Mechanism of sac expansion without evident endoleak analyzed with X ray phase-contrast tomography.

Objective: Synchrotron radiation-based X ray phase-contrast tomography (XPCT) was used in this study to evaluate abdominal aorta specimens from patients with sac expansion without evidence of an endoleak (endotension) following endovascular aortic repair (EVAR) for an abdominal aortic aneurysm (AAA). The aim of this study was to analyze the morphologic structure of the aortic wall in patients with this condition and to establish the cause of the endotension. Methods: Human aortic specimens of the abdominal aorta were obtained during open repair, fixed with formalin, and analyzed among three groups. Group A was specimens from open abdominal aortic aneurysm repairs (n = 7). Group E was specimens from sac expansion without an evident endoleak after EVAR (n = 7). Group N was specimens from non-aneurysmal "normal" cadaveric abdominal aortas (n = 5). Using XPCT (effective voxel size, 12.5 µm; density resolution, 1 mg/cm3), we measured the density of the tunica media (TM) in six regions of each sample. Then, any changes to the elastic lamina and the vasa vasorum were analyzed pathologically. The specimens were immunohistochemically examined with anti-CD31 and vascular endothelial growth factor antibodies. Results: The time from EVAR to open aortic repair was 64.2 ± 7.2 months. There were significant differences in the thickness of the TM among three groups: 0.98 ± 0.03 mm in Group N; 0.31 ± 0.01 mm in Group A; and 0.15 ± 0.03 mm in Group E (P < .005). There were significant differences in the TM density among the groups: 1.087 ± 0.004 g/cm3 in Group N; 1.070 ± 0.001 g/cm3 in Group A; and 1.062 ± 0.007 g/cm3 in Group E (P < .005). Differences in the thickness and density of the TM correlated with the thickness of the elastic lamina; in Group N, uniform high-density elastic fibers were observed in the TM. By contrast, a thinning of the elastic lamina in the TM was observed in Group A. A marked thinness and loss of elastic fibers was observed in Group E. CD31 immunostaining revealed that the vasa vasorum was localized in the adventitia and inside the outer third of the TM in Group N, and in the middle of the TM in Group A. In Group E, the vasa vasorum advanced up to the intima with vascular endothelial growth factor-positive cells in the intimal section. Conclusions: XPCT could be used to demonstrate the densitometric property of the aortic aneurysmal wall after EVAR. We confirmed that the deformation process that occurs in the sac expansion after EVAR without evidence of an endoleak could be explained by hypoxia in the aortic wall.

Structural properties in ruptured mitral chordae tendineae measured by synchrotron-based X-ray phase computed tomography.

The link between the structural properties and the rupturing of chordae tendineae in the mitral valve complex is still unclear. Synchrotron-radiation-based X-ray phase computed tomography (SR-XPCT) imaging is an innovative way to quantitatively analyze three-dimensional morphology. XPCT has been employed in this study to evaluate the chordae tendineae from patients with mitral regurgitation and to analyze structural changes in the ruptured chordae tendineae in patients with this condition. Six ruptured mitral chordae tendineae were obtained during surgical repairs for mitral regurgitation and were fixed with formalin. In addition, 12 healthy chordae tendineae were obtained from autopsies. Employing XPCT (effective pixel size, 3.5 µm; density resolution, 1 mg cm-3), the density of the chordae tendineae in each sample was measured. The specimens were subsequently analyzed pathologically. The mean age was 70.2 ± 3.0 in the rupture group and 67.2 ± 14.1 years old in the control group (p = 0.4927). All scans of chorda tendineae with SR-XPCT were performed successfully. The mean densities were 1.029 ± 0.004 in the rupture group and 1.085 ± 0.015 g cm-3 in the control group (p < 0.0001). Density based on SR-XPCT in the ruptured mitral chordae tendineae was significantly lower compared with the healthy chorda tendinea. Histological examination revealed a change in the components of the connective tissues in ruptured chorda tendinea, in accordance with the low density measured by SR-XPCT. SR-XPCT made it possible to measure tissue density in mitral chordae tendineae. Low density in mitral chordae tendineae is associated with a greater fragility in ruptured mitral chordae tendineae.

The role and timing of cardiopulmonary bypass in the surgical repair of traumatic cardiac injury.

OBJECTIVES: The management of traumatic cardiac injury (TCI) may require a prompt treatment, including the use of cardiopulmonary bypass (CPB) followed by surgical repair. This study evaluated the surgical outcomes among TCI patients. METHODS: From August 2003, 21 patients with TCI were underwent emergent surgical repair. TCI was classified as grade I to VI according to the Cardiac Injury Organ Scale (CIS) of the American Association for Surgery of Trauma, and severity was evaluated using the Injury Severity Score (ISS). RESULTS: Of the 21 patients, the mean age and ISS were 54.8 ± 18.8 years and 26.5 ± 6.3, respectively, including13 blunt and eight penetrating injuries. A CIS grade of IV or greater was observed in 17 patients and unstable hemodynamics in 16. CPB or extracorporeal membranous oxygenation (ECMO) were used in three patients before they underwent surgery and in seven patients after undergoing sternotomy, including three on whom a canular access route was prepared preoperatively. There was a significant correlation between the preoperative width of pericardial effusion and the use of CPB (p < 0.05). Overall hospital mortality was 14.3%, and 100% in patients with uncontrolled bleeding during surgery. All patients who underwent CPB before or during surgery, in whom a standby canular access route had been established, survived. CONCLUSIONS: TCI is associated with a high mortality rate, and survival depends on efficient diagnosis and the rapid mobilization of the operating room. Preparations for CPB or establishing a canular access route should be made before surgical procedures in cases in which the hemodynamics are unstable.

Impact of time from symptom onset to operation on outcome of repair of acute type A aortic dissection with malperfusion.

OBJECTIVES: We analyzed patients with acute type A aortic dissection complicated by malperfusion syndrome to establish whether the timing of operative treatment and the location of malperfusion are factors in determining outcomes. METHODS: A total of 331 patients with acute type A aortic dissection were treated surgically between August 2003 and May 2019. Eighty-four patients (25%) presented with preoperative malperfusion syndrome. Fifty-eight patients with malperfusion syndrome (69%) were transferred to the operating room within 5 hours of the onset of symptoms (immediate repair); 26 patients (31%) were transferred after 5 hours (later repair). We analyzed the effects of immediate aortic repair on surgical outcomes. RESULTS: There was no significant difference in the early mortality rates between patients with immediate and later aortic repair, which were 20.0% (n = 11/58) and 26.9% (n = 7/19), respectively (P = .12). Preoperative coronary malperfusion was the only predictor of early mortality. The cumulative 5-year survivals of patients with malperfusion syndrome in the immediate and later repair groups were 76.7% and 45.4%, respectively. A significant difference was noted in the long-term outcomes between the 2 groups (P = .02). On multivariable Cox survival analysis, coronary malperfusion and shock on arrival were associated with increased long-term mortality (P < .01 and P = .04). Conducting surgery within 5 hours of the onset of symptoms was a significant predictor of favorable long-term outcome (P = .03). CONCLUSIONS: Although preoperative coronary malperfusion and shock on arrival worsened the long-term outcomes in patients undergoing aortic repair for acute type A aortic dissection with preoperative malperfusion syndrome, conducting an operation within 5 hours of the onset of symptoms significantly improved their long-term outcomes.

Decision making and management of acute type-A dissection presenting with shock or cardiac arrest.

While the outcome of aortic repair for acute type-A aortic dissection has improved, overall mortality among patients who developed acute type-A aortic dissection remains extremely high. Hypotension in acute type-A aortic dissection patients is a critical condition that is associated with increased in-hospital mortality and neurologic events. The underlying causes of shock include acute aortic regurgitation, cardiac tamponade, and myocardial infarction. The most reasonable initial approach is to administer intravenous fluids to improve blood pressure, increase preload and cardiac output, and ensure adequate end-organ perfusion. Cardiac tamponade-induced hypotension associated with aortic rupture has been identified as a major risk factor for perioperative mortality in patients with acute type-A aortic dissection. In addition, the most serious complications of acute type-A aortic dissection include preoperative cardiopulmonary arrest, especially out-of-hospital cardiopulmonary arrest. Recent advances in rapid transportation and diagnosis, and the introduction of extracorporeal cardiopulmonary resuscitation, have resulted in an increase in the number of patients with cardiopulmonary arrest related to acute type-A aortic dissection. However, controversy continues to surround treatment strategies, surgical indications, and the timing of surgery on such patients. This review, therefore, discusses decision-making and the managerial issues surrounding acute type-A dissection presenting with shock, cardiac tamponade, and cardiac arrest.

Giant Patent Ductus Arteriosus Aneurysm Compressing the Esophagus.

It is extremely rare to observe aneurysmal changes in patients with patent ductus arteriosus (PDA), especially in adults. If left untreated, a PDA aneurysm can increase the risk of life-threatening complications, including rupture, dissection, esophageal fistula, and infection. Following is a description of successful surgical repair in a 55-year-old man with PDA aneurysm compressing the esophagus. Histologically, the aneurysmal wall showed mild thickening of the intima and media with hyperplastic smooth muscle cells, but no destructive changes were observed.

Impact of High-Resolution Epiaortic Ultrasonographic Imaging on Evaluating Aortic Wall Pathology.

There has been no definitive method, other than pathological findings, to identify the degeneration of the tunica media in the aortic wall (TM). We describe how high-resolution intraoperative epiaortic ultrasonographic imaging identifies changes in the TM of patients with aortic dissection. This method shows great promise in facilitating presymptomatic diagnoses of various aortic wall pathologies.

Retrospective study of thoracic endovascular aortic repair as a first-line treatment for traumatic blunt thoracic aortic injury.

OBJECTIVE: This study sought to confirm if thoracic endovascular aortic repair (TEVAR) was an appropriate therapeutic strategy for blunt thoracic aortic injury (BTAI). METHODS: Between 3/2005 and 12/2020, 104 patients with BTAI were brought to our hospital. The severity of each trauma case was evaluated using the Injury Severity Score (ISS); aortic injuries were classified as type I to IV according to Society for Vascular Surgery guidelines. Initial treatment was categorized into four groups: nonoperative management (NOM), open aortic repair (OAR), TEVAR, or emergency room thoracotomy/cardiopulmonary resuscitation (ERT/CPR). RESULTS: The patients' mean age and ISS were 56.7 ± 20.9 years and 48.3 ± 20.4, respectively. Type III or IV aortic injury were diagnosed in 82 patients. The breakdown of initial treatments was as follows: NOM for 28 patients, OAR for four, TEVAR for 47, and ERT/CPR for 25. The overall early mortality rate was 32.7%. Logistic regression analysis confirmed ISS > 50 and shock on admission as risk factors for early mortality. The cumulative survival rate of all patients was 61.2% at 5 years after treatment. After initial treatment, eight patients receiving TEVAR required OAR. The cumulative rate of freedom from reintervention using TEVAR at 5 years was higher in approved devices than in custom-made devices (96.0 vs. 56.3%, p = 0.011). CONCLUSIONS: Using TEVAR as an initial treatment for patients with BTAI is a reasonable approach. Patients with severe multiple traumas and shock on admission had poor early outcomes, and those treated with custom-made devices required significant rates of reintervention.

Evaluation of Ductal Tissue in Coarctation of the Aorta Using X-Ray Phase-Contrast Tomography.

We assessed the histological accuracy of X-ray phase-contrast tomography (XPCT) and investigated three-dimensional (3D) ductal tissue distribution in coarctation of the aorta (CoA) specimens. We used nine CoA samples, including the aortic isthmus, ductus arteriosus (DA), and their confluences. 3D images were obtained using XPCT. After scanning, the samples were histologically evaluated using elastica van Gieson (EVG) staining and transcription factor AP-2 beta (TFAP2B) immunostaining. XPCT sectional images clearly depicted ductal tissue distribution as low-density areas. In comparison with EVG staining, the mass density of the aortic wall positively correlated with elastic fiber formation (R = 0.69, P < 0.001). TFAP2B expression was consistent with low-density area including intimal thickness on XPCT images. On 3D imaging, the distances from the DA insertion to the distal terminal of the ductal media and to the intima on the ductal side were 1.63 ± 0.22 mm and 2.70 ± 0.55 mm, respectively. In the short-axis view, the posterior extension of the ductal tissue into the aortic lumen was 79 ± 18% of the diameter of the descending aorta. In three specimens, the aortic wall was entirely occupied by ductal tissue. The ductal intima spread more distally and laterally than the ductal media. The contrast resolution of XPCT images was comparable to that of histological assessment. Based on the 3D images, we conclude that complete resection of intimal thickness, including the opposite side of the DA insertion, is required to eliminate residual ductal tissue and to prevent postoperative re-coarctation.

Acute Type A Aortic Dissection With Cardiopulmonary Arrest at Presentation.

BACKGROUND: Management of acute type A aortic dissection (AADA) presenting with cardiopulmonary arrest (CPA) may require aggressive cardiopulmonary resuscitation (CPR), including extracorporeal CPR followed by aortic repair. This study evaluated the early and long-term outcomes of patients with preoperative CPA related to AADA. METHODS: Between September 2003 and August 2019, 474 patients with AADA were brought to our hospital, 157 (33.1%) presenting with CPA. Their mean age was 74.3 ± 11.3 years and prevalence of out-of-hospital CPA 90%, and causes of CPA were cardiac tamponade in 75%, hemothorax in 10%, and coronary malperfusion in 10% of cases. In the same time periods 2974 patients with CPA were transported, and AADA was 4.8% of all cause of CPA. RESULTS: Return of spontaneous circulation was achieved in 26 patients (17%) and extracorporeal CPR was required in 31 (20%); 131 CPA patients (83%) died before surgery, 24 (15%) underwent aortic repair, and 2 (1%) received nonsurgical care. Hospital mortality was 90%, and none survived without aortic repair. Of patients achieving return of spontaneous circulation 17 underwent aortic repair, 13 survived, and 5 fully recovered. All patients with extracorporeal CPR died: 24 before surgery and 7 postoperatively. There were significant differences in hospital mortality between patients who did and did not undergo aortic repair (P < .01). Aortic repair was the only significant predictor of long-term survival (P < .01). CONCLUSIONS: AADA with CPA is associated with significantly high mortality; however aortic repair can be performed with a 30% likelihood of functional recovery, if return of spontaneous circulation is achieved. Preoperative extracorporeal membrane oxygenation is not recommended in this patient cohort.

Effect of Long-term Administration of Prostaglandin E1 on Morphologic Changes in Ductus Arteriosus.

BACKGROUND: To improve survival of patients with hypoplastic left heart syndrome, combination therapy with bilateral pulmonary artery banding and prostaglandin E1 (PGE1)-mediated ductal patency was developed as an alternative for high-risk neonates in Japan. However, the effect of long-term PGE1 administration on ductus arteriosus remains unclear. Synchrotron radiation-based X-ray phase-contrast tomography (XPCT) enables clear visualization of soft tissues at an approximate spatial resolution of 12.5 µm. We aimed to investigate morphologic changes in ductus arteriosus after long-term PGE1 infusion using XPCT. METHODS: Seventeen ductus arteriosus tissue samples from patients with hypoplastic left heart syndrome were obtained during the Norwood procedure. The median duration of lipo-prostaglandin E1 (lipo-PGE1) administration was 48 days (range, 3 to 123). Structural analysis of ductus arteriosus was performed and compared with conventional histologic analysis. RESULTS: The XPCT was successfully applied to quantitative measurements of ductal media. Significant correlation was found between the duration of lipo-PGE1 infusion and mass density of ductal media (R = 0.723, P = .001). The duration of lipo-PGE1 administration was positively correlated with elastic fiber staining (R = 0.799, P < .001) and negatively correlated with smooth muscle formation (R = -0.83, P < .001). No significant increase in intimal cushion formation was found after long-term lipo-PGE1 administration. Expression of ductus arteriosus dominant PGE2-receptor EP4 almost disappeared in specimens when lipo-PGE1 was administered over 3 days. CONCLUSIONS: Disorganized elastogenesis and little intimal cushion formation after long-term lipo-PGE1 administration suggest that ductus arteriosus remodeled to the elastic artery phenotype. Because EP4 was downregulated and ductus arteriosus exhibited elastic characteristics, the dosage of lipo-PGE1 might be decreased after a definite administration period.

Long-term Outcomes After Controlled Pericardial Drainage for Acute Type A Aortic Dissection.

BACKGROUND: Cardiac tamponade with acute aortic dissection type A can cause fatal outcomes. We previously reported excellent outcomes using percutaneous pericardial drainage with controlled volumes of aspirated pericardial effusion (controlled pericardial drainage [CPD]) to stabilize patients with critical cardiac tamponade. This study evaluates the early and late outcomes using this approach. METHODS: Between September 2003 and July 2018, 308 patients with acute aortic dissection type A were treated surgically, including 76 patients who presented with cardiac tamponade on hospital arrival. Forty-nine patients who did not respond to intravenous volume resuscitation underwent CPD in the emergency room, including 14 patients (28.6%) who presented with cardiopulmonary arrest. After CPD 39 patients (79.6%) were transferred to the operating room to undergo immediate aortic repair. The remaining 10 patients (20.4%) received medical treatment on arrival, followed by aortic repair within several days. RESULTS: In 49 patients the mean systolic blood pressure before CPD was 64.4 ± 10.3 mm Hg. Blood pressure rose significantly in all patients after CPD. The total volume of aspirated pericardial effusion was 46.8 ± 56.2 mL, and 30 of 49 patients (61%) required only 30 mL or less of aspiration to improve their blood pressure. All patients underwent successful aortic repair. Early hospital mortality was 16%. However there was no mortality related to CPD. The mean follow-up period was 52.9 ± 54.3 months. The cumulative survival rate was 63.4% after 5 years. CONCLUSIONS: CPD for critical cardiac tamponade with acute type A aortic dissection produced satisfactory early and late outcomes.

Synchrotron Radiation-based X-ray phase-contrast imaging of the aortic walls in acute aortic dissection.

OBJECTIVE: Synchrotron radiation-based X-ray phase-contrast tomography (XPCT) imaging is an innovative modality for the quantitative analysis of three-dimensional morphology. XPCT has been used in this study to evaluate ascending aorta specimens from patients with acute type A aortic dissection (ATAAD) and to analyze the morphologic structure of the aortic wall in patients with this condition. METHODS: Aortic specimens from 12 patients were obtained during repairs for ATAAD and were fixed with formalin. Five patients had Marfan syndrome (MFS), and seven did not. In addition, six normal aortas were obtained from autopsies. Using XPCT (effective pixel size, 12.5 µm; density resolution, 1 mg/cm3), the density of the tunica media (TM) in each sample was measured at eight points. The specimens were subsequently analyzed pathologically. RESULTS: The density of the TM was almost constant within each normal aorta (mean, 1.081 ± 0.001 g/cm3). The mean density was significantly lower in the ATAAD aortas without MFS (1.066 ± 0.003 g/cm3; P < .0001) and differed significantly between the intimal and adventitial sides (1.063 ± 0.003 vs 1.074 ± 0.002 g/cm3, respectively; P < .0001). The overall density of the TM was significantly higher in the ATAAD aortas with MFS than those without MFS (1.079 ± 0.008 g/cm3; P = .0003), and greater variation and markedly different distributions were observed in comparison with the normal aortas. These density variations were consistent with the pathologic findings, including the presence of cystic medial necrosis and malalignment of the elastic lamina in the ATAAD aortas with and without MFS. CONCLUSIONS: XPCT exhibited differences in the structure of the aortic wall in aortic dissection specimens with and without MFS and in normal aortas. Medial density was homogeneous in the normal aortas, markedly varied in those with MFS, and was significantly lower and different among those without MFS. These changes may be present in the TM before the onset of aortic dissection.

Intact Imaging of Human Heart Structure Using X-ray Phase-Contrast Tomography.

Structural examination of human heart specimens at the microscopic level is a prerequisite for understanding congenital heart diseases. It is desirable not to destroy or alter the properties of such specimens because of their scarcity. However, many of the currently available imaging techniques either destroy the specimen through sectioning or alter the chemical and mechanical properties of the specimen through staining and contrast agent injection. As a result, subsequent studies may not be possible. X-ray phase-contrast tomography is an imaging modality for biological soft tissues that does not destroy or alter the properties of the specimen. The feasibility of X-ray phase-contrast tomography for the structural examination of heart specimens was tested using infantile and fetal heart specimens without congenital diseases. X-ray phase-contrast tomography was carried out at the SPring-8 synchrotron radiation facility using the Talbot grating interferometer at the bending magnet beamline BL20B2 to visualize the structure of five non-pretreated whole heart specimens obtained by autopsy. High-resolution, three-dimensional images were obtained for all specimens. The images clearly showed the myocardial structure, coronary vessels, and conduction bundle. X-ray phase-contrast tomography allows high-resolution, three-dimensional imaging of human heart specimens. Intact imaging using X-ray phase-contrast tomography can contribute to further structural investigation of heart specimens with congenital heart diseases.

Three Dimensional Visualization of Human Cardiac Conduction Tissue in Whole Heart Specimens by High-Resolution Phase-Contrast CT Imaging Using Synchrotron Radiation.

BACKGROUND: The feasibility of synchrotron radiation-based phase-contrast computed tomography (PCCT) for visualization of the atrioventricular (AV) conduction axis in human whole heart specimens was tested using four postmortem structurally normal newborn hearts obtained at autopsy. METHODS: A PCCT imaging system at the beamline BL20B2 in a SPring-8 synchrotron radiation facility was used. The PCCT imaging of the conduction system was performed with "virtual" slicing of the three-dimensional reconstructed images. For histological verification, specimens were cut into planes similar to the PCCT images, then cut into 5-µm serial sections and stained with Masson's trichrome. RESULTS: In PCCT images of all four of the whole hearts of newborns, the AV conduction axis was distinguished as a low-density structure, which was serially traceable from the compact node to the penetrating bundle within the central fibrous body, and to the branching bundle into the left and right bundle branches. This was verified by histological serial sectioning. CONCLUSION: This is the first demonstration that visualization of the AV conduction axis within human whole heart specimens is feasible with PCCT.

Impact of synchrotron radiation-based X-ray phase-contrast tomography on understanding various cardiovascular surgical pathologies.

At SPring-8, synchrotron radiation-based X-ray phase-contrast tomography (PCXI) has been developed to measure the inner structures of biological soft tissue without destroying them. To resolve the three-dimensional (3D) morphology, we have applied PCXI to various cardiovascular tissue samples, including the thoracic aorta, ductus arteriosus, and cardiac conduction system. In the aortic walls, PCXI demonstrated differences in 3D structures of tunica media of aortic dissection. These findings correlated well with the irregularity of the structure of the media. In the surgically excised sample of coarctation of the aorta, PCXI showed 3D morphological changes in transition from the ductus arteriosus to the descending aorta. PCXI is also useful for examining abnormalities of the cardiac conduction system in congenital heart defects. Synchrotron radiation-based X-ray phase-contrast tomography has strong modality for analyzing 3D morphology and is useful for understanding the pathophysiology of various cardiovascular surgical pathologies.

Mutations in a TGF-ß ligand, TGFB3, cause syndromic aortic aneurysms and dissections.

BACKGROUND: Aneurysms affecting the aorta are a common condition associated with high mortality as a result of aortic dissection or rupture. Investigations of the pathogenic mechanisms involved in syndromic types of thoracic aortic aneurysms, such as Marfan and Loeys-Dietz syndromes, have revealed an important contribution of disturbed transforming growth factor (TGF)-ß signaling. OBJECTIVES: This study sought to discover a novel gene causing syndromic aortic aneurysms in order to unravel the underlying pathogenesis. METHODS: We combined genome-wide linkage analysis, exome sequencing, and candidate gene Sanger sequencing in a total of 470 index cases with thoracic aortic aneurysms. Extensive cardiological examination, including physical examination, electrocardiography, and transthoracic echocardiography was performed. In adults, imaging of the entire aorta using computed tomography or magnetic resonance imaging was done. RESULTS: Here, we report on 43 patients from 11 families with syndromic presentations of aortic aneurysms caused by TGFB3 mutations. We demonstrate that TGFB3 mutations are associated with significant cardiovascular involvement, including thoracic/abdominal aortic aneurysm and dissection, and mitral valve disease. Other systemic features overlap clinically with Loeys-Dietz, Shprintzen-Goldberg, and Marfan syndromes, including cleft palate, bifid uvula, skeletal overgrowth, cervical spine instability and clubfoot deformity. In line with previous observations in aortic wall tissues of patients with mutations in effectors of TGF-ß signaling (TGFBR1/2, SMAD3, and TGFB2), we confirm a paradoxical up-regulation of both canonical and noncanonical TGF-ß signaling in association with up-regulation of the expression of TGF-ß ligands. CONCLUSIONS: Our findings emphasize the broad clinical variability associated with TGFB3 mutations and highlight the importance of early recognition of the disease because of high cardiovascular risk.