Clinical Outcomes of Angiotensin Converting Enzyme Inhibitors and Angiotensin II Receptor Blockers in COVID-19 Patients With Pre-existing Cardiac Comorbidities: A Literature Review

The growing research regarding the implementation of angiotensin-converting enzyme-2 inhibitors (ACEi) and angiotensin receptor blockers (ARBs) in the treatment of COVID-19 in patients with pre-existing cardiac comorbidities has become a large topic of discussion since the onset of the pandemic. Previous research primarily associates positive outcomes to the use of these drug classes due to their mechanism of action, which involves the downregulation of angiotensin I-converting enzyme 2 (ACE2) in the renin-angiotensin-aldosterone-system (RAAS) pathway, inflammatory mediators, and cytokines. Thus, these medications can convey preventative and protective effects in patients suffering from a SARS-CoV-2 infection. While we explored the studies that supported the positive outcomes of the use of these drugs in the first half of this review, we also expanded on the limitations of these studies in the latter portion. We also further explored the contradictory studies that indicated that using these antihypertensives can paradoxically increase the severity of COVID-19 infection as well. The studies in support of the use of these medications should consider epigenetic variations, ACE2 variants and acknowledge inherent genetic variations in certain ethnic groups as some have a predisposition for a severe COVID-19 infection. Additionally, mortality rates need to be taken into consideration in these studies as they naturally differ throughout the trajectory of the COVID-19 pandemic. While some studies are in support of the use of these antihypertensives despite other studies suggesting otherwise, further research is needed to explore the long-term effects of these antihypertensives and observe whether they are truly beneficial or not in reducing the severity of COVID-19 infections.

The effects of QX-314 on medullary respiratory neurones

The synaptic and current-evoked responses of respiratory neurones located in the nucleus of the tractus solitarius, the para- and retroambigual regions and the nucleus ambiguus, were examined after voltage-dependent sodium currents were blocked by intracellular application of the quaternary lidocaine derivative QX-314. (1) QX-314 abolished orthodromically and antidromically evoked action potential discharge. Only antidromic action potentials recovered during negative DC current injection. (2) QX-314 did not alter the amplitude or duration of small and short excitatory and inhibitory postsynaptic potentials evoked by vagus or superior laryngeal nerve stimulation. Larger and longer waves of spontaneous membrane depolarizations, however, were slightly diminished. (3) The repetitive discharge evoked by depolarizing current pulses was blocked by QX-314. Positive current pulses produced less membrane depolarization than under control and often evoked only a single action potential at the beginning of the pulse, indicating that QX-314 interferes with the processes responsible for repetitive firing. (4) When fast spike discharges were completely blocked, positive current pulses occasionally evoked depolarizing 'spikes' and potentials which were followed by a hyperpolarization. We conclude that a noninactivating sodium inward current and calcium currents contribute to the electroresponsiveness of respiratory neurones.

Dynamic discharge characteristics of low pressure receptors in the rat

An in vitro preparation was used to examine the discharge of low pressure receptors in response to a pulsatile pressure stimulus. Both slowly adapting and rapidly adapting receptors were examined. After an increase in the mean level and/or dP/dt of a pulsatile pressure input, the discharge of slowly adapting receptors reached a steady state level within 2 minutes. An increase in mean pressure, with constant pulsatile amplitude and dP/dt, produced a sustained increase in the number of spikes/cycle, in the average number of spikes/second, and in the average frequency during the burst within the cycle. In slowly adapting receptors, an increase in dP/dt at a constant mean pressure and pulsatile amplitude produced a decrease in the number of spikes/cycle and an increase in both the average number of spikes/second and the frequency during the burst. Rapidly adapting receptors showed very different characteristics, responding to a pulsatile pressure input above threshold with an irregular discharge which averages approximately 1 spike/cycle at all mean pressure levels. Although rapidly adapting receptors transiently increased their discharge in response to an increase in mean pressure, within 60 seconds discharge returned to the pre-increase level. Rapidly adapting receptors continued to discharge with an average of 1 spike/cycle as dP/dt was increased. Since dP/dt was raised by increasing the frequency of the pulsatile input, the number of spikes/second increased. In the steady state, slowly adapting receptors transmit information reflecting mean pressure levels. They are also sensitive to the rate of change of pressure. Rapidly adapting receptors, on the other hand, are insensitive to mean pressure but do respond to the frequency of the stimulus.

Evidence for a monosynaptic connection between slowly adapting pulmonary stretch receptor afferents and inspiratory beta neurones

The synaptic connection between slowly adapting pulmonary stretch receptor afferents and inspiratory neurones within a region ventral to the tractus solitarius was determined using intracellular recording and spike triggered averaging techniques. When the vagus nerve was stimulated at intensities eliciting a Hering-Breuer reflex, the difference in mean latency between centrally recorded action potentials of slowly adapting pulmonary stretch receptor afferents and e.p.s.p.s of inspiratory beta neurones was 0.2 ms. This difference is indicative of a monosynaptic connection. Extracellular single unit spikes of slowly adapting pulmonary stretch receptors recorded from the nodose ganglion were used to trigger the averaging of synaptic noise recorded from inspiratory neurones. A prominent wave of synaptic depolarization was observed in all inspiratory beta neurones even when a small number of sweeps were averaged. This depolarization was absent from inspiratory alpha neurones. The shape indices of these depolarizations are consistent with a monosynaptic connection between slowly adapting pulmonary stretch receptor afferents and inspiratory beta neurones. In addition, the data raise the possibility that this connection is multiple and distributed.