Chest wall kinematics during chemically stimulated breathing in healthy man

Expiratory load compensation is associated with electroencephalographic premotor potentials in humans

In normal humans during quiet breathing, expiration is mostly driven by elastic recoil of the lungs. Expiration becomes active when ventilation must be increased to meet augmented metabolic demands, or in response to expiratory loading, be it experimental or disease-related. The response to expiratory loading is considered to be mediated by both reflex and cortical mechanisms, but the latter phenomenon have not been neurophysiologically characterized. We recorded the EEG in 20 healthy volunteers (9 men, 11 women, age: 22 to 50 yr) during unloaded breathing, voluntary expirations, and in response to 50 cmH2O·l(-1)·s expiratory resistive load (ERL), 20 cmH2O expiratory threshold load (high ETL), and 10 cmH2O expiratory threshold load (low ETL). EEGs were processed by ensemble averaging expiratory time-locked segments and examined for pre-expiratory potentials, defined as a slow negative shift from the baseline signal preceding expiration, and suggestive of cortical preparation of expiration involving the supplementary motor area. Four subjects were excluded because of technical EEG problems. Pre-expiratory potentials were present in one subject at baseline and in all subjects during voluntary expirations. They were present in eight subjects during low ETL, in 15 subjects during high ETL, and in 13 subjets during ERL (control vs. low ETL, P = 0.008; control vs. high ETL, P < 0.001; and control vs. ERL, P < 0.001). Respiratory discomfort was more intense in the presence of pre-expiratory potentials (P < 0.001). These results provide a neurophysiological substrate to a cortical component of the physiological response to experimental expiratory loads in humans.

Postural control and ventilatory drive during voluntary hyperventilation and carbon dioxide rebreathing

The present study sought to establish links between hyperventilation and postural stability. Eight university students were asked to stand upright under two hyperventilation conditions applied randomly: (1) a metabolic hyperventilation induced by 5 min of hypercapnic-hyperoxic rebreathing (CO(2)-R); and, (2) a voluntary hyperventilation (VH) of 3 min imposed by a metronome set at 25 cycles per min. Recordings were obtained with eyes open, with the subjects standing on a force plate over 20-s periods. Ventilatory response, displacements in the centre of pressure in both the frontal and sagittal planes and fluctuations in the three planes of the ground reaction force were monitored in the time and frequency domains. Postural changes related to respiratory variations were quantified by coherence analysis. Myoelectric activities of the calf muscles were recorded using surface electromyography. Force plate measurements revealed a reduction in postural stability during both CO(2)-R and VH conditions, mainly in the sagittal plane. Coherence analysis provided evidence of a ventilatory origin in the vertical ground reaction force fluctuations during VH. Electromyographic analyses showed different leg muscles strategies, assuming the existence of links between the control of respiration and the control of posture. Our results suggest that the greater disturbing effects caused by voluntary hyperventilation on body balance are more compensated when respiration is under automatic control. These findings may have implications for understanding the organisation of postural and respiratory activities and suggest that stability of the body may be compromised in situations in which respiratory demand increases and requires voluntary control.

Chest wall kinematics and respiratory muscle coordinated action during hypercapnia in healthy males

The present study was designed to verify whether during hypercapnic stimulation, as we had previously found during exercise or walking, the partitioning of the respiratory motor output is equally distributed to the muscles of chest wall compartments to assist diaphragm function. We studied chest wall kinematics and respiratory muscle recruitment in seven healthy men during rebreathing of a hypercapnic-hyperoxic gas mixture (CO(2) RT). Data were compared with those previously obtained during either cycling exercise or walking. The chest wall volume ( Vcw), assessed by optoelectronic plethysmography (OEP), was modeled as the sum of the volumes of the lung-apposed rib cage ( Vrc,p), diaphragm-apposed rib cage ( Vrc,a) and abdomen ( Vab). Esophageal ( Pes), gastric ( Pga) and transdiaphragmatic ( Pdi= Pga- Pes) pressures were simultaneously recorded. Velocity of shortening ( V') and power ( W'= Px V') of the diaphragm ( W'di), rib cage muscles ( W'rcm) and abdominal muscles ( W'abm) were also calculated. During CO(2) RT the progressive increase in end-inspiratory Vcw resulted from an increase in both end-inspiratory Vrc,p and Vrc,a, while the progressive decrease in end-expiratory Vcw was entirely due to the decrease in end-expiratory Vab. The increase in Vrc,p was proportionally slightly greater than that in Vrc,a. The end-inspiratory increase and end-expiratory decrease in Vcw were accounted for by inspiratory rib cage (RCM,i) and abdominal (ABM) muscle recruitment, respectively. W'di, W'rcm and W'abm progressively increased. However, while most of W'di was expressed in terms of velocity of shortening, most of W'rcm and W'abm was expressed as force or pressure. A comparison of CO(2) results with data obtained during exercise revealed: (1). a gradual vs. an immediate response, (2). a similar decrease in Vab,e and Pabm, (3). an apparent lack of any difference in ABM recruitment, (4). less gradual ABM relaxation, (5). no drop in Pdi but a similar Wdi change and decrease in pressure-to-velocity ratio of the diaphragm. We have found that in healthy humans: (1). the increased motor output with hypercapnia is equally distributed between RCM and ABM to minimize transdiaphragmatic pressure and (2). data on chest wall kinematics and respiratory muscle recruitment are only partly in line with those obtained during walking or cycling exercise.