Paul J. Niklewski

Ionic mechanisms of cellular electrical and mechanical abnormalities in Brugada syndrome

The Brugada syndrome (BrS) is a right ventricular (RV) arrhythmia that is responsible for up to 12% of sudden cardiac deaths. The aims of our study were to determine the cellular mechanisms of the electrical abnormality in BrS and the potential basis of the RV contractile abnormality observed in the syndrome. Tetrodotoxin was used to reduce cardiac Na(+) current (I(Na)) to mimic a BrS-like setting in canine ventricular myocytes. Moderate reduction (<50%) of I(Na) with tetrodotoxin resulted in all-or-none repolarization in a fraction of RV epicardial myocytes. Dynamic clamp and modeling show that reduction of I(Na) shifts the action potential (AP) duration-transient outward current (I(to)) density curve to the left and has a biphasic effect on AP duration. In the presence of a large I(to), I(Na) reduction either prolongs or collapses the AP, depending on the exact density of I(to). These repolarization changes reduce Ca(2+) influx and sarcoplasmic reticulum load, resulting in marked attenuation of myocyte contraction and Ca(2+) transient in RV epicardial myocytes. We conclude that I(Na) reduction alters repolarization by reducing the threshold for I(to)-induced all-or-none repolarization. These cellular electrical changes suppress myocyte excitation-contraction coupling and contraction and may be a contributing factor to the contractile abnormality of the RV wall in BrS.

SERCA2a superinhibition by human phospholamban triggers electrical and structural remodeling in mouse hearts

Phospholamban (PLN), the reversible inhibitor of the sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA2a), is a key regulator of myocyte Ca(2+) cycling with a significant role in heart failure. We previously showed that the single amino acid difference between human and mouse PLN results in increased inhibition of Ca(2+) cycling and cardiac remodeling and attenuated stress responses in transgenic mice expressing the human PLN (hPLN) in the null background. Here we dissect the molecular and electrophysiological processes triggered by the superinhibitory hPLN in the mouse. Using a multidisciplinary approach, we performed global gene expression analysis, electrophysiology, and mathematical simulations on hPLN mice. We identified significant changes in a series of Na(+) and K(+) homeostasis genes/proteins (including Kcnd2, Scn9a, Slc8a1) and ionic conductance (including L-type Ca(2+) current, Na(+)/Ca(2+) exchanger, transient outward K(+) current). Simulation analysis suggests that this electrical remodeling has a critical role in rescuing cardiac function by improving sarcoplasmic reticulum Ca(2+) load and overall Ca(2+) dynamics. Furthermore, multiple structural and transcription factor gene expression changes indicate an ongoing structural remodeling process, favoring hypertrophy and myogenesis while suppressing apoptosis and progression to heart failure. Our findings expand current understanding of the hPLN function and provide additional insights into the downstream implications of SERCA2a superinhibition in the mammalian heart.

A Novel Index of Hypoxemia for Assessment of Risk During Procedural Sedation

BACKGROUND:Procedural sedation is essential for many procedures. Sedation has an excellent safety profile; however, it is not without risks. Assessment of risk using clinical outcomes in clinical studies is difficult due to their rare occurrence. Therefore, surrogate end points are frequently used in a clinical study in lieu of clinical outcomes. As a clinician integrates multiple aspects of a physiological variable to determine potential risk, a surrogate end point should consider a similar approach. In this study, we identified and tested the appropriateness of a new surrogate end point that may be used in clinical studies, area under the curve of oxygen desaturation (AUCDesat). A review of patient sedation records by anesthesiologists was conducted to assess its relationship to the anesthesia professional perception of risk. METHODS:This study was a post hoc analysis and assessment of perceived risk by anesthesiologists. It consisted of 13 U.S.-trained board-certified anesthesiologists ranking physiological variables as indicators of risk and then reviewing 204 records from 3 completed sedation studies involving the SEDASYS® System. After review, each anesthesiologist assigned a Likert score based on his or her perception of risk for oversedation-related sequelae in each record. These scores were analyzed to determine their relationship to desaturation presence/absence, duration, depth, number of events, and AUCDesat that incorporates each component. RESULTS:Anesthesiologists ranked arterial oxygenation to be the most important factor in assessing risk post hoc (mean rank of 4.69 of 5, P = 0.0007 compared with next highest ranked factor—respiratory rate, N = 13). AUCDesat was better correlated to the Likert scores (rs = 0.85) when compared with the individual elements of AUCDesat, binary assessment of desaturation (rs = 0.73), desaturation depth (rs = −0.70), desaturation duration (rs = 0.70), and incidence of desaturations (rs = 0.55) (all 4 comparisons versus rs = 0.85, P < 0.0001). CONCLUSIONS:Anesthesiologists determined arterial oxygenation to be the most important physiological variable in assessing sedation risk and the potential for adverse clinical outcomes. AUCDesat, a composite index that incorporates duration, incidence, and depth of oxygen desaturation, was better correlated to the Likert scores. AUCDesat, given that it is a single numerical variable, is an ideal end point for assessment of risk of adverse clinical outcomes in clinical sedation studies. Future studies using AUCDesat and actual physiological outcomes may be useful in further defining this end point.