Medicine
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Normothermic Ex Situ Heart Perfusion in Working Mode: Assessment of Cardiac Function and Metabolism
Chapters
Summary January 12th, 2019
Normothermic ex situ heart perfusion (ESHP), preserves the heart in a beating, semi-physiologic state. When performed in a working mode, ESHP provides the opportunity to perform sophisticated assessments of donor heart function and organ viability. Here, we describe our method for myocardial performance evaluation during ESHP.
Transcript
This method can help answer key questions about the optimal preservation of donor hearts, as well as their functional and metabolic evaluation for subsequent transplantation suitability. The main advantage of this technique is it allows a functional evaluation of the donor heart that is not based solely on indirect metabolic assessments. The implications of this technique extend towards the therapy of dysfunctional donor hearts, as the heart function can be easily and repeatedly assessed over time.
Generally, individuals new to this matter will struggle because some technical expertise is required for connecting the heart in a safe and secure manner. Visual demonstration of this method is critical as the technical aspects of the heart retrieval and connection to the device can be challenging to learn by text instruction alone. After confirming a lack of response to noxious stimulus, perform a median sternotomy on the anesthetized pig and once the heart has been exposed, use Metzenbaum scissors to open the pericardium.
Use 1-0 silk sutures to fix the pericardial edges to the sternum and gradually collect 750 milliliters of whole blood from the two-stage venous cannula placed in the right atrium in an autoclaved glass container over a period of 15 minutes while simultaneously replacing the volume with one liter of an isotonic crystalloid solution. Add the blood to a perfusion circuit primed with 750 milliliters of Krebs-Henseleit buffer supplemented with 8%albumin and place a 14 gauge cardioplegia needle and cannula into the ascending aorta. Secure the cardioplegia needle on the aorta with a snare and connect the cardioplegia cannula to the cardioplegia bag.
Add 100 milliliters of blood to 400 milliliters of cardioplegia. After the end of exsanguination, cross-clamp the ascending aorta with an aortic clamp. Next, deliver the cardioplegic solution into the aortic root.
When all the solution has been administered, remove the clamp and excise the heart, including all of the aortic arch vessels and a segment of the descending aorta. To prevent the deleterious effects of prolonged ischemia, the heart should be procured and mounted on the apparatus for perfusion as quickly as possible. After removing any excess tissue around the left atrium, use a 3-0 polypropylene suture to place a purse string suture around the left atrial orifice.
Suture and close the inferior vena cava with a 3-0 polypropylene suture. Place the left atrial cannula into the left atrial orifice and secure the cannula with a snare. Place the left atrial cannula over the silicone membrane embedded magnet of the apparatus, taking care that the magnet and the corresponding metal ring in the left atrial cannula are properly engaged.
Attach the aorta to the aortic cannula embedded in the silicone membrane and use a silk tie to secure the aorta around the cannula. Turn the aorta to achieve a proper lie without tension or kinking and increase the aortic pump speed to about 1600 RPM and gently squeeze the ventricles to de-air the heart. The remaining air in the aortic root will be ejected through the innominate and subclavian branches.
Connect the aortic purge line to the innominate artery and secure the connection with a silk tie. Snare the left subclavian artery orifice with a silk tie and secure the closure with a snare and a snap. Place a 5 French introducer sheath through the orifice of the subclavian artery, adjusting the length and orientation of the catheter so that the catheter does not interfere with the aortic valve function.
Adjust the aortic pump speed to reach a mean pressure of 30 millimeters of mercury to start the perfusion in the non-working Langendorff mode. The appearance of a dark, deoxygenated perfusate in the pulmonary arterial line indicates the re-establishment of coronary flow. Turn the heat exchanger to 38 degrees Celsius to warm the perfusate and use a blood gas analyzer to check the dissolved gas status, adjusting the gas mixture to the appropriate parameters.
After an hour in non-working mode, enter a suitable left atrial pressure in the desired left atrial pressure section of the software and initiate the feedback loop. The activated working mode will appear as a green button and the left atrial pump speed will automatically increase and decrease to reach and maintain the selected left atrial pressure. For the assessment and recording of the steady-state data, place a fluid-filled pigtail catheter through the introducer sheath into the left ventricle.
For assessment of the preload recruitable stroke work, remove the pigtail catheter from the left ventricle and adjust the desired rate of drop in the left atrial pump speed during the analysis and the desired time period during which the analysis will take place. Click record pressure volume loop. The software will automatically exit the working mode and gradually reduce the left atrial pump RPM, while simultaneously recording the left ventricle stroke work and the left atrial pressure.
At the conclusion of data collection, the software will perform a linear regression on the newly acquired data set to yield the preload recruitable stroke work. After the ex situ apparatus heart software has completed the analysis, a message will appear on the main page showing the correlation coefficient of the analysis. Click okay if the coefficient is greater than 0.95.
The preload recruitable stroke work analysis results will be recorded. To assess the metabolic state of the heart and the perfusate, use the information obtained from the blood gas analysis of the perfusate samples collected from both the arterial and pulmonary artery lines every one to two hours. In a Yorkshire Pig model with a starting heart weight of 180 to 220 grams, the initial left atrial flow will be approximately 2, 000 milliliters minute, typically approaching about 2, 250 milliliters per minute during the first one to two hours of perfusion in the working mode.
Blood gas analysis and metabolic assessments performed on the perfusate samples obtained during ex situ heart perfusion provide extensive information on the metabolic status of the heart over time, indicating the perfusate lactate concentration and the information needed to calculate the myocardial oxygen consumption. In the absence of the organs that naturally metabolize their clear cardiac biomarkers, the accumulation of these factors within the perfusate solution over time is typically observed. In addition to load dependent parameters, the left ventricle preload recruitable stroke work can be evaluated during the computer-controlled linear reduction in the left atrial pressure, as demonstrated.
Echocardiography during ex situ heart perfusion can also provide additional information about myocardial function and anatomical parameters. While attempting this procedure, it's important to remember that ex situ heart perfusion is still in its infancy and that more studies are warranted to design better metabolic heart support for better preservation of cardiac viability and function. Following this procedure, other methods like angiographic imaging can be performed to answer additional questions about donor heart's coronary vascular status.
After it's development, this technique paved the way for researchers in the field of organ transplantation to explore the possibility of the safe transplantation of extended criteria donated hearts in patients awaiting a heart transplant. Don't forget that working with biomaterials can be extremely hazardous and that precautions, such as wearing gloves and protective glasses, should always be taken while performing this procedure.
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