9 unit 6 Foundations of Perfusion Technology and Techniques (1).pptx

Perfusion Program Foundations of Perfusion Technology & Techniques What we will cover: Unit 6 - Describe the design characteristics of heat exchangers - Discuss the placement of heat exchangers in the CPB circuit Heat Exchange: Chemistry and Physics Heat Exchange: I know the basics, but what is...

Perfusion Program Foundations of Perfusion Technology & Techniques What we will cover: Unit 6 - Describe the design characteristics of heat exchangers - Discuss the placement of heat exchangers in the CPB circuit Heat Exchange: Chemistry and Physics Heat Exchange: I know the basics, but what is really going on? • The principles within the laws of physics that regulate the transfer of heat (energy) are similar to those which control the movement of gases • direction and rate of movement are determined by the gradient, the area of interaction, and the resistance to movement or transfer based on the properties of the materials involved Heat Exchange: Chemistry and Physics Temperature Effect on Gas Solubility • The solubility of gases is inversely proportional to the temperature • so gases are more soluble in colder temperatures and less so at warmer temperatures • *Why we care careful with rewarming temperature gradient • Le Chatelier's Principle (Henry Louis Le Châtelier circa 1899) • Also known as “the equilibrium law,” it describes the effect on equilibrium due to changes in temperature, pressure, volume, and so forth. • It states that when changes in the environment stress an equilibrium, the equilibrium will shift to relieve that stress. • Fourier’s Law (Jean-Baptiste Joseph Fourier, 1822) • The rate of heat transfer through a particular material is proportional to the temperature gradient, and the area of transfer. Heat Exchange: Chemistry and Physics Heat is transferred, ok. . . Tell me more about how. . . • Heat is transferred as kinetic energy from an area of higher temperature to an area of lower temperature by three modes known as conduction, convection, and radiation • Radiation involves the transfer of energy by the movement of charged particles within the atom as it is converted to electromagnetic radiation • Conduction involves the transfer of energy between two objects that are in physical contact, and the rate of transfer is determined by the thermal conductivity property of the material conducting the energy, or heat. * Blood touching a heat exchange filled with water* • Convection involves the movement of molecules within “fluids” (liquids or gases) through the properties of diffusion (random movement of individual particles or molecules), or by advection (related to bulk movement of fluids or currents) during which the *Transfer of heat from water, influencing the temp of the heat exchanger material itself* Heat Exchange: Chemistry and Physics Still transferring. . . . • In all modes of transfer, the energy moves from the area of higher temperature to lower temperature until equilibrium is reached. • In systems of extracorporeal support, this primarily involves the transfer of heat out of the blood into cooler water in the exchange system during a cooling phase and from warmer water back into the blood during active warming. • *How does each activity transfer heat? Heat Exchange: Chemistry and Physics Heat Transfer Rate • The exact amount of energy (as heat) that is transferred from water to blood within the oxygenator system can be accurately quantified using the laws of thermodynamics if one knows the specific heat of blood, the blood flow rate, and the temperature at the beginning and end of the transfer process • More important to design and manufacturing • For Perfusionist, its more important to have a firm understanding of the basic principles of heat exchange, and how they are affected by the patient and the local environment of the circuit • Highly conductive materials are chosen for use in circumstances where rapid heat transfer is desired, such as in CPB, whereas poorly conductive materials provide insulation by minimizing the transfer of heat • Assuming the material between the water and blood has high thermal conductivity, the rate of transfer depends on the temperature gradient and the area, which in moving fluids also includes the rate of flow and diameter of the channel Heat Exchange: Chemistry and Physics Heat transfer rate (cont) • In the clinical application, this means the larger the gradient between the water bath and the patient’s blood, the quicker we can warm or cool them • Because solubility of gases is directly affected by temperature, excessive rates of transfer can cause gas to come out of solution, causing gas emboli or organ dysfunction, particularly in the brain. • Excessive heating of the blood can cause direct damage to proteins and cellular elements. • Although the physics would permit very rapid exchange of heat, the biology limits the practical use of the principles Heat Exchange: Chemistry and Physics Heat Transfer: Area • In static systems, this is the actual surface area where the contact occurs for heat transfer, but in systems with fluid in motion, the situation is more complex • Closest to the site of diffusion, the greater the heat exchange gradient • The diameter and length of the conduit as well as the flow rates all affect the rate of transfer as well • If there is not sufficient time for equilibrium to be reached locally at the wall, then very little to no heat will be transferred to the flow as you move toward the center of the stream • efficiency in heat transfer is provided by the countercurrent mechanism as previously described (unit 5) • Manufacturers have taken all these factors into consideration in the design of the modern heat exchanger to maximize the efficiency of heat transfer with the smallest possible prime volume. Heat Exchange: Chemistry and Physics Heat Exchangers in Bypass: The forgotten stepchild? • Efficient, reliable, and relatively simplistic in function. • minimal direct interaction and impact on the blood other than the exchange of energy in the form of heat • Designs of the devices have changed very little over the years, and there is little if any difference in the efficiency of the many commercial options • Traditionally in longer-term support with ECMO, the heat exchanger was a separate unit through which the blood was directed after the oxygenator, allowing rewarming of the blood back to normothermia just prior to reinfusion into the patient to prevent loss of heat over time • The introduction of the PMP oxygenator and integration of the heat exchanger changed that • In most systems of integrated heat exchangers, the blood passes through the heat exchanger just prior to moving into the oxygenator compartment for gas exchange. Heat Exchange: Chemistry and Physics Heat Exchangers: details, details. . . • The structure of the heat exchanger component of the oxygenator complex consists of a highly conductive material, usually stainless steel or aluminum which allows rapid and efficient transfer of heat (high thermal conductivity), yet which will be inert to the fluids to which it is exposed • blood passage portion is usually coated with silicone or another polymer coating to minimize activation of coagulation and inflammation (typically same coating as the entire bypass circuit) • An Outside water source is provided and circulated through a heater–cooler unit with specific control of the temperature in a countercurrent direction from the passage of the blood • Energy transfer is from blood to the water during cooling phases and from water to blood during warming • It has been generally recommended that cooling and warming strategies should include a maximum temperature gradient between the patient’s core temperature and the arterial inflow temperature of *no more than 10°C, due to concerns over gas bubble formation and subsequent cerebral emboli due to the changes in gas solubility based on temperature  Wait. . . But you said before. . . Don’t worry, I’ll explain. . . Heat Exchange: Chemistry and Physics Heat Exchangers: more about quantum physics and bubble-nomics • Oxygen is more soluble in colder blood and will come out of solution as the blood is warmed, potentially forming bubbles if warmed too quickly. • Concerns during cooling that with higher gradients in temperature, gas will come out of solution when much cooler blood from the circuit mixes with still warm blood in the patient’s aorta, potentially leading to gas emboli to the central circulation and the brain. • While many perfusionists continue to use the 10° rule of thumb, others have reduced the maximum gradient to 4° to 6°. In a 2002 study, data suggested that perhaps even this was too great a gradient when compared to much slower warming (no more than a 2° gradient); more rapid warming was shown to potentially contribute to poorer neurologic outcomes based on detailed neuropsychological testing (although there was no difference in stroke rate between the groups)  There it is Heat Exchange: Chemistry and Physics Heat Exchangers: about what I said before. . . • On the basis of their findings, the authors cautioned practitioners on using rapid rewarming strategies and recommended considering lower acceptable gradients. • However, general concerns over the inherent risk to organ function from prolonged bypass times that might be required for much longer rewarming periods at the slower rate, particularly after deeper hypothermia, have led many to continue with the generally accepted 4° to 6° gradient. • It is also recommended to not allow the heat exchanger temperature to exceed 40°C–42°C because of concerns over the potential denaturation of proteins in the blood if exposed to more excessive heat, a concept recently validated with an in vitro study of temperature and blood protein denaturation Heat Exchange: Chemistry and Physics Heat Exchangers: Isolated, Integrated, Cardioplegia-ated. . . • Heat exchangers are used as a stand-alone device placed within an extracorporeal circuit, integrated within an oxygenator for use in CPB, and used in Cardioplegia circuits to delivered cold and warm cardioplegia per Surgical protocol. • Heat Exchangers are vital to the performance of cardiac surgery, as well as the use of ECMO in ICUs. • Heater-Cooler devices are used to circulate water through the devices to maintain or reach desired temperature. • These H/C devices contain heating elements, and often cooling condensers as well in an integrated single machine. • There are several commercially available Heat Exchange: Chemistry and Physics Heat Exchange: Chemistry and Physics