1. Ex-Situ Heart Perfusion: Assessing Organ Function
Alexandra 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
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
1/31/14
2/5/14
2/12/14
2/26/14
3/5/14
LV Wet/Dry Ratio
NORMAL LV WET/DRY RATIO RANGE
REPORTED IN THE LITERATURE 2.6±3
0
10
20
30
40
50
60
70
80
90
100
0
100
200
300
400
500
600
0 1 2 3 4 5 6 7 8 9 10 11 12
mmHg
mL/min
Time (hrs)
PERFUSION FLOW PERFUSION PRESSURE
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0 1 2 3 4 5 6 7 8 9 10 11 12
WoodsUnitsmmHg/mL/min)
Time (hrs)
RESISTANCE = CHANGE (Δ) PRESSURE / FLOW
0
2
4
6
8
10
12
14
16
18
20
0 1 2 3 4 5 6 7 8 9 10 11 12
mLO2/dL
Time (hrs)
Average Oxygen Delivery Average Oxygen Consumption
N = 5
Error bars = SD
N = 5
Error bars = SD
N = 5
Error bars = SD