Effect of progressive hypoxia on breathing during sleep

We examined the effects of progressive hypoxia on breathing during wakefulness and quiet (NREM) sleep in 17 healthy young adults (11 males and 6 females). Ventilation was determined from quantitative measurements of abdominal and rib cage excursions using magnetometers or inductive plethysmography. Hypoxia was induced by blending N2 into the inspiratory line of a loose-fitting mask while O2 saturation was monitored with an ear oximeter. No attempt was made to maintain isocapnia. Ventilatory responses to hypoxia were depressed in 2 male subjects by sleep, but were unchanged or increased in the others. Thus on the average, sleep produced no change in ventilatory responses to hypoxia. There was no consistent difference in the frequencies or tidal volumes attained at a given level of ventilation during hypoxia awake and asleep. During sleep there was relatively more rib cage than abdominal movement. Relief of hypoxia was followed by periodic breathing during sleep in 12 subjects but in only 2 subjects when awake. In two thirds of the trials, hypoxia failed to produce arousal even though arterial O2 saturation was allowed to fall below 75%. The results suggest that (1) sleep potentiates apnea producing effects of O2 changes but has inconsistent effects on ventilatory responses to hypoxia, and (2) hypocapnic hypoxia is not an invariably potent stimulus for arousal.

Pattern of diaphragmatic activity during forced expiratory vital capacity

We measured transdiaphragmatic pressure (Pdi) during forced expiratory vital capacity (FVC) maneuvers in 13 normal subjects and electromyographic activity of the diaphragm (edi) in 8 of these subjects. In all subjects, Pdi increased at the initiation of the FVC. In most, this increase lasted 30--50 ms and reached levels well above the Pdi observed at total lung capacity (TLC). After the initial transient increase, approximately half of the subjects demonstrated a substantial fall in Pdi to values near the relaxation level in the mid-vital capacity (VC) volume range, while half showed a second large increase in Pdi in this volume range. Seven of eight subjects tested showed a rapid decrease in Edi at the onset of the FVC, reaching a minimum in 30--50 ms. After this initial transient decrease, Edi increased in six subjects in the mid-VC volume range, in association with secondary rises in Pdi. In two subjects, Edi remained low throughout the remainder of the FVC, and Pdi in the mid VC range was generally lower. These results are consistent with the conclusion that the diaphragm is neither electromyographically silent nor mechanically unimportant during the FVC. Changes in abdominothoracic configuration, superimposed upon "antagonistic" activity of the diaphragm, result in substantial reductions in pleural (esophageal) pressure that may influence regional lung emptying during the FVC.

Effect of quiet sleep on resting and CO2-stimulated breathing in humans

We examined the effects of different levels of inspired CO2 on ventilation and the pattern of breathing in healthy adults during the awake and the stage II quiet-sleep states. During both states, subjects were studied supine with their heads enclosed in a canopy. Tidal volume (VT) was determined from quantitative measurements of abdominal and rib cage excursions with magnetometers. Inspired CO2 was raised by blending CO2-enriched gas into the airflow, which continuously flushed the canopy. During sleep, while room air was breathed, VT decreased significantly from 410 to 360 ml, and respiratory rate also fell from 17 to 16 breaths/min. As a consequence, ventilation was significantly reduced from 6.5 to 5.8 l/min, and end-tidal CO2 partial pressure (PCO2) rose from 39.1 to 42.5 Torr. Ventilatory responses to CO2 were reduced, on the average, during sleep to 79% of waking levels. The change in average inspiratory flow produced by CO2 was also less during sleep. Waking and sleeping ventilatory responses to CO2 correlated inversely with the rise in end-tidal PCO2 when room air was breathed during sleep. At all levels of VT, the rib cage contribution to VT was greater during quiet sleep than during wakefulness. These findings suggest that quiet sleep, in addition to depressing ventilation and the response to CO2 alters the manner in which VT is attained by rib cage and abdominal displacements.

Intercostal muscle reflexes and sleep breathing patterns in the human infant

Breathing variability and apnea characteristic of rapid eye movement (REM) sleep was investigated in a newborn infant with complete interruption of intercostal to phrenic neural pathways due to intrapartem transection of the cervical spinal cord. Breath-to-breath variability in inspiratory duration (TI), breath duration (Ttot), tidal volume (VT), and ventilation (VI) was significantly greater in REM than in quiet sleep and was similar to the variability in these parameters seen in normal infants. In addition, brief periods of diaphragmeatic apnea were observed during REM sleep. The phenomenon of shortened TI during airway occlusion previously attributed to intercostal-to-phrenic reflexes was examined in the quadriplegic infant and in seven healthy term infants. The frequency of this response was increased when airway occlusion was delayed until after onset of inspiration. Shortening of TI by occlusion occurred no less frequently in the quadriplegic than in the control infants. The constant paradoxical inward movement of the rib cage during inspiration observed in the quadriplegic infant suggests that supraspinal innervation of intercostal muscle limits such paradoxical movements in the normal infant. The quadriplegic infant's end-expiratory volume was consistently above his passive functional residual capacity, as inferred from respiratory volume and pressure measurements.

Lung volume changes, occlusion pressure and chest wall configuration in human infants

Measurements of respiration were compared in normal, full term and premature infants before and after application of 5 cm H2O continuous negative pressure (CNEG) around the body below the neck. Mean minute ventilation in the full term infants decreased when CNEG was applied, secondary to a decrease in respiratory rate, with tidal volume relatively unchanged. The premature infants showed no consistent ventilatory response to CNEG. There were variable changes in occlusion pressure at increased lung volume. Observation with rib cage and abdomen anterior-posterior (A-P) magnetometers showed that the increase in end-expiratory lung volume was accounted for largely by expansion of the rib cage so that the configuration of the diaphragm and its mechanical advantage were maintained. These results suggest that the diaphragm is the major determinant of mask occlusion pressure in infants.

SPECULATION

The effects of increased lung volume on the configuration of the chest wall may relate to the success of continuous distending pressure in the treatment of respiratory disorders in infants, including apnea of prematurity.

Mechanics of the human diaphragm during voluntary contraction: statics

We related diaphragm electromyographic activity (Edi) to transdiaphragmatic pressure (Pdi) in man during graded inspiratory efforts. Estimates of rib cage and abdominal volume displacements were based on their anteroposterior (AP) diameter changes. The diaphragm was assumed to contract isometrically when subjects performed inspiratory efforts against a closed airway at specified abdominothoracic configurations, increasing Edi and Pdi while holding lung volume and rib case and abdominal AP diameters constant. The relationship between Pdi and Edi depends primarily on abdominothoracic configuration rather than lung volume. For equal increments in lung volume, the Pdi developed at constant Edi is four to eight times more sensitive to changes in abdominal than in rib cage AP diameter. We demonstrate an isofunctional state of the diaphragm at different lung volumes, when increases in lung volume and rib cage AP diameter are compensated for by slight decreases in abdominal AP diameter, resulting in a constant relationship between Edi and Pdi. We conclude that diaphragm shortening is reflected more directly in abdominal displacement than in lung volume change.

Mechanics of the human diaphragm during voluntary contraction: dynamics

We determined the static, isometric relationship between diaphragmatic EMG (Edi) and transdiaphragmatic pressure (Pdi) in man at specified abdominothoracic configurations, assessed with magnetometers. During inspiratory airflow, measurements of Edi and Pdi were taken as the respiratory system passed through the same configuration as obtained during the static isometric contractions, allowing a comparison of static and dynamic contractions of the diaphragm at a given length and curvature. When voluntary inspiratory maneuvers are performed with no associated outward displacement of the abdominal wall, or with slight inward displacement, the Pdi developed dynamically is the same as that developed statically at a given Edi. When outward movement of the abdominal wall occurs during inspiratory airflow, the Pdi developed dynamically depends on the rate of abdominal displacement, not on overall inspiratory airflow rate. We conclude that the velocity of shortening of the diaphragm increases directly as the rate of abdominal displacement. We construct a pressure-flow analogue of the force-velocity relationship for the diaphragm and discuss the functional implications of these observations in relation to spontaneous breathing.

Vocal cord closure. A cause of upper airway obstruction during controlled ventilation

Studies of vocal cord function were undertaken in a quadriplegic patient requiring ventilatory assistance, and in 2 normal subjects during controlled ventilation in a tank-type respirator. When the patient and the normal subjects relaxed and made no conscious effort to assist the respirator, the vocal cords were observed to close during inspiration and a large pressure gradient (12 to 19 cm H2O) developed across the cords. When the subjects made a slight inspiratory effort ("assist" mode), the cords opened widely during inspiration. There were large increases in flow and tidal volume in the "assist" mode compared with passive ventilation. Measurements of transdiaphragmatic pressure and esophageal pressure showed that these variables did not increase with the slight assist. Thus, increase in ventilation during the "assist" mode appeared to be due to alleviation of inspiratory obstruction at the level of the vocal cords. The same phenomenon was observed in the patient during phrenic nerve pacing. A pacemaker was designed to be triggered by the electromyographic impulse from an accessory muscle of respiration. In this manner, vocal cord opening could be coordinated with the mechanical assist given by the phrenic nerve pacer.

A digital computer technique for analyzing respiratory muscle EMG's

A method is described for extracting from the electromyograms of respiratory muscles a continuous signal which has primarily the periodicity of respiratory pressure and flow wave forms. The EMG is first band-pass-filtered from 50 to 500 Hz, then digitized, full-wave rectified, passed through a nonlinear voltage window to reduce noise (particularly ECG) artifacts, then low-pass filtered with a digital continuous, or moving, averager. An average wave form corresponding to one respiratory cycle is produced by ensemble averaging of the wave forms from several consecutive breaths. Diaphragmatic electromyograms from a human and from a rabbit are processed in this manner, and the effect on the processed wave forms of changes in inspired CO2 and of a change in end-expiratory lung volume are demonstrated.

Mechanical work of breathing derived from rib cage and abdominal V-P partitioning

Estimates of the mechanical work of breathing derived from measurements of separate rib cage and abdominal volume displacements, each plotted against transthoracic pressure, include the elastic cost of chest wall distortion which may occur during breathing. Inspiratory work is partitioned between the diaphragm and the rib cage musculature by adding measurements of transabdominal pressure. The mechanical work of breathing derived from separate rib cage and abdominal volume-pressure (V-P) tracings (the sum of work done by the diaphragm, rib cage, and abdominal musculature) is compared with ventilatory work estimated from the Campbell diagram (which does not include any distortional work). During resting breathing the two estimates are closely comparable, consistent with little or no distortion of the chest wall during quiet breathing. As ventilation increases, the estimate developed from rib cage and abdominal tracings reveals systematically greater mechanical work than is estimated from the Campbell diagram, consistent with distortion of the chest wall from the relaxed thoracoabdominal configuration at higher levels of ventilation. At ventilations achieved during exercise, the Campbell diagram may underestimate the work of breathing by up to 25%.

Dynamics of the chest wall during speech production: function of the thorax, rib cage, diaphragm, and abdomen

Anteroposterior diameters of the rib cage and abdomen and esophageal and gastric pressures were measured in normal subjects in upright and supine body positions during respiratory maneuvers and utterance tasks. Data were charted in relative motion diagrams and various motion-pressure diagrams which enabled graphic solution for muscular pressures exerted by the chest wall and individually by the thorax, rib cage, diaphragm, and abdomen during utterances. Behaviors of the chest wall and its parts were found to depend upon lung volume, utterance loudness, body position, and utterance task. For utterances encompassing most of the vital capacity, chest wall effort was at first net inspiratory and later net expiratory. The former was governed predominately by the rib cage and the abdomen in the upright body position and by the diaphragm in the supine position. For conversational speech, chest wall effort was continuously expiratory, control being vested in the rib cage and the abdomen in the upright body position and typically in the rib cage alone in the supine position. Mechanisms operating during the utterances are discussed, particularly those involved with conversational speech production. We conclude that the abdomen occupies an especially important role in running conversational speech in that it mechanically tunes the diaphragm to increase the latter's inspiratory efficiency and thus enables man to minimally interrupt his ongoing speech for needed inspiratory pauses. We also discuss the relevance of our findings to clinical endeavors.