Phrenic afferents and ventilatory control

It has been understood since the late 1800s that the diaphragm has significant sensory innervation. The role of phrenic afferents in the control of breathing, however, has been obscure. The phrenic nerve has been shown to contain a full array of afferent fibers. However, proprioceptive (group 1 fibers) afferents are few compared to postural muscles or the intercostals. The diaphragm, unlike the inspiratory intercostal muscles, has a small complement of spindle afferents and not all of these spindles are supplied with fusorial fibers. Reduced spindle afferents under gamma control help to explain previous studies of the diaphragm that have failed to reveal autogenic facilitation, that is, a reflex-mediated increase in drive during inspiratory loading. Nevertheless, some clinical studies have revealed increased activation of the diaphragm when its length is reduced. Group 1 fibers, which are predominantly tendon organ afferents in the diaphragm, have been shown to have a phasic inhibitory function. A reduction in this inhibition brought about by a reduction in diaphragmatic length during lung inflation may explain the increased diaphragmatic activation reported in clinical studies. Phrenic afferents have been shown to have multiple spinal and supraspinal projections. Recent studies have explored the ventilatory effects of thin fiber afferents (group III and IV fibers) in the phrenic nerve. Stimulation of these afferents has been shown both to inhibit and excite ventilation. These afferents arise from polymodal receptors that respond to both mechanical and chemical stimulation. Activation of these receptors may occur in a variety of conditions and the ventilatory response may be determined by the specific receptor activated.     

Thin-fiber phrenic afferents mediate the ventilatory response to diaphragmatic ischemia

We assessed the role of groups III and IV phrenic afferents in the ventilatory response to diaphragmatic ischemia in mechanicalldy ventilated, chloralose-anesthetized dogs using thein-situ isolated and innervated left hemidiaphragm preparation. The inspiratory motor drive to the right (Rt Edi) and left (Lt Edi) diaphragms, parasternal (Eps), and alae nasi (Ean) muscles was measured from the peak integrated EMG activities. When left diaphragmatic ischemia was produced in the control group (n = 6) by occluding the left phrenic artery for 20 min, Lt Edi increased to 158%, Rt Edi to 160%, Eps to 150% and Ean to 135% of baseline values. Left diaphragmatic tension, however, remained unchanged during the ischemia period. In the capsaicin-treated group (n = 6), we injected repeated doses of capsaicin, a selective stimulant of groups III and IV afferents, into the left phrenic artery to eliminate inputs from these afferents. Repeated injections of capsaicin are known to induce prolonged periods of afferent dysfunction. The first two injections of capsaicin (1 mg each) produced transient activation of the inspiratory muscles and higher breathing frequencies. Subsequent injections, however, failed to elicit any ventilatory changes. When diaphragmatic ischemia was induced after the last injection of capsaicin, no changes in the Right Edi, Eps and Ean were observed, whereas Left Edi and left diaphragmatic tension declined significantly. We conclude that increased inspiratory motor drive during selective diaphragmatic ischemia is mediated through the activation of groups III and IV phrenic afferents.     

Reflex inhibition of crural diaphragmatic activity by esophageal distention in cats

Distention of the esophagus has been shown to result in selective inhibition of phasic inspiratory activity in the crural portion of the diaphragm, with no effect on costal diaphragmatic activity. The purpose of this study was to determine rigourously the afferent pathways that mediate this response. Bipolar EMG electrodes were placed in the costal and crural portions of the diaphragm in decerebrate, spontaneously breathing cats. Distention of the esophagus by inflation of a Foley catheter balloon with 20 ml of air resulted in a selective inhibition of crural hiatal EMG activity, while costal EMG activity was maintained at predistention levels. The distention was accompanied by a reduction in respiratory frequency. Transection of the spinal cord at the C8-T1 level did not obliterate the crural inhibition produced by inflation. Section of the C4-C8 dorsal roots also failed to abolish the response. However, after bilateral cervical vagotomy, esophageal distention no longer influenced diaphragmatic EMG activity. These results indicate that the crural inhibition observed with esophageal distention is vagally mediated and is not influenced importantly by intercostal or phrenic afferents. Records of activity of the phrenic nerve branch innervating the crural portion of the diaphragm showed a similar response pattern, confirming that the inhibition is central in origin and that the crural fibers inhibited by distention are only a fraction of the total population of crural phrenic motoneurons.     

Inhibitory and excitatory effects on respiration by phrenic nerve afferent stimulation in cats

Respiratory effects of electrical stimulation of phrenic nerve afferents were studied in anesthetized cats, either spontaneously breathing or paralyzed and ventilated. The type of phrenic afferent fibers activated was controlled by recording the evoked action potentials from dorsal root fibers. In both preparations, stimulation at a strength sufficient to activate small diameter myelinated phrenic nerve afferents induced a biphasic response. The first phase lasted a few respiratory cycles and was inhibitory and consisted of a decrease in tidal volume (VT) or phrenic activity (NA), inspiratory time (TI), respiratory duty cycle (TI/Ttot) and instantaneous ventilation (VE) or minute phrenic activity (NMA). Expiratory time (TE) increased and breathing frequency (f) and mean inspiratory flow (VT/TI) or mean inspiratory neural activity (NA/TI) did not change. This short-term response was suppressed in animals pretreated with bicuculline. The second phase was a long-term excitation in which VT or NA, f, VE or NMA and VT/TI increased whereas both TI and TI/Ttot decreased and TE did not change. Unlike published data, our results suggest that small-diameter myelinated phrenic nerve afferents are involved in these responses. These phrenic fibers, like afferents from other muscles, affect respiratory output and may play a role in the control of breathing.     

Mechanism of rib cage inspiratory muscle recruitment in diaphragmatic paralysis

Paralysis of the diaphragm promotes an increase in the activation of the rib cage inspiratory muscles, and previous studies have suggested that this compensation is primarily due to vagal mechanisms (6). To test this hypothesis, we have assessed the effect of diaphragmatic paralysis on the electrical response of 19 parasternal intercostal muscles in eight anesthetized, vagotomized, spontaneously breathing dogs in the supine posture. Complete diaphragmatic paralysis was induced by section of the C5, C6, and C7 phrenic nerve roots in the neck. With the animals breathing room air, diaphragmatic paralysis resulted in a mean 94% increase in the peak height of integrated parasternal activity (p less than 0.001) associated with a 14 mm Hg decrease in arterial PO2 (p less than 0.05) and an 8 mm Hg increase in arterial PCO2 (p less than 0.001). The augmented parasternal activity was unrelated to the duration of inspiration and persisted when the animals were given a hyperoxic gas mixture. Thus the rib cage inspiratory muscles still compensate for diaphragmatic paralysis in the absence of vagal signals and of hypoxemia. This compensation probably results from the considerably augmented CO2 load placed on the extradiaphragmatic muscles.     

Diaphragmatic weakness and paralysis

Diaphragmatic weakness implies a decrease in the strength of the diaphragm. Diaphragmatic paralysis is an extreme form of diaphragmatic weakness. Diaphragmatic paralysis is an uncommon clinical problem while diaphragmatic weakness, although uncommon, is probably frequently unrecognized because appropriate tests to detect its presence are not performed. Weakness of the diaphragm can result from abnormalities at any site along its neuromuscular axis, although it most frequently arises from diseases in the phrenic nerves or from myopathies affecting the diaphragm itself. Presence of diaphragmatic weakness may be suspected from the complaint of dyspnea (particularly on exertion) or orthopnea; the presence of rapid, shallow breathing or, more importantly, paradoxical inward motion of the abdomen during inspiration on physical examination; a restrictive pattern on lung function testing; an elevated hemidiaphragm on chest radiograph; paradoxical upward movement of 1 hemidiaphragm during fluoroscopic imaging; or reductions in maximal static inspiratory pressure. The diagnosis of diaphragmatic weakness is confirmed, however, by a reduction in maximal static transdiaphragmatic pressure (Pdimax). The diagnosis of diaphragmatic paralysis is confirmed by the absence of a compound diaphragm action potential on phrenic nerve stimulation. There are many causes of diaphragmatic weakness and paralysis. In this review we outline an approach we have found useful in attempting to determine a specific cause. Most frequently the cause is either a phrenic neuropathy or diaphragmatic myopathy. Often the neuropathy or myopathy affects other nerves or muscles that can be more easily investigated to determine the specific pathologic basis, and, by association, it is presumed that the diaphragmatic weakness or paralysis is secondary to the same disease process.     

Diaphragmatic weakness and paralysis

Diaphragmatic weakness implies a decrease in the strength of the diaphragm. Diaphragmatic paralysis is an extreme form of diaphragmatic weakness. Diaphragmatic paralysis is an uncommon clinical problem while diaphragmatic weakness, although uncommon, is probably frequently unrecognized because appropriate tests to detect its presence are not performed. Weakness of the diaphragm can result from abnormalities at any site along its neuromuscular axis, although it most frequently arises from diseases in the phrenic nerves or from myopathies affecting the diaphragm itself. Presence of diaphragmatic weakness may be suspected from the complaint of dyspnea (particularly on exertion) or orthopnea; the presence of rapid, shallow breathing or, more importantly, paradoxical inward motion of the abdomen during inspiration on physical examination; a restrictive pattern on lung function testing; an elevated hemidiaphragm on chest radiograph; paradoxical upward movement of 1 hemidiaphragm during fluoroscopic imaging; or reductions in maximal static inspiratory pressure. The diagnosis of diaphragmatic weakness is confirmed, however, by a reduction in maximal static transdiaphragmatic pressure (Pdimax). The diagnosis of diaphragmatic paralysis is confirmed by the absence of a compound diaphragm action potential on phrenic nerve stimulation. There are many causes of diaphragmatic weakness and paralysis. In this review we outline an approach we have found useful in attempting to determine a specific cause. Most frequently the cause is either a phrenic neuropathy or diaphragmatic myopathy. Often the neuropathy or myopathy affects other nerves or muscles that can be more easily investigated to determine the specific pathologic basis, and, by association, it is presumed that the diaphragmatic weakness or paralysis is secondary to the same disease process.     

Dyspnea and respiratory reflexes]

It has been suggested that afferents from intercostal muscles may play a role in the genesis of dyspnea. In this study, lower intercostal muscles were tapped or vibrated to induce a reflex, evoked potentials, and sensation. The tapping stimulus induced H reflex in the same muscle with a latency of 12 msec. Also the same stimulus induced evoked potentials in the cerebral cortex (N1: 19.8 +/- 1.2 msec). This suggests projection of the intercostal muscle spindle afferents to the cerebral cortex. 100 Hz vibration induced a later component, presumably an event-related potential, at 250 msec after the onset of both the inspiratory and expiratory phase. Thus, it may be possible that intercostal muscle spindle afferents project to the cerebral cortex and play a role in respiratory sensation. It has been suggested that dyspnea is reduced by increasing inspiratory and expiratory intercostal muscle spindle afferents during the inspiratory and expiratory phases, respectively. Thus, stretching the inspiratory and expiratory intercostal muscles during the respective muscular contraction phase may be effective in reducing dyspnea.     

Activity of the respiratory muscles during natural defecation: a study on experimental animals]

Activity of the respiratory muscles during natural defecation was studied in two anesthetized and two decerebrate dogs. In anesthetized dogs, excitation of the abdominal muscles and an increase in gastric pressure were observed during defecation. However, pleural pressure was little influenced by such increase in abdominal pressure, maintaining the same rhythmic changes as observed during spontaneous respiration. The rhythmic changes in pleural pressure were associated with rhythmic activity of the diaphragm. When gastric pressure increased during defecation, the diaphragmatic activity also increased during both the inspiratory and expiratory phases. In a decerebrate dog, airflow and airway pressure changed similarly to during defecation. The diaphragm was continuously active, with superimposed rhythmic augmentation. In a paralyzed and artificially ventilated dog with open-chest, the phrenic nerve similarly developed discharges. We conclude that the non-respiratory activity and rhythmic augmentation of phrenic nerve discharge during defecation is pre-programmed in the command for defecation. The activity of phrenic motoneurons may be further modulated by changes in thoracic and abdominal pressure. These mechanisms may act together to coordinate respiration and defecation.     

Lambert-Eaton myasthenic syndrome involving the diaphragm

Inspiratory muscle function was assessed in a patient with the Lambert-Eaton myasthenic syndrome that developed in association with a bronchogenic carcinoma. Repetitive maximal inspiratory pressure measurements and the electromyographic response to phrenic nerve stimulation established involvement of the inspiratory muscles in general and the diaphragm specifically in this condition.     

Bilateral phrenic paralysis in a patient with systemic lupus erythematosus

Respiratory manifestations of systemic lupus erythematosus (SLE) are frequent. They include respiratory muscle abnormalities, which have been implicated in the pathogenesis of the "shrinking lung syndrome" (SLS). We report the case of a patient with this syndrome, in whom diaphragmatic paralysis due to demyelinating phrenic lesions was diagnosed at the same time as SLE. Follow-up studies showed a favorable clinical and diaphragmatic outcome with corticosteroid therapy, but little change in spirometry. It is concluded that severe diaphragm palsy is possibly due to phrenic nerve lesions in SLE, and that the link between diaphragm dysfunction and the SLS is probably not a straightforward one.     

Differential cytokine gene expression in the diaphragm in response to strenuous resistive breathing

Strenuous resistive breathing induces plasma cytokines that do not originate from circulating monocytes. We hypothesized that cytokine production is induced inside the diaphragm in response to resistive loading. Anesthetized, tracheostomized, spontaneously breathing Sprague-Dawley rats were subjected to 1, 3, or 6 hours of inspiratory resistive loading, corresponding to 45-50% of the maximum inspiratory pressure. Unloaded sham-operated rats breathing spontaneously served as control animals. The diaphragm and the gastrocnemius muscles were excised at the end of the loading period, and messenger ribonucleic acid expression of tumor necrosis factor-alpha, tumor necrosis factor-beta, interleukin (IL)-1alpha, IL-1beta, IL-2, IL-3, IL-4, IL-5, IL-6, IL-10, IFN-gamma, and two housekeeping genes was analyzed using multiprobe RNase protection assay. IL-6, IL-1beta, and, to lesser extents, tumor necrosis factor-alpha, IL-10, IFN-gamma, and IL-4 were significantly increased in a time-dependent fashion in the diaphragms but not the gastrocnemius of loaded animals or in the diaphragm of control animals. Elevation of protein levels of IL-6 and IL-1beta in the diaphragm of loaded animals was confirmed with immunoblotting. Immunostaining revealed IL-6 protein localization inside diaphragmatic muscle fibers. We conclude that increased ventilatory muscle activity during resistive loading induces differential elevation of proinflammatory and antiinflammatory cytokine gene expression in the ventilatory muscles.     

呼吸机引起的横膈功能障碍的研究进展

呼吸机引起的横膈功能障碍是由机械通气后膈肌张力负荷下降、神经刺激减少、肌肉萎缩、结构损伤和肌纤维重塑等多因素引起的横膈肌收缩能力不全的临床现象,主要表现为脱机困难。机械通气时间、通气模式和潮气量等对呼吸机引起的横膈功能障碍的发生有显著影响。通过治疗原发病、合理通气、刺激膈神经和抗氧化干预等方法可以对其进行有效防治。     

Transmission fatigue of the rabbit diaphragm

This study evaluates the role of transmission fatigue of the diaphragm in rabbits subjected to inspiratory resistive loading (IRL) sufficiently severe to increase peak tidal airway pressure to about 50% of that elicited by 100 Hz phrenic nerve stimulation. After 58 +/- 14 min of IRL, the transdiaphragmatic pressure (Pdi) responses to phrenic nerve stimulation at 20, 60, and 100 Hz were reduced by approximately one third. In contrast, IRL induced no significant change in the response to direct diaphragm stimulation (in the presence of transient neuromuscular blockade). Although respiratory acidosis occurred during IRL (pH 7.04 +/- 0.04, PCO2 90 +/- 10, PO2 131 +/- 38), it was not sufficient to explain the reduced contractility. In a separate series of experiments, the diaphragm compound action potential elicited by unilateral phrenic nerve stimuli was recorded by implanted diaphragm electrodes and the Pdi elicited by contralateral phrenic nerve stimulation at 100 Hz was measured. Both action potential amplitude and Pdi declined during IRL and both improved after 10 min of recovery. These findings demonstrate that transmission fatigue plays a major role in rabbit diaphragm fatigue induced by spontaneous breathing against inspiratory resistance.     

Respiratory muscle activation by limb muscle afferent stimulation in anesthetized dogs

In 10 chloralose anaesthetized and spontaneously breathing dogs, we assessed the effect of limb muscle afferents on the peak integrated EMG activities of the genioglossus, alae nasi, costal diaphragm, parasternal intercostal, triangularis sterni, and transverse abdominis muscles. The influence of vagal and baroreceptor afferents were eliminated by vagotomy and perfusion of carotid sinuses at a constant pressure. Muscle afferents were activated by stimulating the central end of the gastrocnemius nerve for 1 min at 40 Hz and at different voltages. Stimulation at voltages equal to 5, 10 and 20 times twitch-threshold increased minute ventilation to 165, 216 and 250% of pre-stimulation values, respectively, which was achieved by increasing breathing frequency (shortening of the inspiratory and expiratory times) and tidal volume. The activity of the parasternal intercostal and alae nasi muscles increased by a similar degree to that of the diaphragm while the activities of the genioglossus and transverse abdominis were augmented to a greater degree than that of the diaphragm. On the other hand, the motor drive to triangularis sterni increased significantly only at 20 times twitch-threshold and to a lesser degree than that to the diaphragm. These results suggest that upper airway, inspiratory and expiratory rib cage and abdominal muscles may be independently regulated. Differences in the sensitivity of these muscles to the activation of limb muscle afferents can be explained by a complex pattern of central projections of these afferents on the central respiratory controllers or by intrinsic properties of the motor output of these controllers.     

In normal subjects bracing impairs the function of the inspiratory muscles.

Normal subjects can increase their capacity to sustain hyperpnoea by bracing their arms on fixed objects, a procedure which is also known to reduce dyspnoea in patients with chronic obstructive pulmonary disease (COPD). In the present study, it was tested whether bracing per se could improve the function of the diaphragm. The effect of bracing on diaphragm function was studied in six normal subjects by recording changes in oesophageal (delta Poes) and transdiaphragmatic (delta Pdi) pressure during inspiratory capacity (IC) manoeuvres in the seated and upright postures, and in the seated posture, also during bilateral phrenic nerve stimulation (BPNS) at functional residual capacity (FRC). The pattern of ribcage motion and deformation associated with bracing and with diaphragm contraction was also evaluated using inductance plethysmography and magnetometers. Bracing increased FRC by >300 mL and reduced IC by approximately 200 mL, in both postures. Delta Pdi during BPNS decreased on average by 15% indicating an impaired diaphragmatic function. The ribcage was deformed with bracing and was more distortable during BPNS. In conclusion, in normal subjects, bracing impairs the function of the inspiratory muscles and reduces ribcage stability. These negative effects cannot explain the improved capacity to sustain hyperpnoea when the arms are braced.     

Phrenic nerve pacing via intramuscular diaphragm electrodes in tetraplegic subjects

CONTEXT: Diaphragm pacing in ventilator-dependent tetraplegic subjects is usually achieved by the placement of phrenic nerve electrodes via thoracotomy. However, this technique may be accomplished less invasively via laparoscopic placement of IM electrodes, at a lower cost and with less risk of injury to the phrenic nerve. OBJECTIVE: To assess the feasibility of laparascopic placement of IM diaphragm electrodes to achieve long-term ventilatory support in ventilator-dependent tetraplegic subjects. DESIGN, SETTING, AND PARTICIPANTS: Two IM diaphragm electrodes were placed laparoscopically in each hemidiaphragm in five subjects with ventilator-dependent tetraplegia. Studies were performed either on an outpatient basis or with a single overnight hospitalization. Ventilator-dependent tetraplegic subjects were identified in whom bilateral phrenic nerve function was present, as determined by phrenic nerve conduction studies. Following electrode placement, subjects participated in a conditioning program to improve the strength and endurance of the diaphragm over a period of 15 to 25 weeks. The duration of the study was variable depending on the time necessary to determine the maximum duration that individuals could be maintained without mechanical ventilation support. MAIN OUTCOME MEASURES: Magnitude of inspired volume generation and duration of ventilatory support with bilateral diaphragm pacing alone. RESULTS: In four of the five subjects studied, initial bilateral diaphragm stimulation resulted in inspired volumes between 430 and 1,060 mL. Reconditioning of the diaphragm over several weeks resulted in substantial increases in inspired volumes to 1,100 to 1,240 mL. These subjects were comfortably maintained without mechanical ventilatory support for prolonged time periods by diaphragm pacing, by full-time ventilatory support in three subjects, and 20 h per day, in the fourth subject. No response to stimulation was observed in one subject, most likely secondary to denervation atrophy. CONCLUSIONS: Diaphragm pacing in ventilator-dependent tetraplegic subjects can be successfully achieved via laparascopic placement of IM electrodes.     

Effect of head-up tilt on neural inspiratory drive in the anesthetized dog

To examine how anesthetized dogs compensate for the diaphragmatic shortening that occurs during head-up tilting, we measured the electroneurogram (ENG) of the C5 phrenic root and the electromyographic (EMG) activity of the parasternal intercostal and transversus abdominis muscles in eight spontaneously breathing animals during postural changes between supine (0 degree) and 80 degrees head-up. Both steady state ENG and EMG activities and first breath responses to tilting from 80 degrees head-up to supine were studied. These experiments have shown that: (1) anesthetized dogs respond to head-up tilting by increasing the neural drive to the costal diaphragm and parasternal intercostals; (2) this response, however, does not occur on the first breath and therefore cannot compensate for the immediate changes in diaphragmatic length; (3) the abdominal muscles, in contrast, show a first breath response to tilting and their activation is primarily responsible for the maintenance of tidal volume. Unlike in humans, increases in neural inspiratory drive in head-up anesthetized dogs are mediated by a chemoreceptive, rather than proprioceptive, feedback mechanism.     

The effects of chemical versus locomotory stimuli on respiratory muscle activity in the awake dog

Using 6 chronically instrumented awake dogs, we contrasted the ventilatory and respiratory muscle EMG responses to steady-state normoxic hypercapnia with those occurring at similar levels of ventilation during steady-state treadmill exercise. During hypercapnia, increases in ventilatory output were primarily due to increments in VT whereas during exercise, increases in minute ventilation were due primarily to increases in frequency. With either hyperpnea, augmentation of both inspiratory and expiratory muscle EMG activity occurred but only diaphragmatic EMG activity was strongly correlated with tidal volume changes in both conditions. Using both EMGs and thoraco-abdominal pressures, we found that with increasing chemical or locomotory stimuli, during inspiration (1) diaphragmatic EMG activation occurred sooner relative to the onset of mechanical flow and (2) that respiratory muscles other than the diaphragm contributed significantly to the generation of inspiratory flow and VT either actively or passively through recoil. Post-inspiratory inspiratory activity of the crural diaphragm shortened relative to control in hypercapnia but did not change in exercise. Finally, during mild hyperpneas, end-expiratory lung volume did not change significantly in either hypercapnia or exercise but decreased at the higher levels of hyperpnea only during exercise. This may reflect differences not in the magnitude of phasic expiratory muscle activity, but rather in the degree of tonic abdominal muscle activation between the two conditions.     

A comparative analysis of contractile characteristics of the diaphragm and of respiratory system mechanics

The mean inspiratory flow rate (VT/TI) is used as an index of central respiratory 'drive', and, at rest, it varies interspecifically in proportion to body weight (BW) raised to the 0.74 power (Boggs and Tenney, 1984). VT/TI is determined by the level of central neural respiratory output, the velocity of contraction of respiratory muscles, and the mechanical characteristics of the respiratory system. We have examined the last two factors in 13 species ranging in weight from 0.025 to 515 kg. We determined the 'effective' inspiratory mechanical characteristics of the respiratory system (time constant, resistance, and compliance) and the time course of diaphragmatic contraction during bilateral supramaximal phrenic nerve stimulation in anesthetized animals. We also measured passive expiratory mechanical variables and made morphometric measurements of the diaphragm. We found that VT/TI during phrenic nerve stimulation was proportional to BW0.82. The 'effective' respiratory time constant (tau'rs) and passive expiratory time constant (tau rs) scaled in proportion to body weight with nearly similar exponents: tau'rs alpha BW0.26 and tau rs alpha BW0.21. In addition, the time constant of diaphragmatic contraction (tau mc) was proportional to BW0.20. Inspiratory time is proportional to tau'rs and tau mc, and tidal volume during stimulation was almost directly proportional to body weight. Thus, interspecific changes in VT/TI during stimulation were related to interspecific changes in the mechanical characteristics of the respiratory system and the velocity of muscular contraction. We conclude that interspecific changes in VT/TI need not reflect interspecific variation in central respiratory drive under resting conditions. We found that diaphragm weight and volume and diaphragm muscle thickness were geometrically similar in all species studied. Inspiratory pressure is an interspecific constant; therefore, by the Law of Laplace, smaller animals must develop greater tension per unit of muscle mass.