Effect of fatiguing resistive loads on the level and pattern of respiratory activity in awake goats

The effect of respiratory muscle fatigue on inspiratory muscle electrical activity (EMG), transdiaphragmatic pressure and ventilation during spontaneous breathing was examined in three awake goats. Studies were performed during progressive hypercapnia before and immediately after inspiratory muscle fatigue induced by flow resistive loading (IRL). IRL caused a decrease in the high-low ratio of the diaphragm and intercostal EMG and a decrease in Pdi during electrophrenic stimulation. After IRL, inspiratory time, the breathing duty cycle (inspiratory time/total breath cycle time), peak integrated activity of the diaphragm and external intercostal EMG per breath and per minute were all decreased at any given level of PCO2. Changes in the timing of respiratory motor activity and reduced muscle performance after IRL resulted in a decrease in transdiaphragmatic pressure and ventilation during hypercapnia. In conscious goats studied during spontaneous, chemically stimulated breathing, inspiratory muscle fatigue is associated with reductions in diaphragm and external intercostal muscle electrical activity and reductions in transdiaphragmatic pressure and ventilation.     

Improved efficacy of spontaneous breathing with inspiratory pressure support

During inspiratory pressure support (IPS) ventilation, first a negative airway pressure is produced by the patient to open a demand valve and then a constant positive airway pressure is maintained at a present level while the patient inhales. The aim of this study was to assess the ability of 10 cm H2O IPS to improve the efficacy of spontaneous ventilation. We studied 8 intubated patients recovering from acute respiratory failure, all were breathing spontaneously via 3 different systems: a Servo 900 C ventilator (SCV) without IPS, a Servo 900 C ventilator with 10 cm H2O IPS, and a continuous flow system (CFS). Compared with the CFS, breathing with the SVC without IPS resulted in an increased respiratory rate (RR), increased tidal Volume (VT), increased transdiaphragmatic pressure (Pdi), and no significant change in PaO2 or PaCO2. Ventilation with IPS resulted in significant improvements in VT, PaO2, and PaCO2 with a decreased RR and Pdi when compared with both the other modes of spontaneous ventilation. A significant decrease in the pressure-time index of the diaphragm (i.e., the product of the mean transdiaphragmatic pressure and the inspiratory duty cycle) occurred during IPS. In 2 patients, we recorded diaphragmatic electromyographic activity during both SVC and IPS. In both patients during IPS, an increased VT and a decreased Pdi coincided with a major reduction of electromyographic activity. We conclude that IPS at a level of 10 cm H2O markedly increases the efficacy of spontaneous breathing while reducing the activity of the inspiratory muscles.     

Mathematical Methods in the Study of Respiration in Newborn Babies

In non-REM sleep, breathing is thought to be regulated by an automatic feedback control system independent of input from higher brain centers. Time series analysis of respiratory recordings has been used for a long time to characterize this system in new-born infants and has revealed oscillations in the respiratory pattern of a longer period than the breathing cycle. Studies of mathematical models incorporating the chemical feedback loops show oscillations consistent with these results. In particular periodic breathing is modeled by unstable equilibrium points of these models. Some choices of parameters give oscillations in arterial oxygen, which are asymmetric with respect to the equilibrium point such that the average is higher than the equilibrium value. This supports the hypothesis that the transition to periodic breathing could be a protective mechanism preventing low arterial oxygen. Continuous recordings of respiratory and esophageal pressure waveforms have been used to derive breath-by-breath tidal volume, inspiratory period, mechanical parameters and external inspiratory work during periodic breathing. A minimization procedure has been used to derive an optimum pattern of tidal volumes requiring minimum external inspiratory work to achieve the given ventilation. Comparison of the observed and calculated patterns shows the observed work oscillating about the optimum pattern, which suggests the existence of a control loop that minimizes the work.     

Ventilatory muscle support in respiratory failure with nasal positive pressure ventilation

Long-term intermittent mechanical ventilation results in improvements in ventilatory performance and clinical status between ventilation sessions in patients with chronic respiratory failure. The application of intermittent positive pressure ventilation through a nasal mask (NPPV) is a simple, noninvasive method for the provision of chronic intermittent ventilatory support. We investigated the effects of NPPV on inspiratory muscle activity in three normal subjects and nine patients with acute or chronic ventilatory failure due to restrictive (four subjects) or obstructive (five subjects) respiratory disorders. NPPV resulted in reductions of phasic diaphragm electromyogram amplitude to 6.7 +/- 0.7 percent (mean +/- SEM) of values obtained during spontaneous breathing in the normal subjects, 6.4 +/- 3.2 percent in the restrictive group, and 8.3 +/- 5.1 percent in the obstructive group. Simultaneous decreases in activity of accessory respiratory muscles were observed. The reductions in inspiratory muscle activity were confirmed by the finding of positive intrathoracic pressure swings on inspiration in all subjects. With NPPV, oxygen saturation and PCO2 remained stable or improved as compared with values obtained during spontaneous breathing. These results indicate that NPPV can noninvasively provide ventilatory support while reducing inspiratory muscle energy expenditure in acute and chronic respiratory failure of diverse etiology. Long-term assisted ventilation with NPPV may be useful in improving ventilatory performance by resting the inspiratory muscles.     

The effect of sleep deprivation on activity of the genioglossus muscle

Sleep deprivation appears to increase the severity of obstructive sleep apnea, and inadequate activation of the genioglossus muscle may play an important role in the pathogenesis of obstructive sleep apnea. Therefore, we investigated the effect of sleep deprivation on genioglossal electromyographic (EMG) activity. Eleven men were studied during room air breathing and CO2 rebreathing before sleep deprivation (control), after 1 night of sleep deprivation, and the day after sleep recovery. We measured inspired minute ventilation, tidal volume, respiratory frequency, and peak integrated inspiratory genioglossal EMG activity. After sleep deprivation, no significant changes in inspiratory minute ventilation or tidal volume occurred during room air breathing or CO2 rebreathing, but the breathing frequency during CO2 rebreathing increased significantly after sleep deprivation. Genioglossal EMG activity was diminished during CO2 rebreathing after sleep deprivation, but this was significant only in subjects 30 yr of age and older. The fall in EMG activity was independent of changes in tidal volume. All variables returned towards control levels after sleep recovery. We conclude that sleep deprivation selectively decreases genioglossal EMG activity during CO2 rebreathing in awake older subjects. This influence of sleep deprivation may play a role in the pathogenesis or severity of obstructive sleep apnea.     

Clinical manifestations of inspiratory muscle fatigue

Twelve patients exhibiting difficulties during discontinuation of artificial ventilation permitted us to investigate physical examination techniques used in diagnosing inspiratory muscle fatigue. Diaphragmatic and intercostal electromyographic tracings, arterial blood gases, rate and depth of ventilation, and thoracoabdominal motion were monitored during spontaneous breathing. Six patients showed electromyographic evidence of inspiratory muscle fatigue. A sequence of events leading to respiratory acidemia emerged--namely electromyographic evidence of fatigue, accompanied or followed by an increased respiratory rate, in turn followed by alternation between abdominal and rib cage breathing (respiratory alternans), paradoxical inward abdominal motion during inspiration (abdominal paradox), and finally an increase in PaCO2 associated with a fall in minute ventilation and respiratory rate, and worsening of respiratory acidemia. The abnormalities of respiratory movements may be reliable clinical signs of inspiratory muscle fatigue, particularly when accompanied by tachypnea and hypercapnia.     

Inspiratory muscle resistive training in respiratory failure

In this study, 4 patients who had failed weaning attempts using traditional methods underwent inspiratory muscle resistive training (IRT). Three patients showed improvements in inspiratory muscle strength (mean peak negative inspiratory pressure improved from 38 to 54 cm H2O) and in respiratory muscle endurance (mean PCO2 at the end of the same or a longer period of spontaneous breathing improved from 70 to 52 mmHg). These 3 patients were successfully weaned from mechanical ventilation after 10 to 24 days of IRT. A fourth patient showed no improvement and could not be weaned. This pilot study demonstrated that IRT can be useful in preparing patients with chronic respiratory failure for weaning from mechanical ventilation.     

Physiologic evaluation of pressure support ventilation by nasal mask in patients with stable COPD.

We evaluated the physiologic effects of pressure support ventilation by nasal route (NPSV) in eight patients with severe stable COPD and chronic hypercapnia who were randomly submitted to 2-h sessions of NPSV both with a portable ventilator (Respironics BIPAP device) and with a standard ventilator (Bird 6400ST device) at an inspiratory airway pressure of 22 cm H2O. Two sessions with each ventilator were performed using an FIO2 of 0.21 in each patient on two consecutive days. One patient did not tolerate either form of ventilation. Comparison of spontaneous with BIPAP ventilation showed a significant improvement in pH, PaCO2, and PaO2. Ventilatory pattern assessed by a respiratory inductive plethysmograph showed a significant increase in minute ventilation (VE), VT, and Ttot. Integrated surface diaphragmatic EMG activity measured only during BIPAP device ventilation decreased from that measured during spontaneous breathing. Similar changes in blood gases and ventilatory pattern were observed during ventilation by the Bird 6400ST except for VT/Ti ratio, which significantly increased. Comparison of baseline with measurements performed 12 h after the whole cycle of treatment showed a significant increase in pH and VE and a decrease in PaCO2. We conclude that short-term NPSV may be useful in improving respiratory pattern and blood gases in stable COPD patients with chronic hypercapnia.     

Steady-state response of conscious man to small expiratory resistive loads

To determine the predominant steady-state ventilatory responses to mild expiratory flow-resistive loads, we subjected 14 normal subjects to expiratory resistances of 0-10 cm H2O/L/sec (R0-R3). Breathing patterns and abdominal muscle activity (EMG) were recorded during quiet breathing, and when ventilation was augmented by dead space breathing (7 subjects) or treadmill walking (7 subjects). Expiratory loading increased expiratory time (TE), tidal volume and mean inspiratory flow rate, while decreasing inspiratory duty cycle and respiratory frequency. Minute ventilation (VI) remained constant. These load responses were most prominent during quiet breathing, and were attenuated or abolished as VI increased. Abdominal EMG was negligible during quiet breathing, increased when VI increased, but showed no consistent response to R1-R3. Thus, the principal defense against mild expiratory loads is prolongation of expiration, accompanied by enhanced inspiratory drive. Abdominal muscle expiratory activity is elicited by increasing ventilation, but occurs only sporadically with expiratory loading of the magnitude studied.     

Oxygen cost of breathing. Changes dependent upon mode of mechanical ventilation

We describe a patient with respiratory failure who demonstrated marked increases in O2 consumption (VO2) when breathing with synchronized intermittent mandatory mechanical ventilation (SIMV). When the mode of ventilation was changed to facilitate inspiratory gas flow (pressure-support) during spontaneous breathing, O2 consumption decreased 27 percent. Several important factors contributing to the increased O2 cost of breathing in patients requiring mechanical ventilation are reviewed, including the high internal resistance of demand-flow SIMV systems.     

Effects of different ventilator settings on sleep and inspiratory effort in patients with neuromuscular disease

Patients with neuromuscular disease (NMD) who require long-term ventilation normally have the ventilation set using empirical daytime parameters. We evaluated arterial blood gases (ABG), breathing pattern, respiratory muscle function, and sleep architecture during ventilation with two noninvasive Pressure Support Ventilation (nPSV) settings in nine patients with NMD. The two settings were randomly applied: the usual (US), with the nPSV setting titrated on simple clinical parameters, and the physiological (PHYS), tailored to the patient's respiratory effort. During wakefulness, nPSV significantly improved ABG and minute ventilation and reduced the diaphragmatic pressure-time product (PTPdi/breath), independently of the type of setting (PTPdi/breath spontaneous breathing 5.7 +/- 2.4, US 3.2 +/- 2, PHYS 3.6 +/- 1.6 cm H2O . seconds(-1), p < 0.001). However, during sleep, PHY nPSV resulted in a significant improvement of gas exchange, sleep efficiency (71.7% +/- 14 US vs. 80.6% +/- 8.3 PHYS, p < 0.01) and % of REM sleep (9.1% +/- 7 US vs. 17.3% +/- 5.4 PHYS, p < 0.01). This improvement was significantly correlated with the reduction in ineffective efforts. In NMD, nPSV is effective in improving daytime ABG and in unloading inspiratory muscles independently of whether it is set on the basis of the patient's comfort or the patient's respiratory mechanics. However, PHYS was associated with better sleep architecture and nighttime gas exchange.     

Comparison of supine and prone noninvasive measurements of breathing patterns in fullterm newborns

The current study was undertaken to ascertain whether shift from supine to prone posture alters the pattern of natural breathing in healthy fullterm newborns. Breathing patterns were measured in the supine and prone posture in 20 healthy fullterm infants using calibrated, noninvasive respiratory inductive plethysmography (RIP). The values for breathing pattern components in supine and prone postures expressed as means (± SD) were, respectively, tidal volume (V_T), 14.1(± 3.2) and 18.9(± 4.9)mL; mean inspiratory flow 26.7(± 11.5) and 32.8(± 13.0)mL/s; and minute ventilation 232(± 75) and 288(± 96)mL/kg/min (P < 0.01). Less thoracoabdominal incoordination, expressed as the labored breathing index (LBI), occurred with shift from supine to prone posture in infants studied in the active behavioral stage, changing from 2.0(± 0.5) to 1.3(± 0.4) (P < 0.01). Placement of a facemask-pneumotachograph system increased V, measured with RIP by 26% in the supine, and 18% in the prone posture. Neither respiratory rate nor inspiratory time (T_i) changed with the postural shift. Therefore, change from supine to prone posture improved ventilation and increased respiratory drive as expressed by V_T/T_i. Further, thoracoabdominal incoordination which took place during active sleep in the supine posture, lessened with change to the prone posture. Pediatr Pulmonol. 1994; 18:8–12. © 1994 Wiley-Liss. Inc.     

Chronic hypoventilation syndromes and sleep-related hypoventilation

Chronic hypoventilation affects patients with disorders on any level of the respiratory system. The generation of respiratory impulses can be impaired in congenital disorders, such as central congenital alveolar hypoventilation, in alterations of the brain stem or complex diseases like obesity hypoventilation. The translation of the impulses via spinal cord and nerves to the respiratory muscles can be impaired in neurological diseases. Thoraco-skeletal or muscular diseases may inhibit the execution of the impulses. All hypoventilation disorders are characterized by a reduction of the minute ventilation with an increase of daytime hypercapnia. As sleep reduces minute ventilation substantially in healthy persons and much more pronounced in patients with underlying thoraco-pulmonary diseases, hypoventilation manifests firstly during sleep. Therefore, sleep related hypoventilation may be an early stage of chronic hypoventilation disorders. After treatment of any prevailing underlying disease, symptomatic therapy with non-invasive ventilation (NIV) is required. The adaptation of the treatment should be performed under close medical supervision. Pressure support algorithms have become most frequently used. The most recent devices automatically apply pressure support and vary inspiratory and expiratory pressures and breathing frequency in order to stabilize upper airways, normalize ventilation, achieve best synchronicity between patient and device and aim at optimizing patients’ adherence.     

睡眠对慢性阻塞性肺疾病患者上气道阻力的影响

目的 探讨睡眠对慢性阻塞性肺疾病(COPD)患者上气道阻力及呼吸动力学的影响.方法 选择19例COPD急性加重期住院患者,治疗后病情稳定,通过食道-胃囊管法,检测气道开口压、食道压和胃内压,层流速仪测呼吸流速和容量改变.同步监测多导睡眠图,采集患者在清醒、睡眠仰卧及侧卧位呼吸动力学的变化.结果 睡眠时吸气峰流速、平均吸...     

Electroencephalographic changes during respiratory cycles predict sleepiness in sleep apnea

Common polysomnographic measures of sleep-disordered breathing have shown a disappointing ability to predict important consequences such as excessive daytime sleepiness. Using novel analytic techniques, this study tested the hypothesis that numerous, brief disruptions in cortical activity could occur on a breath-to-breath basis during nonapneic sleep. Spectral analysis proved the existence of respiratory cycle-related electroencephalographic changes in each of 38 adult patients evaluated by polysomnography for sleep-disordered breathing. Furthermore, the tendency for sigma (13-15 Hz) electroencephalographic power to vary with the respiratory cycle predicted next-day sleepiness as measured by the multiple sleep latency test. The predictive value was enhanced when the analysis was limited to those 27 patients who had sleep-disordered breathing (more than 5 apneas or hypopneas per hour of sleep). In contrast, nocturnal rates of apneas and hypopneas, as well as minimal oxygen saturation, did not predict sleepiness as well. On average, sigma power increased notably during inspiration, whereas delta (1-4 Hz) power showed a simultaneous decrease. We conclude that electroencephalographic activity shows detectable changes during nonapneic respiratory cycles in adults evaluated for sleep-disordered breathing. Quantification of these changes, which may reflect numerous inspiratory microarousals, could prove useful in prediction of excessive daytime sleepiness.     

经鼻持续性气道正压对阻塞性睡眠呼吸暂停综合征患者呼吸中枢驱动性的影响

为了解经鼻持续性气道正压（ｎＣＰＡＰ）通气治疗对阻塞性睡眠呼吸暂停综合征（ＯＳＡＳ）患者呼吸中枢驱动性的影响，研究了２０例无二氧化碳（ＣＯ＿２）储留的ＯＳＡＳ患者（Ｏ组）及２０例单纯鼾症患者（Ｓ组）夜间睡眠前后呼吸方式及口腔阻断压（Ｐ＿（０．１））的改变，并观察了ｎＣＰＡＰ治疗对ＯＳＡＳ，患者呼吸方式及Ｐ＿（０．１）的影响。结果显示：Ｏ组患者睡前的Ｐ＿（０．１）、呼吸频率、有效吸气阻抗明显高于Ｓ组，潮气量则显著低于Ｓ组。ｎＣＰＡＰ治疗组患者经一夜睡眠后的Ｐ＿（０．１）、每分通气量、潮气量、呼吸频率等较睡前显著增高。经ｎＣＰＡＰ治疗后Ｏ组的呼吸紊乱指数较治疗前明显降低，夜间最低氧饱和度明显提高，Ｐ＿（０．１）较睡前则无明显升高。提示ＯＳＡＳ患者睡前的呼吸中枢驱动性高于单纯鼾症患者，其呼吸形式为浅快呼吸；经过一夜睡眠后，其呼吸中枢驱动水平较睡前明显增高，呼吸形式更为浅快；ｎＣＰＡＰ治疗可以有效地解除睡眠呼吸暂停及其继发的低氧血症，从而逆转睡眠前后呼吸方式和呼吸中枢驱动性的改变。     

The effect of pressure support ventilation on breathing patterns and the work of breathing

We assessed breathing patterns during pressure support ventilation (PSV) and its relationship with the work of breathing in 10 postoperative patients. With increasing levels of pressure support, minute ventilation and tidal volume increased with a decrease in respiratory frequency. Increased minute ventilation was achieved by increased mean inspiratory flow. Duty cycle, however, decreased with PSV. This decrease might allow the diaphragm a longer rest period between contractions, which might decrease the risk of diaphragmatic fatigue. Furthermore, PSV reduced the inspiratory work added by a ventilator to near zero. Oxygen consumption was also decreased with PSV. We conclude that PSV improved the breathing patterns and minimized the work of breathing spontaneously via a ventilator.     

Entrainment of respiration in humans by periodic lung inflations. Effect of state and CO(2).

Lack of synchrony between a patient and the mechanical ventilator occurs when the respiratory rhythm of the patient fails to entrain to machine inflations. Entrainment implies a resetting of the respiratory rhythm such that a fixed temporal relationship exists between the onset of inspiratory activity and the onset of a mechanical breath. We examined the entrainment response to mechanical ventilation of normal humans over a range of machine rates during wakefulness and during isocapnic and hypercapnic NREM sleep. Wakefulness facilitated 1:1 entrainment of the respiratory rhythm to the mechanical ventilator over a wider range of machine frequencies than during NREM sleep (p < 0.001); isocapnic and hypercapnic conditions did not differ (p = 0.95). To evaluate the Hering-Breuer reflexes in the resetting of the respiratory rhythm during sleep, we examined changes in neural inspiratory time (TI) as the relationship between inspiratory efforts and onset of machine inflations changed. As inspiratory efforts extended into the machine inflation cycle, neural TI shortened. We conclude that entrainment responses of normal humans to mechanical ventilation differ depending on state, but mild increases in respiratory drive caused by CO(2) stimulation do not affect these entrainment responses. Furthermore, the changes in neural TI are consistent with observations in animal studies in which Hering-Breuer reflexes mediated entrainment.     

Sleep and breathing in neuromuscular disease.

Respiratory muscle weakness in neuromuscular disease causes significant morbidity and mortality. The published data on respiratory muscle activity and breathing during sleep in normal subjects, the impact of respiratory muscle weakness on sleep and breathing and the relations to daytime respiratory function in neuromuscular disease are reviewed here. In normal subjects during sleep upper airway resistance increases, chemosensitivity is reduced and the wakefulness drive to breathe is lost, resulting in a fall in ventilation. During rapid eye movement (REM) sleep, ribcage and accessory breathing muscles are suppressed, particularly during bursts of eye movements, and breathing is more irregular, rapid and shallow, with a further fall in ventilation. In subjects with respiratory muscle weakness sleep is fragmented, with shorter total sleep time, frequent arousals, an increase in stage 1 sleep and a reduction in, or complete suppression of, REM sleep. Sleep-disordered breathing and nocturnal desaturation are common and most severe during REM sleep. Correlations between daytime respiratory function and nocturnal desaturation are moderate or weak, but daytime respiratory function has greater prognostic value than nocturnal measurements. Noninvasive ventilation improves sleep quality and breathing in subjects with respiratory muscle weakness. However, the optimal criteria for initiation of ventilation and its role in rapidly progressive neuromuscular diseases are unclear.     

Inspiratory pressure support prevents diaphragmatic fatigue during weaning from mechanical ventilation

Persistent inability to tolerate discontinuation from mechanical ventilation is frequently encountered in patients recovering from acute respiratory failure. We studied the ability of inspiratory pressure support, a new mode of ventilatory assistance, to promote a nonfatiguing respiratory muscle activity in eight patients unsuccessful at weaning from mechanical ventilation. During spontaneous breathing, seven of the eight patients demonstrated electromyographic signs of incipient diaphragmatic fatigue. During ventilation with pressure support at increasing levels, the work of breathing gradually decreased (p less than 0.02) as well as the oxygen consumption of the respiratory muscles (p less than 0.01), and electrical signs suggestive of diaphragmatic fatigue were no longer present. In addition, intrinsic positive end-expiratory pressure was progressively reduced. For each patient an optimal level of pressure support was found (as much as 20 cm H2O), identified as the lowest level maintaining diaphragmatic activity without fatigue. Above this level, diaphragmatic activity was further reduced and untoward effects such as hyperinflation and apnea occurred. When electrical diaphragmatic fatigue occurred, the activity of the sternocleidomastoid muscle was markedly increased, whereas it was minimal when the optimal level was reached. We conclude that in patients demonstrating difficulties in weaning from the ventilator: (1) pressure support ventilation can assist spontaneous breathing and avoid diaphragmatic fatigue (pressure support allows adjustment of the work of each breath to provide an optimal muscle load); (2) clinical monitoring of sternocleidomastoid muscle activity allows the required level of pressure support to be determined to prevent fatigue.