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

Sleep apnea pathophysiology

OBJECTIVE

The purpose of this study is to examine the pathophysiology underlying sleep apnea (SA).

BACKGROUND

We consider several critical features of SA including the roles played by the ascending reticular activating system (ARAS) that controls vegetative functions and electroencephalographic findings associated with both SA and normal sleep. We evaluate this knowledge together with our current understanding of the anatomy, histology, and physiology of the mesencephalic trigeminal nucleus (MTN) and mechanisms that contribute directly to normal and disordered sleep. MTN neurons express γ-aminobutyric acid (GABA) receptors which activate them (make chlorine come out of the cells) and that can be activated by GABA released from the hypothalamic preoptic area.

METHOD

We reviewed the published literature focused on sleep apnea (SA) reported in Google Scholar, Scopus, and PubMed databases.

RESULTS

The MTN neurons respond to the hypothalamic GABA release by releasing glutamate that activates neurons in the ARAS. Based on these findings, we conclude that a dysfunctional MTN may be incapable of activating neurons in the ARAS, notably those in the parabrachial nucleus, and that this will ultimately lead to SA. Despite its name, obstructive sleep apnea (OSA) is not caused by an airway obstruction that prevents breathing.

CONCLUSIONS

While obstruction may contribute to the overall pathology, the primary factor involved in this scenario is the lack of neurotransmitters.

Effect of Venlafaxine on Apnea-Hypopnea Index in Patients With Sleep Apnea: A Randomized, Double-Blind Crossover Study

BACKGROUND

One of the key mechanisms underlying OSA is reduced pharyngeal muscle tone during sleep. Data suggest that pharmacologic augmentation of central serotonergic/adrenergic tone increases pharyngeal muscle tone.

RESEARCH QUESTION

We hypothesized that venlafaxine, a serotonin-norepinephrine reuptake inhibitor, would improve OSA severity.

STUDY DESIGN AND METHODS

In this mechanistic, randomized, double-blind, placebo-controlled crossover trial, 20 patients with OSA underwent two overnight polysomnograms ≥ 4 days apart, receiving either 50 mg of immediate-release venlafaxine or placebo before bedtime. Primary outcomes were the apnea-hypopnea index (AHI) and peripheral oxygen saturation (Spo₂) nadir, and secondary outcomes included sleep parameters and pathophysiologic traits with a view toward understanding the impact of venlafaxine on mechanisms underlying OSA.

RESULTS

Overall, there was no significant difference between venlafaxine and placebo regarding AHI (mean reduction, -5.6 events/h [95% CI, -12.0 to 0.9]; P = .09) or Spo₂ nadir (median increase, +1.0% [-0.5 to 5]; P = .11). Venlafaxine reduced total sleep time, sleep efficiency, and rapid eye movement (REM) sleep, while increasing non-REM stage 1 sleep (P_all < .05). On the basis of exploratory post hoc analyses venlafaxine decreased ("improved") the ventilatory response to arousal (-30%; P = .049) and lowered ("worsened") the predicted arousal threshold (-13%; [P = .02]; ie, more arousable), with no effects on other pathophysiologic traits (P_all ≥ .3). Post hoc analyses further suggested effect modification by arousal threshold (P = .002): AHI improved by 19% in patients with a high arousal threshold (-10.9 events/h [-3.9 to -17.9]) but tended to increase in patients with a low arousal threshold (+7 events/h [-2.0 to 16]). Other predictors of response were elevated AHI and less collapsible upper airway anatomy at baseline (|r| > 0.5, P ≤ .02).

INTERPRETATION

In unselected patients, venlafaxine simultaneously worsened and improved various pathophysiologic traits, resulting in a zero net effect. Careful patient selection based on pathophysiologic traits, or combination therapy with drugs countering its alerting effects, may produce a more robust response.

TRIAL REGISTRY

ClinicalTrials.gov; No.: NCT02714400; URL: www.clinicaltrials.gov.

Effect of acetazolamide on susceptibility to central sleep apnea in chronic spinal cord injury

Spinal cord injury (SCI) is an established risk factor for central sleep apnea. Acetazolamide (ACZ), a carbonic anhydrase inhibitor, has been shown to decrease the frequency of central apnea by inducing mild metabolic acidosis. We hypothesized that ACZ would decrease the propensity to develop hypocapnic central apnea and decrease the apneic threshold. We randomized 16 participants with sleep-disordered breathing (8 SCI and 8 able-bodied controls) to receive ACZ (500 mg twice a day for 3 days) or placebo with a 1-wk washout before crossing over to the other drug arm. Study nights included polysomnography and determination of the hypocapnic apneic threshold and CO₂ reserve using noninvasive ventilation. For participants with spontaneous central apnea, CO₂ was administered until central apnea was abolished, and CO₂ reserve was measured as the difference in end-tidal Pco₂ (PETCO2) before and after. Steady-state plant gain, the response of end-tidal Pco₂ to changes in ventilation, was calculated from PETCO2 and V̇e ratio during stable sleep. Controller gain, the response of ventilatory drive to changes in end-tidal Pco₂, was defined as the ratio of change in V̇e between control and hypopnea to the ΔCO₂ during stable non-rapid eye movement sleep. Treatment with ACZ for three days resulted in widening of the CO₂ reserve (-4.0 ± 1.2 vs. -3.0 ± 0.7 mmHg for able-bodied, -3.4 ± 1.9 vs. -2.2 ± 2.2 mmHg for SCI, P < 0.0001), and a corresponding decrease in the hypocapnic apnea threshold (28.3 ± 5.2 vs. 37.1 ± 5.6 mmHg for able-bodied, 29.9 ± 5.4 vs. 34.8 ± 6.9 mmHg for SCI, P < 0.0001), respectively. ACZ significantly reduced plant gain when compared with placebo (4.1 ± 1.7 vs. 5.4 ± 1.8 mmHg/L min for able-bodied, 4.1 ± 2.0 vs. 5.1 ± 1.7 mmHg·L^-1·min for SCI, P < 0.01). Acetazolamide decreased apnea-hypopnea index (28.8 ± 22.9 vs. 39.3 ± 24.1 events/h; P = 0.05), central apnea index (0.6 ± 1.5 vs. 6.3 ± 13.1 events/h; P = 0.05), and oxyhemoglobin desaturation index (7.5 ± 8.3 vs. 19.2 ± 15.2 events/h; P = 0.01) compared with placebo. Our results suggest that treatment with ACZ decreases susceptibility to hypocapnic central apnea due to decreased plant gain. Acetazolamide may attenuate central sleep apnea and improve nocturnal oxygen saturation, but its clinical utility requires further investigation in a larger sample of patients.NEW & NOTEWORTHY Tetraplegia is a risk factor for central sleep-disordered breathing (SDB) and is associated with narrow CO₂ reserve (a marker of susceptibility to central apnea). Treatment with high-dose acetazolamide for 3 days decreased susceptibility to hypocapnic central apnea and reduced the frequency of central respiratory events during sleep. Acetazolamide may play a therapeutic role in alleviating central SDB in patients with cervical spinal cord injury, but larger clinical trials are needed.

Spinal cord injury is associated with enhanced peripheral chemoreflex sensitivity

Sleep‐disordered breathing (SDB) is prevalent in individuals with chronic spinal cord injury (SCI), but the exact mechanism is unknown. The aim of this study was to investigate whether peripheral chemoreceptors activity is enhanced in individuals with chronic SCI compared to abled‐bodied control subjects using CO₂ and O₂ chemical tests. In protocol (1) 30 subjects (8 cervical [cSCI], 7 thoracic [tSCI] and 15 able‐bodied [AB]) were studied to determine the ventilatory response to hyperoxia during wakefulness in the supine position. In protocol (2) 24 subjects (6 cSCI, 6 tSCI, and 12 AB subjects) were studied to determine the ventilatory response to a single breath of CO₂ (SBCO₂). The chemoreflex response to SBCO₂ was calculated as ∆V_E/∆CO₂ (L/min/mmHg). The ventilatory response to hyperoxia was defined as the % change in V_T following acute hyperoxia compared to preceding baseline. During hyperoxia SCI subjects had a significant decrease in V_T and V_E (63.4 ± 21.7% and 63.1 ± 23.0% baseline, respectively, P < 0.05) compared to AB (V_T: 87.1 ± 14.3% and V_E: 91.38 ± 15.1% baseline, respectively, P < 0.05). There was no significant difference between cSCI and tSCI in the V_T or V_E during hyperoxia (P = NS). There was no significant correlation between AHI and V_E% baseline (r = −0.28) in SCI and AB (n = 30). SCI participants had a greater ventilatory response to an SBCO₂ than AB (0.78 ± 0.42 L/min/mmHg vs. 0.26 ± 0.10 L/min/mmHg, respectively, P < 0.05). Peripheral ventilatory chemoresponsiveness is elevated in individuals with chronic SCI compared to able‐bodied individuals.

Clonidine or remifentanil for adequate surgical conditions in patients undergoing endoscopic sinus surgery: a randomized study

Background

Deliberate hypotension is one way to achieve a bloodless surgical field in endoscopic sinus surgery (ESS). We compared two anaesthesia regimens to induce deliberate hypotension and attempted to determine the most efficient one.

Methods

Fifty-nine patients undergoing ESS were minimized into two groups. In the CLO group, patients received I.V. sufentanil 0.15 µg/kg together with I.V. clonidine 2–3 µg/kg. In the REMI group, patients received remifentanil at a rate of up to 1 µg/kg/min. Fromme scores were collected 15 min after the incision and at the end of the procedure. Mean arterial pressure readings (MAP), heart rate readings, time to eyes opening, time to extubation, pain scores, analgesic requirements, and oxygen needs were collected and compared.

Results

There were no significant differences in Fromme scores between the two groups. The averaged MAP from 15 min to the end of the procedure was significantly lower in the REMI group; these patients also received more ephedrine. Significantly fewer patients in the CLO group needed oxygen therapy to keep their Pulse Oximeter Oxygen Saturation within 3% of their preoperative values. Patients in this group also needed less piritramide in the recovery room, and their pain scores were lower at discharge from the recovery room.

Discussion

Although both anaesthesia regimens offered a similar quality of surgical field, this study suggests that clonidine had a better average safety profile. Furthermore, patients who received this regimen required fewer painkillers immediately after surgery.

Control of Ventilation in Health and Disease

Control of ventilation occurs at different levels of the respiratory system through a negative feedback system that allows precise regulation of levels of arterial carbon dioxide and oxygen. Mechanisms for ventilatory instability leading to sleep-disordered breathing include changes in the genesis of respiratory rhythm and chemoresponsiveness to hypoxia and hypercapnia, cerebrovascular reactivity, abnormal chest wall and airway reflexes, and sleep state oscillations. One can potentially stabilize breathing during sleep and treat sleep-disordered breathing by identifying one or more of these pathophysiological mechanisms. This review describes the current concepts in ventilatory control that pertain to breathing instability during wakefulness and sleep, delineates potential avenues for alternative therapies to stabilize breathing during sleep, and proposes recommendations for future research.

Comparison of lumbar epidural bupivacaine with fentanyl or clonidine for postoperative analgesia in children with cerebral palsy after single-event multilevel surgery

AIM

To compare diazepam use, muscle spasm, analgesia, and side effects when clonidine or fentanyl are added to epidural bupivacaine in children with cerebral palsy after multilevel orthopaedic surgery.

METHOD

Fifty children were prospectively randomized to receive clonidine (n=24, mean age 10y 10mo [SD 2y 11mo]) or fentanyl (n=26, mean age 10y 11mo [SD 2y 10mo]).

RESULTS

There was no difference in primary outcome measures: median diazepam use (fentanyl 0, interquartile range [IQR] 0-0; clonidine 0, IQR 0-0; p=0.46), any muscle spasm (no muscle spasms in: fentanyl, 36%; clonidine, 62%; p=0.11), painful muscle spasm (fentanyl 40%; clonidine 25%; p=0.46), or pain score ≥6 (none: fentanyl 44%; clonidine 42%; p=0.29). There were differences in secondary outcome measures: no vomiting (clonidine 63%; fentanyl 20%); vomiting occurred more frequently with fentanyl (32% vomited more than three times; clonidine none; p=0.001). Fentanyl resulted in more oxygen desaturation (at least two episodes: fentanyl 20%; clonidine 0; p<0.001). Clonidine resulted in lower mean (SD) area under the curve for systolic blood pressure (fentanyl 106.5 [11.0]; clonidine 95.7mmHg [7.9]) and heart rate (fentanyl 104.9 beats per minute [13.6]; clonidine 85.3 [11.5]; p<0.001).

INTERPRETATION

Clonidine and fentanyl provide adequate analgesia with low rates of muscle spasm, resulting in low diazepam use. The choice of epidural additive should be based upon the most tolerable side-effect profile.

Alternative approaches to treatment of Central Sleep Apnea

Divergent approaches to treatment of hypocapnic central sleep apnea syndromes reflect the difficulties in taming a hyperactive respiratory chemoreflex. As both sleep fragmentation and a narrow CO₂ reserve or increased loop gain drive the disease, sedatives (to induce longer periods of stable non-rapid eye movement (NREM) sleep and reduce the destabilizing effects of arousals in NREM sleep) and CO₂-based stabilization approaches are logical. Adaptive ventilation reduces mean hyperventilation yet can induce ventilator-patient dyssynchrony, while enhanced expiratory rebreathing space (EERS, dead space during positive pressure therapy) and CO₂ manipulation directly stabilize respiratory control by moving CO₂ above the apnea threshold. Carbonic anhydrase inhibition can provide further adjunctive benefits. Provent and Winx may be less likely to trigger central apneas or periodic breathing in those with a narrow CO₂ reserve. An oral appliance can meaningfully reduce positive pressure requirements and thus enable treatment of complex apnea. Novel pharmacological approaches may target mediators of carotid body glomus cell excitation, such as the balance between gas neurotransmitters. In complex apnea patients, single mode therapy is not always successful, and multi-modality therapy might need to be considered. Phenotyping of sleep apnea beyond conventional scoring approaches is the key to optimal management.

Complex sleep apnea

OPINION STATEMENT

Complex sleep apnea currently refers to the emergence and persistence of central apneas and hypopneas following the application of positive airway pressure therapy in patients with obstructive sleep apnea. However, this narrow definition is an "outcome" and does not capture the spectrum of pathological activation of the respiratory chemoreflex in sleep apnea. The International Classification of Sleep Disorders - 3rd edition recognizes the phenomenon of Treatment-Related Central Sleep Apnea, but the phenotype is usually evident prior to onset of therapy. The key polysomnographic characteristics of chemoreflex modulated and mediated sleep apnea are nonrapid eye movement (NREM) dominance of respiratory events, short (<30 seconds) or long (>60 seconds) cycle time with a self-similar metronomic timing, and spontaneous improvement during rapid eye movement (REM) sleep. Thus, the majority of chemoreflex effects go unrecognized due to the bias toward obstructive sleep apnea's current scoring criteria. Any treatment of apparently obstructive sleep apnea, including surgery and oral appliances, can expose chemoreflex-driven instabilities. As both sleep fragmentation and a narrow CO2 reserve or increased loop gain drive the disease, sedatives (to induce longer periods of stable NREM sleep and reduce the destabilizing effects of arousals in NREM sleep) and CO2-based stabilization approaches are logical. Adaptive ventilation reduces mean hyperventilation yet can induce ventilator-patient desynchrony, while enhanced expiratory rebreathing space (EERS, dead space during positive pressure therapy) and CO2 manipulation directly stabilize respiratory control by moving CO2 above the apnea threshold. Carbonic anhydrase inhibition can provide further adjunctive benefits. Novel pharmacological approaches may target mediators of carotid body hypoxic sensitization, such as the balance between gas neurotransmitters. In complex apnea patients, single mode therapy is unlikely to be successful, and the power of multi-modality therapy should be harnessed for optimal outcomes.

Physiology in medicine: obstructive sleep apnea pathogenesis and treatment--considerations beyond airway anatomy

We review evidence in support of significant contributions to the pathogenesis of obstructive sleep apnea (OSA) from pathophysiological factors beyond the well-accepted importance of airway anatomy. Emphasis is placed on contributions from neurochemical control of central respiratory motor output through its effects on output stability, upper airway dilator muscle activation, and arousability. In turn, we consider the evidence demonstrating effective treatment of OSA via approaches that address each of these pathophysiologic risk factors. Finally, a case is made for combining treatments aimed at both anatomical and ventilatory control system deficiencies and for individualizing treatment to address a patient's own specific risk factors.

Drug effects on ventilatory control and upper airway physiology related to sleep apnea

Understanding the inter-relationship between pharmacological agents, ventilatory control, upper airway physiology and their consequent effects on sleep-disordered breathing may provide new directions for targeted drug therapy. Where available, this review focuses on human studies that contain both drug effects on sleep-disordered breathing and measures of ventilatory control or upper airway physiology. Many of the existing studies are limited in sample size or comprehensive methodology. At times, the presence of paradoxical findings highlights the complexity of drug therapy for OSA. The existing studies also highlight the importance of considering inter-individual pharmacokinetics and underlying causes of sleep apnea in interpreting drug effects on sleep-disordered breathing. Practical ways to assess an individual's ventilatory control and how it interacts with upper airway physiology is required for future targeted pharmacotherapy in sleep apnea.