Experimentally induced Cheyne-Stokes breathing

Sevoflurane and Hypercapnia Blunt the Physiological Variability of Spontaneous Breathing: A Comparative Interventional Study

Background: Although spontaneous breathing is known to exhibit substantial physiological fluctuation that contributes to alveolar recruitment, changes in the variability of the respiratory pattern following inhalation of carbon dioxide (CO2) and volatile anesthetics have not been characterized. Therefore, we aimed at comparing the indices of breathing variability under wakefulness, sleep, hypercapnia and sedative and anesthetic concentrations of sevoflurane.

Methods: Spontaneous breathing pattern was recorded on two consecutive days in six rabbits using open whole-body plethysmography under wakefulness and spontaneous sleep and following inhalation of 5% CO2, 2% sevoflurane (0.5 MAC) and 4% (1 MAC) sevoflurane. Tidal volume (VT), respiratory rate (RR), minute ventilation (MV), inspiratory time (TI) and mean inspiratory flow (VT/TI) were calculated from the pressure fluctuations in the plethysmograph. Means and coefficients of variation were calculated for each measured variable. Autoregressive model fitting was applied to estimate the relative contributions of random, correlated, and oscillatory behavior to the total variance.

Results: Physiological sleep decreased MV by lowering RR without affecting VT. Hypercapnia increased MV by elevating VT. Sedative and anesthetic concentrations of sevoflurane increased VT but decreased MV due to a decrease in RR. Compared to the awake stage, CO2 had no effect on VT/TI while sevoflurane depressed significantly the mean inspiratory flow. Compared to wakefulness, the variability in VT, RR, MV, TI and VT/TI were not affected by sleep but were all significantly decreased by CO2 and sevoflurane. The variance of TI originating from correlated behavior was significantly decreased by both concentrations of sevoflurane compared to the awake and asleep conditions.

Conclusions: The variability of spontaneous breathing during physiological sleep and sevoflurane-induced anesthesia differed fundamentally, with the volatile agent diminishing markedly the fluctuations in respiratory volume, inspiratory airflow and breathing frequency. These findings may suggest the increased risk of lung derecruitment during procedures under sevoflurane in which spontaneous breathing is maintained.

Modulation-demodulation hypothesis of periodic breathing in human respiration

Periodic breathing (PB) is a diseased condition of the cardiorespiratory system, and mathematically it is modelled as an oscillation. Modeling approaches replicate periodic oscillation in the minute ventilation due to a higher than normal gain of the feedback signals from the chemoreceptors coupled with a longer than normal latency in feedback, and do not consider the waxing-waning pattern of the oronasal airflow. In this work, a noted regulation model is extended by integrating respiratory mechanics and respiratory central pattern generator (rCPG) model, using modulation-demodulation¹ hypothesis. This is a top-down modeling approach, and it is assumed that the sensory feedback signal from the chemoreceptors modulates the output of the rCPG model. It is also assumed that the brainstem network is responsible for the demodulation process. The respiratory mechanics is modeled as a multi-input multi-output (MIMO) system, where modulated and demodulated neural signals are applied as input and the minute ventilation and the oronasal airflow are specified as output. The minute ventilation signal drives the regulation model, completing the feedback loop. The proposed model is validated by comparing the model output with the clinical data. Using the modulation-demodulation hypothesis, a respiratory mechanics model is formulated in the form of a linear state-space model, which can be useful for providing assisted ventilation in clinical conditions.

2017 ERS/ATS standards for single-breath carbon monoxide uptake in the lung

This document provides an update to the European Respiratory Society (ERS)/American Thoracic Society (ATS) technical standards for single-breath carbon monoxide uptake in the lung that was last updated in 2005. Although both D_LCO (diffusing capacity) and T_LCO (transfer factor) are valid terms to describe the uptake of carbon monoxide in the lung, the term D_LCO is used in this document. A joint taskforce appointed by the ERS and ATS reviewed the recent literature on the measurement of D_LCO and surveyed the current technical capabilities of instrumentation being manufactured around the world. The recommendations in this document represent the consensus of the taskforce members in regard to the evidence available for various aspects of D_LCO measurement. Furthermore, it reflects the expert opinion of the taskforce members on areas in which peer-reviewed evidence was either not available or was incomplete. The major changes in these technical standards relate to D_LCO measurement with systems using rapidly responding gas analysers for carbon monoxide and the tracer gas, which are now the most common type of D_LCO instrumentation being manufactured. Technical improvements and the increased capability afforded by these new systems permit enhanced measurement of D_LCO and the opportunity to include other optional measures of lung function.

Executive Summary: 2017 ERS/ATS standards for single-breath carbon monoxide uptake in the lung

This document summarises an update to the European Respiratory Society (ERS)/American Thoracic Society (ATS) technical standards for single-breath carbon monoxide uptake in the lung that was last updated in 2005. The full standards are also available online as https://doi.org/10.1183/13993003.00016-2016 The major changes in these technical standards relate to D_LCO measurement with systems using rapidly responding gas analysers for carbon monoxide and the tracer gas, which are now the most common type of D_LCO instrumentation being manufactured. Technical improvements and the increased capability afforded by these new systems permit enhanced measurement of D_LCO and the opportunity to include other optional measures of lung function.

Neurologic Causes of Acute Respiratory Dysfunction

Many neurologic diseases can cause acute respiratory decompensation, therefore a familiarity with these diseases is critical for any clinician managing patients with respiratory dysfunction. In this article, we review the anatomy of the respiratory system, focusing on the neurologic control of respiration. We discuss general mechanisms by which diseases of the peripheral and central nervous systems can cause acute respiratory dysfunction, and review the neurologic diseases which can adversely affect respiration. Lastly, we discuss the diagnosis and general management of acute respiratory impairment due to neurologic disease.

The effect of partial acclimatization to high altitude on loop gain and central sleep apnoea severity

BACKGROUND AND OBJECTIVE

Loop gain is an engineering term that predicts the stability of a feedback control system, such as the control of breathing. Based on earlier studies at lower altitudes, it was hypothesized that acclimatization to high altitude would lead to a reduction in loop gain and thus central sleep apnoea (CSA) severity.

METHODS

This study used exposure to very high altitude to induce CSA in healthy subjects to investigate the effect of partial acclimatization on loop gain and CSA severity. Measurements were made on 12 subjects (age 30 ± 10 years, body mass index 22.8 ± 1.9, eight males, four females) at an altitude of 5050 m over a 2-week period upon initial arrival (days 2-4) and following partial acclimatization (days 12-14). Sleep was studied by full polysomnography, and resting arterial blood gases were measured. Loop gain was measured by the 'duty cycle' method (duration of hyperpnoea/cycle length).

RESULTS

Partial acclimatization to high-altitude exposure was associated with both an increase in loop gain (duty cycle fell from 0.60 ± 0.05 to 0.55 ± 0.06 (P = 0.03)) and severity of CSA (apnoea-hypopnoea index increased from 76.8 ± 48.8 to 115.9 ± 20.2 (P = 0.01)), while partial arterial carbon dioxide concentration fell from 29 ± 3 to 26 ± 2 (P = 0.01).

CONCLUSIONS

Contrary to the results at lower altitudes, at high-altitude loop gain and severity of CSA increased.

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.

Dynamic CO2 therapy in periodic breathing: a modeling study to determine optimal timing and dosage regimes

We examine the potential to treat unstable ventilatory control (seen in periodic breathing, Cheyne-Stokes respiration, and central sleep apnea) with carefully controlled dynamic administration of supplementary CO₂, aiming to reduce ventilatory oscillations with minimum increment in mean CO₂. We used a standard mathematical model to explore the consequences of phasic CO₂ administration, with different timing and dosing algorithms. We found an optimal time window within the ventilation cycle (covering ∼1/6 of the cycle) during which CO₂ delivery reduces ventilatory fluctuations by >95%. Outside that time, therapy is dramatically less effective: indeed, for more than two-thirds of the cycle, therapy increases ventilatory fluctuations >30%. Efficiency of stabilizing ventilation improved when the algorithm gave a graded increase in CO₂ dose (by controlling its duration or concentration) for more severe periodic breathing. Combining gradations of duration and concentration further increased efficiency of therapy by 22%. The (undesirable) increment in mean end-tidal CO₂ caused was 300 times smaller with dynamic therapy than with static therapy, to achieve the same degree of ventilatory stabilization (0.0005 vs. 0.1710 kPa). The increase in average ventilation was also much smaller with dynamic than static therapy (0.005 vs. 2.015 l/min). We conclude that, if administered dynamically, dramatically smaller quantities of CO₂ could be used to reduce periodic breathing, with minimal adverse effects. Algorithms adjusting both duration and concentration in real time would achieve this most efficiently. If developed clinically as a therapy for periodic breathing, this would minimize excess acidosis, hyperventilation, and sympathetic overactivation, compared with static treatment.

Upper airway mechanics

This review discusses the pathophysiological aspects of sleep-disordered breathing, with focus on upper airway mechanics in obstructive and central sleep apnoea, Cheyne-Stokes respiration and obesity hypoventilation syndrome. These disorders constitute the end points of a spectrum with distinct yet interrelated mechanisms that lead to substantial pathology, i.e. increased upper airway collapsibility, control of breathing instability, increased work of breathing, disturbed ventilatory system mechanics and neurohormonal changes. Concepts are changing. Although sleep apnoea is considered more and more to be an increased loop gain disorder, the central type of apnoea is now considered as an obstructive event, because it causes pharyngeal narrowing, associated with prolonged expiration. Although a unifying concept for the pathogenesis is lacking, it seems that these patients are in a vicious circle. Knowledge of common patterns of sleep-disordered breathing may help to identify these patients and guide therapy.

Dorsomedial medullary 5-HT2 receptors mediate immediate onset of initial hyperventilation, airway dilation, and ventilatory decline during hypoxia in mice

The dorsomedial medulla oblongata (DMM) includes the solitary tract nucleus and the hypoglossal nucleus, to which 5-HT neurons project. Effects of 5-HT in the DMM on ventilatory augmentation and airway dilation are mediated via 5-HT2 receptors, which interact with the CO(2) drive. The interaction may elicit cycles between hyperventilation with airway dilation and hypoventilation with airway narrowing. In the present study, effects of 5-HT2 receptors in the DMM on hypoxic ventilatory and airway responses were investigated, while 5-HT release in the DMM was monitored. Adult male mice were anesthetized, and then a microdialysis probe was inserted into the DMM. The mice were placed in a double-chamber plethysmograph. After recovery from anesthesia, the mice were exposed to hypoxic gas (7% O(2) in N(2)) for 5 min with or without a 5-HT2 receptor antagonist (LY-53857) perfused in the DMM. 5-HT release in the DMM was increased by hypoxia regardless of the presence of LY-53857. Immediate onset and the peak of initial hypoxic hyperventilatory responses were delayed. Subsequent ventilatory decline and airway dilation during initial hypoxic hyperventilation were suppressed with LY-53857. These results suggest that 5-HT release increased by hypoxia acts on 5-HT2 receptors in the DMM, which contributes to the immediate onset of initial hypoxic hyperventilation, airway dilation, and subsequent ventilatory decline. Hypoxic ventilatory and airway responses mediated via 5-HT2 receptors in the DMM may play roles in immediate rescue and defensive adaptation for hypoxia and may be included in periodic breathing and the pathogenesis of obstructive sleep apnea.

Effect of inspired oxygen on periodic breathing in methy-CpG-binding protein 2 (Mecp2) deficient mice

Rett syndrome (RTT) is a neurodevelopmental disorder caused by mutations in the X-linked gene methyl-CpG-binding protein 2 (Mecp2) that encodes a DNA binding protein involved in gene silencing. Periodic breathing (Cheyne-Stokes respiration) is commonly seen in RTT. Freely moving mice were studied with continuous recording of pleural pressure by telemetry. Episodes of periodic breathing in heterozygous Mecp2 deficient (Mecp2(+/-)) female mice (9.4 +/- 2.2 h(-1)) exceeded those in wild-type (Mecp2(+/+)) animals (2.5 +/- 0.4 h(-1)) (P = 0.010). Exposing Mecp2(+/-) animals to 40% oxygen increased the amount of periodic breathing from 118 +/- 25 s/30 min in air to 242 +/- 57 s/30 min (P = 0.001), and 12% oxygen tended to decrease it (67 +/- 29 s/30 min, P = 0.14). Relative hyperoxia and hypoxia did not affect the incidence of periodic breathing in Mecp2(+/+) animals. The ventilation/apnea ratio (V/A) was less at all levels of oxygen in heterozygous Mecp2(+/-) females compare with wild type (P = 0.003 to P < 0.001), indicating that their loop gain is larger. V/A in Mecp2(+/-) fell from 2.42 +/- 0.18 in normoxia to 1.82 +/- 0.17 in hyperoxia (P = 0.05) indicating an increase in loop gain with increased oxygen. Hyperoxia did not affect V/A in Mecp2(+/+) mice (3.73 +/- 0.28 vs. 3.5 +/- 0.28). These results show that periodic breathing in this mouse model of RTT is not dependent on enhanced peripheral chemoreceptor oxygen sensitivity. Rather, the breathing instability is of central origin.

Ventilatory instability during sleep: new insights from the computational model

We developed a computational model of the human respiratory system and its chemoreflex control during sleep [1] [2] Our model, which is an extension of the model of Grodins et al. [3], combines an accurate description of a plant with a novel controller design. The controller consists of two feedback loops (central and peripheral) each with its own delay and gain. The overall minute ventilation is a sum of central and peripheral components. The model was employed to develop a new graphical method for stability analysis of the respiratory control system similar in concept to the phase plane. The relative chemosensitivities of the peripheral and central loops serve as the plane's coordinates. A region of stability exists with the normal operating point for the system lying well inside its boundaries. Changes to the sensitivities of either loop, caused by known pathologies, displace the operating point toward the border of the stability region. Furthermore, the shape and area of the stability region is significantly influenced by changes in the cerebral blood flow.

Development of respiratory control instability in heart failure: a novel approach to dissect the pathophysiological mechanisms

Observational data suggest that periodic breathing is more common in subjects with low F(ETCO(2)), high apnoeic thresholds or high chemoreflex sensitivity. It is, however, difficult to determine the individual effect of each variable because they are intrinsically related. To distinguish the effect of isolated changes in chemoreflex sensitivity, mean F(ETCO(2)) and apnoeic threshold, we employed a modelling approach to break their obligatory in vivo interrelationship. We found that a change in mean CO(2) fraction from 0.035 to 0.045 increased loop gain by 70 +/- 0.083% (P < 0.0001), irrespective of chemoreflex gain or apnoea threshold. A 100% increase in the chemoreflex gain (from 800 l min(-1) (fraction CO(2))(-1)) resulted in an increase in loop gain of 275 +/- 6% (P < 0.0001) across a wide range of values of steady state CO(2) and apnoea thresholds. Increasing the apnoea threshold F(ETCO(2)) from 0.02 to 0.03 had no effect on system stability. Therefore, of the three variables the only two destabilizing factors were high gain and high mean CO(2); the apnoea threshold did not independently influence system stability. Although our results support the idea that high chemoreflex gain destabilizes ventilatory control, there are two additional potentially controversial findings. First, it is high (rather than low) mean CO(2) that favours instability. Second, high apnoea threshold itself does not create instability. Clinically the apnoea threshold appears important only because of its associations with the true determinants of stability: chemoreflex gain and mean CO(2).

Causes of Cheyne-Stokes respiration

Cheyne-Stokes respiration (CSR) is one of several types of unusual breathing with recurrent apneas (dysrhythmias). Reported initially in patients with heart failure or stroke, it was then recognized both in other diseases and as a component of the sleep apnea syndrome. CSR is potentiated and perpetuated by changing states of arousal that occur during sleep. The recurrent hypoxia and surges of sympathetic activity that often occur during the apneas may have serious health consequences. Heart failure and stroke are risk factors for sleep apnea. The recurrent apneas and intermittent hypoxia occurring with sleep apnea further damage the heart and brain. Although all breathing dysrhythmias do not have the same cause, instability in the feedback control involved in the chemical regulation of breathing is the leading cause of CSR. Mathematical models have helped greatly in the understanding of the causes of recurrent apneas.

Mathematical models of periodic breathing and their usefulness in understanding cardiovascular and respiratory disorders

Periodic breathing is an unusual form of breathing with oscillations in minute ventilations and with repetitive apnoeas or near apnoeas. Reported initially in patients with heart failure or stroke, it was later recognized to occur especially during sleep. The recurrent hypoxia and surges of sympathetic activity that often occur during the apnoeas have serious health consequences. Mathematical models have helped greatly in the understanding of the causes of recurrent apnoeas. It is unlikely that every instance of periodic breathing has the same cause, but many result from instability in the feedback control involved in the chemical regulation of breathing caused by increased controller and plant gains and delays in information transfer. Even when it is not the main cause of the periodic breathing, unstable control modifies the ventilatory pattern and sometimes intensifies the recurrent apnoeas. The characteristics of disturbances to breathing and their interaction with the control system can be critical in determining ventilation responses and the occurrence of periodic breathing. Large abrupt changes in ventilation produced, for example, in the transition from waking to sleep and vice versa, or in the transition from breathing to apnoea, are potent factors causing periodic breathing. Mathematical models show that periodic breathing is a 'systems disorder' produced by the interplay of multiple factors. Multiple factors contribute to the occurrence of periodic breathing in congestive heart failure and cerebrovascular disease, increasing treatment options.

Impact of changes in inspired oxygen and carbon dioxide on respiratory instability in the lamb

We examined the effect of hypoxia and hypercapnia administered during deliberately induced periodic breathing (PB) in seven lambs following posthyperventilation apnea. Based on our theoretical analysis, the sensitivity or loop gain (LG) of the respiratory control system of the lamb is directly proportional to the difference between alveolar Po2 and inspired Po2. This analysis indicates that during PB, when by necessity LG is >1, replacement of the inspired gas with one of reduced Po2 lowers LG; if we made inspired Po2 approximate alveolar Po2, we predict that LG would be approximately zero and breathing would promptly stabilize. In six lambs, we switched the inspired gas from an inspiratory oxygen fraction of 0.4 to one of 0.12 during an epoch of PB; PB was immediately suppressed, supporting the view that the peripheral chemoreceptors play a pivotal role in the genesis and control of unstable breathing in the lamb. In the six lambs in which we administered hypercapnic gas during PB, breathing instability was also suppressed, but only after a considerable time lag, indicating the CO2 effect is likely to have been mediated through the central chemoreceptors. When we simulated both interventions in a published model of the adult respiratory controller, PB was immediately suppressed by CO2 inhalation and exacerbated by inhalation of hypoxic gas. These fundamentally different responses in lambs and adult humans demonstrate that PB has differing underlying mechanisms in the two species.

Role of central sympathoexcitation in enhanced hypercapnic chemosensitivity in patients with heart failure

BACKGROUND

Enhanced central hypercapnic chemosensitivity is known to mediate excessive exercise ventilation and to indicate a poor prognosis in patients with chronic heart failure. The present study was designed to elucidate the role of central sympathetic activity in the enhancement of hypercapnic chemosensitivity.

METHODS

Central hypercapnic chemosensitivity and plasma norepinephrine were measured in 99 patients with chronic heart failure. In 40 patients, the alpha index was derived from simultaneous analysis of R-R interval and systolic blood pressure variability. The effects of a central sympatholytic agent, guanfacine (0.25 mg/day), on hypercapnic chemosensitivity and exercise ventilatory response were studied in 20 of these patients.

RESULTS

Hypercapnic chemosensitivity was enhanced in 76% of the patients and correlated significantly with plasma norepinephrine levels (r = 0.49, P < .01) at rest. There was a significant inverse relationship between central chemosensitivity and the alpha index (r = -0.41, P < .01). Guanfacine significantly reduced plasma norepinephrine levels by 29% (P < .01) and chemosensitivity by 31% (P < .01). The beneficial effect of central sympathoinhibition with guanfacine was observed specifically in patients who had enhanced chemosensitivity prior to drug administration. Similarly, the patients with excessive exercise ventilation showed a greater reduction in exercise ventilation with this agent.

CONCLUSIONS

The present findings suggest that central sympathoexcitation could play an important role in the pathogenesis of enhanced hypercapnic chemosensitivity and a resultant increase in exercise ventilation in chronic heart failure.

Central and obstructive sleep apnoea during ascent to high altitude

OBJECTIVE

The aim of the study was to investigate the relationship between central sleep apnoea (CSA) at high altitude and arterial blood gas tensions, and by inference, ventilatory responsiveness.

METHODOLOGY

Fourteen normal adult volunteers were studied by polysomnography during sleep, and analysis of awake blood gases during ascent over 12 days from sealevel to 5050 m in the Nepal Himalayas.

RESULTS

Thirteen subjects developed CSA. Linear regression analysis showed tight negative correlations between mean CSA index and mean values for sleep SaO2, PaCO2 and PaO2 over the six altitudes (r2 > or = 0.74 for all, P < 0.03). Paradoxically there was poor correlation between the individual data for CSA index and those parameters at the highest altitude (5050-m) where CSA was worst (r2 < 0.12 for all, NS), possibly due to variation in degree of acclimatization between subjects. In addition, CSA replaced mild obstructive sleep apnoea during ascent. Obstructive sleep apnoea index fell from 5.5 +/- 6.9/h in rapid eye movement sleep at sealevel to 0.1 +/- 0.3/h at 5050 m (P < 0.001, analysis of variance), while CSA index rose from 0.1 +/- 0.3/h to 55.7 +/- 54.4/h (P < 0.001).

CONCLUSION

There was a general relationship between decreasing PaCO2 and CSA, but there were significant effects from variations in acclimatization that would make hypoxic ventilatory response an unreliable predictor of CSA in individuals.

Lessons from chronic intermittent and sustained hypoxia at high altitudes

Recurrent sleep apnea (RSA), mimicking chronic intermittent hypoxia (CIH), may trigger unique adaptations in oxygen sensing in the carotid body, and consequent cellular functions unlike the effects of sustained hypoxia (SH). As a mechanism, an augmented generation of reactive oxygen species (ROS) in CIH has been invoked at the exclusion of SH effects. The ROS might act at hypoxia inducible factors (HIF-1s), giving rise to various genes whose function is to restore the tissue P(O(2)) close to the original. In a spate, review articles on the CIH effects at sea level have appeared but little on high altitude (HA). Their views have been reexamined with the primary focus on the peripheral chemoreception. At HA, RSA is more common in the lowlanders because of a high ventilatory sensitivity to hypoxia (with the consequent effects) unlike the high altitude natives (HAN). Undoubtedly, the HIF-1s play a central role at HA, the mechanisms of which are unknown and explorable.

Evaluation of variables to characterize respiratory periodicity during sleep in older adults

Reliable markers of early neurological decline might guide interventions to prevent or reverse cognitive decline in older adults. Because cognitive decline is associated with hypoxemia during sleep, the authors examined 3 respiratory periodicity variables in 5 older adults. Subjects were monitored overnight using standard polysomnography. From the inductance band signal, the authors calculated the variability in duration of breathing cycles measured by standard deviation of interbreath intervals (sdIBI), frequency of breathing cycles measured by standard deviation of interbreath frequencies (sdIBF), and amplitude of breathing cycles measured by standard deviation of breathing cycle amplitudes (sdAMP). Logistic regression analysis and kappa coefficients identified variables that reliably detected 5-minute segments having central or obstructive apneas or body movements. An sdIBF > or = 4.5 cpm identified body movements (sensitivity = 0.96, specificity = 0.96, kappa = 0.90). An sdIBI > 1.2 seconds identified central apneas (sensitivity = 0.86, specificity = 0.99, kappa = 0.86), and an sdIBI > or = 1.68 seconds identified segments with 3 central apneas (sensitivity = 0.90, specificity = 0.89, kappa = 0.89). An sdAMP > or = 0.1 V and an sdIBF > or = 1.5 cpm identified obstructive apneas (kappa = 0.91). Data support the potential of these variables to identify central and obstructive apneas and to classify individuals according to different patterns of respiratory periodicity.

Periodic breathing in the mouse

The hypothesis was that unstable breathing might be triggered by a brief hypoxia challenge in C57BL/6J (B6) mice, which in contrast to A/J mice are known not to exhibit short-term potentiation; as a consequence, instability of ventilatory behavior could be inherited through genetic mechanisms. Recordings of ventilatory behavior by the plethsmography method were made when unanesthetized B6 or A/J animals were reoxygenated with 100% O(2) or air after exposure to 8% O(2) or 3% CO(2)-10% O(2) gas mixtures. Second, we examined the ventilatory behavior after termination of poikilocapnic hypoxia stimuli in recombinant inbred strains derived from B6 and A/J animals. Periodic breathing (PB) was defined as clustered breathing with either waxing and waning of ventilation or recurrent end-expiratory pauses (apnea) of > or = 2 average breath durations, each pattern being repeated with a cycle number > or = 3. With the abrupt return to room air from 8% O(2), 100% of the 10 B6 mice exhibited PB. Among them, five showed breathing oscillations with apnea, but none of the 10 A/J mice exhibited cyclic oscillations of breathing. When the animals were reoxygenated after 3% CO(2)-10% O(2) challenge, no PB was observed in A/J mice, whereas conditions still induced PB in B6 mice. (During 100% O(2) reoxygenation, all 10 B6 mice had PB with apnea.) Expression of PB occurred in some but not all recombinant mice and was not associated with the pattern of breathing at rest. We conclude that differences in expression of PB between these strains indicate that genetic influences strongly affect the stability of ventilation in the mouse.

Noninvasive pressure preset ventilation for the treatment of Cheyne-Stokes respiration during sleep

Cheyne-Stokes respiration (CSR) during sleep is common in patients with congestive heart failure (CHF). This pattern of breathing fragments sleep, leading to daytime symptoms of sleepiness and fatigue. It was hypothesized that by controlling CSR with noninvasive pressure preset ventilation (NPPV), there would be a decrease in sleep fragmentation and an improvement in sleep quality. Nine patients (eight males, one female; mean +/- SD 65 +/- 11 yrs) with symptomatic CSR diagnosed on overnight polysomnography (apnoea/hypopnoea index (AHI) 49 +/- 10 x h(-1), minimum arterial oxygen saturation (Sa,O2, 77 +/- 7%) and CHF (left ventricular ejection fraction 25 +/- 8%) were studied. After a period of acclimatization to NPPV (variable positive airway pressure (VPAP) II ST, Sydney, NSW, Australia and bilevel positive airway pressure (BiPAP), Murraysville, PA, USA), sleep studies were repeated on therapy. NPPV almost completely abolished CSR in all patients with a reduction in AHI from 49 +/- 10 to 6 +/- 5 x h(-1) (p<0.001). Residual respiratory events were primarily due to upper airway obstruction at sleep on-set. Arousal index was markedly decreased from 42 +/- 6 to 17 +/- 7 x h(-1) (p <0.001). Sleep architecture showed a trend toward improvement with a reduction in stage 1 and 2 (79 +/- 7% during the diagnostic night versus 72 +/- 10% during NPPV, (p=0.057)), whilst sleep efficiency, slow-wave sleep (SWS), and rapid eye movement (REM) were not altered. Controlling Cheyne-Stokes respiration with noninvasive pressure preset ventilation resulted in reduced arousal and improved sleep quality in the patients with congestive heart failure. Noninvasive pressure preset ventilation should be considered a potential therapy for Cheyne-Stokes respiration in congestive heart failure in those patients who do not respond or fail to tolerate nasal continuous positive airway pressure therapy.

Chemical control stability in patients with obstructive sleep apnea

The role of chemical control instability in the pathogenesis of obstructive sleep apnea (OSA) is not clear. We studied 32 patients with OSA during sleep while their upper airway was stabilized with continuous positive airway pressure. Twelve patients had repetitive OSA whenever they were asleep, regardless of body position or sleep stage, and were classified as having severe OSA (apnea-hypopnea index [AHI] = 88 +/- 19). The remaining 20 patients had sporadic OSA or repetitive OSA for only part of the time (mild/moderate OSA; AHI = 27 +/- 16). Susceptibility to periodic breathing (PB) was assessed by gradually increasing controller gain, using proportional assist ventilation. The increase in loop gain (LG) at each assist level was quantified from the ratio of assisted tidal volume (VT) to the VT obtained during single-breath reloading tests (VT amplification factor [VTAF]). Nine of 12 patients with severe OSA developed PB, with recurrent central apneas, whereas only six of 20 patients in the mild/moderate group developed PB (p < 0.02). This difference was observed despite the subjection of the mild/moderate group to greater amplification of LG; the highest values of VTAF in the mild/moderate and severe groups were 2.7 +/- 1.0 and 1.9 +/- 0.7, respectively (p < 0.01). We conclude that the chemical control system is more unstable in patients with severe OSA than in patients with milder OSA. We speculate that this may contribute to the severity of OSA, at least in some patients.

Periodic breathing in heart failure patients: testing the hypothesis of instability of the chemoreflex loop

In this study, we applied time- and frequency-domain signal processing techniques to the analysis of respiratory and arterial O(2) saturation (Sa(O(2))) oscillations during nonapneic periodic breathing (PB) in 37 supine awake chronic heart failure patients. O(2) was administered to eight of them at 3 l/min. Instantaneous tidal volume and instantaneous minute ventilation (IMV) signals were obtained from the lung volume signal. The main objectives were to verify 1) whether the timing relationship between IMV and Sa(O(2)) was consistent with modeling predictions derived from the instability hypothesis of PB and 2) whether O(2) administration, by decreasing loop gain and increasing O(2) stores, would have increased system stability reducing or abolishing the ventilatory oscillation. PB was centered around 0.021 Hz, whereas respiratory rate was centered around 0.33 Hz and was almost stable between hyperventilation and hypopnea. The average phase shift between IMV and Sa(O(2)) at the PB frequency was 205 degrees (95% confidence interval 198-212 degrees). In 12 of 37 patients in whom we measured the pure circulatory delay, the predicted lung-to-ear delay was 28.8 +/- 5.2 s and the corresponding observed delay was 30.9 +/- 8.8 s (P = 0.13). In seven of eight patients, O(2) administration abolished PB (in the eighth patient, Sa(O(2)) did not increase). These results show a remarkable consistency between theoretical expectations derived from the instability hypothesis and experimental observations and clearly indicate that a condition of loss of stability in the chemical feedback control of ventilation might play a determinant role in the genesis of PB in awake chronic heart failure patients.

Determinants of ventilatory instability and variability

This paper reviews the major mechanisms that can give rise to various forms of variability in the ventilatory pattern. First, an elevated controller gain, coupled with the presence of delays and response lags in the chemoreflex loops, can lead to instability in feedback control and give rise to periodic breathing. This form of ventilatory stability can be assessed quantitatively by employing the concept of 'loop gain'. Several different methods of estimating loop gain from steady state or dynamic respiratory measurements are discussed. An inherently stable respiratory control system can also exhibit periodic behavior due to the influence of primary fluctuations in sleep-wake state and other physiological variables, such as cardiac output and cerebral blood flow. Self-sustained, irregular ventilatory fluctuations may be generated by nonlinear dynamic interactions between various components of the respiratory control system, such as the lung vagal afferents and the respiratory pattern generator, or through the propagation of stochastic disturbances around the chemoreflex loops.

Unusual respiratory response to oxygen in an infant with repetitive cyanotic episodes

High inspired oxygen concentrations have recently been recommended to control Cheyne-Stokes respiration in adults, with the intention of averting periodic apnea and its attendant arterial desaturation. We report a case study on an infant presenting with recurrent apnea and cyanosis in which oxygen treatment led to a gross form of respiratory instability we call episodic breathing, in which a breathing phase of 60 to 90 s alternated with an apnea lasting up to 60 s. When oxygen was discontinued, a profound arterial desaturation developed before breathing recommenced and restored oxygen levels. We propose that episodic breathing is an unusual respiratory pattern that involves the central chemoreceptors and results from the ventilatory threshold (the central PCO(2) at which breathing starts) lying considerably above the apneic threshold (the central PCO(2 )at which breathing stops). This feature predisposes to lengthy periods of hyperpnea alternating with lengthy periods of apnea. We suggest that when the case infant returned to air during episodic breathing, termination of apnea was entirely dependent upon carotid body activity, which reached a sufficient level to restart breathing only when arterial desaturation was severe. We conclude that oxygen therapy involves potential risks when employed to treat respiratory disorders involving unstable breathing patterns in the infant.

Low-frequency respiratory rhythms in infants during the first six months of life

The aim of the study was to investigate characteristics of low-frequency components in respiration. Sixteen healthy term infants were examined from the first day up to the 6th month of life. The respirogram, instantaneous respiratory frequency and respiratory amplitude of undisturbed segments of quiet sleep phases and periodic breathing (PB) were analysed via fast Fourier transformation. The peak frequency (PF) in the low-frequency range (0.04-0.2 Hz) was determined. PF for PB ranged from 0.056 to 0.1 Hz. Further, low-frequency rhythms (LFR) of the respirogram, which were stable during the recordings as well as during development, were found ranging from 0.045 to 0.067 Hz. The LFR of the respirogram is correlated with rhythmic changes in the relationship between inspiratory and expiratory amplitudes. The frequency of the LFR was significantly lower than that of the PB. The data indicate that LFR and PB are low-frequency respiratory rhythms which are separately controlled and perform independently.

Ventilatory instability during sleep onset in individuals with high peripheral chemosensitivity

Previous work has shown that the magnitude of state-related ventilatory fluctuations is amplified over the sleep-onset period and that this amplification is partly due to peripheral chemoreceptor activity, because it is reduced by hyperoxia (J. Dunai, M. Wilkinson, and J. Trinder. J. Appl. Physiol. 81: 2235-2243, 1996). These data also indicated considerable intersubject variability in the magnitude of amplification. A possible source of this variability is individual differences in peripheral chemoreceptor drive (PCD). We tested this hypothesis by measuring state-related ventilatory fluctuations throughout sleep onset under normoxic and hyperoxic conditions in subjects with high and low PCD. Results demonstrated that high-PCD subjects experienced significantly greater amplification of state-related ventilatory fluctuations than did low-PCD subjects. In addition, hyperoxia significantly reduced the amplification effect in high-PCD subjects but had little effect in low-PCD subjects. These results indicate that individuals with high PCD are likely to experience greater sleep-related ventilatory instability and suggest that peripheral chemoreceptor activity can contribute to sleep-disordered breathing.

Central sleep apnea

A central sleep apnea is the absence of respiratory effect, and, this, the absence of airflow during sleep. Central hypopnea, a related disorder, is also discussed. The sensory component of central sleep apnea; defects involving the integrative and executive neurons; non-neurologic causes of central sleep apneas, including chronic obstructive pulmonary disease and congestive heart failure; diagnosis; treatment; and other topics are reviewed.

Coma and confusional states: emergency diagnosis and management

Coma and confusion signal a failure of brain function with many possible causes. Since many of the potential causes may quickly lead to death or severe disability, it is important to develop a focused and ordered approach to facilitate the rapid diagnosis and early institution of proper therapies. This requires an understanding of the localizing features of the neurologic examination and of the syndromes likely to cause coma and confusion, a predetermined plan for empiric therapies in certain cases of doubt when diagnostic confirmation will be delayed, and a careful consideration of cases when the diagnosis is not revealed by the initial neuroimaging, lumbar puncture, or EEG.

Effects of mechanical unloading/loading on respiratory loop gain and periodic breathing in man

We investigated the effect of mechanical unloading and loading on Cheyne-Stokes respiration (CSR) in seven intubated patients with preexisting CSR. For mechanical loading patients had to breathe against the resistance of the endotracheal tube. For mechanical unloading patients were supported with a volume-proportional pressure support in the proportional assist ventilation (PAV) mode whilst the flow-dependent (nonlinear) endotracheal tube resistance was continuously compensated for by means of the automatic tube compensation (ATC) mode. Mechanical unloading aggravated CSR as revealed by a prolongation of apnea time and by an increase in the so-called strength index whereas mechanical loading shortened apnea time and decreased strength index. To test whether the observed changes are caused by the effect of mechanical unloading/loading on respiratory loop gain (relationship between minute ventilation and arterial CO2 tension), the response of respiratory loop gain on mechanical unloading/loading was determined in five healthy subjects (without CSR). In each subject, mechanical unloading increased respiratory loop gain whereas mechanical loading decreased it.

Effects of cardiac dysfunction on non-hypercapnic central sleep apnea

INTRODUCTION

Non-hypercapnic central sleep apnea (CSA) commonly occurs during nonrapid eye movement (non-REM) sleep in adults with congestive heart failure (CHF) and in some subjects without signs or symptoms of CHF. Hyperventilation, reduced lung volume, and circulatory delay are known to contribute to CSA, but to differing degrees depending on presence or absence of CHF.

AIM

To determine whether the pattern of ventilation during sleep could be used to determine the presence of CHF.

METHODS

Full polysomnographs demonstrating CSA were examined in 10 consecutive subjects with CHF and in 10 without CHF. Ventilatory, apnea, and cycle lengths, and circulation time (from the onset of ventilatory effort to the nadir of oximeter trace) were measured from cyclic apneas during non-REM sleep.

RESULTS

The non-CHF group had a greater left ventricular ejection fraction (LVEF) (59.7+/-1.9% vs 19.2+/-2.2%). Circulation time (11.8+/-0.5 s vs 24.9+/-1.7 s; p < 0.001) and cycle length (35.1+/-2.8 s vs 69.5+/-4.5 s; p < 0.001) were significantly greater in the CHF group compared with the non-CHF group, but not apnea length (21.3+/-1.8 s vs 26.8+/-2.0 s; p=0.06). Ventilatory length to apnea length ratio (VL:AL) was uniformly > 1.0 in the CHF group (mean, 1.65; range, 1.02 to 2.33), and in the non-CHF group < 1.0 (mean, 0.66; range, 0.54 to 0.89). LVEF correlated negatively with both circulation time (r=-0.86; p < 0.001) and cycle length (r=-0.79; p < 0.001).

CONCLUSION

The VL:AL ratio > 1.0, as well as both circulation time > 15 s and cycle length > 45 s, can be used to recognize the presence of CHF in subjects with CSA.

Central sleep apnoea and heart failure (Part I)

Central sleep apnoea (CSA) in congestive heart failure is sleep state dependent and occurs typically in stages I and II of non-REM sleep. The pre-requisites are hypocapnia and some prolongation of the circulation time. It is not certain whether abnormalities in after-discharge activity in the brainstem are also important. The presence of CSA in patients with left ventricular dysfunction is a poor prognostic sign and associated with a higher mortality in that group compared to age, sex and ejection fraction matched patients with congestive cardiac failure alone. It is reasonable to speculate that the CSA causes an increase in sympathetic nervous system activity which would maintain afterload at a high level or tend to increase it with time. The application of a high afterload to an impaired left ventricle leads over time to a further reduction in ejection fraction. From other studies, particularly ACE inhibitor studies, it is known that ejection fraction and prognosis are almost linearly related. It could therefore be said that once CSA has developed it may lead to a vicious circle of increasing afterload and further reduction in ejection fraction, causing worsening CSA and further increases in afterload. A number of treatments have been shown to be of benefit: supplemental nocturnal oxygen therapy, acetazolamide and nasal CPAP therapy have all been shown to reduce CSA. In addition nasal continuous positive airways pressure (CPAP) has been shown by two groups in Canada to also improve ejection fraction. The beneficial effects on ejection fraction in particular, persist after the treatment has been withdrawn, which suggests either remodelling of the left ventricular musculature or a resetting of the baseline sympathetic nervous system activity. The impressive increase in ejection fraction due to three months nasal CPAP therapy in one study (an average 35% increase) is both dramatic and exciting for the future. It is reasonable to expect improvement in prognosis for patients with CCF whose ejection fraction rises with CPAP treatment. Finally, only a limited number of studies have been published. Unfortunately the impressive results from Canada have not yet been reproduced in other centres around the world.

Paradoxical effect of oxygen administration on breathing stability following post-hyperventilation apnoea in lambs

1. Oxygen administration is thought to suppress periodic breathing (PB) by reducing carotid body activity, and yet earlier experiments in neonates have shown that PB incidence may be increased following the application of hyperoxia. To clarify this paradox, we studied the changes in the pattern of PB that occur following administration of oxygen in a lamb model of PB. 2. PB was induced in eleven of seventeen anaesthetized lambs following passive hyperventilation with air. When oxygen was administered during PB, the pattern was first enhanced, as evidenced by a sudden decrease in the ratio of the ventilatory duration to the apnoeic pause duration, and then suppressed, as evidenced by a progressive return to stable breathing which was associated with an increase in minute ventilation. 3. Five of the six lambs that did not show PB following passive hyperventilation with air could be made to do so if oxygen was substituted for air as the inspired gas following passive hyperventilation. 4. Five of the eleven lambs that showed PB following hyperventilation with air responded to the application of oxygen during PB by switching to a gross form of episodic breathing consisting of long apnoeic pauses followed by equally long periods of breathing during which minute ventilation fell progressively with time. 5. We conclude that when applied against a background of arterial hypoxaemia, oxygen has a destabilizing influence on ventilation in that (a) it accentuates the unstable breathing that occurs during PB, (b) it induces PB in lambs that exhibited stable breathing in air, and (c) it may precipitate episodic breathing.

Estimation of chemoreflex loop gain using pseudorandom binary CO2 stimulation

We have developed a method for deriving estimates of the chemoreflex control loop gain (LG) from the ventilatory response to inhaled CO2, modulated between 0% and 5% in the form of a pseudorandom binary sequence. The corresponding changes in alveolar (and thus, arterial) CO2 result from two components: 1) the direct effect of breath-to-breath changes in inhaled CO2 and 2) the chemoreflex-mediated changes in ventilation. LG between 0.01 and 0.03 Hz, the frequency range pertinent to periodic breathing, was estimated by computationally delineating the first component from the overall ventilatory response. The method was tested against simulated and experimental data. In both cases, we found strong correlations between our predictions and LG magnitude estimates derived by other methods. However, LG phase estimates were considerably more variable when compared to model predictions based on small-signal analysis. We propose that our method, which uses data from a single test procedure lasting < 10 min, may be more useful than traditional tests of chemoresponsiveness for the quantitative assessment of respiratory control stability during changes in sleep-wake state.

Central sleep apnea

A central sleep apnea is the absence of respiratory effect, and, thus, the absence of airflow during sleep. Central hypopnea, a related disorder, is also discussed. The sensory component of central sleep apnea; defects involving the integrative and executive neurons; non-neurologic causes of central sleep apneas, including chronic obstructive pulmonary disease and congestive heart failure; diagnosis; treatment; and other topics are reviewed in detail.

Post-hyperventilation hypopnea in humans during NREM sleep

We wished to determine if mild hypocapnia above the "apneic threshold" would result in apnea or hypopnea during NREM sleep. Hypocapnia was induced by nasal mechanical hyperventilation for 1 min either under normoxia (51 trials, n = 7) or hyperoxia (43 trials, n = 5). Cessation of mechanical ventilation resulted in hypopnea due to reduced VT without a change in f. Central apnea occurred mostly under hyperoxic conditions (9/43 versus 2/51 trials under normoxic conditions), and only when complete inhibition of ventilatory motor output occurred during mechanical ventilation. Significant correlation between the magnitude of hypocapnia and nadir VE was noted under both normoxic and hyperoxic conditions. However, nadir VE was variable when hypocapnia was modest (-2 mmHg); further hypocapnia (-4 mmHg) was associated with consistent reduction in nadir VE below 30% of control under normoxic conditions, and central apnea under hyperoxic conditions. We conclude that: (1) Brief hyperventilation during NREM sleep is followed by hypocapnic hypopnea due to reduced VT and not breathing frequency; (2) Hypocapnia due to brief mild hyperventilation does not cause central apnea unless peripheral chemoreceptors are also inhibited; (3) Sustained hyperventilation or more severe hypocapnia may be required for the development of hypocapnic central apnea during NREM sleep.

Pressure support ventilation during isoflurane anaesthesia

We have studied the respiratory effects of 5 and 10 cmH2O pressure support ventilation during anaesthesia with 1.5% end-tidal concentration of isoflurane in nine healthy, spontaneously breathing, adult patients. Some of the patients demonstrated an irregular respiratory pattern with periods of apnoea and we therefore went on to study a further seven patients with a continuous 500 s recording of airflow. Pressure support ventilation augmented mean (SD) tidal volume from 212 (56) ml to 360 (88) ml at 5 cmH2O and to 509 (108) ml at 10 cmH2O (n = 16, p < 0.05). Mean (SD) respiratory rate decreased from 26 (6) min-1 to 22 (6) min-1 at 5 cmH2O and 17 (5) min-1 at 10 cmH2O pressure support (n = 16, p < 0.05). Mean (SD) inspiratory work of breathing decreased from 1.77 (0.70) J. min-1 to 0.31 (0.36) J.min-1 at 5 cmH2O and 0.16 (0.26) J.min-1 at 10 cmH2O pressure support ventilation (n = 9, p < 0.05). Analysis of the respiratory rhythm in the second group of seven patients revealed an oscillating respiratory pattern in four patients at 5 cmH2O and six of the seven patients at 10 cmH2O pressure support ventilation. The metabolic advantage of the decreased work of breathing during pressure support ventilation during anaesthesia is unlikely to balance the disadvantage of an oscillating respiratory rhythm.

Post-hyperventilation apnoea in conscious humans

1. In nine normal subjects, analysis was performed of the number, length and location of apnoeic pauses during 20 min of recovery following voluntary overbreathing (VHV). Four different rates of recovery of end-tidal PCO2 (PET,CO2), studied in randomized order, were induced by overbreathing to 15 or 25 mmHg, each for 3 or 6 min. Subjects breathed mildly hyperoxic gas mixtures (inspired PO2 approximately 250 mmHg) to and fro into an open circuit via a mouthpiece and pneumotachograph. 2. Apnoeic pauses rarely occurred immediately after the end of VHV but gradually increased in number and length. When averaged across all subjects and protocols, the largest pauses occurred 2.0 +/- 0.3 min (S.D.; range 1.6-2.4 min) after the end of VHV. Based on a definition of apnoea as expiratory time greater than 6 s, apnoeas occurred between mean times of 0.8 and 5.6 min after the end of VHV, the end of this period being associated with a mean PET,CO2 value of 36.4 mmHg, which was below the initial mean resting value of 39.8 mmHg. 3. Within this apnoeic period, 80% of experiments produced apnoeas of less than 10 s duration, 61% of between 10 and 20 s duration and 42% of between 20 and 30 s duration. Only one out of nine subjects consistently failed to show apnoeas. 4. The range of lengths of individual apnoeas and the number per minute were independent of the length and level of VHV and were not significantly different between the four protocols. 5. The number and length of apnoeas did not change in repeated runs in each subject. We were not able to confirm previous reports that apnoeas occurred more frequently in subjects familiar with the experiment. 6. These results reconciled previous studies showing either apnoea or hyperpnoea following voluntary overbreathing in conscious humans. They showed an initial period of heightened breathing lasting about a minute with few apnoeas, consistent with 'after-discharge'. Beyond that, apnoeas occurred as an 'all-or-nothing' phenomenon as long as PET,CO2 was on average less than 3.4 mmHg below resting PET,CO2. The occurrence and length of apnoeas was consistent in individual subjects with no evidence of a learning effect.