Louis R. DiBernardo

Readily Available Tissue-Engineered Vascular Grafts

Nonimmunogenic, tissue-engineered vascular grafts stored long-term maintain their patency, strength, and function after transplant in large-animal models. Grow Your Own Blood Vessels Growing your own vegetables may be a well-established approach for a healthier life, but growing blood vessels for surgical transplantation is a more unusual pastime. But the idea of growing a readily available supply of blood vessels for surgical transplant into patients requiring, for example, a cardiac bypass or dialysis is not as far-fetched as it sounds. Although a patient’s own blood vessels can sometimes be used for the graft, often this is not possible. Engineered autologous blood vessels can be grown from endothelial cells taken from the patient and cultured on scaffolds, but this process takes 9 months or more, and often the patients cannot wait that long for surgery. Enter Dahl and her team with a new approach that provides readily available, off-the-shelf vascular grafts that retain their strength and patency during long-term storage and function successfully after vascular surgery in baboon and dog animal models. The authors grew their human vascular grafts by culturing smooth muscle cells from human cadavers (that is, allogeneic cells) on tubular scaffolds made from a biodegradable polymer called polyglycolic acid (PGA). The smooth muscle cells produced collagen and other molecules that formed an extracellular matrix. When the scaffold degraded, fully formed vascular grafts were left behind. The investigators then stripped the cells from the grafts, using detergent to make sure the grafts would not elicit an immune response when transplanted. These human vascular grafts were 6 mm or greater in diameter and retained their strength, elasticity, and patency even after storage in phosphate-buffered saline solution for a year. The human vascular grafts were tested in a baboon model of arteriovenous bypass in which the graft formed a direct conduit between an artery and a vein (an approach that enables human patients with kidney disease to undergo dialysis). The authors showed that the grafts in baboons restored blood flow and retained their patency and strength for up to 6 months. When the grafts were removed and examined histologically, they did not show evidence of fibrosis, calcification, or thickening of the vessel wall intima. But the authors wanted to test engineered vascular grafts with smaller diameters, which are often plagued by thrombi (blood clots) after transplant. To do this, they turned to a dog model of peripheral and coronary artery bypass, surgeries that require smaller-diameter vascular grafts. Using dog smooth muscle cells cultured on PGA scaffolds, they created canine vascular grafts with small diameters (3 to 4 mm). They then seeded these grafts with endothelial cells (from the dogs due to be recipients) because an endothelial cell lining helps to prevent blood clot formation. Using the engineered grafts, the investigators then conducted either peripheral or coronary artery bypass in the dog recipients and showed that they functioned effectively for at least 1 month. Together, these results demonstrate that durable vascular grafts derived from allogeneic donors and rendered nonimmunogenic by removal of donor cells are suitable for surgical transplant. The added advantage of being able to store these off-the-shelf vascular grafts long-term in a simple saline solution means that these can be made ahead of time and then are ready to go whenever they are needed. Growing blood vessels for a healthier life is as real as the home-grown asparagus in your garden. Autologous or synthetic vascular grafts are used routinely for providing access in hemodialysis or for arterial bypass in patients with cardiovascular disease. However, some patients either lack suitable autologous tissue or cannot receive synthetic grafts. Such patients could benefit from a vascular graft produced by tissue engineering. Here, we engineer vascular grafts using human allogeneic or canine smooth muscle cells grown on a tubular polyglycolic acid scaffold. Cellular material was removed with detergents to render the grafts nonimmunogenic. Mechanical properties of the human vascular grafts were similar to native human blood vessels, and the grafts could withstand long-term storage at 4°C. Human engineered grafts were tested in a baboon model of arteriovenous access for hemodialysis. Canine grafts were tested in a dog model of peripheral and coronary artery bypass. Grafts demonstrated excellent patency and resisted dilatation, calcification, and intimal hyperplasia. Such tissue-engineered vascular grafts may provide a readily available option for patients without suitable autologous tissue or for those who are not candidates for synthetic grafts.

Modified ultrafiltration improves cerebral metabolic recovery after circulatory arrest

Modified ultrafiltration uses hemofiltration of the patient and bypass circuit after separation from cardiopulmonary bypass to reverse hemodilution and edema. This study investigated the effect of modified ultrafiltration on cerebral metabolic recovery after deep hypothermic circulatory arrest. Twenty-six 1-week-old piglets (2 to 3 kg) were supported by cardiopulmonary bypass (37 degrees C) at 100 ml.kg-1.min-1 and cooled to 18 degrees C. Animals underwent 90 minutes of circulatory arrest followed by rewarming to 37 degrees C. After being weaned from cardiopulmonary bypass, animals were divided into three groups: controls (n = 10); modified ultrafiltration for 20 minutes (n = 9); transfusion of hemoconcentrated blood for 20 minutes (n = 7). Global cerebral blood flow was measured by xenon 133 clearance methods: stage I--before cardiopulmonary bypass; stage II--5 minutes after cardiopulmonary bypass; and stage III--25 minutes after cardiopulmonary bypass. Cerebral metabolic rate of oxygen consumption, cerebral oxygen delivery, and hematocrit value were calculated for each time point. At point III, the hematocrit value (percent) was elevated above baseline in the ultrafiltration and transfusion groups (44 +/- 1.8, 42 +/- 1.8 versus 28 +/- 1.7, 30 +/- 0.7, respectively, p < 0.05). Cerebral oxygen delivery (ml.100 gm-1.min-1) increased significantly above baseline at point III after ultrafiltration (4.98 +/- 0.32 versus 3.85 +/- 0.16, p < 0.05) or transfusion (4.59 +/- 0.17 versus 3.89 +/- 0.06, p < 0.05) and decreased below baseline in the control group (2.77 +/- 0.19 versus 3.81 +/- 0.16, p < 0.05). Ninety minutes of deep hypothermic circulatory arrest resulted in impaired cerebral metabolic oxygen consumption (ml.100 gm-1.min-1) at point III in the control group (1.95 +/- 0.15 versus 2.47 +/- 0.07, p < 0.05) and transfusion group (1.72 +/- 0.10 versus 2.39 +/- 0.15, p < 0.05). After modified ultrafiltration, however, cerebral metabolic oxygen consumption at point III had increased significantly from baseline (3.12 +/- 0.24 versus 2.48 +/- 0.13, p < 0.05), indicating that the decrease in cerebral metabolism immediately after deep hypothermic circulatory arrest is reversible and may not represent permanent cerebral injury. Use of modified ultrafiltration after cardiopulmonary bypass may reduce brain injury associated with deep hypothermic circulatory arrest.

Effects of cardiopulmonary bypass and circulatory arrest on endothelium-dependent vasodilatation in the lung ☆ ☆☆ ★ ★★ ♢

Endothelial injury with failure of pulmonary endothelium-dependent vasodilatation has been proposed as a possible cause for the increased pulmonary vascular resistance observed after cardiopulmonary bypass, but the mechanisms underlying this response are not understood. An in vivo piglet model was used to investigate the role of endothelium-dependent vasodilatation in postbypass pulmonary hypertension. The pulmonary vascular responses to acetylcholine, a receptor-mediated endothelium-dependent vasodilator, and nitric oxide, an endothelium-independent vasodilator, were studied in one group of animals after preconstriction with the thromboxane A2 analog U46619 (n = 6); a second group was studied after bypass with 30 minutes of deep hypothermic circulatory arrest (n = 6). After preconstriction with U46619, both acetylcholine and nitric oxide caused significant decreases in pulmonary vascular resistance (34% +/- 6% decrease, p = 0.007, and 39% +/- 4% decrease, p = 0.001). After cardiopulmonary bypass with circulatory arrest, acetylcholine did not significantly change pulmonary vascular resistance (0% +/- 8% decrease, p = 1.0), whereas nitric oxide produced a 32% +/- 4% decrease in pulmonary vascular resistance (p = 0.007). These results demonstrate a loss of receptor-mediated endothelium-dependent vasodilatation with normal vascular smooth muscle function after circulatory arrest. Administration of the nitric oxide synthase blocker Ngamma-nitro-L-arginine-methyl-ester after circulatory arrest significantly increased pulmonary vascular resistance; thus, although endothelial cell production of nitric oxide may be diminished, it continues to be a major contributor to pulmonary vasomotor tone after cardiopulmonary bypass with deep hypothermic circulatory arrest. In summary, cardiopulmonary bypass with deep hypothermic circulatory arrest results in selective pulmonary endothelial cell dysfunction with loss of receptor-mediated endothelium-dependent vasodilatation despite preserved ability of the endothelium to produce nitric oxide and intact vascular smooth muscle function.

Blockade of endothelin-converting enzyme reduces pulmonary hypertension after cardiopulmonary bypass and circulatory arrest.

BACKGROUND Pulmonary dysfunction associated with elevated pulmonary vascular resistance is a significant problem after cardiopulmonary bypass (CPB) and circulatory arrest. Mediators of the pulmonary hypertensive response to CPB have not been fully elucidated. The purpose of this study was to examine the contribution of the endothelium-derived vasoconstrictor endothelin-1 to postbypass pulmonary hypertension. METHODS Twelve 1-month-old piglets were instrumented with left atrial and pulmonary artery (PA) micromanometers and a PA flow probe. Phosphoramidon (Phos, n = 6) pigs received a 30 mg/kg bolus of Phos, an endothelin converting enzyme inhibitor. Controls (n = 6) received saline solution. All animals were placed on CPB and underwent a 60-minute period of circulatory arrest. The indexed pulmonary vascular resistance (PVRI) was calculated at baseline for controls, both before and 10 minutes after drug infusion in the Phos group, and 15 minutes after separation from CPB in both groups. RESULTS Pre-CPB, mean PA pressure, and PVRI were not different between the control and Phos groups (14.6 +/- 1.1 versus 14.5 +/- 1.1 mm Hg and 7322 +/- 1269 versus 7260 +/- 947 dyne/sec/kg/cm-5, respectively). After CPB mean PA pressure was significantly higher in control than Phos animals (32.1 +/- 1.1 versus 22.5 +/- 1.3 mm Hg, p = 0.0003). PVRI was also significantly higher in the controls (30896 +/- 4714 versus 14972 +/- 1710, dyne/sec/kg/cm-5, p = 0.02). CONCLUSIONS Production of endothelin-1 during CPB and circulatory arrest is a mediator of postbypass pulmonary hypertension.

Low-flow cardiopulmonary bypass produces greater pulmonary dysfunction than circulatory arrest

BACKGROUND Deep hypothermic circulatory arrest (DHCA) is used during the repair of congenital heart disease in neonates. However, because of concern about neurologic injury after DHCA, there is increasing use of continuous deep hypothermic low-flow cardiopulmonary bypass (DHCPB). This study examines the effects of DHCPB versus DHCA on pulmonary dynamics in 1-week-old piglets (weight range, 2.5 to 3.5 kg). METHODS Animals were placed on CPB (37 degrees C) at 100 mL.kg-1.min-1, cooled to 18 degrees C, and then assigned to one of two groups: DHCPB (n = 7), 25 to 50 mL.kg-1.min-1 DHCPB for 90 minutes; or DHCA (n = 8), DHCA for 90 minutes. Animals were rewarmed to 37 degrees C, weaned from CPB, and observed for 30 minutes. Static pulmonary compliance and pulmonary vascular resistance index were assessed before CPB, 5 minutes after CPB, and 30 minutes after CPB. RESULTS There was greater impairment of static pulmonary compliance after DHCPB compared with 90 minutes of DHCA. There was a trend toward higher pulmonary vascular resistance index in the DHCPB group; however, significance was not reached. CONCLUSIONS Deep hypothermic low flow cardiopulmonary bypass produces greater pulmonary dysfunction than DHCA, manifested by decreased static pulmonary compliance. If DHCPB is used in place of DHCA in congenital heart operations, close attention to ventilatory and fluid management is mandatory in the postoperative period to prevent further worsening of pulmonary dysfunction.

pH-stat cooling improves cerebral metabolic recovery after circulatory arrest in a piglet model of aortopulmonary collaterals☆☆☆★★★♢

Cardiopulmonary bypass with deep hypothermic circulatory arrest increases the risk of neurologic injury in patients with aortopulmonary collaterals. Experimental studies have demonstrated that such collaterals decrease the rate of cerebral cooling before arrest and cerebral metabolic recovery after circulatory arrest. Use of pH-stat blood gas management has been shown to increase cerebral blood flow during cooling. The current study was designed to test whether cooling with pH-stat blood gas management can decrease the cerebral metabolic impact of aortopulmonary collaterals. Twenty 4- to 6-week-old piglets underwent placement of a shunt between the left subclavian artery and main pulmonary artery. In control animals (n = 10) the shunts were immediately ligated, whereas in the shunt animals (n = 10) the shunts were left patent. All animals were supported with cardiopulmonary bypass, cooled to 18 degrees C by means of either alpha-stat (five control and five shunt animals) or pH-stat (five control and five shunt animals) blood gas management, subjected to circulatory arrest for 90 minutes, and rewarmed to 37 degrees C. The cerebral metabolic rate of oxygen consumption (a marker for neurologic function) was significantly lower after circulatory arrest in the shunt animals cooled with alpha-stat blood gas management than in the control animals subjected to alpha-stat management (1.2 +/- 0.2 vs 2.3 +/- 0.2 ml oxygen per 100 gm/min, p < 0.05). By contrast, there was no difference between the pH-stat shunt animals and either control group (2.1 +/- 0.2 vs 2.3 +/- 0.2 [alpha-stat] and 2.0 +/- 0.3 [pH-stat] ml oxygen per 100 gm/min, p = not significant). pH-Stat cooling protected the brain from shunt-related injury. When circulatory arrest is used in the presence of aortopulmonary collaterals, the use of pH-stat blood gas management during cooling results in better cerebral protection than alpha-stat blood gas management.

Effects of Aortopulmonary Collaterals on Cerebral Cooling and Cerebral Metabolic Recovery After Circulatory Arrest

BACKGROUND Aortopulmonary collaterals (APC) have been associated with an increased risk of choreoathetosis after deep hypothermic circulatory arrest (DHCA). To study the effects of APC on cerebral hemodynamics and metabolism before and after DHCA, a piglet model was developed. METHODS AND RESULTS Protocol 1: Eight 4- to 6-week-old piglets underwent placement of a left subclavian-to-main pulmonary artery shunt. Control shunts (n = 4) were ligated, APC shunts (n = 4) were left patent. All animals were placed on cardiopulmonary bypass (CPB) and cooled in identical fashion for 20 minutes. Temperature probes were placed in the nasopharynx, cortex, and deep brain. Control animals achieved significantly lower temperatures in all three areas by the end of cooling (17.5 degrees C versus 20.1 degrees C, 19.0 degrees C versus 22.3 degrees C, and 17.5 degrees C versus 21.0 degrees C, respectively, P < .005). Protocol 2: Six control and six APC animals were instrumented as described. All were placed on CPB, cooled to 18 degrees C, arrested for 90 minutes, and rewarmed to 37 degrees C. Cerebral blood flow (CBF) was measured with radioactive microspheres while warm on CPB, after cooling, and after rewarming. Arterial and sagittal sinus blood gases and CBF were used to calculate the cerebral metabolic rate of oxygen consumption (CMRO2). Both CBF and CMRO2 were significantly higher after rewarming to 37 degrees C in control versus APC animals (28 +/- 3 versus 14 +/- 2 mL/100 g per minute and 1.72 +/- 0.21 versus 1.04 +/- 0.14 mL O2/100 g per minute, respectively, P < .05). CONCLUSIONS APC decrease the rate of cerebral cooling on CPB and even if temperature is controlled result in increased cerebral metabolic derangement after DHCA. Patients with such collaterals may need additional measures to optimize cerebral protection.

Acute functional consequences of left ventriculotomy

BACKGROUND Left ventriculotomies are sometimes used during intracardiac congenital defect repair. Acute changes in left ventricular function after longitudinal or apical ventriculotomy were assessed using dynamic pressure-dimensional data. METHODS Ultrasonic dimension transducers along the major, minor, and septal free wall axes and micromanometers were placed in 24 piglets. Pressure-volume data were collected during caval occlusions at baseline and 60 minutes after warm cardiopulmonary bypass alone or with longitudinal ventriculotomy or apical left ventriculotomy. Hemodynamics, contractility, and contraction geometry were analyzed. RESULTS Cardipulmonary bypass caused decreased compliance in all groups, with equally decreased preload and cardiac output. Heart rate increased, but ventriculotomy led to a significantly greater increase. Longitudinal ventriculotomy produced a greater loss of stroke volume and ejection fraction than apical ventriculotomy. Contractility assessed by the preload recruitable stroke work relationship showed no difference between groups; however, all groups showed a slight increase in unit myocardial power at 60 minutes. Axis fractional shortening revealed that the septal freewall is responsible for 50% of stroke volume and that this axis is significantly impaired after longitudinal ventriculotomy. CONCLUSION Apical left ventriculotomy impairs the less important major axis only and is predicted to be better tolerated.

Recurrent Inflammatory Myofibroblastic Tumor of the Heart

A previously healthy 15-year-old boy was admitted after 6 months of intermittent fevers, night sweats, and 9 kg weight loss. A systolic murmur was appreciated at the left upper sternal border. Laboratory data revealed elevated white blood cells (11 800/μL), erythrocyte sedimentation rate (75 mm/h), and C-reactive protein (10.5 mg/dL). The results of blood cultures were negative. Imaging demonstrated a hypermetabolic 2×2×4.5 cm pedunculated mass in the right ventricle, suggestive of a neoplasm with overlying thrombus (Figures 1A, 2A and 2B and online-only Data Supplement Movie I). Figure 1. Cardiac magnetic resonance imaging demonstrating the initial IMT originating from the interventricular septum near the right ventricular outflow tract ( A ) and the recurrent IMT near the tricuspid annulus ( B ). IMT indicates inflammatory myofibroblastic tumor. Figure 2. Transthoracic echocardiography before and after resection of the initial inflammatory myofibroblastic tumor. A and B , Preoperative images showed a large echogenic mass in the apex of the right ventricle extending superiorly through the right ventricular outflow tract. C and D , Postoperative echocardiography revealed no evidence of residual tumor. A4C indicates apical 4-chamber view; Ao, aorta; LA, left atrium; LV, left ventricle; PA, pulmonary artery; PSAX, parasternal short axis view; RA, right atrium; RV, right ventricle; RVOT, right ventricular outflow tract; Pre-op, preoperative; and Post-op, postoperative. Arrow denotes the tumor. At surgery, the mass was found to originate from the midportion of the interventricular septum and was covered with thrombus. …

Reproducibility of left atrial ablation with high-intensity focused ultrasound energy in a calf model.

OBJECTIVE Achieving transmural tissue ablation might be necessary for successful treatment of atrial fibrillation. The purpose of this study was to evaluate the reproducibility of transmural left atrial ablation using a high-intensity focused ultrasound energy system in a calf model. METHODS Nine heparinized bovines underwent a beating-heart left atrial ablation with a single application of the high-intensity focused ultrasound device. All animals were acutely killed, and the left atrium was fixed in formalin. Protocolized histological sections (5 μm) were obtained throughout each lesion and prepared with Masson trichrome and hematoxylin and eosin staining. Measurements were performed on a total of 359 slides from the 9 lesions. In addition, fresh left atrial tissues from 18 unused human donor hearts that did not meet the criteria for cardiac transplantation were measured at the site where the high-intensity focused ultrasound device is normally applied. RESULTS Calf left atrial thickness ranged between 2.5 and 20.1 mm, with a mean of 9.10 mm. High-intensity focused ultrasound ablation consistently produced a 100% transmural lesion in left atrial thickness up to 6 mm. In addition, a transmural lesion was observed in 91% of tissues that were up to 10 mm thick and in 85% that were up to 15 mm thick. Human left atrial thickness ranged between 1.2 to 6 mm, with a mean of 3.7 mm. CONCLUSIONS Calf left atrial thickness in this study was greater than human left atrial thickness. Human left atrial thickness is generally less than 6 mm, and in this range high-intensity focused ultrasound ablation achieved 100% transmurality. These histological results might correlate with a high success rate of atrial fibrillation ablation by using the high-intensity focused ultrasound system.