Abstract
Obstructive sleep apnea (OSA) is a common health problem among surgical patients. Human evidence supports that important components of OSA like sleep fragmentation and intermittent hypoxia may enhance pain behavior and also increase sensitivity to opioid analgesia. To the extent that these effects might affect postoperative opioid pharmacology, OSA may impact the risk for opioid-induced ventilatory impairment (OIVI), a potentially serious complication in the postoperative patient. On the other hand, certain pathophysiological features of OSA might promote the development of OIVI due to enhancing respiratory compromise and/or through suppressing arousal from sleep in response to an airway obstruction event. Nonetheless, possible determinants of OIVI are not limited to factors associated with sleep-disordered breathing and current evidence does not support a direct relationship between an isolated preoperative diagnosis of OSA and increased risk for OIVI during postoperative analgesic therapy. Older age, comorbidity burden, and increased postoperative sedation, seem to be important promoters of potentially severe OIVI in the postoperative patient. Accepted strategies to prevent OIVI without interfering with postoperative analgesia include adopting opioid-sparing analgesic techniques, as well as establishing intense patient monitoring with emphasis on the respiratory and mental capacities.
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Introduction
Obstructive sleep apnea (OSA) is a common health problem; approximately 30 % of the general [1] and surgical [2, 3] population have OSA with most of them lacking formal disease diagnosis [2–5]. Sleep-disordered breathing has been linked to cardiovascular [6, 7] and metabolic [8] morbidity in the general population, and accumulating evidence further suggests that OSA increases the risk for postoperative pulmonary complications [9, 10, 11 •, 12, 13, 14 ••, 15 ••]. Although the causal nature of these complications is unknown, drug-induced ventilatory compromise has been proposed as one of the likely mechanisms [16, 17••].
Several features of the disease suggest that postoperative patients with OSA might be especially vulnerable to opioid side effects such as sedation, diminution of central respiratory drive, and/or increase airway collapsibility. On the other hand, certain physiological components of the disease including nocturnal hypoxemia, sleep disruption, and systemic inflammation, have been associated with either pain processing or opioid analgesia mechanisms. A closer look at these relationships, as well as the variability involved in the compensatory responses to sleep-induced airway obstruction, may allow us to better characterize the risk-versus-benefit ratio regarding the opioid response in the postoperative patient with OSA, and plan better prevention strategies for opioid-related adversities in this population.
Opioid Analgesia
Experimental and clinical evidence suggest that two distinct pathophysiological components of OSA, namely sleep disruption and nocturnal intermittent hypoxemia, either acting as sole inflictors or via exciting inflammatory pathways, posses the biologic capacity to enhance pain. Total [18–21], moderate [22••, 23], as well as stage-selective [23–25], restriction of sleep have been shown to enhance sensitivity to experimental pain [18, 19, 22••, 23] and increase spontaneous pain complaints in volunteers [20, 21, 26•]. Furthermore, physiological sleepiness (measured by the maintaining wakefulness test, MWT) predicted increased sensitivity to experimental pain in healthy volunteers [27], while an extended sleep opportunity in sleepy individuals resulted in decreased sleepiness and reduced pain sensitivity [28]. Consistently, insomniacs [29] and temporomandibular joint disorder patients with primary insomnia [30] demonstrated hyperalgesia, while insomnia symptoms [31] and degraded sleep quality [32] predicted daily intensity [32], as well as chronicity [31], of pain in hospitalized burn patients. Although the exact mechanistic details through which disturbed sleep could exacerbate pain remain unclear, experiments on healthy and clinical populations suggest that disrupted sleep may enhance pain behavior either via increasing the expression of hyperalgesic inflammatory mediators [20, 21, 33, 34•], or by acting on central pain-modulatory networks, i.e., weakening central inhibition and/or reinforcing pain-facilitating mechanisms [22, 35, 36].
In striking contrast with the main finding that sleep disruption enhances pain, temporomandibular joint disorder patients with OSA presented with hypoalgesia to experimental pain [30], suggesting that sleep fragmentation and recurrent nocturnal hypoxemia, both encountered in OSA, may exert different or even opposite effects on pain processing. Even though Smith et al. did not report information on the nocturnal hypoxemia status of these patients, a recent analysis of Cleveland Family Study (CFS; a longitudinal cohort designed to evaluate the familial aggregation of OSA) showed that intermittent hypoxia significantly increased pain reporting in subjects with OSA, independently of the effects of sleep fragmentation and systemic inflammation [37••]. Characteristically, a decrease in the minimum nocturnal arterial saturation (SaO2) from 92 to 75 % approximately doubled the odds for experiencing pain. This suggests that the brief periods of hypoxia, which occur repeatedly during sleep in patients with OSA, could either initiate or contribute to enhanced pain experience [38].
While the effect of disturbed sleep on surgical pain and opioid analgesia has yet to be evaluated in the postoperative patient, several lines of evidence support that chronic intermittent hypoxia may increase the sensitivity to the analgesic effect of opioids. In children undergoing adenotonsillectomy, the morphine requirement for postoperative analgesia was inversely associated with the magnitude of preoperative nocturnal recurrent hypoxemia during sleep [39, 40••]. These findings were supported by independent experiments showing that intermittent hypoxia upregulated μ-opioid receptors in the developing rat [41, 42] and hence it might be responsible for an increased sensitivity to the analgesic and respiratory effects of opioids [43, 44]. In a recent human experiment, both minimum nocturnal SaO2 and insulin growth factor binding protein-1 (IGFBP-1), a serum marker of hypoxia [45], were significantly associated with an increased sensitivity to the opioid analgesic effect [46••]. In contrast to this evidence, others [47, 48] have shown that children with OSA, presented more pain [47], required higher dose of morphine for postoperative analgesia, and experienced prolonged post-anesthetic recovery due to inadequate pain control [48]. In these studies, OSA diagnosis was based upon a polysomnography-determined abnormal apnea/hypopnea index, which might not correlate with either nocturnal hypoxemia or sleep fragmentation phenotypes, making unsafe any inference regarding the possible relationship between nocturnal intermittent hypoxia and postoperative pain and/or opioid consumption.
Because of the immense variability encountered both in OSA phenotypes [49, 50••] and pain/analgesia responses in humans, it would require large systematic investigations to fully characterize the effect of OSA on postoperative pain and opioid analgesia in surgical populations. The advantage of identifying the various parameters involved in enhancing pain and/or opioid analgesia in OSA patients is obvious since it could enhance our ability to predict opioid analgesic pharmacology in this population.
Obstructive Apnea Mechanisms
Current experimental and clinical evidence postulates that OSA is a disorder of ventilatory control [51, 52•]; its severity (as measured by the frequency of apnea/hypopnea events during sleep) is largely determined by the type and effectiveness of compensatory mechanisms that are engaged in response to airway obstruction rather than the anatomical narrowing of the airway, per se [53].
Local anatomical structure and a complex activation pattern of pharyngeal dilator muscles are the primary determinants of upper airway patency during both wakefulness and sleep. Pharyngeal dilators receive input from at least three different types of sources: (a) central respiratory drive (i.e., rising PaCO2 and declining PaO2), (b) local negative airway pressure during inspiration (negative pressure reflex), and (c) wakefulness drive [52, 54]. In OSA patients, the anatomically compromised airway [55••], compounded with a physiologically diminished pharyngeal dilator activity during sleep (loss of wakefulness drive), undergoes repetitive occlusion, partial or complete, as a result of the negative inspiratory pressure exerted by the diaphragm [56, 57]. Airway obstruction is followed by a gradual and steady rise in the contracting force of pharyngeal dilators as a result of both the negative airway pressure and the rising chemical respiratory drive (i.e., due to rising PaCO2 and declining PaO2) with the essential aim of restoring airway patency. Occasionally, arousal from sleep (cortical arousal) would also assist to that end by reinstating the wakefulness drive [50••, 52, 58, 59].
When the rising chemical drive reaches a certain level that can effectively recruit pharyngeal dilators and open the airway (effective recruitment threshold), [50, 60], airway patency is restored. But because of circulatory lung-carotid delays (chemoreceptors sense normal arterial gases with a hysteresis of two to three respiratory cycles), hyperventilation continues (post-obstruction hyperventilation), resulting in hypocapnia and diminished respiratory drive, thus setting the stage for the next obstructive event. Interestingly, in certain cases post-obstruction hyperventilation could be fairly excessive since both intermittent hypoxia and hypercapnia have been shown to promote long-term sensory facilitation of peripheral and central chemoreceptors in humans [61–63] and may thus disproportionately increase the ventilatory response to hypoxic and/or hypercapnic stimuli. Furthermore, cortical arousals, which frequently occur before, at, or even after airway opening, can also promote ventilatory instability via enhancing post-obstruction hyperventilatory response. Although cortical arousals might be the last resort for restoring airway patency and normalize severe hypoxemia associated with certain apnea/hypopnea events, when they occur prematurely (i.e., long before the respiratory drive reaches effective recruitment threshold), would rather precipitate ventilatory instability and promote apneas [64].
Using data from human experiments, Younes [50••] has demonstrated that, depending on the operation of ventilatory control parameters like chemical responsiveness, effective recruitment threshold, and cortical arousal threshold, subjects with identical anatomical airway narrowing of the airway may develop quite different pathophysiological and polysomnographic disease phenotypes, ranging from stable snoring to hypercapnic OSA. The complex dynamics involved, as well as the large inter-individual variability that characterizes the control of ventilator responses to obstructive events, make it difficult to predict the exact pattern and frequency of airway obstruction in individual patients and may further complicate effective management of the disease [65, 66]. For example, whereas therapies like the administration of oxygen [67] and sedatives [68, 69] may benefit patients with increased chemosensory sensitivity and low arousal thresholds, they could be detrimental for others with decreased ventilatory responses to hypoxia/hypercapnia and high arousal thresholds. In the former, oxygen therapy and sedation may act toward stabilizing ventilatory control, whereas in the latter, the same therapeutic measures could prolong the duration of airway obstruction, potentially leading to severe hypoxemia [70, 71]. Although these patients represent a minority among current OSA populations [71], they might be at a greater risk for opioid-related respiratory events in the postoperative period because they rely heavily on arousal to restore adequate airflow and oxygenation; any sedative agent, including opioids, that could further raise arousal thresholds, has the potential to prolong airway obstruction and precipitate hypoxemia. Conversely, in the majority of OSA patients, where frequent obstructive episodes and mild-to-moderate hypoxemia prevail, the sedative effect of opioids may stabilize airway patency and breathing. Interestingly, this contrast between the two main disease phenotypes (i.e., low versus high arousal thresholds associated with the respective pattern of mild versus severe recurrent hypoxemia) is in agreement with the observed juxtaposition between the high prevalence of OSA in surgical populations and the very low incidence of opioid-related critical respiratory events in postoperative patients; i.e., lending support to the hypothesis that only a small subset of OSA patients are at increased risk for opioid-induced respiratory compromise.
Opioid-Induced Ventilatory Impairment
Opioids interfere with the chemical, behavioral, and motor control of respiration [54, 72]. At analgesic doses, μ-opioid receptor agonists: (a) increase the apneic PaCO2 threshold and suppress ventilatory responses to hypercapnia/hypoxia by acting on the carotid bodies and/or central chemoreceptors located in the brainstem [73••, 74, 75], (b) decrease wakefulness respiratory drive through their action on higher cortical centers [76], and (c) inhibit ventilatory capacity by acting on brainstem motor nuclei, which coordinate the muscular apparatus of breathing, including the diaphragm, chest, and upper airway muscles [77, 78, 79••]. These side effects of opioids could collectively be labeled as “opioid-induced ventilatory impairment” (OIVI), and result in alveolar hypoventilation with potentially severe hypoxemia, which in rare, unrecognized cases can also be fatal [17, 80]. Among the various biological elements that could possibly affect the severity of OIVI, comorbidity burden and other factors that are operational in the postoperative setting, including pain, residual anesthetic effects, and disturbed sleep may also be modifiers of OIVI [81–84, 85 ••, 86 ••, 87].
Retrospective analyses and targeted investigations in large perioperative patient registries suggest that OSA, among other conditions like diabetes [86••], obesity, congestive heart failure, and postoperative renal failure, may present a risk marker for potentially serious or even fatal opioid-related respiratory events that occur in the first 24 h after surgery [11•, 14••] when the effect of opioid analgesics are compounded with the residual effects of general anesthesia [16, 17••, 80, 88]. The observation that fatal outcomes were more likely to occur during night time in patients who were difficult to arouse [11•], although not an indicator of direct association with OSA, reinforces the belief that OSA patients with high arousal thresholds, longer obstructive events, and potentially larger desaturations, may have very low reserve to withstand a serious respiratory event in the postoperative period. Admittedly, because of the scarcity of such events [80, 85••], it is difficult to discern the relative contribution of OSA in the risk for OIVI. Even more so, when obesity, insulin resistance, and diabetes, all frequent comorbidities in patients with OSA [1, 89, 90], have also been associated with altered control of ventilation characterized by a decreased response to hypercapnic/hypoxic challenges, and periodic breathing during sleep [91–93].
Opioids decrease the quantity and alter the architecture of sleep [94•, 95]. Although several other potential factors may disturb sleep in the postoperative patient [96], the effects of opioids may have implications on the severity of OIVI, by modifying the risk-versus-benefit relationship in opioid pharmacology. Opioid analgesics impair basic sleep-wake mechanisms [97••] by inhibiting cholinergic [98, 99], adenosinergic [100, 101], and hypocretinergic [102] transmission. These neurochemical effects of opioids have been causally related to inhibition of rapid eye movement (REM) sleep [103], overall sleep disruption, and decreased sleep consolidation [94] that are implicated in promoting sleepiness and hyperalgesia in humans [104]. As a consequence, opioids, by their effects on sleep-wake mechanisms, have the potential to decrease the safety margin between effective analgesia and ventilatory compromise, especially in patients who are prone to OIVI.
Although the effect of opioids on sleep has not been formally assessed in surgical patients with OSA, the administration of a short-acting opioid during sleep in volunteers with moderate OSA resulted in decreased REM and slow wave sleep, and increased arousals [105•]. Interestingly, the obstructive episodes were decreased, whereas central apneas increased during sleep, a finding probably related to the REM-suppressing effect of the opioid. Theoretically, in certain OSA patients who demonstrate REM-predominant apneas, the obstructive events may recur with increased frequency and severity, during an intense REM sleep rebound after the third postoperative day [106, 107] nonetheless the clinical impact of this phenomenon has yet to be shown [17].
Are OSA patients more sensitive to the ventilatory effects of opioids? Current evidence does not support a straightforward answer to this question. To the extent that opioids could further impact the pathophysiology of airway obstruction leading to ventilatory compromise, these agents may precipitate the development of OIVI in the postoperative patient with OSA. However, based on the large variability and dynamics of OSA [50••, 65], it is unlikely that all disease phenotypes share the same risk for OIVI. In certain patients with severe OSA, overlap syndrome (coexistence of OSA with chronic obstructive pulmonary disease), pulmonary arterial hypertension, or obesity hypoventilation syndrome [67, 108], high arousal thresholds and large arterial desaturations, even the slightest opioid-induced suppression of compensatory mechanisms, including their capacity for arousal during airway obstruction, may lead to severe and protracted hypoxemia. Nevertheless, for the typical OSA patient who demonstrates brief obstructive events with mild desaturations and no daytime blood gas abnormalities, the frequent (cortical) arousals from sleep and increased sensitivity to hypoxia and/or hypercapnia, have been described as rather destabilizing factors, potentially aggravating airway obstruction. In these patients, careful titration of opioids to analgesic effect may in fact stabilize airway function by increasing the arousal threshold and dampening chemical responsiveness.
Mitigating Opioid-Related Respiratory Toxicity
Postoperative hypoxemia is a common side effect of opioids, especially in the immediate postoperative period when patients are still under the residual influence of general anesthetics [85••, 109–111]. Characteristically, in a recent prospective cohort, 15 % of the patients experienced a SaO2 ≤ 85 % for more than a continuous hour during the first 48 h after elective surgery [112]. Not all patients who present with hypoxemia after surgery have OSA [85, 109–111]; on the other hand, not all OSA patients present postoperatively with apneas of increased frequency and nocturnal desaturations of increased severity compared to their preoperative status [113••]. As stated above, based on the high prevalence of OSA among surgical patients [2, 3] in contrast to the very low incidence of respiratory events requiring medical intervention [80, 85••], OSA diagnosis cannot be considered a reliable risk marker for the latter. Although certain OSA phenotypes might inherently carry a risk for drug-induced respiratory compromise, these might be difficult to detect using traditional tests and the development of severe OIVI in the postoperative patient seems to be influenced, among other, largely unknown, parameters, also by age and comorbidity burden. Thus, focusing only on the isolated aspect of sleep-disordered breathing would not be sufficient to predict the overall risk for this complication, even more so when a large fraction of OSA patients remain undiagnosed [114]. Presently, no clinical or laboratory test can outperform clinical judgment in predicting potentially severe respiratory compromise.
Because of the poor risk prediction in this domain, efforts have become more attentive either in minimizing factors that are implicated in precipitating postoperative OIVI, or in detecting the development of OIVI early enough so that an appropriate intervention could prevent an escalation to potentially grave outcomes [17••, 54]. The use of general anesthetics with short elimination half-life as well as implementation of multimodal analgesic techniques have been proposed to reduce sedation and minimize requirement for opioids in the immediate postoperative period. In the same context, drugs or adjuncts that could counteract opioid-induced respiratory depression without interfering with the analgesic effect of opioids have already been tested in humans with promising results [115–118]. Furthermore, agents that could decrease sedation [119, 120] in the postoperative period or provide analgesia without suppressing the function of airway dilator muscles [121] may also be useful adjuncts toward eliminating potentially severe OIVI. Although the development of postoperative hypoxemia in patients with a preoperative diagnosis of OSA cannot be regarded as a surrogate risk marker for severe OIVI, continuous positive airway pressure (CPAP) has been efficacious in reducing hypoxemia related to obstructive events, when applied postoperatively [113••]. Nonetheless, the low patient adherence to CPAP, as well as the ineffectiveness of this therapeutic modality to treat opioid-induced central apneas, may limit its value in preventing serious respiratory events. On the other hand, the use of oxygen in the postoperative patient treated with opioids might also impact our ability to recognize OIVI since a normal SaO2 may mask or enhance [122] central respiratory depression, therefore, obscuring an imminent, potentially severe respiratory event. As noted above, this effect may be especially pronounced in patients with decreased ventilatory responses to hypoxia/hypercapnia stimuli and high arousal thresholds [67]. Oxygen therapy in those patients might further depress respiration [122] since the latter may be critically dependent on the hypoxic ventilator drive.
Various monitoring modalities, based on objective signals and clinical signs, including arterial oxy-hemoglobin saturation by pulse oximetry, respiratory rate, sedation level, and end-tidal or transcutaneous carbon dioxide monitoring, have been proposed for the early detection of OIVI [17••, 54, 87, 111]. However, the efficacy of those methods in eliminating or reducing postoperative OIVI remains to be tested.
Conclusion
Experimental evidence shows that in patients with obstructive sleep apnea (OSA), intermittent hypoxia and sleep fragmentation may affect opioid pharmacology by enhancing pain and/or sensitivity to opioid analgesia. These effects may have a clinical impact on the determination of risk-versus-benefit ratio for opioid analgesia in the postoperative patient, with emphasis on opioid-induced respiratory toxicity. Ventilatory impairment can be a serious adverse effect of opioids, potentially leading to grave outcomes. Current evidence does not support a direct relationship between preoperative diagnosis of OSA and an increased risk for opioid-induced ventilatory impairment (OIVI) during postoperative analgesic therapy. However, certain pathophysiological features of OSA could be compatible with enhancement of OIVI due to either respiratory compromise or suppression of arousal mechanisms in response to airway obstruction. Because the determinants of OIVI are not limited to factors pertaining to the physiology of sleep-disordered breathing but also include age, comorbidity burden, as well as other, so far unknown, parameters, accurate prediction of potentially severe OIVI in the postoperative patient, remains a challenge. Minimizing perioperative opioid analgesia and postoperative sedation, as well as establishing a close monitoring for postoperative patients, may prove beneficial in preventing OIVI without decreasing the quality of provided analgesia.
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Doufas, A.G. Obstructive Sleep Apnea, Pain, and Opioid Analgesia in the Postoperative Patient. Curr Anesthesiol Rep 4, 1–9 (2014). https://doi.org/10.1007/s40140-013-0047-0
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DOI: https://doi.org/10.1007/s40140-013-0047-0