Patterns of neural and muscular electrical activity in costal and crural portions of the diaphragm

To determine whether the central respiratory drives to costal and crural portions of the diaphragm differ from each other in response to chemical and mechanical feedbacks, activities of costal and crural branches of the phrenic nerve were recorded in decerebrate paralyzed cats, studied either with vagi intact and servo-ventilated in accordance with their phrenic nerve activity or vagotomized and ventilated conventionally. Costal and crural electromyograms (EMGs) were recorded in decerebrate spontaneously breathing cats. Hypercapnia and hypoxia resulted in significant increases in peak integrated costal, crural, and whole phrenic nerve activities when the vagi were either intact or cut. However, there were no consistent differences between costal and crural neural responses. Left crural EMG activity was increased significantly more than left costal EMG activity in response to hypercapnia and hypoxia. These results indicate that the central neural inputs to costal and crural portions of the diaphragm are similar in eupnea and in response to chemical and mechanical feedback in decerebrate paralyzed cats. The observed differences in EMG activities in spontaneously breathing animals must arise from modulation of central respiratory activity by mechanoreceptor feedback from respiratory muscles, likely the diaphragm itself.     

Postinspiratory activity of costal and crural diaphragm.

Because the first stage of expiration or "postinspiration" is an active neurorespiratory event, we expect some persistence of diaphragm electromyogram (EMG) after the cessation of inspiratory airflow, as postinspiratory inspiratory activity (PIIA). The costal and crural segments of the mammalian diaphragm have different mechanical and proprioceptive characteristics, so postinspiratory activity of these two portions may be different. In six canines, we implanted chronically EMG electrodes and sonomicrometer transducers and then sampled EMG activity and length of costal and crural diaphragm segments at 4 kHz, 10.2 days after implantation during wakeful, resting breathing. Costal and crural EMG were reviewed on-screen, and duration of PIIA was calculated for each breath. Crural PIIA was present in nearly every breath, with mean duration 16% of expiratory time, compared with costal PIIA with duration -2. 6% of expiratory time (P < 0.002). A linear regression model of crural centroid frequency vs. length, which was computed during the active shortening of inspiration, did not accurately predict crural EMG centroid frequency values at equivalent length during the controlled relaxation of postinspiration. This difference in activation of crural diaphragm in inspiration and postinspiration is consistent with a different pattern of motor unit recruitment during PIIA.     

Velocity of shortening of inspiratory muscles and inspiratory flow

Respiratory muscle length was measured with sonomicrometry to determine the relation between inspiratory flow and velocity of shortening of the external intercostal and diaphragm. Electromyographic (EMG) activity and tidal shortening of the costal and crural segments of the diaphragm and of the external intercostal were recorded during hyperoxic CO2 rebreathing in 12 anesthetized dogs. We observed a linear increase of EMG activity and peak tidal shortening of costal and crural diaphragm with alveolar CO2 partial pressure. For the external intercostal, no consistent pattern was found either in EMG activity or in tidal shortening. Mean inspiratory flow was linearly related to mean velocity of shortening of costal and crural diaphragm, with no difference between the two segments. Considerable shortening occurred in costal and crural diaphragm during inspiratory efforts against occlusion. We conclude that the relation between mean inspiratory flow and mean velocity of shortening of costal and crural diaphragm is linear and can be altered by an inspiratory load. There does not appear to be a relationship between inspiratory flow and velocity of shortening of external intercostals.     

In vivo length and shortening of canine diaphragm with body postural change

Using sonomicrometry, we measured the in vivo tidal shortening and velocity of shortening of the costal and crural segments of the diaphragm in the anesthetized dog in the supine, upright, tailup, prone, and lateral decubitus postures. When compared with the supine position, end-expiratory diaphragmatic length varied by less than 11% in all postures, except the upright. During spontaneous breathing, the tidal shortening and the velocity of shortening of the crural segment exceeded that of the costal segment in all postures except the upright and was maximal for both segments in the prone posture. We noted the phasic integrated electromyogram to increase as the end-expiratory length of the diaphragm shortened below and to decrease as the diaphragm lengthened above its optimal length. This study shows that the costal and crural segments have a different quantitative behavior with body posture and both segments show a compensation in neural drive to changes in resting length.     

Relationship between diaphragm length and abdominal dimensions

We examined the relationship between changes in abdominal cross-sectional area, measured by respiratory inductive plethysmography, and changes in length in the costal and crural parts of the diaphragm, measured by sonomicrometry, in nine supine, anesthetized dogs. During passive inflation, both parts of the diaphragm shortened and abdominal cross-sectional area increased. During passive deflation, both parts of the diaphragm lengthened and abdominal cross-sectional area decreased. We subsequently used the relationship between costal and crural diaphragmatic length, respectively, and abdominal cross-sectional area during passive inflation-deflation to predict the length changes in the costal and crural diaphragm during quiet breathing before and after bilateral phrenicotomy. In the intact animal the inspiratory shortening in the crural diaphragm was almost invariably greater than predicted from the relationship during passive inflation. During inspiration after phrenicotomy the crural diaphragm invariably lengthened, whereas the costal diaphragm often shortened. In general there was a good correlation between the measured and predicted length change for the crural diaphragm (r = 0.72 before and 0.79 after phrenicotomy) and a poor one for the costal diaphragm (r = 0.05 before and 0.19 after phrenicotomy).     

Effects of progressive hypoxia on parasternal, costal, and crural diaphragm activation

The distribution of motor drive to the costal and crural diaphragm and parasternal intercostal muscles was evaluated during progressive isocapnic hypoxia in anesthetized dogs. Bipolar stainless steel wire electrodes were placed unilaterally into the costal and crural portions of the diaphragm and into the parasternal intercostal muscle in the second or third intercostal space. Both peak and rate of rise of electromyographic activity of each chest wall muscle increased in curvilinear fashion in response to progressive hypoxia. Both crural and parasternal intercostal responses, however, were greater than those of the costal diaphragm. The onset of crural activation preceded that of the costal portion of the diaphragm and parasternal intercostal muscle activation. Despite differences in the degree of activation among the various chest wall muscles, the rate of increase in activation for any given muscle was linearly related to the rate of increases for the other two. This suggests that respiratory drive during progressive hypoxia increases in fixed proportion to the different chest wall inspiratory muscles. Our findings lend further support to the concept that the costal and crural diaphragm are governed by separate neural control mechanisms and, therefore, may be considered separate muscles.     

Posture effects on timing of abdominal muscle activity during stimulated ventilation.

In humans during stimulated ventilation, substantial abdominal muscle activity extends into the following inspiration as postexpiratory expiratory activity (PEEA) and commences again during late inspiration as preexpiratory expiratory activity (PREA). We hypothesized that the timing of PEEA and PREA would be changed systematically by posture. Fine-wire electrodes were inserted into the rectus abdominis, external oblique, internal oblique, and transversus abdominis in nine awake subjects. Airflow, end-tidal CO2, and moving average electromyogram (EMG) signals were recorded during resting and CO2-stimulated ventilation in both supine and standing postures. Phasic expiratory EMG activity (tidal EMG) of the four abdominal muscles at any level of CO2 stimulation was greater while standing. Abdominal muscle activities during inspiration, PEEA, and PREA, were observed with CO2 stimulation, both supine and standing. Change in posture had a significant effect on intrabreath timing of expiratory muscle activation at any level of CO2 stimulation. The transversus abdominis showed a significant increase in PEEA and a significant decrease in PREA while subjects were standing; similar changes were seen in the internal oblique. We conclude that changes in posture are associated with significant changes in phasic expiratory activity of the four abdominal muscles, with systematic changes in the timing of abdominal muscle activity during early and late inspiration.     

Regional distribution of blood flow within the diaphragm

We investigated the regional distribution of blood flow (Q) within the costal and crural portions of the diaphragm in a total of eight anesthetized supine mongrel dogs. Q was measured with 15-microns microspheres, radiolabeled with three different isotopes, injected into the left ventricle during spontaneous breathing (SB), inspiratory resistive loading (IR), and mechanical ventilation after paralysis (P). At necropsy, the costal and crural portions of each hemidiaphragm were arbitrarily subdivided along a sagittal plane into five to seven and three sections, respectively. During P, there was a dorsoventral Q gradient within the costal part of the diaphragm. During SB there was a fourfold increase in the gradient of Q. Furthermore, during IR, in which mouth pressures of -16 +/- 4 cmH2O were generated, there was a further increase in the gradient of Q. During both SB and IR, Q to the most ventral portion of the costal diaphragm was 26 +/- 6% less than the peak value. In two dogs, studied prone and supine, there was no difference in the Q gradients between the two postures. Over the dorsal 80% of the costal diaphragm there was also a dorsoventral gradient of muscle thickness, such that the most dorsal part was 54 +/- 2% (n = 5) that of the ventral portion. In contrast, there was no consistent gradient of Q or muscle thickness within the crural diaphragm. Our results demonstrate a topographical gravity-independent distribution of Q in the costal, but not the crural, diaphragm.(ABSTRACT TRUNCATED AT 250 WORDS)     

Mechanical role of expiratory muscles during breathing in upright dogs

To examine the mechanical effects of the abdominal and triangularis sterni expiratory recruitment that occurs when anesthetized dogs are tilted head up, we measured both before and after cervical vagotomy the end-expiratory length of the costal and crural diaphragmatic segments and the end-expiratory lung volume (FRC) in eight spontaneously breathing animals during postural changes from supine (0 degree) to 80 degrees head up. Tilting the animals from 0 degree to 80 degrees head up in both conditions was associated with a gradual decrease in end-expiratory costal and crural diaphragmatic length and with a progressive increase in FRC. All these changes, however, were considerably larger (P less than 0.005 or less) postvagotomy when the expiratory muscles were no longer recruited with tilting. Alterations in the elastic properties of the lung could not account for the effects of vagotomy on the postural changes. We conclude therefore that 1) by contracting during expiration, the canine expiratory muscles minimize the shortening of the diaphragm and the increase in FRC that the action of gravity would otherwise introduce, and 2) the end-expiratory diaphragmatic length and FRC in upright dogs are thus actively determined. The present data also indicate that by relaxing at end expiration, the expiratory muscles make a substantial contribution to tidal volume in upright dogs; in the 80 degrees head-up posture, this contribution would amount to approximately 60% of tidal volume.     

Role of costal and crural diaphragm and parasternal intercostals during coughing in cats

The inspiratory phase of coughs often consists of large inspired volumes and increased motor discharge to the costal diaphragm. Furthermore, diaphragm electrical activity may persist into the early expiratory portion of coughs. To examine the role of other inspiratory muscles during coughing, electromyograms (EMG) recorded from the crural diaphragm (Dcr) and parasternal intercostal (PSIC) muscles were compared to EMG of the costal diaphragm (Dco) in anesthetized cats. Tracheal or laryngeal stimulation typically produced a series of coughs, with variable increases in peak inspiratory EMGs of all three muscles. On average, peak inspiratory EMG of Dco increased to 346 +/- 60% of control (P less than 0.001), Dcr to 514 +/- 82% of control (P less than 0.0002), and PSIC to 574 +/- 61% of control (P less than 0.0005). Augmentations of Dcr and PSIC EMG were both significantly greater than of Dco EMG (P less than 0.05 and P less than 0.002, respectively). In most animals, EMG of Dco correlated significantly with EMG of Dcr and of PSIC during different size coughs. Electrical activity of all three muscles persisted into the expiratory portions of many (but not all) coughs. The duration of expiratory activity lasted on average 0.17 +/- 0.03 s for Dco, 0.25 +/- 0.06 s for Dcr, and 0.31 +/- 0.09 s for PSIC. These results suggest that multiple respiratory muscles are recruited during inspiration of coughs, and that the persistence of electrical activity into expiration of coughs is not unique to the costal diaphragm.     

Absence of regional differences in the size and oxidative capacity of diaphragm muscle fibers

The oxidative capacity and cross-sectional area of muscle fibers were compared between the costal and crural regions of the cat diaphragm and across the abdominal-thoracic extent of the muscle. Succinate dehydrogenase (SDH) activity of individual fibers was quantified using a microphotometric procedure implemented on an image-processing system. In both costal and crural regions, population distributions of SDH activities were unimodal for both type I and II fibers. The continuous distribution of SDH activities for type II fibers indicated that no clear threshold exists for the subclassification of fibers based on differences in oxidative capacity (e.g., the classification of fast-twitch glycolytic and fast-twitch oxidative glycolytic fiber types). No differences in either SDH activity or cross-sectional area were noted between fiber populations of the costal and crural regions. Differences in SDH activity and cross-sectional area were noted, however, between fiber populations located on the abdominal and thoracic sides of the costal region. Both type I and II fibers on the abdominal side of the costal diaphragm were larger and more oxidative than comparable fibers on the thoracic side.     

Regional differences in myosin heavy chain isoforms and enzyme activities of the rat diaphragm.

Myosin heavy chain isoforms and enzyme activities were compared between the costal and crural regions of the rat diaphragm. The percentage of heavy chain (HC) IIb in the crural region of the diaphragm was significantly (P less than 0.05) higher than that in the costal region (mean 7.3 vs. 3.0%), and the percentage of HCI was significantly lower in the crural than in the costal diaphragm (22.7 vs. 27.9%). The distributions of HCIIa and HCIId were relatively homogeneous in both regions. Succinate dehydrogenase activity in the costal diaphragm was 21% greater (P less than 0.01) than in the crural diaphragm. In contrast, there was no significant difference in the activity of phosphofructokinase in the crural and costal diaphragms. These results demonstrate that a difference in myosin heavy chain isoforms and oxidative capacity exists between the costal and crural regions of the rat diaphragm.     

Postural changes in spontaneous and evoked regional diaphragmatic activity in dogs

We addressed the question whether gravity-dependent differences in passive tension and length of the diaphragm are associated with differences in its regional activation. By using intramuscular electrodes, we measured the electromyographic activity of different parts of the diaphragm (Edi) during quiet breathing in several postures in 13 anesthetized mongrel dogs. The Edi of the left and right costal hemi-diaphragm was compared between the left and right lateral decubitus postures, whereas that from the substernal and crural regions was compared between the supine and prone positions. On changing posture, the Edi of the dependent part of the diaphragm decreased in both cases, whereas that of the non-dependent part increased. The results were consistent with reflex modulation of regional diaphragm activation in response to postural changes in local resting length. However, these changes in Edi persisted after bilateral vagotomy, cordotomy (C7-T1) and dorsal rhizotomy of the C5-C7 roots. Compound muscle action potentials, recorded in different regions of the diaphragm and evoked by supramaximal stimulation of the phrenic nerves, were altered with changes in posture in the same direction as Edi. Because the stimuli were supramaximal, these changes reflected systematic changes in the recording conditions with posture, possibly because of a combination of 1) changes in the electrical environment surrounding the intramuscular electrodes and 2) passive changes in muscle length. Our results demonstrate systematic, reproducible, posture-dependent changes in regional Edi that may not be due to different neural drive.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of acute hyperinflation on chest wall mechanics in dogs

We studied chest wall mechanics at functional residual capacity (FRC) and near total lung capacity (TLC) in 14 supine anesthetized and vagotomized dogs. During breathing near TLC compared with FRC, tidal volume decreased (674 +/- 542 vs. 68 +/- 83 ml; P less than 0.025). Both inspiratory changes in gastric pressure (4.5 +/- 2.5 vs. -0.2 +/- 2.0 cmH2O; P less than 0.005) and changes in abdominal cross-sectional area (25 +/- 17 vs. -1.0 +/- 4.2%; P less than 0.001) markedly decreased; they were both often negative during inspiration near TLC. Parasternal intercostal shortening decreased (-3.0 +/- 3.7 vs. -2.0 +/- 2.7%), whereas diaphragmatic shortening decreased slightly more in both costal and crural parts (costal -8.4 +/- 2.9 vs. -4.3 +/- 4.1%, crural -22.8 +/- 13.2 vs. -10.0 +/- 7.5%; P less than 0.05). As a result, the ratio of parasternal to diaphragm shortening increased near TLC (0.176 +/- 0.135 vs. 0.396 +/- 0.340; P less than 0.05). Electromyographic (EMG) activity in the parasternals slightly decreased near TLC, whereas the EMG activity in the costal and crural parts of the diaphragm slightly increased. We conclude that 1) the mechanical outcome of diaphragmatic contraction near TLC is markedly reduced, and 2) the mechanical outcome of parasternal intercostal contraction near TLC is clearly less affected.     

Ratio of active to passive muscle shortening in the canine diaphragm.

Active and passive shortening of muscle bundles in the canine diaphragm were measured with the objective of testing a consequence of the minimal-work hypothesis: namely, that the ratio of active to passive shortening is the same for all active muscles. Lengths of six muscle bundles in the costal diaphragm and two muscle bundles in the crural diaphragm of each of four bred-for-research beagle dogs were measured by the radiopaque marker technique during the following maneuvers: a passive deflation maneuver from total lung capacity to functional residual capacity, quiet breathing, and forceful inspiratory efforts against an occluded airway at different lung volumes. Shortening per liter increase in lung volume was, on average, 70% greater during quiet breathing than during passive inflation in the prone posture and 40% greater in the supine posture. For the prone posture, the ratio of active to passive shortening was larger in the ventral and midcostal diaphragm than at the dorsal end of the costal diaphragm. For both postures, active shortening during quiet breathing was poorly correlated with passive shortening. However, shortening during forceful inspiratory efforts was highly correlated with passive shortening. The average ratios of active to passive shortening were 1.23 +/- 0.02 and 1.32 +/- 0.03 for the prone and supine postures, respectively. These data, taken together with the data reported in the companion paper (T. A. Wilson, M. Angelillo, A. Legrand, and A. De Troyer, J. Appl. Physiol. 87: 554-560, 1999), support the hypothesis that, during forceful inspiratory efforts, the inspiratory muscles drive the chest wall along the minimal-work trajectory.     

Crural diaphragm inhibition during esophageal distension correlates with contraction of the esophageal longitudinal muscle in cats

Esophageal distension causes simultaneous relaxation of the lower esophageal sphincter (LES) and crural diaphragm. The mechanism of crural diaphragm relaxation during esophageal distension is not well understood. We studied the motion of crural and costal diaphragm along with the motion of the distal esophagus during esophageal distension-induced relaxation of the LES and crural diaphragm. Wire electrodes were surgically implanted into the crural and costal diaphragm in five cats. In two additional cats, radiopaque markers were also sutured into the outer wall of the distal esophagus to monitor esophageal shortening. Under light anesthesia, animals were placed on an X-ray fluoroscope to monitor the motion of the diaphragm and the distal esophagus by tracking the radiopaque markers. Crural and costal diaphragm electromyograms (EMGs) were recorded along with the esophageal, LES, and gastric pressures. A 2-cm balloon placed 5 cm above the LES was used for esophageal distension. Effects of baclofen, a GABA(B) agonist, were also studied. Esophageal distension induced LES relaxation and selective inhibition of the crural diaphragm EMG. The crural diaphragm moved in a craniocaudal direction with expiration and inspiration, respectively. Esophageal distension-induced inhibition of the crural EMG was associated with sustained cranial motion of the crural diaphragm and esophagus. Baclofen blocked distension-induced LES relaxation and crural diaphragm EMG inhibition along with the cranial motion of the crural diaphragm and the distal esophagus. There is a close temporal correlation between esophageal distension-mediated LES relaxation and crural diaphragm inhibition with the sustained cranial motion of the crural diaphragm. Stretch caused by the longitudinal muscle contraction of the esophagus during distension of the esophagus may be important in causing LES relaxation and crural diaphragm inhibition.     

Costal and crural diaphragm in early inspiration: free breathing and occlusion

Changes in length of costal and crural segments of the canine diaphragm were measured by sonomicrometry within the first 100-300 ms of inspiration during CO2 rebreathing in anesthetized animals. Both segments showed small but significant decreases in end-expiratory length during progressive hypercapnia. Although both costal and crural segments showed electromyographic activity within the first 100 ms of inspiration, in early inspiration crural shortening predominated with minimal costal shortening. Neither segment contracted isometrically early in inspiration in the presence of airway occlusion. The amount of crural shortening during airway occlusion exceeded costal shortening; both segments showed increased shortening with prolonged occlusion and increasing CO2. Costal and crural shortening at 100 ms was not different for unoccluded and occluded states. These observations suggest that neural control patterns evoke discrete and unequal contributions from the diaphragmatic segments at the beginning of an inspiration; the crural segment may be predominately recruited in early inspiration. Despite traditional assumptions about occlusion pressure measurement (P0.1), diaphragm segments do not contract isometrically during early inspiratory effort against an occluded airway.     

Effects of PGF2 alpha on the EMG of costal and crural parts of the diaphragm of the newborn pig

We investigated the effects of PGF2 alpha on the breathing patterns and electric activity of costal and crural parts of the diaphragm in 9 anesthetized newborn pigs. The change in diaphragmatic tension was evaluated as the change in transdiaphragmatic pressure. Because PGF2 alpha induces bronchoconstriction and an increase in respiratory resistances, the changes induced by prostaglandin were evaluated as differences between bronchoconstriction after PGF2 alpha and resistive load obtained by applying gradual occlusion to the inspiratory line of the breathing circuit. Our results show that PGF2 alpha decreased respiratory frequency with lengthening of expiratory time, while the resistive load increased both respiratory phases. The changes in breathing pattern were associated with different electrical activities of the diaphragm. While resistive load did not significantly change the EMG power spectrum, PGF2 alpha recruited new motor units. Furthermore, resistive load induced synchronization of the inspiratory time discharge of the costal and crural parts of the diaphragm, while after PGF2 alpha infusion there was an early inspiratory discharge of the crural part.     

Effect of posture on inspiratory muscle electromyogram response to hypercapnia

The aim of our study was to examine the effect of posture on inspiratory muscle activity response to hypercapnia. Recent research has revealed that in normal subjects the activation of the rib cage muscles and of the diaphragm is actually greater in the upright than in the supine position during resting tidal breathing. In this study we examined whether the upright position necessarily entails a greater activation of the inspiratory muscles also under conditions of ventilatory stress. For this purpose we compared the responses to CO2-rebreathing in the supine and sitting positions in five volunteers, by simultaneously recording the electromyogram of the diaphragm (EMGdi) and the intercostal muscles (EMGint). The electromyogram was recorded by means of surface electrodes to measure the EMG amplitude. While the slopes of ventilatory (VE) response to increasing arterial CO2 tension (PaCO2) were similar in the two positions, both the EMGdi-VE and EMGint-VE relationship showed steeper slopes in the supine than in the sitting position. In each CO2 run the increases in EMGdi were linearly related to those in EMGint. This relationship was not affected by the body position. These results suggested that, in spite of similar ventilatory responses to CO2-rebreathing in the lying and sitting positions, the supine position, in humans, required a higher activation of the inspiratory muscles.     

In vivo length-force relationship of canine diaphragm

Diaphragmatic length was measured by sonomicrometry and transdiaphragmatic pressure (Pdi) by conventional latex balloons in eight dogs anesthetized with pentobarbital sodium under passive conditions and during supramaximal phrenic stimulation. The passive length-pressure relationship indicates that the crural part of the diaphragm is more compliant than the costal part. With supramaximal stimulation the costal diaphragm showed a length-pressure relationship similar in shape to in vitro length-tension curves previously described for the canine diaphragm. The crural part has a smaller pressure-length slope than the costal part in the length range from 80% of optimum muscle length (Lo) to Lo. At supine functional residual capacity (FRC) the resting length (LFRC) of the costal and crural diaphragms are not at Lo. The costal part is distended to 105% of Lo, and crural is shortened to 92% of Lo. Tidal shortening will increase the force output of costal while decreasing that of the crural diaphragm. The major forces setting the passive supine LFRC are the abdominal weight (pressure) and the elastic recoil of the lungs. The equilibrium length (resting length of excised diaphragmatic strips) was 79 +/- 3.6% LFRC for the costal diaphragm and 87 +/- 3.9% LFRC for the crural diaphragm. Similar shortening was obtained in the upright position, indicating passive diaphragmatic stretch at supine LFRC.