic this functionality.
The natural human heart is controlled via a complex process involving a natural electrical system of sorts. The heart’s electrical system works to maintain a steady heart rate which varies to meet the body’s physical demand for oxygen at different levels of activity. With the help of conducting cells within the heart, which carry this electrical signal, and muscle cells, which allow for expansion and contraction of the heart, the heartbeat can be controlled.
Other processes within the human body also work to control the heartbeat. The brain, for instance, is integral in sending signals to the heart when a faster or slower heartbeat is necessary. The brain essentially works to control the initial electrical signal to the heart, which is fired off by the heart’s sinoatrial node. Hormones (i.e. adrenaline) and several components of the nervous system can also have a key impact in how the human body works to alter heart rate.
Approaching the design: how can an optimal artificial heart be developed?
The purpose of this journal is to design and study an artificial mechanical heart that can physically replace the natural human heart. Over the course of the past few years, both external and internal artificial hearts have been developed for people who have suffered chronic heart failure and have been ineligible for a total heart transplant. However, many of these artificial hearts, especially the earliest prototypes, have almost never been implanted successfully. Most prototypes have worked to produce a constant flow of blood throughout the human body. Only very recently have scientists began to develop more complicated control systems that change the output blood flow rate based on an increase in human activity (i.e. during periods of exercise). The design proposed in this journal works to control the oxygen concentration within blood by manipulating the blood flow rate and respiration rate.
Both an optimal material design and vigorous process control mechanism need to be considered in the development of the artificial heart proposed. Biocompatible materials are essential when designing a device to be implanted into the human body. In early artificial heart designs, plastic compounds and titanium were often used. In more recent design efforts, scientists have begun to incorporate biological components, i.e. valves made from chemically treated animal tissues.
Why does it need control?
In terms of process control, an artificial heart can serve to regulate the average oxygen concentration within the body. Depending on the user’s level of activity, physical condition, or human mass, the control system will help to manipulate the volumetric flow rate of blood being pumped throughout the body and the level of rate of oxygen intake. Signals are sent to the mechanical heart and to the brain in order to control these variables.
Maintaining the oxygen concentration in the blood within the body is vital to support human life. Nearly all of the body’s activities, from brain function to elimination, are regulated by oxygen. Oxidation is the essential factor for metabolic function, digestion, and circulation. Thus the proposed mechanical heart must be able to efficiently pump flow through the body, as well as be able to be manipulated by the controller to maintain healthy blood oxygen levels. Heart disease is considered a major health problem internationally. Fabricating a mechanical heart will allow people to live longer lives and maintain increased levels of activity.
The major protein that aids in the transport of oxygen in the blood is hemoglobin. In normal arteries, hemoglobin is about 97% saturated with oxygen. There are about 15 grams of hemoglobin per 100 mL of blood. Each gram of hemoglobin can bind to 1.34 mL of oxygen. Thus, the normal concentration of oxygen in arterial blood is 2.45*10^(-4) g/mL. According to the American Journal of Cardiology, blood concentration below 90% (or 2.27*10^(-4) g/mL) is considered to be potentially dangerous and is thus the lower limit of this operation.
How it works.
A concentration sensor-transmitter measures the remaining concentration of oxygen within blood after the blood cells deposit the oxygen throughout the body. This transmitter is located right before the right atrium in which the oxygen-depleted blood enters. The electrical signal is sent to two locations. First, it is sent to the proposed mechanical controller, which changes the speed of the artificial heartbeat via a pump. This action manipulates the flow of blood, thus controlling the amount of oxygen that can be delivered to the body. Additionally, the electrical signal is sent to a novel transducer, in which the signal can be transformed into a neurological impulse that is sent to the brain. The brain functions as a natural control system that changes the breathing pattern in the lungs. This manipulates the flow rate of air, thereby controlling the oxygen concentration within the blood.
The measured control variable, the oxygen concentration, changes upon disturbances such as an increased or decreased level of activity. The feedback process described above works eliminate rapid changes in oxygen concentration and to approach a set point.
One potential disturbance variable to consider in this process system is variation in human activity. Increased physical activity, such as cardiovascular exercise, will result in depletion of oxygen within the blood. The change in oxygen saturation will cause a deviation from the set point and theoretically, a feedback loop should cause the mechanical heart to pump at a faster rate to achieve the set point.
