Physics of a Total Artificial Heart (TAH)

A Change of Heart

Abstract

 The essential part of the circulatory system is the heart. In other words, any organism cannot live without this organ properly functioning. The duties and mechanisms of these functions must be performed correctly or else there could be potential serious consequences to the rest of the body. When the human heart has failing left and right ventricles, then they can be replaced by a temporary Total Artificial Heart (TAH). It then becomes a critical point to make sure that this machine entering the body can perform safely and in a healthy manner that will not put the rest of the internal organs at risk for damage or failure. Through physics concepts like, observation of Ohm’s Law, pressure, flow rate, flow rate-pressure relationship, and Boyles Law; surgeons and medical experts are able to then observe, protect, and coordinate the adaptability of the temporary Total Artificial Heart with each of the individual patient’s lifestyle. Similarly, to any other kind of transplant of a foreign object into the body, there can be negative outcomes or side effects that come along, but this gives patients another option and more time. This has become another milestone within the medical world and medical history due to the fact that those who are on the verge of dying of biventricular heart failure no longer need to risk and wait to have a transplant of a real full heart but instead can choose to receive the TAH.

           Main Content

The Heart:

 As stated previously, the heart is one of the most vital organs within the human body. The reason that this muscle pumping organ is so thoroughly studied and cautiously observed is because any type of abnormality, or issue that may come up will then highly affect the entire rest of the body This organ is composed of a unique kind of muscle, referred to as a cardiac muscle. This special kind of muscle is able to perform its own rhythmic contractions and pump blood (Monster, 2017). These contractions are able to occur through the help of the heart’s sinoatrial node. This part of the heart sends out a wave of electricity to make the heart contract (Monster, 2017). The heart is also composed of various veins and arteries, and this can be shown in better detail in Figure 1.  The figure shows veins in blue and arteries in red. The veins carry deoxygenated blood cells and the arteries carry oxygenated blood cells. It is necessary to have both types of blood cells, which will be further explained later. Something that is important to understand, is studying the human heart, are the duties that it has to the human body. The hearts main job is to distribute nutrients and oxygenated blood to the other internal organs and tissues, while also removing wastes that cycles through the bloodstream (Dale, 2009). With this in mind, the heart then goes through something called the cardiac cycle. This cycle is how the blood runs through the heart and into the rest of the body. The cardiac cycle starts by deoxygenated blood coming into the right atrium, then to the right ventricle, and then into the lungs. From there the oxygen carried form the cells are delivered while releasing the waste of carbon dioxide. After that the blood then runs through the left chambers (Miranda, 2018). An additional way to understand and view the heart and the circulatory system can be viewing it is as an electrical circuit (Laurenson, 2011). A voltage-current type of behavior is considered to be similar to a pressure-flow behavior. Through physics, the voltage-current relationship is shown in Ohm’s Law, shown below.

 In this equation, the ‘ΔV’ is known as the change in voltage, which is measured in units of volts (V). The ‘I’ is known as current and is measured in amps (A). The ‘R’ is resistance and is measured in ohms (Ω). Similarly, with pressure-flowrate relationship is the equation below (Laurenson, 2011).

 Although slightly different, both equations still involve resistance (R). The only difference is that it involves change in pressure (ΔP) which is measured in pascals (Pa) and flowrate (Q) which is measured in meters cubed per seconds (m^3/s). This is showing that the circulatory system is a basic resistive electrical circuit (Laurenson, 2011).

Total Artificial Heart:

 A Total Artificial Heart (TAH), as shown in Figure 2, is where surgeons replace the left and right ventricles with either a biocompatible plastic or another man-made material (Laurenson, 2011). The biocompatible plastic that is typically used is called, “Angioflex”, and it is composed up of special polyurethane and titanium (Avula, 2013). The biocompatible plastic ventricles are the connected by Velcro. The reason that Velcro is used is so that the doctor is able to place the ventricles in a way that is comfortable and appropriate for the patient (Syncardia, 2018). Those two ventricles are also then connected two tubes, containing specific coils, exiting the body to then be connected to an external power source that helps the artificial valves to contract and ensure blood flow. The power source is placed in a bag and carried around anywhere the patient goes because without the power source the patient will have a heart that cannot contract and the patient will die. As shown in Figure 3, there are various part the temporary Total Artificial Heart. Something that medical experts did while creating these parts was establishing the material to be gentle yet durable to help this power organ sustain its normal functioning properties. Although this is helpful, it does not certify a patient to go without being watched or observed by a surgeon or medical expert.

Flow Rate:

 As this new approach to keeping a stable heart transforms patients’ lives by giving them a second chance to live, there are factors regarding the TAH that need to be considered tomato sure that the other organs in the body are kept safe. Things like blood flow, are a part of which should be carefully examined (Green, 2011). Blood flow of the heart can be observed through an ultrasound. As the heart contracts and blood is passing in and out of the heart, the blood flow monitor emits sound waves at high frequencies. Not only can these high frequencies be observed through the ultrasound, but it can also be understood through the mathematics of physics equations. One of the most crucial things is that the temporary Total Artificial Heart must be able to do it adapt its blood flow to the individual’s activity. For instance, if the patient is jogging the lungs will require more oxygenated blood cells, meaning the blood flow must be quicker. In other cases, when the patient is resting and the blood flow will slow down because no physical activity is being done and the lungs are not in need of an extra amount of oxygen. Through the mathematics of physics, using the following equations of flow rate can help to calculate and understand the blood flow (Hewlett, 2013).

 Flow rate (Q) is calculated in meters cubed per second (m^3/s). This first equation is for the generic flow rate, while the second equation is specific for flow going through a cylindrical area, such as a vein or artery. The top equation is expressing flow rate as being equal to the area (A), in units of meters squared (m^2), multiplied by the average velocity (v), which is in units of m/s. In some occasions surgeons and other medical experts are in need of understanding more information about the blood flow. This would then require breaking down the variables and rearranging the formula. This means that multiplying the area by the distance, measured in meters (m), being traveled by the blood while overall being divide by the time (t), units of seconds (s or sec), can also produce the flow rate. In this situation, the bottom equation is more specific for examining the blood flow through the arteries, veins, and various chambers of the heart. This would also be more beneficial for calculating the flow rate of other areas the heart is directly connected to and that would have immediate impact from the change in ventricles. Similar to the top equation, ‘Q’ is the same variable and the only difference is the substitution of the specific formula for the area (A) along with ‘V’ is referring to volume, which is in units of meters cubed per second (m^3/s). Instead of just calculating area through the width multiplied by length, the cylindrical area of arteries and veins have an area formula of pi multiplied by the radius, in units of meters (m), which is then squared.

Pressure-Volume Relationship:

 Another area of which surgeons and other medical experts should be cautious of while implanting the temporary Total Artificial Heart is amount of work being done by the coils that are within the biocompatible tubes connecting the artificial ventricles to the external power source. The reason being, is that through work heat can be created and if heat is being emitted from the artificial ventricles the surrounding tissue could experience damage or more fatal affects. Another point where work, or the heat being created through work, should be cautiously examined is the point where the coils exit the body to then connect to the external power source. If the coils begin to heat up, they are covered by biocompatible tubes which can prevent any initial damages. The issue is if the coils heat up too much then the biocompatible tubes will also heat up and could not only cause the surrounding internal organs to have minimal to severe damage but could cause an infection to the wound where the tubes are exiting the body. A way to express this through physics is by the pressure-volume relationship, also known as Boyles Law (Schaldach, 1999). The work can be calculated through physics calculations, as shown below. To be more specific, for better understanding, the work being done revolves around the pressure-volume relationship. This will give more specific information for medical experts to then possibly adjust the power source connecting to the coils within the biocompatible material tubes. A visual of the pressure-volume relationship for different hearts in varying conditions can be shown in four graphs in Figure 4.

  In the generic work formula above, the variable ‘F’ stands for Force and ‘d’ stands for distance. Work is in units of joules (J), force is in units of Newtons (N), and distance is measured in units of meters (m). The reason that work is measured in joules is because one joule is equivalent to a Newton over a meter (N/m). Now to review the more specific work equations dealing with the pressure-volume relationship. In both of the equations, ‘P’ is the variable for pressure and it is calculated in units of Pascals (Pa). The other variables are r, ro,  ri, l, σ, σspt, and dspt. The ‘r’, ‘ro’, and ‘ ri’ represents various radii measured in meters (m). The ‘ro’ variable represents the outside radius from one ventricle to the other, whereas ‘ ri’  represents the internal radius of the ventricle. The variable of ‘l’ represents the wall circumference of the ventricle, which is measured in meters squared (m^2). The variables σ, and σspt relate to the muscle stress upon the heart or septum and calculated as an average stress. These variables are unit-less. Then finally there is dspt , and this variable and this is for the wall thickness and more specifically the wall thickness of the septum which is also calculated in meters (m). The reason that there are two equations needed are because if there is a concern for multiple pressures within the same system, or understanding the stress of which the septum or another organ is enduring through the work being done by the temporary Total Artificial Heart parts then there can be adjustments made through the understanding of these calculations. On the other hand, they can be understood more thoroughly through the understanding of the graphs, in Figure 4. Although the graphs all represent differing hearts, medical experts can read all of them in the same way. Though difficult to see in Figure 4, Figure 5 shows a more in-depth explanation of what to see on the pressure-volume graphs of a heart. Figure 5 can prove that the external power source connected to the left and right ventricles, send a signal of pressure to induce a contraction. After the ventricles have contracted, and ejected the majority of the blood to flow to its designated location, the ventricles then will want to relax. During this period of relaxation, the pressure is declining and the volume is rising. This then starts the entire cardiac cycle over again with another signal from the external power source to then contract and eject the blood. As previously stated, this is very crucial for medical experts to understand and program their patients external power sources to signal the correct amount of pressure and work for the ventricles to be expected of. Not only should they be programmed correctly but they need to have the adaptability to then adjust when a patient is going from inactive to active because these different physicality’s will require different levels of work. If the temporary Total Artificial Heart cannot keep up with the lifestyle of the owner then there could be possible fatal issues because the heart is not able to circulate the blood well enough where the body can continue with the change from inactivity to activity.

Conclusion:

 The heart is a complex organ within the human body. For some, it can be difficult to understand, but if one understands the physics concept of a voltage-current relationship then they can easily interpret what the heart and the circulatory system. The heart also is one of the most critical parts of the human body, and its role within the circulatory system is incredibly significant and also essential for life to be possible for all organisms. Thankfully, the temporary Total Artificial Heart (TAH) has created many new opportunities for numerous individuals who had been diagnosed with biventricular failure. This new machine gives patients a new left and a new right ventricle made from biocompatible materials, which are then connected through coils within biocompatible tubes to an external power source with multiple backup generators to ensure their heart never stops contracting and pumping blood through their system. The external power sources’ duties are to keep track of the heart and to adjust accordingly to the activity of the individual. This means that the external power source is sending signals to the artificial ventricles when to contract. Through this option that patients now have to receive new ventricles, there has been phenomenal long-term success, but there are many day to day factors that have to do with this machine that should be thoroughly and cautiously watched or addressed immediately before leaving a medical expert’s presence. These factors include the flow rate of blood, the pressure- volume relationship within the ventricles, the work being done by the external power source connected to the coils within the biocompatible tubes connected to the left and right artificial ventricles. These can all be observed through various physics equations shown throughout the various sub-sections of this paper, while also being observed through the high-frequency sound waves of an ultrasound, or an echocardiogram (Siegel, 2003). This has been an immense impact of transformation within cardiology and for those patients who could not wait for entire heart donor to be ready, but can live through just some pieces of biocompatible machinery parts.

 Through this research I have done, I quickly learned that there hasn’t always been such an option of safe, temporary health. Although there are various types of alternative heart pieces for the different issues towards the heart, this one has shown clear and safe results (Cook, 2015).  Not only have I discovered its credibility but I have now understood the incredible need for this machine and opportunity. The number of patients who are in need of a heart transplant outweighs, by an enormous amount, the number of patients who actually receive a full heart transplant (Cook, 2015). By continuing to grow and develop new options with this machinery and other temporary Total Artificial Heart parts, can give more people a chance and give them up to a year more of time to wait until an official full heart donor arrives.

 The Total Artificial Heart has now been used, clinically, for thirty-five years and has made significant headway within the medical world through those years (Cook, 2015). This has now become a point in which a patient looking for an immediate source of help can achieve that help. Instead of patients not being able to wait until the next heart donor comes along, if they suffer from biventricular heart failure they can obtain a Total Artificial Heart temporarily, typically up to a year’s worth of time, until the appropriate heart donor comes through the door. As time goes by, medical experts and specialist will be able to find better ways of creating the temporary Total Artificial Heart. For instance, by adjusting the size in proportion to the chest cavity that is in need of two new, viable ventricles; even more than they have created today. This means that not only will adults be able to have the appropriate option of this transition point between their two ventricles failing and a heart donor, but young children and babies will be able to have this option as well. This is not to say that children and babies haven’t been given this chance yet, but there are just heavier negative outcomes due to the fact of their size and the more work needs to be done by the external power source due to quicker heart rates. Similar to all medical treatments or implants, there are always side effects that can appear immediately or over time for anyone who receives this. Medical experts can begin to analyze their statistics of side effects from receiving the temporary Total Artificial Heart and find new ways to decrease those side effects. This could include the decrease of infection where either the biocompatible tubes are exiting the body to connect to the external power source or where the ventricles are then place in the chest. Medical experts could also be more aware and attempt fix the results of patients who just reject the heart completely, which then would mean taking the temporary Total Artificial Heart out of their chest and continuing to try to live through the failing left and right ventricles, which wouldn’t give them more time to wait for a heart donor.  Although this isn’t a permanent replacement for those who receive it, it sure is a safer choice than just staying with the fatal heart pieces they currently have. This is a blessing for all those in cardiology and their patients that they serve.

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References

1. Green M. How Does an Artificial Heart Work? How does GPS work?| Explore. [accessed 2018 Nov 10]. http://www.physics.org/article-questions.asp?id=74

2. Avula R. How Things Work: The Artificial Heart. The Tartan. 2013 Jan 21 [accessed 2018 Nov 10]. https://thetartan.org/2013/1/21/scitech/artificialheart

3. Unknown. Temporary Total Artificial Heart. SynCardia. 2018 Apr 3 [accessed 2018 Nov 11]. https://syncardia.com/patients/home/?gclid=EAIaIQobChMIktizs9HM3gIViK_sCh3EIQ2hEAAYASAAEgJjK_D_BwE

4. Laurenson E. Building a Heart: The Function and Mechanics. Yale National Initiative.

 2011 [accessed 2018 Nov 11]. http://teachers.yale.edu/curriculum/viewer/initiative_11.07.04_u

5. Siegel PB. Putting Your Heart Into Physics. The American Journal of Physics. 2003 Aug 8 [accessed 2018 Nov 10]. https://pdfs.semanticscholar.org/3924/4b5fff502d9bcbc1c076e9a66429a32e26d6.pdf

6. Cook JA, Shah KB, Quader MA. The Total Artificial Heart. NCBI. 2015 Dec 7 [accessed 2018 Nov 11]. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4703693/

7. Dale B. The Heart and How It Functions. A Guide to Kentucky’s Mammoth Cave National Park. 2009 Oct 19 [accessed 2018 Nov 11]. https://www.nationalgeographic.com/science/health-and-human-body/human-body/heart/

8. Miranda GM. Structure and Function of the Heart. News-Medical.net. 2018 Aug 23 [accessed 2018 Nov 11]. https://www.news-medical.net/health/Structure-and-Function-of-the-Heart.aspx

9. Monster F. Heart. World History | FactMonster. 2017 [accessed 2018 Nov 11]. https://www.factmonster.com/dk/encyclopedia/human-body/heart

10. Hewlett W, Hewlett F. Flow Rate and It’s Relation to Velocity. OpenStax CNX. 2013 Sep 9 [accessed 2018 Nov 12]. https://cnx.org/contents/g9Jw9bqg@4/Flow-Rate-and-Its-Relation-to-Velocity

11. Schaldach M. Physics of Heart Circulation. MSBT. 1999 Oct [accessed 2018 Nov 11]. http://www.msbt.nat.fau.de/PBMR/documents/199904050475.pdf