Effect of body position on regional diaphragm function in dogs

The in situ lengths of muscle bundles of the crural and three regions of the costal diaphragm between origin and insertion were determined with a video roentgenographic technique in dogs. At total lung capacity (TLC) in both the prone and supine positions, the length of the diaphragm is not significantly different from the unstressed excised length, suggesting that the diaphragm is not under tension at TLC and that there is a hydrostatic gradient of pleural pressure on the diaphragmatic surface. Except for the ventral region of the costal diaphragm, which does not change length at lung volumes greater than 70% TLC, all other regions are stretched during passive deflations from TLC. Therefore below TLC the diaphragm is under passive tension and supports a transdiaphragmatic pressure (Pdi). The length of the diaphragm relative to its unstressed length is not uniform at functional residual capacity (FRC) and does not follow a strict vertical gradient that reverses when the animal is changed from the supine to the prone position. By inference, the length of muscle bundles is determined by factors other than the vertical gradient of Pdi. During mechanical ventilation, regional shortening is identical to the passive deflation length-volume relationship near FRC. Prone and supine FRC is the same, but the diaphragm is slightly shorter in the prone position. In both positions, during spontaneous ventilation there are no consistent differences in regional fractional shortening, despite regional differences in initial length relative to unstressed length.     

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)     

Relationships between abdominal and diaphragmatic volume displacements

We investigated the relationship between the volumes displaced by the diaphragm and the abdominal wall during spontaneous breathing in supine anesthetized dogs. Diaphragmatic volume displacement (Vdi) was calculated from measurements taken from anteroposterior fluoroscopic images employing a previously described geometric model. The volume displacement of the abdominal wall (Vabd) was measured with a calibrated Respitrace. Shortening of single diaphragm muscle bundles in costal and crural regions was measured as the distance between radiopaque beads sutured to the peritoneal surface of the muscle. We found that Vdi always exceeded Vabd, but Vabd/Vdi was larger in animals in which the abdominal wall was more compliant. In this preparation, Vdi is better correlated with costal than with crural shortening. Vabd did not correlate with either costal or crural shortening. We infer that the difference between Vdi and Vabd reflects the volume displacement of the lower rib cage caused by diaphragm contraction. This volume difference was tightly correlated with costal shortening. We conclude from these data that coupling between Vdi and Vabd is influenced by the relative compliances of the chest wall and abdomen. Shortening of regions of the diaphragm may have variable relationships to the measured volume displacement, but costal shortening is intimately related to expansion of the lower rib cage.     

Regional ventilation during spontaneous breathing and mechanical ventilation in dogs

We evaluated the effects of the different patterns of chest wall deformation that occur with different body positions and modes of breathing on regional lung deformation and ventilation. Using the parenchymal marker technique, we determined regional lung behavior during mechanical ventilation and spontaneous breathing in five anesthetized recumbent dogs. Regional lung behavior was related to the patterns of diaphragm motion estimated from X-ray projection images obtained at functional residual capacity (FRC) and end inspiration. Our results indicate that 1) in the prone and supine positions, FRC was larger during mechanical ventilation than during spontaneous breathing; 2) there were significant differences in the patterns of diaphragm motion and regional ventilation between mechanical ventilation and spontaneous breathing in both body positions; 3) in the supine position only, there was a vertical gradient in lung volume at FRC; 4) in both positions and for both modes of breathing, regional ventilation was nonlinearly related to changes in lobar and overall lung volumes; and 5) different patterns of diaphragm motion caused different sliding motions and differential rotations of upper and lower lobes. Our results are inconsistent with the classic model of regional ventilation, and we conclude that the distribution of ventilation is determined by a complex interaction of lung and chest wall shapes and by the motion of the lobes relative to each other, all of which help to minimize distortion of the lung parenchyma.     

Regional deformation of the canine diaphragm.

To follow regional deformation of the diaphragm in dogs, radiopaque markers were implanted under surgical anesthesia into different anatomic regions of the muscle in triangular arrays (approximately 1 cm to a side). After recovery from surgery, changes in area and shape of the triangles were followed with biplane cinefluorography during quiet breathing and during inspiratory efforts against an occluded airway (Mueller maneuvers). From changes in shape of the triangles during contraction, area changes were decomposed into a major direction and magnitude of shortening (Eg1) and a minor length change (Eg2) perpendicular to Eg1, both expressed as a fraction of initial length at end expiration. With the use of these techniques, systematic differences in regional area change were observed in different parts of the diaphragm during inspiratory efforts at different lung volumes. Regional area always decreased during contraction in the crural and midcostal zones of apposition to the rib cage. Area decreased less and often increased during inspiratory efforts in the costal dome near the central tendon and in the costal region near its rib cage insertion. Differences in regional area change were not due to differences in the Eg1 in different parts of the diaphragm but were a consequence of differences in widening of the muscle along Eg2 perpendicular to the direction of Eg1. As lung volume was passively increased above functional residual capacity, regional area decreased in all parts of the diaphragm except in the costal regions near rib cage insertion, where area increased.     

Respiratory muscle length measured by sonomicrometry

The use of sonomicrometry to study the mechanical properties of the diaphragm in vivo is presented. This method consists of the implantation of piezoelectric transducers between muscle fibers to measure the fibers' changes in length. Ultrasonic bursts are produced by one transducer upon electrical excitation and sensed by a second transducer placed 1-2 cm away. The time elapsed between the generation of the ultrasound burst and its detection is used to calculate the intertransducer distance. Excitation and sampling are done at 1.5 kHz and the output is a DC signal proportional to the length change between the transducers. Neither irreversible injury to the diaphragm nor regional differences within an anatomical part or segment were noted. Measurements were stable within the physiological range of temperature. We measured costal and crural length and velocity of contraction in anesthetized dogs during spontaneous breathing, occluded inspirations, passive lung inflation, and supramaximal phrenic nerve stimulation. We found that shortening during spontaneous breathing was 11 and 6% for crural and costal, respectively. The crural leads the costal in velocity of shortening. Supramaximal stimulation results in a velocity of shortening of 5 resting lengths X s-1. During an occluded inspiration crural shortens as much as in the nonoccluded breath, whereas costal shortens less. During passive lung inflation there is a nearly linear relationship between lung volume and diaphragm length; however, the relationships of chest wall dimensions with diaphragm length are nonlinear and cannot be described by any simple function. Some of the implications of these data on the present understanding of diaphragmatic mechanics are discussed.     

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.     

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.     

Videofluoroscopic assessment of muscle fiber shortening in the in situ canine diaphragm

We tested the hypothesis that dynamic shortening of the costal diaphragm can be accurately estimated from measurements of the radiographic width of the zone of apposition (WZapp) by studying seven supine anesthetized dogs. Both muscle fiber length, represented by the distance between implanted radiopaque markers, and WZapp were measured from digitized recordings of fluoroscopic images utilizing interactive computer software. The WZapp was highly correlated with the length of costal fibers during active respiration in all animals (mean R2 = 0.94). The accuracy in the prediction of fiber length and shortening during breathing is enhanced by inclusion of additional variables describing the displacement of the abdominal wall and the resting geometric orientation of the fibers. We conclude that dynamic fluoroscopic measurement of WZapp is a valuable technique for estimating dynamic diaphragm fiber length and shortening. Depending on the experimental circumstances, WZapp may be a more easily acquired indicator of diaphragm shortening than other variables that have been previously utilized. As such, it may provide a suitable approach to assess active shortening of the diaphragm in humans.     

Electrical and mechanical activity of respiratory muscles during hypercapnia

In nine anesthetized supine spontaneously breathing dogs, we compared moving average electromyograms (EMGs) of the costal diaphragm and the third parasternal intercostal muscles with their respective respiratory changes in length (measured by sonomicrometry). During resting O2 breathing the pattern of diaphragm and intercostal muscle inspiratory shortening paralleled the gradually incrementing pattern of their moving average EMGs. Progressive hypercapnia caused progressive increases in the amount and velocity of respiratory muscle inspiratory shortening. For both muscles there were linear relationships during the course of CO2 rebreathing between their peak moving average EMGs and total inspiratory shortening and between tidal volume and total inspiratory shortening. During single-breath airway occlusions, the electrical activity of both the diaphragm and intercostal muscles increased, but there were decreases in their tidal shortening. The extent of muscle shortening during occluded breaths was increased by hypercapnia, so that both muscles shortened more during occluded breaths under hypercapnic conditions (PCO2 up to 90 Torr) than during unoccluded breaths under normocapnic conditions. These results suggest that for the costal diaphragm and parasternal intercostal muscles there is a close relationship between their electrical and mechanical behavior during CO2 rebreathing, this relationship is substantially altered by occluding the airway for a single breath, and thoracic respiratory muscles do not contract quasi-isometrically during occluded breaths.     

Diaphragm curvature modulates the relationship between muscle shortening and volume displacement

During physiological spontaneous breathing maneuvers, the diaphragm displaces volume while maintaining curvature. However, with maximal diaphragm activation, curvature decreases sharply. We tested the hypotheses that the relationship between diaphragm muscle shortening and volume displacement (VD) is nonlinear and that curvature is a determinant of such a relationship. Radiopaque markers were surgically placed on three neighboring muscle fibers in the midcostal region of the diaphragm in six dogs. The three-dimensional locations were determined using biplanar fluoroscopy and diaphragm VD, curvature, and muscle shortening were computed in the prone and supine postures during spontaneous breathing (SB), spontaneous inspiration efforts after airway occlusion at lung volumes ranging from functional residual capacity (FRC) to total lung capacity, and during bilateral maximal phrenic nerve stimulation at those same lung volumes. In supine dogs, diaphragm VD was approximately two- to three-fold greater during maximal phrenic nerve stimulation than during SB. The contribution of muscle shortening to VD nonlinearly increases with level of diaphragm activation independent of posture. During submaximal diaphragm activation, the contribution is essentially linear due to constancy of diaphragm curvature in both the prone and supine posture. However, the sudden loss of curvature during maximal bilateral phrenic nerve stimulation at muscle shortening values greater than 40% (ΔL/L(FRC)) causes a nonlinear increase in the contribution of muscle shortening to diaphragm VD, which is concomitant with a nonlinear change in diaphragm curvature. We conclude that the nonlinear relationship between diaphragm muscle shortening and its VD is, in part, due to a loss of its curvature at extreme muscle shortening.     

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).     

Determinants of transdiaphragmatic pressure in dogs

We measured the transdiaphragmatic pressure (Pdi) during bilateral phrenic nerve stimulation and evaluated the determinants of its change with lung volume, chest wall geometry, and respiratory system impedance in supine dogs. Four rows of radiopaque markers were sewn onto muscle bundles of the costal and crural diaphragm between their origin on the central tendon and their insertion on the rib cage and spine. The length of the diaphragm (L) was determined from the projection images of marker rows using biplane fluoroscopy. Measurements were made at lung volumes between total lung capacity and functional residual capacity before and after the infusion of Ringer lactate solution into the abdominal cavity. In contrast to relaxation, during tetanic stimulation the active lengths of the muscle bundles were similar at all volumes, but the diaphragm assumed different shapes. Although the small differences in active muscle length with volume and liquid loads are consistent with only small changes in muscle force output, Pdi varied by a factor of greater than or equal to 5. There was no single L/Pdi curve that fitted all data during 50-Hz stimulations. We conclude that under these experimental conditions Pdi is not a unique measure of the force produced by the diaphragm and that lung volume, chest wall geometry, and respiratory system impedance are important determinants of the mechanical efficiency of the diaphragm as a pressure generator.     

Chest wall motion during spontaneous breathing and mechanical ventilation in dogs

We measured the volume change of the thoracic cavity (delta Vth) and the volumes displaced by the diaphragm (delta Vdi) and rib cage (delta Vrc) in six pentobarbital-anesthetized dogs lying supine. A high-speed X-ray scanner (dynamic spatial reconstructor) provided three-dimensional images of the thorax during spontaneous breathing and during mechanical ventilation with paralysis. Tidal volume (VT) was measured by integrating gas flow. Changes in thoracic liquid volume (delta Vliq, presumably caused by changes in thoracic blood volume) were calculated as delta Vth - VT. Absolute volume displaced by the rib cage was not significantly different during the two modes of ventilation. During spontaneous breathing, thoracic blood volume increased during inspiration; delta Vliq was 12.3 +/- 4.1% of delta Vth. During mechanical ventilation, delta Vliq was nearly zero. Configuration of the relaxed chest wall was similar during muscular relaxation induced by either pharmacological paralysis or hyperventilation. Expiratory muscle activity produced 50 +/- 11% of the delta Vth during spontaneous breathing. We conclude that at constant VT the volume displaced by the rib cage is remarkably similar during the transition from spontaneous breathing to mechanical ventilation, while both diaphragmatic volume displacement and changes in intrathoracic blood volume decrease by a similar amount.     

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.     

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.     

Effect of lung inflation on diaphragmatic shortening

The effect of lung inflation on chest wall mechanics was studied in 11 vagotomized pentobarbital sodium-anesthetized dogs. Diaphragmatic shortening (percent change from initial length at functional residual capacity, %LFRC) and transdiaphragmatic pressure swings (delta Pdi) were compared with control values over a range of positive-pressure breathing that produced a maximum increase in lung volume to 40% of inspiratory capacity. There was no change in the electromyogram of the diaphragm or parasternal intercostals during positive-pressure breathing. delta Pdi and tidal volume (VT) fell to 52 +/- 3.3 and 42.5 +/- 5% (SE) of control. This was associated with a reduction in the initial resting length of 13 +/- 1.9 and 21 +/- 2.2%LFRC (SE) in the costal and crural diaphragms, respectively. Tidal diaphragmatic shortening, however, decreased to 66 +/- 7 and 57 +/- 7 and the mean velocity decreased to 78 +/- 10 and 63 +/- 8% (SE) of control for the costal and crural diaphragms, respectively. We conclude that the reduction in diaphragmatic shortening is the main determinant of the reduced delta Pdi and VT during lung inflation and relate this to what is currently known about diaphragmatic contractile properties.     

Diaphragmatic perfusion heterogeneity during exercise with inspiratory resistive breathing

Regional distribution of diaphragmatic blood flow (Q; 15-microns-diam radionuclide-labeled microspheres) was studied in normal (n = 7) and laryngeal hemiplegic (LH; n = 7) ponies to determine whether the added stress of inspiratory resistive breathing during maximal exercise may cause 1) redistribution of diaphragmatic Q and 2) crural diaphragmatic Q to exceed that in maximally exercising normal ponies. LH-induced augmentation of already high exertional work of breathing resulted in diminished locomotor exercise capacity so that maximal exercise in LH ponies occurred at 25 km/h compared with 32 km/h for normal ponies. The costal and crural regions received similar Q in both groups at rest. However, exercise-induced increments in perfusion were significantly greater in the costal region of the diaphragm. At 25 km/h, costal diaphragmatic perfusion was 154 and 143% of the crural diaphragmatic Q in normal and LH ponies. At 32 km/h, Q in costal diaphragm of normal ponies was 136% of that in the crural region. Costal and crural diaphragmatic Q in LH ponies exercised at 25 km/h exceeded that for normal ponies but was similar to the latter during exercise at 32 km/h. Perfusion pressure for the three conditions was also similar. It is concluded that diaphragmatic perfusion heterogeneity in exercising ponies was preserved during the added stress of inspiratory resistive breathing. It was also demonstrated that vascular resistance in the crural and costal regions of the diaphragm in maximally exercised LH ponies remained similar to that in maximally exercising normal ponies.     

Effects of body position and lung volume on in situ operating length of canine diaphragm

The performance of the diaphragm is influenced by its in situ length relative to its optimal force-generating length (Lo). Lead markers were sutured to the abdominal surface of the diaphragm along bundles of the left ventral, middle, and dorsal regions of the costal diaphragm and the left crural diaphragm of six beagle dogs. After 2-3 wk postoperative recovery, the dogs were anesthetized, paralyzed, and scanned prone and supine in the Dynamic Spatial Reconstructor (DSR) at a total lung capacity (TLC), functional residual capacity (FRC), and residual volume (RV). The location of each marker was digitized from the reconstructed DSR images, and in situ lengths were determined. After an overdose of anesthetic had been administered to the dogs, each marked diaphragm bundle was removed, mounted in a 37 degrees C in vitro chamber, and adjusted to Lo (maximum tetanic force). The operating length of the diaphragm, or in situ length expressed as percent Lo, varied from region to region at the lung volumes studied; variability was least at RV and increased with increasing lung volume. At FRC, all regions of the diaphragm was shorter in the prone posture compared with the supine, but there was no clear gravity-dependent vertical gradient of in situ length in either posture. Because in vitro length-tension characteristics were similar for all diaphragm regions, regional in vivo length differences indicate that the diaphragm's potential to generate maximal force is nonuniform.     

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.