Neurologic Dysfunction and Neuroprotection in Transcatheter Aortic Valve Implantation.

Transcatheter aortic valve implantation (TAVI) is a fast-growing procedure. Expanding to low-risk patients, it has surpassed surgical aortic valve implantation in frequency and has been associated with excellent outcomes. Stroke is a devastating complication after transcatheter aortic valve implantation. Silent brain infarcts identified by diffusion-weighted magnetic resonance imaging are present in most patients following TAVI. Postoperative delirium and cognitive dysfunction are common neurologic complications. The stroke and silent brain infarcts are likely caused by particulate emboli released during the procedure. Intravascularly positioned cerebral embolic protection devices are designed to prevent debris from entering the aortic arch vessels to avoid stroke. Despite promising design, randomized clinical trials have not demonstrated a reduction in stroke in patients receiving cerebral embolic protection devices. Similarly, the association of cerebral embolic protection devices with silent brain infarcts, postoperative delirium, and cognitive dysfunction is uncertain. Monitored anesthesia care or conscious sedation is as safe as general anesthesia and is associated with lower cost, but different anesthetic techniques have not been shown to decrease stroke risk, postoperative delirium, or cognitive dysfunction. Anesthesiologists play important roles in providing perioperative care including management of neurologic events in patients undergoing TAVI. Large randomized clinical trials are needed that focus on the correlation between perioperative interventions and neurologic outcomes.

Brain Protection in Aortic Arch Surgery: An Evolving Field.

Despite advances in cardiac surgery and anesthesia, the rates of brain injury remain high in aortic arch surgery requiring circulatory arrest. The mechanisms of brain injury, including permanent and temporary neurologic dysfunction, are multifactorial, but intraoperative brain ischemia is likely a major contributor. Maintaining optimal cerebral perfusion during cardiopulmonary bypass and circulatory arrest is the key component of intraoperative management for aortic arch surgery. Various brain monitoring modalities provide different information to improve cerebral protection. Electroencephalography gives crucial data to ensure minimal cerebral metabolism during deep hypothermic circulatory arrest, transcranial Doppler directly measures cerebral arterial blood flow, and near-infrared spectroscopy monitors regional cerebral oxygen saturation. Various brain protection techniques, including hypothermia, cerebral perfusion, pharmacologic protection, and blood gas management, have been used during interruption of systemic circulation, but the optimal strategy remains elusive. Although deep hypothermic circulatory arrest and retrograde cerebral perfusion have their merits, there have been increasing reports about the use of antegrade cerebral perfusion, obviating the need for deep hypothermia. With controversy and variability of surgical practices, moderate hypothermia, when combined with unilateral antegrade cerebral perfusion, is considered safe for brain protection in aortic arch surgery performed with circulatory arrest. The neurologic outcomes of brain protection in aortic arch surgery largely depend on the following three major components: cerebral temperature, circulatory arrest time, and cerebral perfusion during circulatory arrest. The optimal brain protection strategy should be individualized based on comprehensive monitoring and stems from well-executed techniques that balance the major components contributing to brain injury.

A new general method for designing affinity chromatography processes.

Affinity chromatography is widely used for selectively recovering a target solute from a complex mixture. The challenge in designing a capture process is to achieve high yield, high column utilization, and high sorbent productivity while satisfying loading time and pressure limit requirements. The conventional design method based on constant-pattern waves cannot be used for small feed batches or short columns, which do not allow the formation of such waves. Other design methods in the literature rely on simulations or experimental trials, and can be time-consuming and costly. In this study, a new design method with no need of simulations is developed for constant pattern and non-constant pattern systems with Langmuir isotherms. Given feed conditions, loading time, and desired yield, the design requires only the values of certain intrinsic parameters, which can be estimated from a small number of bench-scale experiments. The minimum column volume for capture can be estimated either graphically or analytically. The method is tested with Protein A chromatography data for antibody purification. It is applicable to a wide range of production scales and design requirements. The effects of material properties, feed composition, feed volume, and design requirements on the column volume for capture can be found graphically. When the loading time relative to a characteristic diffusion time is 0.5 or larger, the minimum column volume approaches that of an ideal system. A short loading time increases sorbent productivity, but increases the minimum column volume. A high feed concentration, a high equilibrium capacity, and a small diffusion time relative to the loading time can reduce the minimum column volume and increase productivity.