Respiration Physiology, 90 (1992) 87-98 © 1992 Elsevier Science Publishers B.V. All rights reserved. 0034-5687/92/$05.00
87
RESP 01948
Mechanical characteristics and functional length of canine expiratory muscles Gaspar A. Farkas Thoracic Disease Research Unit, Mayo Clinic and Foundation, Rochester, MN, USA (Accepted 18 May 1992) Abstract. The expiratory muscles of the abdominal region actively contribute to breathing. In dogs, the transversus abdominis appears to be the main abdominal muscle of expiration. The in vitro mechanical properties of the transversus abdominis have not been reported to date, and formed the basis of the present investigation. Moreover, in order to understand better the nature of the mechanical interplay between the various abdominal muscle groups, we also evaluated the effects of posture on the operational length of canine transversus abdominis and external oblique muscles and related their in situ length to optimal length. The experiments were performed on twelve mongrel dogs, anesthetized with pentobarbital sodium. Contractile properties of excised transversus abdominis muscle strips were evaluated at 37 °C and revealed similar twitch, force-frequency and length-tension properties as previously reported in canine external oblique (Farkas and Rochester, J. Appl. Physiol. 65: 2427-2433, 1988). In the supine posture, we noted that external oblique was operating at 88% Lo, while the transversus abdominis was operating at a significantly shorter length of 74~o Lo. Thus, in the supine posture, the external oblique is better situated than the transversus abdominis to generate tension. In the prone posture, however, we noted that both abdominal muscles were located at similar positions along their length-tension curve, operating at a length of roughly 77% Lo. Since both muscles share common length-tension characteristics, the present results indicate that the tension generating potential of both muscles in prone dogs is equal for a given neural input. However, we conclude that the preferential recruitment of the transversus abdominis in prone animals must be related to factors other than simple tension generation.
Mammals, dog; Muscles, expiratory, mechanical properties
The expiratory muscles of the abdominal region actively contribute to breathing. In supine anesthetized dogs, the abdominal muscles exhibit phasic activity (De Troyer and Ninane, 1987; Estenne et al., 1990; Farkas et aL, 1988; Gilmartin et al., 1987; Schroeder et al., 1991) actively shorten below their relaxation length (Arnold et al., 1988; Ninane et al., 1988) and may generate from 20-45% of tidal volume (Schroeder et al., 1991; Warner et aL, 1989). The phasic nature of abdominal activation is maintained in head-up or prone dogs (De Troyer and Ninane, 1987; Farkas et al., 1988, 1989; Farkas and Schroeder, 1990; Smith et al., 1989a,b), and they clearly participate in generating well over half of the tidal volume (Farkas et al., 1988, 1989; Farkas and Schroeder, Correspondence to: G.A. Farkas, Thoracic Disease Research Unit, Mayo Clinic, Rochester, MN 55905 USA.
88
G.A. FARKAS
1990). Thus, the important mechanical role of the expiratory musculature in dogs has been clearly established. In dogs, the transversus abdominis appears to be the main abdominal muscle of expiration (Gilmartin et al., 1987; Ninane et al., 1988). The in vitro mechanical properties of the transversus abdominis have not been reported in the dog and form the basis of the present investigation. In the hamster, Arnold et ai. (1987) noted that the lengthtension characteristic of the transversus abdominis was compressed along the length axis as compared to that recorded for the external oblique, suggesting when the transversus abdominis is operating at shorter length, its force potential is compromised more than that of the external oblique. In order to understand better the nature of the mechanical interplay between the various abdominal muscle groups, we also evaluated the effects of posture on the operational length of canine transversus abdominis and external oblique muscles and related in situ length to the muscles optimal force producing length.
Methods
The experiments were performed on twelve mongrel dogs (15-22 kg), anesthetized with pentobarbital sodium (initial dose: 25-30 mg/kg i.v.). The animals were placed supine on a V-shaped board and intubated with a cuffed endotracheal tube. A cannula was inserted in a femoral vein to give supplemental doses of anesthetic. The level of anesthesia was regulated to maintain the palpebral reflex. Body temperature was maintained between 36 and 38 °C by means of a heating lamp. Protocol A. Determination of the in vitro courractile properties In four dogs, the contractile properties of the transversus abdominis muscle were evaluated in vitro. The muscle was exposed by an incision of the skin of the abdominal wall at an area near the mid-axiUary line, between the costal margin and iliac crest. Segments of the transversus abdominis muscle were removed one at a time and bleeding was controlled. Initial muscle segments were 3-5 cm wide and were oriented parallel to the long axis of the muscle fibers. Two segments of transversus abdominis muscle were removed from each dog. The sequence of gross dissection was first to remove one segment, dissect out a smaller muscle bundte, and measure its contractile properties. The entire sequence required approximately 30-45 min, after which it was repeated for the second muscle segment. Immediately on removal, each muscle segment was immersed in cooled (4 °C) oxygenated Krebs solution in preparation for further dissection. From each segment, a smaller bundle (5-7 mm wide) was carefully dissected under × 3 magnification. Sutures were then firmly tied to both ends of the bundle and served as anchoring points to the muscle bath and force transducer. The bundle was then mounted between large platinum stimulating electrodes and placed in an in vitro muscle bath containing oxygenated Krebs solution maintained at 37 ° C. After a 15-min thermal equilibration period,
OPERATIONAL LENGTH OF CANJ:NE EXPIRATORY MUSCLES
89
the bundle was adjusted to its optimal length (Lo), defined as the length at which active tension was maximum. Lo was measured in triplicate by means of a micrometer. For each bundle, a twitch and a force frequency response were measured at Lo, and a maximum tetanic (100 Hz) length-tension curve was measured at resting lengths ranging from 55-125 % of Lo. On completion of these measurements, the bundle was removed from the apparatus, blotted dry, and weighed on an analytic balance. Muscle cross.sectional area was estimated by dividing the muscle mass by its length and specific gravity (1056 g/cm3), and muscle tensions were expressed as Newtons of force per square centimeter of cross-sectional area. Details regarding the in vitro measurements of contractile properties for other respiratory muscles have been previously published (Farkas and Rochester, 1988; Farkas et al., 1985). Protocol B. Determination of in situ length in relation to the in vitro Lo
Following our initial preparation, eight animals were secured to a body postural frame designed to support anesthetized animals in various postures. The frame has been described previously (Farkas and Schroeder, 1990) and consists of an assembly of aluminium bars, adjustable in all dimensions to match the size of the animal. The animals were supported by four to five r~ylon tie straps which were passed through the supraspinous ligament to affix the spine to a horizontal support bar. The limbs of the animals were subsequently taped to four vertical bars. The frame was designed not to restrict the motion of either the rib cage or abdominal compartments of the chest wall. TI~ ~',~.~
87
RESP 01948
Mechanical characteristics and functional length of canine expiratory muscles Gaspar A. Farkas Thoracic Disease Research Unit, Mayo Clinic and Foundation, Rochester, MN, USA (Accepted 18 May 1992) Abstract. The expiratory muscles of the abdominal region actively contribute to breathing. In dogs, the transversus abdominis appears to be the main abdominal muscle of expiration. The in vitro mechanical properties of the transversus abdominis have not been reported to date, and formed the basis of the present investigation. Moreover, in order to understand better the nature of the mechanical interplay between the various abdominal muscle groups, we also evaluated the effects of posture on the operational length of canine transversus abdominis and external oblique muscles and related their in situ length to optimal length. The experiments were performed on twelve mongrel dogs, anesthetized with pentobarbital sodium. Contractile properties of excised transversus abdominis muscle strips were evaluated at 37 °C and revealed similar twitch, force-frequency and length-tension properties as previously reported in canine external oblique (Farkas and Rochester, J. Appl. Physiol. 65: 2427-2433, 1988). In the supine posture, we noted that external oblique was operating at 88% Lo, while the transversus abdominis was operating at a significantly shorter length of 74~o Lo. Thus, in the supine posture, the external oblique is better situated than the transversus abdominis to generate tension. In the prone posture, however, we noted that both abdominal muscles were located at similar positions along their length-tension curve, operating at a length of roughly 77% Lo. Since both muscles share common length-tension characteristics, the present results indicate that the tension generating potential of both muscles in prone dogs is equal for a given neural input. However, we conclude that the preferential recruitment of the transversus abdominis in prone animals must be related to factors other than simple tension generation.
Mammals, dog; Muscles, expiratory, mechanical properties
The expiratory muscles of the abdominal region actively contribute to breathing. In supine anesthetized dogs, the abdominal muscles exhibit phasic activity (De Troyer and Ninane, 1987; Estenne et al., 1990; Farkas et aL, 1988; Gilmartin et al., 1987; Schroeder et al., 1991) actively shorten below their relaxation length (Arnold et al., 1988; Ninane et al., 1988) and may generate from 20-45% of tidal volume (Schroeder et al., 1991; Warner et aL, 1989). The phasic nature of abdominal activation is maintained in head-up or prone dogs (De Troyer and Ninane, 1987; Farkas et al., 1988, 1989; Farkas and Schroeder, 1990; Smith et al., 1989a,b), and they clearly participate in generating well over half of the tidal volume (Farkas et al., 1988, 1989; Farkas and Schroeder, Correspondence to: G.A. Farkas, Thoracic Disease Research Unit, Mayo Clinic, Rochester, MN 55905 USA.
88
G.A. FARKAS
1990). Thus, the important mechanical role of the expiratory musculature in dogs has been clearly established. In dogs, the transversus abdominis appears to be the main abdominal muscle of expiration (Gilmartin et al., 1987; Ninane et al., 1988). The in vitro mechanical properties of the transversus abdominis have not been reported in the dog and form the basis of the present investigation. In the hamster, Arnold et ai. (1987) noted that the lengthtension characteristic of the transversus abdominis was compressed along the length axis as compared to that recorded for the external oblique, suggesting when the transversus abdominis is operating at shorter length, its force potential is compromised more than that of the external oblique. In order to understand better the nature of the mechanical interplay between the various abdominal muscle groups, we also evaluated the effects of posture on the operational length of canine transversus abdominis and external oblique muscles and related in situ length to the muscles optimal force producing length.
Methods
The experiments were performed on twelve mongrel dogs (15-22 kg), anesthetized with pentobarbital sodium (initial dose: 25-30 mg/kg i.v.). The animals were placed supine on a V-shaped board and intubated with a cuffed endotracheal tube. A cannula was inserted in a femoral vein to give supplemental doses of anesthetic. The level of anesthesia was regulated to maintain the palpebral reflex. Body temperature was maintained between 36 and 38 °C by means of a heating lamp. Protocol A. Determination of the in vitro courractile properties In four dogs, the contractile properties of the transversus abdominis muscle were evaluated in vitro. The muscle was exposed by an incision of the skin of the abdominal wall at an area near the mid-axiUary line, between the costal margin and iliac crest. Segments of the transversus abdominis muscle were removed one at a time and bleeding was controlled. Initial muscle segments were 3-5 cm wide and were oriented parallel to the long axis of the muscle fibers. Two segments of transversus abdominis muscle were removed from each dog. The sequence of gross dissection was first to remove one segment, dissect out a smaller muscle bundte, and measure its contractile properties. The entire sequence required approximately 30-45 min, after which it was repeated for the second muscle segment. Immediately on removal, each muscle segment was immersed in cooled (4 °C) oxygenated Krebs solution in preparation for further dissection. From each segment, a smaller bundle (5-7 mm wide) was carefully dissected under × 3 magnification. Sutures were then firmly tied to both ends of the bundle and served as anchoring points to the muscle bath and force transducer. The bundle was then mounted between large platinum stimulating electrodes and placed in an in vitro muscle bath containing oxygenated Krebs solution maintained at 37 ° C. After a 15-min thermal equilibration period,
OPERATIONAL LENGTH OF CANJ:NE EXPIRATORY MUSCLES
89
the bundle was adjusted to its optimal length (Lo), defined as the length at which active tension was maximum. Lo was measured in triplicate by means of a micrometer. For each bundle, a twitch and a force frequency response were measured at Lo, and a maximum tetanic (100 Hz) length-tension curve was measured at resting lengths ranging from 55-125 % of Lo. On completion of these measurements, the bundle was removed from the apparatus, blotted dry, and weighed on an analytic balance. Muscle cross.sectional area was estimated by dividing the muscle mass by its length and specific gravity (1056 g/cm3), and muscle tensions were expressed as Newtons of force per square centimeter of cross-sectional area. Details regarding the in vitro measurements of contractile properties for other respiratory muscles have been previously published (Farkas and Rochester, 1988; Farkas et al., 1985). Protocol B. Determination of in situ length in relation to the in vitro Lo
Following our initial preparation, eight animals were secured to a body postural frame designed to support anesthetized animals in various postures. The frame has been described previously (Farkas and Schroeder, 1990) and consists of an assembly of aluminium bars, adjustable in all dimensions to match the size of the animal. The animals were supported by four to five r~ylon tie straps which were passed through the supraspinous ligament to affix the spine to a horizontal support bar. The limbs of the animals were subsequently taped to four vertical bars. The frame was designed not to restrict the motion of either the rib cage or abdominal compartments of the chest wall. TI~ ~',~.~
