Dexmedetomidine and clonidine induce long-lasting activation of the respiratory rhythm generator of neonatal mice: possible implication for critical care

Is the Sympathetic System Detrimental in the Setting of Septic Shock, with Antihypertensive Agents as a Counterintuitive Approach? A Clinical Proposition

Mortality in the setting of septic shock varies between 20% and 100%. Refractory septic shock leads to early circulatory failure and carries the worst prognosis. The pathophysiology is poorly understood despite studies of the microcirculatory defects and the immuno-paralysis. The acute circulatory distress is treated with volume expansion, administration of vasopressors (usually noradrenaline: NA), and inotropes. Ventilation and anti-infectious strategy shall not be discussed here. When circulation is considered, the literature is segregated between interventions directed to the systemic circulation vs. interventions directed to the micro-circulation. Our thesis is that, after stabilization of the acute cardioventilatory distress, the prolonged sympathetic hyperactivity is detrimental in the setting of septic shock. Our hypothesis is that the sympathetic hyperactivity observed in septic shock being normalized towards baseline activity will improve the microcirculation by recoupling the capillaries and the systemic circulation. Therefore, counterintuitively, antihypertensive agents such as beta-blockers or alpha-2 adrenergic agonists (clonidine, dexmedetomidine) are useful. They would reduce the noradrenaline requirements. Adjuncts (vitamins, steroids, NO donors/inhibitors, etc.) proposed to normalize the sepsis-evoked vasodilation are not reviewed. This itemized approach (systemic vs. microcirculation) requires physiological and epidemiological studies to look for reduced mortality.

Clinical Practice: Should we Radically Alter our Sedation of Critical Care Patients, Especially Given the COVID-19 Pandemics?

The high number of patients infected with the SARS-CoV-2 virus requiring care for ARDS puts sedation in the critical care unit (CCU) to the edge. Depth of sedation has evolved over the last 40 years (no-sedation, deep sedation, daily emergence, minimal sedation, etc.). Most guidelines now recommend determining the depth of sedation and minimizing the use of benzodiazepines and opioids. The broader use of alpha-2 adrenergic agonists ('alpha-2 agonists') led to sedation regimens beginning at admission to the CCU that contrast with hypnotics+opioids ("conventional" sedation), with major consequences for cognition, ventilation and circulatory performance. The same doses of alpha-2 agonists used for 'cooperative' sedation (ataraxia, analgognosia) elicit no respiratory depression but modify the autonomic nervous system (cardiac parasympathetic activation, attenuation of excessive cardiac and vasomotor sympathetic activity). Alpha-2 agonists should be selected only in patients who benefit from their effects ('personalized' indications, as opposed to a 'one size fits all' approach). Then, titration to effect is required, especially in the setting of systemic hypotension and/or hypovolemia. Since no general guidelines exist for the use of alpha-2 agonists for CCU sedation, our clinical experience is summarized for the benefit of physicians in clinical situations in which a recommendation might never exist (refractory delirium tremens; unstable, hypovolemic, hypotensive patients, etc.). Because the physiology of alpha-2 receptors and the pharmacology of alpha-2 agonists lead to personalized indications, some details are offered. Since interactions between conventional sedatives and alpha-2 agonists have received little attention, these interactions are addressed. Within the existing guidelines for CCU sedation, this article could facilitate the use of alpha-2 agonists as effective and safe sedation while awaiting large, multicentre trials and more evidence-based medicine.

Building on the Shoulders of Giants: Is the use of Early Spontaneous Ventilation in the Setting of Severe Diffuse Acute Respiratory Distress Syndrome Actually Heretical?

Acute respiratory distress syndrome (ARDS) is not a failure of the neurological command of the ventilatory muscles or of the ventilatory muscles; it is an oxygenation defect. As positive pressure ventilation impedes the cardiac function, paralysis under general anaesthesia and controlled mandatory ventilation should be restricted to the interval needed to control the acute cardio-ventilatory distress observed upon admission into the critical care unit (CCU; "salvage therapy" during "shock state"). Current management of early severe diffuse ARDS rests on a prolonged interval of controlled mechanical ventilation with low driving pressure, paralysis (48 h, too often overextended), early proning and positive end-expiratory pressure (PEEP). Therefore, the time interval between arrival to the CCU and switching to spontaneous ventilation (SV) is not focused on normalizing the different factors involved in the pathophysiology of ARDS: fever, low cardiac output, systemic acidosis, peripheral shutdown (local acidosis), supine position, hypocapnia (generated by hyperpnea and tachypnea), sympathetic activation, inflammation and agitation. Then, the extended period of controlled mechanical ventilation with paralysis under general anaesthesia leads to CCU-acquired pathology, including low cardiac output, myoneuropathy, emergence delirium and nosocomial infection. The stabilization of the acute cardio-ventilatory distress should primarily itemize the pathophysiological conditions: fever control, improved micro-circulation and normalized local acidosis, 'upright' position, minimized hypercapnia, sympathetic de-activation (normalized sympathetic activity toward baseline levels resulting in improved micro-circulation with alpha-2 agonists administered immediately following optimized circulation and endotracheal intubation), lowered inflammation and 'cooperative' sedation without respiratory depression evoked by alpha-2 agonists. Normalised metabolic, circulatory and ventilatory demands will allow one to single out the oxygenation defect managed with high PEEP (diffuse recruitable ARDS) under early spontaneous ventilation (airway pressure release ventilation+SV or low-pressure support). Assuming an improved overall status, PaO₂/FiO₂≥150-200 allows for extubation and continuous non-invasive ventilation. Such fast-tracking may avoid most of the CCU-acquired pathologies. Evidence-based demonstration is required.

Hypothesis: Fever control, a niche for alpha-2 agonists in the setting of septic shock and severe acute respiratory distress syndrome?

During severe septic shock and/or severe acute respiratory distress syndrome (ARDS) patients present with a limited cardio-ventilatory reserve (low cardiac output and blood pressure, low mixed venous saturation, increased lactate, low PaO2/FiO2 ratio, etc.), especially when elderly patients or co-morbidities are considered. Rescue therapies (low dose steroids, adding vasopressin to noradrenaline, proning, almitrine, NO, extracorporeal membrane oxygenation, etc.) are complex. Fever, above 38.5-39.5°C, increases both the ventilatory (high respiratory drive: large tidal volume, high respiratory rate) and the metabolic (increased O2 consumption) demands, further limiting the cardio-ventilatory reserve. Some data (case reports, uncontrolled trial, small randomized prospective trials) suggest that control of elevated body temperature ("fever control") leading to normothermia (35.5-37°C) will lower both the ventilatory and metabolic demands: fever control should simplify critical care management when limited cardio-ventilatory reserve is at stake. Usually fever control is generated by a combination of general anesthesia ("analgo-sedation", light total intravenous anesthesia), antipyretics and cooling. However general anesthesia suppresses spontaneous ventilation, making the management more complex. At variance, alpha-2 agonists (clonidine, dexmedetomidine) administered immediately following tracheal intubation and controlled mandatory ventilation, with prior optimization of volemia and atrio-ventricular conduction, will reduce metabolic demand and facilitate normothermia. Furthermore, after a rigorous control of systemic acidosis, alpha-2 agonists will allow for accelerated emergence without delirium, early spontaneous ventilation, improved cardiac output and micro-circulation, lowered vasopressor requirements and inflammation. Rigorous prospective randomized trials are needed in subsets of patients with a high fever and spiraling toward refractory septic shock and/or presenting with severe ARDS.

Effects of dexmedetomidine on cardiorespiratory regulation in spontaneously breathing newborn rats

BACKGROUND

Dexmedetomidine, a selective α2-adrenoceptor agonist, is a new sedative agent.

OBJECTIVE

To examine the dexmedetomidine-associated changes in cardiorespiratory indices in spontaneously breathing newborn rats.

METHODS

An abdominal catheter to administer drugs and subcutaneous electrodes to record electrocardiographic data were inserted into 2- to 4-day-old rats under isoflurane anesthesia; the rats were then placed in individual chambers. After recovery from the anesthesia, the rats received intraperitoneal administrations of normal saline (NS, vehicle), dexmedetomidine (50 μg·kg(-1)), or dexmedetomidine (50 μg·kg(-1)) followed 5 min later with NS or the selective α2-adrenoceptor antagonist atipamezole (1 mg·kg(-1)) (n = 10 in each group). Cardiorespiratory indices were recorded for each animal throughout the experiment.

RESULTS

Dexmedetomidine administration significantly decreased heart rate (HR) and minute ventilation (V'E) (P < 0.05) compared with control, whereas NS administration did not. The decrease in HR and V'E after dexmedetomidine administration was significantly less in rats that received atipamezole (P < 0.05) than in those that received NS after dexmedetomidine administration. The dexmedetomidine-associated V'E depression was attributed to a significant decrease in respiratory frequency (fR) but not tidal volume (VT ). The change in fR was reversed by atipamezole administration, which itself induced no significant changes in HR and fR.

CONCLUSION

In spontaneously breathing immature rats, dexmedetomidine administration significantly reduced HR and V'E. Because atipamezole fully reversed decreases in fR and therefore V'E, dexmedetomidine-related respiratory suppression occurs predominantly through α2-adrenoceptor-related suppression of fR.

Effects of COX inhibition and LPS on formalin induced pain in the infant rat

In the adult, immune and neural processes jointly modulate pain. During development, both are in transition and little is known about the role that the immune system plays in pain processing in infants and children. The objective of this study was to determine if inhibition or augmentation of the immune system would alter pain processing in the infant rat, as it does in the adult. In Experiment 1, rat pups aged 3, 10, or 21 (PN3, PN10, and PN21) days of age were pretreated with NS398 (selective cyclooxygenase (COX)-2 inhibitor) or SC560 (selective COX-1 inhibitor) and tested in the intraplantar formalin test to assess effects of COX inhibition on nociception. Neither drug had an effect on the behavioral response at PN3 or PN10 pups but both drugs attenuated nociceptive scores in PN21 pups. cFos expression in the spinal cord likewise was reduced only at PN21. In Experiment 2, pups were injected with lipopolysaccharide (LPS) prior to the formalin test at PN3 or PN21. LPS increased the nociceptive response more robustly at PN21 than at PN3, while increasing cytokine mRNA equally at both ages. The augmentation of pain responding at PN21 was largely during the late stages of the formalin test, as reported in the adult. These data support previous findings demonstrating late maturing immune modulation of nociceptive behaviors.

Continuous infusion of clonidine in ventilated newborns and infants: a randomized controlled trial

OBJECTIVES

To assess the influence of an infusion of clonidine 1 μg/kg/hr on fentanyl and midazolam requirement in ventilated newborns and infants.

DESIGN

Prospective, double-blind, randomized controlled multicenter trial. Controlled trials.com/ISRCTN77772144.

SETTING

Twenty-eight level 3 German PICUs/neonatal ICUs.

PATIENTS

Ventilated newborns and infants: stratum I (1-28 d), stratum II, (29-120 d), and stratum III (121 d to 2 yr).

INTERVENTIONS

Patients received clonidine 1 μg/kg/hr or placebo on day 4 after intubation. Fentanyl and midazolam were adjusted to achieve a defined level of analgesia and sedation according to Hartwig score.

MEASUREMENTS AND MAIN RESULTS

Two hundred nineteen infants were randomized; 212 received study medication, 69.7% were ventilated in the postoperative care and 30.3% for other reasons. Primary endpoint: consumption of fentanyl and midazolam in the 72 hours following the onset of study medication (main observation period) in the overall study population. The confirmatory analysis of the overall population showed no difference in the consumption of fentanyl and midazolam. Explorative age-stratified analysis demonstrated that in stratum I (n = 112) the clonidine group had a significantly lower consumption of fentanyl (clonidine: 2.1 ± 1.8 μg/kg/hr, placebo: 3.2 ± 3.1 μg/kg/hr; p = 0.032) and midazolam (clonidine: 113.0 ± 100.1 μg/kg/hr, placebo: 180.2 ± 204.0 μg/kg/hr; p = 0.030). Strata II (n = 43) and III (n = 46) showed no statistical difference. Sedation and withdrawal-scores were significantly lower in the clonidine group of stratum I (p < 0.001). Frequency of severe adverse events did not differ between groups.

CONCLUSIONS

Clonidine 1 μg/kg/hr in ventilated newborns reduced fentanyl and midazolam demand with deeper levels of analgesia and sedation without substantial side effects. This was not demonstrated in older infants, possibly due to lower clonidine serum levels.

Suppression of the cough reflex by α 2-adrenergic receptor agonists in the rabbit

The α₂-adrenergic receptor agonist clonidine has been shown to inhibit citric acid-induced cough responses in guinea pigs when administered by aerosol, but not orally. In contrast, oral or inhaled clonidine had no effect on capsaicin-induced cough and reflex bronchoconstriction in humans. In addition, intravenous administration of clonidine has been shown to depress fentanyl-induced cough in humans. We investigated the effects of the α₂-adrenergic receptor agonists, clonidine and tizanidine, on cough responses induced by mechanical and chemical (citric acid) stimulation of the tracheobronchial tree. Drugs were microinjected (30–50 nL) into the caudal nucleus tractus solitarii (cNTS) and the caudal ventral respiratory group (cVRG) as well as administered intravenously in pentobarbital sodium-anesthetized, spontaneously breathing rabbits. Bilateral microinjections of clonidine into the cNTS or the cVRG reduced cough responses at 0.5 mmol/L and abolished the cough reflex at 5 mmol/L. Bilateral microinjections of 0.5 mmol/L tizanidine into the cNTS completely suppressed cough responses, whereas bilateral microinjections of 5 mmol/L into the cVRG only caused mild reductions in them. Depressant effects on the cough reflex of clonidine and tizanidine were completely reverted by microinjections of 10 mmol/L yohimbine. Intravenous administration of clonidine (80–120 μg/kg) or tizanidine (150–300 μg/kg) strongly reduced or completely suppressed cough responses. These effects were reverted by intravenous administration of yohimbine (300 μg/kg). The results demonstrate that activation of α₂-adrenergic receptors in the rabbit exerts potent inhibitory effects on the central mechanism generating the cough motor pattern with a clear action at the level of the cNTS and the cVRG.

Is there a place for pressure-support ventilation and high positive end-expiratory pressure combined to alpha-2 agonists early in severe diffuse acute respiratory distress syndrome?

Acute respiratory distress syndrome (ARDS) is associated with a high mortality linked primarily to co-morbidities (sepsis, cardiac failure, multiple organ failure, etc.). When the lung is the single failing organ, quick resolution of ARDS should skip some complications arising from a prolonged stay in the critical care unit. In severe ARDS (PaO2/FIO2=P/F<100 with positive end-expiratory pressure (PEEP) ≥ 5 cm H2O), current recommendations are to intubate the trachea of the patient and use mechanical ventilation, low tidal volume, high PEEP, prone positioning and possibly neuromuscular blockade in association with intravenous sedation. Another strategy is possible. Firstly, spontaneous ventilation (SV) coupled with pressure support (PS) ventilation and high PEEP is possible from tracheal intubation onwards, with the possible exception of the short period following immediately tracheal intubation. Secondly, using alpha-2 adrenergic agonists (e.g. clonidine, dexmedetomidine) can provide first-line sedation from the beginning of mechanical ventilation, as they preserve respiratory drive, lower oxygen consumption and pulmonary hypertension and increase diuresis. Alpha-2 agonists are to be supplemented, if appropriate, by drugs devoid of effect on respiratory drive (neuroleptics, etc.). The expected benefits would be to prevent acquired diaphragmatic weakness, accumulation of sedation, cognitive dysfunction, and presumably improved outcome. This hypothesis should be tested in a double blind randomized controlled trial.

[Dexmedetomidine and clonidine: a review of their pharmacodynamy to define their role for sedation in intensive care patients]

Alpha-2 adrenergic agonists ("alpha-2 agonists") present multiple pharmacodynamic effects: rousable sedation, decreased incidence of delirium in the setting of critical care, preservation of respiratory drive, decreased whole body oxygen consumption, decreased systemic and pulmonary arterial impedance, improved left ventricular systolic and diastolic function, preserved vascular reactivity to exogenous catecholamines, preserved vasomotor baroreflex with lowered set point, preserved kidney function, decreased protein catabolism. These pharmacodynamic effects explain the interest for these drugs in the critical care setting. However, their exact role for sedation in critically ill-patients remains open for further studies. Given the few double-blind randomized multicentric trials available, the present non exhaustive analysis of the literature aims at presenting the utilization of alpha-2 agonists as potential first-line sedative agents, in the critical care setting. Suggestions regarding the use of alpha-2 agonists as sedatives are detailed.

Effects of clonidine on breathing during sleep and susceptibility to central apnoea

We hypothesized that administration of clonidine would decrease the hypocapnic apnoeic threshold (HAT) and widen the CO(2) reserve during non-REM sleep.

METHODS

Ten healthy subjects (4 females) (age 22.3 ± 3.0 years; BMI 25.5 ± 3.4 kg/m(2)) were randomized to receive placebo or 0.1 mg/45 kg of clonidine on 2 separate nights. Ventilation and upper airway resistance were monitored during wakefulness and sleep. Two separate experiments were performed: Protocol 1 (n=8), CO(2) reserve, HAT and HcVR were determined using non-invasive hyperventilation (NIV) to induce hypocapnia for at least 3 min; Protocol 2 (n=6), peripheral hypocapnic ventilatory response (HcVR) was determined by NIV using short (3 breaths) hyperventilation.

RESULTS

Clonidine decreased the systolic blood pressure by 12 ± 10 mmHg but did not affect baseline ventilation or upper airway resistance during wakefulness or sleep. Protocol (1), clonidine was associated with decreased HAT relative to placebo (37.3 ± 3.3 mmHg vs. 39.7 ± 3.4 mmHg, P<0.05), increased CO(2) reserve (-3.8 ± 1.3 mmHg vs. -2.8 ± 1.2 mmHg, P<0.05), and decreased HcVR (1.6 ± 0.6 L/min/mmHg vs. 2.5 ± 1.3 L/min/mmHg, P<0.05). Protocol (2), administration of clonidine did not decrease peripheral HcVR compared to placebo (0.5 ± 0.3 L/min/mmHg vs. 0.7 ± 0.3 L/min/mmHg, P=NS).

CONCLUSION

Clonidine is associated with diminished susceptibility to hypocapnic central apnoea without significant effect on ventilation or upper airway mechanics. Reduced susceptibility to hypocapnic central apnoea is not explained by the peripheral chemoreceptor pathway. This suggests a central rather than a peripheral effect of clonidine on the susceptibility to hypocapnic central apnoea.