Spatially homogeneous ground state of the two-dimensional Hubbard model

We investigate the stability with respect to phase separation or charge density-wave formation of the two-dimensional Hubbard model for various values of the local Coulomb repulsion and electron densities using Green-function Monte Carlo techniques. The well known sign problem is particularly serious in the relevant region of small hole doping. We show that the difference in accuracy for different doping makes it very difficult to probe the phase separation instability using only energy calculations, even in the weak-coupling limit ($U=4t$) where reliable results are available. By contrast, the knowledge of the charge correlation functions allows us to provide clear evidence of a spatially homogeneous ground state up to $U=10t$.Comment: 7 pages and 5 figures. Phys. Rev. B, to appear 200

Low energy and dynamical properties of a single hole in the t-Jz model

We review in details a recently proposed technique to extract information about dynamical correlation functions of many-body hamiltonians with a few Lanczos iterations and without the limitation of finite size. We apply this technique to understand the low energy properties and the dynamical spectral weight of a simple model describing the motion of a single hole in a quantum antiferromagnet: the $t-J_z$ model in two spatial dimension and for a double chain lattice. The simplicity of the model allows us a well controlled numerical solution, especially for the two chain case. Contrary to previous approximations we have found that the single hole ground state in the infinite system is continuously connected with the Nagaoka fully polarized state for $J_z \to 0$. Analogously we have obtained an accurate determination of the dynamical spectral weight relevant for photoemission experiments. For $J_z=0$ an argument is given that the spectral weight vanishes at the Nagaoka energy faster than any power law, as supported also by a clear numerical evidence. It is also shown that spin charge decoupling is an exact property for a single hole in the Bethe lattice but does not apply to the more realistic lattices where the hole can describe closed loop paths.Comment: RevTex 3.0, 40 pages + 16 Figures in one file self-extracting, to appear in Phys. Rev

Opioid-induced constipation in mixed chronic pain patients: Prevalence and predictors analysis

Objectives: Assessment of opioid-induced constipation (OIC) prevalence and relationship with demographic, clinical, and drug predictors in our daily practice. Design: Observational and retrospective study. Setting: Chronic pain (CP) center of Bologna\u2019s Teaching Hospital, Italy. Subjects: Mixed consecutive CP opioid-user outpatients (n = 128). Main outcome measure(s): OIC was assessed with the Bowel Function Index (BFI) in three consecutive visits. Absolute difference and Student\u2019s t-test were used to compare BFI scores. Predictors (opioid compound and type, morphine-equivalent daily-dose [MEDD], and laxatives) were retrieved from the patients\u2019 charts. BFI and predictors relationships were checked by multinomial logistic regression (MLR); independent predictors of BFI scores were assessed with \u3c72 analysis. Results: Of the 384 evaluations, 85 percent were on strong opioids with a MEDD range of 11-50 mg per day in the majority (60 percent) and 64 percent showed moderate constipation; 42 percent did not use laxatives while 24 percent used macrogol with significant decrease in the BFI. MLR showed that oxycodone was associated with a risk for moderate constipation. Lactulose and glycerin suppositories were associated with severe constipation. Non-opioid users and cancer patients were associated with normal bowel function and severe constipation, respectively. Conclusions: OIC was found in almost all evaluations of weak or strong opioid-users (97 percent); moderate to severe OIC was found in 72 percent of the evaluations. Cancer patients were associated with severe constipation. Macrogol was superior to other laxatives. In our experience, macrogol relieved constipation in those on the combination of oxycodone and naloxone and in those on fentanyl patches. Lactulose and glycerol suppositories were associated with severe constipation

VALIDAZIONE DI UN QUESTIONARIO PER LA DIAGNOSI DEL DOLORE EPISODICO INTENSO (BTP)

INTRODUZIONE Il Breakthrough pain (BTP) \ue8 un\u2019esacerbazione transitoria del dolore che si verifica dal 51% al 95% dei pazienti che presentano dolore cronico di base (background pain), moderato-severo, cancro correlato, ben controllato con oppiacei. I pazienti oncologici con BTP presentano in genere, dolore pi\uf9 intenso, una maggiore compromissione funzionale, una minore qualit\ue0 di vita e pi\uf9 importanti distress psicologici, rispetto a quelli senza BTP. (1) Inoltre il BTP, presente anche nei pazienti non oncologici, determina un maggiore impatto socio-economico in termini di ricoveri ospedalieri, visite in pronto soccorso e visite ambulatoriali. (2) Lo scopo dello studio \ue8 la validazione di un questionario diagnostico del BTP, attraverso il quale si potrebbe sia quantificare il fenomeno che perfezionare coerentemente l\u2019analgesia. METODI Il q-BTP, composto da 12 quesiti, \ue8 stato costruito ad hoc sul modello proposto da Portenoy. Sono stati arruolati nello studio pilota (2^ settimana del mese di Giugno 2010) pazienti con dolore cronico moderato-severo trattati con oppiacei. La somministrazione del q-BTP avveniva durante la prima visita in regime di consulenza intra-ospedaliera oppure presso l\u2019Ambulatorio di Terapia Antalgica. RISULTATI Tra le n=28 nuove visite effettuate nel periodo considerato, n=9 pazienti sono stati arruolati nello studio. L\u2019et\ue0 media del campione era 62,4 (\ub19,6) anni; n=6 era di sesso femminile e n=7 presentava patologia neoplastica. In base al q-BTP, n=7 pazienti presentavano dolore episodico intenso. L\u2019intensit\ue0 media del BTP era moderata-forte. DISCUSSIONE E NOTE CONCLUSIVE Il BTP \ue8 un fenomeno clinico complesso (3) che richiede un'accurata diagnosi. Il q-BTP si \ue8 rivelato di facile applicazione e di alta specificit\ue0. Somministrato sottoforma d\u2019intervista, aiuta il medico nella diagnosi di dolore episodico intenso e facilita l\u2019adeguamento della terapia antalgica nei pazienti neoplastici e non. Il questionario indaga sulla presenza di esacerbazioni transitorie di dolore, sulla loro frequenza, sede e durata. Il q-BTP ha permesso di evidenziare un\u2019elevata prevalenza del BTP nel campione studiato. In seguito a questo studio pilota \ue8 prevista l\u2019estensione dello studio su un campione maggiore. BIBLIOGRAFIA: 1. Portenoy RK, Paine D, Jacobsen P. Breakthrough pain: characteristics and impact in patient with cancer pain. Pain 1999;81:129-134 2. Portenoy RK, Hagen NA. Breakthrou pain:definition, prevalence and characteristics pain.1990;41:273-281 3. Caraceni A, Martini C, Zecca E, et al; Working Group ofan IASP Task Force on Cancer Pain. Breakthrough pain characteristics and syndromes in patients with cancer pain.An international survey. Palliat Med. 2004;18:177-183

Non-invasive haemodynamic monitoring with clearsight in patients undergoing spinal anaesthesia for total hip replacement. A prospective observational cohort study

Summary. Background and aim of the work: Patients undergoing elective primary total hip replacement and spinal anesthesia may encounter significant hemodynamic instability. Objective: The study is aimed at observing the haemodynamic modifications after spinal anaesthesia during total hip replacement in patients managed to \u201cpreload independence\u201d with goal directed fluid therapy (GDFT) and monitored non-invasively with Clearsight. Methods: Thirty patients, aged 50-80 years, with an American Society of Anaesthesiologists\u2019 (ASA) score II-III, undergoing elective primary total hip replacement and spinal anaesthesia were enrolled. Patients were monitored with the EV1000 platform and the Clearsight finger-cuff and managed with a goal directed fluid therapy. Results: The 79% of the population showed preload independence at baseline. After spinal, the 93% did not show a significant mean arterial pressure reduction. In our population, 79% reported a decrease >10% of the stroke volume during surgery, while 66% in the Recovery Room. Patients showed an improvement in mean arterial pressure, systemic vascular resistances indexed (SVRI), stroke volume (SV) and stroke volume indexed (SVI) at spinal resolution compared to baseline. Conclusions: Our cohort population showed hemodynamic stability throughout the study period, with increased SV and decreased SVRI at spinal resolution compared to basal values. Further randomized prospective studies are advocated in the same setting. (www.actabiomedica.it)