James S. Titus

Active Adaptation of the Tethered Mitral Valve Insights Into a Compensatory Mechanism for Functional Mitral Regurgitation

Background— In patients with left ventricular infarction or dilatation, leaflet tethering by displaced papillary muscles frequently induces mitral regurgitation, which doubles mortality. Little is known about the biological potential of the mitral valve (MV) to compensate for ventricular remodeling. We tested the hypothesis that MV leaflet surface area increases over time with mechanical stretch created by papillary muscle displacement through cell activation, not passive stretching. Methods and Results— Under cardiopulmonary bypass, the papillary muscle tips in 6 adult sheep were retracted apically short of producing mitral regurgitation to replicate tethering without confounding myocardial infarction or turbulence. Diastolic leaflet area was quantified by 3-dimensional echocardiography over 61±6 days compared with 6 unstretched sheep MVs. Total diastolic leaflet area increased by 2.4±1.3 cm2 (17±10%) from 14.3±1.9 to 16.7±1.9 cm2 (P=0.006) with stretch with no change in the unstretched valves despite sham open heart surgery. Stretched MVs were 2.8 times thicker than normal (1.18±0.14 versus 0.42±0.14 mm; P<0.0001) at 60 days with an increased spongiosa layer. Endothelial cells (CD31+) coexpressing &agr;-smooth muscle actin were significantly more common by fluorescent cell sorting in tethered versus normal leaflets (41±19% versus 9±5%; P=0.02), indicating endothelial-mesenchymal transdifferentiation. &agr;-Smooth muscle actin-positive cells appeared in the atrial endothelium, penetrating into the interstitium, with increased collagen deposition. Thickened chordae showed endothelial and subendothelial &agr;-smooth muscle actin. Endothelial-mesenchymal transdifferentiation capacity also was demonstrated in cultured MV endothelial cells. Conclusions— Mechanical stresses imposed by papillary muscle tethering increase MV leaflet area and thickness, with cellular changes suggesting reactivated embryonic developmental pathways. Understanding such actively adaptive mechanisms can potentially provide therapeutic opportunities to augment MV area and reduce ischemic mitral regurgitation.

Regional Myocardial Perfusion After Experimental Subarachnoid Hemorrhage

BACKGROUND AND PURPOSE The pathophysiology of cardiac injury after subarachnoid hemorrhage (SAH) remains controversial. Data from animal models suggest that catecholamine-mediated injury is the most likely cause of cardiac injury after SAH. However, researchers also have proposed myocardial ischemia to be the underlying cause, as a result of coronary artery disease, coronary artery spasm, or hypertension and tachycardia. To test the hypothesis that SAH-induced cardiac injury occurs in the absence of myocardial hypoperfusion, we developed an experimental canine model that reproduces the clinical and pathological cardiac lesions of SAH and defines the epicardial and microvascular coronary circulation. METHODS Serial ECG, hemodynamic measurements, coronary angiography, regional myocardial blood flow measurements by radiolabeled microspheres, 2D echocardiography, and myocardial contrast echocardiography were performed in 9 dogs with experimental SAH and 5 controls. RESULTS Regional wall motion abnormalities were identified in 8 of 9 SAH dogs and 1 of 5 controls (Fishers Exact Test, P=0.02) but no evidence was seen of coronary artery disease or spasm by coronary angiography and of significant myocardial hypoperfusion by either regional myocardial blood flow or myocardial contrast echocardiography. CONCLUSIONS In this experimental model of SAH, a unique form of regional left ventricular dysfunction occurs in the absence of myocardial hypoperfusion. Future studies are justified to determine the cause of cardiac injury after SAH.

Relation between ionized calcium concentration and ventricular pump performance in the dog under hemodynamically controlled conditions

The effect of plasma ionized calcium concentration on left ventricular function was studied in the canine heart on right heart bypass. Stroke volume, mean arterial pressure and heart rate were controlled. Plasma ionized calcium was lowered to 0.58 +/- 0.01 mM by citrate infusion and raised to 1.70 +/- 0.01 mM by calcium chloride infusion in random order in each dog. Left ventricular function at each of these ionized calcium levels was compared with that in an immediately preceding normocalcemic period. At a constant stroke work (16.9 +/- 0.2 g-m), sustained hypercalcemia was associated with a small decrease in left ventricular end-diastolic pressure (1.7 +/- 0.7 cm H2O, p less than 0.05) despite a marked increase in peak left ventricular dP/dt (first derivative of ventricular pressure) averaging 34 percent (p less than 0.001). Coronary blood flow, tension-time index and myocardial oxygen consumption were not significantly altered. Stroke work determined at a left ventricular end-diastolic pressure of 14 cm H2O, by interpolation in left ventricular function curves, was 11 +/- 4.4 percent above that at control normocalcemia (p less than 0.05). At a constant stroke work (16.9 +/- 0.2 g-m), sustained hypocalcemia was associated with a marked depression of left ventricular function as demonstrated by a substantial increase (from 4.9 +/- 0.3 to 12.7 +/- 1.1 cm H2O, p less than 0.0001) in left ventricular end-diastolic pressure (p less than 0.0001), decreased mean systolic ejection rate (p less than 0.01) and decreased peak left ventricular dP/dt (p less than 0.0001). Coronary blood flow increased (p less than 0.05) whereas myocardial oxygen consumption did not change significantly. A marked displacement of left ventricular function curves to the right (compared with curves obtained during normocalcemia) was observed, and stroke work determined at a left ventricular end-diastolic pressure of 14 cm H2O was 52 +/- 5.4 percent below control level (p less than 0.001). It appears that hypercalcemia, when initiated from a normal control level, provides only a small enhancement of ventricular pump performance (as indexed by the stroke work-left ventricular end-diastolic pressure relation) despite a marked increase in peak left ventricular dP/dt, whereas marked improvement of left ventricular performance may be expected when calcium infusion is initiated from an ionized calcium level that is below normal.

Rapid cooling contracture with cold cardioplegia

BACKGROUND Cold cardioplegia can induce rapid cooling contracture. The relations of cardioplegia-induced cooling contracture to myocardial temperature or myocyte calcium are unknown. METHODS Twelve crystalloid-perfused isovolumic rat hearts received three 2-minute cardioplegic infusions (1 mmol/L calcium) at 4 degrees, 20 degrees, and 37 degrees C in random order, each followed by 10 minutes of beating at 37 degrees C. Finally, warm induction of arrest by a 1-minute cardioplegic infusion at 37 degrees C was followed by a 1-minute infusion at 4 degrees C. Indo-1 was used to measure the intracellular Ca2+ concentration in 6 of these hearts. Additional hearts received hypoxic, glucose-free cardioplegia at 4 degrees or 37 degrees C. RESULTS After 1 minute of cardioplegia at 4 degrees, 20 degrees, and 37 degrees C, left ventricular developed pressure rose rapidly to 54% +/- 3%, 43% +/- 3%, and 18% +/- 1% of its prearrest value, whereas the intracellular Ca2+ concentration reached 166% +/- 23%, 94% +/- 4%, and 37% +/- 10% of its prearrest transient. Coronary flow was 5.7 +/- 0.2, 8.7 +/- 0.3, and 12.6 +/- 0.6 mL/min, respectively. Warm cardioplegia induction at 37 degrees C reduced left ventricular developed pressure and [Ca2+]i during subsequent 4 degrees C cardioplegia by 16% (p = 0.001) and 34% (p = 0.03), respectively. Adenosine triphosphate and phosphocreatine contents were lower after 4 degrees C than after 37 degrees C hypoxic, glucose-free cardioplegia. CONCLUSIONS Rapid cooling during cardioplegia increases left ventricular pressure, [Ca2+]i and coronary resistance, and is energy consuming. The absence of rapid cooling contracture may be a benefit of warm heart operations and warm induction of cardioplegic arrest.

Funnel-Tipped Aortic Cannula for Reduction of Atheroemboli

BACKGROUND Atheroemboli caused by aortic manipulation poses a risk for stroke in patients undergoing cardiopulmonary bypass (CPB) surgery. One potential cause is the high velocity jet from aortic perfusion cannulae. This study describes the flow patterns of a novel funnel-tip cannula designed to reduce emboli by decreasing fluid velocity and resultant shear force on the aortic wall. METHODS A funnel-tip cannula was constructed and compared with standard straight-tip cannulae and the Dispersion (Research Medical Inc, Midvale, UT) and Sarns Soft Flow (Terumo Cardiovascular Systems Corp, Ann Arbor, MI) cannulae. Pressure drop measurements were collected at 1 to 6 L/minute flows. Velocity flow profiles were created using phase contrast magnetic resonance imaging. Absolute velocity was measured in a phantom aorta at 5 L/minute flow. Each cannula was further studied in a synthetic model of an atherosclerotic aorta to determine the mass of dislodged particulate matter generated at 2, 3, and 5 L/minute flows. RESULTS The funnel-tip cannula demonstrated significantly lower values (p < 0.05) in pressure drop (55 mm Hg), exit velocity (309 cm/second, 167 cm/second for center axis and wall, respectively), and particulate dislodgement (0.15 +/- 0.05 g) than other tested cannulae. The Soft Flow cannula generated the next lowest pressure drop but exhibited twice the exit velocity and particulate dislodgement of the funnel-tip cannula. The Dispersion cannula did not demonstrate a reduction in velocity or particulate dislodgement compared with the standard straight-tip cannulae. CONCLUSIONS The results of this study suggest that a low-angled funnel-tip cannula has favorable flow characteristics warranting further investigation. Design development may reduce the risk of atheroemboli generation during CPB surgery.

Myocardial preservation related to magnesium content of hyperkalemic cardioplegic solutions at 8 °C

Abstract This study investigates whether the addition of magnesium to a hyperkalemic cardioplegic solution containing 0.1 mM ionized calcium improves myocardial preservation, and whether there is an optimal magnesium concentration in this solution. Isolated perfused rat hearts were arrested for two hours by this cardioplegic solution, which was fully oxygenated and infused at 8 °C every 15 minutes to simulate clinical conditions. The cardioplegic solution contained either 0, 2, 4, 8, 16, or 32 mM magnesium. At end-arrest, the myocardial creatine phosphate concentration (nanomoles per milligram of dry weight) was 20.7 ± 2.1, 22.9 ± 1.7, 24.8 ± 2.0, 31.3 ± 1.4, 33.1 ± 1.8, and 31.6 ± 0.8, respectively, in hearts given cardioplegic solution containing these magnesium concentrations. Thus, the concentration of creatine phosphate was significantly higher at end-arrest when the cardioplegic solution contained 8, 16, or 32 mM than 0 or 2 mM magnesium ( p p p

Oxygenated cardioplegia: The metabolic and functional effects of glucose and insulin

Reports differ as to the efficacy of glucose and insulin as cardioplegic additives. Although deliberate oxygenation of crystalloid cardioplegic solutions improves myocardial protection, little is known about the protection afforded by glucose and insulin in such oxygenated solutions. In the isolated working rat heart, we studied the addition of oxygen, glucose, and insulin, separately and together, to a cardioplegic solution. The solution was equilibrated with O2 or N2, with glucose added as a substrate or sucrose as a nonmetabolizable osmotic control, with or without insulin. Hearts were arrested for 2 hours at 8 degrees C by multidose infusions. Oxygenation decreased lactate production and improved high-energy phosphate and glycogen preservation during arrest, prevented ischemic contracture, and improved functional recovery. The addition of glucose to the oxygenated solution increased the level of adenosine triphosphate at end-arrest from 10.5 +/- 0.5 to 13.9 +/- 0.6 nmol/mg dry weight and glycogen stores from 18.7 +/- 2.5 to 35.7 +/- 5.5 nmol/mg dry weight. The further addition of insulin did not better preserve these metabolites. Improvements in functional recovery due to glucose or insulin in the oxygenated solution attained statistical significance when both additives were included. Glucose increased lactate production significantly only when the solution was nitrogenated. Insulin added to the nitrogenated glucose-containing solution increased adenosine triphosphate and glycogen levels after 1 hour of arrest; and, although insulin did not prevent ischemic contracture from developing during the latter part of arrest with profound depletion of these metabolites, functional recovery was improved. The mechanism of improved functional recovery by insulin is not clear.(ABSTRACT TRUNCATED AT 250 WORDS)

Relation of myocardial protection to cardioplegic solution pH: Modulation by calcium and magnesium

The relationship between myocardial preservation and cardioplegic solution pH was assessed in isolated, perfused rat hearts. A base solution without calcium or magnesium and the same solution containing 0.2 mmol/L ionized calcium or 16 mmol/L magnesium or both ions were studied at several values of pH between 6.8 and 8.7. Hearts were arrested at 8 degrees C by multidose infusions of these bicarbonate-buffered solutions bubbled with oxygen and a varying percentage of carbon dioxide to control pH. Diastolic tone (left ventricular balloon) and adenosine triphosphate (ATP) depletion during arrest both increased as the cardioplegic solution became more alkaline. Calcium increased these effects of pH. Magnesium weakened the effect of pH on diastolic tone, maintained ATP at all pH levels, and inhibited the effects of calcium on the relationships of pH to diastolic tone and ATP. When data from all solutions were considered together, ATP depletion was shown to be linearly related to diastolic tone. Calcium depressed functional recovery (left ventricular developed pressure during reperfusion expressed as a percentage of its prearrest value) at all pH levels. With the other solutions, recovery was similar and best within a broad and relatively alkaline pH range. With the solution containing calcium and magnesium, at pH levels of 8.28 +/- 0.02, 7.87 +/- 0.03, 7.58 +/- 0.02, 7.41 +/- 0.01, 7.06 +/- 0.02, and 6.80 +/- 0.01, recovery at 5 minutes of reperfusion was 101.4% +/- 3.7%, 102.9% +/- 2.8%, 107.3% +/- 3.7%, 102.8% +/- 2.9%, 91.8% +/- 3.6%, and 94.3% +/- 3.5%, respectively. This effect of alkalinity was short-lived. Extreme alkalinity of the base, acalcemic solution produced the calcium paradox, as reported previously. Good preservation of ATP by the most acid solutions did not predict good functional recovery. Magnesium increased the persistence of frequent extrasystoles during early reperfusion, but the effect was attenuated by calcium. The data support the inclusion of magnesium in cardioplegic solutions, particularly when they contain calcium, show that cardioplegic solution pH can have major effects on the arrested heart, and suggest that a relatively alkaline pH may modestly benefit functional recovery.

Delivery of proteins to CNS as seen and measured by positron emission tomography.

Presently, there are no effective treatments for several diseases involving the central nervous system (CNS). While several novel molecular approaches are being developed, many of them require delivery of macromolecular or supramolecular agents to the CNS tissues protected by the blood–brain and blood–arachnoid barriers. A variety of approaches that are being developed for overcoming or bypassing the barriers are based on complex transfer processes. The delivery of biopharmaceuticals and other macromolecules and particulates to the CNS, especially through the leptomeningeal (intrathecal) route, includes a variety of stages, such as leptomeningeal propagation, drainage to the systemic circulation, and penetration into the CNS. The investigation of complex pharmacokinetics that includes convective, as well as diffusional and active transfer processes, greatly benefit from real-time non-invasive in vivo monitoring of the drug transport. Pharmacological positron emission tomography (PET) imaging, which enables such monitoring, plays an increasingly significant role in drug delivery and biopharmacology. PET is a powerful tool for quantitative in vivo tracking of molecules labeled with positron-emitting radionuclides. The high sensitivity, format, and accuracy of the data (similar to those of conventional tissue sampling biodistribution studies) make PET a readily adoptable pharmacological technique. In contrast to the conventional studies, PET also allows for longitudinal nonterminal same-animal studies. The latter may not only improve the data statistics, but also enable preclinical studies (especially in large and/or rare animals) not feasible under the conventional approach. This paper is intended to demonstrate the character of data that can be obtained by PET and to demonstrate how the main patterns of the leptomeningeal route pharmacokinetics can be investigated using this method. Examples of data processing are taken from our recent studies of five model proteins in rats and nonhuman primates.

Cardioplegia and ischemia in the canine heart evaluated by 31P magnetic resonance spectroscopy

BACKGROUND Warm continuous blood cardioplegia provides excellent protection, but must be interrupted by ischemic intervals to aid visualization. We hypothesized that (1) as ischemia is prolonged, the reduced metabolic rate offered by cooling gives the advantage to hypothermic cardioplegia; and (2) prior cardioplegia mitigates the deleterious effects of normothermic ischemia. METHODS Isolated cross-perfused canine hearts underwent cardioplegic arrest followed by 45 minutes of global ischemia at 10 degrees C or 37 degrees C, or 45 minutes of normothermic ischemia without prior cardioplegia. Left ventricular function was measured at baseline and during 2 hours of recovery. Metabolism was continuously evaluated by phosphorus-31 magnetic resonance spectroscopy. RESULTS Adenosine triphosphate was 71% +/- 4%, 71% +/- 7%, and 38% +/- 5% of baseline at 30 minutes, and 71% +/- 4%, 48% +/- 5%, and 39% +/- 6% at 42 minutes of ischemia in the cold ischemia, warm ischemia, and normothermic ischemia without prior cardioplegia groups, respectively. Left ventricular systolic function, left ventricular relaxation, and high-energy phosphate levels recovered fully after cold cardioplegia and ischemia. Prior cardioplegia delayed the decline in intracellular pH during normothermic ischemia initially by 9 minutes, and better preserved left ventricular relaxation during recovery, but did not ameliorate the severe postischemic impairment of left ventricular systolic function, marked adenosine triphosphate depletion, and creatine phosphate increase. Left ventricular distensibility decreased in all groups. CONCLUSIONS When cardioplegia is followed by prolonged ischemia, better protection is provided by hypothermia than by normothermia. Prior cardioplegia confers little advantage on recovery after prolonged normothermic ischemia but delays initial ischemic metabolic deterioration, which would contribute to the safety of brief interruptions of warm cardioplegia.