Although isoflurane may cause subendocardial hypoperfusion in the presence of coronary stenosis because of its coronary arteriolar dilatory effects, it is not known how the subendocardial microcirculation is affected. The authors examined the effects of isoflurane on poststenotic subendocardial microvessels with coronary stenosis.
The authors observed subendocardial microvessels in in situ beating swine hearts with or without critical stenosis of the left anterior descending coronary artery (LAD) with a needle-type videomicroscope during isoflurane- (ISO-H), adenosine- (ADE-H), and nitroglycerin- (NTG-H) induced hypotension (mean arterial pressure, 55 mmHg). Regional myocardial function, oxygen balance, and lactate metabolism in the region perfused by the LAD also were determined.
In swine with stenosis, there were no differences in heart rate, cardiac output, and LAD blood flow among the three types of hypotension. Regional lactate production and anterior interventricular venous pO2 were similar during ISO-H and NTG-H but higher during ADE-H. With videomicroscopy, about half as many subendocardial microvessels could be visualized during ADE-H as with ISO-H and NTG-H. The average decrease in the systolic diameter of subendocardial microvessels of greater than 100 microm was 9 +/- 6% during ISO-H and 12 +/- 5% during NTG-H, but no consistent phasic diameter changes were observed during ADE-H. In swine without stenosis, a systolic diameter decrease was observed during all three types of hypotension.
These findings suggest that hypotension induced by isoflurane or nitroglycerin preserves phasic diameter changes in subendocardial microvessels in the presence of critical coronary stenosis, whereas that induced by adenosine does not.
In hearts with coronary artery stenosis, there is a possibility of redistribution of coronary blood flow away from the potentially ischemic myocardium, i.e., a coronary steal phenomenon, which may lead to myocardial ischemia when powerful coronary vasodilators, such as adenosine and dipyridamole, are administered.  Because isoflurane has a more potent coronary and systemic vasodilatory effect than other volatile anesthetics, such as halothane, enflurane, or sevoflurane, [2–4] the question of whether isoflurane is capable of causing coronary steal has been the subject of intense investigation during the past decade. [5–7] Clinical and experimental studies showed that isoflurane anesthesia may be associated with myocardial ischemia of the collateral-dependent area in the heart with multivessel coronary disease if accompanied by significant hypotension. [5–7] However, the possibility that isoflurane might induce subendocardial ischemia downstream of a proximal coronary stenosis in hearts with one-vessel disease is still a subject of controversy. Although previous studies using the microsphere technique have repeatedly shown that subendocardial hypoperfusion occurs with isoflurane in animal models of coronary stenosis, [7–9] subendocardial microvessels have never been evaluated by direct observation in such models.
The subendocardial microcirculation is unique because it is affected by cardiac contraction and intraventricular pressure. Subendocardial blood flow and blood volume change dynamically during the cardiac cycle. Coronary inflow occurs mostly in diastole and is stopped by contraction during systole. Whereas blood in the subendocardial arterioles is squeezed into the arteries retrogradely, venous outflow from subendocardial microvessels is enhanced during systole because of an increase in subendocardial tissue pressure over the actual microvascular intraluminal pressure. During diastole, subendocardial tissue pressure decreases to near the left diastolic pressure, facilitating coronary arterial inflow.  The term intramyocardial pumping has been coined to describe the systolic-diastolic variation in coronary blood flow, which is thought to be greatest in the subendocardium.  A recent study using a newly developed needle-type videomicroscope has confirmed the concept of phasic changes in the subendocardial microcirculation during the cardiac cycle. 
In the present study, we attempted to determine if isoflurane impairs the dynamic changes in the subendocardial microcirculation in hearts with coronary stenosis using the new videomicroscope. Regional myocardial function, the oxygen supply-demand balance, and the lactate metabolism of the region supplied by the left anterior descending coronary artery (LAD) with stenosis were also examined in this animal model. We compared the effects of isoflurane-induced hypotension with those of nitroglycerin- or adenosine-induced hypotension. Adenosine dilates small arterioles less than 20 micro meter and causes transmural redistribution in the presence of coronary stenosis. [3,13,14] Nitroglycerin, on the other hand, is a dilator of large epicardial coronary arteries and does not alter the distribution of intramyocardial perfusion. [15–18]
Our study protocol was approved by the local Governmental Ethical Committee (No. 58/94) for Animal Research. Thirty-three swine of either sex (weight, 18.2–38.6 kg) were used. Anesthesia was induced with intravenous pentobarbital, 8–10 mg/kg, after premedication with ketamine, 40–50 mg/kg, azaperone, 6–8 mg/kg, and atropine, 1.0 mg, intramuscularly. The trachea was intubated, and the lungs were mechanically ventilated with a mixture (FIO2= 0.5) of nitrous oxide and oxygen (Servo B; Siemens-Elema, Solna, Sweden). Tidal volume was adjusted to maintain normocapnia (35–40 mmHg) with a ventilatory rate of 14 breaths/min. Fentanyl and pentobarbital were infused at rates of 1.0 mg/h and 30 mg/h, respectively, throughout the experiment. Paralysis was maintained with intravenous infusion of pancuronium, 8 mg/h. Body temperature was maintained at about 38 [degree sign] Celsius by an electric blanket.
The animals were placed in a supine position. The right femoral artery was cannulated for blood sampling and pressure monitoring. Ringer lactate solution was infused at a rate of 10 ml [center dot] kg sup -1 [center dot] h sup -1 during surgical preparation and at a rate of 5 ml [center dot] kg sup -1 [center dot] h sup -1 after surgical preparation through a catheter in the right femoral vein. A precalibrated micromanometer catheter (PC 370; Millar Instruments, Houston, TX) was inserted through the right carotid artery into the left ventricle to measure left ventricular pressure and its first derivative via an electronic differentiate. After a mid-sternal thoracotomy, a 14-mm ultrasonic transit-time flow probe was placed around the root of the ascending aorta for measurement of cardiac output (CO) via a flowmeter (T208; Transonic Systems Inc., New York, NY). The animals were then turned from the supine to the right lateral position. A left thoracotomy through the fifth intercostal space and a pericardiotomy were performed, and the heart was placed in a pericardial cradle. Then a 22-gauge catheter was inserted into the anterior interventricular vein via a puncture and advanced 0.5–1.0 cm in a retrograde direction.
A pair of ultrasonic dimension crystals was placed into the subendocardium of the region supplied by the LAD in an equatorial plane for the measurement of segmental length. Appropriate alignment of the crystals was monitored on an oscilloscope (V522; Hitachi Denshi, Tokyo, Japan). Percent systolic segment shortening (SS) was calculated off-line from the following formula: SS =(EDL - ESL)[center dot] EDL sup -1 [center dot] 100, where EDL = end-diastolic segment length and ESL = end-systolic segment length during a cardiac cycle.
The LAD was dissected free proximal to the first diagonal branch to measure its blood flow (QLAD) via an ultrasonic flow probe (2 mm in diameter). A thin Teflon-coated copper wire (W.L. Gore and Associates, Pleinfeld, Germany) and a silk thread were placed around the LAD distal to the probe. The former was passed through a highly flexible spring and attached to a micro-manipulator for gradual LAD constriction, and the latter was used for temporary occlusion to test the hyperemic response, as described elsewhere.  We considered a LAD stenosis to be critical when the reactive hyperemic flow increase after 10-s total occlusion was less than 10% of preocclusion QLAD. 
For determination of blood gas/pH (ABL 3000; Radiometer, Copenhagen, Denmark), hemoglobin, hemoglobin oxygen saturation (CO-282 Oximeter; Instrumentation Laboratories, Lexington, MA) and lactate, 2.5-ml blood samples were simultaneously withdrawn from the aorta and the anterior interventricular vein through catheters. Plasma lactate levels were measured enzymatically. Regional myocardial oxygen consumption of the LAD (MVO2) supplied region was calculated as the product of QLAD and the difference in oxygen contents between arterial blood and interventricular vein blood. Regional lactate consumption or production was calculated with the same formula substituting lactate levels for oxygen contents. The values were corrected for the myocardial weight of the LAD-perfused region.
Recording of the Subendocardial Microvessel Image
The device and technique used for observation of subendocardial microvessels are described elsewhere.  Briefly, the videomicroscope system (VMS-1210; Nihon Kohden, Tokyo, Japan) consists of a portable CCD camera, a needle-probe (4.5 mm in diameter, 180 mm in length), a lens (x200), a light guide, and the control unit. The images are displayed on a monitor and stored on a videocassette recorder (NV-SB60W; Panasonic, Osaka, Japan). This system has a spatial resolution of 5 micro meter. The image from the CCD is converted into color video signals at a rate of 30 times/s. To facilitate the introduction of the needle-probe into the left ventricle, a double lumen sheath (5.0 mm in diameter) with a doughnut-shaped balloon on the tip was inserted through a small incision of the left lateral free wall and snared tightly with 3–0 Prolene[registered sign] suture, which is enforced with pledget. The endocardial microvessels were carefully searched for 10 min during induced hypotension (see Protocol) by pressing the doughnut-shaped balloon lightly onto the anterior part of the endocardial surface of the left ventricular septum. Intervening blood was intermittently flushed away with warmed saline. By carefully moving the needle probe, 984 randomly selected subendocardial image fields (1200 micro meter x 1600 micro meter) were recorded during induced hypotension. The recorded images on a videocassette were used for subsequent off-line analysis. All subendocardial microvessels recorded were counted, and their mean diameters were measured from the TV-monitor by observers blinded to the protocol. Phasic changes by the subendocardial microvessels of greater than 100 micro meter were evaluated by frame-to-frame analysis using software (NIH Image 1.41 by Wayne Rasband) on a Macintosh Computer (Apple Computer, Cupertino, CA). End-diastole and end-systole were determined by the use of superimposed electrocardiograph or dP/dt signals.
Two sets of experiments were performed in this study, i.e., with critical LAD stenosis and without LAD stenosis. After operative preparations and instrumentation, hemodynamics were allowed to stabilize for 30 min. Hemodynamic variables including sonomicrometry were recorded, and blood samples were collected to determine blood gas variables and the lactate level. In the first set of experiments, critical stenosis of the LAD proximal to the first diagonal branch was produced. The same measurements were repeated after a 15-min period of stabilization. Then mean aortic blood pressure (MAP) was decreased to 55 mmHg by either isoflurane, nitroglycerin, or adenosine in a randomized sequence. Each induced hypotension was maintained for 30 min. After a 15-min period of stabilization, the subendocardial microvascular images were recorded for 10 min, followed by hemodynamic measurement and blood collection. Between each hypotension, the drugs were withheld, and the animals were allowed to recover for 20 min while the critical stenosis was maintained. The reachievement of a baseline state was confirmed when CO, MAP, and QLAD returned to a range of +/- 15% of the corresponding values after the creation of LAD stenosis and before induced hypotension. Isoflurane was administered through a vaporizer, whereas nitroglycerin and adenosine were infused into the inferior caval vein with a syringe pump.
The second set of experiments was performed in the same setting and with the same protocol except for the LAD stenosis. After hemodynamic measurements and blood sampling, hypotension of 55 mmHg MAP was induced by either isoflurane, nitroglycerin, or adenosine in a randomized sequence. A search for subendocardial microvessels was made, and hemodynamic measurements and blood collection were carried out during each hypotension. At the end of the experiments, the animals were killed with intravenous potassium chloride immediately after injection of ink into the LAD. The heart was removed, the LAD supplied myocardium was weighed, and the endocardial surface was examined for macroscopic changes.
Data are expressed as means +/- SD. Differences during control states with and without critical stenosis were evaluated by the two-tailed paired t test. The data for the three treatments (isoflurane-, adenosine- and nitroglycerin-induced hypotension) with or without critical stenosis of the LAD were compared by one-way analysis of variance (ANOVA) for repeated measures, followed by Fisher's least significant test. Lactate data were analyzed by a nonparametric technique (Friedman's test), which does not assume normality and equality of variances. P values less than 0.05 were considered to be statistically significant.
To obtain data from nine valid experiments in the first set and five in the second set, 33 experiments were necessary. Five swine were needed to establishing transventricular access for the needle probe and were also used to acquire experience with the handling of the videomicroscope. Six swine were lost during surgical preparation, and six were lost as a result of ventricular fibrillation. Two swine were excluded from evaluation because of irreversible ischemic changes in the subendocardium detected at necropsy, as evidenced by a paler appearance compared with that of the surrounding area and by focal hemorrhage. In these swine, fibrin-formation on the endocardial surface and abruption of the microvessels were found by in vivo microscopic observation during induced-hypotension. Such findings were not made in in vivo microscopic observations nor in the postmortem examination of the swine evaluated in this study.
Global and Regional Coronary Hemodynamics during Induced Hypotension in Swine with and without Critical Stenosis of the Left Anterior Descending Coronary Artery
QLAD decreased by 20% with the creation of critical stenosis. There were, however, no significant differences in global and regional coronary venous pO2(pvO2). Although there was no lactate production before coronary stenosis, slight lactate production was observed in four of nine swine after creation of critical coronary stenosis. The global and coronary hemodynamic variables among isoflurane-, nitroglycerin-, and adenosine-induced hypotension with critical stenosis of the LAD were similar except that LVEDP was lowest with nitroglycerin and maxdP/dt was highest with adenosine. Differences in cardiac output during adenosine-induced hypotension did not reach statistical significance. pvO2was significantly greater during adenosine-induced hypotension than during isoflurane- or nitroglycerin-induced hypotension in swine. Lactate production was significantly higher during adenosine-induced hypotension than during isoflurane- or nitroglycerin-induced hypotension. Net lactate production was observed in six, five, and eight of nine swine during isoflurane-, nitroglycerin-, and adenosine-induced hypotension, respectively (Table 1).
In swine without coronary stenosis, adenosine-induced hypotension caused a significant increase in QLAD and thereby a greater pvO2value as compared with isoflurane- or nitroglycerin-induced hypotension, whereas HR, MAP, and CO were similar during all three conditions. Lactate was not produced in any swine without coronary stenosis during induced hypotension (Table 2).
Subendocardial Microvessels during Induced Hypotension
An actual image of a subendocardial microvessel during isoflurane-induced hypotension is shown in Figure 1. Because the subendocardial arterioles and venules could not be separated with certainty in this experimental design in which neither dye was injected nor a long diastole was induced, all microvessels were evaluated together. Yada et al. reported that the subendocardial arterioles and venules showed similar phasic diameter changes in their observation.  During a 10-min period of observation, we recorded similar total numbers of subendocardial image fields (240, 231, and 214 fields during isoflurane-, nitroglycerin-, and adenosine-induced hypotension, respectively) with critical stenosis of the LAD. We visualized a significantly smaller number of microvessels during adenosine-induced hypotension (n = 72 in total, microvessel/field: 0.35 +/- 0.21) than during isoflurane-(n = 157 in total, microvessel/field: 0.62 +/- 0.22) or nitroglycerin-(n = 135 in total, microvessel/field: 0.60 +/- 0.31) induced hypotension, but there were no differences in mean diameters (60 +/- 35, 64 +/- 33, and 69 +/- 45 micro meter during isoflurane, nitroglycerin, and adenosine, respectively;Figure 2(A)). Diameter histograms of the subendocardial microvessels and phasic diameter changes in subendocardial microvessels greater than 100 micro meter during isoflurane-, nitroglycerin-, or adenosine-induced hypotension in swine with critical stenosis of the LAD are shown in Figure 2(B). The mean diameters decreased by 9 +/- 6% during isoflurane-induced hypotension and 12 +/- 5% during nitroglycerin-induced hypotension from end-diastole to end-systole, but no consistent phasic diameter changes were found during adenosine-induced hypotension.
In swine without coronary stenosis, there were no differences in the visualized numbers (106, 95, and 98 in total and microvessel/fields: 0.79 +/- 0.06, 0.67 +/- 0.09 and 0.75 +/- 0.07) and their mean diameters (49 +/- 31, 53 +/- 34, and 56 +/- 35 micro meter) of subendocardial microvessels among three types of induced hypotension (Figure 3(A)). The mean diameters during induced hypotension in these swine were significantly greater than those of corresponding values in swine with critical stenosis. Phasic diameter changes of subendocardial microvessels greater than 100 micro meter from end-diastole to end-systole occurred during the three types of induced hypotension (mean decreases from end-diastole to end-systole: 9 +/- 5, 12 +/- 8, and 9 +/- 5%, during isoflurane-, nitroglycerin-, and adenosine-induced hypotension, respectively;Figure 3(B)).
The results of the current study using a swine model with a fixed critical stenosis of the LAD show that isoflurane and nitroglycerin have similar effects on regional oxygen supply-demand balance, lactate metabolism, and the subendocardial microvessels, and that their effects differed from those of adenosine. There were no differences in these variables among the three drugs in swine without coronary stenosis except for luxury coronary perfusion by adenosine.
Lactate production suggests the presence of anaerobic metabolism and therefore myocardial ischemia, but the amount of its production does not necessarily reflect the severity of ischemia because the severity also depends on the tissue metabolic state and supply of substrates as well as tissue perfusion. Nevertheless, we presume that a perfusion abnormality, i.e., some parts of the myocardium were luxury perfused, whereas in other parts ischemia was intensified. This occurred more in the LAD-supplied myocardium with adenosine-induced hypotension than in that with isoflurane- or nitroglycerin-induced hypotension because adenosine-induced hypotension was associated with significantly higher values of interventricular venous pO2and regional lactate production, whereas SS, QLAD, and MVO2were similar during the three conditions. Although increased perfusion heterogeneity during adenosine-induced hypotension has been reported for hearts without coronary stenosis,  in poststenotic myocardium, such adenosine-induced perfusion abnormalities are enhanced by a redistribution of blood flow away from the endocardium. [9,20] In contrast, despite lower interventricular venous pO2, only marginal regional lactate production occurred during isoflurane- or nitroglycerin-induced hypotension as compared with adenosine-induced hypotension. Apparently, isoflurane- or nitroglycerin-induced hypotension did not cause a perfusion abnormality in the LAD-supplied myocardium.
The data obtained by intravital videomicroscopy also revealed differences between isoflurane- or nitroglycerin-induced hypotension and adenosine-induced hypotension in swine with coronary stenosis. During adenosine-induced hypotension, smaller numbers of microvessels were visible and hence accepted for analysis than during isoflurane- or nitroglycerin-induced hypotension. Microvessels are visualized with the videomicroscope by differences in the reflection of light from red cells and the surrounding myocardial tissue.  Thus the smaller number of visualized microvessels during adenosine-induced hypotension may be regarded as the result of either a derecruitment of perfused subendocardial microvessels or a markedly lowered microvascular hematocrit in the subendocardial vessels. Although the exact mechanism could not be identified from the present data, a reduction in the number of perfused microvessels (i.e., functional microvessel density) seems most likely because intracoronary adenosine does not appear to affect the small vessel hematocrit in the canine heart.  Irrespective of the mechanisms, our results indicate alterations in the subendocardial microvascular perfusion, which would reduce subendocardial oxygen supply during adenosine-induced hypotension, as compared with isoflurane- or nitroglycerin-induced hypotension. This conclusion is consistent with the data of myocardial oxygen supply-demand balance and lactate metabolism.
However, there was difference in the mean diameters of the subendocardial microvessels among the three drug-induced hypotensions despite the different vasodilatory properties of the drugs. Vascular diameters are affected by many factors, such as intrinsic and metabolic smooth muscle tone, intravascular pressure, and surrounding tissue pressure. The absence of any difference in the mean diameters of the subendocardial microvessels may indicate that the microvessels had been maximally dilated on implementation of critical stenosis before the induced hypotension and that these factors were roughly similar among the three types of hypotension. The fact that the mean diameters of subendocardial microvessels during induced hypotension in hearts with coronary stenosis were greater than the corresponding values in hearts without coronary stenosis may also confirm the dilation of subendocardial microvessels in the poststenotic region.
Phasic diameter changes in the microvessels are characteristic of the subendocardial microvessels in beating hearts, reflecting variation of the subendocardial microvascular blood volume during a cardiac cycle. [10–13] Subendocardial blood is squeezed to the subepicardial vessel in systole and refilled in diastole. They were always observed during induced hypotension in this study, but during adenosine-induced hypotension in swine with coronary stenosis. Because adenosine in the presence of a stenosis has no effect on diastolic inflow but increases systolic retrograde flow in the septal artery,  subendocardial phasic diameter changes in the present study should have persisted or been enhanced during adenosine-induced hypotension. The absence of consistent phasic diameter change suggests that systolic blood flow from the endocardium to the epicardium did not occur and, therefore, that subendocardial microcirculation was impaired. We speculate that it was caused by collapse of transmyocardial vessels during systole. This would occur if the normal systolic intramyocardial tissue gradient across the ventricular wall, minimal in the epicardial layer and maximal in the endocardial layer, is lost, and the intramyocardial vessels are compressed by intramyocardial tissue pressure during systole. Because no measurements of intramyocardial pressures or coronary artery pressure distal to the stenosis were performed in the present study, this mechanism remains speculative. It is also possible that a decrease in vascular smooth muscle tone by adenosine may have contributed to the collapse of intramyocardial vessels during adenosine-induced hypotension.  In contrast to adenosine, isoflurane and nitroglycerin appear to preserve subendocardial microvascular perfusion in the poststenotic region because the phasic diameter change in subendocardial microvessels was not lost. Recently, Kajiya's laboratory reported that nitroglycerin injection increases phasic diameter changes in subendocardial arterioles, presumably, by enhancing systolic retrograde flow. 
The findings of the present study may thus indicate that the action of isoflurane on the coronary circulation is similar to that of nitroglycerin rather than that of adenosine. This may be attributed to the fact that isoflurane is not as potent a vasodilator as adenosine, as shown by recent studies. [3,25] In addition, a decrease in intramyocardial tissue pressure may have acted to preserve the phasic diameter changes in the subendocardial microvessels during isoflurane- or nitroglycerin-induced hypotension. A recent study in chronically instrumented dogs showed that isoflurane decreases intramyocardial tissue pressure because of its negative inotropic effect.  The decrease in intramyocardial tissue pressure causes an increase in intramyocardial perfusion pressure and makes collapse of the intramyocardial vessels less likely.  Similar beneficial effects of a vasodilator with a negative inotropic property, nifedipine, on subendocardial perfusion and on lactate metabolism have been reported in swine with coronary stenosis.  A difference in the vascular reserve between the subepicardial and subendocardial layers may have also played a role.  Negative inotropism of isoflurane may have caused relative vasoconstriction in the subepicardial layer via a decrease in metabolic demand, thereby facilitating a blood flow shift to the subendocardium. The two possible factors, intramyocardial tissue pressure and vascular reserve, cannot be quantified.
Left ventricular end diastolic pressure (LVEDP) was the least during nitroglycerin-induced hypotension. Because subendocardial tissue pressure at end-diastole is equal to LVEDP, the decrease in LVEDP may have contributed to preservation of the phasic diameter changes in the subendocardial microvessels during nitroglycerin-induced hypotension through enhancement of diastolic coronary inflow. This effect of nitroglycerin has been postulated as one mechanism for relieving myocardial ischemia. 
Direct comparison of the present study with other studies is not possible because we did not determine regional blood flow distribution. However, other studies have invariably reported subendocardial hypoperfusion in the region distal to coronary stenosis with isoflurane-induced hypotension, as compared with the normal subendocardium. [5,8,9] The findings in the present study regarding the subendocardial perfusion do not appear to be at variance with those of most recent studies. In an experimental set-up with critical stenosis of the left circumflex coronary artery (LCA) in dogs, Tatekawa et al.  observed subendocardial hypoperfusion of the LCA supplied region with anesthetic concentrations of isoflurane, but wall thickness, MVO2and oxygen extraction did not differ between the LCA- and LAD-supplied regions. The authors concluded that isoflurane, albeit inducing transmural coronary steal, does not induce subendocardial ischemia, which suggests a preserved subendocardial circulation. Another recent study comparing the effects of isoflurane, halothane, and enflurane at similar arterial pressure (MAP = 75 mmHg) in the swine heart with coronary stenosis reported that isoflurane did not decrease subendocardial perfusion, as compared with halothane and enflurane.  The authors speculated that worsened ischemic metabolism during isoflurane (increased lactate production) was a result of less myocardial depression with isoflurane than with the other anesthetics. Wilton et al.  showed that 2.1% isoflurane and accompanying hypotension (MAP = 59 mmHg) caused transmural coronary steal (subepicardial hyperperfusion at the expense of subendocardial hypoperfusion) in the myocardium distal to a critical stenosis in dogs. Although their study showed a relation between subendocardial hypoperfusion and a decrease in wall thickening with isoflurane anesthesia, this does not necessarily imply impairment of subendocardial microcirculation with isoflurane.
Critique of Methods and Limitations
This study was conducted in a swine model, the coronary circulation of which is close to that of humans. We produced acutely critical stenosis of the LAD to simulate the condition of one-vessel disease. Although presence of a collateral dependent myocardium is a prerequisite for intercoronary steal, transmural steal occurs downstream from a coronary stenosis as a result of earlier exhaustion of the subendocardial vascular reserve when coronary pressure decreases. Hence, it requires no collateral vessels. We believe, therefore, that this acute model in the current study has clinical relevance to the chronic condition of one-vessel disease, although factors such as vascular reactivity and endothelial function, which may be impaired in such patients, could not be examined in our acute model.
In the present study, we randomized the order of drug administration and placed a period of hemodynamic recovery and stabilization between the induced hypotensions to validate the comparison of the three drugs. We found fibrin-formation and abruption of microvessels by in vivo microscopic examination during induced hypotension in two swine and irreversible ischemic changes in the subendocardium by postmortem examination in the same animals. Those animals were excluded from the study. On the other hand, there were no such findings in the in vivo microvascular observation during induced hypotension or postmortem examination in any of the swine evaluated in the study. In these swine, hemodynamic parameters, such as MAP, cardiac output, and QLAD, soon returned to the near-baseline values on cessation of hypotensive drugs during the recovery phase. We thus believe that there were no irreversible changes or carry-over effects in the subendocardial microcirculation in these swine.
We investigated the effects of isoflurane on myocardial perfusion during induced hypotension. In addition, the swine received several drugs at induction of anesthesia, inhalation of 50% nitrous oxide, and constant infusion of fentanyl and pancuronium during the experiment. We hence admit that these drugs necessary for anesthesia may have minimally influenced the results of the current study and may present some limitations for comparison with other studies in chronically instrumented animals.  In addition, the effects of induced hypotension and isoflurane were indistinguishable in the present study. The preserved subendocardial microvascular perfusion in this experimental condition, however, indicates the safety of clinical application of isoflurane during a normotensive state in patients with one-vessel coronary disease.
For our protocol it was essential to observe subendocardial microvessels within the LAD-supplied region of the left ventricle, i.e., the anterior-free wall or the anterior part of the septal wall. The previously developed approach through the left ventricular atrium  was not suitable in our experimental model because it caused severe hemodynamic alterations when the needle probe was advanced to the anterior-free wall during induced hypotension. We developed a transventricular access through the lateral free wall, which allows for insertion of the needle probe perpendicular to the anterior septal wall, thereby causing only minimal hemodynamic disturbance. The septal myocardium is influenced by right ventricular pressure, contrasting with the free-wall myocardium, which is devoid of such influence. Although the latter part would hence seem to be better for our purpose, we think that there was a sufficient transmural pressure gradient for the intramyocardial pump mechanism in the former part because systolic left pressures reached between 80–90 mmHg at a MAP of 55 mmHg in our animals. Mechanical irritation of the myocardium by the needle probe would cause ventricular extrasystoles, which could alter microvessel diameters. To avoid this influence, electrocardiograph and dP/dt signals were superimposed on the monitor and recorded simultaneously with the microvessel images. Thus diameter measurements were only performed in absence of ventricular extrasystole for several cardiac cycles before the time of measurements. However, it was not possible to obtain control data of the subendocardial microvessels during a normotensive state with and without critical stenosis in this experimental model because of the stronger heart motion. The effects of critical stenosis on the subendocardial microvessels thus remained unknown.
In conclusion, the observation of the subendocardial microvessels in a swine model of a single-vessel coronary disease in this study demonstrated that subendocardial microcirculation is preserved during isoflurane- and nitroglycerin-induced hypotension, as evident from the phasic diameter change in subendocardial microvessels. In contrast, phasic diameter change was absent during adenosine-induced hypotension. Similarities of isoflurane-induced hypotension to nitroglycerin-induced hypotension but not to adenosine-induced hypotension also were found in interventricular pO2and poststenotic lactate production. The present study indicates that isoflurane-induced hypotension is not associated with impairment of subendocardial microcirculation in the heart with critical coronary stenosis and confirms that isoflurane is unlikely to cause subendocardial ischemia through transmural coronary steal.
The authors thank Professor F. Kajiya, Chairman, Department of Medical Engineering and Systems Cardiology, Kawasaki Medical School, for his kind advice and support of the project, and T. Kikuno and S. Miyake, students of Kawasaki College of Allied Health Professions for the evaluation of recorded microvascular images.