Experimentally induced Cheyne-Stokes breathing

Flow-regulatory function of upper airway in health and disease: a unified pathogenetic view of sleep-disordered breathing

Although the Starling resistor behavior of the upper airway during sleep has been well established in health and disease, its physiological implications have not been fully appreciated. The purposes of the present communication are to reassess the current state of knowledge within the framework of the Starling resistor concept and to examine the implications of the concept on homeostatic feedback respiratory control and the pathogenesis of the sleep apnea syndrome. The main inferences drawn from the assessment include: (1) Owing to the Starling resistor properties of the upper airway and the well-organized neurochemical control mechanism, the upper airway performs important homeostatic flow regulatory function; it appropriately dampens the potentially unstable breathing during sleep and prevents the PaCO2 from falling below the apneic threshold; (2) Under certain conditions, the upper airway flow regulatory function fails to achieve appropriate dampening, leading to development of a variety of sleep-related breathing disorders that include underdamping due to overly sensitive central chemoresponsiveness and/or excessive lung to chemoreceptor transport lag--central sleep apnea; overdamping due to upper airway obstructive dysfunction--obstructive sleep apnea and/or hypopnea; and, finally, conditions with mixed features of central underdamping with coexisting collapsible upper airway; and (3) Successful treatment of these conditions requires restoration of appropriate damping. The overdamping imposed by the faulty upper airway is effectively reduced by surgical and medical approaches, and by application of nasal continuous positive airway pressure (CPAP). Reduction of PaCO2 by use of acetalzolamide and/or aminophylline reduces the plant gain, thus effectively offsetting the underdamping of central origin. Owing to the dual effect of nasal CPAP on the upper airway and respiratory pump, use of nasal CPAP can also effectively reduce the plant gain, accounting for the therapeutic effect of nasal CPAP on the central sleep apnea.

Mathematical study of periodic breathing as an instability of the respiratory system

A theoretical study of respiratory stability, based on a simple CO2 model of the respiratory system, investigates each component of respiration: the plant system and the central and peripheral controller systems. Analysis of the dynamic properties of the plant leads to a simplified respiratory model for the study of the influence of the central and peripheral controller components on stability. It is shown that the central component is not involved in respiratory instability phenomena such as periodic breathing whereas the peripheral component plays a major role. The explicit analytical index of stability obtained allows definition of the conditions of occurrence of periodic breathing in terms of the fundamental respiratory parameters. Moreover, this index can be used to evaluate the influence of various respiratory parameters on the stability of respiration.

Pulmonary Manifestations of Neurological Disease

Breathing is a complicated act that requires sophisticated control mechanisms. The nervous system coordinates 3 fundamentally important components of respiration. The central nervous system has a central pattern generator that, along with appropriate feedback mechanisms, establishes both the resiratory rate and the depth of respirations. The peripheral nervous system facilitates transmission of these respiratory commands to the ventilatory muscles. The nervous system also contributes to the control of airway size. Laryngeal function is coordinated with inspiration, and local nerves in the lung have a major role in determining bronchial patency. Finally, the nervous system acts in incompletely understood ways to regulate perfusion of the lungs and to match local pulmonary blood flow with ventilation. Failure of the nervous system to adequately control these 3 different aspects of respiration may result in lifethreatening illness. Understanding how the nervous system affects control of ventilation, airway patency, and pulmonary perfusion therefore will enable intensivists to recognize and manage the pulmonary complications of neurological disease.

Pulmonary status of habitual cocaine smokers

We determined the prevalence of respiratory symptoms and lung dysfunction in a large sample of habitual smokers of freebase cocaine ("crack") alone and in combination with tobacco and/or marijuana. In addition, we compared these findings with those in an age- and race-matched sample of nonusers of crack who did or did not smoke tobacco and/or marijuana. A detailed respiratory and drug use questionnaire and a battery of lung function tests were administered to (1) a convenience sample of 202 habitual smokers of cocaine (cases) who denied intravenous drug abuse and (2) a reference sample of 99 nonusers of cocaine (control subjects). The cocaine smokers (85% black) included the following: 68 never-smokers of marijuana, of whom 43 currently smoked tobacco and 25 did not, and 134 ever-smokers of marijuana (42 current and 92 former), of whom 92 currently smoked tobacco and 42 did not. The control subjects (96% black) included the following: 69 never-smokers of marijuana, of whom 26 currently smoked tobacco and 43 did not, and 30 ever-smokers of marijuana (18 current and 12 former), of whom 21 currently smoked tobacco and 9 did not. Cases smoked an average of 6.5 g cocaine per week for a mean of 53 months. The median time of the most recent use of crack prior to study was 19 days (range less than 1 to 180 days). After controlling for the use of other smoked substances, frequent crack use was associated with: (1) a high prevalence of at least occasional occurrences of acute cardiorespiratory symptoms within 1 to 12 h after smoking cocaine (cough productive of black sputum [43.7%], hemoptysis [5.7%], chest pain [38.5%], usually worse with deep breathing, and cardiac palpitations [52.6%]) and (2) a mild but significant impairment in the diffusing capacity of the lung.(ABSTRACT TRUNCATED AT 250 WORDS)

The abnormal pupil in Cheyne-Stokes respiration. Case report

Cheyne-Stokes respiration commonly induces a rhythmic pupillary dilatation during hyperpnea and constriction during apnea. Failure of a pupil to dilate during hyperventilation indicates underlying sympathetic nerve paralysis. This report deals with an instance in which one pupil failed to constrict during apnea due to oculomotor nerve compression. The periodic respirations and anisocoria disappeared following surgical evacuation of a large ipsilateral subdural hematoma.

Factors affecting respiratory system stability

Periodic breathing (recurrent central apneas) occurs frequently during sleep. Periodic breathing can arise as a result of unstable behavior of the respiratory control system. A mathematical model of the respiratory control system was used to investigate, systematically, the effect of severity of disturbances to respiration and certain system parameters on periodic breathing occurring during sleep. The model consisted of multi-compartment representation of O2 and CO2 stores, a peripheral controller sensitive to O2 and CO2, and a central controller sensitive to CO2. The effects of hypoxia and hypercapnia on the upper airway muscles were not considered in the model. Episodes of hyperventilation or asphyxia were used to disturb the control system and explore the boundaries of stable breathing. Circulation time and metabolic rate were also varied. Simulations with the model produced the following findings: The number of central apneas associated with periodic breathing were greater as circulation time increased; controller gain increases also made the number of apneas greater, although periodic breathing occurs with lower controller gains as circulation time increases. At each level of circulation time there was a range of controller gain changes which caused little change in the number of apneas. There were more apneas with hypoxia; also the number of apneas increased with sleep-associated reductions in metabolic rate. The more rapidly resting PCO2 rose at sleep onset, the greater the likelihood of recurrent apneas. Finally, the more intense the disturbance, the more apneas there were.

Respiratory and arousal responses to hypoxia in apnoeic infants reinvestigated

Respiratory and arousal responses to mild hypoxia (15% oxygen in nitrogen) were recorded in 18 healthy infants and 33 infants who had sustained severe sleep related apnoeic events (ALTE). Respiratory movements and transcutaneous gas pressures (tcPO2 and tcPCO2) were continuously monitored during the 10 min test. The changes in tcPCO2 in relation to the decrease in tcPO2 were used as an index of the ventilatory and metabolic responses to hypoxia. We found that the response of apnoeic infants was within the range of the controls although the distribution of the individual response slopes was shifted towards the lower end of the range. Arousal was observed in 33% of apnoeic infants and 32% of the controls. Regular periodic breathing occurred in 42% of apnoeic infants compared to 28% of controls. In contrast to the controls, periodic breathing in apnoeic infants was not associated with a drop in tcPCO2 to below baseline levels. Apnoeic infants also alternated between regular and periodic breathing during the test. These findings are suggestive of a weak feed back control of breathing but do not support former views of a deficient hypoxic response in infants with ALTE.

CO2 control of the respiratory system: plant dynamics and stability analysis

A stability analysis of respiratory chemical control is developed using a mathematical model of CO2 mass transport dynamics. Starting with a 3-compartment model of CO2 stores that distinguishes alveolar, muscle, and other tissue, model reduction techniques are applied to obtain a first-order representation of the respiratory plant. This model contains an effective tissue volume for CO2, whose derived value is much smaller than previously predicted. To investigate oscillatory instabilities, a controller which incorporates only peripheral chemoreceptor responses was added to the first-order plant model. An explicit stability index (SI) is obtained analytically from a linearized version of this model. SI varies directly with the controller gain and circulation delay time and inversely with the effective tissue volume and inspired CO2 concentration. Numerical simulations using the first-order nonlinear model show that SI is a good predictor of system stability. According to the linearized model, the system is stable for SI less than 1; from the nonlinear model, the system is stable for SI less than 1.1. For typical normal adults, the SI value is well within the stable region.

Effects of acetazolamide in patients with the sleep apnoea syndrome

There is as yet no convincing evidence that acetazolamide, a carbonic anhydrase inhibitor, is effective in obstructive sleep apnoea. A study was therefore designed to examine the effect of acetazolamide (250 mg/day) on sleep events and ventilatory control during wakefulness in nine patients with the sleep apnoea syndrome. In eight of the nine patients the apnoea index and the total duration of apnoea were reduced by acetazolamide, and the mean (SEM) apnoea index of all patients changed from 25.0 (6.7) to 18.1 (5.8) episodes an hour. Furthermore, the total time of arterial oxygen desaturation (SaO2)--more than 4% depression in SaO2 from the baseline sleeping level--divided by total sleep time was also significantly decreased and its mean (SEM) value improved from 24.1 (7.9) to 13.6 (4.8)% of total sleep time. Five of the seven patients with varying degrees of daytime hypersomnolence had their symptoms obviously improved. There was no patient whose predominant type of apnoea was converted from the obstructive to the central type, or vice versa. In the studies of wakefulness, metabolic acidosis, an increase of arterial oxygen tension (PaO2) and a decrease of arterial carbon dioxide tension (PaCO2) were observed. The slopes of the occlusion pressure response and the ventilatory response to carbon dioxide increased, and the carbon dioxide ventilatory response line shifted to the left. It is suggested that acetazolamide cannot remove apnoea completely but has a beneficial effect in mild cases of obstructive sleep apnoea through an augmentation of central (CO2, H+) drive and a stabilising effect on ventilatory control.

Effects of theophylline on ventilatory response to hypoxic challenge

The respiratory and arousal responses to mild hypoxia during quiet sleep were studied using inductive plethysmography and transcutaneous gas electrodes in 11 apnoeic infants before and after the administration of oral theophylline (3 mg/kg). Theophylline changed the ventilatory response to a more biphasic pattern--that is, ventilation decreased after an initial increase. The relative ventilatory slope (defined as the decrease in transcutaneous carbon dioxide tension (PCO2) in relation to the fall in transcutaneous oxygen tension (PO2)) decreased significantly after theophylline. Four infants were roused during hypoxia before theophylline administration compared with none after treatment. Theophylline abolished the periodic breathing induced by hypoxia in one of six infants. These findings suggest that methylxanthines may not, as previously thought, enhance the respiratory drive during hypoxia.

A possible mechanism for mixed apnea in obstructive sleep apnea

Hypopneas or pauses in respiratory effort frequently precede episodes of obstructive sleep apnea resulting in mixed apneas. We studied five subjects after chronic tracheostomy for obstructive sleep apnea. During stable non-REM (NREM) sleep, subjects breathed entirely through the tracheostomy. Tracheostomy occlusion caused experimental obstructive apnea which lasted 13.9 +/- 4.7 sec and ended with transient arousal and pharyngeal opening. At the end of the apnea there was marked hyperventilation (inspired minute ventilation rose 21.6 +/- 3.5 L on the first breath) followed by hypocapnia, hypopnea, and pauses in inspiratory effort as the subjects resumed NREM sleep. Hypocapnia was greater before inspiratory pauses lasting at least 5 sec than before shorter pauses (PETco2, 4.2 +/- 1.8 mm Hg below baseline vs 1.2 +/- 2.5 mm Hg below baseline). In three patients, pauses in inspiratory effort following experimental obstructive apnea were prevented by administration of 4 percent CO2 and 40 percent O2 inspired gas. This study suggests that: hyperventilation with hypocapnia occurs at the termination of obstructive apneas, and hypocapnia may be responsible for the attenuation or cessation of respiratory effort initiating the subsequent cycle of obstruction.

Hopf bifurcations and the stability of the respiratory control system

A simple model of a feedback loop controlling ventilation is analysed. This model is intended to describe the response of the system, initially at equilibrium, to a sudden fall in CO2 concentration in the lung, brought about by a deep sigh. A previous paper described the model in detail and the general method of analysis. Here we continue the discussion of stability, first in terms of local stability after a small displacement from equilibrium and then by computer simulation to illustrate the behaviour after large displacements. The local analysis is used to select representative sets of system parameters to illustrate the different types of trajectory obtained by computer simulation. When the equilibrium point is stable the response to a disturbance is overdamped, underdamped or critically damped. When the equilibrium point is unstable the system responds by going into a limit cycle. The transition between these two cases proceeds via a Hopf Bifurcation. The limit cycle type of ventilatory pattern, i.e. a periodic, underdamped waxing and waning of ventilation is commonly seen in premature infants and in term infants between 1 and 6 months of age.

The development of stability of respiration in human infants: changes in ventilatory responses to spontaneous sighs

Serial respiratory recordings using impedance pneumography and barometric plethysmography were made from shortly after birth to 7 months in fifteen normal full-term infants. Each recording was made with the infant asleep and sleep state was estimated from records of electroencephalogram and electro-oculogram made in parallel. The respiratory records obtained during non-rapid eye movement (r.e.m.) sleep were analysed with computer assistance and stretches of the record, approximately 1 min before and up to 2 min after a spontaneous sigh and ensuing apnoeic pause, were processed and presented as sequential values of the fractional deviation of VE, the breath by breath minute volume, from the mean. That part of the sequence which represented the respiratory response to the sigh was then fitted with second order equations representing the critically or underdamped response. The results were presented for each curve in terms of xi, the damping ratio and omega n, the frequency of the undamped respiratory oscillation. Three-quarters of the responses could be so fitted with an error of 20% or less. The residual responses were mainly from infants within a few days of birth. In the youngest infants (4 days or less), the respiratory response to a sigh was highly stable but sluggish: during the period 4-8 days to 3-4 months, the oscillatory period diminished from ca. 25-12 s and respiration was potentially unstable since a small reduction in the damping factor would cause prolonged oscillation while, from 3-4 months, the more mature type of response which was stable with a rapid recovery supervened. The possible mechanisms responsible for this trend are discussed in terms of the factors thought to determine respiratory stability in the adult together with the possible relevance of the results to the normal process of respiratory adaptation at birth and to the respiratory difficulties encountered by some infants in the new-born period and early infancy.

Mechanisms of hypoxia-induced periodic breathing during sleep in humans

Ventilation was studied during wakefulness and sleep in six healthy humans in normoxia (mean barometric pressure (PB) = 740 torr), and in hypobaric hypoxia (PB = 455 torr). Hypoxia caused hyperventilation and hypocapnic alkalosis (delta Pa,CO2 = -7 torr) during wakefulness and in all sleep states. Periodic breathing was the predominant pattern of breathing in all stages of non-rapid eye movement (non-r.e.m.) sleep in hypoxia, but was rarely observed during wakefulness or r.e.m. sleep. Periodic breathing was composed of repetitive oscillations of reproducible cycle length characterized by clusters of breaths with augmented inspiratory effort (VT/TI) and highly variable distribution of breath-to-breath minute ventilation (VE) and tidal volume (VT), which alternated regularly with prolongations of the expiratory pause of the last breath of each cluster (apnea duration = 5-18 sec). Hypoxia-induced periodic breathing was eliminated by: (a) acute restoration of normoxia coincident with a 3-6 torr increase in Pa,CO2; and (b) augmented FI,CO2 (at constant arterial oxygen saturation) which rapidly and reversibly eliminated apneas and stabilized breathing pattern with a less than 2 torr increase in Pa,CO2. If hypocapnia was prevented (by augmented FI,CO2) during acute induction of hypoxia in non-r.e.m. sleep, periodic breathing was also prevented. We propose that the genesis of hypoxia-induced periodic breathing requires the combination of hypoxia and hypocapnia. Periodicity results from oscillations in CO2 about a CO2-apnea threshold whose functional expression is critically linked to sleep state.

Neonatal breathing control mediated via the central chemoreceptors

Respiratory changes elicited via the central chemoreceptor system have been studied in anesthetized newborn guinea pigs and newborn rabbits. Periodic breathing was induced by inhibition of the central chemoreceptors by superfusion with alkaline cerebrospinal fluid. The periodic breathing was promptly reversed to steady by increasing the oxygen or carbon dioxide concentration in the inspired air or by intravenous theophylline. Elicitation of periodic breathing simply by exposing the animals to hypoxia succeeded only when very low oxygen concentrations were given. Clearcut respiratory excitation was produced by small amounts of theophylline applied onto the ventral surface of the medulla. Not only theophylline intravenously but also theophylline topically applied on the ventral medullary surface normalized spontaneously developed periodic breathing. Application of meperidine onto the ventral medullary surface gave respiratory inhibition with dosages considerably lower than required when given intravenously. The results emphasize the importance of an adequate respiratory drive from the central chemoreceptors for the maintenance of a regular breathing pattern. The findings support a view that at least part of the respiratory effects seen in the newborn following administration of meperidine or theophylline is due to effect of the drugs on the central chemosensitive system.

Sleep apnea considered as a control system instability

In the present study a mathematical model of the chemical control of respiration is described which attempts to simulate periodic breathing during sleep. The model is an extension of an earlier model which has been shown to successfully reproduce the transient effects of CO2 inhalation on breathing, controlled changes in ventilation on arterial gas tension, and Cheyne-Stokes breathing. Included in the extended model are the effects of chemical stimuli during sleep on both chest wall and upper airway muscle activity. Data is presented indicating that simulations from the model reproduce reasonably well the essential features of the results obtained in eight subjects with periodic respiration during sleep when breathing room air, O2, or low concentrations of CO2. Simulations from the model and the experimental data suggest that periodic breathing during sleep results from unstable operation in the respiratory control system analogous to that seen during instabilities in physical control systems. The model indicates that obstructive as well as central apneas can be produced by control system instability. Furthermore, central apneas increase the likelihood of obstructive apneas while obstructive apneas tend to aggravate the control instability. The model results predict that the characteristics of the periodic breathing seen during sleep, such as apnea length, will depend on circulation time and the sensitivity of both upper airway and chest wall muscles to hypercapnia and hypoxia.

Respiratory sensitivity before and after birth

A description is first given of respiratory activity in the fetus and its control. Evidence suggests that when the fetus makes respiratory movements, it is in a state comparable to REM sleep in the newborn and adult and that in the alternating periods of apnoea, it is in quiet sleep. It does not appear that the respiratory movements are normally regulated by chemical or reflex, e.g. Hering Breuer, inputs though they are enhanced by CO2 and depressed by hypoxia. In the apnoeic periods, breathing movements are virtually impossible to elicit by chemical or reflex means. Evidence from examination of peripheral inputs indicates that: the carotid body chemoreceptors are inhibited at receptor level, stimulation of the aortic chemoreceptors affects the circulation only and although pulmonary stretch receptors are active and are excited by inflation of the fetal lung, such inflation does not affect discharge in medullary respiratory units or phrenic nerve. Since there is no real evidence of immaturity of the respiratory system in late gestation and since chemical and most reflex inputs appear to provide an adequate stimulus, it is most probable that the periods of apnoea are caused by an inhibitory process, possibly of supra-pontine origin which acts close to medullary respiratory units and effectively inhibits the operation of the automatic component. This inhibitory process may operate periodically; or continuously and be periodically overridden in REM sleep. After birth, breathing is normally continuous and sensitive to lung inflation, CO2 and after a variable delay, to hypoxia. This may be due to the lifting of the inhibitory process allowing activation of the automatic component. However, there is evidence that even in normal, full term infants, full maturation of the automatic component is not complete until about three months of age and in the meanwhile, breathing tends to be imperfectly regulated and subject to damped oscillations when disturbed.

Experimental Biot periodic breathing in cats: effects of changes in PiO2 and PiCO2

Periodic breathing of the Biot or cluster type was induced in spontaneously breathing, pentobarbital anesthetized cats by placing bilateral lesions within the pneumotaxic system of the rostral pons. Control lesions positioned outside of the critical nuclei never resulted in Biot breathing. The periodic pattern was characterized by clusters of breaths which were separated by distinct periods of apnea and was clearly not of Cheyne-Stokes quality. Test gas challenges inducing hypoxia and hypercapnia tended to diminish the apneic breatholds, whereas hyperoxia potentiated the periodic breathing by increasing the duration of the non-ventilatory phase. Only hypercapnia significantly altered the tidal volume of Biot breaths by increasing the depth of breathing. No conclusions can be drawn as to whether the Biot pattern arises from an inherent central respiratory controller periodicity, or from oscillations in arterial blood gas tensions and peripheral chemoreceptor (and mechanoreceptor) inputs. It is suggested that the experimental model for Biot breathing may be of unique importance for studying the control of expiratory duration, particularly apnea. Also, it is of interest that similar breathing patterns and gas responses occur in the neonate and adult.

Characteristics and rate of occurrence of spontaneous and provoked augmented breaths

The tidal volume and corresponding efferent phrenic activity of spontaneously occurring and provoked "augmented" breaths, AB, and the subsequent post-augmented breaths were studied in cats anesthetized with pentobarbitone during hypercapnia and hypoxia. The augmentation phase (phase II) begins at, or close to, the crest of what appears as a "normal" inspiration (phase I). The amplitude and duration of phase II remained fairly constant whereas the amplitude and the duration of phase I changed with chemical drive just as in control breaths. The smaller amplitude and shorter duration of post-augmented breaths as compared to control breaths seems to be due to both a lower-than-normal inspiratory "off-switch" threshold following the AB and an increased rate of rise of inspiratory activity. With increasing hypercapnia and hypoxia both the time interval between AB and the refractory period following an AB during which a new AB cannot be provoked were reduced. Following bilateral vagotomy AB was temporarily abolished but reappeared after 1-2 h. The relatively low rates of occurrence after vagotomy still showed the same type of dependence on chemical stimuli. The refractory period was not abolished although usually decreased by gallamine paralysis or high thoracic spinalization.