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.     

Effects of lung volume on diaphragm EMG signal strength during voluntary contractions

The use ofesophageal recordings of the diaphragm electromyogram (EMG) signalstrength to evaluate diaphragm activation during voluntary contractionsin humans has recently been criticized because of a possible artifactcreated by changes in lung volume. Therefore, the first aim of thisstudy was to evaluate whether there is an artifactual influence of lungvolume on the strength of the diaphragm EMG during voluntarycontractions. The second aim was to measure the required changes inactivation for changes in lung volume at a given tension, i.e., thevolume-activation relationship of the diaphragm. Healthy subjects(n = 6) performed contractions of thediaphragm at different transdiaphragmatic pressure (Pdi) targets (range20-160 cmH₂O) whilemaintaining chest wall configuration constant at different lungvolumes. The diaphragm EMG was recorded with a multiple-arrayesophageal electrode, with control of signal contamination andelectrode positioning. The effects of lung volume on the EMG werestudied by comparing the crural diaphragm EMG root mean square (RMS),an index of crural diaphragm activation, with an index of globaldiaphragm activation obtained by normalizing Pdi to the maximum Pdi atthe given muscle length(Pdi/Pdi_max@L) at thedifferent lung volumes. We observed a direct relationship between RMSand Pdi/Pdi_max@L independent of diaphragm length. The volume-activation relationship ofthe diaphragm was equally affected by changes in lung volume as thevolume-Pdi relationship (60% change from functional residual capacityto total lung capacity). We conclude that the RMS of the diaphragm EMGis not artifactually influenced by lung volume and can be used as areliable index of diaphragm activation. The volume-activationrelationship can be used to infer changes in the length-tensionrelationship of the diaphragm at submaximal activation/contractionlevels.

     

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.     

Effect of muscle length on electromyogram in a canine diaphragm strip preparation

The relationship between diaphragm electromyogram (EMG), isometric force, and length was studied in the canine diaphragm strip with intact blood supply and innervation under three conditions: supramaximal tetanic (100 Hz) phrenic nerve stimulation (STPS; n = 12), supramaximal phrenic stimulation at 25 Hz (n = 15), and submaximal phrenic stimulation at 25 Hz (n = 5). In the same preparation, the EMG-length relationship was also examined with direct muscle stimulation when the neuromuscular junction was blocked. EMG from three different sites and via two types of electrodes (direct or sewn-in and surface) were recorded during isometric contraction at different lengths. Direct EMGs were recorded from two bipolar electrodes sutured into the strip, one near its central end and the other near its costal end. A third EMG electrode configuration summed potentials from the whole strip by recording potentials between central and costal sites. Surface EMGs were recorded by a bipolar spring clip electrode that made contact with upper and lower surfaces of the muscle strip with light pressure. In all conditions of stimulation with different types of electrodes, all EMGs decreased significantly (P less than 0.05) when muscle length was changed from 50 to 120% of resting length (L0). Minimal and maximal force outputs were observed at 50 and 120% of L0, respectively, in all experiments. The results of this study indicated that the muscle length is a significant variable that affects the EMG recording and that the diaphragmatic EMG may not be an accurate reflection of phrenic nerve activity.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of uncompensated and compensated metabolic acidosis on canine diaphragm

We investigated the effects of metabolic acidosis and compensated metabolic acidosis on force of contraction of the diaphragm in anesthetized dogs. Mechanically ventilated animals were prepared with an open thorax. A balloon was positioned beneath the diaphragm to measure transdiaphragmatic pressure (Pdi), and a plaster cast was placed around the abdomen to maintain length and geometry of the diaphragm. The force of contraction was evaluated by measuring Pdi during supramaximal phrenic stimulation at different frequencies and also during spontaneous inspiratory efforts. In 13 dogs with an arterial pH (pHa) of 7.38 and arterial PCO2 (PaCO2) of 36.5 Torr, metabolic acidosis was produced by infusion of HCl until pHa equaled 6.98 and PaCO2 equaled 36.4 Torr. Pdi at all frequencies greater than 10 Hz was significantly reduced (P less than 0.05). The dogs were then hyperventilated until pHa was 7.34 and PaCO2 was 12.8 Torr. Pdi was significantly reduced again at all frequencies (P less than 0.05) except 5 Hz. The percent reduction in Pdi by compensated acidosis was significantly greater at low-frequency stimulation than at high (P less than 0.05). Similar qualitative results were observed during spontaneous inspiratory efforts where Pdi was compared at constant magnitudes of diaphragmatic electromyograms. Twitch characteristics revealed that metabolic acidosis led to a significant shortening of twitch relaxation time (P less than 0.05), and compensated metabolic acidosis added to this effect a significant decrease in twitch amplitude (P less than 0.05).     

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.     

Effects of recruitment maneuvers with PEEP on lung volume distribution in canine models of direct and indirect lung injury

Lung recruitment maneuvers can help open collapsed lung units for sufficient oxygenation, and positive end expiratory pressure (PEEP) is used to keep the lung open after recruitment. However, the application of high PEEP levels may play a significant role in causing regional lung hyperinflation during mechanical ventilation. The authors sought to study the effects of PEEP targeting optimal oxygenation on regional lung volume distribution in a direct and an indirect acute respiratory distress syndrome (ARDS) model. ARDS was induced by either surfactant depletion or oleic acid injection in dogs. After lung recruitment, PEEP was decreased from 20 to 10 cmH₂O in 2 cmH₂O steps every 10 min to examine regional lung aeration by using computed tomography. Lung injury appeared to be localized in the model of surfactant depletion while it widely diffused after oleic acid infusion. At PEEP levels that achieved optimal oxygenation, nonaerated lung units decreased and normally aerated lung units enhanced, but hyperinflated areas increased significantly in both models (P < 0.05). Hyperinflated areas were greater in the surfactant depletion model than in the oleic acid model at PEEP levels applied (P < 0.05). Optimal oxygenation guided PEEP may cause hyperinflated in both focal lung injury and diffused lung injury post lung recruitment. Hyperinflation was more susceptible in focal lung injury than in diffused lung injury post lung recruitment.     

Interaction between the canine diaphragm and intercostal muscles in lung expansion.

Changes in intrathoracic pressure produced by the various inspiratory intercostals are essentially additive, but the interaction between these muscles and the diaphragm remains uncertain. In the present study, this interaction was assessed by measuring the changes in airway opening (DeltaPao) or transpulmonary pressure (DeltaPtp) in vagotomized, phrenicotomized dogs during spontaneous inspiration (isolated intercostal contraction), during isolated rectangular or ramp stimulation of the peripheral ends of the transected C(5) phrenic nerve roots (isolated diaphragm contraction), and during spontaneous inspiration with superimposed phrenic nerve stimulation (combined diaphragm-intercostal contraction). With the endotracheal tube occluded at functional residual capacity, DeltaPao during combined diaphragm-intercostal contraction was nearly equal to the sum of the DeltaPao produced by the two muscle groups contracting individually. However, when the endotracheal tube was kept open, DeltaPtp during combined contraction was 123% of the sum of the individual DeltaPtp (P < 0.001). The increase in lung volume during combined contraction was also 109% of the sum of the individual volume increases (P < 0.02). Abdominal pressure during combined contraction was invariably lower than during isolated diaphragm contraction. It is concluded, therefore, that the canine diaphragm and intercostal muscles act synergistically during lung expansion and that this synergism is primarily due to the fact that the intercostal muscles reduce shortening of the diaphragm. When the lung is maintained at functional residual capacity, however, the synergism is obscured because the greater stiffness of the rib cage during diaphragm contraction enhances the DeltaPao produced by the isolated diaphragm and reduces the DeltaPao produced by the intercostal muscles.     

Dependence of diaphragmatic length on lung volume and thoracoabdominal configuration

Changes in lung volume can be partitioned into volume displacements of the rib cage and abdomen. Abdominal displacements are often used as estimates of diaphragmatic displacements and changes in lengthening of diaphragmatic muscle. We used X-rays, ultrasound, and linear measurements of thoracic and abdominal diameters to estimate relationships among lung volume, thoracoabdominal configuration and diaphragmatic length, and we found that diaphragmatic length was strongly dependent on rib cage as well as abdominal displacement. In three subjects, the diaphragm shortened 57-85% as much during a breath made without abdominal displacement as during a normal breath in which the abdominal wall moved outward with the rib cage. We conclude that changes in diaphragmatic length can be estimated from surface measurements without radiation and that the length of the diaphragm cannot be estimated from displacements of the abdominal wall alone.     

Effect of position and lung volume on upper airway geometry

The occurrence of upper airway obstruction during sleep and with anesthesia suggests the possibility that upper airway size might be compromised by the gravitational effects of the supine position. We used an acoustic reflection technique to image airway geometry and made 180 estimates of effective cross-sectional area as a function of distance along the airway in 10 healthy volunteers while they were supine and also while they were seated upright. We calculated z-scores along the airway and found that pharyngeal cross-sectional area was smaller in the supine than in the upright position in 9 of the 10 subjects. For all subjects, pharyngeal cross-sectional area was 23 +/- 8% smaller in the supine than in the upright position (P less than or equal to 0.05), whereas glottic and tracheal areas were not significantly altered. Because changing from the upright to the supine position causes a decrease in functional residual capacity (FRC), six of these subjects were placed in an Emerson cuirass, which was evacuated producing a positive transrespiratory pressure so as to restore end-expiratory lung volume to that seen before the position change. In the supine posture an increase in end-expiratory lung volume did not change the cross-sectional area at any point along the airway. We conclude that pharyngeal cross-sectional area decreases as a result of a change from the upright to the supine position and that the mechanism of this change is independent of the change in FRC.     

The effects of body position and exercise on plasma volume dynamics

We examined the plasma volume changes associated with a protocol of either exercise or controlled rest under identical positional and ambient conditions. Nine healthy adult males rode (E) and on another occasion sat quietly (C) on a cycle ergometer for 30 min. Ten minutes of cycle exercise immediately followed the resting C protocol. Ambient temperature was 30 degrees C (rh = 35%) and exercise load was equal to 50% of peak VO2. Venous blood samples were obtained with subjects both in the supine and seated positions prior to all experiments. Additional blood was drawn during minutes 1, 5, 10, and 30 in both experimental conditions. A final sample was taken during C after the 10 min exercise. Moving from the supine to a seated position resulted in an average loss of 162 ml of plasma across all experiments. During the E condition a further reduction in plasma volume (76 ml) occurred by one minute of exercise. Plasma volume stabilized by 5 min of exercise under the E protocol. During the C condition, subsequent fluid loss (98 ml) was not apparent until 10 min after the first seated sample and totalled 176 ml at the end of 30 min of rest. Ten minutes of cycling at the end of the C experiment resulted in a further plasma volume reduction of 137 ml. Plasma protein and albumin contents decreased by 5 min of exercise in E and by 30 min of rest in C. [Na+] and [Cl-] did not change in either condition but a rapid increase in [K+] during exercise indicated an addition of potassium to the vascular volume.(ABSTRACT TRUNCATED AT 250 WORDS)     

Influence of acute lung volume change on contractile properties of human diaphragm

The effect of stimulus frequency on thein vivo pressure generating capacity of the human diaphragm is unknownat lung volumes other than functional residual capacity. Thetransdiaphragmatic pressure (Pdi) produced by a pair of phrenic nervestimuli may be viewed as the sum of the Pdi elicited by the first (T1Pdi) and second (T2 Pdi) stimuli. We used bilateral anteriorsupramaximal magnetic phrenic nerve stimulation and a digitalsubtraction technique to obtain the T2 Pdi at interstimulus intervalsof 999, 100, 50, 33, and 10 ms in eight normal subjects at lung volumesbetween residual volume and total lung capacity. The reduction in T2Pdi that we observed as lung volume increased was greatest at long interstimulus intervals, whereas the T2 Pdi obtained with short interstimulus intervals remained relatively stable over the 50% ofvital capacity around functional residual capacity. For all interstimulus intervals, the total pressure produced by the pair decreased as a function of increasing lung volume. These data demonstrate that, in the human diaphragm, hyperinflation has a disproportionately severe effect on the summation of pressure responseselicited by low-frequency stimulations; this effect isdistinct from and additional to the known length-tension relationship.

     

Effects of room temperature and body position change on cerebral blood volume and center-of-foot pressure in healthy young adults

This study aimed to examine the effects of room temperature and body position changes on cerebral blood volume, blood pressure and center-of-foot pressure (COP). Cerebral oxygenation kinetics and blood pressure were measured by near infrared spectroscopy (NIRS) and volume-compensation, respectively, in 9 males and 9 females after rapid standing from sitting and supine positions in low (12 degrees C) or normal (22 degrees C) room temperatures. COP was also measured in a static standing posture for 90 s after rapid standing. The total hemoglobin (Hb) decreased just after standing. Blood pressure after standing at normal temperature tended to decrease immediately but at low temperature tended to decrease slightly and then to increase greatly. The decreasing ratio of total Hb and blood pressure upon standing from a supine position at normal room temperatures was the largest of any condition. Total Hb recovered to a fixed level approximately 25 sec after standing from a sitting position and approximately 35 sec after standing from a supine position. All COP parameters after standing tended to change markedly in the supine position compared to the sitting position, especially at normal temperatures. The COP parameters after standing in any condition were not significantly related to the decreasing ratio of total Hb but were related to the recovery time of total Hb after standing. In conclusion, decreasing ratios of total Hb and blood pressure after standing from a supine position at normal temperatures were large and may affect body sway.     

Effects of lung volume, volume history, and methacholine on lung tissue viscance

We examined the effects of lung volume change and volume history on lung resistance (RL) and its components before and during induced constriction. Eleven subjects, including three current and four former asthmatics, were studied. RL, airway resistance (Raw), and, by subtraction, tissue viscance (Vtis) were measured at different lung volumes before and after a deep inhalation and were repeated after methacholine (MCh) aerosols up to maximal levels of constriction. Vtis, which average 9% of RL at base line, was unchanged by MCh and was not changed after deep inhalation but increased directly with lung volume. MCh aerosols induced constriction by increasing Raw, which was reversed by deep inhalation in inverse proportion to responsiveness. such that the more responsive subjects reversed less after a deep breath. Responsiveness correlated directly with the degree of maximal constriction, as more responsive subjects constricted to a greater degree. These results indicate that in humans Vtis comprises a small fraction of overall RL, which is clearly volume-dependent but unchanged by MCh-induced constriction and unrelated to the degree of responsiveness of the subject.