It has been well established that exposure to hypoxia results in local tissue injury, and that ischemic lesions may be aggravated by the return of fresh blood flow to ischemic muscle, a phenomenon referred to as reperfusion syndrome.1 Abrupt reperfusion induces a burst of reactive oxygen species (ROS) in postischemic tissues (particularly in vascular endothelial cells),2 which results in inflammatory-like responses at the onset of reperfusion, such as endothelial dysfunction (decreased endothelium-dependent vasodilation),3 reduced endogenous nitric oxide (NO) generation,4 increased superoxide anion generation,5 and the release of proinflammatory cytokines into the interstitium and vascular space.6 Furthermore, it is widely recognized that the microvasculature, particularly endothelial cells lining microscopic blood vessels, is vulnerable to the consequences of ischemia and reperfusion.7
Hemodynamic energy concept, energy equivalent pressure (EEP) and surplus hemodynamic energy (SHE), is mainly used to evaluate effective pulsatility in pulsatile extracorporeal life support systems,8–12 and inotropic drugs are known to affect both EEP and SHE.13 If adequate pulsatility is generated, EEP is always higher than mean arterial pressure (MAP), and the difference between EEP and MAP represents extra energy. Surplus hemodynamic energy exists only if some pulsatility is created in the pressure or flow, and thus for 100% nonpulsatile flow, SHE is 0.14 Furthermore, pulsatile flow can maintain flow in the microcirculation15 and can be quantified using SHE.
Thus, we aimed at the possibility that the pattern of blood flow pulsatility could be changed after reperfusion injury by increased resistance of microvascular beds or by decreased elasticity of the arterial walls. It would, in turn, affect the tissue perfusion. Through a careful literature search, we have not found any studies directly addressing the changes in blood flow pulsatility after reperfusion injury. Therefore, in this study, we investigated changes in pulsatility after ischemia-reperfusion injury using the hemodynamic energy concept.
Materials and Methods
Anesthesia and the Surgical Method
All animals were treated in accordance with the Guide for the Care and Use of Laboratory Animals issued by Korea University School of Medicine. Twenty-one male New Zealand white rabbits, weighing 2–3.2 kg, were used. Although the role of estrogen as a free radical scavenger is debatable,16 it can influence the oxidative stress in ischemia-reperfusion injury; therefore, we used male rabbits only.
Animals were premedicated with intramuscular xylazine hydrochloride (1 mg/kg), weighed, and placed on a surgical table. After local anesthesia (2 ml, 1% lidocaine), tracheostomy was performed using a 16G catheter. Anesthesia was maintained with 2% isoflurane and 1 L/min oxygen. Mechanical ventilation was maintained at a tidal volume of 5 ml and a respiratory rate of 30 breaths/min.
After laparotomy, the abdominal aorta was bluntly dissected free of surrounding tissues. A 22G catheter (used for monitoring arterial pressure) was inserted into the aorta (2 cm below the origin of the renal artery), and a 2 mm ultrasonic flowmeter (TS420 flow meter, MA2PSB probe, Transonic, Ithaca, NY) was placed around the abdominal aorta to measure blood flow.
Animals were randomly assigned to three groups. a) Ischemia group (I group, n = 7) in which a microvascular clamp was placed immediately above the aortic bifurcation and used to occlude the abdominal aorta for 3 hours. b) Reperfusion group (I/R group, n = 7) in which a microvascular clamp was placed as in the I group but released after 2 hours, and tissues were then reperfused for 1 hour. c) Control group (C group, n = 7) was composed of sham-operated animals.
At the end of the experiment, animals were euthanized while anesthetized in accordance with our institution's guidelines.
Measurement of Hemodynamic Energy
Measurements were repeated six times for 3 minutes, and waveforms were analyzed using MATLAB software (MathWorks, Natick, MA).
Energy equivalent pressure was defined using the following formula9,11,12:
where f is the pump flow rate (L/min) and p is the arterial pressure (mm Hg). The time integrals were over a single pulse cycle. Energy equivalent pressure, which is expressed in millimeters of mercury, is defined at the ratio of the areas beneath the hemodynamic power curve and the pump flow curve at the end of the flow and pressure cycles.
Surplus hemodynamic energy was defined as follows10,11:
The constant, i.e., 1,332, converts the pressure from millimeters of mercury to dynes per square centimeter.
All parameters were measured at baseline, during ischemia (60 minute after clamping, I-60), and during reperfusion at 5 (R-5), 15 (R-15), 30 (R-30), and 60 min (R-60) after clamp release.
SPSS (SPSS version 17 for Window, SPSS Inc., Chicago, IL) was used for statistical analysis. Kruskal-Wallis variance analysis was used to compare inter- and intragroup differences, and significant differences were confirmed using the Mann-Whitney test. Values of p < 0.05 were deemed significant. All results are expressed as means ± SDs.
No significant intergroup difference was found for mean blood flow (MBF) at baseline (Table 1). Mean blood flows in I group during the ischemic period (I-60, R-5, R-15, R-30, and R-60) were significantly lower than in C group at corresponding times. Mean blood flow in the I/R group during ischemia (I-60) was significantly lower than in the C group. However, after reperfusion, MBF in the I/R group recovered to baseline and was similar to that of the C group.
No intergroup difference in pulse pressure (PP) was observed at any time (Table 2).
No intergroup difference was found between MAP, EEP, or SHE at baseline or at I-60 (Tables 3–5), but MAP was significantly lower in the I/R group at R-15 and R-60 than in the C group (p < 0.05, Table 3). Furthermore, EEPs in the I/R group at R-15 and R-60 were significantly lower than in the C group (Table 4), and SHEs in the I/R group at R-5, R-15, and R-30 were significantly lower than in the C group (Table 5). In the C group, MAP, PP, EEP, and SHE did not change significantly with time (Tables 3–5), and in the I group, MAP, PP, EEP, and SHE did not change. In the I/R group, MAP and EEP did not change significantly during whole observation period (Tables 3 and 4). However, after reperfusion start, SHE in the I/R group at R-5 was significantly lower than at I-60 (p < 0.05, Table 5). Furthermore, in the I/R group, taking SHE at baseline as 100%, SHE values at R-5, R-15, R-30, and R-60 were 59.83%, 78.42%, 96.67%, and 88.06%, respectively.
The purpose of this study was to determine the relation between reperfusion injury and blood flow pulsatility. We hypothesized that pulsatility could change caused by microvascular dysfunction. For evaluating this hypothesis, we adopted hemodynamic energy concept.
Kurose et al.17 found that ROS levels dramatically increased 2–5 minutes after reperfusion start. This increase in ROS and in the levels of many inflammatory mediators released when activated leukocytes damage adjacent endothelial cells is thought to play a key role in tissue injury.18 Furthermore, the endothelial dysfunction caused by ischemia-reperfusion injury induces further pathways in the vasculature, which affect the circulation of the affected region.19,20 Ischemia-reperfusion injury seems to be a result of an overproduction of reactive oxygen-derived free radicals (OFR),21 which leads to consumption and depletion of endogenous scavenging antioxidants. A cascade of formation of OFR is initiated by the generation of superoxide. Superoxide can alter vasomotility through two different pathways. First of all, superoxide is a potent vasoconstrictor. The other mechanism is that superoxide annihilates NO.22 Because NO is a potent vasodilator and has an antiaggregatory effect, the annihilation of NO by superoxide may lead to vasoconstriction and thrombus formation in microcirculatory blood vessels (“no-reflow”), constantly observed in reperfusion injury. All theses mechanisms induced decreased microcirculation after ischemia-reperfusion injury.
In this study, we evaluated hemodynamic energy after reperfusion injury. Surplus hemodynamic energy in the I/R group was lower after reperfusion start than in the C group. In the I/R group, SHE was lower after reperfusion start than at I-60, but nevertheless, MBF recovered to the level it is in the C group. In the I/R group, SHE was lowest at R-5 (59.85% of the baseline level). We believe that the decreased SHE observed in our study reflects a decrease in pulsatility caused by microvascular dysfunction.
Decreased pulsatility caused harmful effect on microcirculation. In a study that used pulsatile pump, when the flow pattern was changed from the pulsatile to the nonpulsatile mode, the erythrocyte velocity in many capillaries dropped and remained at a low level, and the number of the perfused capillaries decreased. It is thought that the basal and flow-stimulated endothelium-derived NO release in the microvessels decreased because of the disappearance of pulsatility and that it induced the constriction of arterioles. At the same time, the baroreceptors might sense the decrease in the arterial peak pressure or dp/dt, and this would increase the sympathetic nerve activities and induce the constriction of arterioles.23
It has also been shown that, when extra energy is generated by pulsatile perfusion, regional and global vital organ perfusion is superior to the nonpulsatile perfusion improved.14 Takeda24 described capillary collapse and microcirculatory shunting during ripple-flow or nonpulsatile perfusion at a certain critical closing pressure. Capillary collapse and microcirculatory shunting interrupt the delivery of nutrients to cells and eventually cause a shift toward an anaerobic metabolism. The extra energy of pulsatile flow is believed to maintain peripheral perfusion by keeping capillary beds open and tissue fluids moving. Thus, the extra energy generated by pulsatility increases blood flow in the microcirculation and makes exchanges between cells and circulation possible.
The hemodynamic energy concept is usually used to compare pulsatile flow with nonpulsatile flow. A PP of >15 mm Hg is considered to be pulsatile flow, whereas a PP of <15 mm Hg is considered to be nonpulsatile flow.25 If adequate pulsatility is generated, EEP is always higher than MAP,14 for example, in the normal human heart, there is an ∼10% difference between EEP and MAP.9 In addition, in vitro testing has shown that MAP and EEP increase with increasing vascular resistance, but that SHE values do not substantially alter.26 Because SHE is less dependent on afterload, it is probably a better measure of pulsatility performance in the presence of various afterloads than EEP.27 In this study, in all three groups, MAP, PP, and EEP did not change significantly with time. Only SHE in the I/R group deceased after declamping, which suggests that SHE would provide the best means of evaluating pulsatility in vivo. In particular, SHE should be utilized in the model-related reperfusion studies undertaken to evaluate pulsatility.
Our findings suggest that ischemia-reperfusion injury changes hemodynamic energy, especially SHE, which means that reperfusion injury reduces pulsatility and harms the microcirculation. Clinically, ischemia- and reperfusion-induced microvascular dysfunction have been described in many organs, and ischemia-reperfusion injury is recognized as a potentially serious problem that may be encountered during a variety of standard medical and surgical procedures, such as thrombolytic therapy, organ transplantation, coronary angioplasty, and cardiopulmonary bypass.28 Furthermore, because reperfusion per se can reduce hemodynamic energy, it is more reasonable to use pulsatile flow than nonpulsatile flow when reperfusion injury is expected, as in extracorporeal membrane oxygenation support after acute myocardial infarction, or ventricular assist device support after open heart surgery involving an aortic clamp. In addition, when conducting pulsatile flow studies in vivo, it must be remembered that reperfusion can bias hemodynamic energy measurement.
One of the limitations of this study is that we did not measure cardiac function, and reperfusion injury can change cardiac function and thus affect hemodynamic energy. However, many studies have reported that the cardiac index increased immediately after clamp-off because of a compensatory response in healthy humans29,30; usually, the cardiac index increases at ∼5 minutes after reperfusion.30 However, in this study, presumably due to the compensatory mechanism, hemodynamic energy decreased after reperfusion. The other limitation of this study is that we did not measure level of ROS in the microcirculation or erythrocyte velocity in capillaries, which were representative of state of microcirculation directly. The ischemic-reperfusion injuries present a similar physiopathology between organs. Repercussions for the organ's function depend on the degree and duration of the ischemia and also on the sensitivity of the organ. Repercussions for the general functioning of the body are going to depend on the role of the organ in the homeostasis and on the impact of the ischemia on its function.31 Thus, the changes of microcirculation could be different between organs. In our future studies, we will include other quantitative analysis such as measuring ROS of different organs. Early endothelial dysfunction is, above all, present at the level of small-caliber vessels.32 So, if we measure SHE in the small artery instead of aorta which was the measuring site in this study, the result would be clearer. In our future studies, we will measure SHE in the various sites of vessels simultaneously. Despite these limitations, we believe that the significance of this study is to suggest a new way of approaching reperfusion injury. We think that molecular, pathologic, and functional parameters should be added to further elaborate the exact connection between hemodynamic energy and reperfusion injury in the future studies.
Summarizing, ischemic-reperfusion injury was found to reduce vascular pulsatility. Furthermore, SHE is a more sensitive hemodynamic energy parameter than PP or EEP during ischemia-reperfusion injury.
Supported by a grant from the Korea Health 21 R&D Project of the Ministry of Health and Welfare (grant no. A020609) and by a grant from the Brain Korea 21 Project of the Ministry of Education and Human Resources Development, Republic of Korea.
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