Biomechanical modeling of refractive corneal surgery

The aim of refractive corneal surgery is to modify the curvature of the cornea to improve its dioptric properties. With that goal, the surgeon has to define the appropriate values of the surgical parameters in order to get the best clinical results, i.e., laser and geometric parameters such as depth and location of the incision, for each specific patient. A biomechanical study before surgery is therefore very convenient to assess quantitatively the effect of each parameter on the optical outcome. A mechanical model of the human cornea is here proposed and implemented under a finite element context to simulate the effects of some usual surgical procedures, such as photorefractive keratectomy (PRK), and limbal relaxing incisions (LRI). This model considers a nonlinear anisotropic hyperelastic behavior of the cornea that strongly depends on the physiological collagen fibril distribution. We evaluate the effect of the incision variables on the change of curvature of the cornea to correct myopia and astigmatism. The obtained results provided reasonable and useful information in the procedures analyzed. We can conclude from those results that this model reasonably approximates the corneal response to increasing pressure. We also show that tonometry measures of the IOP underpredicts its actual value after PRK or LASIK surgery.     

Omega-3 fatty acids in dry eye and corneal nerve regeneration after refractive surgery

Dry eye (DE) is a multifactorial condition that affects the surface of the eye and induces an inflammatory response. Corneal nerves play an important role in the maintenance of a healthy ocular surface. Here we review corneal structure, nerve architecture, DE conditions, and nerve regeneration following corneal surgery and discuss how n-3 fatty acids affect the health of the cornea. Animal studies show that resolvins, compounds derived from eicosapentaenoic acid (EPA), increase tear volume and decrease inflammation induced by DE. After corneal surgery in rabbits, treatment with nerve growth factor (NGF) or pigment epithelial derived factor (PEDF) in conjunction with docosahexaenoic acid (DHA) increase nerve density and corneal epithelial cell proliferation. Increased synthesis of the novel docosanoid, neuroprotectin D1 (NPD1), was found in corneas after the animals were treated with PEDF and DHA. Topical application of these lipids derived from n-3 fatty acids could be useful in treating DE and prevent clinical complications such as cornea erosion and ulcerations.     

Patient-specific modeling of corneal refractive surgery outcomes and inverse estimation of elastic property changes

The purpose of this study is to develop a 3D patient-specific finite element model (FEM) of the cornea and sclera to compare predicted and in vivo refractive outcomes and to estimate the corneal elastic property changes associated with each procedure. Both eyes of a patient who underwent laser-assisted in situ keratomileusis (LASIK) for myopic astigmatism were modeled. Pre- and postoperative Scheimpflug anterior and posterior corneal elevation maps were imported into a 3D corneo-scleral FEM with an unrestrained limbus. Preoperative corneal hyperelastic properties were chosen to account for meridional anisotropy. Inverse FEM was used to determine the undeformed corneal state that produced <0.1% error in anterior elevation between simulated and in vivo preoperative geometries. Case-specific 3D aspheric ablation profiles were simulated, and corneal topography and spherical aberration were compared at clinical intraocular pressure. The magnitude of elastic weakening of the residual corneal bed required to maximize the agreement with clinical axial power was calculated and compared with the changes in ocular response analyzer (ORA) measurements. The models produced curvature maps and spherical aberrations equivalent to in vivo measurements. For the preoperative property values used in this study, predicted elastic weakening with LASIK was as high as 55% for a radially uniform model of residual corneal weakening and 65% at the point of maximum ablation in a spatially varying model of weakening. Reductions in ORA variables were also observed. A patient-specific FEM of corneal refractive surgery is presented, which allows the estimation of surgically induced changes in corneal elastic properties. Significant elastic weakening after LASIK was required to replicate clinical topographic outcomes in this two-eye pilot study.     

A simple venous thromboembolism prophylaxis protocol for patients undergoing bariatric surgery

Objective: Pulmonary embolism is a leading cause of death for bariatric patients. Numerous regimens have been proposed, but a comprehensive, simple approach is lacking. This study provides a simple, easily implemented prophylaxis regimen. Research Methods and Procedures: One hundred fifty bariatric surgery patients were evaluated. Patients considered at high risk for venous thromboembolism had heart failure, a BMI of ≥50 kg/m², or a history of venous thromboembolism or pelvic surgery. Preoperatively and postoperatively, all patients received subcutaneous enoxaparin or unfractionated heparin. High‐risk patients received either preoperatively inserted inferior vena cava filters or continuous heparin infusions intraoperatively. All high‐risk patients were anticoagulated with warfarin (Coumadin; Bristol Myers‐Squibb, Princeton, NJ) for at least 3 months postoperatively. Initially, some patients experienced significant hemorrhage; to prevent this, sutures were oversewn into staple lines. Results: No patient experienced venous thromboembolism; a binomial test showed that the regimen reduced the risk of this complication to less than 2% (p < 0.05). Hemorrhage sufficient to require transfusion occurred in 4 of the first 20 patients; of the remaining 130 patients, into whose staple lines sutures were oversewn, none required transfusion (p < 0.05). Discussion: Patients should be divided into those who are at high risk and those who are at low risk for venous thromboembolism. All patients should receive pre‐ and postoperative anticoagulation. High‐risk patients should also receive either an inferior vena cava filter or intraoperative heparin infusions, as well as at least 3 months of Coumadin therapy. Oversewing of staple lines may reduce the risk of hemorrhage.     

Macrophage migration inhibitory factor in pediatric patients undergoing surgery for congenital heart repair

Macrophage migration inhibitory factor (MIF), a proinflammatory mediator, has been shown to be elevated following heart surgery in adults and may be associated with several postoperative complications, including cardiac and pulmonary dysfunction. In this study, we aimed to measure perioperative plasma MIF, interleukin (IL)-8, and free T4 in 20 children age <4 years undergoing surgical repair of congenital heart lesions with left ventricular volume overload, and to determine whether the response of these mediators determined postoperative outcomes. Plasma samples were obtained preoperatively, immediately on arrival in the pediatric intensive care unit (PICU), and at 12, 24, and 48 h. Patients were continuously monitored in the PICU, and data were recorded daily for therapeutic and monitoring procedures that reflected the invasiveness, intensity, and complexity of care rendered (therapeutic interventional scoring system, TISS). Preoperative plasma MIF, IL-8, and free T4 were not different from age-matched healthy children. However, plasma MIF and IL-8 increased significantly 2 h after completion of cardiopulmonary bypass, and then normalized within 24 h. Peak postoperative levels of MIF (48 +/- 24 ng/mL) and IL-8 (79 +/- 57 pg/mL) correlated significantly with duration of cardiopulmonary bypass. The magnitude of the postoperative increase in plasma MIF was associated with increased number of days required for mechanical ventilation (r = 0.553; P = 0.012), and peak plasma IL-8 correlated significantly with the fraction of inhaled oxygen (FiO(2)) required immediately after surgery (r = 0.510; P = 0.02). Higher circulating MIF levels correlated significantly with increased inotropic support requirements on the second postoperative day, whereas higher postoperative IL-8 levels were associated with higher TISS scores, suggesting increased need for postoperative medical care. These data suggest a potential negative effect of high circulating levels of MIF in the immediate postoperative period on respiratory and cardiovascular functions, and support the development of therapeutic strategies targeting MIF function in this clinical setting.     

Isotope limb blood flow measurement in patients undergoing peripheral laser angioplasty

A technique of isotope limb blood flow (ILBF) measurement employing Technetium-labelled human albumin and a gamma camera, was used to assess limb perfusion in 19 patients undergoing percutaneous laser angioplasty, both before and one month after treatment. Twenty-three limbs with femoro-poplited occlusions ranging in length from 3–35 cm (median 8cm) were recanalized using an Nd-YAG laser and sapphire tipped optical fibre. Primary angiographic success was achieved in 19 lesions of which 6 re-occluded within the first month, and 13 remained patent with relief of symptoms. Clinically successful procedures were associated with a large increase in ILBF. However, normal blood flow was restored in only 54% of limbs. There was a slight decrease in limb perfusion after failed laser angioplasty but this was not statistically significant in this small series. Furthermore, there was no clinically apparent circulatory compromise, or need for urgent surgical bypass in failed cases. We conclude that ILBF is a useful means of assessing patients undergoing laser angioplasty, particularly for detecting small flow changes in patients who are unable to complete a standard exercise test. Its use can be recommended for assessing blood flow changes following other forms of limb revascularization.     

小手术青年患者围术期认知功能的变化

目的:以青年健康志愿者为对照,探明青年患者小手术围术期认知功能的变化.方法:以择期行脐部以下部位小手术(手术时间＜2h)、ASA分级Ⅰ～Ⅱ级的青年患者(18-39岁)60人为病例组,青年健康志愿者30人为对照组.分别于术前一天、术后3天行6项认知功能测定.结果:术后各项测验值普遍高于术前测验值(P＜0.05).病例组在术前有2项分测验低于对照组,术后4项分测验低于对照组(P＜0.05).按Z计分诊断术后认知功能障碍,术后第3天发生5例POCD,其中时照组1例,病例组4例.组间差异无统计学意义(x2=0.539,P=0.463).结论:小手术青年患者认知功能下降在术前已经存在,应激反应有可能是导致围术期认知功能下降的主要原因之一.     

The mechanism of reducing thiopentone dose in elderly patients undergoing surgery

The dose of thiopentone required to induce anesthesia in adults decreasing with age is not due to pharmacodynamic change. The change of pharmacokinetic properties of thiopentone with age in undergoing surgery patient's arterial blood was investigated in seven elderly (67-82 yr) and six young (21-33 yr) patients of both sexes. Thiopentone (3 mg kg-1) was administered intravenously and arterial blood samples were obtained immediately after the injection to measure plasma and red blood cell thiopentone concentrations by an HPLC method. Plasma protein binding was studied using ultracentrifuge method. The disappearance of thiopentone from the arterial blood was described by a two-compartment open model. The distribution rate constant (alpha) was significantly larger in the young patients (p less than 0.001). The distribution half-life was longer in the elderly (p less than 0.05). Both the input microscopic rate constant, K21, and the exit microscopic rate constant, K12, with the central compartment were significantly larger in the young patients. (p less than 0.02 and p less than 0.001, respectively). The difference between the exit and input microscopic rate constant, K12-K21, was much larger in the young patients (p less than 0.001). The plasma protein binding was significantly reduced in the elderly (p less than 0.05). The apparent overall volume of distribution, Vd was not significantly different between young and elder patients. However, the volume of distribution of the central compartment was smaller in the young patients (p less than 0.05). This was probably due to the difficulty of estimation of initial thiopentone plasma concentration post-equilibrium in the central compartment after administration of thiopentone.(ABSTRACT TRUNCATED AT 250 WORDS)     

Factor XI deficiency: implications for management of patients undergoing aesthetic surgery.

We report our experience in patients with an abnormal partial thromboplastin time elevation due to factor XI deficiency (Rosenthal syndrome) who presented for aesthetic surgery consideration. Preoperative evaluation included a thorough history, physical examination, coagulation profile, and hematological consultation. Nine of 10 patients underwent 12 elective aesthetic procedures without undue intraoperative or postoperative bleeding. Based on these findings, we stratified patients as low risk or high risk. Low-risk patients were those with greater than 15 percent factor XI levels, or those with 5 to 14 percent factor XI levels but a history of multiple major surgical procedures without bleeding complications. High-risk patients were those with factor XI levels less than 15 percent, history of bleeding either spontaneously or with surgery, and a family history of bleeding diathesis from factor XI deficiency. Low-risk patients had fresh frozen plasma available for the procedure, whereas high-risk patients received fresh frozen plasma 2 hours before surgery. We conclude that (1) in these patients with abnormally high partial thromboplastin time values and no prior known bleeding disorder, we have identified factor XI deficiency as the prevalent coagulopathy; (2) partial thromboplastin time does not necessarily correlate with factor XI levels; (3) patients can be classified as high or low risk for elective surgery based on factor XI levels and prior surgical or family history; (4) recommendations for perioperative management can be made based on this risk profile; and (5) aesthetic surgery can be performed successfully and safely on patients with factor XI deficiency on a case-by-case basis when appropriate guidelines are enforced.     

Involvement of pigment epithelium-derived factor,docosahexaenoic acid and neuroprotectin D1 in corneal inflammation and nerve integrity after refractive surgery

Alterations in corneal innervations result in impaired corneal sensation, severe dry eye and damage to the epithelium that may in turn lead to corneal ulcers, melting and perforation. These alterations can occur after refractive surgery. We have discovered that pigment epithelium-derived factor (PEDF) plus docosahexaenoic acid (DHA or the docosanoid bioactive neuroprotectin D1 (NPD1)) induces nerve regeneration after corneal surgery that damages the stromal nerves. We found that PEDF is released from corneal epithelial cells after injury, and when DHA is provided to the cells it stimulates the biosynthesis of NPD1 by an autocrine mechanism. The combination of PEDF plus DHA also decreased the production of leukotriene B4 (LTB4), a neutrophil chemotactic factor, thereby decreasing the inflammation induced after corneal damage. These studies suggest that PEDF plus DHA and its derivative NPD1 hold promise as a future treatment to restore a healthy cornea after nerve damage.     

Immunonutrition for patients undergoing elective surgery for gastrointestinal cancer: impact on hospital costs

ABSTRACT: BACKGROUND: Oral or enteral dietary supplementation with arginine, omega 3 fatty acids and nucleotides (known as immunonutrition) significantly improve outcomes in patients undergoing elective surgery. The objective of the study was to determine the impact on hospital costs of immunonutrition formulas used in patients undergoing elective surgery for gastrointestinal cancer. METHODS: US hospital costs of stay with and without surgical infectious complications, and average cost per day in the hospital for patients undergoing elective surgery for gastrointestinal cancer were estimated using data from the Healthcare Cost and Utilization Project's 2008 Nationwide Inpatient Sample. These costs were then used to estimate the impact of perioperative immunonutrition on hospital costs using estimates of reduction in infectious complications or length of stay from a meta-analysis of clinical trials in patients undergoing elective surgery for gastrointestinal cancer. Sensitivity of the results to changes in baseline complication rates or length of stay was tested. RESULTS: From the meta-analysis estimates, use of immunonutrition resulted in savings per patient of $3,300 with costs based on reduction in infectious complication rates or $6,000 with costs based on length of hospital stay. Cost savings per patient were present for baseline complication rates above 3.5% or when baseline length of stay and infectious complication rates were reduced to reflect recent US data for those with upper and lower GI elective cancer surgery (range, $1,200 to $6,300). CONCLUSIONS: Use of immunonutrition for patients undergoing elective surgery for gastrointestinal cancer is an effective and cost-saving intervention.     

Compartmentation of energy metabolism in atrial myocardium of patients undergoing cardiac surgery

The parameters of oxidative phosphorylation and its interaction with creatine kinase (CK)- and adenylate kinase (AK)-phosphotransfer networks in situ were studied in skinned atrial fibers from 59 patients undergoing coronary artery bypass surgery, valve replacement/correction and atrial septal defect correction. In atria, the mitochondrial CK and AK are effectively coupled to oxidative phosphorylation, the MM-CK is coupled to ATPases and there exists a direct transfer of adenine nucleotides between mitochondria and ATPases. Elimination of cytoplasmic ADP with exogenous pyruvate kinase was not associated with a blockade of the stimulatory effects of creatine and AMP on respiration, neither could it abolish the coupling of MM-CK to ATPases and direct transfer of adenine nucleotides. Thus, atrial energy metabolism is compartmentalized so that mitochondria form functional complexes with adjacent ATPases. These complexes isolate a part of cellular adenine nucleotides from their cytoplasmic pool for participating in energy transfer via CK- and AK-networks, and/or direct exchange. Compared to atria in sinus rhythm, the fibrillating atria were larger and exhibited increased succinate-dependent respiration relative to glutamate-dependent respiration and augmented proton leak. Thus, alterations in mitochondrial oxidative phosphorylation may contribute to pathogenesis of atrial fibrillation. (Mol Cell Biochem 270: 49–61, 2005)