At pump flow of 50% cardiac output, changes to the normal blood flow were as follows: (1) in the aorta proximal to the outflow graft (proximal aorta), there was a spike of normograde blood flow from the heart, followed by a spike of retrograde blood flow from the pump, followed again by a small peak of normograde blood flow; and (2) in the aorta distal to the outflow graft (distal aorta), there was a larger spike of normograde blood flow corresponding to positive flow from the heart and pump, followed by a long time period of zero or retrograde flow corresponding to the net zero flow through the pump.
At pump flow of 100% of cardiac output, changes to the normal blood flow were as follows: (1) in the proximal aorta, flow patterns were similar to those demonstrated at pump flow of 50% cardiac output, but with less pulsatility to the flow; and (2) in the distal aorta, there was virtually no pulsatility to the flow wave.
When the pump was off, with the circuit open (mimicking pump failure), changes to the normal blood flow were as follows: (1) in the proximal aorta, the flow pattern was generally similar to a normal pattern, although with a greater spike; and (2) in the distal aorta, there was a similar spike of normograde flow corresponding to systole; however, during diastole, all normograde flow from the proximal aorta was directed retrograde through the outflow graft, with the net effect of zero normograde blood flow in the distal aorta for approximately 50% of the cardiac cycle.
To further quantify the effects of different levels of pump support on distal aorta blood flow, pump flow and distal aorta blood flow were normalized to PA blood flow (Figure 5). The results suggest that at pump flows of 25% and 100% of PA flow, there is a normal amount of blood flow through the distal aorta. However, at intermediate pump flows, there is a trend towards a decrease in blood flow through the distal aorta (pump flow = 25% PA flow vs. Pump flow = 50% PA flow, p = 0.08). During conditions of induced heart failure, blood flow through the distal aorta was approximately twice normal at all levels of pump support (65–75%vs. 40% of PA flow).
Continuous flow blood pumps are seen as the next generation of VADs because of their small size, relative simplicity, lack of valves, and minimal moving parts. However, since these pumps have no valves, they operate in synchronicity with left ventricular pressure and contraction. 6 As the blood pressure in the aorta increases, there will be a corresponding decrease in pump flow at constant pump speed. Thus there is a pulsatile characteristic to the flow. Whereas this may not be very relevant in a diseased heart, which cannot generate a significant pulse, in those hearts that can or eventually regain the ability to contract, this change in pump flow characteristics could have profound implications to VAD support.
One of the most startling findings of this study was the considerable decrease in skin tissue perfusion with continuous flow pump support. As this measurement is directly related to the amount of blood flow through the skin capillaries, it appears as if there is lower capillary blood flow with continuous flow pump support. The direct correlation between pump flow rate and tissue perfusion (Figure 2) suggests that this is not due to the loss of pulsatility but a hemodynamic mismatch. Although the pulse pressure is increasing, there is a concurrent decrease in tissue perfusion. In fact, based upon the flow study data, decreasing pump flow from 100% of cardiac output to 50% (support levels similar to that of the PI animals) (Table 1), resulting in an increase in pulse pressure from 24 to 40 mm Hg, actually diminished flow in the aorta distal to the outflow graft by 25%, suggesting a partial mechanism for the 50% decrease in tissue perfusion. Decreased total blood flow is suggested by Figure 4, wherein the total time of regurgitant flow in the cardiac cycle increases from less than 5% under normal flow circumstances to 25% when pump flow is 50% of PA flow. Whereas the volume of retrograde blood flow is small, the energy equivalent pressure may be decreased, as suggested by others. 15–17 A decrease in the energy of the blood flow could decrease the delivery of blood flow to the high resistance capillary beds.
As pump support increased from 25 to 40% of cardiac output, there was a 25% decrease in distal aortic blood flow. This decreased level persisted until the pump support was approximately 100%. This is likely to be due to interaction of pump flow with native heart flow. At lower pump support, the heart is contributing blood flow down the aorta, and the pump is unable to overcome the pressure head from the blood leaving the heart. Thus the blood from the pump tends to be directed distally. As pump support increases, the trend is for blood flow from the pump to become more evenly divided, going both normograde and retrograde, as the pump is able to overcome the pressure head from the heart. This change in aortic blood flow could partially explain the diminished tissue perfusion seen in the PI animals and shows the role of blood pressure in pump flow and aortic hemodynamics.
Alterations in aortic blood flow could potentially explain the findings of other groups. Nishimura et al.8 describe changes to aortic structure and function after exposure to continuous flow pump support. In their study, there was a seemingly paradoxical increase in elastin and a decrease in collagen content in the aorta of goats subjected to continuous flow pump support compared with controls. Based upon our aortic flow study data, in which there is increased pulsatility of the flow wave in the aorta between the heart and the outflow graft, one could predict an increase in elastin content in that section of the aorta. Likewise, distal portions of the aorta, which are subjected to decreased flow pulsatility, may have increased collagen content, a subject of ongoing study.
Even more interesting were the distal aorta blood flow results during mock pump failure. In approximately 50% of the cardiac cycle, there was no normograde blood flow in the distal aorta. During end systole and early diastole, the majority of blood coming from the heart down the aorta goes back through the pump and does not continue down the aorta. Whereas the total volume loss is fairly low, the length of time involved and the subsequent decrease in flow wave energetics are likely significant. This suggests that should the pump fail after being implanted onto the ascending aorta, there could be a substantial drop in blood delivered to the brain, a potentially debilitating scenario.
In the heart failure model, there was an increase in blood flow through the distal aorta with the heart failure model (approximately 2× normal), exactly the opposite of what was shown in the healthy heart aortic hemodynamic studies. Whereas the cause of this is unclear, our hypothesis is that as the blood pressure decreases with the induced heart failure, there is decreased pressure head for the pump to work against in both phases of the cardiac cycle, allowing for less competition with the native heart ejection. The heart producing a small ejection may be enough to preferentially direct blood distally because of a slightly greater pressure head proximal to the outflow graft. Again, the implications are likely not as great for humans when the graft is being attached to the ascending aorta, but it could be significant for a descending thoracic aorta anastomosis.
These data suggest marked effects from implantation with continuous flow blood pumps; however, they should be taken in perspective. Virtually all studies were done with animals with a healthy heart. As such, the relevance to the typical human patient with a diseased heart may be limited. If there is cardiac recovery, these data would certainly become more relevant. Furthermore, the attachment of the outflow graft was on the descending thoracic aorta. Whereas the majority of VADs are likely to be attached to the ascending aorta, the descending thoracic aorta may receive more attention as the drive for less invasive surgery increases. In addition, the data suggesting the implications of pump failure are relevant no matter where the outflow graft is located and may have more profound clinical implications if the anastomosis site is the ascending aorta (retrograde blood flow through the outflow graft may decrease overall blood flow to the brain, leading to increased likelihood of regional ischemia). Further studies are being planned to help elucidate these issues.
This work was supported in part by a grant from the National Institutes of Health, 1R01 HL64950–01. The authors thank Mary Watach and Lisa Gordon for their excellent technical assistance.
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