8 unit 5 Foundations of Perfusion Technology and Techniques (1).pptx

Perfusion Program Foundations of Perfusion Technology & Techniques What we will cover: Unit 5 - Describe the characteristics of an ideal oxygenator Discuss the historical development of oxygenator techniques Describe different types of membrane oxygenators Discuss the oxygen characteristics of d...

Perfusion Program Foundations of Perfusion Technology & Techniques What we will cover: Unit 5 - Describe the characteristics of an ideal oxygenator Discuss the historical development of oxygenator techniques Describe different types of membrane oxygenators Discuss the oxygen characteristics of different membrane oxygenators Oxygenators in the Extracorporeal circuit Oxygenators in the Extracorporeal circuit The Ideal Oxygenator. . . • a method of gas exchange for oxygen and carbon dioxide, and a mechanism for temperature control • referred to as the “oxygenator,” but we must recognize that it is responsible for the movement of both oxygen in, as well as carbon dioxide out. • Efficient flow pathway • Low pressure drop across the membrane • Maximized surface area for integrated gas and heat exchange Oxygenators in the Extracorporeal circuit Fick’s First Law of Diffusion (Adolf Fick, 1855): • J represents the diffusion flux or amount of substance (e.g., O2) moved per unit area, per unit time. • D, the diffusion coefficient, is a constant for the particular barrier, based on its composition • (Difussivity: preceding negative sign simply makes the flux J positive when the movement is down the concentration gradient) • Substance concentration is represented by φ and the length by x • Fick’s first law of diffusion tells us that the movement of O2 and CO2 across a barrier will be in the direction of higher to lower concentration (partial pressure) with a magnitude that is proportional to the gradient, and proportional to the area involved, and inversely proportional to the diffusion coefficient or “diffusivity.” Gas diffusion occurs faster when the gradient across the membrane is higher, and a “thinner” barrier allows more diffusion of gas than a “thicker” one, while a barrier of equal “thickness” but much larger surface area will allow more gas to diffuse during the same time interval. Ex. If the concentration of CO2 is higher in the blood stream than ambient air in the room, then the sweep gas flow across the oxygenator will efficiently remove CO2 from higher (blood) to lower (ambient air) concentrations. Oxygenators in the Extracorporeal circuit Principle of Countercurrent Flow • Given two parallel tubes filled with fluid or gas separated by a membrane with some degree of permeability to components of that fluid or gas, the exchange of molecules or particles across the barrier is more efficient if the movements of the liquids are opposite in direction. • If the two systems are moving in parallel (concurrent), then at the entrance the concentrations are 100% and 0%. • As the two systems move along, there is continued diffusion of X across the barrier with reduction of the concentration of X on the donor side, and increase in the concentration on the recipient side. • Gradually the concentrations change to 90%:10%,--> 80%:20%, and so on until equilibrium is reached at 50%:50%. • Continued movement along the additional length of the tube does not provide any additional exchange of X into the recipient system, and the end result at the exit site remains 50% • If we now reverse the direction of one of the systems so that the donor and recipient systems are flowing in opposite directions (countercurrent), the movement of G can occur throughout the entire length of the system, because the gradient driving the diffusion can be maintained. • If there is sufficient length to the systems, the concentration of X can potentially reach 100% at its exit site Concurrent versus countercurrent flow. Increased transfer occurs due to movement along the entire system with countercurrent flow, rather than the maximum 50:50 equilibrium achievable with concurrent flow. Oxygenators in the Extracorporeal circuit Oxygenator Performance and Capabilities • priming volume is the volume of fluid (crystalloid or blood) required to fill the blood phase of the device, including any integrated heat exchanger. • This may also include some minimal level in the reservoir depending on the device and the manufacturer • The ability of an oxygenator to oxygenate blood is expressed in the oxygen reference blood flow, which is defined as the flow rate of whole blood at normothermia, with a normal hemoglobin (12 g/dL), with a base excess of zero, that will increase the oxygen content of venous blood with an oxygen saturation of 65%, by 45 mL/L of flow. • Rated flow or reference flow is the maximal recommended flow to achieve adequate gas exchange, and is equal to the lowest flow among the oxygen reference blood flow, carbon dioxide reference blood flow, manufacturer’s recommended maximal flow, or 8 L/min --> <-- Oxygenators in the Extracorporeal circuit Direct Contact Oxygenators (Bubble, Screen, Rotating Disc, Drum) • In all of the devices, the blood from the patient’s body would enter the machine and at some point directly mix with a gas mixture of primarily oxygen in the form of very small bubbles generated through several mechanisms, frequently using a device called a sparger that generated the gas bubbles. • Each bubble provided the surface area within the blood for gas exchange • Size of the bubble was very important • the available surface area for gas exchange is inversely proportional to the bubble diameter: smaller bubbles provided more surface area and more efficient transfer of oxygen. However, if the bubbles were too small, the rapid rise in CO2 tension limited the amount of CO2 removal that was possible • Once the bubbles are generated and mix with the venous blood, the mixture has sufficient time for the transfer of oxygen directly onto the hemoglobin molecules in the blood (with a very small proportion going into solution). Simultaneously, the CO2 would leave the solution and enter the bubbles, exiting near the top of the device or in a separator. Oxygenators in the Extracorporeal circuit Membrane Oxygenators • The advantages the membrane oxygenator offered were the separation of the blood and gas, reducing the damage and thrombosis seen with bubble oxygenators, as well as the reduction of gaseous emboli. • Blood no longer pushed upward by the moving gas bubbles, but is pumped through the membrane independent of the gas flow, thus allowing separate regulation of the rate and composition of the gas phase to manage O2 and CO2 exchange • ***Membrane was constructed as a long rolled coil from a sheet of a silicone polymer which completely divided the gas and blood phases. • Gas exchange was not as efficient as through other materials, in that the oxygen had to essentially diffuse into the silicone polymer phase and then diffuse out into the blood, with the same process in reverse for carbon dioxide. • Thus, much larger surface areas were required and concurrently larger priming volumes. Oxygenators in the Extracorporeal circuit Microporous Membrane Oxygenators • Unlike the large, less efficient silicone membrane oxygenators, the so-called microporous hollow fiber oxygenators were designed specifically for the needs of the operating room where short-term use with small devices requiring low priming volumes and low resistances were very advantageous. • Vast majority of oxygenators were made of polypropylene hollow fibers, although a few utilized sheets of polypropylene, where micropores less than 1 μm (micron) are created through a process of heating and stretching the material. • gas moves through the small fibers which are surrounded by blood moving in countercurrent fashion. • Once exposed to blood, the pores in the fibers become coated with plasma proteins through which gas molecules may pass, but through which the plasma proteins and water do not due to the surface tension of the blood. • Over time, the pores eventually allow plasma components across the pores into the gas phase, known as plasma leakage • This usually takes a number of hours or even days (not a concern for CPB usage) Silicone sheet oxygenator Microporous Membrane Oxygenator Oxygenators in the Extracorporeal circuit Microporous Membrane Oxygenators • While the goal of the oxygenator is to mimic the function of the native lung by providing sufficient oxygen supply and carbon dioxide removal, it is not feasible to reproduce the structure of the lung. • The smallest feasible constructible pathway for blood is still over 100 times the size of capillaries, and if made any smaller the resistance becomes prohibitive for the purposes of extracorporeal support. • This is compensated for by increasing the length of the blood path over 1,000-fold. So instead of each cell briefly passing along an individual alveolus, we now have larger amounts of blood passing through wider, but very long distances to create more effective surface area for gas exchange. • If the length of the blood path is short and there is perfectly laminar flow, little to no oxygen would get to the hemoglobin in the central stream of blood. • However, the flow is not laminar and turbulence acts to disrupt the layers of the velocity gradient and increases eddy currents and mixing. In this circumstance, this is highly desirable so as to increase the ability of oxygen to diffuse onto more available hemoglobin molecules. • By putting the gas through the fibers instead of the blood, the resistance is much lower and allows adequate contact for gas exchange. • Unlike oxygen, carbon dioxide is much more soluble in blood where it is quickly converted to bicarbonate. • Additional molecules of CO2 are transported on amine groups of plasma proteins, including hemoglobin, making the excretion of CO2 much less problematic than the delivery of oxygen Oxygenators in the Extracorporeal circuit Plasma-tight Hollow Fiber Oxygenators • In 2008, the first polymethylpentene (PMP) hollow fiber oxygenator was approved by the Food and Drug Administration (FDA) for use in the United States for extracorporeal support. • important new tool for long-term extracorporeal support • Hollow fibers of PMP were truly nonporous, instead of being covered with a very thin membrane, which allowed efficient gas exchange without the eventual plasma leakage, although rare cases have been reported. • It revolutionized ECMO support in this country and around the world by providing a low-volume, low-resistance, biocompatible, and efficient oxygenator that could be used for days or weeks at a stretch. • While the stimulus for coagulation does seem to be less with the PMP and its various coatings, because the surface area and volume are so small, when thrombosis is initiated, it can progress extremely rapidly causing relatively sudden oxygenator failure. • This becomes extremely important in light of the trends in ECMO support and the desire for a simpler and more “automated” and compact system, which has led to less monitoring of pre- and postmembrane pressures and less close observation by experienced staff. Oxygenators in the Extracorporeal circuit Design Characteristics • The movement of O2 and CO2 across the oxygenator is governed by the same principles and laws of physics regarding diffusion coefficients and gradients; however, our strategy on CPB or ECMO is somewhat different since we are driving oxygen in, while removing carbon dioxide. • We use the same terminology for the inflow gas of the oxygenator even though it is technically not inspired or expired like the lungs, but rather enters at one end and exits at the other. • This is referred to as the “sweep gas” and the flow referred to as the “sweep rate,” as given in L/min or mL/min. Physiologically, it is equivalent to the minute ventilation of native lung function, but without the issue of dead space ventilation of the native airways. • Unlike the lung tissue, the artificial surface of the oxygenator is not susceptible to oxygen toxicity, so that flow of 100% oxygen into the sweep gas does not carry the risk of inflammation seen in the alveoli. • Since we do not have direct control over the concentration of CO2 within the blood phase, but only control the concentration in the sweep, we can only control the diffusion by increasing the gradient across the membrane by keeping the sweep gas concentration as low as possible, essentially zero, with the infusion of 100% oxygen. • The quicker the CO2 that diffuses across the membrane from the blood to the gas phase is removed from the oxygen, the quicker the gradient is reestablished, and the quicker more CO2 can be removed. Simply put, the faster the sweep rate, the more the CO2 removal. Oxygenators in the Extracorporeal circuit Design Characteristics (cont) • Modern oxygenators are provided with a predetermined “rated flow” which is an expression of the maximal flow that will still allow appropriate oxygenation. • Flows above the rated flow are more likely to produce blood that is not fully oxygenated. • During CPB, patients are under general anesthesia and have generally decreased metabolism; so the ability to provide sufficient gas exchange is rarely a problem. • During ECMO, however, patients are more commonly awake and mobile, occasionally ambulatory, and so with larger patients it may be difficult to meet their metabolic needs without adding a second oxygenator. • This should be done in parallel rather than in series to maximize gas exchange as well as to not increase the resistance. Oxygenators in the Extracorporeal circuit Pressure Drop (aka Delta P): the pressure difference between the entrance of blood to the oxygenator and the exit. ** Lower upon exit than entrance. .. Why? Prime Volume: the amount of volume required to displace all air from the oxygenator with fluid. Oxygenators in the Extracorporeal circuit Oxygenator Performance • Throughout the surgical procedure the Perfusionist monitors the function of the oxygenator by drawing blood samples and performing blood gas analysis. • Oxygenator specific: PO2 and CO2 values • Trending RPMs to generate a correlated line pressure and flow. • Allows Perfusionist to monitor where the case started and if there is any potential rising pressure within the oxygenator – possibly indicating clot, malfunction, etc. • Oxygenators are designed to maximize their potential for air removal. If an integrated arterial line filter is present, then air removal performance is maximized. • Each brand varies slightly in design, but purpose is Oxygenators in the Extracorporeal circuit Oxygenators in the Extracorporeal circuit