Steven Keller

Cardiac Arrest: the Changing Incidence of Ventricular Fibrillation

Opinion statementThere are more than 300,000 out-of-hospital cardiac arrests (OHCA) in the USA annually, which can be grouped into those presenting with tachyarrhythmic (shockable) rhythms and those presenting with non-tachyarrhythmic rhythms. The incidence of tachyarrhythmic rhythms, which include ventricular fibrillation (VF) and pulseless ventricular tachycardia (VT), has been noted to be progressively decreasing in multiple studies of OHCA. Improved medical and surgical therapies for ischemic heart disease, and the widespread use of implantable cardiac defibrillators (ICDs), have likely contributed to a declining incidence of VF arrest and may result in conversion of an otherwise VF event into a pulseless electrical activity (PEA) arrest. As the incidence of VF has declined, it is unclear if the absolute incidence of non-tachyarrhythmic rhythms has increased or remained largely unchanged. This article discusses the changing rates of presenting rhythms in sudden cardiac arrest, the underlying cellular mechanisms of PEA, the factors contributing to the relative increase in the rate of PEA arrests, and current treatment options.

Tracking Colliding Cells In Vivo Microscopy

Leukocyte motion represents an important component in the innate immune response to infection. Intravital microscopy is a powerful tool as it enables in vivo imaging of leukocyte motion. Under inflammatory conditions, leukocytes may exhibit various motion behaviors, such as flowing, rolling, and adhering. With many leukocytes moving at a wide range of speeds, collisions occur. These collisions result in abrupt changes in the motion and appearance of leukocytes. Manual analysis is tedious, error prone, time consuming, and could introduce technician-related bias. Automatic tracking is also challenging due to the noise inherent in in vivo images and abrupt changes in motion and appearance due to collision. This paper presents a method to automatically track multiple cells undergoing collisions by modeling the appearance and motion for each collision state and testing collision hypotheses of possible transitions between states. The tracking results are demonstrated using in vivo intravital microscopy image sequences. We demonstrate that 1) 71% of colliding cells are correctly tracked; (2) the improvement of the proposed method is enhanced when the duration of collision increases; and (3) given good detection results, the proposed method can correctly track 88% of colliding cells. The method minimizes the tracking failures under collisions and, therefore, allows more robust analysis in the study of leukocyte behaviors responding to inflammatory conditions.

Monitoring of oesophageal pressure.

PURPOSE OF REVIEW Studies in patients with acute respiratory distress syndrome (ARDS) have been unable to demonstrate a survival advantage with higher levels of positive end-expiratory pressure (PEEP) to open atelectatic lung regions or prevent their cyclic collapse. This review will discuss the challenges of accurately measuring pleural pressure with balloon-tipped catheters in the oesophagus, and the utility of such pressure monitoring to set PEEP and assess lung mechanics, focusing on patients with ARDS. RECENT FINDINGS Recent investigations have suggested that the monitoring of oesophageal pressure in ARDS patients may help individualize PEEP settings to optimize lung recruitment based on transpulmonary pressure. SUMMARY Changes in oesophageal pressure likely accurately reflect global changes in pleural pressure in supine patients with ARDS. However, absolute oesophageal pressure values in such patients may be subject to local artefacts and may substantially overestimate pleural pressure in other lung regions. Setting PEEP high enough to achieve a targeted end-expiratory transpulmonary pressure in the region of the oesophageal balloon catheter could overdistend other lung regions. Measurement of oesophageal pressure is feasible, but its clinical utility to titrate PEEP, compared with routine assessment, awaits experimental confirmation.

Noninvasive detection of fibrillation potentials in skeletal muscle

The presence of spontaneous muscle activity was determined by analysis of the power spectra of computer-model-generated sequences of spontaneous activity and additive noise. The modeling results identified the frequency band of 100-300 Hz as the band of peak signal-to-noise ratio for the detection of fibrillation potentials. Animal experiments were conducted in which the left sciatic nerves of three rats were transected. Measurements were taken 14 days following surgery with Ag/AgCl gel electrodes on the skin surface. Data was recorded from the gastrocnemius muscle on both the normal and denervated side for all three rats. The normal data and the denervated data yielded no discernible difference in the time-domain. Spectral analysis, however, demonstrated a clear and quantifiable difference between denervated and normal muscle signals. The average difference between the denervated and normal power spectral densities for the frequency band from 100 Hz to 300 Hz was 3.43, 1.90, and 3.02 dB for the three rats. The additional energy observed in the signals recorded from denervated muscles suggests that the single fiber spontaneous muscle activity that occurs in denervated muscle can be noninvasively detected. The potential diagnostic utility of noninvasive fibrillation potential detection is discussed and suggestions for future experiments are made.

Mechanical circulatory support device-heart hysteretic interaction can predict left ventricular end diastolic pressure

Variations in the driving motor current of a percutaneous ventricular assist device can be used to predict left ventricular end diastolic pressure. Predicting pressure from a pump Mechanical ventricular assist devices help the heart pump blood after cardiac surgery or while awaiting heart transplant. Chang et al. show that the motor current and pressure head recorded by the Impella, a temporary assist device placed within the left ventricle, can be used to measure left ventricular end diastolic pressure (LVEDP). LVEDP indicates cardiac function and is used to titrate mechanical pump support. Chang and colleagues found that Impella-based LVEDPs matched left heart catheter-based LVEDPs more closely than pulmonary capillary wedge pressure–inferred LVEDPs. After testing in a mock circulatory loop and a pig model of cardiac dysfunction, they confirmed their findings with patient data. This method does not require any modification of the Impella assist device, suggesting that it could be easily adopted into clinical practice. The full potential of mechanical circulatory systems in the treatment of cardiogenic shock is impeded by the lack of accurate measures of cardiac function to guide clinicians in determining when to initiate and how to optimally titrate support. The left ventricular end diastolic pressure (LVEDP) is an established metric of cardiac function that refers to the pressure in the left ventricle at the end of ventricular filling and immediately before ventricular contraction. In clinical practice, LVEDP is typically only inferred from, and poorly correlates with, the pulmonary capillary wedge pressure (PCWP). We leveraged the position of an indwelling percutaneous ventricular assist device and advanced data analysis methods to obtain LVEDP from the hysteretic operating metrics of the device. We validated our hysteresis-derived LVEDP measurement using mock flow loops, an animal model of cardiac dysfunction, and data from a patient in cardiogenic shock to show greater measurement precision and correlation with actual pressures than traditional inferences via PCWP. Delineation of the nonlinear relationship between device and heart adds insight into the interaction between ventricular support devices and the native heart, paving the way for continuous assessment of underlying cardiac state, metrics of cardiac function, potential closed-loop automated control, and rational design of future innovations in mechanical circulatory support systems.

De-escalation of support with veno-arterial extracorporeal membrane oxygenation and Impella for cardiogenic shock

We read with interest the recently published paper by Pappalardo and colleagues1 investigating the co-implantation of Impella for left ventricular (LV) ‘venting’ in patients undergoing veno-arterial extracorporeal membrane oxygenation (VA-ECMO) for cardiogenic shock. This approach has been adopted at our institution, particularly in patients with profound LV failure. In VA-ECMO, it has been proposed that LV unloading may be impaired by increased afterload as a consequence of retrograde aortic blood flow. Various mechanisms of LV ‘venting’ have been described, including interatrial septostomy, left atrial cannulation, intra-aortic balloon pumps, and trans-aortic devices such as the Impella. The authors observed that the combination of Impella with VA-ECMO compared with VAECMO alone was associated with higher rates of hospital survival and successful bridges to further therapy.1 In an accompanying editorial, Lorusso asked ‘Are two crutches better than one?’.2 He highlighted that, particularly since there are other mechanisms for LV unloading, the risks of the dual systems must be taken into account, including financial costs, positional monitoring of the axial pump, and haemolysis. However, there is another potential benefit

Motor unit action potentials as a source of noise in the non-invasive detection of fibrillation potentials

Denervated muscle fibers produce spontaneous depolarizations termed fibrillation potentials. These potentials are an indicator of neuromuscular pathology and are detected by inserting a needle electrode into the muscle of interest to detect the time-based signal. A proposed noninvasive method measures the spectral energy corresponding to increased spontaneous muscle activity. This paper examines the impact of normal muscle activity on such a method through the use of a computer model of fibrillation potentials and normal motor unit action potentials. A mathematical expression for the surface recorded signal is proposed and used as the basis for analyzing the temporal and spectral characteristics of spontaneous and normal motor activity. Based on these results, filtering methods for the removal of normal motor activity are proposed and future work needed to implement non-invasive detection of fibrillation potentials is discussed.