Functional and morphological characteristics of compensated and decompensated cardiac hypertrophy in dogs with chronic infrarenal aorto-caval fistulas

Post-hypothermic cardiac left ventricular systolic dysfunction after rewarming in an intact pig model

INTRODUCTION

We developed a minimally invasive, closed chest pig model with the main aim to describe hemodynamic function during surface cooling, steady state severe hypothermia (one hour at 25°C) and surface rewarming.

METHODS

Twelve anesthetized juvenile pigs were acutely catheterized for measurement of left ventricular (LV) pressure-volume loops (conductance catheter), cardiac output (Swan-Ganz), and for vena cava inferior occlusion. Eight animals were surface cooled to 25°C, while four animals were kept as normothermic time-matched controls.

RESULTS

During progressive cooling and steady state severe hypothermia (25°C) cardiac output (CO), stroke volume (SV), mean arterial pressure (MAP), maximal deceleration of pressure in the cardiac cycle (dP/dt(min)), indexes of LV contractility (preload recruitable stroke work, PRSW, and maximal acceleration of pressure in the cardiac cycle, dP/dt(max)) and LV end diastolic and systolic volumes (EDV and ESV) were significantly reduced. Systemic vascular resistance (SVR), isovolumetric relaxation time (Tau), and oxygen content in arterial and mixed venous blood increased significantly. LV end diastolic pressure (EDP) remained constant. After rewarming all the above mentioned hemodynamic variables that were depressed during 25°C remained reduced, except for CO that returned to pre-hypothermic values due to an increase in heart rate. Likewise, SVR and EDP were significantly reduced after rewarming, while Tau, EDV, ESV and blood oxygen content normalized. Serum levels of cardiac troponin T (TnT) and tumor necrosis factor-alpha (TNF-α) were significantly increased.

CONCLUSIONS

Progressive cooling to 25°C followed by rewarming resulted in a reduced systolic, but not diastolic left ventricular function. The post-hypothermic increase in heart rate and the reduced systemic vascular resistance are interpreted as adaptive measures by the organism to compensate for a hypothermia-induced mild left ventricular cardiac failure. A post-hypothermic increase in TnT indicates that hypothermia/rewarming may cause degradation of cardiac tissue. There were no signs of inadequate global oxygenation throughout the experiments.

Growth factor activation in myocardial vascularization: therapeutic implications

A rapid growth of the coronary vasculature occurs during prenatal and early postnatal periods as precursor cells from the epi- and sub-epicardium differentiate, migrate and form vascular structures (vasculogenesis) which then fuse, branch and in some cases recruit cells to form three tunics (angiogenesis). These processes are tightly controlled by temporally and spatially expressed growth factors which are stimulated by metabolic and mechanical factors. The process of angiogenesis in the myocardium is not limited to developmental periods of life, but may occur when the heart is challenged by enhanced loading conditions or during hypoxia or ischemia. This review focuses on the activation of growth factors by metabolic and mechanical stimuli in the developing heart and in the adult heart undergoing adaptive responses. Experimental studies support the hypotheses that both metabolic (hypoxia) and mechanical (stretch) factors serve as powerful stimuli for the up-regulation of growth factors which facilitate angiogenesis and arteriogenesis. Both hypoxia and stretch are powerful inducers of VEGF and its receptors, and provide for paracrine and autocrine signaling. In addition to the VEGF family, bFGF and angiopoietins play major roles in myocardial vascularization. Sufficient evidence supports the hypothesis that mechanical (e.g., bradycardia) and metabolic (e.g., thyroxine analogs) may provide effective non-invasive angiogenic therapies for the ischemic and post-infarcted heart.

Left ventricular remodeling in hearts with tricuspid atresia: morphologic observations and possible basis for ventricular dysfunction after surgery

OBJECTIVES

This study was undertaken to assess the pattern of left ventricular hypertrophy, the myocardial capillary network, and the extracellular matrix in hearts with tricuspid atresia.

METHODS

We examined 32 hearts with tricuspid atresia and 27 normal hearts from control subjects with similar age and sex distribution. Wall thickness, inlet length, and outlet length were obtained from the left ventricle. Immunohistochemical staining for von Willebrand factor was used to label myocardial capillaries. By means of computer-assisted morphometry, the following data were obtained from the inlet, apex, and outlet of the left ventricle: transverse myocyte diameter, myocyte nuclear volume fraction, capillary volume fraction, interstitial fibrous volume fraction, and endocardial thickness.

RESULTS

The wall thickness in affected hearts was not different from that in control hearts. The left ventricular outlet length was significantly greater in the malformed hearts (P =.005). The myocyte diameter did not differ from that in control hearts. The capillary volume fraction was decreased in the malformed hearts (P <.001). The interstitial fibrous deposition was greater in the malformed hearts at all sites analyzed (P <.001). Fibrosis was greater in the inlet and apex (P =.004) and also in the subendocardial half of the ventricular wall than in the subepicardial half. According to a logistic regression model, age was the only variable associated with the probability of occurrence of fibrosis.

CONCLUSIONS

Our findings in hearts with tricuspid atresia possibly represent chronically induced volume overload in the presence of ischemia, rather than a typical model of volume overload. A decreased capillary volume fraction may indicate a greater susceptibility to ischemia. Fibrous deposition probably occurs early in life.

Effect of myocardial volume overload and heart failure on lactate transport into isolated cardiac myocytes

The purpose of this study was to determine lactate transport kinetics in single isolated rat ventricular cardiac myocytes after 1) 8 wk of myocardial volume overload (MVO) and 2) congestive heart failure (CHF). Twenty male Sprague-Dawley rats were assigned to one of four groups: myocardial hypertrophy (MH), MH sham (MHS), CHF, or CHF sham (CHFS). A chronic MVO was induced in the MH and CHF groups by an infrarenal arteriovenous fistula. Postdeath heart and lung weights were significantly greater (P < 0.05) for the MH and CHF groups compared with controls. Isolated cardiac myocytes were loaded with BCECF to determine intracellular pH (pH(i)) changes after the addition of lactate to the extracellular superfusate. Alterations in pH(i) with the addition of varied lactate concentrations were attenuated 72-89% by 5.0 mM alpha-cyano-4-hydroxycinnamate. Significant differences (P < 0.05) were found in estimated maximal lactate transport rates between the experimental and sham groups (MH = 19.4 +/- 1.1 nmol x microl(-1) x min(-1) vs. MHS = 15.1 +/- 1.1 nmol x microl(-1) x min(-1); CHF = 20.2 +/- 2.0 nmol x microl(-1) x min(-1) vs. CHFS = 14.0 +/- 0.9 nmol x microl(-1) x min(-1)). Western blot analysis confirmed a 270% increase in monocarboxylate symport protein 1 (MCT1) protein content in CHF compared with CHFS rats. The results of this study suggest that MH and CHF induced by MVO engender a greater maximal lactate transport capacity across the cardiac myocyte sarcolemma along with an increase in MCT1 protein content. These alterations would likely benefit the cell by attenuating intracellular acidification during a period of increased myocardial load.

Regulation of insulin-like growth factor-1 by the renin-angiotensin system during regression of cardiac eccentric hypertrophy through angiotensin-converting enzyme inhibitor and AT1 antagonist

Angiotensin II (Ang II) mediates its effects through its non-tyrosine-kinase G protein coupled Ang-II type 1 receptor (AT1). Growing evidence indicates that a functional insulin-like growth factor-1 (IGF-1) tyrosine kinase receptor is required for Ang-II-induced mitogenesis. Along with Ang II, we have previously shown that changes in IGF-1 receptor binding at myofibers are causative agents for cardiac eccentric hypertrophy. This study investigated the interaction of the renin-angiotensin system with the IGF-1 receptor during the development and regression of cardiac hypertrophy. Alterations in IGF-1 binding were evaluated in the CHAPS-pretreated perfused heart. Four weeks of aortocaval shunt increased relative heart mass by 76% without a major change in body mass or systolic blood pressure. Binding studies showed that IGF-1 has a higher affinity for the cardiac myofibers of shunt than sham rats. Two weeks of treatment with the angiotensin-converting enzyme (ACE) inhibitor captopril (0.5 g/L in drinking water) or the AT1-antagonist losartan (10 mg/(kg x day)) reduced cardiac hypertrophy by 54 and 42%, respectively. However, while both ACE inhibition and AT1-antagonist treatments produced equivalent regression in ventricular hypertrophy, captopril was more efficacious than losartan in the regression of atrial hypertrophy. Regression of cardiac hypertrophy in the shunt by either captopril or losartan was accompanied with a reduction or normalization of the elevated IGF-1 affinity. Thus, the induction and regression of cardiac eccentric hypertrophy seems to be largely dependent on cross talk between the renin-angiotensin system and the IGF-1 axis at the receptor level.

Evaluation of three peripheral arteriovenous fistulas for hemodialysis access in dogs

OBJECTIVE

To design and create 3 types of arteriovenous fistulas (AVF) in normal dogs, to monitor the dogs for secondary cardiovascular complications, and to verify adequacy of these fistulas for hemodialysis vascular access.

STUDY DESIGN

Experimental study.

ANIMALS

Four normal adult dogs.

METHODS

Cadaveric dissections were performed, and surgical protocols were generated for carotid-jugular (CJ), brachial-cephalic (BC), and distal caudal femoral-lateral saphenous anastomosis (DCFLS) AVF. Each surgical procedure was then performed in 2 live dogs. Echocardiography was performed at days 0, 1, 3, 7, 14, 28, and 56 to evaluate the dogs for evidence of volume overload secondary to AVF formation. Estimation of luminal diameter and confirmation of fistula patency were performed using percutaneous color Doppler ultrasound. At day 56, hemodialysis was performed using each fistula as a vascular access.

RESULTS

No significant changes occurred in the echocardiographic variables over time. All fistulas were patent at day 56 with mean luminal diameters of 4.5 mm (CJ), 4 mm (BC), and 1.5 mm (DCFLS). The BC fistula was superior for ease of needle placement and stabilization and provided adequate blood flow for clinical hemodialysis.

CONCLUSIONS

Based on this short-term study, arteriovenous fistulas appear to be a safe and effective means for hemodialysis access in dogs.

CLINICAL RELEVANCE

The arteriovenous fistulas described provide an alternative to the central venous catheters currently used for chronic hemodialysis access in dogs.

Time course of structural adaptations in chronic AV block dogs: evidence for differential ventricular remodeling

To determine the nature and time course of biventricular hypertrophy and concomitant electrical and mechanical changes after creation of complete atrioventricular block (CAVB), six adult dogs (22–30 kg) were subjected to serial magnetic resonance imaging (MRI) and electrocardiography. After 6 days of CAVB, left ventricular (LV) mass, ejection fraction (EF), and Q-T time at a paced rhythm of 60 beats/min were already significantly increased. Maximal values were reached within 14–21 days of CAVB: LV mass, from 116 ± 11 to 143 ± 12 g; right ventricular (RV) mass, from 40 ± 3 to 55 ± 6 g; EF, from 68 ± 6% to 86 ± 5%; and Q-T time, from 285 ± 25 to 330 ± 35 ms, all P < 0.05. Cardiac output returned to baseline at day 14. End-diastolic wall thickness increased only in the RV, in which angiotensin type 1 (AT1) receptor mRNA expression was significantly greater. The autopsy correlated well with the MRI results ( r = 0.98, P≤ 0.01). In conclusion, electrophysiological, mechanical, and structural adaptation processes after bradycardia-induced volume overload develop rapidly and are completed within 3 wk. The degree of hypertrophy was greater in the RV, which was associated with an increase in AT1receptor mRNA.

Interaction between the renin-angiotensin system and insulin-like growth factor I in aorto-caval fistula-induced cardiac hypertrophy in rats

The effects of angiotensin converting enzyme inhibition and angiotensin II receptor blockade on the development of cardiac hypertrophy and myocardial insulin-like growth factor I (IGF-I) in volume overload were studied in male Wistar rats with aorto-caval fistulas (ACF). Rats were treated with ramipril (RAM, 3 mg kg(-1) day(-1)) for 4-20 days or losartan (LOS, 10 mg kg(-1) day(-1)) for 2-7 days. Myocardial IGF-I and IGF-I receptor (IGF-I-R) mRNA were determined by solution hybridization. ACF caused hypertrophy of left (LV) and right ventricles (RV). Hypertrophy appeared on day 2 and reached maximal values of +60% in LV and +75% in RV at day 12. Systolic blood pressure was initially reduced 15% but recovered by day 12. RAM abolished the recovery of blood pressure. Furthermore, RAM attenuated RV hypertrophy by 17% on day 7 and on day 20, RV weights were close to values found in controls. Beginning on day 9, RAM reduced LV weight back to control levels in parallel to blood pressure. In contrast, LOS affected neither RV nor LV hypertrophy. RV IGF-I mRNA increased 60-100% on day 7 alone in RV in ACF. RAM potentiated the increase in RV IGF-I to +400% and induced an increase in RV IGF-I-R mRNA on day 7 (+90%) in ACF. LOS did not affect RV IGF-I. Development of cardiac hypertrophy in ACF seemed independent of angiotensin II. RV hypertrophy was associated with activation of IGF-I independent of the renin-angiotensin system. IGF-I was further potentiated when development of hypertrophy was attenuated, possibly indicative of a greater urge for compensational growth in a relatively thinner and more volume-distended chamber.

Effect of chronic volume overload on baroreflex control of heart rate and sympathetic nerve activity

Baroreceptor-heart rate reflex sensitivity is decreased in congestive heart failure. The reflex control of heart rate and sympathetic nerve activity in rats with chronic volume overload, an established model for moderate heart failure, is still unknown. Therefore, we investigated the regulation of humoral and neuronal sympathetic activity and the baroreflex control of heart rate and sympathetic nerve activity in conscious, unrestrained rats with aortocaval shunt. Rats with aortocaval shunts had larger hearts (388 +/- 11 vs. 277 +/- 4 mg/100 g body wt), elevated central venous pressures (14 +/- 4 vs. 4 +/- 3 mmHg), and higher atrial natriuretic peptide plasma levels (87 +/- 16 vs. 25 +/- 3 pmol/l) than controls but had similar systemic blood pressure and heart rate values. Plasma epinephrine (0.63 +/- 0.16 vs. 0.21 +/- 0.08 pmol/l, P < 0.05) and norepinephrine concentrations (0.27 +/- 0.03 vs. 0.16 +/- 0.02 pmol/l, P < 0.05) were elevated in shunted rats compared with controls. Nitroprusside-induced hypotension led to a significantly greater increase in efferent splanchnic sympathetic nerve activity in shunted rats than in controls (0.9 +/- 0.1 vs. 2.6 +/- 0.6 microV, P < 0.05), whereas the heart rate responses were not different between the groups. These results indicate that the regulation of the autonomic nervous system is altered in chronically volume-overloaded rats. The arterial baroreflex control of efferent splanchnic sympathetic nerve activity was dissociated from the control of heart rate. Therefore, analysis of the activation of sympathetic nervous system assessed by direct measurements of efferent sympathetic nerve activity appears to be more sensitive for the detection of altered autonomic nervous system function than the analysis of baroreflex control of heart rate.

Blood volume and body fluid compartments in lambs with aortopulmonary left-to-right shunts

A left-to-right shunt is accompanied by an increased plasma and blood volume. Since this is likely realized through renin/aldosterone-mediated salt and water retention, other body fluid compartments may be changed too. Therefore, we studied blood volume and body fluid compartments by a single-injection, triple-indicator dilution technique in nine 8-wk-old lambs with an aortopulmonary left-to-right shunt (55 +/- 3% of left ventricular output; mean +/- SEM) and in 11 control lambs, 2.5 wk after surgery. Systemic blood flow was maintained at the same level as in control lambs, but the aortic pressure of the shunt lambs was lower. Blood volume in shunt lambs was larger than in control lambs (110 +/- 6 vs. 84 +/- 7 ml/kg, P < 0.001) through an increase in plasma volume, which correlated significantly with the magnitude of the left-to-right shunt (r = 0.81, P < 0.01). Red blood cell volume was equal to that of control lambs. Evidence was obtained that the increase in plasma volume was induced by a transient increase in renin (8.0 +/- 2.2 vs. 1.6 +/- 0.2 nmol.l-1.h-1; P < 0.02) and aldosterone (0.51 +/- 0.14 vs. 0.24 +/- 0.09 nmol/liter) concentrations. Interstitial water volume, however, was not significantly different from that in control lambs. The amount of intravascular protein was significantly higher than in control lambs (5.0 +/- 0.3 vs. 3.5 +/- 0.2 g/kg body mass, P < 0.001). There were no significant differences in intracellular and total body water volumes between the two groups. We conclude that the increased amount of intravascular protein confines the fluid retained by the kidneys to the vascular compartment.

Baroreflex and beta-adrenoceptor function are diminished in rat cardiac hypertrophy due to volume overload

We investigated whether cardiac hypertrophy induced by volume loading influences baroreflex sensitivity. Aortic insufficiency (AI) was induced in male Wistar rats by graded disruption of the aortic valve, which, after 2 weeks, resulted in a 30% increase in heart/body weight or left-ventricular/body weight ratio compared with control animals. Baroreflex sensitivity was assessed in conscious animals by measuring the heart rate (HR) responses to the changes in mean arterial pressure (MAP) induced by phenylephrine and nitroprusside sodium at 2 weeks. The slopes of the HR vs MAP plots obtained with phenylephrine and nitroprusside decreased significantly with increasing heart weight/body weight ratio (correlation coefficient r = 0.625 and 0.526, respectively). In isolated right atria from AI animals baseline rate was higher, and the isoproterenol effect on sinus rate was significantly smaller than in atria from control animals, indicating a dysfunction of the beta-adrenoceptor pathway. The data show that baroreflex dysfunction associated with a down-regulation of the beta-adrenoceptor pathway of the sinus node develops simultaneously with volume overload-induced hypertrophy in the absence of overt heart failure.

[Heart volume overload in the rabbit during growth via chronic arteriovenous fistula. Evaluation of an original model]

A heart volume overload model was developed in male New Zealand rabbit nine weeks old, using femorofemoral or jugulo-carotid fistulae, producing 50% increase of the mean cardiac output. An evaluation was performed after 2 and 11 weeks. It was evaluated by dimensional echocardiogram and doppler system flow measurement at the aorta ring, angiography and right cardiac pressure recording. This hemodynamic analysis was completed by a heart and fistulae histopathological study. In some volume overload cases, an increasing volume index parameter was described. Two weeks later an increase of the cardiac frequency and a dilatation of the aorta ring were observed but the stroke volume remained unchanged. 11 old weeks fistula animals presented similar frequency and stroke volume. So, the heart compensatory mechanisms varied with the age of the rabbit. The right catheter and cardiac angiography were not useful for ventricular volume overload estimation. Histopathological study showed parenchymal changes and alveolo capillary congestion. An increase of the heart weight/body weight ratio was described. When analysed by ultrasound and blood flow measurement, data were similar to those obtained by invasive pressure recordings or by dilution system. An experimental model of overload volume in rabbit with non invasive recording methods (bidimensional ultrasound and doppler system) can be proposed.

Geometry of capillary networks in volume overloaded rat heart

Volume overload cardiac hypertrophy was induced in male Sprague-Dawley rats by experimental aortocaval fistula. This procedure resulted in considerable increases in left ventricular mass (70%) by 21-23 days. Our objective was to study the effect of volume overload on the geometry of coronary capillaries in the left ventricular midmyocardium. Tissue sections were stained according to a protocol that distinguished arteriolar (AC) and venular (VC) capillary regions by color. Morphometric data were then collected and compared between AC and VC regions. In sham-operated controls (CON; n = 8), the tissue area (capillary domain) supplied by a single capillary decreased from AC to VC regions (AC = 505 +/- 5 microns 2: VC = 452 +/- 7 microns 2; P less than 0.01; mean +/- SE). In volume overloaded hearts (VOL; n = 8), only VC domain areas were reduced from control values (P less than 0.01) and the differences between AC and VC regions were preserved (AC = 480 +/- 5 microns 2; VC = 395 +/- 6 microns 2; P less than 0.01). Minimal capillary length was significantly longer in volume overloaded hearts (VOL = 723 +/- 18; CON = 581 +/- 20 microns; P less than 0.01). In the control group, AC segment length was longer than VC segment length (AC = 93 +/- 2 microns: VC = 74 +/- 2 microns; P less than 0.01). In volume overload, AC segment length was also longer than VC segment length, but the divergence between AC and VC regions was increased (AC = 108 +/- 3 microns; VC = 71 +/- 2 microns; P less than 0.01). These changes in capillary geometry may be secondary to specific changes in the arrangement and dimension of myocytes in the left ventricular wall following volume overload hypertrophy.

Norepinephrine-induced cardiac hypertrophy of the cat heart

Norepinephrine administration causes progressive hypertrophy of the mammalian heart as measured by myocardial mass. The purpose of this study was to determine the growth response of the myocardial tissue components as well as the myocardial cell itself to norepinephrine. Young, adult cats were given low doses of norepinephrine in dextrose or dextrose alone twice daily for 15 days. On day 16, there were no changes in the animals body weight, right ventricular systolic pressure, right ventricular end-diastolic pressure, heart rate, cardiac index, or blood pressure. However, the right ventricle/body weight, the left ventricle/body weight and the total heart weight/body weight were increased significantly in the norepinephrine treated animals. The increase was on the order of 40%. The cardiac muscle cell was also significantly increased in size and both the right and left ventricular cardiac muscle cells exhibited a dramatic increase in size as measured by cross sectional area. Upon stereological examination it was found that the amount of hypertrophy as seen in the cardiac muscle cells was paralleled by the hypertrophy seen in the other tissue components of the myocardium. The volume density of the muscle cells, the interstitial components, as well as the blood vessel compartment were identical in the control and in the norepinephrine-treated groups. In conclusion, this study demonstrates that the response of the myocardium to norepinephrine is similar to that seen in response to a volume overload rather than that seen in response to pressure overload.

Surgical model of volume overload-induced ventricular myocardial hypertrophy (VHvo) to study a clinical problem in humans

Cardiac patients with aortic or mitral valve incompetence often suffer from VHvo and are at higher risk of surgery compared to other patients undergoing heart surgery. Hypertrophied hearts are known to have altered metabolism and function. Therefore, it is imperative to use hearts with similar pathology as found in clinical patients when studying ways to better protect them. We herein report a model of VHvo developed in our laboratory. Pigs (n = 10) weighing 21 kg were anesthetized, intubated, and mechanically ventilated in dorsal recumbency. Bilateral femoral arterio-venous side-to-side anastomoses were created at least twice the diameter of the vessels. Patency was confirmed and the animals grew to maturity with this pathology, at which time they were sacrificed. Severe hypertrophy was documented with significantly greater heart weight to body weight ratio, LV free wall thickness, and septum. This surgical model of VHvo is associated with minimal complications and no pericardial adhesions. This allows for accurate metabolic and functional heart studies to be conducted at a later date.