Nonchemical Elimination of Inspiratory Motor Output via Mechanical Ventilation in Sleep

Bifurcation of the respiratory response to lung inflation in anesthetized dogs

Numerous studies have demonstrated the effect of lung volume on prolongation of duration of expiration (TE) with limited understanding of the TE shortening and termination of expiration as observed in newborn. In 14 dogs, the effects of varied onset of lung inflation during expiration on the TE were evaluated. When lung inflation was applied in the first part of expiration (20-60% of TE) TE was lengthened. However, in the second portion (60-80% of TE) of expiration, lung inflation either terminated or prolonged TE; whereas in the last portion of expiration (80-90% of TE), lung inflation tended to terminate expiration prematurely. The effects were abolished after bilateral vagotomy. We postulate that prolongation of TE relates to the Breuer-Hering inflation reflex, which increases the time needed for a passive expiration; whereas the ability to shorten TE could relate to Head's paradoxical reflex acting to initiate inspiration or to activate inspiratory motor activity to brake expiratory flow as occurs in the newborn.

Definition, discrimination, diagnosis and treatment of central breathing disturbances during sleep

The complexity of central breathing disturbances during sleep has become increasingly obvious. They present as central sleep apnoeas (CSAs) and hypopnoeas, periodic breathing with apnoeas, or irregular breathing in patients with cardiovascular, other internal or neurological disorders, and can emerge under positive airway pressure treatment or opioid use, or at high altitude. As yet, there is insufficient knowledge on the clinical features, pathophysiological background and consecutive algorithms for stepped-care treatment. Most recently, it has been discussed intensively if CSA in heart failure is a “marker” of disease severity or a “mediator” of disease progression, and if and which type of positive airway pressure therapy is indicated. In addition, disturbances of respiratory drive or the translation of central impulses may result in hypoventilation, associated with cerebral or neuromuscular diseases, or severe diseases of lung or thorax. These statements report the results of an European Respiratory Society Task Force addressing actual diagnostic and therapeutic standards. The statements are based on a systematic review of the literature and a systematic two-step decision process. Although the Task Force does not make recommendations, it describes its current practice of treatment of CSA in heart failure and hypoventilation.

Functional connectivity in raphé-pontomedullary circuits supports active suppression of breathing during hypocapnic apnea

Hyperventilation is a common feature of disordered breathing. Apnea ensues if CO2 drive is sufficiently reduced. We tested the hypothesis that medullary raphé, ventral respiratory column (VRC), and pontine neurons have functional connectivity and persistent or evoked activities appropriate for roles in the suppression of drive and rhythm during hyperventilation and apnea. Phrenic nerve activity, arterial blood pressure, end-tidal CO2, and other parameters were monitored in 10 decerebrate, vagotomized, neuromuscularly-blocked, and artificially ventilated cats. Multielectrode arrays recorded spiking activity of 649 neurons. Loss and return of rhythmic activity during passive hyperventilation to apnea were identified with the S-transform. Diverse fluctuating activity patterns were recorded in the raphé-pontomedullary respiratory network during the transition to hypocapnic apnea. The firing rates of 160 neurons increased during apnea; the rates of 241 others decreased or stopped. VRC inspiratory neurons were usually the last to cease firing or lose rhythmic activity during the transition to apnea. Mayer wave-related oscillations (0.04-0.1 Hz) in firing rate were also disrupted during apnea. Four-hundred neurons (62%) were elements of pairs with at least one hyperventilation-responsive neuron and a correlational signature of interaction identified by cross-correlation or gravitational clustering. Our results support a model with distinct groups of chemoresponsive raphé neurons contributing to hypocapnic apnea through parallel processes that incorporate disfacilitation and active inhibition of inspiratory motor drive by expiratory neurons. During apnea, carotid chemoreceptors can evoke rhythm reemergence and an inspiratory shift in the balance of reciprocal inhibition via suppression of ongoing tonic expiratory neuron activity.

Ventilatory failure, ventilator support, and ventilator weaning

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

Central sleep apnea

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

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.

Ventilation induced apnea and its effect on dorsal brainstem inspiratory neurones in the rat

The purpose of this study was to examine the effect of mechanical ventilation (MV) on inherent breathing and on dorsal brainstem nucleus tractus solitarius (NTS) respiratory cell function. In pentobarbitone-anaesthetised rats, application of MV at combined high frequencies and volumes (representing threshold levels) produced apnea. The apnea persisted as long as MV was maintained at or above the threshold frequency and volume. Following removal of MV, inherent breathing did not resume immediately, with the diaphragm exhibiting post-mechanical ventilation apnea. The fall in arterial P(CO2) (Pa(CO2)) levels evoked by MV-engendered hyperventilation was shown not to be the trigger for initiation of apnea. MV-induced apnea was immediately reversed by bilateral vagotomy. Further, MV-induced apnea could not be evoked in bilaterally vagotomized animals suggesting that vagal feedback is the critical pathway for its initiation. NTS inspiratory neurones were inhibited during both MV-induced apnea and post-mechanical ventilation apnea, implying the involvement of central neural mechanisms in mediating this effect.

Increased propensity for apnea in response to acute elevations in left atrial pressure during sleep in the dog

Periodic breathing is commonly observed in chronic heart failure (CHF) when pulmonary capillary wedge pressure is abnormally high and there is usually concomitant tachypneic hyperventilation. We hypothesized that acute pulmonary hypertension at pressures encountered in CHF and involving all of the lungs and pulmonary vessels would predispose to apnea/unstable breathing during sleep. We tested this in a chronically instrumented, unanesthetized dog model during non-rapid eye movement (NREM) sleep. Pulmonary hypertension was created by partial occlusion of the left atrium by means of an implanted balloon catheter in the atrial lumen. Raising mean left atrial pressure by 5.7 +/- 1.1 Torr resulted immediately in tachypneic hyperventilation [breathing frequency increased significantly from 13.8 to 19.9 breaths/min; end-tidal P(CO2) (P(ET(CO2))) fell significantly from 38.5 to 35.9 Torr]. This tachypneic hyperventilation was present during wakefulness, NREM sleep, and rapid eye movement sleep. In NREM sleep, this increase in left atrial pressure increased the gain of the ventilatory response to CO2 below eupnea (1.3 to 2.2 l.min(-1).Torr(-1)) and thereby narrowed the CO2 reserve [P(ET(CO2)) (apneic threshold) - P(ET(CO2)) (eupnea)], despite the decreased plant gain resulting from the hyperventilation. We conclude that acute pulmonary hypertension during sleep results in a narrowed CO2 reserve and thus predisposes toward apnea/unstable breathing and may, therefore, contribute to the breathing instability observed in CHF.

Crossing the apnoeic threshold: causes and consequences

This brief review addresses the characteristics, lability and the mechanisms underlying the hypocapnic-induced apnoeic threshold which is unmasked during NREM sleep. The role of carotid chemoreceptors as fast, sensitive detectors of dynamic changes in CO2 is emphasized and placed in historical context of the long-held debate over central vs. peripheral contributions to CO2 sensing and to apnoea. Finally, evidence is presented which points to a significant role for unstable, central respiratory motor output as a significant contributor to upper airway narrowing and obstruction during sleep.

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.

Prolonged neural expiratory time induced by mechanical ventilation in infants

Mechanical ventilation may interfere with the spontaneous breathing pattern in infants because they have strong reflexes that play a large role in the control of breathing. This study aimed to answer the following questions: does a ventilator-assisted breath 1) reduce neural inspiratory time, 2) reduce the amplitude of the diaphragm electrical activity, and 3) prolong neural expiration, within the delivered breath? In 14 infants recovering from acute respiratory failure (mean age and weight were 2.3 +/- 1.3 mo and 3.95 +/- 0.82 kg, respectively), we measured 1) the electrical activity of the diaphragm with a multiple-array esophageal electrode, and 2) airway opening pressure, while patients breathed on synchronized intermittent mandatory ventilation (mean rate, 11.2 +/- 6.5 breaths/min). We compared neural inspiratory and expiratory times for the mandatory breaths and for the spontaneous breaths immediately preceding and following the mandatory breath. Although neural inspiratory time was not different between mandatory and spontaneous breaths, neural expiratory time was significantly increased (p < 0.001) for the mandatory breaths (953 +/- 449 ms) compared with the premandatory and postmandatory spontaneous breaths (607 +/- 268 ms and 560 +/- 227 ms, respectively). Delivery of the mandatory breath resulted in a reduction in neural respiratory frequency by 28.6 +/- 6.4% from the spontaneous premandatory frequency. The magnitude of inspiratory electrical activity of the diaphragm was similar for all three breath conditions. For the mandatory breaths, ventilatory assist persisted for 507 +/- 169 ms after the end of neural inspiratory time. Infant-ventilator asynchrony (both inspiratory and expiratory asynchrony) was present in every mandatory breath and constituted 53.4 +/- 26.2% of the total breath duration.

Controlled versus assisted mechanical ventilation effects on respiratory motor output in sleeping humans

Central apneas occur after cessation of mechanical ventilation despite normocapnic conditions. We asked whether this was due to ventilator-induced increases in respiratory rate or VT. Accordingly, we compared the effects of increased VT (135 to 220% of eupneic VT) with and without increased respiratory rate, using controlled and assist control mechanical ventilation, respectively, upon transdiaphragmatic pressure in sleeping humans. Increasing ventilator frequency +1 per minute and VT to 165-200% of baseline eupnea eliminated transdiaphragmatic pressure during controlled mechanical ventilation and prolonged expiratory time (two to four times control) after mechanical ventilation. During and after assist control mechanical ventilation at 135-220% of eupneic VT, transdiaphragmatic pressure was reduced in proportion to the increase in ventilator volume. However, every ventilator cycle was triggered by an active inspiration, and immediately after mechanical ventilation, expiratory time during spontaneous breathing was prolonged less than 20% of that observed after controlled mechanical ventilation at similar VT. We conclude that both increased frequency and VT during mechanical ventilation significantly inhibited respiratory motor output via nonchemical mechanisms. Controlled mechanical ventilation at increased frequency plus moderate elevations in VT reset respiratory rhythm and inhibited respiratory motor output to a much greater extent than did increased VT alone.

Carotid body denervation eliminates apnea in response to transient hypocapnia

We determined the effects on breathing of transient ventilatory overshoots and concomitant hypocapnia, as produced by pressure support mechanical ventilation (PSV), in intact and carotid body chemoreceptor denervated (CBX) sleeping dogs. In the intact dog, PSV-induced transient increases in tidal volume and hypocapnia caused apnea within 10-11 s, followed by repetitive two-breath clusters separated by apneas, i.e., periodic breathing (PB). After CBX, significant expiratory time prolongation did not occur until after 30 s of PSV-induced hypocapnia, and PB never occurred. Average apneas of 8.4 +/- 1-s duration after a ventilatory overshoot required a decrease below eupnea of end-tidal Pco(2) 5.1 +/- 0.4 Torr below eupnea in the intact animal and 10.1 +/- 2 Torr in the CBX dog, where the former reflected peripheral and the latter central dynamic CO(2) chemoresponsiveness, as tested in the absence of peripheral chemoreceptor input. Hyperoxia when the dogs were intact shortened PSV-induced apneas and reduced PB but did not mimic the effects of CBX. We conclude that, during non-rapid eye movement sleep, carotid chemoreceptors are required to produce apneas that normally occur after a transient ventilatory overshoot and for PB.

Response of ventilator-dependent patients to delayed opening of exhalation valve

In conventional mechanical ventilation, the inflation cycle often extends into neural expiration (TEN), potentially exacerbating dynamic hyperinflation (DH). We wished to determine the extent to which patients defend against DH when this happens. Such defense may include prolongation of TEN (timing response) and/or expiratory muscle recruitment (neuromuscular response). Fifty patients were ventilated in the Proportional Assist mode, allowing us to infer these responses noninvasively. At random intervals, exhalation of single breaths was delayed by briefly delaying the opening of exhalation valve (occlusion) (T(occ) = 0.78 +/- 0.34 seconds). Timing response was assessed from the change in TEN. Neuromuscular response was assessed from the difference between volume exhaled after release of occlusion and volume exhaled in unoccluded breaths over a similar expiratory flow duration (DeltaV(iso-time)). There was no evidence of an acute neuromuscular response; DeltaV(iso-time) averaged 0.005 +/- 0.023 L (NS). Forty-five of 50 patients significantly lengthened TEN. However, the timing response offset only 36 +/- 20% of the delay in expiration. Because of absent neuromuscular responses and weak timing responses, DH increased in most patients in postocclusion breaths (DeltaDH = 0.10 +/- 0.08 L, p = 2E-10). We conclude that acute compensatory responses to delays in opening of exhalation value are weak in ventilator-dependent patients. As a result, such nonsynchrony tends to exacerbate DH.

Effect of ventilatory drive on carbon dioxide sensitivity below eupnea during sleep

We determined the effects of changing ventilatory stimuli on the hypocapnia-induced apneic and hypopneic thresholds in sleeping dogs. End-tidal carbon dioxide pressure (PET(CO2)) was gradually reduced during non-rapid eye movement sleep by increasing tidal volume with pressure support mechanical ventilation, causing a reduction in diaphragm electromyogram amplitude until apnea/periodic breathing occurred. We used the reduction in PET(CO2) below spontaneous breathing required to produce apnea (DeltaPET(CO2)) as an index of the susceptibility to apnea. DeltaPET(CO2) was -5 mm Hg in control animals and changed in proportion to background ventilatory drive, increasing with metabolic acidosis (-6.7 mm Hg) and nonhypoxic peripheral chemoreceptor stimulation (almitrine; -5.9 mm Hg) and decreasing with metabolic alkalosis (-3.7 mm Hg). Hypoxia was the exception; DeltaPET(CO2) narrowed (-4.1 mm Hg) despite the accompanying hyperventilation. Thus, hyperventilation and hypocapnia, per se, widened the DeltaPET(CO2) thereby protecting against apnea and hypopnea, whereas reduced ventilatory drive and hypoventilation narrowed the DeltaPET(CO2) and increased the susceptibility to apnea. Hypoxia sensitized the ventilatory responsiveness to CO2 below eupnea and narrowed the DeltaPET(CO2); this effect of hypoxia was not attributable to an imbalance between peripheral and central chemoreceptor stimulation, per se. We conclude that the DeltaPET(CO2) and the ventilatory sensitivity to CO2 between eupnea and the apneic threshold are changeable in the face of variations in the magnitude, direction, and/or type of ventilatory stimulus, thereby altering the susceptibility for apnea, hypopnea, and periodic breathing in sleep.

Effect of hypoxia on the hypopnoeic and apnoeic threshold for CO(2) in sleeping humans

1. Rhythmic breathing during sleep requires that P(CO2) be maintained above a sensitive hypocapnic apnoeic threshold. Hypoxia causes periodic breathing during sleep that can be prevented or eliminated with supplemental CO(2). The purpose of this study was to determine the effect of hypoxia in changing the difference between the eupnoeic P(CO2) and the P(CO2) required to produce hypopnoea or apnoea (hypopnoea/apnoeic threshold) in sleeping humans. 2. The effect of hypoxia on eupnoeic end-tidal partial pressure of CO(2) (P(ET,CO2)) and hypopnoea/apnoeic threshold P(ET,CO2) was examined in seven healthy, sleeping human subjects. A bilevel pressure support ventilator in a spontaneous mode was used to reduce P(ET,CO2) in small decrements by increasing the inspiratory pressure level by 2 cmH2O every 2 min until hypopnoea (failure to trigger the ventilator) or apnoea (no breathing effort) occurred. Multiple trials were performed during both normoxia and hypoxia (arterial O(2) saturation, S(a,O2) = 80 %) in a random order. The hypopnoea/apnoeic threshold was determined by averaging P(ET,CO2) of the last three breaths prior to each hypopnoea or apnoea. 3. Hypopnoeas and apnoeas were induced in all subjects during both normoxia and hypoxia. Hypoxia reduced the eupnoeic P(ET,CO2) compared to normoxia (42.4 +/- 1.3 vs. 45.0 +/- 1.1 mmHg, P < 0.001). However, no change was observed in either the hypopnoeic threshold P(ET,CO2) (42.1 +/- 1.4 vs. 43.0 +/- 1.2 mmHg, P > 0.05) or the apnoeic threshold P(ET,CO2) (41.3 +/- 1.2 vs. 41.6 +/- 1.0 mmHg, P > 0.05). Thus, the difference in P(ET,CO2) between the eupnoeic and threshold levels was much smaller during hypoxia than during normoxia (-0.2 +/- 0.2 vs. -2.0 +/- 0.3 mmHg, P < 0.01 for the hypopnoea threshold and -1.1 +/- 0.2 vs. -3.4 +/- 0.3 mmHg, P < 0.01 for the apnoeic threshold). We concluded that hypoxia causes a narrowing of the difference between the baseline P(ET,CO2) and the hypopnoea/apnoeic threshold P(ET,CO2), which could increase the likelihood of ventilatory instability.