Relation between local myocardial growth and blood flow during chronic ventricular pacing

Several studies have shown that, per unit mass, myocardial blood flow (MBF) and oxygen consumption are similar in hypertrophic and non-hypertrophic ventricles. This observation may be explained by the degree of myocardial growth matching the increase in oxygen demand. Such matching may, however, not be perfect at the local level, because substantial heterogeneity of MBF exists within the ventricular wall. We investigated to what extent local growth and MBF are matched after redistribution of workload within the left ventricular (LV) wall. Redistribution of workload was established by ventricular pacing at physiological heart rate, which induces asynchronous activation and contraction. Local wall mass (2D-echocardiography) and MBF (fluorescent microspheres) were determined in the canine LV wall before (t=0) and after 6 months of normal sinus rhythm (SHAM group, n=5) or 6 months of pacing at the LV free wall (PACE group, n=8). During acute pacing MBF (ml/min/g) increased with increasing distance to the pacing site. Local relative MBF (rMBF, local MBF normalized to mean MBF in the LV wall) varied from 0.8 adjacent to the pacing site to 1.2 in remote regions. After 6 months of pacing these regional differences had disappeared, probably due to changes in wall mass, which increased with increasing distance to the pacing site (by up to 39+/-13%). In SHAM animals rMBF at t=0 correlated well with rMBF 6 months later (r=0.71). In PACE animals, however, this correlation was poor (r=0.33), because rMBF increased in regions close to the pacing site with initial rMBF<1 and rMBF decreased in regions remote from the pacing site with initial rMBF>1.

CONCLUSIONS

After redistribution of workload within the LV wall as induced by ventricular pacing, local load-regulated growth tends to equalize MBF distribution, but local adaptation of MBF also depends on initial MBF.

Effects of single- and biventricular pacing on temporal and spatial dynamics of ventricular contraction

Resynchronization is frequently used for the treatment of heart failure, but the mechanism for improvement is not entirely clear. In the present study, the temporal synchrony and spatiotemporal distribution of left ventricular (LV) contraction was investigated in eight dogs during right atrial (RA), right ventricular apex (RVa), and biventricular (BiV) pacing using tagged magnetic resonance imaging. Mechanical activation (MA; the onset of circumferential shortening) was calculated from the images throughout the left ventricle for each pacing protocol. MA width (time for 20-90% of the left ventricle to contract) was significantly shorter during RA (43.6 +/- 17.1 ms) than BiV and RVa pacing (67.4 +/- 15.2 and 77.6 +/- 16.4 ms, respectively). The activation delay vector (net delay in MA from one side of the left ventricle to the other) was significantly shorter during RA (18.9 +/- 8.1 ms) and BiV (34.2 +/- 18.3 ms) than during RVa (73.8 +/- 16.3 ms) pacing. Rate of LV pressure increase was significantly lower during RVa than RA pacing (1,070 +/- 370 vs. 1,560 +/- 300 mmHg/s) with intermediate values for BiV pacing (1,310 +/- 220 mmHg/s). BiV pacing has a greater impact on correcting the spatial distribution of LV contraction than on improving the temporal synchronization of contraction. Spatiotemporal distribution of contraction may be an important determinant of ventricular function.

Remodeling by ventricular pacing in hypertrophying dog hearts

OBJECTIVE

Asynchronous electrical activation of the left ventricle (LV), induced by ventricular pacing (VP), reduces mechanical load in early- and enhances it in late-activated regions. Consequently, chronic VP leads to asymmetric hypertrophy. We investigated whether such locally induced myocardial hypertrophy also occurs in the presence of pressure overload hypertrophy (POH).

METHODS

POH was induced by aortic banding in puppies. At age 9 months, seven dogs were paced at the right ventricular (RV) apex at physiological heart rate for 6 months (POH-pace group), while four POH dogs served as POH-control group. Changes in volume of the LV cavity and the total LV wall and of five LV wall sectors were measured by means of 2D-echocardiography and X-ray marker detection.

RESULTS

During the last 6 months of the protocol the volume of the five LV wall sectors increased in the POH-control group, ranging from 27+/-9 to 30+/-5% (mean+/-S.D.). In POH-pace animals sector wall volume in the four sectors at intermediate to long distance from the pacing site increased to a similar extent (ranging from 31+/-16 to 35+/-17%), but wall volume in the early-activated apical septum increased significantly less (17+/-21%). In these hearts myocyte diameter was significantly smaller in the apical septum than in the lateral LV wall. The regional difference in wall volume changes (19+/-21%) was significantly smaller in the POH-pace group than in chronically paced, non-hypertrophic, canine hearts in a previous study from our laboratory (43+/-14%).

CONCLUSIONS

In hypertrophying hearts chronic pacing at the RV apex suppresses the development of hypertrophy in the early-activated apical septum but does not cause additional hypertrophy in late-activated regions, as is the case in non-hypertrophic hearts. The latter suggests that the local growth response is reduced in hypertrophying hearts.

High-resolution functional imaging with ultrasound contrast agents based on RF processing in an in vivo kidney experiment

Knowledge of the relative tissue perfusion distribution is valuable in the diagnosis of numerous diseases. Techniques for the assessment of the relative perfusion distribution, based on ultrasound (US) contrast agents, have several advantages compared to established nuclear techniques. These are, among others, a better spatial and temporal resolution, the lack of exposure of the patient to ionizing radiation and the relatively low cost. In the present study, US radiofrequency (RF) image sequences are acquired, containing the signal intensity changes associated with the transit of a bolus contrast agent through the microvasculature of a dog kidney. The primary objective is to explore the feasibility of calculating functional images with high spatial resolution. The functional images characterize the transit of the contrast agent bolus and represent distributions of peak time, peak value, transit time, peak area, wash-in rate and wash-out decay constant. For the evaluation of the method, dog experiments were performed under optimized conditions where motion artefacts were minimized and an IA injection of the contrast agent Levovist was employed. It was demonstrated that processing of RF signals obtained with a 3.5-MHz echo system can provide functional images with a high spatial resolution of 2 mm in axial resolution, 2 to 5 mm in lateral resolution and a slice thickness of 2 mm. The functional images expose several known aspects of kidney perfusion, like perfusion heterogeneity of the kidney cortex and a different peripheral cortical perfusion compared to the inner cortex. Based on the findings of the present study, and given the results of complimentary studies, it is likely that the functional images reflect the relative perfusion distribution of the kidney.

Blood flow distributions by microsphere deposition methods

The art and science of the use of deposition markers for the estimation of blood flow distributions throughout the body and within organs is reviewed. Development of diffusible tracer techniques started 50 years ago. Twenty years later, radioactive 15 micron microspheres became the standard marker. Early studies on small animals, fetal sheep in 1967 and rats in 1976, provoked much of the technical development. Needs for avoiding the use of radioactivity, for having long lasting labels, and for providing higher spatial resolution, are driving the continuing exploration of newer techniques using colored and fluorescent microspheres and molecular deposition markers. Strengths and weaknesses of the various methods are compared.

Kinematic analysis of left ventricular deformation in myocardial infarction using magnetic resonance cardiac tagging

The Magnetic Resonance (MR) tagging technique provides detailed information about 2D motion in the plane of observation. Interpretation of this information as a reflection of the 3D motion of the entire cardiac wall is a major problem. In finite element models of the mechanics of the infarcted heart, an infarcted region causes motional asymmetry, extending far beyond the infarct boundary. Here we present a method to quantify such asymmetry in amplitude and orientation. For this purpose images of a short-axis cross-section of the ejecting left ventricle were acquired from 9 healthy volunteers and 5 patients with myocardial infarction. MR-tags were applied in a 5 mm grid at end-diastole. The tags were tracked by video-image analysis. Tag motion was fitted to a kinematic model of cardiac motion. For the volunteers and the patients the center of the cavity displaced by about the same amount (p = 0.11) during the ejection phase: 3.8 +/- 1.4 and 3.0 +/- 0.9 mm (mean +/- sd), respectively. Cross-sectional rotation and the decrease in cross-sectional area of the cavity were both greater in the volunteers than in the patients: 6.4 +/- 1.5 vs. 3.0 +/- 0.8 degrees (p < 0.001), and 945 +/- 71 vs. 700 +/- 176 mm2 (p = 0.02), respectively. In the patients, asymmetry of wall motion, as expressed by a sine wave dependency of contraction around the circumference, was significantly enlarged (p = 0.02). The proposed method of kinematic analysis can be used to assess cardiac deformation in humans. We expect that by analyzing images of more cross-sections simultaneously, the 3D location and the degree of infarction can be assessed efficiently.

Mapping of regional myocardial strain and work during ventricular pacing: experimental study using magnetic resonance imaging tagging

OBJECTIVES

The purpose of this study was to determine the spatial distribution of myocardial function (myofiber shortening and work) within the left ventricular (LV) wall during ventricular pacing.

BACKGROUND

Asynchronous electrical activation, as induced by ventricular pacing, causes various abnormalities in LV function, perfusion and structure. These derangements may be caused by abnormalities in regional contraction patterns. However, insight into these patterns during pacing is as yet limited.

METHODS

In seven anesthetized dogs, high spatial and temporal resolution magnetic resonance-tagged images were acquired in three orthogonal planes. Three-dimensional deformation data and LV cavity pressure and volume were used to determine midwall circumferential strain and external and total mechanical work at 192 sites around the left ventricle.

RESULTS

During ventricular pacing, systolic fiber strain and external work were approximately zero in regions near the pacing site, and gradually increased to more than twice the normal value in the most remote regions. Total mechanical work, normalized to the value during right atrial pacing, was 38 +/- 13% (right ventricular apex [RVapex] pacing) and 61 +/- 23% (left ventricular base [LVbase] pacing) close to the pacing site, and 125 +/- 48% and 171 +/- 60% in remote regions, respectively (p < 0.05 between RVapex and LVbase pacing). The number of regions with reduced work was significantly larger during RVapex than during LVbase pacing. This was associated with a reduction of global LV pump function during RVapex pacing.

CONCLUSIONS

Ventricular pacing causes a threefold difference in myofiber work within the LV wall. This difference appears large enough to regard local myocardial function as an important determinant for abnormalities in perfusion, metabolism, structure and pump function during asynchronous electrical activation. Pacing at sites that cause more synchronous activation may limit the occurrence of such derangements.

Mapping propagation of mechanical activation in the paced heart with MRI tagging

The temporal evolution of three-dimensional (3-D) strain maps derived from magnetic resonance imaging (MRI) tagging were used to noninvasively evaluate mechanical activation in the left ventricle (LV) while seven canine hearts were paced in situ from three different sites: the base of the LV free wall (LVb), the right ventricular apex (RVa), and the right atrium (RA). Strain maps plotted against time showed the evolution of shortening over the entire LV midwall and were used to generate mechanical activation maps showing the onset of circumferential shortening. RA pacing showed rapid synchronous shortening; LVb pacing showed a wave front of mechanical activation propagating slowly and steadily from the pacing site, whereas RVa pacing showed regions of rapid and slower propagation. The mechanical (M) activation times correlated linearly with the electrical (E) activation (M = 1.06E + 8.4 ms, R = 0.95). The time for 90% activation of the LV was 63.1 +/- 24.3 ms for RA pacing, 130.2 +/- 9.8 ms for LVb pacing, and 121.3 +/- 17.9 ms for RVa pacing. The velocity of mechanical activation was calculated for LVb and RVa pacing and was similar to values reported for electrical conduction in myocardium. The propagation of mechanical activation for RVa pacing showed regional variations, whereas LVb pacing did not.

Hyperoxia and local organ blood flow in the developing chick embryo

1. Hyperoxia can cause local vasoconstriction in adult animal organs as a protective mechanism against hyperoxia-induced toxicity. It is not known at what time during development this vasoconstrictor capacity is present. Therefore, we measured the cardiac output (CO) distribution in different organs during a period of acute hyperoxia (100 % O2) in the developing chick embryo. 2. Fertile eggs were divided into five incubation time groups (10 and 11, 12 and 13, 14 and 15, 16 and 17, and 18 and 19 days of a normal incubation time of 21 days). Eggs were opened at the air cell and a catheter was inserted into a branch of the chorioallantoic vein for injections of 15 microm fluorescent microspheres during normoxia and at the end of 5 min (test group 1; n = 39) or 20 min (test group 2; n = 21) of hyperoxia exposure (100 % O2). The fraction of CO to an organ was calculated as the fluorescence of the organ sample divided by the sum of the fluorescence of all organs. 3. Only in 18- and 19-day-old embryos did hyperoxia cause a decrease in the fractions of CO to the heart and carcass, and an increase in those to the yolk-sac and chorioallantoic membrane. This response was more pronounced after 20 min (test group 2) than after 5 min (test group 1) of hyperoxia with an additional decrease in the fractions of CO to the brain, intestine and liver (test group 2). 4. These data indicate that local mechanisms for hyperoxia-induced vasoconstriction in the heart, brain, liver, intestine and carcass develop late, during the final 15 % of the incubation period, in the developing chick embryo.

Optimization of ventricular function by improving the activation sequence during ventricular pacing

Abnormal electrical activation occurring during ventricular pacing reduces left ventricular (LV) pump function. Two strategies were compared to optimize LV function using ventricular pacing, minimal asynchrony and optimal sequence of electrical activation. ECG and hemodynamics aortic flowprobe, thermodilution cardiac output, LV pressure and its maximal rates of rise (LVdP/dtpos) and fall (LVdP/dtneg) were measured in anesthetized open-chest dogs (n = 7) with healthy hearts. The QRS duration (a measure of asynchrony of activation) was 47 +/- 5 ms during sinus rhythm and increased to 110 +/- 12 ms during DDD pacing at the right ventricular (RV) apex with a short AV interval. During pacing at the LV apex and LV base, the QRS duration was 8% +/- 7% and 15% +/- 7% (P < 0.05) longer than during RV apex pacing, respectively. Stroke volumes, LVdP/dtpos and LVdP/dtneg, however, were higher during LV apex (15% +/- 16%, 10% +/- 12% [P < 0.05], and 15% +/- 10%, respectively) and LV base pacing (11% +/- 12% [P < 0.05], 3% +/- 12%, and 3% +/- 11%, respectively) than during RV apex pacing. Systolic LV pressure was not influenced significantly by the site of pacing. Biventricular pacing (RV apex together with one or two LV sites) decreased the QRS duration by approximately 20% as compared with RV apex pacing, however, it did not improve stroke volumes, LVdP/dtpos and LVdP/dtneg beyond those during pacing at the LV apex alone. In conclusion, the sequence of electrical activation is a stronger determinant of ventricular function than the synchrony of activation. For optimal LV function the selection of an optimal single pacing site, like the LV apex, is more important than pacing from multiple sites.

Estimates of regional work in the canine left ventricle

Assessment of the magnitude of regional myocardial work requires knowledge of regional fiber stress and fiber shortening. The theoretical development and experimental validation of a method is presented which used values of estimated active and passive fiber stress according to a fluid-fiber model, and measured fiber strain values. This enables the construction of regional stress-strain diagrams, a regional analog of the pressure-volume area model by Suga and co-investigators, which can be linked to regional oxygen consumption. In the left ventricle, either normally or asynchronously activated, the method yields reliable data on strain and active and passive fiber stress. The relation between estimated regional work and myocardial oxygen demand is in quantitative agreement with previously reported relations between global oxygen demand and measured pressure-volume area. During coronary artery occlusion, however, these values were less reliable, which might be due to inaqdequate knowledge of the (passive) material properties of the myocardium.

Asynchronous electrical activation induces asymmetrical hypertrophy of the left ventricular wall

BACKGROUND

Asynchronous electrical activation, induced by ventricular pacing, causes regional differences in workload, which is lower in early- than in late-activated regions. Because the myocardium usually adapts its mass and structure to altered workload, we investigated whether ventricular pacing leads to inhomogeneous hypertrophy and whether such adaptation, if any, affects global left ventricular (LV) pump function.

METHODS AND RESULTS

Eight dogs were paced at physiological heart rate for 6 months (AV sequential, AV interval 25 ms, ventricular electrode at the base of the LV free wall). Five dogs were sham operated and served as controls. Ventricular pacing increased QRS duration from 47.2+/-10.6 to 113+/-16.5 ms acutely and to 133.8+/-25.2 ms after 6 months. Two-dimensional echocardiographic measurements showed that LV cavity and wall volume increased significantly by 27+/-15% and 15+/-17%, respectively. The early-activated LV free wall became significantly (17+/-17%) thinner, whereas the late-activated septum thickened significantly (23+/-12%). Calculated sector volume did not change in the LV free wall but increased significantly in the septum by 39+/-13%. In paced animals, cardiomyocyte diameter was significantly (18+/-7%) larger in septum than in LV free wall, whereas myocardial collagen fraction was unchanged in both areas. LV pressure-volume analysis showed that ventricular pacing reduced LV function to a similar extent after 15 minutes and 6 months of pacing.

CONCLUSIONS

Asynchronous activation induces asymmetrical hypertrophy and LV dilatation. Cardiac pump function is not affected by the adaptational processes. These data indicate that local cardiac load regulates local cardiac mass of both myocytes and collagen.

Fluorescent microspheres are superior to radioactive microspheres in chronic blood flow measurements

The accuracy of the fluorescent (FM) and radioactive microsphere (RM) techniques is similar in acute experiments but has not been established in chronic experiments. In the present study various combinations (at least pairs) of FM and/or RM labels were injected simultaneously between 2 mo and 5 min before each animal was killed. Blood flow was determined in many organs. Intramethod mean difference and variation did not change over time for FM but increased significantly for RM (from 1.8 +/- 1.4 to 25.6 +/- 21.8% and from 4.4 +/- 3.2 to 32.4 +/- 23.0% at 5 min and 2 mo, respectively). Also the FM-RM intermethod mean difference and variation increased (from -0.5 +/- 8.5 to 40.8 +/- 23.8% and from 23. 6 +/- 4.6 to 71.8 +/- 34.3%, respectively). After 2 mo, blood flow estimations were 20-50% lower with the various RM, whereas brain and liver blood flow values varied even more between isotopes. Underestimation started within 1 day for 51Cr and within 2 wk for 141Ce, 95Nb, and 85Sr. We conclude that FM are superior to RM for blood flow determination in experiments lasting longer than 1 day, presumably because of leaching of isotopes from RM.

Cardiac output distribution in response to hypoxia in the chick embryo in the second half of the incubation time

1. The fetus develops cardiovascular adaptations to protect vital organs in situations such as hypoxia and asphyxia. These include bradycardia, increased systemic blood pressure and redistribution of the cardiac output. The extent to which they involve maternal or placenta influences is not known. The objective of the present work was to study the cardiac output distribution in response to hypoxia in the chick embryo, which is independent of the mother. 2. Fertilized eggs were studied at three incubation times (10-13 days, 14-16 days and 17-19 days of a normal incubation time of 21 days). Eggs were placed in a Plexiglass box in which the oxygen concentration could be changed. Eggs were opened at the air cell and a chorioallantoic vein was catheterized. Cardiac output distribution was measured with 15 micron fluorescent microspheres injected during normoxia, during the last minute of a 5 min period of hypoxia and after 5 min of subsequent reoxygenation. 3. Hypoxia caused a redistribution of the cardiac output in favour of heart (+17 to +160 % of baseline) and brain (+21 to +57 % of baseline) at the expense of liver (-3 to -65 % of baseline), yolk-sac (-46 to -77 % of baseline) and carcass (-6 to -33 % of baseline). 4. The magnitude of the changes in cardiac output distribution to the heart, brain, liver and carcass in response to hypoxia increased with advancing incubation time. 5. The data demonstrate the development of a protective redistribution of the cardiac output in response to hypoxia in the chick embryo from day 10 of incubation.

Imaging asynchronous mechanical activation of the paced heart with tagged MRI

A method for imaging the rapid temporal-spatial evolution of myocardial deformations in the paced heart is proposed. High time resolution-tagged MR images were obtained after stimulation of the myocardium with an MR-compatible pacing system. The images were analyzed to reconstruct dynamic models of local 3D strains over the entire left ventricle during systole. Normal canine hearts were studied in vivo with pacing sites on the right atrium, left ventricular free wall and right ventricular apex. This method clearly resolved local variations in myocardial contraction patterns caused by ventricular pacing. Potential applications are noninvasive determination of electrical conduction abnormalities and the evaluation of new pacing therapies.

Hemodynamic and coronary vascular effects of dexmedetomidine in the anesthetized goat

BACKGROUND

In phase III trials, the hemodynamic stabilising effect of the alpha 2-adrenergic agonist dexmedetomidine (DEX) is being investigated in patients with coronary artery disease, Coronary vascular effects of alpha 2-agonists have been studied in dogs and pigs, but both species have a different hemodynamic response to DEX than man. The aim of this study was to investigate the hemodynamic and coronary vascular effects in goats.

METHODS

In 6 open-chest goats anesthetized with halothane, central and coronary hemodynamics and oxygen supply and demand were measured before and following IV bolus infusion of DEX in doses ranging from 0.1 to 10 micrograms/kg.

RESULTS

With DEX doses of 1 microgram/kg or higher, mean arterial pressure (MAP), systemic vascular resistance, coronary vascular resistance and arterio-mixed venous oxygen content increased within 2 min, but returned to baseline within 15 min. In contrast, there was a progressive and cumulative decrease in cardiac output (CO), heart rate, and dP/dtmax, Regional coronary venous oxygen extraction (C(a-cv)O2) transiently increased after 3 micrograms/kg DEX and decreased 15 min after 10 micrograms/kg DEX. LVEDP transiently increased after 3 and 10 micrograms/kg DEX. The changes after DEX 10 micrograms/kg differed from those after lower doses: MAP (35%), CO (50%), stroke volume (33%), C(a-cv)O2 (15%) and myocardial oxygen extraction (33%) were all decreased. Myocardial oxygen supply and demand decreased in parallel.

CONCLUSIONS

1) The cardiovascular response to IV DEX in goats is similar to man. 2) In goats after DEX, systemic and coronary vasoconstriction are short-lived, and 3) the balance between myocardial oxygen supply and demand is maintained.

Cardiac output distribution in the chick embryo from stage 36 to 45

OBJECTIVE

The distribution of cardiac output to different organs is well described in the mammalian fetus. Chick embryos are not often used in perinatal cardiovascular research and therefore it is not known whether they can serve as an animal model for this purpose. In this study we documented cardiac output distribution in chick embryos at increasing incubation time.

METHODS

Fertilized eggs from day 10 to 19 with an incubation time of 21 days were studied in 3 increasing incubation time groups (10-13, 14-16 and 17-19 days). For the experiment, the egg was placed in a holder in an incubator. The egg was opened at the air cell and a small vein of the chorioallantoic membrane was catheterized. Twenty thousand fluorescent 15 microns microspheres in 0.2 ml were injected. After 5 min, the embryo was sacrificed and the different organs were dissected and digested for microsphere isolation and subsequent fluorescence analysis.

RESULTS

The chorioallantoic membrane, which is the placenta equivalent of the chick embryo, received a relatively large fraction of the combined cardiac output: 52.08% (interquartile range [IQR] 12.67%) on days 10-13 and 40.95% (IQR 27.24%) on days 17-19. Relatively small fractions were distributed: to the heart 2.03% (IQR 1.58) on days 10-13 and 3.18% (IQR 1.95) on days 17-19, and to the brain 3.20% (IQR 1.80) on days 10-13 and 5.02% (IQR 3.39) on days 17-19. As incubation time advanced, the fraction of the combined cardiac output to the chorioallantoic membrane and yolk-sac decreased significantly in favor of the heart and brain.

CONCLUSION

This distribution shows great similarity to the one found in the mammalian fetus. The chick embryo is an attractive model for perinatal cardiovascular research.

Relation between torsion and cross-sectional area change in the human left ventricle

During the ejection phase, motion of the left ventricular (LV) wall is such that all myocardial fibers shorten to the same extent. In a mathematical model of LV mechanisms it was found that this condition could be satisfied only if torsion around the long axis followed a unique function of the ratio of cavity volume to wall volume. When fiber shortening becomes non-uniform due to cardiac pathology, this pathology may be reflected in aberration of the torsional motion pattern. In the present study we investigated whether the predicted regular motion pattern could be found in nine healthy volunteers, using Magnetic Resonance Tagging. In two parallel short-axis cross-sections, displacement, rotation, and area ejection were derived from the motion of tags, attached non-invasively to the myocardium. Information from both sections was combined to determine area ejection, quantified as the change in the logarithm of the ratio of cavity area to wall area, and torsion, represented by the shear angle on the epicardium. Linear regression was applied to torsion as a function of area ejection. The slope thus found (-0.173 +/- 0.024 rad, mean +/- S.D.) was similar to the slope as predicted by the model of LV mechanics (-0.194 +/- 0.026 rad). In conclusion, the relation between area ejection and torsion could be assessed noninvasively in humans. In healthy volunteers, the relation was close to what was predicted by a mathematical model of LV mechanics, and also close to what was found earlier in experiments on animals.

Alleviation of the peripheral hemodynamic effects of dexmedetomidine by the calcium channel blocker isradipine

BACKGROUND

Alpha 2-adrenergic agonists have peripheral vasoconstrictive effects and central sympatholytic and sedative effects. Whereas the latter are the basis of their use in anesthesia, the former could limit their clinical application.

METHODS

To study whether a vasodilator could alleviate the systemic and coronary vasoconstrictor effects of dexmedetomidine without influencing the central sympatholytic effects, the calcium channel blocker isradipine was infused after a high dose of dexmedetomidine in anesthetized dogs.

RESULTS

Dexmedetomidine 10 micrograms.kg-1 decreased plasma concentrations of norepinephrine and epinephrine by more than 90%, heart rate by 39%, cardiac output by 64%, dp/dtmax by 29% and increased mean arterial pressure by 55% and the left ventricular end-diastolic pressure (LVEDP) 4-fold as compared to baseline. In addition, coronary blood flow decreased by 52% and coronary venous oxygen saturation by 51%. Isradipine could completely antagonize all the coronary and systemic hemodynamic changes induced by dexmedetomidine, but only partially he increase in LVEDP. Isradipine caused no changes in plasma catecholamine levels.

CONCLUSION

Isradipine could alleviate the peripheral hemodynamic actions of dexmedetomidine while having no effect on its central sympatholytic properties.

Regional electrical activation and mechanical function in the partially ischemic left ventricle of dogs

During normoxia, asynchronous electrical activation of the left ventricle significantly affects regional mechanical performance. Regional fiber strain and external work during the ejection phase are found to be lower in early-activated than in late-activated regions. Because electrical activation is known to be delayed during ischemia, the present study was designed to investigate the influence of this electrical asynchrony on regional fiber strain, if any, during moderate and severe myocardial ischemia. Regional electrical activation time (t(ea)) and fiber strain during the ejection phase (ef,e) were measured in the epicardial layers of the left ventricular anterior wall during normoxia and after 15 min of total occlusion (n = 5) or 30, 60, 120, and 180 min of partial occlusion of the left anterior interventricular coronary artery (LAICA; n = 11). Myocardial blood flow (MBF) was assessed with radioactive microspheres. Blood gases, pH, and lactate and Pi contents were determined in arterial, local venous, and coronary sinus blood. During normoxia, t(ea) and ef,e were similar in various epicardial regions of the left ventricular anterior wall. During total LAICA occlusion, in the ischemic area, subepicardial MBF decreased from 0.86 +/- 0.36 (SD) to 0.18 +/- 0.09 ml.g-1.min-1 and subepicardial ef,e decreased from -0.11 +/- 0.02 to -0.01 +/- 0.01, whereas the delay in t(ea) between the normoxic basal-lateral and ischemic apical-medial areas increased slightly but significantly from 1.9 +/- 8.0 to 7.5 +/- 8.0 ms. After a 180-min partial occlusion of the LAICA, in the ischemic area, subepicardial MBF decreased from 0.62 +/- 0.17 to 0.49 +/- 0.18 ml.g-1.min-1 and ef,e decreased from -0.08 +/- 0.01 to -0.03 +/- 0.01. No significant change in the difference in t(ea) between the normoxic and ischemic areas could be detected (5.1 +/- 4.8 and 5.2 +/- 5.8 ms in the control situation and after 180-min partial occlusion, respectively). These findings indicate that in the ischemic epicardium 1) mechanical function is more affected than electrical impulse conduction and 2) delayed activation, if any, is accompanied by decreased instead of increased fiber strain, as found in the normoxic left ventricle.

The effect of dexmedetomidine on nutrient organ blood flow

The alpha 2-adrenergic agonist dexmedetomidine decreases not only heart rate, myocardial contractility, and oxygen demand, but also cardiac output (Q). To investigate whether this reduction in Q could critically impair perfusion of individual organs, we studied the effect of dexmedetomidine on nutrient blood flow to the heart, brain, kidney, spleen, skin, intestine, liver, and arteriovenous anastomoses using the radioactive microsphere technique. Studies were conducted in 14 dogs with an open chest and anesthetized with either chloralose/urethane (CU) or fentanyl/halothane (FH), to create different baseline conditions. Hemodynamic variables, organ blood flow, arterial and mixed venous oxygen, and lactate content were measured before and after administration of 0.1, 1, and 10 micrograms/kg dexmedetomidine intravenously (IV). After 10 micrograms/kg dexmedetomidine Q decreased in both groups by 50%. The decrease in blood flow varied greatly between the organs. While flow through arteriovenous anastomoses and skin decreased by 70% to 90%, renal blood flow decreased by 30%, cerebral blood flow only when baseline blood flow was high (FH dogs), and left ventricular blood flow only in the CU group, where the largest decrease in hemodynamic variables occurred. Oxygen consumption decreased only in CU dogs, but so did arterial lactate levels. These data indicate that dexmedetomidine causes considerable redistribution of Q, predominantly reducing blood flow to less vital organs and shunt flow.

Beneficial effects of dexmedetomidine on ischaemic myocardium of anaesthetized dogs

We have studied the effect of dexmedetomidine during coronary artery stenosis (CAS) in dogs. Three periods of 15 min of CAS were induced at 40-min intervals in two groups of dogs (dexmedetomidine compared with placebo). Dexmedetomidine was administered before the second and third periods of CAS in doses of 1 and 3 micrograms kg-1, respectively. Dexmedetomidine decreased plasma concentrations of noradrenaline by mean 71 (SEM 9)%, heart rate by 8 (4)%, cardiac output by 30 (6)% and increased mean arterial pressure by 23 (10)%. Dexmedetomidine reduced blood flow in non-ischaemic myocardium and in the ischaemic epicardial layer by 16 (8)%, but blood flow was preserved in the ischaemic mid-myocardial and subendocardial layers. Consequently, dexmedetomidine increased the ischaemic-non-ischaemic blood flow ratio. Dexmedetomidine did not change myocardial oxygen consumption but decreased myocardial oxygen demand from 4.91 (0.33) to 3.76 (0.25) mumol min-1 g-1, thereby reducing the oxygen deficiency of the ischaemic myocardium from 1.47 (0.37) to 0.29 (0.32) mumol min-1 g-1.

Coronary vascular effects of dexmedetomidine during reactive hyperemia in the anesthetized dog

OBJECTIVE

The central sympatholytic effects of alpha2-adrenergic agonists are believed to be beneficial during myocardial ischemia, but the peripheral vasoconstrictive effects are controversial. The aim of this study was to investigate the coronary vascular effects of dexmedetomidine (DM) during reactive hyperemia.

DESIGN

The study had a prospective, randomized, open-comparative design.

SETTING

University animal laboratory.

PARTICIPANTS

Nine mongrel dogs.

INTERVENTIONS

Coronary artery occlusions lasting 2 minutes were induced five times at 40-minute intervals. DM, 0.1, 1, and 10 micrograms/kg was administered 15 minutes before the second, third, and fourth coronary occlusion, respectively. The alpha2-antagonist atipamezole was administered before the fifth coronary occlusion.

MEASUREMENTS AND MAIN RESULTS

DM, 1 microgram/kg, significantly decreased heart rate (from 128 +/- 13 to 96 +/- 21 beats/min); 10 micrograms/kg of DM also significantly decreased cardiac output (from 3.4 +/- 1.1 to 1.4 +/- 0.4 L/min). DM decreased myocardial blood flow in all layers of normally perfused myocardium. In hyperemic myocardium, DM significantly decreased epicardial blood flow (from 3.30 +/- 1.43 to 1.44 +/- 0.49 mL/min/g after DM 10 micrograms/kg), whereas endocardial blood flow did not change, hereby significantly increasing the endo/epi blood flow ratio (from 0.99 +/- 0.54 to 2.28 +/- 0.78).

CONCLUSIONS

In the postischemic hyperemic subendocardial layer, coronary blood flow was preserved after DM. DM reduced primary determinants of myocardial oxygen demand. These effects of DM may be beneficial in conditions of temporary coronary artery occlusion and subsequent reperfusion.

The effects of alpha 2-adrenergic stimulation with mivazerol on myocardial blood flow and function during coronary artery stenosis in anesthetized dogs

The central sympatholytic effect of alpha 2 agonists may be beneficial during myocardial ischemia, but could be opposed by their peripheral vasoconstrictive effect. We studied the effects of mivazerol during periods of moderate coronary artery stenosis in anesthetized dogs. Mivazerol decreased heart rate (from 125 +/- 6 to 106 +/- 6 bpm) and cardiac output (from 4.4 +/- 0.6 to 1.8 +/- 0.2L/min) under normal conditions, while mean arterial pressure did not change. Mivazerol reduced blood flow in nonischemic myocardium and in the ischemic epicardial layer, but blood flow was preserved in the ischemic midmyocardial and subendocardial layer. Mivazerol had no effect on myocardial oxygen extraction during the stenoses, and regional myocardial oxygen consumption was unchanged. However, mivazerol decreased myocardial oxygen demand from 4.51 +/- 0.51 to 3.17 +/- 0.24 mumol.min-1.g-1, thereby reducing oxygen deficiency of ischemic myocardium to values significantly lower than in the placebo group (from 1.07 +/- 0.32 to 0.47 +/- 0.41 mumol.min-1.g-1). Mivazerol had no effect on myocardial lactate production during the stenoses. We conclude that mivazerol reduced myocardial oxygen demand while blood flow was preserved in the inner layers of ischemic myocardium.