Effects of slow wave sleep on ventilatory compensation to inspiratory elastic loading

Central Sleep Apnea in Kidney Disease

Sleep is an essential function of life and serves a crucial role in the promotion of health and performance. Poor sleep quality and sleep disorders have been a recurrent finding in patients with chronic kidney disease (CKD). Sleep disorders such as obstructive sleep apnea (OSA) can contribute to hypertension, diabetes, cardiovascular disease, and worsen obesity, all of which are implicated in the etiology of CKD, but CKD itself may lead to OSA. Relationships between CKD/end-stage renal disease (ESRD) and OSA have been the subject of numerous investigations, but central sleep apnea (CSA) also is highly prevalent in CKD/ESRD but remains poorly understood, underdiagnosed, and undertreated in these patients. Emerging literature has implicated CSA as another contributor to morbidity and mortality in CKD/ESRD, and several studies have suggested that CSA treatment is beneficial in improving these outcomes. Patients with CKD/ESRD co-existing with congestive heart failure are particularly prone to CSA, and studies focused on managing CSA in congestive heart failure patients have provided important information concerning how best to manage CSA in kidney disease as well. Adaptive servo-ventilation ultimately may represent the treatment of choice in these patients, although a stepped approach using a variety of therapeutic modalities is recommended.

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.

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.

Abdominal compression increases upper airway collapsibility during sleep in obese male obstructive sleep apnea patients

STUDY OBJECTIVES

Abdominal obesity, particularly common in centrally obese males, may have a negative impact on upper airway (UA) function during sleep. For example, cranial displacement of the diaphragm with raised intra-abdominal pressure may reduce axial tension exerted on the UA by intrathoracic structures and increase UA collapsibility during sleep.

DESIGN

This study aimed to examine the effect of abdominal compression on UA function during sleep in obese male obstructive sleep apnea patients.

SETTING

Participants slept in a sound-insulated room with physiologic measurements controlled from an adjacent room.

PARTICIPANTS

Fifteen obese (body mass index: 34.5 +/- 1.1 kg/m2) male obstructive sleep apnea patients (apnea-hypopnea index: 58.1 +/- 6.8 events/h) aged 50 +/- 2.6 years participated.

INTERVENTIONS

Gastric (PGA) and transdiaphragmatic pressures (P(DI)), UA closing pressure (UACP), UA airflow resistance (R(UA)), and changes in end-expiratory lung volume (EELV) were determined during stable stage 2 sleep with and without abdominal compression, achieved via inflation of a pneumatic cuff placed around the abdomen. UACP was assessed during brief mask occlusions.

MEASUREMENTS AND RESULTS

Abdominal compression significantly decreased EELV by 0.53 +/- 0.24 L (P=0.045) and increased PGA (16.2 +/- 0.8 versus 10.8 +/- 0.7 cm H2O, P < 0.001), P(DI) (11.7 +/- 0.9 versus 7.6 +/- 1.2 cm H2O, P < 0.001) and UACP (1.4 +/- 0.8 versus 0.9 +/- 0.9 cm H2O, P = 0.039) but not R(UA)(6.5 +/- 1.4 versus 6.9 +/- 1.4 cm H2O x L/s, P=0.585).

CONCLUSIONS

Abdominal compression negatively impacts on UA collapsibility during sleep and this effect may help explain strong associations between central obesity and obstructive sleep apnea.

Perioperative treatment of patients with obstructive sleep apnea

PURPOSE OF REVIEW

Obstructive sleep apnea is a common disorder. Despite reports of its role as a risk factor for postoperative morbidity and mortality, only a few investigators have examined the optimal treatment of patients during this vulnerable period. Recognition of obstructive sleep apnea during conscious sedation or in the perioperative period is important to prevent the occurrence of adverse outcomes. This review discusses the influence of sedative, anesthetic, and analgesic agents and other factors during the perioperative period on patients with obstructive sleep apnea. The aim of this article is to emphasize the importance of recognizing and appropriately treating surgical patients with obstructive sleep apnea.

RECENT FINDINGS

Sedative, analgesic, and anesthetic agents used perioperatively play a major role in the development of sleep-disordered breathing during the postoperative period. Postoperative apneic episodes frequently occur even after surgery remote from the upper airway. Sleep apnea predisposes patients to a greater than normal risk for postsurgical complications. Adequate screening of patients preoperatively and initiation of continuous positive airway pressure therapy perioperatively could prevent serious complications, including hypoxemia, arrhythmias, myocardial infarction, and respiratory arrest.

SUMMARY

Obstructive sleep apnea places a significant proportion of surgical patients at increased risk of perioperative complications. Obstructive sleep apnea can be induced, unmasked, or exacerbated by the effects of sedative, analgesic, and anesthetic agents regardless of the site of surgery. The role of sleep apnea as a risk factor for development of postoperative complications needs greater emphasis. Increased awareness of the risk posed by an obstructed upper airway and appropriate management are important to optimize the perioperative care of patients with obstructive sleep apnea.

The ventilatory responsiveness to CO(2) below eupnoea as a determinant of ventilatory stability in sleep

Sleep unmasks a highly sensitive hypocapnia-induced apnoeic threshold, whereby apnoea is initiated by small transient reductions in arterial CO(2) pressure (P(aCO(2))) below eupnoea and respiratory rhythm is not restored until P(aCO(2)) has risen significantly above eupnoeic levels. We propose that the 'CO(2) reserve' (i.e. the difference in P(aCO(2)) between eupnoea and the apnoeic threshold (AT)), when combined with 'plant gain' (or the ventilatory increase required for a given reduction in P(aCO(2))) and 'controller gain' (ventilatory responsiveness to CO(2) above eupnoea) are the key determinants of breathing instability in sleep. The CO(2) reserve varies inversely with both plant gain and the slope of the ventilatory response to reduced CO(2) below eupnoea; it is highly labile in non-random eye movement (NREM) sleep. With many types of increases or decreases in background ventilatory drive and P(aCO(2)), the slope of the ventilatory response to reduced P(aCO(2)) below eupnoea remains unchanged from control. Thus, the CO(2) reserve varies inversely with plant gain, i.e. it is widened with hyperventilation and narrowed with hypoventilation, regardless of the stimulus and whether it acts primarily at the peripheral or central chemoreceptors. However, there are notable exceptions, such as hypoxia, heart failure, or increased pulmonary vascular pressures, which all increase the slope of the CO(2) response below eupnoea and narrow the CO(2) reserve despite an accompanying hyperventilation and reduced plant gain. Finally, we review growing evidence that chemoreceptor-induced instability in respiratory motor output during sleep contributes significantly to the major clinical problem of cyclical obstructive sleep apnoea.

Areas of the brain concerned with ventilatory load compensation in awake man

There is broad agreement that the awake human ventilatory response to a moderate inspiratory load consists of a prolongation of inspiratory time (T(I)) with a maintenance of tidal volume (V(T)) and end-tidal P(C)(O(2)) (P(ET,C)(O(2))), the response being severely blunted in sleep. There is no agreement on the mechanisms underlying this ventilatory response. Six naive healthy males (aged 39-44) were studied supine with their heads in a positron emission tomography (PET) scanner to allow relative regional cerebral blood flow (rCBF) to be measured with H(2)(15)O given intravenously. A linearised resistive load (24 cmH(2)O (l s(-1))(-1)) could be added to the inspiratory limb of a breathing valve inserted into a tightly fitting facemask; inspiratory flow was measured with a pneumotachograph. The load was applied, without alerting the subject, when the radioactivity first reached the head. Six scans were performed with and without the load, in each subject. Relative rCBF contrasts between the loaded and unloaded breathing states showed significant activations in inferior parietal cortex, prefrontal cortex, midbrain, basal ganglia and multiple cerebellar sites. No activations were found in the primary sensorimotor cortex. The findings suggest that there is a pattern of motor behavioural response to the uncomfortable sensation that inspiration is impeded. This results in a prolongation of T(I), the maintenance of V(T) and a reduction in the degree of discomfort, presumably because of the reduction of mean negative pressure in the airways.

Anaesthesia and sleep apnoea

Sleep disordered breathing is a common problem affecting all age groups, particularly in association with certain other medical conditions and syndromes. The pathological consequences of the disorder may be severe, with significant implications for the perioperative management of sufferers. Research into the effects of surgery and anaesthesia on sleep is very much in its infancy. Understanding of the implications of sleep disturbance and sleep disordered breathing for perioperative morbidity and mortality is limited. While several observations have led to considerable speculation in the literature, evidence of a causal relationship is still largely lacking. Anaesthetists are ideally placed to screen large numbers of people for sleep disordered breathing, a source of considerable community morbidity. Recognizing the symptoms, signs and associations of the condition during the preoperative visit is important in planning management, as is recognition of the likelihood of OSA in patients who present difficulty with tracheal intubation or airway maintenance. Particular care is required in the perioperative management of patients with diagnosed or suspected sleep apnoea.

Effects of inspiratory flow on diaphragmatic motor output in normal subjects

Increasing inspiratory flow (V) has been shown to shorten neural inspiratory time (TI(n)) in normal subjects breathing on a mechanical ventilator, but the effect of V on respiratory motor output before inspiratory termination has not previously been studied in humans. While breathing spontaneously on a mechanical ventilator, eight normal subjects were intermittently exposed to 200-ms-duration positive pressure pulses of different amplitudes at the onset of inspiration. Based on the increase in V above control breaths (DeltaV), trials were grouped into small, medium, and large groups (mean DeltaV: 0.51, 1.11, and 1.65 l/s, respectively). We measured TI(n), transdiaphragmatic pressure (Pdi), and electrical activity (electromyogram) of the diaphragm (EMGdi). Transient increases in V caused shortening of TI(n) from 1.34 to 1.10 (not significant), 1.55 to 1.11 (P < 0.005), and 1.58 to 1.17 s (P < 0. 005) in the small, medium, and large DeltaV groups, respectively. EMGdi measured at end TI(n) of the pulse breaths was 131 (P < 0.05), 142, and 155% (P < 0.05) of the EMGdi of the control breaths at an identical time point in the small, medium, and large trials, respectively. The latency of the excitation was 126 +/- 42 (SD) ms, consistent with a reflex effect. Increasing V had two countervailing effects on Pdi: 1) a depressant mechanical effect due primarily to the force-length (11.2 cmH(2)O/l) relation of the diaphragm, and 2) an increase in diaphragm activation. For the eight subjects, mean peak Pdi did not change significantly, but there was significant intersubject variability, reflecting variability in the strength of the excitation reflex. We conclude that increasing inspiratory V causes a graded facilitation of EMGdi, which serves to counteract the negative effect of the force-length relation on Pdi.

The respiratory response to inspiratory resistive loading during rapid eye movement sleep in humans

We investigated the respiratory response to an added inspiratory resistive load (IRL) during rapid eye movement (REM) sleep in humans and compared this with those in non-REM (NREM) sleep and wakefulness. Results were obtained from 7 out of 15 healthy subjects (n = 7; 32 +/- 9 years, mean +/- s.d.). Linearised IRLs (4 and 12 cmH(2)O l(-1) s(-1)) were applied for five breaths during NREM sleep (4-10 trials per subject; total 101), REM sleep (2-5 trials; total 46) and wakefulness (2-3 trials; total 40). Respiratory variables were compared, between unloaded breathing (UL: mean of 5 breaths preceding IRL) and the 1st (B1) and 5th (B5) loaded breaths in each state. During wakefulness, 12 cmH(2)O l(-1) s(-1) IRL produced an immediate respiratory compensation with prolongation of inspiratory time (T(I); UL: 2.0 +/- 0.6; B1: 2.6 +/- 0.7 s) and an increase in tidal volume (V(T); UL: 0.49 +/- 0.12; B1: 0.52 +/- 0.12 l). During REM sleep, T(I) was prolonged (UL: 2.0 +/- 0.3; B1: 2.2 +/- 0.5 s), although V(T) fell (UL: 0.27 +/- 0.15; B1: 0.22 +/- 0.10 l). For both wakefulness and REM sleep the TI response was significantly greater than seen in NREM sleep (UL: 1.9 +/- 0.3; B1: 1.9 +/- 0.3 s.). For VT, only the wakefulness response was significantly different from NREM sleep (UL: 0.31 +/- 0.14; B1: 0.21 +/- 0.10 l). The B5 responses were not significantly different between states for any of the variables. REM sleep is associated with partial respiratory load compensation suggesting that exacerbation of sleep disordered breathing in REM (compared to NREM) sleep is unlikely to be secondary to an inability to overcome increases in upper airway resistance.

Load compensation as a function of state during sleep onset

Ventilation decreases and airway resistance increases with the loss of electroencephalogram alpha activity at sleep onset. The aim of this study was to determine whether reflexive load compensation is lost immediately on the loss of alpha activity. Six healthy male subjects were studied under two conditions (load and control-no load), in three states (continuous alpha, continuous theta, and immediately after a transition from alpha to theta), and in two phases (early and late sleep onset). Ventilation and respiratory timing were measured. A comparison of loaded with control conditions indicated that loading had no effect on inspiratory minute ventilation during continuous alpha (differential effect of 0.00 l/min) and only a small, nonsignificant effect in theta immediately after phase 2 transitions (0.31 l/min), indicating a preservation of load compensation at these times. However, there were significant decreases in inspiratory minute ventilation on loaded trials during continuous theta in phase 2 (0.77 l/min) and phase 3 (1.15 l/min) and during theta immediately after a transition in phase 3 (0.87 l/min), indicating a lack of reflexive load compensation. The results indicate that, because reflex load compensation is state dependent, state-related changes in airway resistance contribute to state-related changes in ventilation during sleep onset. However, this effect was slightly delayed with transitions into theta early in sleep.

Effect of sleep on respiratory muscle activity during mechanical ventilation

The purpose of this study was to determine whether consciousness was critical for the expression of neuromechanical inhibition of breathing during mechanical ventilation. This same mechanical ventilation model also was used to evaluate the relative importance of sleep state in causing CO2 retention during sleep. Positive pressure ventilation was used to suppress respiratory muscle activity; CO2 was then added until a reappearance of inspiratory effort, which defined the recruitment threshold (PCO2RT). Keeping the mechanics of the respiratory system constant through the use of passive mechanical ventilation allowed us to measure the output of the respiratory controller, independent of these parameters. Eight normal subjects were mechanically hyperventilated with a nasal mask during wakefulness and sleep with matched flow rates, frequencies, and tidal volumes. When inspiratory muscle activity was undetectable and end-tidal PCO2 (PETCO2) fell below 30 mm Hg, inspired CO2 was added in stepped increments until inspiration reoccurred. The sleeping state increased both eupneic PETCO2 (42 +/- 4 versus 38 +/- 3 mm Hg) and PCO2RT (48 +/- 3 versus 46 +/- 2 mm Hg) compared with that during wakefulness. Neuromechanical inhibition of inspiratory muscle activity during mechanical ventilation was present during both wakefulness and sleep, as evidenced by the mean difference between PCO2RT and eupneic PETCO2 of 8 and 6 mm Hg, respectively. Recruitment thresholds during wakefulness and sleep were compared to evaluate the effect of sleep on respiratory motor output independent of changes in load, i.e., respiratory mechanics held constant.(ABSTRACT TRUNCATED AT 250 WORDS)