Susceptibility to periodic breathing with assisted ventilation during sleep in normal subjects

Respiratory drive: a journey from health to disease

Respiratory drive is defined as the intensity of respiratory centers output during the breath and is primarily affected by cortical and chemical feedback mechanisms. During the involuntary act of breathing, chemical feedback, primarily mediated through CO2, is the main determinant of respiratory drive. Respiratory drive travels through neural pathways to respiratory muscles, which execute the breathing process and generate inspiratory flow (inspiratory flow-generation pathway). In a healthy state, inspiratory flow-generation pathway is intact, and thus respiratory drive is satisfied by the rate of volume increase, expressed by mean inspiratory flow, which in turn determines tidal volume. In this review, we will explain the pathophysiology of altered respiratory drive by analyzing the respiratory centers response to arterial partial pressure of CO2 (PaCO2) changes. Both high and low respiratory drive have been associated with several adverse effects in critically ill patients. Hence, it is crucial to understand what alters the respiratory drive. Changes in respiratory drive can be explained by simultaneously considering the (1) ventilatory demands, as dictated by respiratory centers activity to CO2 (brain curve); (2) actual ventilatory response to CO2 (ventilation curve); and (3) metabolic hyperbola. During critical illness, multiple mechanisms affect the brain and ventilation curves, as well as metabolic hyperbola, leading to considerable alterations in respiratory drive. In critically ill patients the inspiratory flow-generation pathway is invariably compromised at various levels. Consequently, mean inspiratory flow and tidal volume do not correspond to respiratory drive, and at a given PaCO2, the actual ventilation is less than ventilatory demands, creating a dissociation between brain and ventilation curves. Since the metabolic hyperbola is one of the two variables that determine PaCO2 (the other being the ventilation curve), its upward or downward movements increase or decrease respiratory drive, respectively. Mechanical ventilation indirectly influences respiratory drive by modifying PaCO2 levels through alterations in various parameters of the ventilation curve and metabolic hyperbola. Understanding the diverse factors that modulate respiratory drive at the bedside could enhance clinical assessment and the management of both the patient and the ventilator.

Causes, Consequences, and Treatments of Sleep and Circadian Disruption in the ICU: An Official American Thoracic Society Research Statement

Background: Sleep and circadian disruption (SCD) is common and severe in the ICU. On the basis of rigorous evidence in non-ICU populations and emerging evidence in ICU populations, SCD is likely to have a profound negative impact on patient outcomes. Thus, it is urgent that we establish research priorities to advance understanding of ICU SCD. Methods: We convened a multidisciplinary group with relevant expertise to participate in an American Thoracic Society Workshop. Workshop objectives included identifying ICU SCD subtopics of interest, key knowledge gaps, and research priorities. Members attended remote sessions from March to November 2021. Recorded presentations were prepared and viewed by members before Workshop sessions. Workshop discussion focused on key gaps and related research priorities. The priorities listed herein were selected on the basis of rank as established by a series of anonymous surveys. Results: We identified the following research priorities: establish an ICU SCD definition, further develop rigorous and feasible ICU SCD measures, test associations between ICU SCD domains and outcomes, promote the inclusion of mechanistic and patient-centered outcomes within large clinical studies, leverage implementation science strategies to maximize intervention fidelity and sustainability, and collaborate among investigators to harmonize methods and promote multisite investigation. Conclusions: ICU SCD is a complex and compelling potential target for improving ICU outcomes. Given the influence on all other research priorities, further development of rigorous, feasible ICU SCD measurement is a key next step in advancing the field.

Web Applications for Teaching the Respiratory System: Content Validation

The subject of respiratory mechanics has complex characteristics, functions, and interactions that can be difficult to understand in training and medical education contexts. As such, education strategies based on computational simulations comprise useful tools, but their application in the medical area requires stricter validation processes. This paper shows a statistical and a Delphi validation for two modules of a web application used for respiratory system learning: (I) “Anatomy and Physiology” and (II) “Work of Breathing Indexes”. For statistical validation, population and individual analyses were made using a database of healthy men to compare experimental and model-predicted data. For both modules, the predicted values followed the trend marked by the experimental data in the population analysis, while in the individual analysis, the predicted errors were 9.54% and 25.38% for maximal tidal volume and airflow, respectively, and 6.55%, 9.33%, and 11.77% for rapid shallow breathing index, work of breathing, and maximal inspiratory pressure, respectively. For the Delphi validation, an average higher than 4 was obtained after health professionals evaluated both modules from 1 to 5. In conclusion, both modules are good tools for respiratory system learning processes. The studied parameters behaved consistently with the expressions that describe ventilatory dynamics and were correlated with experimental data; furthermore, they had great acceptance by specialists.

The Effect of Clusters of Double Triggering and Ineffective Efforts in Critically Ill Patients

OBJECTIVES

To characterize clusters of double triggering and ineffective inspiratory efforts throughout mechanical ventilation and investigate their associations with mortality and duration of ICU stay and mechanical ventilation.

DESIGN

Registry-based, real-world study.

BACKGROUND

Asynchronies during invasive mechanical ventilation can occur as isolated events or in clusters and might be related to clinical outcomes.

SUBJECTS

Adults requiring mechanical ventilation greater than 24 hours for whom greater than or equal to 70% of ventilator waveforms were available.

INTERVENTIONS

We identified clusters of double triggering and ineffective inspiratory efforts and determined their power and duration. We used Fine-Gray's competing risk model to analyze their effects on mortality and generalized linear models to analyze their effects on duration of mechanical ventilation and ICU stay.

MEASUREMENTS AND MAIN RESULTS

We analyzed 58,625,796 breaths from 180 patients. All patients had clusters (mean/d, 8.2 [5.4-10.6]; mean power, 54.5 [29.6-111.4]; mean duration, 20.3 min [12.2-34.9 min]). Clusters were less frequent during the first 48 hours (5.5 [2.5-10] vs 7.6 [4.4-9.9] in the remaining period [p = 0.027]). Total number of clusters/d was positively associated with the probability of being discharged alive considering the total period of mechanical ventilation (p = 0.001). Power and duration were similar in the two periods. Power was associated with the probability of being discharged dead (p = 0.03), longer mechanical ventilation (p < 0.001), and longer ICU stay (p = 0.035); cluster duration was associated with longer ICU stay (p = 0.027).

CONCLUSIONS

Clusters of double triggering and ineffective inspiratory efforts are common. Although higher numbers of clusters might indicate better chances of survival, clusters with greater power and duration indicate a risk of worse clinical outcomes.

Central sleep apnea

Central apnea syndrome is a disorder with protean manifestations and concomitant conditions. It can occur as a distinct clinical entity or as part of another clinical syndrome. The pathogenesis of central sleep apnea (CSA) varies depending on the clinical condition. Sleep-related withdrawal of the ventilatory drive to breathe is the common denominator among all cases of central apnea, whereas hypocapnia is the final common pathway leading to apnea in the majority of central apnea. Medical conditions most closely associated with CSA include heart failure, stroke, spinal cord injury, and opioid use, among others. Nocturnal polysomnography is the standard diagnostic method, including measurement of sleep and respiration. The latter includes detection of flow, measurement of oxyhemoglobin saturation and detection of respiratory effort. Management strategy incorporates clinical presentation, associated conditions, and the polysomnographic findings in an individualized manner. The pathophysiologic heterogeneity may explain the protean clinical manifestations and the lack of a single effective therapy for all patients. While research has enhanced our understanding of the pathogenesis of central apnea, treatment options are extrapolated from treatment of obstructive sleep apnea. Co-morbid conditions and concomitant obstructive sleep apnea influence therapeutic approach significantly. Therapeutic options include positive pressure therapy, pharmacologic therapy, and supplemental Oxygen. Continuous positive airway pressure (CPAP) is the initial standard of care, although the utility of other modes of positive pressure therapy, as well as pharmacotherapy and device-based therapies, are currently being investigated.

[Central control malfunction in Wolfram syndrome: Report of one case of difficult weaning]

INTRODUCTION

Wolfram syndrome is a rare autosomal recessive genetic disease characterized by diabetes insipidus, diabetes mellitus, optic atrophy and deafness. A neurodegenerative syndrome is usually associated, including cerebellar ataxia. A few cases of central apnea have been reported in the literature. Here, we report a case of Wolfram syndrome with respiratory symptoms that led to the discovery of central apneas as well as complicated and delayed weaning in an intensive care unit (ICU).

OBSERVATION

The patient is a 39-year-old woman diagnosed with Wolfram syndrome who was admitted to an ICU for septic shock. She experienced difficult weaning before central apneas were observed while spontaneous ventilation was being attempted. After two extubation failures, cerebral MRI was performed and revealed parenchymatous atrophy of the posterior brain fossa involving the cerebral trunk, cerebellar peduncles, as well as both cerebellum hemispheres and the cerebellar vermis. Even after the patient was tracheotomized, central apneas persisted when the patient breathed spontaneously with her tracheotomy, necessitating nocturnal ventilation.

CONCLUSION

While central apneas Wolfram syndrome remain rare, they should be systematically investigated due to their association with severe morbimortality.

COVID-19 ARDS: Points to Be Considered in Mechanical Ventilation and Weaning

The COVID-19 disease can cause hypoxemic respiratory failure due to ARDS, requiring invasive mechanical ventilation. Although early studies reported that COVID-19-associated ARDS has distinctive features from ARDS of other causes, recent observational studies have demonstrated that ARDS related to COVID-19 shares common clinical characteristics and respiratory system mechanics with ARDS of other origins. Therefore, mechanical ventilation in these patients should be based on strategies aiming to mitigate ventilator-induced lung injury. Assisted mechanical ventilation should be applied early in the course of mechanical ventilation by considering evaluation and minimizing factors associated with patient-inflicted lung injury. Extracorporeal membrane oxygenation should be considered in selected patients with refractory hypoxia not responding to conventional ventilation strategies. This review highlights the current and evolving practice in managing mechanically ventilated patients with ARDS related to COVID-19.

The central nervous system during lung injury and mechanical ventilation: a narrative review

Mechanical ventilation induces a number of systemic responses for which the brain plays an essential role. During the last decade, substantial evidence has emerged showing that the brain modifies pulmonary responses to physical and biological stimuli by various mechanisms, including the modulation of neuroinflammatory reflexes and the onset of abnormal breathing patterns. Afferent signals and circulating factors from injured peripheral tissues, including the lung, can induce neuronal reprogramming, potentially contributing to neurocognitive dysfunction and psychological alterations seen in critically ill patients. These impairments are ubiquitous in the presence of positive pressure ventilation. This narrative review summarises current evidence of lung-brain crosstalk in patients receiving mechanical ventilation and describes the clinical implications of this crosstalk. Further, it proposes directions for future research ranging from identifying mechanisms of multiorgan failure to mitigating long-term sequelae after critical illness.

Techniques to monitor respiratory drive and inspiratory effort

PURPOSE OF REVIEW

There is increased awareness that derangements of respiratory drive and inspiratory effort are frequent and can result in lung and diaphragm injury together with dyspnea and sleep disturbances. This review aims to describe available techniques to monitor drive and effort.

RECENT FINDINGS

Measuring drive and effort is necessary to quantify risk and implement strategies to minimize lung and the diaphragm injury by modifying sedation and ventilation. Evidence on the efficacy of such strategies is yet to be elucidated, but physiological and epidemiological data support the need to avoid injurious patterns of breathing effort.Some techniques have been used in research for decades (e.g., esophageal pressure or airway occlusion pressure), evidence on their practical utility is growing, and technical advances have eased implementation. More novel techniques (e.g., electrical activity of the diaphragm and ultrasound) are being investigated providing new insights on their use and interpretation.

SUMMARY

Available techniques provide reliable measures of the intensity and timing of drive and effort. Simple, noninvasive techniques might be implemented in most patients and the more invasive or time-consuming in more complex patients at higher risk. We encourage clinicians to become familiar with technical details and physiological rationale of each for optimal implementation.

NAVA and PAV+ for lung and diaphragm protection

PURPOSE OF REVIEW

Complications of mechanical ventilation, such as ventilator-induced lung injury (VILI) and ventilator-induced diaphragmatic dysfunction (VIDD), adversely affect the outcome of critically ill patients. Although mostly studied during control ventilation, it is increasingly appreciated that VILI and VIDD also occur during assisted ventilation. Hence, current research focuses on identifying ways to monitor and deliver protective ventilation in assisted modes. This review describes the operating principles of proportional modes of assist, their implications for lung and diaphragm protective ventilation, and the supporting clinical data.

RECENT FINDINGS

Proportional modes of assist, proportional assist ventilation, PAV, and neurally adjusted ventilatory assist, NAVA, deliver a pressure assist that is proportional to the patient's effort, enabling ventilation to be better controlled by the patient's brain. This control underlies the potential of proportional modes to avoid over-assist and under-assist, improve patient--ventilator interaction, and provide protective ventilation. Indeed, in clinical studies, proportional modes have been associated with reduced asynchronies, enhanced diaphragmatic recovery, and limitation of excessive tidal volume. Additionally, proportional modes facilitate better monitoring of the delivery of protective assisted ventilation.

SUMMARY

Physiological rationale and clinical data suggest a potential role for proportional modes of assist in providing and monitoring lung and diaphragm protective ventilation.

Abnormal Sleep, Circadian Rhythm Disruption, and Delirium in the ICU: Are They Related?

Delirium is a syndrome characterized by acute brain failure resulting in neurocognitive disturbances affecting attention, awareness, and cognition. It is highly prevalent among critically ill patients and is associated with increased morbidity and mortality. A core domain of delirium is represented by behavioral disturbances in sleep-wake cycle probably related to circadian rhythm disruption. The relationship between sleep, circadian rhythm and intensive care unit (ICU)-acquired delirium is complex and likely bidirectional. In this review, we explore the proposed pathophysiological mechanisms of sleep disruption and circadian dysrhythmia as possible contributing factors in transitioning to delirium in the ICU and highlight some of the most relevant caveats for understanding the relationship between these complex phenomena. Specifically, we will (1) review the physiological consequences of poor sleep quality and efficiency; (2) explore how the neural substrate underlying the circadian clock functions may be disrupted in delirium; (3) discuss the role of sedative drugs as contributors to delirium and chrono-disruption; and, (4) describe the association between abnormal sleep-pathological wakefulness, circadian dysrhythmia, delirium and critical illness. Opportunities to improve sleep and readjust circadian rhythmicity to realign the circadian clock may exist as therapeutic targets in both the prevention and treatment of delirium in the ICU. Further research is required to better define these conditions and understand the underlying physiologic relationship to develop effective prevention and therapeutic strategies.

Nocturnal bilevel positive airway pressure for the treatment of asthma

Nocturnal worsening of asthma may be due to reduced lung volumes and fewer sigh breaths, which have been shown to increase airway resistance and bronchoreactivity. We hypothesized that mimicking deep inspiration using nocturnal mechanical support would improve symptoms in patients with asthma. Subjects with asthma underwent usual care and bilevel positive airway pressure (PAP) therapy for 4 weeks, separated by 4 weeks, and methacholine challenge (PC20) and subjective assessments. 13 patients with asthma alone and 8 with asthma + OSA completed the protocol. Change in bronchoreactivity (ratio of Post/Pre PC20) was not significantly different during usual care and bilevel PAP [0.86 (IQR 0.19, 1.82) vs 0.94 (IQR 0.56, 2.5), p = 0.88], nor was the change in Asthma Control Test different: 0.1 ± 2.2 vs. -0.2 ± 2.9, p = 0.79, respectively. Bilevel PAP therapy for four weeks did not improve subjective or objective measures of asthma severity in patients with asthma or those with asthma and OSA, although there was heterogeneity in response.

A physiology-based mathematical model for the selection of appropriate ventilator controls for lung and diaphragm protection

Mechanical ventilation is used to sustain respiratory function in patients with acute respiratory failure. To aid clinicians in consistently selecting lung- and diaphragm-protective ventilation settings, a physiology-based decision support system is needed. To form the foundation of such a system, a comprehensive physiological model which captures the dynamics of ventilation has been developed. The Lung and Diaphragm Protective Ventilation (LDPV) model centers around respiratory drive and incorporates respiratory system mechanics, ventilator mechanics, and blood acid–base balance. The model uses patient-specific parameters as inputs and outputs predictions of a patient’s transpulmonary and esophageal driving pressures (outputs most clinically relevant to lung and diaphragm safety), as well as their blood pH, under various ventilator and sedation conditions. Model simulations and global optimization techniques were used to evaluate and characterize the model. The LDPV model is demonstrated to describe a CO₂ respiratory response that is comparable to what is found in literature. Sensitivity analysis of the model indicate that the ventilator and sedation settings incorporated in the model have a significant impact on the target output parameters. Finally, the model is seen to be able to provide robust predictions of esophageal pressure, transpulmonary pressure and blood pH for patient parameters with realistic variability. The LDPV model is a robust physiological model which produces outputs which directly target and reflect the risk of ventilator-induced lung and diaphragm injury. Ventilation and sedation parameters are seen to modulate the model outputs in accordance with what is currently known in literature.

The Injurious Effects of Elevated or Nonelevated Respiratory Rate during Mechanical Ventilation

Respiratory rate is one of the key variables that is set and monitored during mechanical ventilation. As part of increasing efforts to optimize mechanical ventilation, it is prudent to expand understanding of the potential harmful effects of not only volume and pressures but also respiratory rate. The mechanisms by which respiratory rate may become injurious during mechanical ventilation can be distinguished in two broad categories. In the first, well-recognized category, concerning both controlled and assisted ventilation, the respiratory rate per se may promote ventilator-induced lung injury, dynamic hyperinflation, ineffective efforts, and respiratory alkalosis. It may also be misinterpreted as distress delaying the weaning process. In the second category, which concerns only assisted ventilation, the respiratory rate may induce injury in a less apparent way by remaining relatively quiescent while being challenged by chemical feedback. By responding minimally to chemical feedback, respiratory rate leaves the control of V. e almost exclusively to inspiratory effort. In such cases, when assist is high, weak inspiratory efforts promote ineffective triggering, periodic breathing, and diaphragmatic atrophy. Conversely, when assist is low, diaphragmatic efforts are intense and increase the risk for respiratory distress, asynchronies, ventilator-induced lung injury, diaphragmatic injury, and cardiovascular complications. This review thoroughly presents the multiple mechanisms by which respiratory rate may induce injury during mechanical ventilation, drawing the attention of critical care physicians to the potential injurious effects of respiratory rate insensitivity to chemical feedback during assisted ventilation.

Monitoring patient-ventilator asynchrony

PURPOSE OF REVIEW

This article describes and discusses the importance of monitoring patient-ventilator asynchrony, and the advantages and limitations of the specific techniques available at the bedside to evaluate it.

RECENT FINDINGS

Signals provided by esophageal catheters (pressure or electromyogram) are the most reliable and accurate instruments to detect asynchronies. Esophageal signals (providing electrical activity of the diaphragm or/and esophageal pressure) have allowed the recent description of reverse triggering, a new kind of asynchrony, in which mechanical insufflation repeatedly triggers diaphragmatic contractions. However, the use of esophageal catheters is not widespread, and data on the prevalence and consequences of asynchronies are still scarce. The development of software solutions that continuously and automatically record breathing waveforms from the ventilator recording is emerging. Using this technology, recent data support the fact that asynchronies are frequent and may be negatively associated with outcome.

SUMMARY

The prevalence and consequences of asynchronies may be largely underestimated because of a frequent lack of monitoring. Dedicated software solutions that continuously and automatically detect asynchronies may allow both clinical research and clinical applications aimed at determining the effects of asynchronies and minimizing their incidence among critically ill patients.

Patient-Ventilator Dyssynchrony

In mechanically ventilated patients, assisted mechanical ventilation (MV) is employed early, following the acute phase of critical illness, in order to eliminate the detrimental effects of controlled MV, most notably the development of ventilator-induced diaphragmatic dysfunction. Nevertheless, the benefits of assisted MV are often counteracted by the development of patient-ventilator dyssynchrony. Patient-ventilator dyssynchrony occurs when either the initiation and/or termination of mechanical breath is not in time agreement with the initiation and termination of neural inspiration, respectively, or if the magnitude of mechanical assist does not respond to the patient's respiratory demand. As patient-ventilator dyssynchrony has been associated with several adverse effects and can adversely influence patient outcome, every effort should be made to recognize and correct this occurrence at bedside. To detect patient-ventilator dyssynchronies, the physician should assess patient comfort and carefully inspect the pressure- and flow-time waveforms, available on the ventilator screen of all modern ventilators. Modern ventilators offer several modifiable settings to improve patient-ventilator interaction. New proportional modes of ventilation are also very helpful in improving patient-ventilator interaction.

Congestive Heart Failure and Central Sleep Apnea

Congestive heart failure (CHF) is among the most common causes of admission to hospitals in the United States, especially in those over age 65. Few data exist regarding the prevalence CHF of Cheyne-Stokes respiration (CSR) owing to congestive heart failure in the intensive care unit (ICU). Nevertheless, CSR is expected to be highly prevalent among those with CHF. Treatment should focus on the underlying mechanisms by which CHF increases loop gain and promotes unstable breathing. Few data are available to determine prevalence of CSR in the ICU, or how CSR might affect clinical management and weaning from mechanical ventilation.

Heart failure and sleep disorders

Awareness of the importance of sleep-related disorders in patients with cardiovascular diseases is growing. In particular, sleep-disordered breathing, short sleep time, and low sleep quality are frequently reported by patients with heart failure (HF). Sleep-disordered breathing, which includes obstructive sleep apnoea (OSA) and central sleep apnoea (CSA), is common in patients with HF and has been suggested to increase the morbidity and mortality in these patients. Both OSA and CSA are associated with increased sympathetic activation, vagal withdrawal, altered haemodynamic loading conditions, and hypoxaemia. Moreover, OSA is strongly associated with arterial hypertension, the most common risk factor for cardiac hypertrophy and failure. Intrathoracic pressure changes are also associated with OSA, contributing to haemodynamic alterations and potentially affecting overexpression of genes involved in ventricular remodelling. HF treatment can decrease the severity of both OSA and CSA. Indeed, furosemide and spironolactone administration, exercise training, cardiac resynchronization therapy, and eventually heart transplantation have shown a positive effect on OSA and CSA in patients with HF. At present, whether CSA should be treated and, if so, which is the optimal therapy is still debated. By contrast, more evidence is available on the beneficial effects of OSA treatment in patients with HF.

Positive and negative effects of mechanical ventilation on sleep in the ICU: a review with clinical recommendations

PURPOSE

Sleep is an essential physiologic process that helps to restore normal body homeostasis. Sleep disturbances have been shown to be associated with poor clinical outcomes, such as a greater risk of cardiovascular disease and increasing mortality. Critically ill patients, particularly those receiving mechanical ventilation, may be more susceptible to sleep disruption.

METHODS AND RESULTS

Mechanical ventilation is an important factor influencing sleep in critically ill patients as it may have positive or negative effects, depending on patient population, mode, and specific settings. Other causes of sleep disruption include the acute illness itself, the daily routine care, and the effects of medications. Improving sleep in patients admitted to an intensive care unit has the potential to improve both short- and long-term clinical outcomes. In this article we review the specific aspects of sleep in critically ill mechanically ventilated patients, including abnormal sleep patterns and loss of circadian rhythm, as well as the effects of mechanical ventilation and intravenous sedatives on sleep quality and quantity.

CONCLUSIONS

We provide recommendations for clinicians regarding optimal ventilatory settings and discuss fields for future research.

Congestive heart failure and central sleep apnea

Congestive heart failure (CHF) is among the most common causes of admission to hospitals in the United States, especially in those over age 65. Few data exist regarding the prevalence CHF of Cheyne-Stokes respiration (CSR) owing to congestive heart failure in the intensive care unit (ICU). Nevertheless, CSR is expected to be highly prevalent among those with CHF. Treatment should focus on the underlying mechanisms by which CHF increases loop gain and promotes unstable breathing. Few data are available to determine prevalence of CSR in the ICU, or how CSR might affect clinical management and weaning from mechanical ventilation.

Effect of sedative-hypnotics, anesthetics and analgesics on sleep architecture in obstructive sleep apnea

The perioperative care of obstructive sleep apnea (OSA) patients is currently receiving much attention due to an increased risk for complications. It is established that postoperative changes in sleep architecture occur and this may have pathophysiological implications for OSA patients. Upper airway muscle activity decreases during rapid eye movement sleep (REMS). Severe OSA patients exhibit exaggerated chemoreceptor-driven ventilation during non-rapid eye movement sleep (NREMS), which leads to central and obstructive apnea. This article critically reviewed the literature relevant to preoperative screening for OSA, prevalence of OSA in surgical populations and changes in postoperative sleep architecture relevant to OSA patients. In particular, we addressed three questions in regard to the effects of sedative-hypnotics, anesthetics and analgesics on sleep architecture, the underlying mechanisms and the relevance to OSA. Indeed, these classes of drugs alter sleep architecture, which likely significantly contributes to abnormal postoperative sleep architecture, exacerbation of OSA and postoperative complications.

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.

Ventilatory failure, ventilator support, and ventilator weaning

The development of acute ventilatory failure represents an inability of the respiratory control system to maintain a level of respiratory motor output to cope with the metabolic demands of the body. The level of respiratory motor output is also the main determinant of the degree of respiratory distress experienced by such patients. As ventilatory failure progresses and patient distress increases, mechanical ventilation is instituted to help the respiratory muscles cope with the heightened workload. While a patient is connected to a ventilator, a physician's ability to align the rhythm of the machine with the rhythm of the patient's respiratory centers becomes the primary determinant of the level of rest accorded to the respiratory muscles. Problems of alignment are manifested as failure to trigger, double triggering, an inflationary gas-flow that fails to match inspiratory demands, and an inflation phase that persists after a patient's respiratory centers have switched to expiration. With recovery from disorders that precipitated the initial bout of acute ventilatory failure, attempts are made to discontinue the ventilator (weaning). About 20% of weaning attempts fail, ultimately, because the respiratory controller is unable to sustain ventilation and this failure is signaled by development of rapid shallow breathing. Substantial advances in the medical management of acute ventilatory failure that requires ventilator assistance are most likely to result from research yielding novel insights into the operation of the respiratory control system.

Central sleep apnea

Neurophysiologically, central apnea is due to a temporary failure in the pontomedullary pacemaker generating breathing rhythm. As a polysomnographic finding, central apneas occur in many pathophysiological conditions. Depending on the cause or mechanism, central apneas may not be clinically significant, for example, those that occur normally at sleep onset. In contrast, central apneas occur in a number of disorders and result in pathophysiological consequences. Central apneas occur commonly in high-altitude sojourn, disrupt sleep, and cause desaturation. Central sleep apnea also occurs in number of disorders across all age groups and both genders. Common causes of central sleep apnea in adults are congestive heart failure and chronic use of opioids to treat pain. Under such circumstances, diagnosis and treatment of central sleep apnea may improve quality of life, morbidity, and perhaps mortality. The mechanisms of central sleep apnea have been best studied in congestive heart failure and hypoxic conditions when there is increased CO2 sensitivity below eupnea resulting in lowering eupneic PCO2 below apneic threshold causing cessation of breathing until the PCO2 rises above the apneic threshold when breathing resumes. In many other disorders, the mechanism of central sleep apnea (CSA) remains to be investigated.

Obstructive sleep apnea

Obstructive sleep apnea (OSA) is a common disorder characterized by repetitive collapse of the pharyngeal airway during sleep. Control of pharyngeal patency is a complex process relating primarily to basic anatomy and the activity of many pharyngeal dilator muscles. The control of these muscles is regulated by a number of processes including respiratory drive, negative pressure reflexes, and state (sleep) effects. In general, patients with OSA have an anatomically small airway the patency of which is maintained during wakefulness by reflex-driven augmented dilator muscle activation. At sleep onset, muscle activity falls, thereby compromising the upper airway. However, recent data suggest that the mechanism of OSA differs substantially among patients, with variable contributions from several physiologic characteristics including, among others: level of upper airway dilator muscle activation required to open the airway, increase in chemical drive required to recruit the pharyngeal muscles, chemical control loop gain, and arousal threshold. Thus, the cause of sleep apnea likely varies substantially between patients. Other physiologic mechanisms likely contributing to OSA pathogenesis include falling lung volume during sleep, shifts in blood volume from peripheral tissues to the neck, and airway edema. Apnea severity may progress over time, likely due to weight gain, muscle/nerve injury, aging effects on airway anatomy/collapsibility, and changes in ventilatory control stability.

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.

Complex sleep apnea syndrome

Complex sleep apnea syndrome (CompSAS) is a distinct form of sleep-disordered breathing characterized as central sleep apnea (CSA), and presents in obstructive sleep apnea (OSA) patients during initial treatment with a continuous positive airway pressure (CPAP) device. The mechanisms of why CompSAS occurs are not well understood, though we have a high loop gain theory that may help to explain it. It is still controversial regarding the prevalence and the clinical significance of CompSAS. Patients with CompSAS have clinical features similar to OSA, but they do exhibit breathing patterns like CSA. In most CompSAS cases, CSA events during initial CPAP titration are transient and they may disappear after continued CPAP use for 4~8 weeks or even longer. However, the poor initial experience of CompSAS patients with CPAP may not be avoided, and nonadherence with continued therapy may often result. Treatment options like adaptive servo-ventilation are available now that may rapidly resolve the disorder and relieve the symptoms of this disease with the potential of increasing early adherence to therapy. But these approaches are associated with more expensive and complicated devices. In this review, the definition, potential plausible mechanisms, clinical characteristics, and treatment approaches of CompSAS will be summarized.

The effect of lung stretch during sleep on airway mechanics in overweight and obese asthma

Both obesity and sleep reduce lung volume and limit deep breaths, possibly contributing to asthma. We hypothesize that increasing lung volume dynamically during sleep would reduce airway resistance in asthma. Asthma (n=10) and control (n=10) subjects were studied during sleep at baseline and with increased lung volume via bi-level positive airway pressure (BPAP). Using forced oscillations, respiratory system resistance (R(rs)) and reactance (X(rs)) were measured during sleep and R(rs) was partitioned to upper and lower airway resistance (R(up), R(low)) using an epiglottic pressure catheter. R(rs) and R(up) increased with sleep (p<0.01) and X(rs) was decreased in REM (p=0.02) as compared to wake. R(rs), R(up), and R(low), were larger (p<0.01) and X(rs) was decreased (p<0.02) in asthma. On BPAP, R(rs) and R(up) were decreased (p<0.001) and X(rs) increased (p<0.01), but R(low) was unchanged. High R(up) was observed in asthma, which reduced with BPAP. We conclude that the upper airway is a major component of R(rs) and larger lung volume changes may be required to alter R(low).

A method for measuring and modeling the physiological traits causing obstructive sleep apnea

There is not a clinically available technique for measuring the physiological traits causing obstructive sleep apnea (OSA). Therefore, it is often difficult to determine why an individual has OSA or to what extent the various traits contribute to the development of OSA. In this study, we present a noninvasive method for measuring four important physiological traits causing OSA: 1) pharyngeal anatomy/collapsibility, 2) ventilatory control system gain (loop gain), 3) the ability of the upper airway to dilate/stiffen in response to an increase in ventilatory drive, and 4) arousal threshold. These variables are measured using a single maneuver in which continuous positive airway pressure (CPAP) is dropped from an optimum to various suboptimum pressures for 3- to 5-min intervals during sleep. Each individual's set of traits is entered into a physiological model of OSA that graphically illustrates the relative importance of each trait in that individual. Results from 14 subjects (10 with OSA) are described. Repeatability measurements from separate nights are also presented for four subjects. The measurements and model illustrate the multifactorial nature of OSA pathogenesis and how, in some individuals, small adjustments of one or another trait (which might be achievable with non-CPAP agents) could potentially treat OSA. This technique could conceivably be used clinically to define a patient's physiology and guide therapy based on the traits.

Control of breathing during mechanical ventilation: who is the boss?

Over the past decade, concepts of control of breathing have increasingly moved from being theoretical concepts to "real world" applied science. The purpose of this review is to examine the basics of control of breathing, discuss the bidirectional relationship between control of breathing and mechanical ventilation, and critically assess the application of this knowledge at the patient's bedside. The principles of control of breathing remain under-represented in the training curriculum of respiratory therapists and pulmonologists, whereas the day-to-day bedside application of the principles of control of breathing continues to suffer from a lack of outcomes-based research in the intensive care unit. In contrast, the bedside application of the principles of control of breathing to ambulatory subjects with sleep-disordered breathing has out-stripped that in critically ill patients. The evolution of newer technologies, faster real-time computing abilities, and miniaturization of ventilator technology can bring the concepts of control of breathing to the bedside and benefit the critically ill patient. However, market forces, lack of scientific data, lack of research funding, and regulatory obstacles need to be surmounted.

An interdependent model of central/peripheral chemoreception: evidence and implications for ventilatory control

In this review we discuss the implications for ventilatory control of newer evidence suggesting that central and peripheral chemoreceptors are not functionally separate but rather that they are dependent upon one another such that the sensitivity of the medullary chemoreceptors is critically determined by input from the carotid body chemoreceptors and vice versa i.e., they are interdependent. We examine potential interactions of the interdependent central and carotid body (CB) chemoreceptors with other ventilatory-related inputs such as central hypoxia, lung stretch, and exercise. The limitations of current approaches addressing this question are discussed and future studies are suggested.

Effect of episodic hypoxia on the susceptibility to hypocapnic central apnea during NREM sleep

We hypothesized that episodic hypoxia (EH) leads to alterations in chemoreflex characteristics that might promote the development of central apnea in sleeping humans. We used nasal noninvasive positive pressure mechanical ventilation to induce hypocapnic central apnea in 11 healthy participants during stable nonrapid eye movement sleep before and after an exposure to EH, which consisted of fifteen 1-min episodes of isocapnic hypoxia (mean O(2) saturation/episode: 87.0 +/- 0.5%). The apneic threshold (AT) was defined as the absolute measured end-tidal PCO(2) (Pet(CO(2))) demarcating the central apnea. The difference between the AT and baseline Pet(CO(2)) measured immediately before the onset of mechanical ventilation was defined as the CO(2) reserve. The change in minute ventilation (V(I)) for a change in Pet(CO(2)) (DeltaV(I)/ DeltaPet(CO(2))) was defined as the hypocapnic ventilatory response. We studied the eupneic Pet(CO(2)), AT Pet(CO(2)), CO(2) reserve, and hypocapnic ventilatory response before and after the exposure to EH. We also measured the hypoxic ventilatory response, defined as the change in V(I) for a corresponding change in arterial O(2) saturation (DeltaV(I)/DeltaSa(O(2))) during the EH trials. V(I) increased from 6.2 +/- 0.4 l/min during the pre-EH control to 7.9 +/- 0.5 l/min during EH and remained elevated at 6.7 +/- 0.4 l/min the during post-EH recovery period (P < 0.05), indicative of long-term facilitation. The AT was unchanged after EH, but the CO(2) reserve declined significantly from -3.1 +/- 0.5 mmHg pre-EH to -2.3 +/- 0.4 mmHg post-EH (P < 0.001). In the post-EH recovery period, DeltaV(I)/DeltaPet(CO(2)) was higher compared with the baseline (3.3 +/- 0.6 vs. 1.8 +/- 0.3 l x min(-1) x mmHg(-1), P < 0.001), indicative of an increased hypocapnic ventilatory response. However, there was no significant change in the hypoxic ventilatory response (DeltaV(I)/DeltaSa(O(2))) during the EH period itself. In conclusion, despite the presence of ventilatory long-term facilitation, the increase in the hypocapnic ventilatory response after the exposure to EH induced a significant decrease in the CO(2) reserve. This form of respiratory plasticity may destabilize breathing and promote central apneas.

Pathophysiology of sleep apnea

Sleep-induced apnea and disordered breathing refers to intermittent, cyclical cessations or reductions of airflow, with or without obstructions of the upper airway (OSA). In the presence of an anatomically compromised, collapsible airway, the sleep-induced loss of compensatory tonic input to the upper airway dilator muscle motor neurons leads to collapse of the pharyngeal airway. In turn, the ability of the sleeping subject to compensate for this airway obstruction will determine the degree of cycling of these events. Several of the classic neurotransmitters and a growing list of neuromodulators have now been identified that contribute to neurochemical regulation of pharyngeal motor neuron activity and airway patency. Limited progress has been made in developing pharmacotherapies with acceptable specificity for the treatment of sleep-induced airway obstruction. We review three types of major long-term sequelae to severe OSA that have been assessed in humans through use of continuous positive airway pressure (CPAP) treatment and in animal models via long-term intermittent hypoxemia (IH): 1) cardiovascular. The evidence is strongest to support daytime systemic hypertension as a consequence of severe OSA, with less conclusive effects on pulmonary hypertension, stroke, coronary artery disease, and cardiac arrhythmias. The underlying mechanisms mediating hypertension include enhanced chemoreceptor sensitivity causing excessive daytime sympathetic vasoconstrictor activity, combined with overproduction of superoxide ion and inflammatory effects on resistance vessels. 2) Insulin sensitivity and homeostasis of glucose regulation are negatively impacted by both intermittent hypoxemia and sleep disruption, but whether these influences of OSA are sufficient, independent of obesity, to contribute significantly to the "metabolic syndrome" remains unsettled. 3) Neurocognitive effects include daytime sleepiness and impaired memory and concentration. These effects reflect hypoxic-induced "neural injury." We discuss future research into understanding the pathophysiology of sleep apnea as a basis for uncovering newer forms of treatment of both the ventilatory disorder and its multiple sequelae.

Effect of air leak on the performance of auto-PAP devices: a bench study

Automatic positive airway pressure (auto-PAP) devices, used in the treatment of patients with obstructive sleep apnea (OSA), may not function optimally in the presence of an air leak. We set out to determine the magnitude of air leak that prevents auto-PAP devices from responding to respiratory events of OSA in a bench model. We simulated apnea, flow limitation, obstructive hypopnea, nonobstructive hypopnea, and snoring events of OSA with an artificial airway and a loudspeaker in a bench model connected to a commercially available auto-PAP device. Four auto-PAP devices were tested, but two of the "older-generation" devices (Tranquility and Virtuoso) did not respond to events of OSA that involved changes in flow contour; hence, we studied the effects of air leak and humidifier in the two "newer-generation" auto-PAP devices only (GoodKnight 418P, Autoset-T). When the air leak was progressively increased from baseline levels recommended by the manufacturer to levels seen clinically--5 to 7, 10, and 30 l/min--the GoodKnight 418P decreased pressure response by 56% (5.6+/-1.8 cm H(2)O, p=0.04). The pressure response of the Autoset-T, however, did not change from baseline during similar levels of air leak. The GoodKnight performed appropriately when the air leak was within 20 l/min, but the corresponding value for the Autoset was higher at 50 l/min. For both devices combined, air leak caused the pressure to drop between the device and the airway: 2.8+/-0.3 cm H(2)O at 30 l/min of air leak (p<0.001). Air leaks cause auto-PAP devices to underestimate the pressure required to treat events of OSA and to overestimate the pressure delivered at the upper airway. Physicians should be aware of performance limitations of auto-PAP devices in the presence of an air leak.

Mechanisms of apnea

This paper focuses on the underlying mechanisms contributing to sleep-disordered breathing. Obstructive sleep apnea (OSA) is the most common sleep-related breathing disorder and is characterized by repetitive narrowing or collapse of the pharyngeal airway during sleep. Conversely, central sleep apnea (CSA), highly prevalent in congestive heart failure, is distinguished by a lack of drive to breathe during sleep, resulting in repetitive periods of insufficient ventilation. Both lead to compromised gas exchange, impaired sleep continuity, and catecholamine surges and are associated with major comorbidities including excessive daytime sleepiness and increased risk of cardiovascular disease. Although OSA and CSA exist on a spectrum of sleep-disordered breathing, the 2 entities may overlap in their underlying pathophysiologies. This brief review summarizes the etiology and current understanding of OSA and CSA pathophysiology and the role that the cardiovascular system may play in contributing to disease pathology and highlights the likely substantial overlap that exists between the various forms of sleep-disordered breathing.