Ex-Situ Heart Perfusion: Assessing Organ FunctionAlexandra Klimovitz, Lucas Witer MD, Brandon Sowell, Amit Iyengar BE, Alvaro Rojas-Pena M.D, Robert H. Bartlett MD, Martin Bocks MD, Gabe Owens MD

The shortage of viable hearts for transplant is a very prominent obstacle in contemporary medicine. Ex-situ organ perfusion as opposed to the current method of cold storage may extend donor criteria. This system could provide a means for preserving and reconditioning a heart, and allow for assessment of its viability over a prolonged period of time. In order to do this, however, we need to develop a model for perfusion that allows us to quantify the function of the heart prior to transplantation. Qualitatively, we can look at a heart and say it has good contractile function. Quantitatively, however, we have been trying to control parameters such as resistance and edema to assess its function. These quantities are measured using mean arterial pressure, flow rate, and left ventricle wet/dry ratio. Additionally, by analyzing the values of oxygen delivery and consumption we can get a good idea of how function is fluctuating from changes in resistance and edema, and thus of how this heart is functioning throughout the perfusion process.

RESULTS

Lines are placed in the porcine donor's femoral artery and vein for monitoring and perfusate collection. An initial ~500cc of blood is collecting from the donor, and is spun down in a centrifuge for 15 minutes. The bottom layer (red blood cells) and top layer (plasma) are collected, while the middle layer of white blood cells are discarded.

Plasma and RBC are stored separately, and then reconstituted at a more dilute ratio (90% plasma 10% RBC) for perfusion. The remainder of donor blood is collected and separated following the procurement of the heart via traditional surgical techniques. After connection to the perfusion apparatus, the heart is slowly rewarmed (after having been cooled with ice) over 20-30 min. An ideal mean perfusion pressure of 35-45 mmHg is maintained throughout the experiment by adjusting flow accordingly, and blood gases are collected hourly.

Figure 1: Development of perfusate for ex situ perfusion of hearts.

Figure 2: Schematic for Perfusion Apparatus

Figure 3: Perfusion Apparatus after priming

4 of 5 preps showed that edema was present in the heart, however resistance did not rise and oxygen delivery and consumption were not effected. This suggests that in spite of edema present we have found a way to adequately supply the organ with oxygen and nutrients, and now we can look at other factors that may effect function.

The implications that a perfected perfusion process under normothermic conditions could have on transplant medicine are huge, and it may one day replace cold storage entirely as it is an effective way to preserve and assess function of an organ prior to transplantation.

1. Our quantitative assessment of the function of a heart was determined by examining resistance (using mean perfusion pressure, perfusion flow rate, organ resistance, edema (using LV wet/dry ratios), and a comparison of oxygen delivery and consumption).

2. Significant improvements have been made in controlling these factors: we have reduced average end-organ resistance by half, and almost eliminated edema in our wet/dry ratios that had previously reached values upward of 7.

For future preps it is possible we will look more in depth at how electrolytes and other perfusate conditions effect the heart function throughout the duration of the perfusion. We may look at adding steroids and antibiotics to our perfusate to improve heart function and perhaps we will experiment with adding certain hormones in order to help maintain normal physiologic conditions.

Resistance was used as a quantitative analysis of how well O2 and nutrients were being delivered to the heart. We targeted a perfusion pressure of 40±5 mmHg and adjusted the flow accordingly in order to maintain this. Higher perfusion pressures could be detrimental to the heart and cause hemorrhage in capillary beds. If flow was decreased to maintain this pressure, we could say that resistance was rising (and thus our ability to deliver nutrients and perfuse the heart was decreasing). Resistance for all preps started high because the heart was cooled down during procurement (vaso-constriction), however as the temperature of the organ rose to target (37°C), the resistance decreases. The average resistance of all five preps increases but was <0.6mmHg/mL/min the duration of the prep went on suggesting that we consistently supplied the organ with oxygen and nutrients throughout the prep and resistance was not a factor in loss of function.

The left ventricle (LV) Wet/Dry Ratio can tell us how much edema is present in the heart after 12 hours of perfusion. In 80% of the experiments a 10% increase in organ weight was seen, and in one experiment a 10% decrease was observed. Increased LV wet/dry ratio is associated with poor LV function.

Normal O2 delivery is around 20mL O2/dL, this value was achieved after normothermic (37°C) perfusion suggesting that adequate oxygen supply is maintained throughout the experiment. Oxygen consumption is ¼ to ⅕ O2 delivery and it will depend on organ activity and metabolism.

CONCLUSION

BACKGROUND

METHODS

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LV Wet/Dry RatioNORMAL LV WET/DRY RATIO RANGE REPORTED IN THE LIT-ERATURE 2.6±3

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N = 5Error bars = SD

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N = 5Error bars = SD