Respiratory and postural changes in intercostal muscle length in supine dogs

Acute Ventilatory Support During Whole-Body Hybrid Rowing in Patients With High-Level Spinal Cord Injury: A Randomized Controlled Crossover Trial

BACKGROUND

High-level spinal cord injury (SCI) results in profound spinal and supraspinal deficits, leading to substantial ventilatory limitations during whole-body hybrid functional electrical stimulation (FES)-rowing, a form of exercise that markedly increases the active muscle mass via electrically induced leg contractions. This study tested the effect of noninvasive ventilation (NIV) on ventilatory and aerobic capacities in SCI.

METHODS

This blinded, randomized crossover study enrolled 19 patients with SCI (level of injury ranging from C4 to T8). All patients were familiar with FES-rowing and had plateaued in their training-related increases in aerobic capacity. Patients performed two FES-rowing peak exercise tests with NIV or without NIV (sham).

RESULTS

NIV increased exercise tidal volume (peak, 1.50 ± 0.31 L vs 1.36 ± 0.34 L; P < .05) and reduced breathing frequency (peak, 35 ± 7 beats/min vs 38 ± 6 beats/min; P < .05) compared with the sham test, leading to no change in alveolar ventilation but a trend toward increased oxygen uptake efficiency (P = .06). In those who reached peak oxygen consumption (Vo₂peak) criteria (n = 13), NIV failed to significantly increase Vo₂peak (1.73 ± 0.66 L/min vs 1.78 ± 0.59 L/min); however, the range of responses revealed a correlation between changes in peak alveolar ventilation and Vo₂peak (r = 0.89; P < .05). Furthermore, those with higher level injuries and shorter time since injury exhibited the greatest increases in Vo₂peak.

CONCLUSIONS

Acute NIV can successfully improve ventilatory efficiency during FES exercise in SCI but may not improve Vo₂peak in all patients. Those who benefit most seem to be patients with cervical SCI within a shorter time since injury.

TRIAL REGISTRY

ClinicalTrials.gov; Nos.: NCT02865343 and NCT03267212; URL: www.clinicaltrials.gov.

Task-dependent output of human parasternal intercostal motor units across spinal levels

KEY POINTS

During breathing, there is differential activity in the human parasternal intercostal muscles and the activity is tightly coupled to the known mechanical advantages for inspiration of the same regions of muscles. It is not known whether differential activity is preserved for the non-respiratory task of ipsilateral trunk rotation. In the present study, we compared single motor units during resting breathing and axial rotation of the trunk during apnoea. We not only confirmed non-uniform recruitment of motor units across parasternal intercostal muscles in breathing, but also demonstrated that the same motor units show an altered pattern of recruitment in the non-respiratory task of trunk rotation. The output of parasternal intercostal motoneurones is modulated differently across spinal levels depending on the task and these results help us understand the mechanisms that may govern task-dependent differences in motoneurone output.

ABSTRACT

During inspiration, there is differential activity in the human parasternal intercostal muscles across interspaces. We investigated whether the earlier recruitment of motor units in the rostral interspaces compared to more caudal spaces during inspiration is preserved for the non-respiratory task of ipsilateral trunk rotation. Single motor unit activity (SMU) was recorded from the first, second and fourth parasternal interspaces on the right side in five participants in two tasks: resting breathing and 'isometric' axial rotation of the trunk during apnoea. Recruitment of the same SMUs was compared between tasks (n = 123). During resting breathing, differential activity was indicated by earlier recruitment of SMUs in the first and second interspaces compared to the fourth space in inspiration (P < 0.01). By contrast, during trunk rotation, the same motor units showed an altered pattern of recruitment because SMUs in the first interspace were recruited later and at a higher rotation torque than those in the second and fourth interspaces (P < 0.05). Tested for a subset of SMUs, the reliability of the breathing and rotation tasks, as well as the SMU recruitment measures, was good-excellent [intraclass correlation (2,1): 0.69-0.91]. Thus, the output of parasternal intercostal motoneurones is modulated differently across spinal levels depending on the task. Given that the differential inspiratory output of parasternal intercostal muscles is linked to their relative mechanical effectiveness for inspiration and also that this output is altered in trunk rotation, we speculate that a mechanism matching neural drive to muscle mechanics underlies the task-dependent differences in output of axial motoneurone pools.

Mechanical effect of muscle spindles in the canine external intercostal muscles

High-frequency mechanical vibration of the ribcage increases afferent activity from external intercostal muscle spindles, but the effect of this procedure on the mechanical behaviour of the respiratory system is unknown. In the present study, we have measured the changes in external intercostal muscle length and the craniocaudal displacement of the ribs during ribcage vibration (40 Hz) in anaesthetized dogs. With vibration, external intercostal inspiratory activity increased by approximately 50 %, but the respiratory changes in muscle length and rib displacement were unaltered. A similar response was obtained after the muscles in the caudal segments of the ribcage were sectioned and the caudally oriented force exerted by these muscles on the rib was removed, thus suggesting that activation of external intercostal muscle spindles by vibration generates little tension. Prompted by this observation, we also examined the role played by the external intercostal muscle spindles in determining the respiratory displacement of the ribs during breathing against high inspiratory airflow resistances. Although resistances consistently elicited prominent reflex increases in external intercostal inspiratory activity, the normal inspiratory cranial displacement of the ribs was reversed into an inspiratory caudal displacement. Also, this caudal rib displacement was essentially unchanged after section of the external intercostal muscles, whereas it was clearly enhanced after denervation of the parasternal intercostals. These findings indicate that stretch reflexes in external intercostal muscles confer insufficient tension on the muscles to significantly modify the mechanical behaviour of the respiratory system.

Contribution of spindle reflexes to post-inspiratory activity in the canine external intercostal muscles

1. The external intercostal muscles have greater post-inspiratory activity than the parasternal intercostal muscles and are more abundantly supplied with muscle spindles. In the present study, the hypothesis was tested that spindle afferent inputs play a major role in determining this activity. 2. The electrical activity of the external and parasternal intercostal muscles in the rostral interspaces was recorded in anaesthetized spontaneously breathing dogs, and the ribs were manipulated so as to alter their normal caudal displacement and the normal lengthening of the muscles in early expiration. 3. Post-inspiratory activity in the external intercostal muscles showed a reflex decrease when the caudal motion of the ribs and the lengthening of the muscles was impeded, and it showed a reflex increase when the rate of caudal rib motion and muscle lengthening was increased. In contrast, the small post-inspiratory activity in the parasternal intercostal muscles remained unchanged. 4. When the two ribs making up the interspace investigated were locked to keep muscle length constant, post-inspiratory activity in the external intercostal muscles was reduced and no longer responded to cranial rib manipulation. 5. These observations confirm that afferent inputs from muscle receptors, presumably muscle spindles, are a primary determinant of post-inspiratory activity in the canine external intercostal muscles. In anaesthetized animals, the contribution of central control mechanisms to this activity is small.

Pulmonary-locomotory interactions in exercising dogs and horses

In exercising quadrupeds, limb movement is often coupled with breathing frequency. This finding has lead some investigators to conclude that locomotory forces, associated with foot plant, abdominal visceral displacements or lumbo-sacral flexion, are the primary determinants of airflow generation. Analysis of respiratory muscle electrical activation (EMG) and contraction profiles in chronically instrumented dogs and horses, along with measurements of esophageal pressure (Pes) changes and limb movements, provide evidence that each breath during the exercise hyperpnea is determined by respiratory neuromuscular events. Specifically: (1) Phasic diaphragmatic EMG and tidal shortening are always synchronous with decreases in Pes; (2) decrements in Pes are always associated with inspiratory flow generation; and (3) strict phase coupling between breathing and stride frequency is not obligatory. Thus, although locomotory-associated forces may minimally assist with flow generation, they are not the primary determinants of breathing during exercise.

Contractility of the ventilatory pump muscles

The ventilatory muscles are striated skeletal muscles, and their in situ function is governed by the same relationships that determine the contractile force of muscles in vitro. The ventilatory muscles, however, are functionally distinct from limb skeletal muscles in several aspects, the most notable being that the ventilatory muscles are the only skeletal muscles upon which life depends. Among the muscles that participate in ventilation, the diaphragm is closest to its optimal resting length at functional residual capacity (FRC) and has the greatest capacity for shortening and volume displacement, making it the primary muscle of inspiration. All inspiratory muscles shorten when the lung is inflated above FRC, but interactions among the various inspiratory muscles make for a wider range of high force output than could be achieved by any one muscle group acting in isolation. The velocity of inspiratory muscle shortening, especially diaphragmatic shortening, causes maximal dynamic inspiratory pressures to be substantially lower than maximal static pressures. This effect is especially pronounced during maximal voluntary ventilation, maximal exercise, and maximal inspiratory flow, volume maneuvers over the full vital capacity. During quiet breathing, the ventilatory muscles operate well below the limits of their neural activation and contractile performance. During intense activity, however, the diaphragmatic excursion approaches its limits over the entire vital capacity, and respiratory pressures may near their dynamic maximum. Because the system may operate near its available capacities during increased ventilatory demands, multiple strategies are available to compensate for deficits. For example, if the diaphragm is acutely shortened, it can still generate the required respiratory pressure if it receives more neural drive. Alternatively, other muscles can be recruited to take over for an impaired diaphragm. Thus, the whole system is highly versatile.

Respiratory muscle stiffness is age- and muscle-specific

We investigated the possible contribution of respiratory muscles to the well documented increase in chest wall stiffness with age. Diaphragm and internal intercostal muscle strips were dissected from male Fischer 344 rats of 3, 6, 12, and 24 months of age. Muscles were subjected, in vitro, to stress-strain and yield point tests. Passive tension data from these tests were normalized to a reference length (Lr), which was defined in terms of absolute stress, 700 Pascals. In general, Lr of diaphragm was found to be 90% of the length (Lo) required for maximal tetanic tension. Within a range of stretch between Lr and 130% Lr, diaphragm muscles from adult rats (6-12 month) were more compliant than those of either young (3 month) or old (24 month) animals. In contrast, intercostal muscles from old rats were stiffer than those of young or adult rats. Yield strength of both muscles was constant with age, but diaphragm muscles were found to have a higher yield strength than intercostal muscles from any age. Thus, only some passive mechanical properties of respiratory muscles vary with age, and this variation in muscle-specific. A surprising finding of this study was that diaphragm muscles of adult animals were more compliant than those of either young or old rats.

Effect of pressure and timing of contraction on human rib cage muscle fatigue

Breathing against inspiratory loads can be accomplished with different degrees of coupling between the diaphragm and the other muscles attached to the rib cage (RCM). Thus, the electromyographic signs of fatigue develop separately in each muscle group. While breathing with diaphragm emphasis, the occurrence of diaphragmatic fatigue was found to be related to the tension-time index TTdi (= Pdi/Pdimax x Ti/Ttot). Above the critical range of 0.15 to 0.18, the endurance of the diaphragm is less than 1 h and it is inversely related to the TTdi value. However, in most loaded breathing conditions, the spontaneous pattern of breathing is characterized by predominant activation of RCM. The tension-time conditions at which fatigue develops during breathing with RCM emphasis are not known. We assessed the critical tension-time value in four normal subjects breathing with RCM emphasis against inspiratory threshold loads. RCM predominance was achieved by developing negative abdominal pressure swings during inspiration, and it was characterized by the tension-time index TTrc (Ppl/Pplmax x Tl/Ttot), where Ppl is pleural pressure developed under this condition. Above a critical TTrc value of 0.30, endurance time was inversely related to TTrc, and it resulted from failure of the RCM rather than of the diaphragm. We conclude that the critical threshold, as assessed by TTrc, is higher for breathing patterns with RCM emphasis than previously described by TTdi for diaphragm emphasis. However, when predominantly recruited, as in breathing patterns commonly adopted in loaded conditions, the RCM fatigue earlier than the diaphragm.

Differential control of the inspiratory intercostal muscles during airway occlusion in the dog

1. The effect of airway occlusion on the electrical activity of the three groups of inspiratory intercostal muscles (external intercostal, levator costae, parasternal intercostal) situated in the cranial portion of the rib-cage has been studied in thirty anaesthetized, spontaneously breathing dogs. 2. The three muscles were active during normal inspiration, and their activity was prolonged similarly during airway occlusion. However, a comparison of activity during occluded and unoccluded inspirations indicated that airway occlusion caused a facilitation of external intercostal and levator costae activities but an inhibition of parasternal intercostal activity. 3. The facilitation of external intercostal and levator costae activities was markedly reduced after section of the phrenic nerves and completely suppressed after section of the appropriate thoracic dorsal roots. 4. The inhibition of parasternal intercostal activity was not affected by section of the phrenic nerves or by section of the thoracic dorsal roots. This phenomenon, however, was abolished after bilateral cervical vagotomy. 5. Activation of the external intercostals and levator costae during inspiratory efforts are thus highly dependent on segmental reflexes arising in these muscles. In contrast, activation of the parasternal intercostals resembles that of the diaphragm in the sense that it depends primarily on the central respiratory drive.

Action of the intercostal muscles on the rib cage

Recent studies suggest that the parasternal muscles (PA) are primarily responsible for rib cage expansion during eupneic breathing with a much lesser role played by the interosseous external intercostals (EI). The purpose of the present investigation was to assess the capacity of the EI to expand the rib cage during spontaneous breathing in the absence of coincident ipsilateral PA activation. In 9 anesthetized dogs, we measured PA EMG and length in the 3rd interspace and EI EMG and length in the 3rd and 4th interspaces. During resting breathing, each muscle was electrically active and shortened to a similar degree, approximately 3% of resting length. Following ipsilateral PA denervation (1st through 6th interspaces), the level of EI shortening in the 3rd and 4th interspaces was maintained, but with an increase in neural drive to these muscles. The parasternal muscle in the 3rd interspace lengthened during inspiration. Subsequent sequential denervation of EI in the 3rd and 4th interspaces resulted in their lengthening. In 4 additional animals, axial motion of the 4th rib was measured in the mid axillary line. Ipsilateral PA denervation had no significant effect on rib motion. External intercostal denervation (3rd interspace), on the other hand, had a substantial impact on rib motion, causing the 4th rib to move in the caudal direction during inspiration. Our results indicate that: (a) the EI of the lateral rib cage are capable of elevating the ribs during inspiration independent of PA contraction; (b) PA contraction contributes to EI shortening during eupneic breathing and (c) regional loss of muscle activation results in local rib cage distortion, suggesting that the upper rib cage has multiple degrees of freedom.