The blockchain folk theorem

Blockchains are distributed ledgers, operated within peer-to-peer networks. If reliable and stable, they could offer a new, cost effective way to record transactions, but are they? We model the proof-of-work blockchain protocol as a stochastic game and analyse the equilibrium strategies of rational, strategic miners. Mining the longest chain is a Markov perfect equilibrium, without forking, in line with Nakamoto (2008). The blockchain protocol, however, is a coordination game, with multiple equilibria. There exist equilibria with forks, leading to orphaned blocks and persistent divergence between chains. We also show how forks can be generated by information delays and software upgrades. Last we identify negative externalities implying that equilibrium investment in computing capacity is excessive

The blockchain folk theorem

Blockchains are distributed ledgers, operated within peer-to-peer networks. If reliable and stable, they could offer a new, cost effective way to record transactions, but are they? We model the proof-of-work blockchain protocol as a stochastic game and analyse the equilibrium strategies of rational, strategic miners. Mining the longest chain is a Markov perfect equilibrium, without forking, in line with Nakamoto (2008). The blockchain protocol, however, is a coordination game, with multiple equilibria. There exist equilibria with forks, leading to orphaned blocks and persistent divergence between chains. We also show how forks can be generated by information delays and software upgrades. Last we identify negative externalities implying that equilibrium investment in computing capacity is excessive

Stratégie d'optimisation hémodynamique des patients à risque (impacts de l'acidose respiratoire et métabolique, du clampage de l'aorte abdominale sous-rénale et du positionnement peropératoire)

L optimisation hémodynamique péri-opératoire est une stratégie qui vise à maximaliser le transport artériel en oxygène et/ou le volume d éjection systolique lors de chirurgie à risque. Ce concept a beaucoup évolué lors de ces trente dernières années, vers une approche plus simple, plus réalisable en pratique clinique et moins invasive. Les principales thérapeutiques utilisées dans les différents protocoles d optimisation hémodynamique sont le remplissage vasculaire, l administration d agents inotropes et de vasopresseurs. Cependant, les conséquences physiopathologiques de l agression chirurgicale peuvent impacter grandement les modalités d administration et l efficacité des thérapeutiques précitées. Dans la première étude, nous avons décrit l impact de l acidose respiratoire et métabolique (fréquemment rencontrées lors de chirurgie majeure et/ou de coeliochirurgie) sur l efficacité des agents a et b-adrénergiques sur le myocarde sain de rat. Dans un deuxième travail nous avons mis en évidence que le remplissage vasculaire ne pouvait pas être guidé par des indices dynamiques de précharge dépendance lors du clampage chirurgicale de l aorte abdominale sous-rénale, dans un modèle porcin. Enfin, dans la troisième étude, nous avons montré dans un modèle clinique, que le positionnement en décubitus ventral lors d une chirurgie du rachis entrainait des modifications majeures des interactions cardiorespiratoires et que les indices dynamiques devaient être interprétés avec prudence pour guider le remplissage vasculaire dans ce contexte. Ces études translationnelles soulignent trois situations fréquentes impactant l efficacité et/ou les modalités d administration des thérapeutiques nécessaires à une optimisation hémodynamique peropératoireThe aim of perioperative haemodynamic optimization is to maximize oxygen delivery and/or stroke volume during high risk surgery. This concept has evolved during the last thirty years, to a simpler, more feasible and less invasive approach. Main treatments used in different hemodynamic optimization protocols are fluid loading, inotropes and vasopressors administration. However, pathophysiological consequences of surgical stress can greatly impact the mode of administration and the efficacy of the above therapeutics. In the first study, we described the impact of respiratory and metabolic acidosis (frequently encountered during major surgery and/or laparoscopic surgery) on the effectiveness of a and b-adrenergic agents in healthy rat myocardium. In a second work, we demonstrated that intravenous fluids cannot be guided by dynamic indices of preload dependency during surgical clamping of the infrarenal abdominal aorta in a porcine model. Finally, in the third study, we demonstrated in a clinical model, that positioning in prone position during spine surgery induced major changes in cardiorespiratory interactions and dynamic indices should be interpreted with caution to guide fluid therapy in this context. These translational studies highlight three common situations impacting the effectiveness and/or administration of therapeutic necessary for intraoperative hemodynamic optimization.BORDEAUX2-Bib. électronique (335229905) / SudocBORDEAUX1-Bib.electronique (335229901) / SudocSudocFranceF

Equilibrium bitcoin pricing

We offer an overlapping generations equilibrium model of cryptocurrency pricing and confront it to new data on bitcoin transactional benefits and costs. The model emphasizes that the fundamental value of the cryptocurrency is the stream of net transactional benefits it will provide, which depend on its future prices. The link between future and present prices implies that returns can exhibit large volatility unrelated to fundamentals. We construct an index measuring the ease with which bitcoins can be used to purchase goods and services, and we also measure costs incurred by bitcoin owners. Consistent with the model, estimated transactional net benefits explain a statistically significant fraction of bitcoin returns

Equilibrium bitcoin pricing

We offer an overlapping generations equilibrium model of cryptocurrency pricing and confront it to new data on bitcoin transactional benefits and costs. The model emphasizes that the fundamental value of the cryptocurrency is the stream of net transactional benefits it will provide, which depend on its future prices. The link between future and present prices implies that returns can exhibit large volatility unrelated to fundamentals. We construct an index measuring the ease with which bitcoins can be used to purchase goods and services, and we also measure costs incurred by bitcoin owners. Consistent with the model, estimated transactional net benefits explain a statistically significant fraction of bitcoin returns

Enteral Feeding in Patients With Open Abdomen and Negative Pressure Therapy: A Propensity Score Analysis

IntroductionIn critically ill surgical patients treated with open abdomen and negative pressure therapy (OA/NPT), the association between nutritional support and clinical outcome is still controversial. The main objective of this study was to assess the effect of enteral nutritional support during the acute phase (i.e., the first 7 days) on clinical outcome (abdominal wall closure rate or fistula formation) in critically ill surgical patients treated by OA/NPT.MethodsOver a 5-year period, every critically ill patient who underwent nutritional support while treated by OA/NPT was retrospectively included. The main study outcome was a composite criterion, defined as delayed abdominal closure ≥8 days and/or secondary abdominal complications (secondary anastomotic leak, intra-abdominal abscess and fascial dehiscence). Inverse probability of treatment weight (IPTW) was derived from a propensity score model. Multivariable logistic regression was used to test the association between clinical outcome and different modalities of nutritional support (enteral nutrition vs. nil per os during the first week after OA/NPT, early vs. late enteral nutrition, normal vs. low caloric/protein intake).ResultsOver the study period, 171 patients were included and 50% underwent delayed abdominal closure and/or secondary abdominal complications. The rate of delayed abdominal closure or secondary abdominal complications was significantly lower in patients who received enteral nutrition versus those who remained nil per os (40% vs. 61%, p = 0.007), with an IPTW-adjusted OR of poor clinical outcome of 0.49 [95%CI: 0.25–0.98]. There was no other statistical association between modalities of nutritional support and the study outcome.ConclusionIn critically ill patients with OA/NPT, the use of enteral feeding within 7 days after surgery was associated with better clinical outcome. Further studies are mandatory to better define the adequate timing for enteral feeding, the energy needs and the protein requirements during the acute phase after OA/NPT