The thymus. Pediatric surgical aspects.

Since the original description of thymic death in an infant 400 years ago, the thymus has been recognized as an important structure to practitioners caring for infants and children. The source of many cysts, masses, and tumors in the neck and mediastinum, the thymus gland merits the pediatric surgeon's attention. The thymus is clearly an important lymphoid organ, the removal of which may be therapeutic in MG, but congenital absence leads to profound cell-mediated immunodeficiency. The immunologic sequelae of its neonatal extirpation remains obscure. It is apparent that further research is needed to clarify the functional role of the thymus gland in the developing immune system. Until better elucidated, a conservative approach to neonatal thymectomy may be justified.

The effect of retrograde cerebral perfusion after particulate embolization to the brain.

Neurologic injury as a consequence of cerebral embolism of either air or atherosclerotic debris during cardiac or aortic surgery is still a major cause of postoperative morbidity and mortality. While exploring various means of improving cerebral protection during complex cardiothoracic procedures, we have developed a chronic porcine model to study retrograde cerebral perfusion. We have previously demonstrated that retrograde perfusion results in a small amount of nutritive flow and provides cerebral protection that appears to be superior to simple prolonged hypothermic circulatory arrest. The current study was designed to evaluate the efficacy of retrograde cerebral perfusion in mitigating the effects of particulate cerebral embolism occurring during cardiac surgery. Four groups of pigs (19 to 28 kg) underwent cardiopulmonary bypass with deep hypothermia at an esophageal temperature of 20 degrees C: an antegrade control group (AC, n = 5), an antegrade embolism group (AE, n = 10), a retrograde control group (RC, n = 5), and a retrograde embolism group (RE, n = 10). In addition, because of extreme heterogeneity in outcome in the initial RE group, an additional group of 10 animals underwent embolism and retrograde perfusion at a later time. Embolization was accomplished by injection of 200 mg of polystyrene microspheres (250 to 750 micrograms in diameter) via the aortic cannula into an isolated aortic arch preparation in the AE and RE groups; the control groups received injections of 10 ml of saline solution. After infusion of the microspheres or saline solution, conventional perfusion, with the aortic arch pressure maintained at 50 mm Hg, was continued for a total of 30 minutes in the antegrade groups; in the retrograde groups, retrograde flow was initiated via a cannula positioned in the superior vena cava, and was continued for 25 minutes. Superior vena caval flow was regulated to maintain a sagittal sinus pressure of approximately 30 mm Hg in the retrograde groups, and blood returning to the isolated aortic arch was collected and measured. All animals were allowed to recover and were evaluated daily according to a quantitative behavioral score in which 9 indicates apparently complete normalcy, with lower numbers indicating various degrees of cerebral injury. At the time of planned death on day 6, half of the brain was used for recovery of embolized microspheres after digestion with 10N sodium hydroxide. The other half was submitted for histologic study. Neurologic recovery in both the antegrade and retrograde control groups appeared to be complete, although mild evidence of histologic damage was present in some animals in the retrograde control group.(ABSTRACT TRUNCATED AT 400 WORDS)

Quantitative electroencephalography: a method to assess cerebral injury after hypothermic circulatory arrest.

Although hypothermic circulatory arrest and low-flow cardiopulmonary bypass are routinely used for surgical correction of congenital cardiac anomalies, use of long durations of arrest, often required for more complex repairs, raises serious concerns about cerebral safety. Searching for an intraoperative assessment that can reliably predict cerebral injury, we have found an excellent correlation between changes in quantitative electroencephalography intraoperatively and immediately postoperatively after prolonged hypothermic arrest, and neurologic and behavioral evidence of cerebral injury. After epidural placement of four recording electroencephalographic electrodes and baseline neurologic/behavioral and electroencephalographic assessment, 32 puppies were randomly assigned to one of four groups: hypothermic controls in which cooling to 18 degrees C was followed immediately by rewarming, 30 minutes of hypothermic circulatory arrest at 18 degrees C, 90 minutes of arrest at 18 degrees C, and 90 minutes of low-flow cardiopulmonary bypass at 25 ml/kg per minute at 18 degrees C. An electroencephalogram was recorded at baseline, after cooling, during rewarming, and at 2, 4, and 8 hours after the start of rewarming, as well as before the operation and 1 week after the operation. Postoperative neurologic and behavioral outcome was assessed 24 hours after cardiopulmonary bypass and daily for 1 week by means of a graded scale in which 0 is normal and 12 and 13 indicate severe neurologic injury (coma and death). Thirty animals survived the experimental protocol: two animals in the 90-minute hypothermic arrest group died before neurologic evaluation could be completed, and the remainder exhibited various degrees of neurologic and behavioral impairment, more severe on day 1 than on day 6. No animal in the remaining groups had a significant neurologic deficit. Quantitative electroencephalographic analysis shows marked differences between the 90-minute arrest group and the controls in the percent electroencephalographic silence during rewarming and at 2 hours, and in the percent recovery of baseline power at 2, 4, and 8 hours. At 2 hours after the start of rewarming, a correlation between electroencephalographic amplitude and neurologic/behavioral score on day 1 was carried out, which predicts with great certainty (p < 0.00001) that if electroencephalographic power at this time is less than 500 microV2, overt neurologic injury will subsequently become apparent. In addition, a significant shift from higher to lower frequency in the day 6 postoperative electroencephalogram compared with baseline occurs only in the 90-minute arrest group.(ABSTRACT TRUNCATED AT 400 WORDS)

Metabolic correlates of neurologic and behavioral injury after prolonged hypothermic circulatory arrest.

Thirty-two inbred weanling puppies were divided into four groups to study the effect on cerebral blood flow and metabolism of different hypothermic strategies for cerebral protection similar to those used during cardiac operations in infancy. All animals were cooled to 18 degrees C. The animals in the hypothermic control group were immediately rewarmed. One group underwent 30 minutes of hypothermic circulatory arrest at 18 degrees C; another group had 90 minutes of hypothermic circulatory arrest at 18 degrees C, and the final group had low-flow cardiopulmonary bypass (25 ml/kg per minute) at 18 degrees C for 90 minutes. All animals had preoperative and postoperative neurologic and behavioral evaluation and extensive intraoperative monitoring of cerebral blood flow, cerebral vascular resistance, and oxygen and glucose uptake and metabolism: quantitative electroencephalography was also monitored before, during and after operation, but those results are reported separately. Two animals in the 90-minute arrest group died, and all the survivors showed evidence of clinical, neurologic, and behavioral impairment on postoperative day 1, with residual abnormalities in all but one animal on day 6. In contrast, the survivors in all the other groups showed no significant clinical or behavioral sequelae. Cerebral metabolism was reduced only to 32% to 40% of baseline values at 18 degrees C in all groups, although systemic metabolism was only 16% of normal. Cerebral metabolism returned promptly to baseline in all groups during rewarming and remained at baseline levels throughout the 8 hours of follow-up. Cerebral blood flow showed marked hyperemia in the hypothermic arrest groups during rewarming but then significant reductions below baseline values in all groups except the controls at 2 and 4 hours after the operation, lasting as late as 8 hours after the operation in the 90-minute arrest group. Cerebral vascular resistance showed increases in all groups at 2 and 4 hours after the operation, which persisted in the 90-minute arrest group at 8 hours. Cerebral metabolism was maintained at baseline levels despite postoperative decreases in cerebral blood flow and increases in cerebral vascular resistance by increases in oxygen and glucose extraction. The result was very low sagittal sinus oxygen saturations in all groups, most marked in the 90-minute arrest groups, which had a saturation of only 24% 8 hours after the operation. Our data show a severe, prolonged disturbance in cerebral blood flow and cerebral vascular resistance after 90 minutes of hypothermic circulatory arrest at 18 degrees C, which correlates with clinical evidence of cerebral injury.(ABSTRACT TRUNCATED AT 400 WORDS)

Three faces of the Bentall procedure.

Since the original description of composite replacement of the aortic valve and ascending aorta by Bentall in 1968, several modifications of the technique have been described. In order to evaluate the results of these different techniques, we have retrospectively reviewed our results with 140 consecutive patients who underwent Bentall operations between October 1986 and March 1994, using three different anastomotic techniques: Classic, n = 30; Button, n = 95, and Cabrol, n = 15. Overall hospital mortality was 5%. In univariate analysis, acute type A dissection, rupture, new preoperative neurological symptoms, and the Cabrol technique were associated with a higher hospital mortality, but by multivariate analysis no independent risk factors were demonstrated. Overall rates of reoperation did not differ among the three techniques (Classic 4.1%/pt-yr, Button 2.7%/pt-yr, Cabrol 0%/pt-yr; p = 0.44). The actuarial freedom from reoperation was 87% at 5 years. The 5-year actuarial survival for all patients was 79% (Classic 85%, Button 82%, Cabrol 52%): the poorer results with the Cabrol modification are likely due to patient selection, complicated by a higher early mortality in this small group of patients. The presence of dissection was associated with a higher mortality in Marfan patients (50% vs 8%, p = 0.03). The rate of aortic valve-related complications was 3.6%/pt per year. Actuarial event-free survival was 67% at 5 years. Current indications for an elective Bentall procedure include an ascending aortic diameter of 6 cm or greater, with significant aortic valvular dysfunction, and dilatation of the ascending aorta greater than 5 cm in patients with Marfan syndrome or a bicuspid aortic valve. The routine procedure of choice is the Button Bentall technique, with the Classic Bentall and the Cabrol variation reserved for use under special circumstances.

Comparison of retrograde cerebral perfusion to antegrade cerebral perfusion and hypothermic circulatory arrest in a chronic porcine model.

Retrograde cerebral perfusion (RCP) is a new method of cerebral protection that has been touted as an improvement over hypothermic circulatory arrest (HCA). However, RCP has been used clinically for durations and at temperatures that are "safe" for HCA alone. This study was designed to compare RCP to HCA and antegrade cerebral perfusion (ACP) deliberately exceeding "safe" limits, in order to determine unequivocally whether RCP provides better cerebral protection than HCA. Four groups of six Yorkshire pigs (20 to 30 kg) were randomly assigned to undergo 90 minutes of RCP, ACP, HCA, or HCA with heads packed in ice (HCA-HP) at an esophageal temperature of 20 degrees C. Arterial, mixed venous and cerebral venous oxygen, glucose and lactate contents; quantitative EEG; were monitored at baseline (37 degrees C); at the end of cooling cardiopulmonary bypass (20 degrees C); during rewarming (30 degrees C); and at two and four hours post intervention. Animals were recovered and were evaluated daily using a quantitative behavioral score (0 to 9). Mean behavioral score was lower in the HCA group than in the other three groups at seven days (HCA 5.8 +/- 1.1; RCP 8.5 +/- 0.2; ACP 9.0 +/- 0.0; HCA-HP 8.5 +/- 0.2, p < 0.05). Recovery of QEEG was better in the ACP group than in all others, but the RCP group had faster EEG recovery than HCA alone, although not better than HCA-HP (HCA 15 +/- 4; RCP 27 +/- 3; ACP 78 +/- 5; HCA-HP 19 +/- 3, p < 0.001). However, histopathological evidence of ischemic injury was present in 5 of 6 HCA animals and also in 4 of 6 of the HCP-HP group, but only in 1 of 6 RCP animals and in none of the ACP group. This study demonstrates that ACP affords the best cerebral protection by all outcome measures, but RCP provides clear improvement compared to HCA.

Pathogenesis of spinal cord injury during simulated aneurysm repair in a chronic animal model.

The pathogenesis of paraplegia after repair of thoracic aortic aneurysms is controversial. Using direct spinal cord evoked potential monitoring, critical intercostal arteries (CICA) were identified to evaluate the impact of backbleeding and ligation versus that of preservation during simulated aneurysm repair. Thirty pigs (40 kg) were randomly assigned to one of five groups. In groups 1 through 4, a thoracic segment containing CICA was cross-clamped for 60 minutes and distal aortic perfusion was provided by a centrifugal pump. In groups 1 and 2, the thoracic segment was vented, maintaining segment pressure at 0 mm Hg; CICA were ligated in group 1 and preserved in group 2. Thoracic segment was perfused at 70 mm Hg in groups 3 and 4; CICA were ligated in group 3 and preserved in group 4. Critical intercostal artery ligations were performed at the end of the cross-clamp period. In group 5 simple cross-clamping at the left subclavian artery was performed as a control. The combination of venting and ligation of CICA correlated with impaired neurologic outcome according to Tarlov's score (median, 1.5 in group 1 versus 3 in group 2; p = 0.015), indicated by a significant difference in median values of direct spinal cord evoked potential amplitude (expressed as a fraction of baseline values) at 120 minutes after cross-clamping (0.76 in group 1 versus 0.98 in group 2; p = 0.0082).(ABSTRACT TRUNCATED AT 250 WORDS)

Aortic dissection involving a double aortic arch with a right descending aorta.

We report the successful surgical treatment of a 67-year-old man with a double aortic arch, right descending aorta, and a subacute dissection of the descending thoracic aorta using hypothermic circulatory arrest.

Evaluation of cerebral metabolism and quantitative electroencephalography after hypothermic circulatory arrest and low-flow cardiopulmonary bypass at different temperatures.

Although widely used for repair of complex cardiovascular pathologic conditions, long intervals of hypothermic circulatory arrest and low flow cardiopulmonary bypass may both result in cerebral injury. This study examines cerebral hemodynamics, metabolism, and electrical activity to evaluate the risks of cerebral injury after 60 minutes of hypothermic circulatory arrest at 8 degrees C, 13 degrees C, and 18 degrees C, compared with 60 minutes of low flow cardiopulmonary bypass at 18 degrees C. Thirty-two puppies were randomly assigned to one of four experimental groups and centrally cooled to the appropriate temperature. Serial evaluations of quantitative electroencephalography, radioactive microsphere determinations of cerebral blood flow, calculations of cerebral oxygen consumption, cerebral glucose consumption, cerebral vascular resistance, cerebral oxygen extraction, systemic oxygen metabolism, and systemic vascular resistance were done. Measurements were obtained at baseline (37 degrees C), at the end of cooling, at 30 degrees C during rewarming, and at 2, 4, and 8 hours after hypothermic circulatory arrest or low flow cardiopulmonary bypass. At the end of cooling, cerebral vascular resistance remained at baseline levels in all groups, but systemic vascular resistance was increased in all groups. Cerebral oxygen consumption became progressively lower as temperature was reduced: it was only 5% of baseline at 8 degrees C; 20% at 13 degrees C; and 34% and 39% at 18 degrees C. Quantitative electroencephalography was silent in the 8 degrees C and 13 degrees C groups, but significant slow wave activity was present at 18 degrees C. Systemic vascular resistance and cerebral oxygen consumption returned to baseline values in all groups by 2 hours after hypothermic circulatory arrest or low flow cardiopulmonary bypass, but cerebral vascular resistance remained elevated at 2 and 4 hours, not returning to baseline until 8 hours after hypothermic circulatory arrest or low flow cardiopulmonary bypass. All but two of the long-term survivors (27 of 32) appeared neurologically normal; after hypothermic circulatory arrest at 8 degrees and 18 degrees C two animals had an unsteady gait. Comparison of quantitative electroencephalography before operation and 6 days after operation showed a significant increase in slow wave activity (delta activity) after hypothermic circulatory arrest and low flow cardiopulmonary bypass at 18 degrees C, a change that suggests possible cerebral injury. Although undetected after operation by simple behavioral and neurologic assessment, significant differences in cerebral metabolism, vasomotor responses, and quantitative electroencephalography do exist during and after hypothermic circulatory arrest and low flow cardiopulmonary bypass at various temperatures and may be implicated in the occurrence of cerebral injury.(ABSTRACT TRUNCATED AT 400 WORDS)

Computer-generated 3D representations of the aorta: a new tool in the management of aortic aneurysm patients.

In order to facilitate care of patients with thoracic aortic aneurysms, we have developed a computer program which allows three dimensional (3D) reconstructions of the aorta from serial sections of CT scans. Each completed study, which can be displayed on a single data sheet, provides a 3D drawing of the aorta, the diameter of each aortic slice, the volume of each aortic segment, the total volume and surface area of the aorta, and a quantity we have called the tortuosity index. The method enables appreciation of transverse as well as longitudinal increases in aneurysm size, and eccentricities in aortic pathology can easily be recognized. The data are stored on floppy discs: this allows a second program to provide comparisons of the important findings from multiple and long-term serial CT studies.