The impact of COVID-19 on the sensitivity of D-dimer for pulmonary embolism.

OBJECTIVE: This study seeks to evaluate the test characteristics of D-dimer for pulmonary embolism (PE) in patients with a concurrent diagnosis of COVID-19. We hypothesized that the sensitivity of D-dimer for PE at current institutional cut points would be similar to those without COVID-19. METHODS: This is a multicenter retrospective observational cohort study across five urban and suburban EDs in the same health care system. The electronic health record was queried for all computed tomography pulmonary angiography (CTPA) studies from December 1, 2019, to October 22, 2020. All ED patients who underwent CTPA had D-dimer and COVID-19 testing completed in a single encounter were included in the study. Baseline demographics were obtained. Test characteristics of D-dimer for PE were calculated for patients with and without COVID-19. Additionally, receiver operator characteristics (ROC) curves were constructed for two different D-dimer assays. RESULTS: There were 1158 patient encounters that met criteria for analysis. Performance of D-dimer testing for PE was similar between COVID-19-positive and -negative patients. In COVID-19-positive patients, the sensitivity was 100% (95% confidence interval [CI] = 87.6%-100%), specificity was 11.9% (95% CI = 7.9%-17.1%), and negative predictive value (NPV) was 100%. In COVID-19-negative patients the sensitivity was 97.6% (95% CI = 91.5%-99.7%), specificity was 14.4% (95% CI = 12.1%-17%), and NPV was 98.3% (95% CI = 93.8%-99.6%). For assay 1 the area under the curve (AUC) for COVID-19-positive patients was 0.76 (95% CI = 0.68-0.83), and for COVID-19-negative patients, 0.73 (95% CI = 0.69-0.77). For assay 2, AUC for COVID-19-positive patients was 0.85 (95% CI = 0.77-0.92), and for COVID-19-negative patients, 0.80 (95% CI = 0.77-0.84). Inspection of the ROC curve for assay 1 revealed that 100% sensitivity was maintained up to a threshold of 0.67 FEU (fibrinogen equivalent units; from 0.50 FEU) with an increase in specificity to 29% (from 18.7%), and for assay 2, 100% sensitivity was maintained up to a threshold of 662 D-dimer units (DDU; from 230 DDU) with an increased specificity to 59% (from 6.1%). CONCLUSION: Results from this multicenter retrospective study did not find a significant difference in sensitivity of D-dimer for PE due to concomitant COVID-19 infection. Further study is required to determine if PE can safely be excluded based on D-dimer results alone in patients with suspected or proven COVID-19 or if adjusted D-dimer levels could have a role in management.

Herpes Zoster Meningitis in a Young, Immunocompetent Adult.

BACKGROUND: Varicella-zoster virus is typically encountered in the emergency department (ED) in two forms: varicella (chickenpox) in children and zoster (shingles) in older adults. Zoster is infrequently encountered in young, healthy adults, and neurological complications are extremely rare. CASE REPORT: We describe a case of a previously healthy 36-year-old woman who presented to the ED with fever, nuchal rigidity, and headache 4 days after being diagnosed with herpes zoster and started on oral valacyclovir. Lumbar puncture confirmed herpes zoster meningitis. Despite initiation of antivirals within 48 h of symptom onset, progression to zoster meningitis occurred. WHY SHOULD AN EMERGENCY PHYSICIAN BE AWARE OF THIS?: Emergency physicians must be aware that neurological complications of varicella zoster can develop despite initiation of oral antivirals. These patients must be identified in the ED, as admission for intravenous antivirals is indicated.

Oxytocin neuron activation prevents hypertension that occurs with chronic intermittent hypoxia/hypercapnia in rats.

Hypertension is a common outcome associated with obstructive sleep apnea (OSA), a prevalent yet poorly treated cardiovascular disease. Recent studies showed oxytocin (OXT), released from hypothalamic paraventricular nucleus (PVN) neurons, activates cardiac vagal neurons in the dorsal motor nucleus of the vagus (DMNX) and may blunt cardiovascular responses to stress. This study tests whether the release of OXT from PVN fibers in the DMNX is diminished with chronic intermittent hypoxia-hypercapnia (CIH/H) exposure, an animal model of OSA, and whether activation of PVN OXT neurons restores OXT release in the DMNX and prevents the hypertension resulting from CIH/H. To assess OXT release from PVN fibers, Chinese hamster ovarian (CHO) cells were engineered to be highly sensitive to OXT by stable expression of the human recombinant OXT receptor and the calcium indicator R-GECO1. PVN fibers in the DMNX were selectively photoactivated in vitro by expression of channelrhodopsin. The release of OXT onto CHO cells in the DMNX was blunted in rats exposed to 21 days of CIH/H. Chronic activation of PVN OXT neurons in vivo, using designer receptors exclusively activated by designer drugs, restored the release of OXT onto CHO cells in the DMNX. Chronic PVN OXT neuron activation in vivo also prevented the hypertension that occurred in conscious unrestrained telemetry-equipped sham rats exposed to 3 wk of CIH/H. These results demonstrate that chronic activation of OXT neurons restores the release of OXT from PVN fibers in the DMNX and prevents the hypertension that occurs with 3 wk of CIH/H exposure.

Evidence that BDNF regulates heart rate by a mechanism involving increased brainstem parasympathetic neuron excitability.

Autonomic control of heart rate is mediated by cardioinhibitory parasympathetic cholinergic neurons located in the brainstem and stimulatory sympathetic noradrenergic neurons. During embryonic development the survival and cholinergic phenotype of brainstem autonomic neurons is promoted by brain-derived neurotrophic factor (BDNF). We now provide evidence that BDNF regulates heart rate by a mechanism involving increased brainstem cardioinhibitory parasympathetic activity. Mice with a BDNF haploinsufficiency exhibit elevated resting heart rate, and infusion of BDNF intracerebroventricularly reduces heart rate in both wild-type and BDNF+/- mice. The atropine-induced elevation of heart rate is diminished in BDNF+/- mice and is restored by BDNF infusion, whereas the atenolol-induced decrease in heart rate is unaffected by BDNF levels, suggesting that BDNF signaling enhances parasympathetic tone which is diminished with BDNF haploinsufficiency. Whole-cell recordings from pre-motor cholinergic cardioinhibitory vagal neurons in the nucleus ambiguus indicate that BDNF haploinsufficiency reduces cardioinhibitory vagal neuron activity by increased inhibitory GABAergic and diminished excitatory glutamatergic neurotransmission to these neurons. Our findings reveal a previously unknown role for BDNF in the control of heart rate by a mechanism involving increased activation of brainstem cholinergic parasympathetic neurons.

Optogenetic approaches to characterize the long-range synaptic pathways from the hypothalamus to brain stem autonomic nuclei.

Recent advances in optogenetic methods demonstrate the feasibility of selective photoactivation at the soma of neurons that express channelrhodopsin-2 (ChR2), but a comprehensive evaluation of different methods to selectively evoke transmitter release from distant synapses using optogenetic approaches is needed. Here we compared different lentiviral vectors, with sub-population-specific and strong promoters, and transgenic methods to express and photostimulate ChR2 in the long-range projections of paraventricular nucleus of the hypothalamus (PVN) neurons to brain stem cardiac vagal neurons (CVNs). Using PVN subpopulation-specific promoters for vasopressin and oxytocin, we were able to depolarize the soma of these neurons upon photostimulation, but these promoters were not strong enough to drive sufficient expression for optogenetic stimulation and synaptic release from the distal axons. However, utilizing the synapsin promoter photostimulation of distal PVN axons successfully evoked glutamatergic excitatory post-synaptic currents in CVNs. Employing the Cre/loxP system, using the Sim-1 Cre-driver mouse line, we found that the Rosa-CAG-LSL-ChR2-EYFP Cre-responder mice expressed higher levels of ChR2 than the Rosa-CAG-LSL-ChR2-tdTomato line in the PVN, judged by photo-evoked currents at the soma. However, neither was able to drive sufficient expression to observe and photostimulate the long-range projections to brainstem autonomic regions. We conclude that a viral vector approach with a strong promoter is required for successful optogenetic stimulation of distal axons to evoke transmitter release in pre-autonomic PVN neurons. This approach can be very useful to study important hypothalamus-brainstem connections, and can be easily modified to selectively activate other long-range projections within the brain.

Anaesthetics differentially modulate the trigeminocardiac reflex excitatory synaptic pathway in the brainstem.

The trigeminocardiac reflex (TCR) occurs upon excitation of the trigeminal nerve with a resulting bradycardia and hypotension. While several anaesthetics and analgesics have been reported to alter the incidence and strength of the TCR the mechanisms for this modulation are unclear. This study examines the mechanisms of action of ketamine, isoflurane and fentanyl on the synaptic TCR responses in both neurones in the spinal trigeminal interpolaris (Sp5I) nucleus and cardiac vagal neurones (CVNs) in the Nucleus Ambiguus (NA). Stimulation of trigeminal afferent fibres evoked an excitatory postsynaptic current (EPSC) in trigeminal neurones with a latency of 1.8 ± 0.1 ms, jitter of 625 µs, and peak amplitude of 239 ± 45 pA. Synaptic responses further downstream in the reflex pathway in the CVNs occurred with a latency of 12.1 ± 1.1 ms, jitter of 0.8-2 ms and amplitude of 57.8 ± 7.5 pA. The average conduction velocity to the Sp5I neurones was 0.94 ± 0.18 mm ms(-1) indicating a mixture of A-δ and C fibres. Stimulation-evoked EPSCs in both Sp5I and CVNs were completely blocked by AMPA/kainate and NMDA glutamatergic receptor antagonists. Ketamine (10 µm) inhibited the peak amplitude and duration in Sp5I as well as more distal synapses in the CVNs. Isoflurane (300 µm) significantly inhibited, while fentanyl (1 µm) significantly enhanced, EPSC amplitude and area in CVNs but had no effect on the responses in Sp5l neurones. These findings indicate glutamatergic excitatory synaptic pathways are critical in the TCR, and ketamine, isoflurane and fentanyl differentially alter the synaptic pathways via modulation of both AMPA/kainate and NMDA receptors at different synapses in the TCR.

Clonidine, an alpha2-receptor agonist, diminishes GABAergic neurotransmission to cardiac vagal neurons in the nucleus ambiguus.

In hypertension, there is an autonomic imbalance in which sympathetic activity dominates over parasympathetic control. Parasympathetic activity to the heart originates from cardiac vagal neurons located in the nucleus ambiguus. Presympathetic neurons that project to sympathetic neurons in the spinal cord are located in the ventral brainstem in close proximity to cardiac vagal neurons, and many of these presympathetic neurons are catecholaminergic. In addition to their projection to the spinal cord, many of these presympathetic neurons have axon collaterals that arborize into neighboring cardiorespiratory locations and likely release norepinephrine onto nearby neurons. Activation of alpha(2)-adrenergic receptors in the central nervous system evokes a diverse range of physiological effects, including reducing blood pressure. This study tests whether clonidine, an alpha(2)-adrenergic receptor agonist, alters excitatory glutamatergic, and/or inhibitory GABAergic or glycinergic synaptic neurotransmission to cardiac vagal neurons in the nucleus ambiguus. Cardiac vagal neurons were identified in an in vitro brainstem slice preparation, and synaptic events were recording using whole cell voltage clamp methodologies. Clonidine significantly inhibited GABAergic neurotransmission but had no effect on glycinergic or glutamatergic pathways to cardiac vagal neurons. This diminished inhibitory GABAergic neurotransmission to cardiac vagal neurons would increase parasympathetic activity to the heart, decreasing heart rate and blood pressure. The results presented here provide a cellular substrate for the clinical use of clonidine as a treatment for hypertension as well as a role in alleviating posttraumatic stress disorder by evoking an increase in parasympathetic cardiac vagal activity, and a decrease in heart rate and blood pressure.