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Nikki Gillum Posnack

Research team develops new and improved method for studying cardiac function

Nikki Gillum Posnack

While researching how plastic affects heart function in sensitive populations, such as children born with congenital heart defects, Children’s National researcher Nikki Posnack, Ph.D., led a team that developed a new and improved, replicable method of performing simultaneous dual optical mapping to examine electrical activity and calcium for the study of cardiac function.

Since arriving at the Sheikh Zayed Institute for Pediatric Surgical Innovation, researcher Nikki Gillum Posnack, Ph.D., a principal investigator with the institute and assistant professor of pediatrics at the George Washington University School of Medicine and Health Sciences, has been focused on examining how exposure to plastic affects heart function in sensitive populations, such as children born with congenital heart defects. She performs optical mapping to conduct this research, but the industry standard approaches of either using dual cameras or sequential single cameras were cost prohibitive and technically challenging while also diminishing the quality of the imaging results.

Fast forward to July 2019 when Dr. Posnack and her team published “Plasticizer Interaction With the Heart” in the journal Arrhythmia and Electrophysiology, which used imaging techniques to reveal the impact of plastic chemicals on the electrical activity of the heart. Dr. Posnack’s laboratory has since expanded this technique and revealed a new replicable method of performing simultaneous dual optical mapping to examine electrical activity and calcium handling in the heart.

Sharing a new method for studying cardiac function

This groundbreaking method is itself the focus of a new BMC Biomedical Engineering journal article titled “Lights, camera, path splitter: a new approach for truly simultaneous dual optical mapping of the heart with a single camera.”

The article compares and contrasts the current standard for dual camera simultaneous configurations and single camera sequential configurations to Dr. Posnack’s new single camera simultaneous configuration.

Simultaneous dual mapping systems use two probes and dual dyes – one for electrical voltage and the other for calcium. While dual-dye combinations like Di-4-ANEPPS with Indo-1, Di-2-ANEPEQ and calcium green have been developed to separate fluorescence signals by emission, these dye combinations can have spectral overlap, creating the need for non-ideal emission bandpass to negate spectral overlap and/or the inclusion of a calcium probe with an inferior dissociation constant. Additionally, dual-sensor systems require proper alignment to ensure that fluorescence signals are being analyzed from the same tissue region on each individual detector, which could lead to erroneous results. The dual-camera optical setup is expensive, technically challenging and requires a large physical footprint that is often not feasible for basic science and teaching laboratories conducting critical research.

As an alternative, some researchers use a single camera configuration to sequentially image the voltage and calcium probes using excitation light patterning. This approach also has limitations. These single-sensor designs use dual-dye combinations that require two or more excitation light sources, but share a single emission band. Like the dual camera system, this platform design is also technically challenging since the different excitation light wavelengths require light source triggering, camera synchronization and frame interleaving. Due to timing coordination, decreased frame rates, excitation light ramp up/down times and shutter open/close times, single system setups require shorter exposure times compared to dual sensor setups, diminishing the signal-to-noise quality without offering the same temporal fidelity. There is a cost advantage to the single camera system, however, because the additional camera is often one of the most expensive components.

This new single camera, simultaneous dual optical mapping approach is the first multiparametric mapping system that simultaneously acquires calcium and voltage signals from cardiac preparations, using a commercially available optical path splitter, single camera and single excitation light. Using a large field of view sCMOS sensor that is faster and more sensitive, this configuration separates the two emission bands for voltage and calcium probes and simultaneously directs them to either sides of the single, large camera sensor. This protocol employs a commonly used dual-dye combination (RH237 and Rhod2-AM). In contrast, other protocols may require genetically-encoded indicators or fluorescent probes that are not yet commercially available.

The team validated the utility of the approach by performing high-speed simultaneous dual imaging with sufficient signal-to-noise ratio for calcium and voltage signals and specificity of emission signals with negligible cross-talk. Demonstrating the need for simultaneous electrical and calcium sensors, they found that when ventricular tachycardia is induced, there is spatially discordant calcium alternans present in different regions of the heart even when the electrical alternans remain concordant.

Having eliminated the second camera as well as the need for multiple excitation light sources, light pattering and frame interleaving, this system is more cost effective, simpler, and can be easily setup by various types of researchers, not just those with engineering backgrounds.

With a limited research budget and a background in physiology, Dr. Posnack worked collaboratively with her post-doctoral fellow Rafael Jaimes III, an engineer in the Sheikh Zayed Institute for Pediatric Surgical Innovation, to develop a cost-effective system that would enable her to truly study the effects of plastics on the heart.

Multidisciplinary approach

“We’re fortunate to have a multidisciplinary team in the Sheikh Zayed Institute so that I could work with an engineer to develop the technology and system we needed to propel our research,” said Dr. Posnack. “There are so many researchers who have the science background, but not necessarily the technical aptitude, and they get stymied in their research, so we’re proud that this paper will help other researchers replicate the system to study cardiac function.”

The research paper was funded by a grant from the National Institutes of Health as well as support from the Children’s Research Institute, Children’s National Heart Institute and the Sheikh Zayed Institute for Pediatric Surgical Innovation.

The applications for this optical mapping system are significant and Dr. Posnack has been consulted by other research teams looking to implement it in their labs. Additionally, Dr. Posnack has collaborated with several neuroscience teams at Children’s National Hospital, including one that is investigating the effects of hypoxia on brain and heart development, and another that is interested in using image modalities and data processing to analyze calcium as an indicator of neuron firing.

Dr. Posnack continues to use this new dual optical mapping system to further her research as she anticipates the publication of a new article about age-dependent changes in cardiac electrophysiology and calcium handling.

Nikki Gillum Posnack

What are the health effects of plastics?

Nikki Gillum Posnack

Nikki Posnack, Ph.D., assistant professor at the Children’s National Heart Institute, is an early-stage investigator examining the impact plastic chemical exposure has on the developing hearts of newborns and young children.

For newborns or children in the pediatric intensive care unit, plastic tubing is part of daily life. It delivers life-sustaining blood transfusions, liquid nutrition and air to breathe. But small amounts of the chemicals in the plastic of this tubing and other medical devices can leak into the patient’s bloodstream. The potential effects of these chemicals on the developing hearts of newborns and very young children are not well understood.

One researcher, Nikki Posnack, Ph.D., an assistant professor at the Children’s National Heart Institute, aims to change that and shares her early insights, funded by the National Center for Advancing Translation Science (NCATS), in an NCATS news feature.

“While plastics have revolutionized the medical field, we know chemicals in plastics leach into the body and may have unintended effects,” Posnack said. “The heart is sensitive to toxins, so we want to look at the effect of these plastics on the most sensitive patient population: kids who are recovering from heart surgery and already prone to cardiac complications.”

NPosnack-Heart-image

NIH funding to improve devices and safeguard cardiovascular health

Nearly 15 million blood transfusions are performed each year in the U.S., and pediatric patients alone receive roughly 425,000 transfused units. Endocrine-disrupting chemicals, such as bisphenol A and di-2-ethylhexyl-phthalate (DEHP), can leach from some plastic devices used in such transfusions. However, it remains unclear how many complications following a transfusion can be attributed to the interplay between local and systemic reactions to these chemical contaminants.

NPosnack-Heart-image

Top: Live, excised heart that is being perfused with a crystalloid buffer via the aorta. The heart is stained with a voltage-sensitive fluorescent dye, which is excited by an LED light source. Bottom, right: Cardiac action potentials are optically mapped across the epicardial surface in real-time by monitoring changes in the fluorescence signal that are proportional to changes in transmembrane voltage. Bottom, left: An activation map (middle) depicts the speed of electrical conduction across the heart surface. Credit: Rafael Jaimes, Ph.D.; Luther Swift, Ph.D.; Manelle Ramadan, B.S.; Bryan Siegel, M.D.; James Hiebert, B.S., all of Children’s National Health System; and Daniel McInerney, student at The George Washington University.

The National Heart, Lung and Blood Institute within the National Institutes of Health has awarded a $3.4 million, five-year grant to Nikki Gillum Posnack, Ph.D., assistant professor at the Children’s National Heart Institute within the Sheikh Zayed Institute for Pediatric Surgical Innovation (SZI) at Children’s National Health System, to answer that question and to provide insights that could accelerate development of safer biomaterials.

According to the Food and Drug Administration, patients who are undergoing IV therapy, blood transfusion, cardiopulmonary bypass or extracorporeal membrane oxygenation or who receive nutrition through feeding support tubes have the potential to be exposed to DEHP.

Posnack led a recent study that found that an experimental model exposed to DEHP experienced altered autonomic regulation, heart rate variability and cardiovascular reactivity and reported the findings Nov. 6, 2017, in the American Journal of Physiology. The pre-clinical model study is the first to show such an association between phthalate chemicals used in everyday medical devices like IV tubing and cardiovascular health.

In the follow-on research, Posnack and colleagues will:

  • Use in vivo and whole heart models to define the extent to which biomaterial leaching and chemical exposure alters cardiovascular and autonomic function in experimental models
  • Determine whether biocompatibility and incidental chemical exposure are linked to cardiovascular and autonomic abnormalities experienced by pediatric patients post transfusion
  • Compare and contrast alternative biomaterials, chemicals and manufacturing techniques to identify safer transfusion device options.

“Ultimately, we hope to strengthen the evidence base used to inform decisions by the scientific, medical and regulatory communities about whether chemical additives that have endocrine-disrupting properties should be used to manufacture medical devices,” Posnack says. “Our findings also will highlight incentives that could accelerate development of alternative biomaterials, additives and fabrication techniques to improve safety for patients undergoing transfusion.”

Nikki Gillum Posnack

Experimental model study links phthalates and cardiovascular health

Nikki Gillum Posnack

“Because phthalate chemicals are known to migrate out of plastic products, our study highlights the importance of adopting safer materials, chemical additives and/or surface coatings for use in medical devices to reduce the risk of inadvertent exposure,” explains study senior author Nikki Gillum Posnack, Ph.D.

An experimental model exposed to di-2-ethylhexyl-phthalate (DEHP), a chemical that can leach from plastic-based medical devices, experienced altered autonomic regulation, heart rate variability and cardiovascular reactivity, according to a study published online Nov. 6, 2017 by the American Journal of Physiology. The pre-clinical model study is the first to show such an association between phthalate chemicals used in everyday medical devices like IV tubing and cardiovascular health.

“Plastics have revolutionized medical devices, transformed how we treat blood-based diseases and helped to make innovative cardiology procedures possible,” says Nikki Gillum Posnack, Ph.D., study senior author and assistant professor at the Children’s National Heart Institute within the Sheikh Zayed Institute for Pediatric Surgical Innovation (SZI) at Children’s National Health System. “Because phthalate chemicals are known to migrate out of plastic products, our study highlights the importance of adopting safer materials, chemical additives and/or surface coatings for use in medical devices to reduce the risk of inadvertent exposure.”

According to the Food and Drug Administration, patients who are undergoing IV therapy, blood transfusion, cardiopulmonary bypass or extracorporeal membrane oxygenation or who receive nutrition through feeding support tubes have the potential to be exposed to DEHP.

Patients undergoing extensive interventions to save their lives may be exposed to multiple plastic-based devices that supply oxygen and nutrition or that pump newly oxygenated blood to oxygen-starved organs.

“These interventions keep very fragile kids alive. What’s most important is getting patients the care they need when they need it,” Posnack says. “In the biomaterials field, our ultimate goal is to reduce inadvertent risks to patients that can result from contact with plastic products by identifying replacement materials or safer coatings to lower overall risk.”

In order to assess the safety of phthalate chemicals used in such medical devices, the Children’s-led research team implanted adult experimental models with radiofrequency transmitters that monitored their heart rate variability, blood pressure and autonomic regulation. Then, they exposed the experimental models to DEHP, a softener used in making the plastic polymer, polyvinyl chloride, flexible.

DEHP-treated pre-clinical models had decreased heart rate variability with lower-than-normal variation in the intervals between heart beats. The experimental models also showed an exaggerated mean arterial pressure response to ganglionic blockade. And in response to a stressor, the experimental models in the treatment group displayed enhanced cardiovascular reactivity as well as prolonged blood pressure recovery, according to the study team.

“The autonomic nervous system is a part of the nervous system that automatically regulates such essential functions as blood pressure and breathing rate without any conscious effort by the individual,” Posnack adds. “Because alterations in the autonomic balance provide an early warning sign of trouble – before symptoms of hypertension or atherosclerosis manifest – our findings underscore the importance of additional studies to explore the potential impact of phthalate chemicals on organ function.”

Billie Lou Short, M.D., chief of Children’s Division of Neonatology, called the paper an “important study” that builds on a foundation laid in the late 199os by Children’s researchers who were the first to show that plasticizers migrated from tubing in the extracorporeal membrane oxygenation (ECMO) circuit. Children’s researchers also led a study published in 2004 that evaluated the effect of plasticizers on the human reproductive system. A small number of adolescents who had undergone ECMO as newborns did not experience the complications that had been seen in in experimental models, Dr. Short says.

Posnack’s study co-authors include Rafael Jaimes III, Ph.D., SZI staff scientist; Meredith Sherman, SZI research technician; and Adam Swiercz, Narine Muselimyan and Paul J. Marvar, all of The George Washington University.