A transition from solid effect to indirect cross effect with broadband microwave irradiation.

Dynamic nuclear polarization (DNP) at high magnetic fields has become a prominent technique for signal enhancement in nuclear magnetic resonance (NMR). In static samples, the highest DNP enhancement is usually observed for high radical concentrations in the range of 15-40 mM. Under these conditions, the dominant DNP mechanism for broad-line radicals is the electron-electron spectral-diffusion-based indirect cross effect (iCE). To further increase the DNP performance, broadband microwave irradiation is often applied. Until now, the theory of iCE was not rigorously combined with broadband microwave irradiation. This paper fills this gap by extending the iCE theory to explicitly include broadband irradiation. We demonstrate that our theory allows for quantitative fitting of the DNP spectra lineshapes using four different datasets acquired at 3.4 T and 7 T. We find that the DNP mechanism changes with an increase in the excitation bandwidth. While with narrowband continuous-wave irradiation the DNP mechanism is a combination of the solid effect (SE) and iCE, it shifts toward iCE with increasing excitation bandwidth until, at high bandwidth, the iCE completely dominates the DNP spectrum - this effect was not accounted for previously.

Heteronuclear DNP of protons and deuterons with TEMPOL.

Dynamic nuclear polarization (DNP) experiments on samples with several types of magnetic nuclei sometimes exhibit "cross-talk" between the nuclei, such as different nuclei having DNP spectra with similar shapes and enhancements. In this work we demonstrate that while at 20 K the DNP spectra of (1)H and (2)H nuclei, in a sample composed of 50% v/v (1)H2O/DMSO-d6 and containing 40 mM TEMPOL, are different and can be analyzed using the indirect cross effect (iCE) model, at 6 K the DNP spectra of both (1)H and (2)H nuclei become identical. In addition we experimentally demonstrate that there exists an efficient polarization exchange between the two nuclear pools at this temperature. Both of these results are hallmark predictions of the thermal mixing (TM) formalism. However, the origin of these observations cannot, in our case, be explained using the standard TM formalism, as in our sample the electron reservoir cannot be described by a single non-Zeeman spin temperature, which is a prerequisite of TM. This conclusion follows from the analysis of the electron electron double resonance (ELDOR) experiments on our sample and is similar to the previously published results. Consequently, another mechanism must be used in order to explain these "cross-talk" effects. The heteronuclear cross effect (hnCE) DNP mechanism, previously introduced based on the simulations of the spin evolution in small model systems, results in "cross-talk" effects between two types of nuclei that are similar to the experimental ones seen in this work. In particular we show that the hnCE mechanism exhibits polarization transfer between the nuclei and that there exists a clear relationship between the steady state polarizations of the two types of nuclei which may, in the future, be correlated with the phenomenon observed in the two types of bulk nuclear signals in samples during DNP experiments. It is suggested that the hnCE electrons are a possible source for the process that equalizes the bulk enhancements of the (1)H and (2)H nuclei and are responsible for the observed cross-talk effects.

Effects of the electron polarization on dynamic nuclear polarization in solids.

Dynamic Nuclear Polarization (DNP) experiments on solid dielectrics can be described in terms of the Solid Effect (SE) and Cross Effect (CE) mechanisms. These mechanisms are best understood by following the spin dynamics in electron-nuclear and electron-electron-nuclear model systems, respectively. Recently it was shown that the frequency swept DNP enhancement profiles can be reconstructed by combining basic SE and CE DNP spectra. However, this analysis did not take into account the role of the electron spectral diffusion (eSD), which can result in a dramatic loss of electron polarization along the EPR line. In this paper we extend the analysis of DNP spectra by including the influence of the eSD process on the enhancement profiles. We show for an electron-electron-nuclear model system that the change in nuclear polarization can be caused by direct MW irradiation on the CE electron transitions, resulting in a direct CE (dCE) enhancement, or by the influence of the eSD process on the spin system, resulting in nuclear enhancements via a process we term the indirect CE (iCE). We next derive the dependence of the basic SE, dCE, and iCE DNP spectra on the electron polarization distribution along the EPR line and on the MW irradiation frequency. The electron polarization can be obtained from ELDOR experiments, using a recent model which describes its temporal evolution in real samples. Finally, DNP and ELDOR spectra, recorded for a 40 mM TEMPOL sample at 10-40 K, are analyzed. It is shown that the iCE is the major mechanism responsible for the bulk nuclear enhancement at all temperatures.

The electron depolarization during dynamic nuclear polarization: measurements and simulations.

Dynamic nuclear polarization is typically explained either using microscopic systems, such as in the solid effect and cross effect mechanisms, or using the macroscopic formalism of spin temperature which assumes that the state of the electrons can be described using temperature coefficients, giving rise to the thermal mixing mechanism. The distinction between these mechanisms is typically made by measuring the DNP spectrum - i.e. the nuclear enhancement profile as a function of irradiation frequency. In particular, we have previously used the solid effect and cross effect mechanisms to explain temperature dependent DNP spectra. Our past analysis has however neglected the effect of depolarization of the electrons resulting from the microwave (MW) irradiation. In this work we concentrate on this electron depolarization process and perform electron-electron double resonance (ELDOR) experiments on TEMPOL and trityl frozen solutions, using a 3.34 Tesla magnet and at 2.7-30 K, in order to measure the state of the electron polarization during DNP. The experiments indicate that a significant part of the EPR line is affected by the irradiation due to spectral diffusion. Using a theoretical framework based on rate equations for the polarizations of the different electron spin packets and for those of the nuclei we simulated the various ELDOR line-shapes and reproduced the MW frequency and irradiation time dependence. The obtained electron polarization distribution cannot be described using temperature coefficients as required by the classical thermal mixing mechanism, and therefore the DNP mechanism cannot be described by thermal mixing. Instead, the theoretical framework presented here for the analysis of the ELDOR data forms a basis for future interpretation of DNP spectra in combination with EPR measurements.

The (13)C solid DNP mechanisms with perchlorotriphenylmethyl radicals--the role of (35,37)Cl.

The microwave frequency swept DNP enhancement, referred to as the DNP spectrum, is strongly dependent on the EPR spectrum of the polarizing radical and it reveals the underlying DNP mechanisms. Here we focus on two chlorinated trityl radicals that feature axially symmetric powder patterns at 95 GHz, the width of which are narrower than those of TEMPOL or TOTAPOL but broader than that of the trityl derivative OX63. The static DNP lineshapes of these commonly used radicals in DNP, have been recently analyzed in terms of a superposition of basic Solid Effect (SE) and Cross Effect (CE)-DNP lineshapes, with their relative contributions as a fit parameter. To substantiate the generality of this approach and further investigate an earlier suggestion that a (35,37)Cl-(13)C polarization transfer pathway, termed "hetero-nuclear assisted DNP", may be in effect in the chlorinated radicals (C. Gabellieri et al., Angew. Chem., Int. Ed., 2010, 49, 3360-3362), we measured the static (13)C-glycerol DNP spectra of solutions of ca. ∼10 mM of the two chlorinated trityl radicals as a function of temperature (10-50 K) and microwave power. Analysis of the DNP lineshapes was first done in terms of the SE/CE superposition model calculated assuming a direct e-(13)C polarization transfer. The CE was found to prevail at the high temperature range (40-50 K), whereas at the low temperature end (10-20 K) the SE dominates, as was observed earlier for (13)C DNP with OX63 and (1)H DNP with TEMPOL and TOTAPOL, thus indicating that this is rather general behavior. Furthermore, it was found that at low temperatures it is possible to suppress the SE, and increase the CE by merely lowering the microwave power. While this analysis gave a good agreement between experimental and calculated lineshapes when the CE dominates, some significant discrepancies were observed at low temperatures, where the SE dominates. We show that by explicitly taking into account the presence of (35/37)Cl nuclei through a e-(35,37)Cl-(13)C polarization pathway in the SE-DNP lineshape calculations, as proposed earlier, we can improve the fit significantly, thus supporting the existence of the "hetero-nuclear assisted DNP" pathway.

Shimon Vega in the eyes of his students and postdocs.

Professor Shimon Vega (1943-2021) of the Weizmann Institute of Science passed away on the 16-th of November. Shimon Vega established theoretical frameworks to develop and explain solid-state nuclear magnetic resonance (NMR) and dynamic nuclear polarization (DNP) techniques and methodologies. His departure left a profound mark on his many students, postdocs, and colleagues. Shortly after his passing, we all assembled spontaneously for an international online meeting to share our reflections and memories of our experiences in Shimon's lab and how they affected us deeply during that period of timeand throughout our scientific careers. These thoughts and feelings were put here into writing.

In vitro activity of tigecycline against multidrug-resistant Acinetobacter baumannii and selection of tigecycline-amikacin synergy.

Polymyxin B, minocycline, and tigecycline were the most potent of 10 antibiotics against 170 isolates of multidrug-resistant Acinetobacter baumannii. In time-kill studies, the exposure of a highly tigecycline-resistant isolate to tigecycline resulted in enhanced susceptibility to amikacin and synergistic bactericidal activities of the two drugs.

Pericardial meshing: an effective method for prevention of pericardial adhesions and epicardial reaction after cardiac operations.

Cardiac reoperations, particularly for coronary revascularization, are becoming more frequent and carry increased risk of damage to the heart during resternotomy. We experimentally evaluated a pericardial meshing technique to facilitate primary pericardial closure. In 18 mongrel dogs, an 8 by 5 cm pericardial flap was fashioned through a left thoracotomy. A standardized procedure for induction of pericardial adhesions was carried out in all animals. Animals were divided into three groups of six animals each: Group I (control)--the pericardial flap was primarily resutured; Group II--the flap was meshed and then resutured; and Group III--the flap was replaced by a pericardial substitute. Animals were put to death 8 weeks postoperatively and the pericardial space was examined for adhesions and epicardial reaction. The extent of adhesions and epicardial reaction was graded as: 0--none; 1--minimal; 2--moderate; and 3--severe. Both in Group I and Group III severe pericardial adhesions (grade 2-3) and epicardial reactions (grade 2-3) were formed, which obscured the underlying coronary anatomy. In Group II pericardial adhesions and epicardial reactions were none to minimal (grade 0-1) and the underlying coronary anatomy was not obscured. The meshed pericardium was completely regenerated by normal pericardium within several weeks. This study demonstrates that pericardial meshing facilitates primary tension-free pericardial closure. Free drainage of intrapericardial blood is achieved. A complete anatomic layer between heart and sternum is restored. Pericardial meshing is superior to the pericardial substitutes examined, as adhesions and epicardial reactions are significantly reduced, and the coronary anatomy is readily identifiable.

Pericardial meshing: a new technique to facilitate primary tension-free pericardial closure.

A method is described to facilitate pericardial approximation at the conclusion of open-heart surgical procedures. Before sternal closure, the anterior pericardium is meshed by multiple longitudinal incisions until tension-free closure is possible. The technique was developed by animal experimentation and is now being performed in patients. It has proven to be simple, safe, and effective.

Coronary pathology predicts conduction disturbances after coronary artery bypass grafting.

Conduction disturbances after coronary artery bypass grafting may result from compromised septal blood flow. To examine this hypothesis we reviewed the preoperative coronary angiography of 55 consecutive patients undergoing coronary artery bypass grafting. Thirty-five patients had either no lesion or a discrete lesion in the left anterior descending coronary artery that did not include the septal perforator (type I anatomy). Twenty patients had a lesion of the left anterior descending coronary artery at the origin of the first septal branch, a lesion of the first septal artery, or a pair of lesions in the left anterior descending coronary artery that straddled the origin of the first septal artery; all lesions were proximal to the graft site (type II anatomy). None of the patients with type I anatomy had a major conduction disturbance after coronary artery bypass grafting. Eleven of the patients with type II anatomy had major conduction disturbances after coronary artery bypass grafting; right bundle-branch block in 1, right bundle-branch block and left anterior hemiblock in 2, left bundle-branch block in 5, and complete atrioventricular block that required pacemaker implantation in 3 (p less than 0.001). In the 20 patients with type II anatomy, the appearance of conduction disturbances correlated well with the absence of retrograde flow to the septal branches from the right coronary artery (p less than 0.01). Pathological lesions in the left anterior descending coronary artery that compromise flow in the first perforator and that do not provide an adequate circulation produce localized damage and conduction disturbances after coronary artery bypass grafting. This can be predicted from the preoperative angiographic anatomy.

Delayed sternal closure following cardiac operations.

In 13 patients, sternal closure was delayed at the end of open heart procedures. Seven patients underwent coronary artery bypass surgery (CAB), 5 valve replacements, and one left ventricular aneurysmectomy and closure of post myocardial infarction VSD. In all, primary closure of the sternum was considered impossible or inadvisable. The major indications for delaying sternal closure were: cardiac dilatation with tamponade-like behaviour upon attempted sternal closure (8 patients); intractable bleeding (2); intractable arrhythmia (1); insertion of mediastinal assist devices (3) and intraoperative non-cardiogenic pulmonary edema (1). In all, only the skin was closed. Delayed sternal closure (DSC) was performed 36-120 hours later on 10 of the patients, when their condition had stabilized. Nine patients are long term survivors. None of these patients has developed mediastinitis, wound infection, osteomyelitis or instability of the sternum. The judicious use of DSC in selected situations has several advantages: hemodynamic deterioration from pressure upon the heart may be prevented; a quick access to the heart in case of tamponade or intractable arrhythmia is obtained; insertion of mediastinal assist devices is facilitated. With careful technique the risk of infection is low.

Value of dobutamine/nitrate radionuclide angiography in predicting revascularization effects on ventricular function.

OBJECTIVE: Dobutamine and Nitrate Radionuclide Angiography (RNA) may help to distinguish viable from nonviable myocardium and to predict revascularization effects on LV function. SETTING: Ambulatory. EXPERIMENTAL DESIGN: Prospective. PATIENTS AND METHODS: We studied 12 patients (11 male, 1 female; mean age 56 +/- 10 years) with three-vessel disease, severe LV dysfunction, and prior MI for a mean follow-up time of 10 +/- 3.4 months. Thallium-201 scintigraphy favored potential viability in all patients: inducible ischemia with satisfactory perfusion in more than one coronary arterial territory. Global and regional LVEF's were calculated from preoperative RNA (baseline rest study and with continuous dobutamine infusion with gradual rate increase plus oral nitrates) and postoperative RNA at 1 and 6 months. RESULTS: There was no operative mortality, but two late deaths occurred. Symptomatically, most patients showed improvement. Global LVEF increased during dobutamine and nitrates preoperatively (p < 0.01), but not at 1 and 6 months postoperatively (without pharmacological intervention). Mean LVED volume was not significantly changed postoperatively. Regional EF improvement was found in 4 out of 9 LV wall segments under dobutamine and nitrates, and this increase persisted postoperatively at 1 and 6 months. CONCLUSIONS: Mild but significant increase in global LVEF during dobutamine and nitrates administration is not predictive of postoperative LVEF improvement. However, regional EF improvement appears to be predictive of post-revascularization LV functional improvement.

Luke warm blood cardioplegia for CAB surgery in patients with severely impaired LV function. Improved results.

Ward blood cardioplegia (WBC) has recently been reported to improve myocardial protection in adult open heart surgery, especially in high risk cases. However, WBC has been reported to have some disadvantages including narrow safety margins concerning brain and kidney perfusion. We therefore modified our technique to utilize luke-warm blood cardioplegia (LWBC). We carried out 470 open heart procedures using luke-warm cardioplegia (anterograde + retrograde perfusion) from 1/2/1991 - 30/9/1992; 94 had LVEF < 30% and form the basis of this study. Other major risk factors in this group included: > 70 yrs - 26 patients, L main > 50% - 14 patients, emergencies - 11 patients, redo's - 3 patients. Eightyone patients underwent CAB only; 3 had additional MVR, 3 additional closure acute VSD of whom one underwent additional LV aneurysmectomy, one additional AVR; 4 patients underwent AVR only, and 2 MVR. Average number of grafts/patient for the 81 isolated CAB's was 4.5. IABP was necessary postbypass in 4 patients (9 emergencies were on IABP support at time of operation). Thirty day mortality was 3 patients (3.2%). Late mortality was 5 cases. These results are superior to those achieved using cold protection and warm blood cardioplegia. LWBC is a safe method for myocardial protection in patients undergoing CAB, particularly when LV function is severely compromised.

[Surgical pulmonary embolectomy and thrombolytic therapy].

Acute, massive pulmonary embolism is life-threatening and must be treated immediately. Since the early 1970's when thrombolytic therapy was shown to hasten resolution of pulmonary emboli, there has been a debate in the literature as to new indications for surgical pulmonary embolectomy. Some authors believed that there are no longer any indications for embolectomy, while others justify surgery for certain indications. Although the debate is still on, this operation is very rarely performed today. We present a patient who developed massive pulmonary embolism, with continuing extreme hemodynamic and respiratory disturbances despite full thrombolytic treatment. Embolectomy was successfully performed.

Normothermic continuous blood cardioplegia improves electrophysiologic recovery after open heart surgery.

BACKGROUND: Myocardial protection during open heart surgery is based on administration of oxygenated blood cardioplegia, the preferred temperature of which is still under debate. The current randomized study was designed to prospectively evaluate the quality of myocardial protection and the functional recovery of the heart with either normothermic (group N) or hypothermic (group H) oxygenated blood cardioplegia. METHODS: Under continuous electrocardiographic Holter monitoring, 42 patients were randomly scheduled to receive either normothermic (33.5 degrees C) or hypothermic (10 degrees C) cardioplegia solutions during coronary bypass grafting surgery. Blood samples for creatinine phosphokinase, creatinine phosphokinase-MB, lactate, epinephrine, and norepinephrine were withdrawn during cardiopulmonary bypass via a coronary sinus cannula. RESULTS: Active cooling in group H on initiation of cardiopulmonary bypass was characterized by transition through ventricular fibrillation in 75% of patients, whereas in group N atrial fibrillation occurred in 65% of patients. On myocardial reperfusion, sinus rhythm spontaneously resumed in 95% of group N patients compared to 25% in group H (P = 0.0003). In the latter, 75% of patients developed ventricular fibrillation often followed by complete atrioventricular block, which necessitated temporary pacing for a mean duration of 168 +/- 32 min. Both groups showed a similar incidence of intraventricular block and ST segment changes. However, the incidence of ventricular premature beats in the first 16 h after cardiopulmonary bypass was significantly greater in group H (P < 0.05), 20 +/- 26/h, compared to 3 +/- 5/h in group N. Blood concentrations of lactate, creatinine phosphokinase, epinephrine, and norepinephrine increased gradually during the operation, but the differences between the groups were not significant. CONCLUSIONS: The current prospective human study suggests that the increased susceptibility for ventricular fibrillation and dysrhythmia, and the delayed recovery of the conduction system after hypothermic myocardial protection, are related to temperature-induced changes in vital cellular functions of the conduction tissue in the postischemic period. Both cardioplegic methods provide adequate myocardial protection but normothermic oxygenated blood cardioplegia may accelerate recovery of the heart after cardiopulmonary bypass.